GaN-based surface-emitting lasers using high contrast grating

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GaN-based surface-emitting lasers using

high contrast grating

Tien-Chang Lu*, Shing-Chung Wang, Tzeng-Tsong Wu, Shu-Hsien Wu and Yu-Cheng Syu

Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung

University,

Hsinchu 30050, Taiwan

*[email protected]

ABSTRACT

GaN-based surface-emitting lasers (SELs) using high contrast grating (HCG) with AlN/GaN distributed Bragg reflectors were reported. The laser device achieved a threshold energy density of about 0.56 mJ/cm2_{and the lasing} wavelength was at 393.6 nm with a high degree of polarization of 73% at room temperature. The resonant mode and polarization characteristics matched to the theoretical prediction. GaN-based SELs using HCG supported by the Fano resonance can be potential for development of blue surface emitting laser sources

Keywords- GaN, surface-emitting laser, high contrast grating, Fano resonance.

1. INTRODUCTION

In recent years, sub-wavelength high contrast grating (HCG) has been widely investigated owing to its advantageous optical properties. By changing the HCG parameters such as grating period, height and width, high reflectivity reflectors with broad stopband width and specific polarization characteristics could be obtained and are useful for many applications [1-5]. Based on the superior properties, HCGs not only serve as high reflectivity reflectors for vertical cavity surface-emitting lasers (VCSELs) but also provide unique characteristics including polarization selection, wavelength tuning and fast modulation speed [2, 5].

On the other hand, for development of high power surface-emitting lasers (SELs), sub-wavelength HCGs could be employed for high-quality factor (Q) resonators with in-plane oscillation over a large area [6, 7]. In 2008, Zhou et al. have demonstrated the GaAs-based membrane HCG high-Q resonators at low temperature, where the GaAs-based membrane HCGs were fabricated by selectively etching and e-beam lithography. High Q factor supported by the Fano resonance was obtained to be 14000 by optical pumping at 4K. However, the lasing action was not achieved in GaAs-based membrane HCGs [6].

In the development of SELs toward to the short wavelength region, GaN-based VCSELs and photonic crystal surface emitting lasers (PCSELs) have received much attention recently [8-14]. The simple geometry of membrane HCG high-Q resonators can be applied for realizing high power SELs. However, it is still challenge to fabricate GaN-based membrane HCG structures due to the immature etching process and specifically epitaxial structure [15]. In this report, GaN-based SELs using HCG with AlN/GaN distributed Bragg reflectors (DBRs) were designed, fabricated and demonstrated. Without using the suspended membrane structure, the AlN/GaN DBRs can play an important role as the low refractive index layer for confining the optical mode in the HCG cavity. Finally, the specific lasing mode was compared to the band diagram and mode pattern calculated by plane wave expansion (PWE) [16] and finite element methods (FEM) [17].

2. SIMULATION

Schemes of GaN-based SELs using HCG are shown in Fig. 1(a). We used rigorous coupled-wave analysis (RCWA) to calculate the corresponding asymmetric reflectivity spectra with respect to the different duty cycle (the grating width divided by the grating period) and the grating height for the TE polarization of the GaN-based HCG. First, the grating period was designed to be 345 nm to bring the Fano resonance approaching to the gain peak of multiple quantum wells (MQWs). Figure 1(b) shows the calculated reflectivity spectra mapping with respect to the duty cycle. To meet the target wavelength of about 400 nm, the duty cycle was set to be around 0.8 to 0.86 for fabrication tolerance. Figure 1(c) shows the calculated reflectivity spectra mapping versus the grating height. The calculated results show the number of reflectivity peaks would increase while increasing the grating height due to the onset of higher order Fabry-Perot modes. In order to fit

Invited Paper

CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2042064 Proc. of SPIE Vol. 8995 89950E-1

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poly-on the PMMA 490 to 530 nm ght and duty c ne shape from optical channel band width s difference in t channels wo confine the op calculated refl Ω , rrowband, q i urve in Fig. 1 ivity spectra o y spectra mappin versus grating h m. The olive and

ENT ic chemical va a 25-pair AlN/ and a 100 d at 405 nm w m-thick SiNx la -methyl metha A, and then et m which was c cycle were set m 0 to 1 which

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on (MOCVD) 320 nm-thick p-GaN layer. dth of about ma-enhanced c MA) photores tive ion etchin

λ cavity. Figure m, 520 nm (3λ d to the feature duced by HCG nd, the second he Fabry-Pero ate resulting in well and resul ompared to the

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(the grating ectra for the ation results system on a c k n-GaN layer The typica 25 nm. In the chemical vapo sist. The HCG ng (RIE) down e λ e G d ot n lt e Q e s c-r, al e or G n

Proc. of SPIE Vol. 8995 89950E-2

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(a) The input-o d SELs using H Eth and 1.75 Eth -output charac en the thresho GaN-based H about 1.15 mJ confinement HCG was abov e lasing area c y. When it wa ssion spectrum the HCG patt a (ICP) dry etc electron micro mated to be 34 repetition rate ed out at room he HCG patte a 600 μm cor on IHR320 S nd (b) tilted ang be 345 nm, 0.82 output character HCG was abov h. The full width

4. cteristics curv old energy de HCG SELs wa J/cm2 _{[9]. The} in HCG stru ve the thresho covered most as above the th m right below

terns were etc ching. The tot oscope (SEM) 45 nm, 0.82 a e of 1 kHz was m temperature ern. The μ-PL re in the norm Spectrometer)

gle SEM images and 490 nm, re ristics of GaN-e thGaN-e thrGaN-eshold h at half maxim RESULT ve of GaN-bas ensity was ab as superior to e low threshol uctures. The ri old condition. of the HCG p hreshold cond the threshold ched down to tal area of HC images of the and 490 nm, r s used as the o . The laser be signal was co mal plane of t . The spectra s of GaN-based espectively. based SELs us condition. (b) mum (FWHM) o TS AND DI

sed SELs usin out 0.56 mJ/c the GaN-base ld condition c ight inset of F The white da pattern. Figure dition, one dom d condition of o penetrate th CG region was e device. It ca espectively. T optical pumpin am pumped o ollected by a the sample an al resolution d SELs using HC sing HCG. The The measured of lasing peak at SCUSSION g HCG is sho cm2_{. Compare} ed PCSELs o can be attribut Fig. 3(a) show ash line repres

e 3(b) shows t minated lasing f GaN-based S he MQWs lay s about 20 μm2 an be seen tha The 355 nm p ng source in th obliquely onto 15X objective nd fed into a s was about 0. CG. The period inset shows th spectra with di t 1.75 Eth is 0.88 NS own in Fig. 3( ed to the prev of about 2.7 m ted to the Fan ws the CCD i ents the whol the lasing spe g peak was ob SELs using HC yer of about 4 2_{. Figure 2 sho} at the period, d pulse Nd:YVO he measureme the devices w e lens normal spectrometer w .07 nm for s

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ased SELs usin different order using HCG us CONCLUS th AlN/GaN ow threshold e ymmetric PL s e 394. Further be 12˚ and 73 onic band dia ions such as la

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the PWE meth blue circle indi element method demonstrated y was measure observed ind ing characteri ely. Finally, th ode pattern. Si laser printer, h Ls using HCG w ed SELs using

hod. The blue d cates the forth o d. The paramet d and investig ed to be 0.56 m dicating the F istics such as t he specific su ince GaN-bas high density o when it was HCG. The dashed line order band. ter a is the gated at room mJ/cm2_{and the} ano resonance the divergence urface emitting sed SELs have optical storage m e e e g e e, and the next generation micro/pico-projectors, we believe the GaN-based SELs using HCG have the great potential for accomplishment of low threshold, high power short wavelength coherent light sources in the near future.

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ACKNOWLEDGMENT

The authors would like to gratefully acknowledge Prof. Koyama at Tokyo Institute of Technology for his fruitful suggestion. This work was supported in part by the Ministry of Education Aim for the Top University program and by the National Science Council of Taiwan under Contract No. NSC99-2622-E009-009-CC3 and NSC98-2923-E-009-001-MY3.

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