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Wavelength and intensity switching in directly coupled semiconductor microdisk lasers Gustavo E. Fernandes, 1, * Laurent Guyot, 1 Grace D. Chern, 2 Michael Kneissl, 3 Noble M. Johnson, 4 Qinghai Song, 5 Lei Xu, 5 and Richard K. Chang 1 1 Department of Applied Physics and Center for Laser Diagnostics, Yale University, New Haven, Connecticut, 06511, USA 2 U.S. Army Research Laboratory, Adelphi, Maryland 20783, USA 3 Institute for Solid State Physics, Technical University of Berlin, D-10623 Berlin, Germany 4 Palo Alto Research Center Inc. (PARC), Palo Alto, California 94304, USA 5 Department of Optical Science and Engineering, Fudan University, Shanghai 2004333, China * Corresponding author: [email protected] Received October 26, 2007; revised February 1, 2008; accepted February 5, 2008; posted February 13, 2008 (Doc. ID 89070); published March 14, 2008 We demonstrate output wavelength and intensity switching in a three-element directly coupled microdisk device consisting of one spiral microdisk coupled to two semicircle microdisks. The gapless coupling mecha- nism used allows individual elements to achieve lasing while achieving optimal transfer of optical power between adjacent microdisks. By controlling the transparency of the center element via injection current, the edge elements can be allowed to exchange their amplified spontaneous emission. In this manner, on– off–on switching of the output intensity, as well as discontinuous shifts in the output wavelength, can be achieved as a function of increasing injection current. © 2008 Optical Society of America OCIS codes: 140.4780, 140.5960, 230.3990. In recent years, devices consisting of two or more coupled microdisk resonators [17] have led to a host of interesting phenomena and functionality, includ- ing mode locking [1], optical delay lines [2], optical logic gates [3], and bistability [4]. Coupling schemes used thus far have relied on evanescent-field cou- pling through a fixed-width air or substrate gap [5]. These methods offer little control over the coupling efficiency during run time and require gap widths in the submicrometer range [8,9] for optimal coupling efficiencies. By using gapless (end-to-end) coupling via a current-controlled waveguide, we have achieved run- time control of the coupling between individual mi- crodisks. The high coupling efficiency achieved via this method allows coupling not only of the lasing modes but also of the amplified spontaneous emission (ASE). In this Letter we report the switching of out- put wavelength and intensity, achieved by varying the coupling in a three-element directly coupled de- vice consisting of a spiral and two semicircle micro- disks. The devices considered here (see Fig. 1) are single- quantum-well AlGaAs devices grown by low-pressure metalorganic vapor phase epitaxy, as described else- where [10]. Direct coupling is achieved by seamlessly joining the spiral and semicircle-shaped microdisks at the region where the whispering-gallery modes (WGMs) occur [Fig. 1(a)]. Variable amplitude current pulses (1 s, 50 ns rise and fall times) are simultaneously applied to each of the three microdisks in the coupled device. The injec- tion current is confined to the perimeter of the micro- disks [11] by a SiO 2 insulation layer that covers the central portion of the microdisks [Fig. 1(b)]. Electri- cal insulation between the optically coupled micro- disk elements is achieved by separating the p-metal electrodes with SiO 2 pads [Fig. 1(c)]. The series resis- tance between two adjacent microdisks (p-contact to p-contact) was measured to be approximately 600 , and the current versus voltage (I–V) forward curve on one of the microdisks indicated that the differen- tial series resistance was approximately 8 . The light emitted from the coupled microdisk laser in the direction indicated by the arrow in Fig. 1(c) is collected by means of a camera lens (f = 50 mm, d = 25 mm) into a spectrometer (500 mm focal length, 0.05 nm resolution with a 1200 g / mm grating). The Fig. 1. (a) Scanning electron microscope (SEM) image. (b) AlGaAs microdisk heterostructure. (c) Top view showing p-contacts and insulation pads. Note: SEM figure shows equivalent GaN device. March 15, 2008 / Vol. 33, No. 6 / OPTICS LETTERS 605 0146-9592/08/060605-3/$15.00 © 2008 Optical Society of America

Wavelength and intensity switching in directly coupled semiconductor microdisk lasers

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March 15, 2008 / Vol. 33, No. 6 / OPTICS LETTERS 605

Wavelength and intensity switching in directlycoupled semiconductor microdisk lasers

Gustavo E. Fernandes,1,* Laurent Guyot,1 Grace D. Chern,2 Michael Kneissl,3 Noble M. Johnson,4

Qinghai Song,5 Lei Xu,5 and Richard K. Chang1

1Department of Applied Physics and Center for Laser Diagnostics, Yale University, New Haven,Connecticut, 06511, USA

2U.S. Army Research Laboratory, Adelphi, Maryland 20783, USA3Institute for Solid State Physics, Technical University of Berlin, D-10623 Berlin, Germany

4Palo Alto Research Center Inc. (PARC), Palo Alto, California 94304, USA5Department of Optical Science and Engineering, Fudan University, Shanghai 2004333, China

*Corresponding author: [email protected]

Received October 26, 2007; revised February 1, 2008; accepted February 5, 2008;posted February 13, 2008 (Doc. ID 89070); published March 14, 2008

We demonstrate output wavelength and intensity switching in a three-element directly coupled microdiskdevice consisting of one spiral microdisk coupled to two semicircle microdisks. The gapless coupling mecha-nism used allows individual elements to achieve lasing while achieving optimal transfer of optical powerbetween adjacent microdisks. By controlling the transparency of the center element via injection current,the edge elements can be allowed to exchange their amplified spontaneous emission. In this manner, on–off–on switching of the output intensity, as well as discontinuous shifts in the output wavelength, can beachieved as a function of increasing injection current. © 2008 Optical Society of America

OCIS codes: 140.4780, 140.5960, 230.3990.

In recent years, devices consisting of two or morecoupled microdisk resonators [1–7] have led to a hostof interesting phenomena and functionality, includ-ing mode locking [1], optical delay lines [2], opticallogic gates [3], and bistability [4]. Coupling schemesused thus far have relied on evanescent-field cou-pling through a fixed-width air or substrate gap [5].These methods offer little control over the couplingefficiency during run time and require gap widths inthe submicrometer range [8,9] for optimal couplingefficiencies.

By using gapless (end-to-end) coupling via acurrent-controlled waveguide, we have achieved run-time control of the coupling between individual mi-crodisks. The high coupling efficiency achieved viathis method allows coupling not only of the lasingmodes but also of the amplified spontaneous emission(ASE). In this Letter we report the switching of out-put wavelength and intensity, achieved by varyingthe coupling in a three-element directly coupled de-vice consisting of a spiral and two semicircle micro-disks.

The devices considered here (see Fig. 1) are single-quantum-well AlGaAs devices grown by low-pressuremetalorganic vapor phase epitaxy, as described else-where [10]. Direct coupling is achieved by seamlesslyjoining the spiral and semicircle-shaped microdisksat the region where the whispering-gallery modes(WGMs) occur [Fig. 1(a)].

Variable amplitude current pulses (1 �s, 50 ns riseand fall times) are simultaneously applied to each ofthe three microdisks in the coupled device. The injec-tion current is confined to the perimeter of the micro-disks [11] by a SiO2 insulation layer that covers thecentral portion of the microdisks [Fig. 1(b)]. Electri-

cal insulation between the optically coupled micro-

0146-9592/08/060605-3/$15.00 ©

disk elements is achieved by separating the p-metalelectrodes with SiO2 pads [Fig. 1(c)]. The series resis-tance between two adjacent microdisks (p-contact top-contact) was measured to be approximately 600 �,and the current versus voltage (I–V) forward curveon one of the microdisks indicated that the differen-tial series resistance was approximately 8 �.

The light emitted from the coupled microdisk laserin the direction indicated by the arrow in Fig. 1(c) iscollected by means of a camera lens (f=50 mm, d=25 mm) into a spectrometer (500 mm focal length,0.05 nm resolution with a 1200 g/mm grating). The

Fig. 1. (a) Scanning electron microscope (SEM) image. (b)AlGaAs microdisk heterostructure. (c) Top view showingp-contacts and insulation pads. Note: SEM figure shows

equivalent GaN device.

2008 Optical Society of America

606 OPTICS LETTERS / Vol. 33, No. 6 / March 15, 2008

spectrally dispersed emission is then detected withan intensified CCD detector.

When operated individually, the output of each ac-tive element follows the traditional light output ver-sus current (L–I) curve found in textbooks on lasers[12]. For small injection currents, the elements areoptically absorbing, because the electroluminescencepeak coincides with the bandgap energy [12] of thesemiconductor. As the injection current is increased,the elements become optically transparent and, fi-nally, enter the gain regime, where both the WGMsand the spontaneous emission can be amplified.

Table 1 lists the peak output wavelengths andlasing-threshold currents for each individual ele-ment. These values were obtained by measuring theoutput spectra of the device with the active elementpumped above lasing threshold and the other ele-ments set to 0 A/cm2. Only the center semicircle(SC-1) did not lase when pumped independently, be-cause the direct-coupling junctions on either side ofSC-1 do not provide enough feedback. An extendedcavity composed of SC-1+SC-2 (Table 1) can beformed by simultaneously setting the injection cur-rents in both SC-1 and SC-2 (ISC-1 and ISC-2, respec-tively) above their lasing threshold.

More interesting L–I curves result when two or allthree elements are operated simultaneously. WithISC-1 in the transparency or gain regime, both the spi-ral and SC-2 can exchange their ASE and, as a result,deplete each other’s inverted population. This causesthe device L–I curve to develop dips (regions of lowoutput intensity) as either ISC-1 or ISC-2 is increased.For large enough values of ISC-1 and ISC-2, lasing canbe achieved in SC-1+SC-2, and the device recoversfrom the dip region with the emergence of new lasingpeaks in the output spectrum. SC-1 thus functions asa control channel for the coupling between the spiraland SC-2.

The output spectra in Fig. 2(a) show two operatingon states, separated by an off state, as a function ofincreasing ISC-1. The first on state, corresponding tothe lasing in SC-2, lasts until transparency isachieved in SC-1. This is because the feedback inSC-2 is provided partially by the interface with SC-1[6,7]. The off state begins when ISC-1�250 A/cm2. Inthe off state no lasing peak is observed in the spec-trum, and the net light output from the device con-sists entirely of ASE. Further increase of ISC-1 causesthe device to enter the second on state, as a new

Table 1. Lasing Wavelength and Threshold Currentof Individual Elements in the Three-Element Device

ActiveElement �Lasing�nm� Ithresh�A/cm2�

Spiral (Sp) 733.4 462Center semicircle

(SC-1)No lasingobserved

Edge semicircle(SC-2)

725.0 990

SC-1 + SC-2 727.5 990 (both)

group of lasing peaks emerges in the spectrum. The

new group of peaks coincides with the lasing wave-length of the SC-1+SC-2 cavity shown in Table 1 andis thus redshifted by approximately 2.5 nm relativeto the first on state. The relative peak output inten-sities in the two on stages can be controlled by oper-ating the spiral in the ASE regime. With ISp=0 A/cm2, the intensity in the second on stage be-comes approximately 40% larger relative to the firston stage.

A different pumping configuration, with ISC-2 var-ied [Fig. 2(b)], causes the device to exhibit a blueshiftof approximately 5 nm in the output wavelength.Here, the initial spectrum �ISC-2=0 A/cm2� corre-sponds to spiral lasing modes ��733 nm�. The secondon state corresponds again to the lasing of the SC-1+SC-2 cavity, at 727.5 nm. Inspection of Table 1 andFig. 2 reveals a possible application for the three-element device as optical logic OR, AND, and NOTgates, the latter two requiring the addition of an op-tical filter to block one of the emission wavelengthsand transmit the other.

The wavelength shifts result from different effec-tive cavities becoming active as the injection currentis changed in one of the elements. The size of thewavelength shifts can thus be set via the relative

Fig. 2. (Color online) Output spectra as a function of in-jection current for different pumping configurations. Thecurrent in the darkened element is ramped from 0 to 1700

2

A/cm .

March 15, 2008 / Vol. 33, No. 6 / OPTICS LETTERS 607

sizes of the elements but is limited by the gain band-width of the semiconductor (e.g., devices with bothsemicircles having the same radii were tested, but nowavelength shift was observed with the on–off–on in-tensity switching).

Another interesting set of L–I curves results inboth ISp and ISC-2 kept above threshold while ISC-1 isvaried (Fig. 3). Here, two separate groups of lasingpeaks are observed in the spectrum when ISC-1�0 A/cm2. The lasing peaks at 731.5 nm correspondto the spiral, and the peaks at 723.9 nm correspondto SC-2. While SC-1 is kept in the absorbing regime,the peaks corresponding to the spiral and SC-2 can

Fig. 3. (Color online) (a) Spiral and SC-2 in the lasing re-gime. (b) Peak intensity of the main peaks appearing in thespectra.

coexist, and their intensities can be independently

controlled. The off state for this configuration is alsoreached at ISC-1�250 A/cm2. At large values of ISC−1,lasing reestablishes in the device, now with SC-1+SC-2 acting as an extended cavity with a lasingwavelength of 730.3 nm.

The reason for the poor signal-to-noise ratio inFigs. 2(b) and 3 is related to the fact that the lasingmodes excited by such pumping configurations havelow Q. In principle this may be remedied by optimiz-ing such parameters as the length of overlap betweenelements, as well as the relative sizes of the ele-ments.

We have demonstrated wavelength and intensityswitching on three-element directly coupled AlGaAsmicrodisks. The current controlled coupling mecha-nism used does not depend on any critical parameteror demanding operating conditions and provides real-time control of the emission of light from these de-vices. The spectral and L–I characteristics revealpossible applications as logic gates and opticalswitches.

This work was partially supported by the DefenseAdvanced Research Projects Agency SemiconductorUltraviolet Sources (SUVOS) Program under Spaceand Naval Warfare (SPAWAR) Systems Center con-tract N66001-02-C-8017.

References

1. W. T. Tsang, N. A. Olsson, and R. A. Logan, Appl. Phys.Lett. 42, 650 (1983).

2. J. K. S. Poon, J. Scheuer, Y. Xu, and A. Yariv, J. Opt.Soc. Am. B 21, 1665 (2004).

3. T. A. Ibrahim, K. Amarnath, L. C. Kuo, R. Grover, V.Van, and P. T. Ho, Opt. Lett. 29, 2779 (2004).

4. N. A. Olsson, W. T. Tsang, and R. A. Logan, Appl. Phys.Lett. 44, 375 (1984).

5. O. Skorka and J. Salzman, Opt. Lett. 28, 1939 (2003).6. G. D. Chern, G. E. Fernandes, R. K. Chang, Q. Song, L.

Xu, M. Kneissl, and N. M. Johnson, Opt. Lett. 32, 1093(2007).

7. R. K. Chang, G. E. Fernandes, and M. Kneissl, inICTON 2006 Conference Proceedings, (IEEE 2006), Vol.1, pp. 47–51.

8. A. Yariv, IEEE Photon. Technol. Lett. 14, 483 (2002).9. Y. Pan and R. K. Chang, Appl. Phys. Lett. 82, 487

(2002).10. M. Kneissl, M. Teepe, N. Miyahsita, N. M. Johnson, G.

D. Chern, and R. K. Chang, Appl. Phys. Lett. 84, 2485(2004).

11. N. B. Rex, R. K. Chang, and L. J. Guido, IEEE Photon.Technol. Lett. 13, 1 (2001).

12. O. Svelto, Principles of Lasers (Plenum, 1998).