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Single Transverse Whispering-Gallery Mode AlGaInAs/InP Hexagonal Resonator

Microlasers

Article in IEEE Photonics Journal · August 2011

DOI: 10.1109/JPHOT.2011.2163498

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Single Transverse Whispering-Gallery Mode AlGaInAs/InP Hexagonal Resonator MicrolasersVolume 3, Number 4, August 2011

J. D. LinY. Z. Huang, Senior Member, IEEEY. D. YangQ. F. YaoX. M. LvJ. L. XiaoY. Du

DOI: 10.1109/JPHOT.2011.21634981943-0655/$26.00 ©2011 IEEE

Single Transverse Whispering-GalleryMode AlGaInAs/InP Hexagonal

Resonator MicrolasersJ. D. Lin, Y. Z. Huang, Senior Member, IEEE, Y. D. Yang,

Q. F. Yao, X. M. Lv, J. L. Xiao, and Y. Du

State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors,Chinese Academy of Sciences, Beijing 100083, China

DOI: 10.1109/JPHOT.2011.21634981943-0655/$26.00 �2011 IEEE

Manuscript received June 7, 2011; revised July 23, 2011; accepted July 26, 2011. Date of publicationAugust 4, 2011; date of current version August 23, 2011. This work was supported by the NationalNature Science Foundation of China under Grant 60777028, Grant 60838003, Grant 61006042, andGrant 61061160502. Corresponding author: Y. Z. Huang (e-mail: [email protected]).

Abstract: AlGaInAs/InP hexagonal resonator microlasers with an output waveguideconnected to one vertex of the hexagon are fabricated using standard photolithographyand inductively coupled plasma (ICP) etching process. Room-temperature continuous-waveelectrically injected operation with a threshold current of 18 mA is realized for a hexagonlaser with an edge length of 16 �m and an output waveguide width of 2 �m. Single-modeoperation is achieved with a side mode suppression ratio of 21 and 33 dB at the injectioncurrent of 30 and 60 mA, respectively. The peak wavelength intervals of the laser spectrumagree very well with the longitudinal mode intervals of the whispering-gallery modes, whichindicates single transverse mode operation. The modeQ factor of 6:53� 103 is measuredfor the lasing mode at 1547 nm at the threshold current, which is in the same magnitude asthe Q factor obtained by finite-difference time-domain (FDTD) simulation. The numericalsimulations also indicate that the hexagonal resonator with an output waveguide is suitableto realize single transverse mode operation.

Index Terms: Semiconductor lasers.

1. IntroductionWhispering-gallery mode (WGM) resonators have attracted great attention for fabricating microlasersand optical add-drop filters. Circular resonators [1], [2] are most common WGM resonators, andequilateral-polygonal resonators, such as equilateral triangular [3], square [4], and hexagonalresonators [5], can also support highQ WGMs. Deformedmicrodisks [6], spiral-shapedmicropillars [7],ring-spiral coupled microcavity [8], and rounded isosceles triangle shape microcavity [9] wereproposed to realize unidirectional emission microlasers. Furthermore, unidirectional emissiontriangle, square and circular microlasers were designed and fabricated by connecting an outputwaveguide to the resonators [10]–[12]. Naturally, hexagonal microcavities are formed in a lot ofnanowires with hexagonal cross section. The mode characteristics of hexagonal resonators wereanalyzed by the boundary element method [5] and the symmetry analysis of group theory and thefinite-difference time-domain (FDTD) technique [13], and recently, the effect of metal for increasingthe quality factor and confinement factor for ZnO hexagonal nanocavities were simulated [14]. Inaddition, an optical pumping hexagonal-facet GaAs/AlGaAs laser with optical waveguides wasrealized by selective area metal organic chemical vapor deposition [15], ultraviolet lasing was

Vol. 3, No. 4, August 2011 Page 756

IEEE Photonics Journal AlGaInAs/InP Hexagonal Resonator Microlasers

observed in optically pumped ZnO nanonails with a hexagonally shaped head [16], and frequency-upconverted WGM lasing was realized in ZnO hexagonal nanodisks [17].

In a resonator, the longitudinal mode intervals are inversely proportional to the resonator size,and therefore, single longitudinal mode operation is easy to realize for microresonators, such asvertical-cavity surface-emitting lasers. However, the wavelength intervals of transverse modes canbe much smaller than that of the longitudinal modes. Therefore, transverse mode controls areusually realized by increasing the loss or using the cutoff condition of the higher order transversemodes. The introduction of air trenches in triangular lasers was used to increase the radiationlosses for single-mode operation [18], and optically pumped lasing actions of extremely small GaNhexagonal nanorings were achieved with the higher transverse modes suppressed by narrowingthe wall of the nanorings [19]. In this paper, we study the transverse mode control for unidirectionalemission hexagonal resonator microlasers with an output waveguide connected to a vertex.Electrically injected AlGaInAs/InP hexagon microlasers are fabricated by planar technology, andsingle-mode operation is realized with a side mode suppression ratio up to 33 dB for a 16-�m-side-length hexagon microlaser with a 2-�m-wide output waveguide. The mode wavelength intervals ofthe laser spectrum agree with the longitudinal mode interval of the hexagon resonator, whichreveals the single transverse mode lasing. Furthermore, the FDTD simulations indicate that thehexagonal resonator is easy to realize the fundamental transverse mode operation.

2. Laser CharacteristicsAn AlGaInAs/InP multiple-quantum-well laser wafer is used to fabricate the hexagonal microlasers.The active region consists of six Al0:24GaIn0:71As/Al0:44GaIn0:49As quantum wells, with the thick-nesses of the quantum wells and barrier layers of 6 and 9 nm, respectively, with the lower claddinglayers of 100-nm undoped graded AlGaInAs and 140 nm N-AlInAs, upper cladding layers of 150-nmundoped graded AlGaInAs and InAlAs, and the upper confinement layers of 1.8 �m InP andP-InGaAs contacting layer. The N-doped density is about 1� 1018 cm�3, and the P-doped densitiesincrease from 5 �1017 cm�3 to larger than 1� 1019 cm�3 in the P-contacting layer. The total growththickness is about 2.3 �m, except for the N-InP buffer layer. The hexagonal resonator microlasers,with a side length of 16 �m and a 2-�m wide output waveguide connected to one vertex of thehexagon, are fabricated under the technique process similar to [12]. First, an 800-nm SiO2 wasdeposited by plasma-enhanced chemical vapor deposition (PECVD) on the laser wafer, and thehexagonal resonator patterns were transferred onto the SiO2 layer using standard photolithographyand inductively coupled plasma (ICP) etching techniques. Then, the laser wafer was etched by about5 �m using the ICP technique with the patterned SiO2 as masks. After the ICP etching process, achemical etching process was used to improve the smoothness of the etched side walls, and then,the residual SiO2 hard masks on the microresonators were etched using diluted HF solution. Finally,a 450-nm SiO2 insulating layer was deposited on the wafer, and the SiO2 layer on the top ofhexagonal resonators was etched using the ICP etching process again for electrical injection,and Ti–Au and Au–Ge–Ni were used as p-contact and n-contact metals, respectively. A scanning

Fig. 1. (a) SEM image of a hexagon resonator with the side length of 16 �m after the inductively coupledplasma etching process and (b) the microscopic picture of a fabricated laser.



electron microscope (SEM) image of a hexagonal resonator is shown in Fig. 1(a), and a microscopicpicture of a fabricated hexagon laser is shown in Fig. 1(b) with the cleaved output waveguide. Thecircular pattern on the top of the hexagonal resonator is formed during opening the contactingwindow for electrical injection.

The microlaser is mounted in a cryostat using a heater to raise the temperature without pouringliquid nitrogen into the cryostat for measuring the total output power, as in [10]. The output power ismeasured by butt-coupling a 5-mm-diameter optical detector to the cleaved port of the outputwaveguide of the microlasers. The output power and applied voltage versus continuous-wave (cw)injection current are measured at the temperatures of 297, 300, 303, and 306 K and plotted in Fig. 2,respectively, for a hexagonal resonator microlaser with the side length of 16 �m and the outputwaveguide width of 2 �m. The threshold current is about 18mA at room temperature of 297 K, and themaximumcw lasing operation temperature is 306K. Themeasuredmaximumoutput powers are 48.6,36.9, 28.2, and 19.2 �W at 297, 300, 303, and 306 K, respectively. A series resistance of 9.2 � isestimated from the applied voltage versus the injecting current around the threshold current at 297 K.

The laser spectra measured at 297 K with the cw injecting currents of 30 and 60 mA are plotted inFig. 3(a), respectively, and the detail laser spectra around the modes at 1538.7 and 1547.1 nm areplotted in Fig. 3(b) at 297 K with the injecting currents of 16, 18, and 20 mA. Evident peaks appearat the wavelengths of 1506.8, 1514.7, 1522.7, 1530.8, 1539.1, 1547.4, and 1555.9 nm at 297 K and30 mA with the wavelength intervals of 7.9, 8.0, 8.1, 8.3, 8.3, and 8.5 nm, respectively. Themeasured wavelength intervals can be fitted by the longitudinal mode interval �2=3

ffiffiffi

3p

nga for theWGMs [16] by taking the group index ng as 3.44 at 1540 nm, which are corresponding to the modes

Fig. 2. Applied voltage and output power versus continuous-wave injection current at the temperature of297, 300, 303, and 306 K for the hexagonal resonator microlasers.

Fig. 3. Laser spectra of the AlGaInAs/InP hexagon resonator microlaser with the side length of 16 �m at(a) the injection current of 30 and 60 mA at 297 K and (b) 16, 18, and 20 mA at 297 K.



with the incident angle of 60� for the mode light ray as shown in the inset of Fig. 3(a). Single-modeoperation is achieved with side mode suppression ratio of 21 and 33 dB at the injection current of30 and 60 mA, respectively, at 297 K. The mode wavelength variation with the temperature at0.097 nm/K is obtained from the mode wavelength shift of 0.29 nm from 297 to 300 K at 60 mA. Inaddition, the mode wavelength shift is 1.96 nm as the injection current increases from 30 to 60 mA,which is mainly caused by the temperature rise with the increase of the injection current. In addition,Fig. 3(a) displays a periodic spectral feature underneath the sharp emission peaks, which may bethe Fabry–Pérot modes arising from light bouncing back-and-forth between two opposite faces ofthe hexagon. Fitting the peak of the laser spectrum in Fig. 3(b) at the threshold current of 18 mAwith a Lorentzian function, we obtain the full-width at half maximum (FWHM) of 0.228 and 0.237 nmfor the modes at the wavelengths of 1538.76 and 1547.14 nm, respectively. The correspondingmode Q factors of 6:75� 103 and 6:53� 103 are calculated as the ratio of the mode wavelengthto the FWHM, but the FWHM of 0.303 nm is obtained for the mode at the peak wavelength of1547.14 nm from the spectrum at 16 mA, and the corresponding Q factor is 5:10� 103. In fact, wecannot confirm that the absorption loss is compensated by the material gain at the threshold.Therefore, the mode Q factor is difficult to measure exactly for lasers with gain and absorption loss.

3. Numerical Simulation of Mode CharacteristicsIn this section, 2-D FDTD technique [20] was used to investigate the mode characteristics for thehexagon resonator. First, a hexagon resonator with the side length of 16 �m and a 2-�m wideoutput waveguide, as shown in Fig. 4 is considered, which is laterally confined by 0.4 �m SiO2,0.04 �m titanium, and 0.2 �m gold layers with the refractive indices of 1:45; 3:7þ 4:5i , and0:18þ 10:2i , respectively, and the refractive index of the resonator is taken to be 3.2. The grayareas in Fig. 4 are the perfectly matched layer (PML) absorbing boundaries to terminate the FDTDcomputing window. Symmetric and anti-symmetric TE modes relative to the output waveguide aresimulated, respectively, using the mirror symmetry relative to y ¼ 0 in the simulation. A Gaussianmodulated cosine impulse PðtÞ ¼ exp½�ðt � t0Þ2=t2w �cosð2�f0tÞ is used as the exciting source,which is applied as a linear source inside the resonator. The half width of the pulse tw ¼ 95:6 fs, thepulse center t0 ¼ 2tw , and center frequency f0 ¼ 195:94 THz are used for a wide band excitingsource to excite multiple modes over a wide frequency range, and the spatial cell size and the timestep are taken to be 20 nm and 0.0467 fs, respectively. In the FDTD simulation, the time-domainoutputs of Hz are recorded as FDTD outputs at four monitor points ðx ; yÞ ¼ ð13; 3Þ; ð�10:6; 7:6Þ;ð�12:4; 2Þ; and ð�4:8; 3:4Þ �m inside the hexagon resonator. Then, the Padé approximation withBaker’s algorithm [21] is used to transform the FDTD output from the time-domain to the frequency-domain.

The intensity spectra obtained from the 219-step FDTD output at ðx ; yÞ ¼ ð13; 3Þ �m are plotted inFig. 5(a) as the solid and dashed lines for symmetric and anti-symmetric TE modes. The peakwavelengths of the symmetric modes agree very well with those of the anti-symmetric modes with

Fig. 4. Schematic diagram of a 2-D hexagonal resonator with an output waveguide at a vertex.



the peak wavelength intervals of 8.7, 8.8, 8.6, 9.0, and 9.0 nm from 1510 to 1570 nm, which arecorresponding to the longitudinal mode intervals of the WGMs. The simulated mode intervals arelarger than the experimental mode intervals, because the simulated results are based on a constantrefractive index n ¼ 3:2 instead of the group index ng ¼ n � �dn/d�. Except for degenerate modesof the different symmetries and an addition peak at 1532.4 nm for the symmetry state, the results

Fig. 6. Field patterns of the magnetic field components of (a) the symmetric mode and (b) theantisymmetric mode at the wavelength of 1551.9 nm, and (c) the symmetric mode at the wavelength of1532.4 nm.

Fig. 5. (a) Intensity spectra for symmetric and antisymmetric TE modes are plotted as the solid anddashed lines, and (b) the detail intensity spectrum for symmetric TE modes obtained under a narrowband exciting source for the hexagonal resonator with the side length of 16 �m and a 2-�m wide outputwaveguide. The inset shows the mode light rays for the second order WGM.



show that the hexagonal resonator with the side length of 16 �m can support single transversemode operation. The detail intensity spectrum obtained under a narrow band exciting source isplotted in Fig. 5(b) around the addition peak of the symmetry state, and the corresponding mode Qfactors of 4:0� 103 and 4:6� 103 are obtained for the modes at the wavelengths of 1532.4 and1534.0 nm, respectively.

To further investigate the mode characteristics, a narrow band exciting source with tw ¼ 3:06 psand t0 ¼ 2tw is used to simulate the mode field distribution. The magnetic field intensity distributionsobtained for the symmetric and antisymmetric modes at the frequency f0 ¼ 193:18 THz (1551.9 nm)

Fig. 7. Intensity spectra of symmetric TE modes in the hexagonal resonators with the side length of(a) 16 �m, (b) 8 �m, and (c) 4 �m, with the width of the output waveguides are 2, 2, and 1 �m,respectively. The solid lines and the dashed lines are results for the resonators with and without theoutput waveguide.



are plotted in Fig. 6(a) and (b), which concentrate around the hexagon edges and are rather weak inthe corner regions. Therefore, the confined modes can have high Q factors as an output waveguideconnected to a vertex of the hexagon to realize unidirectional emission. The mode Q factors of6:3� 103 and 3:7� 103 are calculated for the symmetric and antisymmetric modes as the ratio ofthe peak frequency to the corresponding FWHM. As the thickness of titanium layer increases from0.04 to 0.06 �m, the Q factor of the symmetric mode at 1551.9 nm drops from 6:3� 103 to5:3� 103. The simulated magnetic filed intensity for the symmetric mode at the additional peak of1532.4 nm of Fig. 5(b) is shown in Fig. 6(c), and the mode field pattern for the peak at 1534.0 nm inFig. 5(b) is similar to that of Fig. 6(a). The modes in Fig. 6(a) and (b) are the first-order WGMs (thefundamental transverse mode), and that of Fig. 6(c) is the second-order WGM (the first-ordertransverse mode). The Q factor of the second-order WGM at 1532.4 nm in Fig. 6(c) is 4:0� 103,which is lower than the Q factor of 4:5� 103 for the first-order WGM at 1534.0 nm. The numericalresults indicate that the hexagonal resonator with the side length of 16 �m is suitable to realizesingle transverse mode operation, which are in good agreement with the lasing spectra of Fig. 3(a).Considering the uncertainty in the measured Q factor and the influence of the Ti layer thickness, wethink that the simulated mode Q factors agree well with the measured mode Q factors.

Finally, the mode characteristics are simulated for hexagon resonators confined by 0.4-�minsulating SiO2 and air, and the intensity spectra for symmetric TE modes are plotted in Fig. 7 forthe resonators with the side length and output waveguide width of (a) 16 and 2 �m, (b) 8 and 2 �m,and (c) 4 and 1 �m, respectively. The solid and the dashed lines are results for the hexagonresonators with and without the output waveguide. The intensity spectra in Fig. 7(a) and (c) haveuniform peaks with wavelength intervals of the longitudinal mode intervals, except for an additionalpeak at 1548.6 nm in the dashed line of Fig. 7(a). Based on the simulated mode field patterns, wecan assign the uniform peaks as the first-order WGMs. However, three transverse modes existover a longitudinal mode interval in the dashed lines of Fig. 7(b). The higher order transverse modepeaks in the dashed line at 1523.3, 1536.7, 1557.0, and 1578.4 nm with the Q factors of 4:3� 103,9:5� 103, 2:6� 103, and 1:2� 104 disappear as the 8-�m side hexagonal connected with theoutput waveguide. However, the second-order WGMs at 1529.0, 1549.4, and 1570.2 nm are stillobserved in the solid lines of Fig. 7(b). The results indicate that hexagonal resonators with the sidelength of 16 and 4 �m are easier to realize single transverse mode operation than that with the sidelength of 8 �m.

The mode field patterns are also simulated for hexagon resonators with the side length of 4 and8 �m. The magnetic field distribution for the TE symmetric mode at 1523.6 nm is plotted in Fig. 8(a)for the 4-�m side length hexagonal resonator with a 1-�m wide output waveguide. The magneticfield distribution for the TE symmetric mode at 1540.8 nm is plotted in Fig. 8(b) for the 8-�m sidelength hexagonal resonator with a 2-�m wide output waveguide, and that at 1549.4 nm is plotted in

Fig. 8. Magnetic field patterns of the symmetric state at the wavelengths of (a) 1523.6 nm in the 4-�mside hexagon resonator with the 1-�m-wide waveguide, (b) 1540.8 nm in the 8-�m side hexagonresonator with the 2-�m-wide waveguide, and (c) 1549.4 in the 8-�m side hexagon resonator without anoutput waveguide.



Fig. 8(c) for the 8-�m side length hexagonal resonator without the output waveguide. The filedpatterns in Fig. 8(a) and (b) are the first-order WGMs, and that of Fig. 8(c) is the second-orderWGMs. The mode Q factors are 3:3� 103, 8:0� 103, and 9:4� 103 for modes in Fig. 8(a)–(c). Byintroducing the output waveguide, the mode Q factor of the second-order WGM in Fig. 8(c) will dropfrom 9:4� 103 to 3:4� 103, which is much smaller than 8:0� 103 of the first-order WGM inFig. 8(b). For the perfect hexagonal resonators with the side length of 16, 8, and 4 �m without anoutput waveguide, the first-order symmetric WGMs at 1560.4, 1561.4, and 1564.5 nm have the Qfactors of 1:1� 104, 1:8� 104, and 4:7� 103, respectively. Compared with equilateral triangle,square, and circular microresonators [10], [12], [22], we find that WGMs of the hexagonmicroresonator have relative low mode Q factors in the order of 103 to 104. The low mode Q factorsin the same magnitude as that determined by the absorption loss can result in a large Q factordifference between the first and the second-order transverse WGMs. Therefore, the hexagonmicroresonator is suitable to realize single transverse mode operation.

4. SummaryIn conclusion, we have fabricated 16-�m side length AlGaInAs/InP hexagon resonator microlaserswith a 2-�m-wide output waveguide by planar technology. Continuous-wave electrically injectedoperation with the threshold current of 18 mA is realized at room temperature, and the single-modeoperation with the side mode suppression ratio of 33 dB is achieved at the injection current of 60 mA.The FDTD simulation results also indicate that the hexagon resonator microlaser is suitable torealize single transverse mode operation.

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