Flexible nanofiber-coupled hybrid plasmonic Bragg grating

Sheng Liu,1 Linjie Zhou,1,* Jian Xu,2 Xinyi Wang,1 and Jianping Chen1 1State Key Laboratory of Advanced Optical Communication Systems and Networks, Department of Electronic

Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 2Center for Advanced Electronic Materials and Devices, Shanghai Jiao Tong University, Shanghai 200240, China

*ljzhou@sjtu.edu.cn

Abstract: We report a hybrid plasmonic Bragg grating composed of a nanofiber coupled with orthogonally oriented metal strips. Numerical simulations are performed to study the transmission and reflection spectra of the grating. It shows that the TM polarization has much stronger Bragg reflection due to the excitation of hybrid plasmonic modes. The dependence of reflection peaks on several key device parameters is analyzed. Light propagation simulation further reveals that both fundamental and first-order TM modes are excited upon Bragg reflection, leading to two separate peaks in the spectrum. We implement the prototype device by attaching a nanofiber onto the surface of an array of sub-micrometer-wide metal strips. The main reflection peak is measured to have a 3-dB bandwidth of 15 nm and out-of-band rejection of more than 30 dB. The effects of nanofiber radius, alignment angle and coupling length on the device performance are also experimentally investigated.

©2016 Optical Society of America

OCIS codes: (250.5300) Photonic integrated circuits; (250.5403) Plasmonics.

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1. Introduction

Surface plasmon-polaritons (SPP) [1–7] are attracting considerable attentions due to the field intensity enhancement at the interface and the light confinement ability beyond the diffraction limit. SPPs can be excited in a subwavelength scale and guided at the interface between metal and dielectric, providing a promising solution to realize an optical device unbounded by the diffraction limit [8–12]. Sub-wavelength metal gratings with the SPP or hybrid SPP-dielectric modes excited close to the metal surfaces [13–15] have many potential applications in making functional optical components. The metal gratings have been widely used in band-pass filters [16], chemical and biological sensors [17], reflective polarizers [18], polarization-independent and omnidirectional absorbers [19], and color filters [20–22] etc.

In this paper, we report a Bragg grating composed of a nanofiber situated on an array of gold (Au) strips. A hybrid plasmonic mode is excited due to the coupling between the nanofiber dielectric mode and the metal SPP mode. The nanofiber can be freely moved on the metal grating surface, making it flexible to adjust the coupling position, length and angle. We investigate the device performance both theoretically and experimentally. The paper is organized as follows. In section 2, we introduce the device design and simulation results.

#258679 Received 1 Feb 2016; revised 8 Apr 2016; accepted 15 Apr 2016; published 20 Apr 2016 © 2016 OSA 2 May 2016 | Vol. 24, No. 9 | DOI:10.1364/OE.24.009316 | OPTICS EXPRESS 9317

Section 3 describes the device fabrication and experimental results. Section 4 concludes our work.

2. Device structure and simulation results

As the nanofiber is very gentle and flexible, it could be bent to rest on top of the metal grating. The metal grating is made of an alternative array of metal strips and air slits as schematically shown in Fig. 1. Once the nanofiber touches the metal surface, it will firmly adhere to the surface through Van der Waals and electrostatic forces [23]. A good merit of such a grating structure lies in its easy mobility and re-configurability. The nanofiber can be drawn from a standard single mode fiber (SMF) [24-26], resulting in low loss connection to other fiber-based devices and systems. With an adiabatic taper, the conversation efficiency between the SMF and the nanofiber can exceed 99% [27]. The metal grating is formed on top of the SiO2 substrate. We denote the width, length and thickness of the metal strip as d, L and h, respectively, the width of the air slit as a, the radius of the nanofiber as R and its alignment angle with respect to the grating longitudinal direction as θ. Therefore, the grating period along the nanofiber is Λ = (a + d) /cos(θ) with a filling factor of f = d/(a + d). The coupling length between nanofiber and metal grating is denoted as Lc, which can be expressed by the number of grating period N as Lc = N·Λ.

Fig. 1. Schematic diagrams of the hybrid plasmonic Bragg grating. (a) Top view; (b) Side view; (c) Perspective view.

The coupled system is essentially a Bragg grating with the waveguide effective index periodically perturbed by the underlying metal strips. The Bragg wavelength λΒ given by [28]

( ) ( )1 22 2 cosB eff eff effn dn anλ θ= Λ = + (1)

where neff = (dneff1 + aneff2)/(d + a) is the average effective index, neff1 and neff2 are the modal effective indices of the nanofiber loaded with and without the metal strip, respectively. The effective index is dependent on the nanofiber radius. The reflection bandwidth Δλ is given by [29]

( )2

2 2ac c

eff c

Ln L

λλ κ ππ

Δ = + (2)

where κac is the coupling coefficient. If κacLc << π, then the bandwidth is an inverse function of the grating length as

#258679 Received 1 Feb 2016; revised 8 Apr 2016; accepted 15 Apr 2016; published 20 Apr 2016 © 2016 OSA 2 May 2016 | Vol. 24, No. 9 | DOI:10.1364/OE.24.009316 | OPTICS EXPRESS 9318

2

eff cn L

λλΔ ≈ (3)

The expression for the peak reflectivity is given by [30]

( ) ( )2tan hc ac cR L Lκ= (4)

The reflectivity increases with the coupling length. When the coupling length is long enough (i.e., Lc > π/κac), it gives nearly total reflection.

We first use the 3D finite-difference time-domain (FDTD) simulation to numerically study the device performance. The permittivity of Au is assumed to be εAu = −95.92 + 10.97i around 1550 nm wavelength [31]. The silica fiber has a permittivity of εsilica = 2.08. The height of the Au film is set as h = 500 nm. Figure 2 shows the normalized transmission and reflection spectra of the device with Λ = 650 nm, R = 0.7 μm, f = 0.4, N = 50 and θ = 0 for the TM and TE polarizations. Here, the TM polarization is defined as the one whose major electric field is perpendicular to the Au film, and the TE polarization is the one with its major electric field parallel to the metal film. Both TM and TE spectra show two Bragg reflection bands, and yet the TM one has much stronger peak reflection (<3 dB loss) and slightly longer central wavelengths than the TE one. The difference could be attributed to the hybrid plasmonic mode excitation for TM mode, which leads to the much stronger interaction between the light field and the metal grating. In our following simulation study, we only focus on the TM mode as it provides a superior performance than the TE mode.

Fig. 2. Simulated transmission and reflection spectra of the nanofiber-attached hybrid plasmonic grating for (a) TM and (b) TE polarizations.

The performance of the grating depends on multiple device parameters, including the grating period, the filling factor, the fiber radius, and the coupling length, etc. We further study the effect of these design parameters on the grating reflection spectrum. The results are shown in Fig. 3. We only vary one parameter each time while the others are kept the same as those used above. The reflection peaks shift to longer wavelengths when the grating period increases, which is expectable from the Bragg wavelength formula given by Eq. (1). The change of filling factor also shifts the two peaks. In particular, the left one is shifted by 30 nm and right one 20 nm when the filling factor increases from 0.3 to 0.6. The nanofiber radius affects both of the Bragg reflection wavelengths and the peak profiles (width and height). When the radius is small, e.g., R = 0.4 μm, the left peak is very low and broad. It gradually grows up with the increasing radius. Finally, the number of grating periods or the coupling length also affects the reflection peak profile. A longer coupling length results in stronger peak reflection and a narrower bandwidth, which is consistent with Eqs. (2) and (4). It is worth noting that the coupling length has little effect on the reflection spectrum when it becomes long enough (N > 65), as light is fully reflected by the front part of the grating. The simulation reveals that a very short metal grating (10’s microns) can induce a strong reflection

#258679 Received 1 Feb 2016; revised 8 Apr 2016; accepted 15 Apr 2016; published 20 Apr 2016 © 2016 OSA 2 May 2016 | Vol. 24, No. 9 | DOI:10.1364/OE.24.009316 | OPTICS EXPRESS 9319

owning to the excitation of the hybrid plasmonic mode, which is favorable in making an ultra-compact device.

Fig. 3. Simulated reflection spectra in response to various design parameters: (a) grating period, (b) filling factor, (c) nanofiber radius, and (d) number of grating periods.

To better understand the Bragg reflection behavior of the device, we simulate the steady-state light electric-field intensity pattern in the device at the two Bragg wavelengths for the set of parameters used in Fig. 2. The results are shown in Fig. 4. Clear standing wave patterns are observed with its period consistent with the metal grating period. Light intensity reduces along the nanofiber axial direction with partial light absorbed by the metal and partial light reflected back to the incident port. A difference is also discernable from the field patterns at the two Bragg wavelengths. At the longer wavelength of 1650 nm, the standing wave node is located above the metal strip while the antinode is located in the trench. The reflected light is the fundamental TM mode of the nanofiber. In contrast, at the shorter wavelength of 1525 nm, the field pattern exhibits a zig-zag path where the light wave node is pushed outwards above the metal strip and pulled inwards the substrate in the trench. Examination of the cross-sectional field pattern of the reflected light beyond the light launch point reveals that a first-order TM mode is excited by the metal grating. As the first-order TM mode is close to cut-off, its reflection peak therefore gradually disappears when the nanofiber radius reduces.

#258679 Received 1 Feb 2016; revised 8 Apr 2016; accepted 15 Apr 2016; published 20 Apr 2016 © 2016 OSA 2 May 2016 | Vol. 24, No. 9 | DOI:10.1364/OE.24.009316 | OPTICS EXPRESS 9320

Fig. 4. (a) Light electric-field intensity pattern (|E|2) in the x-y plane at the 1650 nm wavelength. Nanofiber fundamental mode is launched from the intersection plane at x = −17 μm. The Inset shows the cross-sectional distribution of the reflected light intensity. (b) Magnified view showing the standing wave light field pattern at 1650 nm. The cross-sectional field patterns along the dashed lines (i) and (ii) are also illustrated. (c) Light electric-field intensity pattern in the x-y plane at the 1525 nm wavelength. (d) Magnified view of the light field pattern at 1525 nm.

3. Device preparation and experiments

Figure 5(a) shows the fabrication process flow of the metal grating. We started with a silicon wafer coated with a thick layer of silicon dioxide. It was first deposited with a 500-nm-thick Au film using e-beam evaporation. A layer of 440-nm-thick e-beam resist (Zep 520) was spin-coated on the gold film and baked on a hotplate at 180°C for 90s. The grating was patterned on the resist layer by electron-beam lithography (EBL). The metal layer was then dry etched by ion-beam etching (IBE) with the resist serving as the etching soft-mask. The ion-beam energy was 550 eV and the chamber pressure was 5 × 10−4 Pa. The etch rates of the gold and resist films are 45 nm/min and 22 nm/min, respectively. After dry etch, the remained e-beam resist was subsequently removed by N-Methyl-2-Pyrrolidinone (NMP) and acetone. The entire fabrication was done at the Center for Advanced Electronic Materials and Devices (AEMD) platform of our university.

Figure 5(b) shows the scanning electron micrograph (SEM) image of the metal grating. The white and dark lines are the Au strips and air slits, respectively. The grating period was

#258679 Received 1 Feb 2016; revised 8 Apr 2016; accepted 15 Apr 2016; published 20 Apr 2016 © 2016 OSA 2 May 2016 | Vol. 24, No. 9 | DOI:10.1364/OE.24.009316 | OPTICS EXPRESS 9321

measured to be 651.3 nm and the air slit width 260.5 nm. Thus the filling factor is 0.6. In order to make it more convenient to attach the nanofiber to the surface of metal grating, the grating pattern was designed to be large enough with an area of 1 mm × 1.5 mm.

Fig. 5. (a) Fabrication process flow of the metal grating. (b) SEM image of the metal grating.

The nanofiber was drawn from a standard SMF using the custom nanofiber drawing platform [32]. The parameters, such as the drawing speed and the resulting nanofiber section length, were set in advance. The radius of the nanofiber was measured from the SEM images. A flat spectrum with no ripples was measured, indicating that the mode transition from the SMF to the nanofiber was smooth without excitation of back reflection. The measured insertion loss of the nanofiber was less than 1 dB.

The coupling of the nanofiber to the metal grating was realized as follows. The fiber was first clamped and pulled straight by two high-precision translation stages. The metal grating chip was held by another translation stage and positioned beneath the nanofiber. We then moved the fiber clamp stages closer so that the nanofiber gradually bent downward. Next, we moved the chip up to approach the nanofiber. The nanofiber can tightly cling to the metal grating surface once they were touched together due to the Van der Waals and electrostatic forces. The contact length was decided by the vertical position of the chip and the distance between the fiber clamp stages. The actual contact length was measured from the images taken by the top and backside CCD cameras. The nanofiber alignment angle was adjustable by rotating the chip.

Figure 6 shows the experimental setup to characterize the device. A broadband light emitting diode (LED) source was used and the output light was polarized by using a polarization beam splitter (PBS). A polarization controller was followed to control the light to TM polarization before entering the grating. The transmission power was monitored by an optical power meter. The reflection spectra were recorded by an optical spectrum analyzer (OSA). A 3-dB coupler was put in front of the device in order to separate the reflected light. Although it has a 3-dB extra loss, it has very high isolation of 63 dB (much higher than that of a typical optical circulator with 40 dB isolation).

#258679 Received 1 Feb 2016; revised 8 Apr 2016; accepted 15 Apr 2016; published 20 Apr 2016 © 2016 OSA 2 May 2016 | Vol. 24, No. 9 | DOI:10.1364/OE.24.009316 | OPTICS EXPRESS 9322

Fig. 6. Experimental setup to characterize the device. PC: polarization controller; PBS: polarization beam splitter; OSA: optical spectrum analyzer.

Figure 7(a) shows the measured reflection spectra for different nanofiber radii ranging from 0.45 μm to 0.73 μm. The coupling length is around 0.8 mm. With the increase of the nanofiber radius, the filter central wavelength experiences red-shift as the larger nanofiber radius increases the effective index. The peak has a 3-dB bandwidth of 15 nm and out-of-band rejection of >30 dB. The change trend is consistent with the simulation results shown in Fig. 3(c). The peak reflection loss is about 10 dB, caused by the metal absorption and scattering. It should be noted that a second reflection peak is also discernable for R = 0.7 μm and 0.73 μm. Compared to the simulation, it is much weaker perhaps because of the high bending loss. We also studied the influence of the nanofiber alignment angle on the filter performance as shown in Fig. 7(b). It can be seen that the central wavelength of the reflection band slightly increases with the angle and meanwhile the peak broadens with a reduced height. Such a change trend could be understood as follows. A larger angle is equivalent to an increased grating period and a stronger coupling coefficient. According to Eqs. (2) and (4), the reflection bandwidth increases with a reduced peak power. Finally, we also investigated the effect of coupling length. As stated above, the coupling length can be changed by moving the chip vertically. Figure 7(c) shows the reflection spectra for four coupling lengths. The nanofiber radius is 0.45 μm. The coupling length is an appropriate value estimated from the microscope image. The measurement reveals that the reflection spectrum is relatively stable and almost not changeable with the coupling length. It hence suggests that the reflection mainly occurs at the front of the metal grating. The effective interaction length is less than the shortest contact length in the experiment. Due to the strong attractive force existing between the nanofiber and the metal grating, we were unable to further shorten the contact length.

Fig. 7. Measured input-normalized TM reflection spectra for various device parameters: (a) nanofiber radius, (b) nanofiber alignment angle and (c) coupling length.

The nanofiber coupled Bragg grating can work as a bandpass optical filter for multiplexing/de-multiplexing wavelength division multiplexing (WDM) optical signals. In this regard, its function is similar to a conventional fiber Bragg grating, but the grating length is much shorter and the passband is much broader in our device because of the strong coupling enabled by the metal grating. Another key feature that we would like to highlight is

#258679 Received 1 Feb 2016; revised 8 Apr 2016; accepted 15 Apr 2016; published 20 Apr 2016 © 2016 OSA 2 May 2016 | Vol. 24, No. 9 | DOI:10.1364/OE.24.009316 | OPTICS EXPRESS 9323

that strong optical field is localized outside the nanofiber, especially in the air between the nanofiber and metal grating, which makes it quite attractive in optical sensing applications. The enhancement of optical field at the metal interface can also be exploited to study the light-matter interaction when a thin layer of material is coated on the metal. For example, a graphene can be sandwiched between the nanofiber and the metal grating to facilitate the excitation of four-wave mixing in graphene.

4. Conclusions

We have presented a flexible hybrid plasmonic grating composed of a nanofiber situated on an array of metal strips. The nanofiber is bent to touch and couple with the metal grating. The TM mode has much strong reflection than the TE mode. Numerical simulations show that the reflection band is strongly dependent on the nanofiber radius and metal grating period. The interaction length affects the peak reflectivity and bandwidth. A higher order mode nanofiber mode could be excited upon reflection by the grating even when the incident light is the fundamental mode. We experimentally realized such a hybrid plasmonic grating device and characterized the device with different nanofiber radii, alignment angles and coupling lengths. A typical device exhibits a strong reflection peak with a 3-dB bandwidth of 15 nm and out-of-band rejection of more than 30 dB. The reflection spectrum is almost unchangeable when the coupling length is long enough, facilitating to form a stable and repeatable device. The flexibility in nanofiber attachment makes it cost-effective to build such a hybrid plasmonic grating device. It could find potential applications in optical filters and biochemical sensors.

Acknowledgment

This work was supported in part by the National Natural Science Foundation of China (NSFC) (61422508), the Shanghai Rising-Star Program (14QA1402600).

#258679 Received 1 Feb 2016; revised 8 Apr 2016; accepted 15 Apr 2016; published 20 Apr 2016 © 2016 OSA 2 May 2016 | Vol. 24, No. 9 | DOI:10.1364/OE.24.009316 | OPTICS EXPRESS 9324