Vrije Universiteit Brussel

Low-loss millimeter-length waveguides and grating couplers in single-crystal diamond

Gao, Fei; Huang, Zhigong; Feigel, Benjamin; Van Erps, Jürgen Albert; Thienpont, Hugo;Beausoleil, Raymond G.; Vermeulen, NathaliePublished in:J. Lightwave Technol.

DOI:10.1109/JLT.2016.2622620

Publication date:2016

Link to publication

Citation for published version (APA):Gao, F., Huang, Z., Feigel, B., Van Erps, J. A., Thienpont, H., Beausoleil, R. G., & Vermeulen, N. (2016). Low-loss millimeter-length waveguides and grating couplers in single-crystal diamond. J. Lightwave Technol., 34(23),5576-5582. https://doi.org/10.1109/JLT.2016.2622620

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Low-Loss Millimeter-Length Waveguides andGrating Couplers in Single-Crystal Diamond

Fei Gao, Zhihong Huang, Member, IEEE, Benjamin Feigel, Student Member, IEEE, Jurgen Van Erps, Member, IEEE,Hugo Thienpont, Member, IEEE, Raymond G. Beausoleil, Senior Member, IEEE,

and Nathalie Vermeulen, Member, IEEE

Abstract—We report on the design, fabrication, and charac-terization of millimeter-length strip waveguides with monolithicgrating couplers in commercially available synthetic single-crystaldiamond. To minimize the device footprint and the influenceof the wafer wedge of the single-crystal diamond thin plate, weadopt a curled waveguide layout. The devices are fabricated usingelectron-beam lithography and reactive-ion etching. To improvethe e-beam patterning accuracy of the grating etch masks, weapply proximity-effect compensation on the gratings and tapers.The linear characterization results indicate a waveguide attenu-ation of 6.5 dB/mm and a grating transmission of −6.3 dB in thefiber-optic communication C band. These results demonstrate thefeasibility of fabricating long waveguides and integrated gratingcouplers in single-crystal diamond. Our research findings wouldbe beneficial for further exploring quantum and nonlinear opticsin integrated single-crystal diamond devices.

Index Terms—Diamond, electron beam lithography, gratings,integrated optics, optical waveguides, proximity effects.

I. INTRODUCTION

IN RECENT years, synthetic single-crystal diamond (SCD)has been identified as a promising optical material for

integrated photonics [1], [2]. The individual nitrogen-vacancy(NV) centers, which are present only in SCD and cannot befound in poly-crystal diamond materials [3], have attractedmuch attention for realizing quantum-photonic on-chip de-vices [4]–[9]. Also, for nonlinear optical applications suchas wavelength conversion, SCD possesses many attractiveproperties such as large transparency windows from 0.23 μmto 2.6 μm and from 6.2 μm to 10 μm and beyond [10].Furthermore, it does not suffer from multi-photon absorptionin the visible and infrared (IR) ranges, yet exhibits, as a resultof its single-crystal structure, a relatively large Raman gain

Manuscript received February 5, 2016; revised September 12, 2016; acceptedOctober 19, 2016. This work was supported in part by the Research FoundationFlanders under the project G.A002.13N, in part by BELSPO-IAP, in part bythe Hercules and Methusalem foundations, and in part by the OZR of theVrije Universiteit Brussel. The work of N. Vermeulen was supported by ERC-FP7/2007-2013 grant 336940.

F. Gao, B. Feigel, J. Van Erps, H. Thienpont, and N. Vermeulen are withthe Brussels Photonics Team, Department of Applied Physics and Photonics,Vrije Universiteit Brussel, 1050 Brussels, Belgium (e-mail: fgao@b-phot.org;bfeigel@b-phot.org; jverps@b-phot.org; hthienpo@vub.ac.be; nvermeul@b-phot.org).

Z. Huang and R. G. Beausoleil are with the Large-Scale Integrated PhotonicsResearch Group, Hewlett Packard Labs, Palo Alto, CA 94304 USA (e-mail:zhihong.huang@hpe.com; ray.beausoleil@hpe.com).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2016.2622620

(13.5 cm/GW at 1030 nm) and a considerable Kerr nonlinearity(n2 = 1.3 × 10−19 m2/W at 545 nm) [2]. In addition, it has arelatively high refractive index of n = 2.39 at 1550 nm [2]. Thisenables strong confinement of light in diamond waveguides, asrequired by integrated photonic applications.

Integrated photonic devices have been demonstrated insingle-, poly-, and nano-crystal diamond materials [4]–[19].Different fabrication approaches have been used by severalresearch groups worldwide, either relying on electron-beamlithography (EBL) with inductively-coupled plasma reactive-ion etching (ICP RIE), or focused-ion-beam (FIB) milling [14],[15]. Compared to FIB milling, the combination of EBL andICP RIE has the advantage of achieving higher precision andoffers the possibility of realizing more sophisticated devicedesigns. Most of the devices realized thus far are fabricated in alayered diamond-on-insulator (DOI) substrate with a diamondmembrane on top of a buried oxide (SiO2) layer.

High performance ring and photonic-crystal resonators madein single-crystal DOI substrates have been realized, from whichquality-factors up to, respectively, 106 and 6000 have been mea-sured [8], [12]. In addition, a very low propagation loss of0.34 dB/cm has been derived from the high Q-factor of the ringresonator [12]. Usually, SCD thin plates contain a wafer wedge(>300 nm/mm [12]) which is caused by the diamond polishingprocess. To minimize the influence of the wafer wedge, those de-vices mentioned above were relatively small (with their longestedge up to about 300 μm) and their path lengths were similarlyshort (sub-millimeter). Recently, a new technique for preparingSCD thin plates without wafer wedge was introduced [3]. Thisapproach consists of ion slicing in conjunction with homoepitax-ial re-growing to create single-crystal diamond membrane win-dows (SCDMWs) with a homogeneous thickness. High-qualityring resonators with a loaded Q-factor of 6.6 × 104 were fabri-cated in these SCDMW samples. Whereas such samples withoutwafer wedge would be a very suitable platform for fabricatingrelatively large devices, the longest path length realized so farin SCDMW films is well below 1 mm [3]. However, in large-scale integrated circuits, devices with longer path lengths suchas waveguides of at least a few millimeters long are essential.

To date, the methods used to couple light into and out ofsingle-crystal DOI photonic devices are mainly based on edgecoupling [12], optical fiber taper coupling [16], or grating-assisted microscope objective coupling [7], [9]. Among thevarious light coupling approaches, a grating-enabled fiber-to-chip coupling method offers a relatively high efficiency anddevice layout flexibility, as well as the possibility of automated

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wafer-scale testing. A further advantage is that by integratingmonolithic grating couplers in the devices, one avoids addi-tional lithography steps to overlay the diamond waveguides withpolymer waveguide extensions required for edge coupling [12].However, grating couplers optimized for flat-cleaved opticalfiber coupling at telecom wavelengths have, to our knowledge,rarely been demonstrated in single-crystal DOI photonic de-vices. Moreover, the grating couplers recently demonstrated onthe aforementioned SCDMW samples featured a relatively lowcoupling efficiency of −15 dB [3], which should be futher in-creased for allowing practical applications.

Fabricating integrated devices in poly/nano-crystal DOI sub-strates is less challenging than in single-crystal DOI substrates,as the former can be prepared by directly growing a dia-mond layer through plasma-enhanced chemical vapor deposi-tion (PECVD) on an existing wafer-scale SiO2 substrate [17].The poly/nano-crystal DOI substrates fabricated in such a waydo not suffer from wafer wedge. Various high-quality on-chipdevices have been realized in poly-crystal DOI substrates, in-cluding ring resonators with Q-factors up to 11000, waveguidesup to 4.6 mm long with a transmission loss of 5.3 dB/mm, andmonolithic grating couplers with a coupling efficiency of −5 dB[17], [18].

To be able to fully exploit the potential of SCD in, amongothers, quantum and nonlinear optics, both millimeter-lengthwaveguides and efficient grating couplers should be developedalso in SCD. In this paper, we present the design, fabrication,and characterization of millimeter-length strip waveguides withmonolithically integrated grating couplers in SCD. The devicesare fabricated using EBL and ICP RIE techniques on SCD sam-ples with a wafer wedge. Cutback measurement results indicatethat we have achieved single-mode operation in the fiber-opticcommunication C band with a waveguide attenuation of about6.5 dB/mm and a grating coupling efficiency around −6.3 dB.Compared to the devices fabricated in poly-crystal diamond (forwhich one can use a more advanced, partially-etched gratingand waveguide designs [17]), our more simplified SCD devicesachieve comparable performances.

II. DESIGN

A. Waveguide Cross Section and Layout Design

Due to the wafer wedge, partially-etched rib waveguides can-not be fabricated accurately. In this work, we only consider stripwaveguide designs. To ensure low-loss propagation and single-mode operation, we choose a 900 nm × 500 nm rectangular dia-mond core surrounded by a 2 μm thick silica cladding. Althoughnot strictly required, we have added the silica top cladding soas to reduce the waveguide scattering losses [11]. The opticalmodes of the waveguide at 1550 nm are numerically simulatedin a fully vectorial mode solver (Lumerical MODE Solutions).The mode profiles of the fundamental transverse-electric (TE)and transverse-magnetic (TM) modes are shown in Fig. 1.

We design a set of curled waveguide layouts with devicelengths up to about 2 mm as shown in Fig. 2. There are severaladvantages to using this type of layout. Firstly, as aforemen-tioned, the SCD membranes often present a wedge which is

Fig. 1. Numerically simulated optical mode profiles of a diamond strip wave-guide with a width of 900 nm and a height of 500 nm. The wavelength used is1550 nm.

Fig. 2. Four types of waveguide layouts with device lengths of: (a) 0.642 mm,(b) 1.134 mm, (c) 1.575 mm, and (d) 2.016 mm.

caused by polishing during the diamond plate processing. Ourcurled waveguide layout brings the input and output gratingscloser to each other so that the two gratings have a minimizedheight difference in the wedged wafer. This way, the trans-mission windows of the two gratings (on the same device) donot show a significant difference, which is beneficial towardsensuring a high device transmission. Secondly, in the worstcase scenario, millimeter-long straight waveguides fabricatedin a wedged wafer could exhibit more than 300 nm deviationin waveguide height. The compactness of the curled layout istherefore ideal to reduce the height error for long waveguides.Thirdly, In order to fit a long waveguide within a small devicefootprint, a meander layout usually takes many more sharp turnsthan our curled layout. Through preliminary measurements wefound that the total device transmission loss for millimeter-longwaveguides could be up to 20 dB higher when using a meanderlayout instead of the curled layout. Therefore it is preferable toadopt the curled layouts to minimize the number of sharp bends,thus reducing the amount of waveguide bending loss. Finally, all

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our waveguide layouts use the same grating-coupler interfaceconfiguration. This enables an efficient wafer-scale characteri-zation using an automated testing stage.

B. Grating Design

We employ grating couplers to couple the TE polarized light at1550 nm between the on-chip diamond waveguides and G.652standard telecom single-mode optical fibers. As the wedgeddiamond wafer is not suitable for realizing an accurate partially-etched grating profile, we design a binary grating coupler, withthe grating trenches fully etched throughout the diamond layer.

To design the grating coupler, i.e. to determine the optimalgrating period and fill factor, we maximize the coupling effi-ciency, while taking into account the manufacturability. We usea finite-difference time-domain (FDTD) simulation software(Lumerical FDTD Solutions) for this optimization. To deter-mine a suitable starting point for the numerical optimization ofthe grating parameters, we first carry out an approximate calcu-lation of the grating period using the phase matching conditiongiven by [20]

Λ =λ

neff − sin(θ)ncla, (1)

where Λ is the grating period, λ = 1550 nm is the free-spacewavelength, neff = 1.915 is approximated as the average valueof the refractive index of diamond (n = 2.39) and SiO2 (n =1.44), and ncla = 1.44 is the refractive index of the silicacladding. The light emission angle of the grating coupler isθ degrees away from the normal to the grating surface. This isnecessary to avoid that the grating coupler reflects the outgoinglight back to the on-chip waveguide and reduces the couplingefficiency [21]. We choose θ = 9◦, which corresponds to thefiber angle of the Cascade Microtech LWP optical probe in ourmeasurement setup. Equation (1) then yields an initial gratingperiod of Λ = 917 nm.

We then carry out an FDTD simulation to determine the opti-mal grating fill factor for the initial grating period. By calculatingthe grating coupling efficiency for different grating fill factors,while keeping the grating period fixed at 917 nm, we obtain themaximal coupling efficiency at the fill factor of 0.67.

To improve its manufacturability without compromisingmuch grating performance, we search for grating designs withlower fill factors and larger grating periods by performing a finetwo-parameter sweep in the neighborhood of their initial values.The simulation results shown in Fig. 3 indicate a range of grat-ing periods and fill factors yielding high coupling efficienciesbetween −3.81 dB and −3.74 dB. We choose the combinationof a relatively large grating period (950 nm) and a relatively lowfill factor (0.6) within the optimal range as is indicated by “X”in Fig. 3. This combination yields a local maximal efficiency of−3.81 dB.

III. FABRICATION

A. DOI Wafer Preparation

The devices are fabricated in the single-crystal DOI sub-strates which are prepared using a similar procedure previously

Fig. 3. Results of a grating coupling efficiency FDTD simulation showingthe coupling efficiency at 1550 nm as a function of the grating period and fillfactor. Suitable grating period and fill factor combinations yielding coupling ef-ficiencies between −3.81 dB and −3.74 dB are indicated by the lighter tone andfollow a diagonal line from the lower left corner to the upper right corner in thefigure. To benefit from a higher manufacturability, we choose the combinationof 950 nm and 0.6 as indicated by “×”.

developed by HP Laboratories [7]. We use a Type IIa (100) di-amond plate which is purchased from MB Optics and has a sur-face of 3 mm × 3 mm and a thickness of 5 μm. In the first step,the diamond plate is cleaned in subsequently acetone, methanol,and isopropyl alcohol. The cleaned diamond plate is then frag-mented by using a diamond-tip scribing pen into smaller pieceswith edge lengths of approximately a few hundred micrometers.We place each diamond piece (sample) onto a 10 mm × 10 mmlarge silicon carrier wafer that has been thermally oxidized toform a 2 μm thick SiO2 top layer. The diamond samples aremerely attached to their carrier wafers via van der Waals forces.In the second step, the diamond samples are thinned to a thick-ness of about 500 nm by RIE in Ar/Cl2 plasma. We choose thechlorine-based recipe because it allows us to reduce the rough-ness of the diamond surface [22]. The RIE thinning process alsoetches the top surface of the carrier wafers. Consequently, afterthe thinning process, the height difference between the surfacesof the diamond samples and their carrier wafers can increasetens of micrometers. Such a height difference is problematic inthe subsequent spin coating process, because it results in a lessuniform e-beam resist film. Therefore, we carry out the thinningprocess in several iterations, between which we can examine thethinning progress and replace the carrier wafers. A polyethyleneterephthalate (PET) film and a vacuum pick-up pen are used tohandle the diamond samples. We also use these tools to flip thediamond samples such that both the top and bottom surfaces areetched and smoothed.

B. E-Beam Lithography

The e-beam resist (Dow Corning XR-1541, 6%) is appliedvia spin coating (at 3000 rpm during 45 s) to form a 100 nmthick film on top of the thinned diamond samples. We usea Raith150-two electron-beam writing tool to define the etchmasks in the e-beam resist. The main patterning settings on the

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Fig. 4. A qualitative demonstration of the electron-dose settings for a grat-ing with its taper before and after the proximity-effect compensation in theRaith150-two software. The latter increases the dose values near the edges ofthe pattern, and decreases the dose values near the center of the pattern. Thisway, the exposed area is subjected to an effective electron dose which is uniformat all points. The total length of the grating and taper is 100 μm.

Raith150-two are: a working distance of 10 mm, an accelerationvoltage of 30 kV, an aperture size of 10 μm, and a writefield sizeof 200 μm × 200 μm. To produce high-quality waveguide etchmasks, we use the fixed-beam moving-stage (FBMS) patterningmode provided by the Raith150-two. Compared to the scan-ning mode mostly used in e-beam patterning, the FBMS modeoffers two advantages for long waveguide patterning. Firstly,it reduces the roughness of the waveguide edges and, conse-quently, reduces the scattering losses. Secondly, it can exposean entire waveguide pattern in one “stroke” across multiplewritefields. This is beneficial to avoiding those potential discon-tinuities, typically occurring in the scanning mode, such as gaps,lateral misalignments and overlaps at stitching points betweenwritefields.

An important challenge in e-beam patterning is the proximityeffect which limits the patterning accuracy. This effect is due tothe e-beam broadening caused by forward- and back-scatteredelectrons during electron-matter interaction. The software of theRaith150-two allows us to modify the electron dose settings tocompensate for the proximity effect. Since the pattern of thegrating couplers contains fine features and is heavily affectedby this effect, we need to compensate the proximity effect on thegratings. We also include the tapers in the proximity-effect com-pensation because their large exposure areas result in the scat-tering of electrons towards the adjacent grating areas during thee-beam patterning. Fig. 4 qualitatively shows the dose settings ofthe grating and taper areas before and after the proximity-effectcompensation in the Raith150-two software.

To determine the adequate electron dose adjustment forproximity-effect compensation, the software uses a function tocalculate the scattered-electron distribution. The basic format ofthis function is the sum of two Gaussian terms [23]

f(x) =1

π(1 + η)

(1α2 e−

x 2

α 2 +η

β2 e− x 2

β 2

). (2)

In (2), x is the lateral distance from the e-beam, α is the range ofthe forward-scattered electrons, β is range of the back-scatteredelectrons, and η is the back-scattering efficiency. These param-eters are strongly dependent on the acceleration voltage of theincident electron beam, the material and the thickness of boththe e-beam resist and the substrate [23]. We calculate α by using

Fig. 5. SEM images of two grating etch masks generated with (lower image)and without (upper image) proximity-effect compensation. This comparisonshows that this e-beam patterning technique reduces the grating line swelling inthe central grating area and improves the e-beam patterning accuracy.

the following equation [24]

α = 0.9(Rt/Vb)1.5 , (3)

where Rt is the thickness of the e-beam resist (about 100 nm)and Vb is the acceleration voltage of the electron beam (30 kV).This yields α ≈ 5 nm. However, we do not find any significantdifference in our proximity-compensation results using eitherα = 5 nm or the default α = 2 nm. The values of β and η aredetermined empirically by e-beam patterning experiments inthe following way. We pattern multiple grating and taper ar-eas with the proximity effect compensated by using differentcombinations of β and η values. By visually examining the e-beam patterned grating areas with scanning electron microscopy(SEM), we identify the parameter combination which generatesthe most uniform grating lines. The optimal parameters we haveidentified for our substrate are β = 3500 nm and η = 0.75.

Fig. 5 shows a comparison between two grating masks pat-terned with and without proximity-effect compensation. It isclear that this e-beam patterning technique reduces the line-width swelling in the central grating area.

The e-beam patterned samples are developed in the AZ300MIF developer. To transfer the mask pattern to the diamondlayer underneath, we perform a second RIE step in Ar/O2plasma. Compared to the aforementioned chlorine-based recipe,the oxygen-based recipe is more suitable for pattern transfer be-cause it provides a higher selectivity between the etch mask andthe diamond, resulting in a reduced mask erosion. After patternetching, we grow a 2 μm thick SiO2 film via PECVD on top ofthe chip to form an upper cladding, as required by our design.

IV. CHARACTERIZATION

The newly fabricated diamond devices are examined visuallyunder an optical microscope as shown in Fig. 6. The dimensionsof the waveguide width and grating period are measured usingan atomic force microscope (AFM) in tapping mode before theSiO2 is deposited on the chip. These measurement results are

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Fig. 6. Microscope images of two diamond chips containing four differentdevice layouts.

Fig. 7. AFM measurement results of a typical SCD device indicating a wave-guide width of about 920 nm and a grating period of about 950 nm. Due tothe finite size of the AFM probe, the gaps between adjacent grating lines showa “V”-shaped profile which is slightly different from the actual gap profile.Nonetheless, the results of the grating period measurement are not affected.

given in Fig. 7, and indicate a waveguide width of about 920 nmand a grating period of about 950 nm, both of which are veryclose to the design values.

Fig. 8 shows a SEM image of a typical diamond grating,before the silica encapsulation, from which we estimate a gratingfill factors of about 0.64. This is larger than our designed valueof 0.6. We anticipate that this is one of the major fabricationerrors which affects the grating performance.

Next, we measure the optical transmission of the diamond de-vices to characterize the waveguide loss and the grating couplingefficiency. Two flat-cleaved G.652 single-mode fibers mountedon CascadeMicrotech LWP optical probes are aligned with themeasured device. The light source is a continuous-wave tun-able laser (TUNICS T100S-HP) with a wavelength range from1440 nm to 1640 nm. We use an input power of 1mW to ex-clude optical nonlinear effects. The devices’ peak transmissionwavelengths are centered around 1563 nm (instead of 1550 nm)with a standard deviation of 22 nm. We anticipate that this shiftis mainly caused by the fabrication error in grating fill factor. Asshown in Fig. 9, by applying the cutback technique on the peaktransmissions measured from eight diamond devices with four

Fig. 8. SEM image of a typical diamond grating taken before the depositionof SiO2 . The estimated grating fill factor is about 0.64.

Fig. 9. The waveguide attenuation and the grating transmission are estimatedusing the cutback technique with eight different devices featuring four differentlengths. The data points are fitted linearly resulting in a high R2 value of 0.99.The fitted line indicates a waveguide attenuation of about 6.5 dB/mm and agrating transmission around −6.3 dB.

different waveguide lengths, we estimate a waveguide attenu-ation of about 6.5 dB/mm and a grating transmission around−6.3 dB. We anticipate that our SCD device performance ismainly limited by the fully-etched profile for the following rea-sons. First of all, fully-etched waveguides (i.e. strip waveguides)usually suffer from more optical losses caused by the sidewallroughness, because the waveguide mode typically has a strongeroverlap with the waveguide sidewalls in such strip waveguidesthan in the rib waveguides where the diamond waveguide coreis only half-etched and rests on a wide diamond pedestal [17],[18]. Secondly, compared to a partially-etched grating couplerdesign, our fully-etched grating coupler design is less optimalfor TE mode coupling [25]. As such, we expect that SCD deviceswill outperform those made of poly-crystal diamond if they canalso be realized with partially-etched waveguides and gratingcouplers. Despite the fabrication challenges and less sophisti-cated fully-etched grating and waveguide designs, our devicesachieve a comparable performance to the ones fabricated ear-lier in poly-crystal diamond. The latter have a partially-etchedprofile and feature a waveguide attenuation and a grating trans-mission of 5.3 dB/mm and -5 dB, respectively [17].

The devices’ transmission spectra are shown in Fig. 10 fromwhich we estimate a -3 dB bandwidth of 50 nm wide. This isthe same as the bandwidth achieved with poly-crystal diamonddevices [17].

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Fig. 10. Transmission spectra (wavelength ranging from 1440 nm to 1640nm) measured from four typical diamond devices denoted as Device 1 to 4 withincreasing waveguide lengths from 0.642 mm up to 2.016 mm, showing a−3 dBbandwidth of about 50 nm.

V. CONCLUSION

In summary, we have demonstrated the design, fabrica-tion, and characterization of millimeter-length waveguides withmonolithic grating couplers in SCD. One of the major impedi-ments of utilizing the SCD material is the lack of uniform DOIwafers. To minimize the fabrication error in device height, espe-cially for grating couplers, we used a special waveguide layoutto mitigate the influence of the wafer wedge. In the EBL step,we experimentally performed the proximity-effect compensa-tion which has effectively improved the grating accuracy. Com-pared to the millimeter-length diamond devices fabricated inpoly-crystal diamond [17], our SCD devices have achieved com-parable specifications in grating coupling efficiency (−6.3 dB),waveguide transmission attenuation (6.5 dB/mm), and band-width (50 nm).

Long single-mode on-chip waveguides are essential for pho-ton routing in large-scale integrated circuits. Furthermore, ow-ing to SCD’s superior optical properties, such single-passwaveguides with long light-matter interaction lengths are espe-cially interesting for nonlinear applications. Compared to othercoupling techniques used in SCD integrated devices, the mono-lithic grating couplers provide a lower fabrication complexity,a relatively high efficiency, and more device layout flexibility.Our findings will be beneficial for further exploring quantumand nonlinear optics in SCD integrated platforms.

ACKNOWLEDGMENT

The authors would like to thank Ranojoy Bose, Jason Pelc,Tho Tran, Xuema Li, and Xiaoge Zeng for the helpful discus-sions on lithography.

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[21] D. Taillaert et al., “An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides and single-mode fibers,”Quantum Electron., IEEE J., vol. 38, no. 7, pp. 949–955, Jul. 2002.

[22] C. Lee, E. Gu, M. Dawson, I. Friel, and G. Scarsbrook, “Etching and micro-optics fabrication in diamond using chlorine-based inductively-coupledplasma,” Diamond Related Mater., vol. 17, no. 7, pp. 1292–1296, 2008.

[23] Raith, “NanoSuit software reference,” 2012. [Online]. Available:http://www.raith.de

[24] P. Rai-Choudhury, Handbook of Microlithography, Micromachining, andMicrofabrication: Microlithography. Hertfordshire, U.K.: IET, 1997,vol. 1.

[25] D. Taillaert et al., “Grating couplers for coupling between optical fibersand nanophotonic waveguides,” Japanese J. Appl. Phys., vol. 45, no. 8R,2006, Art. no. 6071.

Fei Gao received the B.E. degree in electronic science and technology fromSichuan University, Chengdu, China, in 2010, and the M.Sc. degree in photonicscience and engineering from Ghent University and Vrije Universiteit Brussel,Belgium, in 2013. He is currently working toward the Ph.D. degree within thePhotonics Team at the Department of Applied Physics and Photonics, Vrije Uni-versiteit Brussel. His research focusses on the fabrication of integrated photonicdevices into single-crystal diamond material using electron beam lithography.This research is undertaken in partnership with Hewlett Packard Labs, PaloAlto, CA, USA, where he makes frequent research visits.

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Zhihong Huang (S’01–M’06) received the B.S. degree in applied physics fromPeking University, Beijing, China, and the M.S. and Ph.D. degrees in electricalengineering from the University of Texas at Austin, TX, USA. She is a staffResearch Scientist at Hewlett Packard Labs, Palo Alto, CA, USA, leading thedevelopment of low-power optical transceivers for optical interconnects. Herresearch interests include optical sensors, nano-photonics, silicon photonics, aswell as quantum information processing using diamond defect centers. She hasauthored and coauthored more than 60 journal and conference papers, and wasgranted 7 US/international patents with another 10+ pending.

Benjamin Feigel (S’2013) was born in Antwerp, Belgium, in 1989. He receivedthe B.Sc. degree in electronics and information technology engineering fromthe Vrije Universiteit Brussel (VUB), Brussels, Belgium, in 2011, and theM.Sc. degree in photonics engineering from the VUB and Universiteit Gent,Ghent, Belgium, in 2013.

As part of his M.Sc. degree, he did an internship at TE Connectivity (nowCommScope), Kessel-Lo, Belgium, around simulations of optical-time domainreflectometry. He is currently working toward the Ph.D degree in PhotonicsEngineering at the Brussels Photonics Team, Department of Applied Physicsand Photonics, VUB. His research interests are the exploration of nonlinearoptical effects in novel integrated materials, as diamond and graphene. Moreparticularly, he is investigating spectral broadening and wavelength conversionin graphene.

He is a member of the Society of Photo-Optical Instrumentation Engineersand the IEEE Photonics Society.

Jurgen Van Erps (M’03) was born in Etterbeek, Belgium, in 1980. He receivedthe degree in electrotechnical engineering with majors in photonics from theVrije Universiteit Brussel, Brussels, Belgium, in 2003, where he received thePh.D. degree (summa cum laude) in 2008. Since February 2013, he is a Profes-sor at VUB, teaching general photonics and optical communication systems.

He was an invited speaker at several international conferences. He(co-)authored 54 SCI-stated papers and more than 110 papers in internationalconference proceedings. He is a co-inventor of three patents. He serves as a Re-viewer for several international journals. His research interests include micro-optical systems for optical interconnects and optofluidics applications, and theirfabrication by means of deep proton writing, ultraprecision diamond tooling,and hot embossing. Next to that, he performs experimental work on nonlinearapplications of integrated photonics devices, including high-resolution opticalsampling of ultrahigh bitrate signals, and automatic dispersion monitoring andcompensation of 1.28 Tbaud links and on-chip supercontinuum generation. Heis a senior member of the Society of Photo-Optical Instrumentation Engineersand a member of the IEEE Photonics Society.

Hugo Thienpont (M’99) was born in Ninove, Belgium, in 1961. He received thedegree in electrotechnical engineering in 1984 and the Ph.D. degree in appliedsciences in 1990, both from the Vrije Universiteit Brussel (VUB), Brussels,Belgium. In 1994, he became Professor at the Faculty of Engineering. In 2004,he was elected Chair of the Department of Applied Physics and Photonics. Cur-rently, he coordinates several research and networking projects at the Europeanlevel and manages microphotonics related industrial projects in collaborationwith companies such as Barco, Melexis, Best, Tyco, and Umicore. He has au-thored more than 800 SCI-stated journal papers and international conferenceproceedings publications.

He has received the International Commission for Optics Prize ICO in 1999and the Ernst Abbe medal from Carl Zeiss. In 2003, he was awarded the ti-tle of IEEE Photonics Distinguished Lecturer. In 2005, he received the SPIEPresidents Award for dedicated services to the European Photonics Community,and in 2007 the international MOC Award for his contributions in the fieldof micro-optics and the prize Prof. Roger van Geen for his scientific achieve-ments during his research career at VUB. In 2008, he obtained the prestigiousMethusalem status for top scientist from the Flemish government for his re-search track-record in photonics. In 2011, he received the University medalAlma Mater Bene Merentibus of the Warsaw University of Technology. In 2013he was elected Member to the Royal Flemish Academy of Belgium for Scienceand Art and in 2013 he received the prize for science communication. He is amember of the Board of Stakeholders of the Technology Platform Photonics21.Since 2012, he has been Vice-Rector for innovation and industrial policy of theVrije Universiteit Brussel.

Raymond G. Beausoleil (M’86–SM’06) received the B.S. degree from Caltech,Pasadena, CA, USA, in 1980, and the Ph.D. degree from Stanford University,Stanford, CA, in 1986, both in physics. He is currently an HPE Senior Fellowin the System Architecture Laboratory at Hewlett Packard Labs, Palo Alto, CA,where he leads the Large-Scale Integrated Photonics Research Group. Prior toHPE, his research was focused on high-power all-solid-state laser and nonlinearoptical systems, as well as numerical algorithms for computer firmware (leadingto the navigation algorithms for the optical mouse). At Hewlett Packard Labs, heperforms basic research in microscale and nanoscale quantum optics for classicaland quantum information processing. He is an Adjunct Professor of appliedphysics at Stanford University, Stanford, CA, USA, where he conducts researchon applications of classical and quantum optics to information technology. Heis a Fellow of the American Physical Society. He has contributed to morethan 300 papers and conference proceedings (including many invited papersand plenary/keynote addresses) and five book chapters. He has more than 130patents issued, and more than four dozen pending.

Nathalie Vermeulen (S’06–M’08) was born in Duffel, Belgium, in 1981. Shereceived the M.Sc. degree in electrical engineering with majors in photonicsfrom the Vrije Universiteit Brussel (VUB), Brussels, Belgium, in 2004, and thePh.D. degree from VUB in 2008. Since October 2013 she is a Tenure TrackProfessor in the Brussels Photonics Team (B-PHOT) at VUB.

She is (co-)author of 35 peer-reviewed journal publications, 32 conferenceproceedings, and 4 patents. Her research interests include modeling and demon-strating novel concepts for nonlinear optics in photonic integrated circuits, anddeveloping mid-infrared solid-state lasers.

She has been an invited speaker at eight international conferences. In 2007,she was awarded the Newport Spectra-Physics Research Excellence Award andin 2010 she received the European Photonics21 Innovation Award. In 2013,she received a Starting Grant from the European Research Council (ERC), andsince then she has also been coordinating a European Future and EmergingTechnologies (FET) project. Also, in 2013, she was elected member to theYoung Academy of Belgium. In 2014, she received the VUB I. VanderschuerenAward and in 2015 she received the international LIGHT2015 Young Womenin Photonics prize. She is a member of the International Society for OpticalEngineers, Optical Society of America and IEEE Photonics Society.

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Low-Loss Millimeter-Length Waveguides andGrating Couplers in Single-Crystal Diamond

Fei Gao, Zhihong Huang, Member, IEEE, Benjamin Feigel, Student Member, IEEE, Jurgen Van Erps, Member, IEEE,Hugo Thienpont, Member, IEEE, Raymond G. Beausoleil, Senior Member, IEEE,

and Nathalie Vermeulen, Member, IEEE

Abstract—We report on the design, fabrication, and charac-terization of millimeter-length strip waveguides with monolithicgrating couplers in commercially available synthetic single-crystaldiamond. To minimize the device footprint and the influenceof the wafer wedge of the single-crystal diamond thin plate, weadopt a curled waveguide layout. The devices are fabricated usingelectron-beam lithography and reactive-ion etching. To improvethe e-beam patterning accuracy of the grating etch masks, weapply proximity-effect compensation on the gratings and tapers.The linear characterization results indicate a waveguide attenu-ation of 6.5 dB/mm and a grating transmission of −6.3 dB in thefiber-optic communication C band. These results demonstrate thefeasibility of fabricating long waveguides and integrated gratingcouplers in single-crystal diamond. Our research findings wouldbe beneficial for further exploring quantum and nonlinear opticsin integrated single-crystal diamond devices.

Index Terms—Diamond, electron beam lithography, gratings,integrated optics, optical waveguides, proximity effects.

I. INTRODUCTION

IN RECENT years, synthetic single-crystal diamond (SCD)has been identified as a promising optical material for

integrated photonics [1], [2]. The individual nitrogen-vacancy(NV) centers, which are present only in SCD and cannot befound in poly-crystal diamond materials [3], have attractedmuch attention for realizing quantum-photonic on-chip de-vices [4]–[9]. Also, for nonlinear optical applications suchas wavelength conversion, SCD possesses many attractiveproperties such as large transparency windows from 0.23 μmto 2.6 μm and from 6.2 μm to 10 μm and beyond [10].Furthermore, it does not suffer from multi-photon absorptionin the visible and infrared (IR) ranges, yet exhibits, as a resultof its single-crystal structure, a relatively large Raman gain

Manuscript received February 5, 2016; revised September 12, 2016; acceptedOctober 19, 2016. This work was supported in part by the Research FoundationFlanders under the project G.A002.13N, in part by BELSPO-IAP, in part bythe Hercules and Methusalem foundations, and in part by the OZR of theVrije Universiteit Brussel. The work of N. Vermeulen was supported by ERC-FP7/2007-2013 grant 336940.

F. Gao, B. Feigel, J. Van Erps, H. Thienpont, and N. Vermeulen are withthe Brussels Photonics Team, Department of Applied Physics and Photonics,Vrije Universiteit Brussel, 1050 Brussels, Belgium (e-mail: fgao@b-phot.org;bfeigel@b-phot.org; jverps@b-phot.org; hthienpo@vub.ac.be; nvermeul@b-phot.org).

Z. Huang and R. G. Beausoleil are with the Large-Scale Integrated PhotonicsResearch Group, Hewlett Packard Labs, Palo Alto, CA 94304 USA (e-mail:zhihong.huang@hpe.com; ray.beausoleil@hpe.com).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2016.2622620

(13.5 cm/GW at 1030 nm) and a considerable Kerr nonlinearity(n2 = 1.3 × 10−19 m2/W at 545 nm) [2]. In addition, it has arelatively high refractive index of n = 2.39 at 1550 nm [2]. Thisenables strong confinement of light in diamond waveguides, asrequired by integrated photonic applications.

Integrated photonic devices have been demonstrated insingle-, poly-, and nano-crystal diamond materials [4]–[19].Different fabrication approaches have been used by severalresearch groups worldwide, either relying on electron-beamlithography (EBL) with inductively-coupled plasma reactive-ion etching (ICP RIE), or focused-ion-beam (FIB) milling [14],[15]. Compared to FIB milling, the combination of EBL andICP RIE has the advantage of achieving higher precision andoffers the possibility of realizing more sophisticated devicedesigns. Most of the devices realized thus far are fabricated in alayered diamond-on-insulator (DOI) substrate with a diamondmembrane on top of a buried oxide (SiO2) layer.

High performance ring and photonic-crystal resonators madein single-crystal DOI substrates have been realized, from whichquality-factors up to, respectively, 106 and 6000 have been mea-sured [8], [12]. In addition, a very low propagation loss of0.34 dB/cm has been derived from the high Q-factor of the ringresonator [12]. Usually, SCD thin plates contain a wafer wedge(>300 nm/mm [12]) which is caused by the diamond polishingprocess. To minimize the influence of the wafer wedge, those de-vices mentioned above were relatively small (with their longestedge up to about 300 μm) and their path lengths were similarlyshort (sub-millimeter). Recently, a new technique for preparingSCD thin plates without wafer wedge was introduced [3]. Thisapproach consists of ion slicing in conjunction with homoepitax-ial re-growing to create single-crystal diamond membrane win-dows (SCDMWs) with a homogeneous thickness. High-qualityring resonators with a loaded Q-factor of 6.6 × 104 were fabri-cated in these SCDMW samples. Whereas such samples withoutwafer wedge would be a very suitable platform for fabricatingrelatively large devices, the longest path length realized so farin SCDMW films is well below 1 mm [3]. However, in large-scale integrated circuits, devices with longer path lengths suchas waveguides of at least a few millimeters long are essential.

To date, the methods used to couple light into and out ofsingle-crystal DOI photonic devices are mainly based on edgecoupling [12], optical fiber taper coupling [16], or grating-assisted microscope objective coupling [7], [9]. Among thevarious light coupling approaches, a grating-enabled fiber-to-chip coupling method offers a relatively high efficiency anddevice layout flexibility, as well as the possibility of automated

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wafer-scale testing. A further advantage is that by integratingmonolithic grating couplers in the devices, one avoids addi-tional lithography steps to overlay the diamond waveguides withpolymer waveguide extensions required for edge coupling [12].However, grating couplers optimized for flat-cleaved opticalfiber coupling at telecom wavelengths have, to our knowledge,rarely been demonstrated in single-crystal DOI photonic de-vices. Moreover, the grating couplers recently demonstrated onthe aforementioned SCDMW samples featured a relatively lowcoupling efficiency of −15 dB [3], which should be futher in-creased for allowing practical applications.

Fabricating integrated devices in poly/nano-crystal DOI sub-strates is less challenging than in single-crystal DOI substrates,as the former can be prepared by directly growing a dia-mond layer through plasma-enhanced chemical vapor deposi-tion (PECVD) on an existing wafer-scale SiO2 substrate [17].The poly/nano-crystal DOI substrates fabricated in such a waydo not suffer from wafer wedge. Various high-quality on-chipdevices have been realized in poly-crystal DOI substrates, in-cluding ring resonators with Q-factors up to 11000, waveguidesup to 4.6 mm long with a transmission loss of 5.3 dB/mm, andmonolithic grating couplers with a coupling efficiency of −5 dB[17], [18].

To be able to fully exploit the potential of SCD in, amongothers, quantum and nonlinear optics, both millimeter-lengthwaveguides and efficient grating couplers should be developedalso in SCD. In this paper, we present the design, fabrication,and characterization of millimeter-length strip waveguides withmonolithically integrated grating couplers in SCD. The devicesare fabricated using EBL and ICP RIE techniques on SCD sam-ples with a wafer wedge. Cutback measurement results indicatethat we have achieved single-mode operation in the fiber-opticcommunication C band with a waveguide attenuation of about6.5 dB/mm and a grating coupling efficiency around −6.3 dB.Compared to the devices fabricated in poly-crystal diamond (forwhich one can use a more advanced, partially-etched gratingand waveguide designs [17]), our more simplified SCD devicesachieve comparable performances.

II. DESIGN

A. Waveguide Cross Section and Layout Design

Due to the wafer wedge, partially-etched rib waveguides can-not be fabricated accurately. In this work, we only consider stripwaveguide designs. To ensure low-loss propagation and single-mode operation, we choose a 900 nm × 500 nm rectangular dia-mond core surrounded by a 2 μm thick silica cladding. Althoughnot strictly required, we have added the silica top cladding soas to reduce the waveguide scattering losses [11]. The opticalmodes of the waveguide at 1550 nm are numerically simulatedin a fully vectorial mode solver (Lumerical MODE Solutions).The mode profiles of the fundamental transverse-electric (TE)and transverse-magnetic (TM) modes are shown in Fig. 1.

We design a set of curled waveguide layouts with devicelengths up to about 2 mm as shown in Fig. 2. There are severaladvantages to using this type of layout. Firstly, as aforemen-tioned, the SCD membranes often present a wedge which is

Fig. 1. Numerically simulated optical mode profiles of a diamond strip wave-guide with a width of 900 nm and a height of 500 nm. The wavelength used is1550 nm.

Fig. 2. Four types of waveguide layouts with device lengths of: (a) 0.642 mm,(b) 1.134 mm, (c) 1.575 mm, and (d) 2.016 mm.

caused by polishing during the diamond plate processing. Ourcurled waveguide layout brings the input and output gratingscloser to each other so that the two gratings have a minimizedheight difference in the wedged wafer. This way, the trans-mission windows of the two gratings (on the same device) donot show a significant difference, which is beneficial towardsensuring a high device transmission. Secondly, in the worstcase scenario, millimeter-long straight waveguides fabricatedin a wedged wafer could exhibit more than 300 nm deviationin waveguide height. The compactness of the curled layout istherefore ideal to reduce the height error for long waveguides.Thirdly, In order to fit a long waveguide within a small devicefootprint, a meander layout usually takes many more sharp turnsthan our curled layout. Through preliminary measurements wefound that the total device transmission loss for millimeter-longwaveguides could be up to 20 dB higher when using a meanderlayout instead of the curled layout. Therefore it is preferable toadopt the curled layouts to minimize the number of sharp bends,thus reducing the amount of waveguide bending loss. Finally, all

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our waveguide layouts use the same grating-coupler interfaceconfiguration. This enables an efficient wafer-scale characteri-zation using an automated testing stage.

B. Grating Design

We employ grating couplers to couple the TE polarized light at1550 nm between the on-chip diamond waveguides and G.652standard telecom single-mode optical fibers. As the wedgeddiamond wafer is not suitable for realizing an accurate partially-etched grating profile, we design a binary grating coupler, withthe grating trenches fully etched throughout the diamond layer.

To design the grating coupler, i.e. to determine the optimalgrating period and fill factor, we maximize the coupling effi-ciency, while taking into account the manufacturability. We usea finite-difference time-domain (FDTD) simulation software(Lumerical FDTD Solutions) for this optimization. To deter-mine a suitable starting point for the numerical optimization ofthe grating parameters, we first carry out an approximate calcu-lation of the grating period using the phase matching conditiongiven by [20]

Λ =λ

neff − sin(θ)ncla, (1)

where Λ is the grating period, λ = 1550 nm is the free-spacewavelength, neff = 1.915 is approximated as the average valueof the refractive index of diamond (n = 2.39) and SiO2 (n =1.44), and ncla = 1.44 is the refractive index of the silicacladding. The light emission angle of the grating coupler isθ degrees away from the normal to the grating surface. This isnecessary to avoid that the grating coupler reflects the outgoinglight back to the on-chip waveguide and reduces the couplingefficiency [21]. We choose θ = 9◦, which corresponds to thefiber angle of the Cascade Microtech LWP optical probe in ourmeasurement setup. Equation (1) then yields an initial gratingperiod of Λ = 917 nm.

We then carry out an FDTD simulation to determine the opti-mal grating fill factor for the initial grating period. By calculatingthe grating coupling efficiency for different grating fill factors,while keeping the grating period fixed at 917 nm, we obtain themaximal coupling efficiency at the fill factor of 0.67.

To improve its manufacturability without compromisingmuch grating performance, we search for grating designs withlower fill factors and larger grating periods by performing a finetwo-parameter sweep in the neighborhood of their initial values.The simulation results shown in Fig. 3 indicate a range of grat-ing periods and fill factors yielding high coupling efficienciesbetween −3.81 dB and −3.74 dB. We choose the combinationof a relatively large grating period (950 nm) and a relatively lowfill factor (0.6) within the optimal range as is indicated by “X”in Fig. 3. This combination yields a local maximal efficiency of−3.81 dB.

III. FABRICATION

A. DOI Wafer Preparation

The devices are fabricated in the single-crystal DOI sub-strates which are prepared using a similar procedure previously

Fig. 3. Results of a grating coupling efficiency FDTD simulation showingthe coupling efficiency at 1550 nm as a function of the grating period and fillfactor. Suitable grating period and fill factor combinations yielding coupling ef-ficiencies between −3.81 dB and −3.74 dB are indicated by the lighter tone andfollow a diagonal line from the lower left corner to the upper right corner in thefigure. To benefit from a higher manufacturability, we choose the combinationof 950 nm and 0.6 as indicated by “×”.

developed by HP Laboratories [7]. We use a Type IIa (100) di-amond plate which is purchased from MB Optics and has a sur-face of 3 mm × 3 mm and a thickness of 5 μm. In the first step,the diamond plate is cleaned in subsequently acetone, methanol,and isopropyl alcohol. The cleaned diamond plate is then frag-mented by using a diamond-tip scribing pen into smaller pieceswith edge lengths of approximately a few hundred micrometers.We place each diamond piece (sample) onto a 10 mm × 10 mmlarge silicon carrier wafer that has been thermally oxidized toform a 2 μm thick SiO2 top layer. The diamond samples aremerely attached to their carrier wafers via van der Waals forces.In the second step, the diamond samples are thinned to a thick-ness of about 500 nm by RIE in Ar/Cl2 plasma. We choose thechlorine-based recipe because it allows us to reduce the rough-ness of the diamond surface [22]. The RIE thinning process alsoetches the top surface of the carrier wafers. Consequently, afterthe thinning process, the height difference between the surfacesof the diamond samples and their carrier wafers can increasetens of micrometers. Such a height difference is problematic inthe subsequent spin coating process, because it results in a lessuniform e-beam resist film. Therefore, we carry out the thinningprocess in several iterations, between which we can examine thethinning progress and replace the carrier wafers. A polyethyleneterephthalate (PET) film and a vacuum pick-up pen are used tohandle the diamond samples. We also use these tools to flip thediamond samples such that both the top and bottom surfaces areetched and smoothed.

B. E-Beam Lithography

The e-beam resist (Dow Corning XR-1541, 6%) is appliedvia spin coating (at 3000 rpm during 45 s) to form a 100 nmthick film on top of the thinned diamond samples. We usea Raith150-two electron-beam writing tool to define the etchmasks in the e-beam resist. The main patterning settings on the

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Fig. 4. A qualitative demonstration of the electron-dose settings for a grat-ing with its taper before and after the proximity-effect compensation in theRaith150-two software. The latter increases the dose values near the edges ofthe pattern, and decreases the dose values near the center of the pattern. Thisway, the exposed area is subjected to an effective electron dose which is uniformat all points. The total length of the grating and taper is 100 μm.

Raith150-two are: a working distance of 10 mm, an accelerationvoltage of 30 kV, an aperture size of 10 μm, and a writefield sizeof 200 μm × 200 μm. To produce high-quality waveguide etchmasks, we use the fixed-beam moving-stage (FBMS) patterningmode provided by the Raith150-two. Compared to the scan-ning mode mostly used in e-beam patterning, the FBMS modeoffers two advantages for long waveguide patterning. Firstly,it reduces the roughness of the waveguide edges and, conse-quently, reduces the scattering losses. Secondly, it can exposean entire waveguide pattern in one “stroke” across multiplewritefields. This is beneficial to avoiding those potential discon-tinuities, typically occurring in the scanning mode, such as gaps,lateral misalignments and overlaps at stitching points betweenwritefields.

An important challenge in e-beam patterning is the proximityeffect which limits the patterning accuracy. This effect is due tothe e-beam broadening caused by forward- and back-scatteredelectrons during electron-matter interaction. The software of theRaith150-two allows us to modify the electron dose settings tocompensate for the proximity effect. Since the pattern of thegrating couplers contains fine features and is heavily affectedby this effect, we need to compensate the proximity effect on thegratings. We also include the tapers in the proximity-effect com-pensation because their large exposure areas result in the scat-tering of electrons towards the adjacent grating areas during thee-beam patterning. Fig. 4 qualitatively shows the dose settings ofthe grating and taper areas before and after the proximity-effectcompensation in the Raith150-two software.

To determine the adequate electron dose adjustment forproximity-effect compensation, the software uses a function tocalculate the scattered-electron distribution. The basic format ofthis function is the sum of two Gaussian terms [23]

f(x) =1

π(1 + η)

(1α2 e−

x 2

α 2 +η

β2 e− x 2

β 2

). (2)

In (2), x is the lateral distance from the e-beam, α is the range ofthe forward-scattered electrons, β is range of the back-scatteredelectrons, and η is the back-scattering efficiency. These param-eters are strongly dependent on the acceleration voltage of theincident electron beam, the material and the thickness of boththe e-beam resist and the substrate [23]. We calculate α by using

Fig. 5. SEM images of two grating etch masks generated with (lower image)and without (upper image) proximity-effect compensation. This comparisonshows that this e-beam patterning technique reduces the grating line swelling inthe central grating area and improves the e-beam patterning accuracy.

the following equation [24]

α = 0.9(Rt/Vb)1.5 , (3)

where Rt is the thickness of the e-beam resist (about 100 nm)and Vb is the acceleration voltage of the electron beam (30 kV).This yields α ≈ 5 nm. However, we do not find any significantdifference in our proximity-compensation results using eitherα = 5 nm or the default α = 2 nm. The values of β and η aredetermined empirically by e-beam patterning experiments inthe following way. We pattern multiple grating and taper ar-eas with the proximity effect compensated by using differentcombinations of β and η values. By visually examining the e-beam patterned grating areas with scanning electron microscopy(SEM), we identify the parameter combination which generatesthe most uniform grating lines. The optimal parameters we haveidentified for our substrate are β = 3500 nm and η = 0.75.

Fig. 5 shows a comparison between two grating masks pat-terned with and without proximity-effect compensation. It isclear that this e-beam patterning technique reduces the line-width swelling in the central grating area.

The e-beam patterned samples are developed in the AZ300MIF developer. To transfer the mask pattern to the diamondlayer underneath, we perform a second RIE step in Ar/O2plasma. Compared to the aforementioned chlorine-based recipe,the oxygen-based recipe is more suitable for pattern transfer be-cause it provides a higher selectivity between the etch mask andthe diamond, resulting in a reduced mask erosion. After patternetching, we grow a 2 μm thick SiO2 film via PECVD on top ofthe chip to form an upper cladding, as required by our design.

IV. CHARACTERIZATION

The newly fabricated diamond devices are examined visuallyunder an optical microscope as shown in Fig. 6. The dimensionsof the waveguide width and grating period are measured usingan atomic force microscope (AFM) in tapping mode before theSiO2 is deposited on the chip. These measurement results are

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Fig. 6. Microscope images of two diamond chips containing four differentdevice layouts.

Fig. 7. AFM measurement results of a typical SCD device indicating a wave-guide width of about 920 nm and a grating period of about 950 nm. Due tothe finite size of the AFM probe, the gaps between adjacent grating lines showa “V”-shaped profile which is slightly different from the actual gap profile.Nonetheless, the results of the grating period measurement are not affected.

given in Fig. 7, and indicate a waveguide width of about 920 nmand a grating period of about 950 nm, both of which are veryclose to the design values.

Fig. 8 shows a SEM image of a typical diamond grating,before the silica encapsulation, from which we estimate a gratingfill factors of about 0.64. This is larger than our designed valueof 0.6. We anticipate that this is one of the major fabricationerrors which affects the grating performance.

Next, we measure the optical transmission of the diamond de-vices to characterize the waveguide loss and the grating couplingefficiency. Two flat-cleaved G.652 single-mode fibers mountedon CascadeMicrotech LWP optical probes are aligned with themeasured device. The light source is a continuous-wave tun-able laser (TUNICS T100S-HP) with a wavelength range from1440 nm to 1640 nm. We use an input power of 1mW to ex-clude optical nonlinear effects. The devices’ peak transmissionwavelengths are centered around 1563 nm (instead of 1550 nm)with a standard deviation of 22 nm. We anticipate that this shiftis mainly caused by the fabrication error in grating fill factor. Asshown in Fig. 9, by applying the cutback technique on the peaktransmissions measured from eight diamond devices with four

Fig. 8. SEM image of a typical diamond grating taken before the depositionof SiO2 . The estimated grating fill factor is about 0.64.

Fig. 9. The waveguide attenuation and the grating transmission are estimatedusing the cutback technique with eight different devices featuring four differentlengths. The data points are fitted linearly resulting in a high R2 value of 0.99.The fitted line indicates a waveguide attenuation of about 6.5 dB/mm and agrating transmission around −6.3 dB.

different waveguide lengths, we estimate a waveguide attenu-ation of about 6.5 dB/mm and a grating transmission around−6.3 dB. We anticipate that our SCD device performance ismainly limited by the fully-etched profile for the following rea-sons. First of all, fully-etched waveguides (i.e. strip waveguides)usually suffer from more optical losses caused by the sidewallroughness, because the waveguide mode typically has a strongeroverlap with the waveguide sidewalls in such strip waveguidesthan in the rib waveguides where the diamond waveguide coreis only half-etched and rests on a wide diamond pedestal [17],[18]. Secondly, compared to a partially-etched grating couplerdesign, our fully-etched grating coupler design is less optimalfor TE mode coupling [25]. As such, we expect that SCD deviceswill outperform those made of poly-crystal diamond if they canalso be realized with partially-etched waveguides and gratingcouplers. Despite the fabrication challenges and less sophisti-cated fully-etched grating and waveguide designs, our devicesachieve a comparable performance to the ones fabricated ear-lier in poly-crystal diamond. The latter have a partially-etchedprofile and feature a waveguide attenuation and a grating trans-mission of 5.3 dB/mm and -5 dB, respectively [17].

The devices’ transmission spectra are shown in Fig. 10 fromwhich we estimate a -3 dB bandwidth of 50 nm wide. This isthe same as the bandwidth achieved with poly-crystal diamonddevices [17].

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Fig. 10. Transmission spectra (wavelength ranging from 1440 nm to 1640nm) measured from four typical diamond devices denoted as Device 1 to 4 withincreasing waveguide lengths from 0.642 mm up to 2.016 mm, showing a−3 dBbandwidth of about 50 nm.

V. CONCLUSION

In summary, we have demonstrated the design, fabrica-tion, and characterization of millimeter-length waveguides withmonolithic grating couplers in SCD. One of the major impedi-ments of utilizing the SCD material is the lack of uniform DOIwafers. To minimize the fabrication error in device height, espe-cially for grating couplers, we used a special waveguide layoutto mitigate the influence of the wafer wedge. In the EBL step,we experimentally performed the proximity-effect compensa-tion which has effectively improved the grating accuracy. Com-pared to the millimeter-length diamond devices fabricated inpoly-crystal diamond [17], our SCD devices have achieved com-parable specifications in grating coupling efficiency (−6.3 dB),waveguide transmission attenuation (6.5 dB/mm), and band-width (50 nm).

Long single-mode on-chip waveguides are essential for pho-ton routing in large-scale integrated circuits. Furthermore, ow-ing to SCD’s superior optical properties, such single-passwaveguides with long light-matter interaction lengths are espe-cially interesting for nonlinear applications. Compared to othercoupling techniques used in SCD integrated devices, the mono-lithic grating couplers provide a lower fabrication complexity,a relatively high efficiency, and more device layout flexibility.Our findings will be beneficial for further exploring quantumand nonlinear optics in SCD integrated platforms.

ACKNOWLEDGMENT

The authors would like to thank Ranojoy Bose, Jason Pelc,Tho Tran, Xuema Li, and Xiaoge Zeng for the helpful discus-sions on lithography.

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Fei Gao received the B.E. degree in electronic science and technology fromSichuan University, Chengdu, China, in 2010, and the M.Sc. degree in photonicscience and engineering from Ghent University and Vrije Universiteit Brussel,Belgium, in 2013. He is currently working toward the Ph.D. degree within thePhotonics Team at the Department of Applied Physics and Photonics, Vrije Uni-versiteit Brussel. His research focusses on the fabrication of integrated photonicdevices into single-crystal diamond material using electron beam lithography.This research is undertaken in partnership with Hewlett Packard Labs, PaloAlto, CA, USA, where he makes frequent research visits.

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Zhihong Huang (S’01–M’06) received the B.S. degree in applied physics fromPeking University, Beijing, China, and the M.S. and Ph.D. degrees in electricalengineering from the University of Texas at Austin, TX, USA. She is a staffResearch Scientist at Hewlett Packard Labs, Palo Alto, CA, USA, leading thedevelopment of low-power optical transceivers for optical interconnects. Herresearch interests include optical sensors, nano-photonics, silicon photonics, aswell as quantum information processing using diamond defect centers. She hasauthored and coauthored more than 60 journal and conference papers, and wasgranted 7 US/international patents with another 10+ pending.

Benjamin Feigel (S’2013) was born in Antwerp, Belgium, in 1989. He receivedthe B.Sc. degree in electronics and information technology engineering fromthe Vrije Universiteit Brussel (VUB), Brussels, Belgium, in 2011, and theM.Sc. degree in photonics engineering from the VUB and Universiteit Gent,Ghent, Belgium, in 2013.

As part of his M.Sc. degree, he did an internship at TE Connectivity (nowCommScope), Kessel-Lo, Belgium, around simulations of optical-time domainreflectometry. He is currently working toward the Ph.D degree in PhotonicsEngineering at the Brussels Photonics Team, Department of Applied Physicsand Photonics, VUB. His research interests are the exploration of nonlinearoptical effects in novel integrated materials, as diamond and graphene. Moreparticularly, he is investigating spectral broadening and wavelength conversionin graphene.

He is a member of the Society of Photo-Optical Instrumentation Engineersand the IEEE Photonics Society.

Jurgen Van Erps (M’03) was born in Etterbeek, Belgium, in 1980. He receivedthe degree in electrotechnical engineering with majors in photonics from theVrije Universiteit Brussel, Brussels, Belgium, in 2003, where he received thePh.D. degree (summa cum laude) in 2008. Since February 2013, he is a Profes-sor at VUB, teaching general photonics and optical communication systems.

He was an invited speaker at several international conferences. He(co-)authored 54 SCI-stated papers and more than 110 papers in internationalconference proceedings. He is a co-inventor of three patents. He serves as a Re-viewer for several international journals. His research interests include micro-optical systems for optical interconnects and optofluidics applications, and theirfabrication by means of deep proton writing, ultraprecision diamond tooling,and hot embossing. Next to that, he performs experimental work on nonlinearapplications of integrated photonics devices, including high-resolution opticalsampling of ultrahigh bitrate signals, and automatic dispersion monitoring andcompensation of 1.28 Tbaud links and on-chip supercontinuum generation. Heis a senior member of the Society of Photo-Optical Instrumentation Engineersand a member of the IEEE Photonics Society.

Hugo Thienpont (M’99) was born in Ninove, Belgium, in 1961. He received thedegree in electrotechnical engineering in 1984 and the Ph.D. degree in appliedsciences in 1990, both from the Vrije Universiteit Brussel (VUB), Brussels,Belgium. In 1994, he became Professor at the Faculty of Engineering. In 2004,he was elected Chair of the Department of Applied Physics and Photonics. Cur-rently, he coordinates several research and networking projects at the Europeanlevel and manages microphotonics related industrial projects in collaborationwith companies such as Barco, Melexis, Best, Tyco, and Umicore. He has au-thored more than 800 SCI-stated journal papers and international conferenceproceedings publications.

He has received the International Commission for Optics Prize ICO in 1999and the Ernst Abbe medal from Carl Zeiss. In 2003, he was awarded the ti-tle of IEEE Photonics Distinguished Lecturer. In 2005, he received the SPIEPresidents Award for dedicated services to the European Photonics Community,and in 2007 the international MOC Award for his contributions in the fieldof micro-optics and the prize Prof. Roger van Geen for his scientific achieve-ments during his research career at VUB. In 2008, he obtained the prestigiousMethusalem status for top scientist from the Flemish government for his re-search track-record in photonics. In 2011, he received the University medalAlma Mater Bene Merentibus of the Warsaw University of Technology. In 2013he was elected Member to the Royal Flemish Academy of Belgium for Scienceand Art and in 2013 he received the prize for science communication. He is amember of the Board of Stakeholders of the Technology Platform Photonics21.Since 2012, he has been Vice-Rector for innovation and industrial policy of theVrije Universiteit Brussel.

Raymond G. Beausoleil (M’86–SM’06) received the B.S. degree from Caltech,Pasadena, CA, USA, in 1980, and the Ph.D. degree from Stanford University,Stanford, CA, in 1986, both in physics. He is currently an HPE Senior Fellowin the System Architecture Laboratory at Hewlett Packard Labs, Palo Alto, CA,where he leads the Large-Scale Integrated Photonics Research Group. Prior toHPE, his research was focused on high-power all-solid-state laser and nonlinearoptical systems, as well as numerical algorithms for computer firmware (leadingto the navigation algorithms for the optical mouse). At Hewlett Packard Labs, heperforms basic research in microscale and nanoscale quantum optics for classicaland quantum information processing. He is an Adjunct Professor of appliedphysics at Stanford University, Stanford, CA, USA, where he conducts researchon applications of classical and quantum optics to information technology. Heis a Fellow of the American Physical Society. He has contributed to morethan 300 papers and conference proceedings (including many invited papersand plenary/keynote addresses) and five book chapters. He has more than 130patents issued, and more than four dozen pending.

Nathalie Vermeulen (S’06–M’08) was born in Duffel, Belgium, in 1981. Shereceived the M.Sc. degree in electrical engineering with majors in photonicsfrom the Vrije Universiteit Brussel (VUB), Brussels, Belgium, in 2004, and thePh.D. degree from VUB in 2008. Since October 2013 she is a Tenure TrackProfessor in the Brussels Photonics Team (B-PHOT) at VUB.

She is (co-)author of 35 peer-reviewed journal publications, 32 conferenceproceedings, and 4 patents. Her research interests include modeling and demon-strating novel concepts for nonlinear optics in photonic integrated circuits, anddeveloping mid-infrared solid-state lasers.

She has been an invited speaker at eight international conferences. In 2007,she was awarded the Newport Spectra-Physics Research Excellence Award andin 2010 she received the European Photonics21 Innovation Award. In 2013,she received a Starting Grant from the European Research Council (ERC), andsince then she has also been coordinating a European Future and EmergingTechnologies (FET) project. Also, in 2013, she was elected member to theYoung Academy of Belgium. In 2014, she received the VUB I. VanderschuerenAward and in 2015 she received the international LIGHT2015 Young Womenin Photonics prize. She is a member of the International Society for OpticalEngineers, Optical Society of America and IEEE Photonics Society.