Optical bistability in GaInAsP/InP coupled-circular resonator microlasers
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Optical bistability in GaInAsP/InP coupled-circular resonator microlasers Jian-Dong Lin, Yong-Zhen Huang,* Yue-De Yang, Qi-Feng Yao, Xiao-Meng Lv, Jin-Long Xiao, and Yun Du State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China *Corresponding author: [email protected] Received June 15, 2011; revised August 8, 2011; accepted August 9, 2011; posted August 9, 2011 (Doc. ID 149294); published September 1, 2011 Optical bistability is realized in GaInAsP/InP coupled-circular resonator microlasers, which are fabricated by planar technology. For a coupled-circular resonator microlaser with the radius of 20 μm and a 2 μm-wide bus waveguide, hysteresis loops are observed for the output power coupling into an optical fiber versus the cw injection cur- rent at room temperature. The laser output spectra of the upper and lower states of the hysteresis loop indicate that the bistability is related to mode competitions. The optical bistability can be explained as the mode compe- tition between the symmetry and antisymmetry coupled modes relative to the bus waveguide. © 2011 Optical Society of America OCIS codes: 140.2020, 140.3948, 230.3120, 230.5750, 250.5960. Optical bistability in semiconductor microlasers are required as a fundamental element for optical memory and optical flip-flop in optical signal processing [1]. Bistable semiconductor lasers were first predicted and realized in two-section semiconductor lasers with ab- sorption saturation [2–5], and in twin microdisk lasers [6]. Furthermore, polarization bistabilities in vertical- cavity surface-emitting lasers [7,8] and unidirectional bistability in semiconductor ring lasers [1,9,10] and microdisk lasers [11] were reported and attracted great attention, due to the advantage of the ultrahigh switching speed of mode competition via gain saturation, and a fast bistable operation was predicted in photonic-crystal microcavities based on mode competition [12]. In addi- tion, optical bistability in an equilateral triangle resonator microlaser was observed at low temperatures due to the mode competition [13]. Recently, high Q symmetry and antisymmetry modes were predicted for coupled-circular microresonators with a middle bus waveguide by two-dimensional (2D) finite-difference time-domain (FDTD) technique [14]. The symmetry and antisymmetry modes have different output coupling efficiencies from the bus waveguide, and the output efficiency of the antisymmetry coupled mode is zero if the bus waveguide is a single-mode waveguide. Thus, the mode transition between the symmetry and antisymmetry modes can result in a great variation of output power from the bus waveguide. In this Letter, we report the observation of the optical bistability in a GaInAsP/InP coupled-circular microlaser with a radius of 20 μm at room temperature. A GaInAsP/InP multiple-quantum-well laser wafer is used for fabricating the microlasers. The active region of the laser wafer is five compressively strained quantum wells sandwiched by up and down 120 nm-thick GaInAsP confinement layers, and the thicknesses of the quantum wells and the barrier layers are 10 and 12 nm, respec- tively. The upper layers are a 1:5 μm-thick p-InP cladding layer and a pþ-InGaAs contacting layer. The coupled- circular resonator microlasers with the radius of 20 μm are fabricated by photolithography and the inductively coupled-plasma (ICP) etching technique processes [13]. An 800 nm SiO 2 layer was first deposited by plasma- enhanced chemical vapor deposition, and the resonator patterns were transferred onto the SiO 2 layer using standard photolithography and ICP etching techniques. The laser wafer was etched by about 6 μm using the ICP technique with the patterned SiO 2 as masks, then a chemically etching process was used to improve the smoothness of the etched side walls. Then the residual SiO 2 masks on the resonators were removed by using diluted HF solution, and a 450 nm SiO 2 insulating layer was deposited on the wafer. Finally, the SiO 2 layer on the top of resonators was etched using the ICP etching process for opening an electrical injection window, and p- and n-electrodes were deposited. The sidewalls of the coupled-circular microlasers and the two ends of the bus waveguide were covered by an SiO 2 insulating layer and p-electrode Ti-Au layers. Figure 1 shows (a) the micro- scope image of the microlaser, where the upper side of the bus waveguide is cleaved for testing, and (b) the schematic diagram of the microlaser. The InGaAsP/InP coupled-circular resonators are tangentially coupled with the middle bus waveguide in a photo mask, although a small gap appears between the p-electrodes in the circular resonators and the bus waveguide in Fig. 1(a). The output powers coupled into an optical fiber versus cw injection currents are plotted in Fig. 2 at (a) 293 K and (b) 308 K, respectively, where the dashed and the solid lines are measured by increasing and decreasing cw injection currents, respectively. The hysteresis loops are Fig. 1. (Color online) (a) Microscope image of a coupled- circular resonator microlaser with the bus waveguide cleaved for testing, and (b) schematic diagram of a coupled-circular microlaser. September 1, 2011 / Vol. 36, No. 17 / OPTICS LETTERS 3515 0146-9592/11/173515-03$15.00/0 © 2011 Optical Society of America
Transcript of Optical bistability in GaInAsP/InP coupled-circular resonator microlasers
Optical bistability in GaInAsP/InP coupled-circularresonator microlasers
Jian-Dong Lin, Yong-Zhen Huang,* Yue-De Yang, Qi-Feng Yao, Xiao-Meng Lv, Jin-Long Xiao, and Yun DuState Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors,
Chinese Academy of Sciences, Beijing 100083, China*Corresponding author: [email protected]
Received June 15, 2011; revised August 8, 2011; accepted August 9, 2011;posted August 9, 2011 (Doc. ID 149294); published September 1, 2011
Optical bistability is realized in GaInAsP/InP coupled-circular resonator microlasers, which are fabricated by planartechnology. For a coupled-circular resonator microlaser with the radius of 20 μm and a 2 μm-wide bus waveguide,hysteresis loops are observed for the output power coupling into an optical fiber versus the cw injection cur-rent at room temperature. The laser output spectra of the upper and lower states of the hysteresis loop indicatethat the bistability is related to mode competitions. The optical bistability can be explained as the mode compe-tition between the symmetry and antisymmetry coupled modes relative to the bus waveguide. © 2011 OpticalSociety of AmericaOCIS codes: 140.2020, 140.3948, 230.3120, 230.5750, 250.5960.
Optical bistability in semiconductor microlasers arerequired as a fundamental element for optical memoryand optical flip-flop in optical signal processing [1].Bistable semiconductor lasers were first predicted andrealized in two-section semiconductor lasers with ab-sorption saturation [2–5], and in twin microdisk lasers[6]. Furthermore, polarization bistabilities in vertical-cavity surface-emitting lasers [7,8] and unidirectionalbistability in semiconductor ring lasers [1,9,10] andmicrodisk lasers [11] were reported and attracted greatattention, due to the advantage of the ultrahigh switchingspeed of mode competition via gain saturation, and a fastbistable operation was predicted in photonic-crystalmicrocavities based on mode competition [12]. In addi-tion, optical bistability in an equilateral triangle resonatormicrolaser was observed at low temperatures due to themode competition [13].Recently, high Q symmetry and antisymmetry modes
were predicted for coupled-circular microresonatorswith a middle bus waveguide by two-dimensional (2D)finite-difference time-domain (FDTD) technique [14]. Thesymmetry and antisymmetry modes have different outputcoupling efficiencies from the bus waveguide, and theoutput efficiency of the antisymmetry coupled mode iszero if the bus waveguide is a single-mode waveguide.Thus, the mode transition between the symmetry andantisymmetry modes can result in a great variation ofoutput power from the bus waveguide. In this Letter,we report the observation of the optical bistability in aGaInAsP/InP coupled-circular microlaser with a radiusof 20 μm at room temperature.A GaInAsP/InP multiple-quantum-well laser wafer is
used for fabricating the microlasers. The active regionof the laser wafer is five compressively strained quantumwells sandwiched by up and down 120 nm-thick GaInAsPconfinement layers, and the thicknesses of the quantumwells and the barrier layers are 10 and 12 nm, respec-tively. The upper layers are a 1:5 μm-thick p-InP claddinglayer and a pþ-InGaAs contacting layer. The coupled-circular resonator microlasers with the radius of 20 μmare fabricated by photolithography and the inductivelycoupled-plasma (ICP) etching technique processes [13].
An 800 nm SiO2 layer was first deposited by plasma-enhanced chemical vapor deposition, and the resonatorpatterns were transferred onto the SiO2 layer usingstandard photolithography and ICP etching techniques.The laser wafer was etched by about 6 μm using theICP technique with the patterned SiO2 as masks, thena chemically etching process was used to improve thesmoothness of the etched side walls. Then the residualSiO2 masks on the resonators were removed by usingdiluted HF solution, and a 450 nm SiO2 insulating layerwas deposited on the wafer. Finally, the SiO2 layer onthe top of resonators was etched using the ICP etchingprocess for opening an electrical injection window, andp- and n-electrodes were deposited. The sidewalls of thecoupled-circular microlasers and the two ends of the buswaveguide were covered by an SiO2 insulating layer andp-electrode Ti-Au layers. Figure 1 shows (a) the micro-scope image of the microlaser, where the upper side ofthe bus waveguide is cleaved for testing, and (b) theschematic diagram of the microlaser. The InGaAsP/InPcoupled-circular resonators are tangentially coupledwith the middle bus waveguide in a photo mask, althougha small gap appears between the p-electrodes in thecircular resonators and the bus waveguide in Fig. 1(a).
The output powers coupled into an optical fiber versuscw injection currents are plotted in Fig. 2 at (a) 293K and(b) 308K, respectively, where the dashed and the solidlines are measured by increasing and decreasing cwinjection currents, respectively. The hysteresis loops are
Fig. 1. (Color online) (a) Microscope image of a coupled-circular resonator microlaser with the bus waveguide cleavedfor testing, and (b) schematic diagram of a coupled-circularmicrolaser.
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observed with two bistable regions at the injectioncurrent from 70 to 97mA and from 106 to 145mA at293K, and from 82 to 105mA and from 115 to 151mAat 308K. As shown in Fig. 2, the output power appearsfirst at the lower state with a low output power in thefirst hysteresis loop, and then jumps to the upper stateof the second hysteresis loop with the increase of theinjection current. The ratios of the output power in theupper state to that of the lower states are about 2 and10 for the first and the second hysteresis loops at 293K,respectively, and the ratio is about 4 at 308K.Figure 3 shows the laser output spectra of the upper
and the lower states at (a) 100mA and (b) 130mA mea-sured at 308K with an optical spectrum analyzer at aresolution of 0:1 nm. In Fig. 3(a), the main lasing peakat the wavelength of 1538:89 nm is about 15 dB largerthan the second peak at 1535:35 nm in the upper state,but the main peak is only 5:5 dB larger than the secondpeak in the lower state. As the current increases from100 to 130mA, the third peak at 1544:40 nm in Fig. 3(a)becomes the main lasing peak at 1545:11 nm with anintensity 12 dB larger than that of the second peak at1535:93 nm in the upper state of Fig. 3(b), and the mainpeak is only 5 dB larger than the second peak in the lowerstate. In addition, the main lasing peak in Fig. 3(a)becomes the third peak at 1539:63 nm in Fig. 3(b). Thedetailed laser spectra of the main peak in Fig. 3(c), mea-sured at 130mA with a resolution of 0:06 nm, show thatthe laser spectrum of the upper state is much wider thanthat of the lower state; the main peak in the upper statemay contain several modes. The wavelengths of thelasing peaks increase about 0:65 nm as the injectioncurrent increases from 100 to 130mA; the correspondingtemperature increase is 6:5K; by assuming the mode
wavelength versus the temperature at 0:1nm=K, even thetemperature of the device holder is set at 308K. The tem-perature increase of 6:5K can result in peak wavelengthshift of the gain spectrum, which can partly explain themode jump with the wavelength interval of 5:5 nm from100 to 130mA. Assuming the wavelength interval of5:5 nm as the angular (longitudinal) mode interval, wecan get the mode group index of 3.44 with one period ofthe mode light path as the perimeter of the circular reso-nator. The laser spectra indicate that strong mode com-petitions exist between the modes at the wavelengths of1538.89 ð1539:63Þ nm and 1544.40 ð1545:11Þ nm at 100ð130ÞmA. The peak at 1535.35 ð1535:93Þnm is alwaysthe second peak at 100 ð130ÞmA with a constant intensityat the upper and the lower states, which can be attributedto the mode of different radial mode number or thecoupled mode of two different whispering-galley modes(WGMs) [14], because the wavelength interval 3:54 nmbetween the major and the second peaks in Fig. 3(a)is smaller than the angular mode wavelength interval.
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Fig. 2. (Color online) Output power coupled into an opticalfiber versus the increased and decreased cw injection currentfor a coupled-circular microlaser with a radius of 20 μm at(a) 293K and (b) 308K, respectively.
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Fig. 3. (Color online) Laser spectra in the lower and the upperstates at 308K with the injection currents of (a) 100mA,(b) 130mA, and (c) detail spectra at 130mA, for the coupled-circular microlaser.
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We also measure the output power versus the injectioncurrent by butt-coupling a 5mm-diameter optical detec-tor with the cleaved end of the bus waveguide. Theobtained output powers versus the injection currentsare plotted in Fig. 4, which shows little variation of theoutput power with the increase and decrease of theinjection current.Furthermore, the far field patterns of the microlaser
show great variation from the upper to lower states ofthe hysteresis loops. The results indicate that the opticalbistability can be attributed to the mode competitionbetween the symmetry and the antisymmetry coupled-modes in the coupled-circular resonator. The antisymme-try modes have very low coupling efficiencies to theoptical fiber, so the mode jumps between the symmetryand antisymmetry modes result in the optical bistability.A large power jump can be expected as the bus wave-guide is a single-mode waveguide instead of the 2 μm-wide waveguide. Comparing to the triangle bistablemicrolaser, the coupled-circular microlaser has two portsfor signal processing. Furthermore, the high Qmodes arehigh radial order WGMs in the coupled-circular resona-tor, which can reduce the requirement of the smoothnessof the sidewall for realizing optical bistability relativeto the microdisk lasers [11]. Because of the techniqueproblem, the yield of our devices is not very high. Weonly get some devices with bistability behavior. Thetemperature dependence is a problem that needs to beconsidered practically. We will consider these problemsin our future investigation.Finally, we simulate mode field patterns by 3D FDTD
technique for a coupled-circular microresonator with aradius of 2 μm and a 0:4 μm-wide middle bus waveguideconfined by air. The thickness of the microresonator is0:4 μm with the refractive index of 3.4. The obtainedmagnetic field patterns in the middle plane are plotted inFig. 5 for (a) antisymmetry coupledmode betweenWGMsTE15;3 and TE18;2 and (c) symmetry coupled-modes ofTE15;3 at the mode frequencies of 208.52 and 211:04THz.The subscripts are angular and radial mode numbers forWGMs. Figures 5(b) and 5(d) are mode field patterns inthe plane 15 nm shift from the middle of the bus wave-guide corresponding to Figs. 5(a) and 5(c), respectively.In conclusion, we have demonstrated optical bistabil-
ity in a coupled-circular microlaser with a middle bus
waveguide at room temperature. The hysteresis loops areobserved from the output power coupled into an opticalfiber versus the cw injection current. The laser outputspectra indicate that the optical bistability is caused bythe mode competitions. We can expect that such opticalbistability can realize a fast optical switch due to the gainsaturation without a big variation of the carrier density.
This work was support by National Natural ScienceFoundation of China (NSFC) under grants 60723002,60838003, 61021003, and 61061160502.
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Fig. 4. (Color online) Output power versus cw injectioncurrent for the coupled-circular microlasers measured by adetector butt-coupling to the bus waveguide at 293K.
Fig. 5. (Color online) Field patterns of the magnetic field forTE modes at the frequencies of (a) 208.52 and (c) 211:04THz inthe middle plane; (b) and (d) mode field patterns in the plane15nm shift from the middle of the bus waveguide correspond-ing to (a) and (c).
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