Fano resonances GaN-based high contrast grating surface-emitting lasers

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Fano resonances GaN-based high contrast grating surface-emitting

lasers

Tzeng-Tsong Wu, Shu-Hsien Wu, Tien-Chang Lu, Hao-Chung Kuo and Shing-Chung Wang

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

University,

Hsinchu 30050, Taiwan

ABSTRACT

GaN-based high contrast grating surface-emitting lasers (HCG SELs) with AlN/GaN distributed Bragg reflectors were reported. The device exhibited a low threshold pumping 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 specific lasing mode and polarization characteristics agreed well with the theoretical modeling. The low threshold characteristics of our GaN-based HCG SELs utilized by the Fano resonance can be potential for development of blue surface emitting laser sources Keywords- GaN, Fano resonance, high contrast grating, surface-emitting laser

1. INTRODUCTION

Sub-wavelength high contrast gratings (HCGs) have been widely investigated in the recent years owing to their advantageous properties. By modification of HCG parameters such as grating height, period and width, high reflectivity reflectors with broad stopband width and specific polarization could be achieved and applied for many applications. [1-5] Owing to the superior properties, the vertical cavity surface-emitting lasers (VCSELs) integrated with the HCGs as the top mirrors which not only serve as high reflectivity reflectors for VCSELs but also provide unique characteristics including polarization selection, wavelength tuning and fast modulation speed. [2, 5]

Being a potential 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 selective etching and e-beam lithography. High Q value of the resonator supported by the Fano resonance [6] was obtained to be 14000 by optical pumping at 4K.

In the development of SELs toward the short wavelength region, GaN-based VCSELs and photonic crystal surface emitting lasers (PCSELs) have received much research attention recently. [8-14] The simple geometry of membrane HCG high-Q resonators can be applied for realizing SELs. However, it is still challenge to fabricate GaN-based membrane HCG structures due to the immature etching process to fabricate realized at room temperature by optical pumping. [15] In this report, we have designed, fabricated and demonstrated the GaN-based HCG SELs with AlN/GaN distributed Bragg reflectors (DBRs). 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

Schematics of GaN-based HCG SELs are shown in Fig. 1(a). We used RCWA to analyze the corresponding asymmetric reflectivity spectra as functions of different duty cycle (the grating width divided by the grating period) and 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 GaN gain peak. Fig. 1(b) shows the calculated reflectivity spectra mapping as a function of duty cycle. To match the target wavelength of about 400 nm, the duty cycle was set to be around 0.8 to 0.86 for fabrication tolerance. Fig. 1(c) shows the calculated reflectivity spectra mapping versus grating height. The calculated results show the number of reflectivity peaks would increase with increasing the grating height due to the onset of higher

CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2002911 Proc. of SPIE Vol. 8633 86330B-1

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which was clo were set to be which was ref scattering ind other hand, th Fabry-Perot s resulting in as and result in compared to t where ω0 an factor of the c curve based o in this case w Figure 1 ( width divi grating wi RCWA an In experim system on a c n-GaN layer, photolumines fabrication ste deposition (P structure was Perot modes. I osed to a 3λ c e 345 nm, 520 ferred to the f duced by HCG he second opti scattering wou symmetric lin a high Q fa the Fano reson

nd γ are the ce cavity can be c on Eq. (1) mat

as calculated t

(a) Schematics ided by the grat ith period: 345n nd fitting results

ment, the GaN c-plane 2-in sa

10 pairs of I scence (PL) sp

eps, the as-gro PECVD) as th patterned by In order to fit o cavity. Fig. 1( nm (3λ cavity feature of Fan G in lateral dir ical channel i uld generate a e shape of the ctor which is nance equation enter frequenc calculated by tches quite we to be 533. of GaN-based ting period). (c) nm, filling facto s by the Fano re N-based HCG apphire substr InGaN/GaN m pectrum of M own sample w he hard mask electron-beam

our target wav d) shows the r y) and 0.82. T o resonance. [ ection, the Br s the Fabry-P flat band spec e spectrum. Th s beneficial fo n: F Ω cy and the wi using Q = ω0/

ell to the simul

HCG SELs. (b) ) Reflectivity sp or: 0.82, grating esonance equati 3. E G SELs were ate. The epita multiple quant MQWs had a p was first depos

and spin-coa m lithography velength, the g reflectivity sp The reflectivity [5] For GaN-b ragg scattering Perot scattering ctrum. These t his phenomen or the laser o and Ω

idth of the nar /γ. The blue cu ulated reflectiv ) Reflectivity s pectra mapping g height: 520 nm on. EXPERIM grown by a axial structure tum wells (M peak centered sited a 300 nm ated the poly-on the PMMA grating height pectrum when y spectrum sho based HCG SE g would cause g induced by two different c non could conf

operation. The Ω , rrowband, q i urve in Fig. 1( vity spectra ob pectra mapping versus grating h m. The red and

ENT metal-organi consisted of a MQWs), and a d at 405 nm w m-thick SiNx l -methyl metha A, and then et was selected the grating pe ows an asymm ELs, the first o the narrow b index differen channels wou fine the optica e simulated re (1) s the Fano as (b) representin btained by RCW g as a function height. (d) The navy line show

c chemical v a 25-pair AlN 100 nm-thick with a linewid ayer by plasm acrylate (PMM tched by react to be around eriod, height a metric line sha

optical chann band width spe

nce in vertical uld reach the s

al mode in the eflectivity spe

symmetry para ng the fitted F WA. The extr

of duty cycle ( reflectivity spe w the simulation vapor depositi N/GaN DBR, a k p-GaN laye dth of about ma-enhanced c MA) photores tive ion etchin

490 to 530 nm and duty cycle ape from 0 to

el is the Bragg ectrum. On the l direction, the ame final state e structure wel ectrum can b ameter. The Q Fano resonance racted Q facto (the grating ectra for the n results by

on (MOCVD a 320 nm-thick er. The typica

25 nm. In the chemical vapo sist. The HCG ng (RIE) down m e 1 g e e e ll e Q e or D) k al e or G n

Proc. of SPIE Vol. 8633 86330B-2

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(a) CÜ >, CT) E C.) E 11

gl

5000 4000 3000 2000 1fm

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(b; 1490 nm (oï Ç 380 100 75Eth 875Eth 400 420 44 3velength (nm) nm -3 4 0 to SiNx layer coupled plasm angle scannin HCG were est ~0.5 ns at a experiments w devices with a normal to the spectrometer for spectral ou Figure 2 T were estim Figure 3 (a HCG SEL 1.75 Eth. T The pump clear threshol reports, the th GaN-based V supporting the GaN-based H be discovered pumping ener . Then, the HC ma (ICP) dry ng electron mi timated to be repetition rat were carried o a spot size of e sample surf with a charge utput measure

The (a) planer an mated to be 345

a) The input-ou L was above the The full width at

ping energy de ld can be obs hreshold condi VCSELs of ab e perfect mod HCG SEL was d that the lasi rgy density. W CG patterns w etching. The icroscope (SE 345 nm, 0.82 te of 1 kHz out at room t about 20 μm face or by a -coupled devi ement. nd (b) tilted ang nm, 0.82 and 4 utput characteris e threshold con t half maximum 4. ensity versus t served when t ition of GaN-b bout 1.15 mJ/ de confinemen above the thr ing area cove When it was ab were etched do total area of EM) images of and 490 nm, r was used as emperature. T to cover the H fiber with a ice(Jobin-Yvo

gle SEM images 490 nm, respecti

stics of GaN-ba ndition. (b) The m (FWHM) of la

RESULT

the output cha the threshold based HCG SE /cm2_{. The ultr} nt in HCG stru reshold conditi red most of t bove the thresh

own to penetra HCG region f the device. I respectively. T

the optical p The laser beam HCG pattern. 600 μm cor on IHR320 Sp s of GaN-based ively. ased HCG SELs e measured spec asing peak at 1. TS AND DI aracteristics cu energy densit ELs was super ralow thresho uctures. The r ion. The white the HCG patt hold condition ate the MQWs was about 20 It can be seen The 355 nm p pumping sour m pumped ob The μ-PL sign e in the norm ectrometer). T d HCG SELs. Th

s. The inset show ctra with differe .75 Eth is 0.88 n

SCUSSION

urve of GaN-b ty was about rior to the GaN old condition right inset of F

e dash line rep tern. Fig. 3(b) n, one domina

s layer of abou 0 μm2_{. Fig. 2}

that the perio ulse Nd:YVO rce in the me bliquely onto nal was collec mal plane of The spectral re he period, duty ws the CCD im ent pumping po nm. NS based HCG SE 0.56 mJ/cm2_. N-based PCSE can be attribu Fig. 3(a) show

presents the w ) shows the la ated lasing pea

ut 490 nm usi shows the pla od, duty cycle O4 laser with a easurement sy the GaN-base cted by a 15X the sample a esolution was

cycle and heigh

mages when the ower of 0.875 E ELs is shown . Compared to ELs of about 2 uted to the F ws the CCD im whole HCG are asing spectra ak was observ ing inductively aner and tilted e and height o

pulse width o ystem. All the ed HCG SEL

objective len and fed into a

about 0.07 nm

ht of HCG

GaN-based Eth, Eth and

in Fig. 3(a). A o the previou 2.7 mJ/cm2_and

ano resonance mage when the ea and it could with differen ved and located

y d of of e s ns a m A us d e e d nt d

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N 0 . 2

a--00

L 388 392 Vaveleng

- *- Ex

- *- Fa

396

th (nm)

periment

Ino-fittin

1

_i_l

400

at 393.6 nm. T spectrum with can thus be e shown in Fig. In order to divergence an The DOP is d peak. The DO electrical field intensity was resolution of based HCG S Figure 4 Th result by Fa Since the the photonic experimental lower band-e approached to be identified a bar.In additio mode pattern indicates the A field is parall provide perfec [7]. The perf threshold con The emission h black curve estimated by f 3 as the red c o further unde ngle were mea defined as (Ima OP can be calc ds only paralle collected by the rotation s EL was measu he PL spectra (bl ano-resonance equ HCG structur band structu lasing mode o dge position o zero in the f as the anti-sym on, the FEM w

shows severa AlN/GaN DB el to the grati ctly mode con fect mode con ndition of GaN

spectrum righ exhibits an as fitting the spec curve. The ext erstand the las asured at the n

ax-Imin)/(Imax+Im

culated to be 7 el to the gratin a fiber with stage was abo ured to be 12˚

lue curve) of GaN uation. The Q fac re can be treat ure of our Ga of our GaN-ba in Fig. 5(a). T fourth order ba mmetric mode was also used al nodes and a BRs could prov ing bar which nfinement in H nfinement sup N-based HCG S ht below the th symmetric lin ctrum with th racted Q facto ser characteris normal directio min) where Ima 73%. The high ng bar. Fig. 4( a 600 μm cor ut 0.5 degree . N-based HCG SE ctor was calculated

ted as one dim aN-based HCG ased HCG SEL The Bragg sc and. The lowe e. On the contr to calculate th antinodes whi vide the mode can also conf HCG, resulting pported by Fa SELs achieved hreshold cond ne shape, whic he Fano resona or was estimat stics of GaN-b on to the samp

ax and Imin are

h DOP of HC (b) describes re normal to for divergenc

ELs when it was b d to be 394. mensional pho G SEL as sh L was located cattering of su er band-edge p trary, the symm he mode patter ich can be refe e confinement firm the DOP g in destructiv ano resonance d in this exper dition of GaN-ch is the GaN-chara ance equation ted to be 394, based HCG SE ple surface. Fi the maximum CG SELs can b the angle vers the sample su ce angle meas

below the thresho

otonic crystal, hown in Fig. d at 0.877, corr urface emissio position revea metric mode w rn of GaN-bas ferred to the an in the HCG c P results. Furth ve interference e could facili riment. -based HCG S acteristic of Fa n described in which was clo EL, the degree ig. 4(a) shows m and the mini be attributed to sus measured urface on the surement. The old condition. Th we can use th 5(a). The no responding to on would occu als the resonan would only sh sed HCG SEL nti-symmetric cavity. The res

hermore, the a in the far-fiel itate the high

SEL is plotted ano resonance Eq. (1). The osed to the des e of polarizati s the measured imum intensit to the resonan output intensi rotational sta e divergence a

he red curve show

he PWE meth ormalized freq

the fourth ord ur when the g nce mode in th how the nodes L, as shown in c mode. The s sonance direct anti-symmetri ld with little o h Q factor and d in Fig. 3. Th e. The Q facto fitted curve i signed value. ion (DOP) and d DOP results ty of the lasing

ce direction o ity. The outpu age. The angl

angle of GaN ws the fitting hod to simulate quency of the der band in th group velocity he HCG could in one grating Fig. 5(b). The simulation also tion of electric ic mode would output coupling d the ultralow e or s d s. g of ut e N-e e e y d g e o c d g w

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(a) Figure 5 ( the thresh divergence Figure 6 ( the light li calculated In summa Supporting by wavelength w phenomenon angle and deg lasing mode c drawn much a and the next accomplishme

(a) The degree hold condition.

e angle were m

(a) The calculat ine. The red do d mode pattern o

ary, GaN-base y the Fano res was located a and the qualit gree of polari conformed to attention due t generation m

ent of low thre

of polarization The detection easured to be 73

ted band structu ot lines represen of GaN-based H ed HCG SELs sonance, the u at 393.6 nm. ty factor was e ization were m

the one dime to many prac micro/pico-pro eshold, high p (DOP) and (b) direction was 3% and 12°, res ure of GaN-bas nt the different HCG SEL using 5. C s with AlN/Ga ultralow thresh The asymm extracted to b measured to b ensional photo tical applicati ojectors, we power short wa the divergence normal to the spectively. sed HCG SEL u order bands an g the finite elem

CONCLUS aN DBRs wer hold energy d metric PL spe e 394. Further be 12˚ and 73 onic band dia ions such as la

believe the G avelength coh

e angle of the G surface of the

using the PWE nd the blue circl ment method. Th

SION

re demonstrat density was me

ectrum was o rmore, the las 3%, respective agram and mo aser display, l GaN-based H herent light sou

GaN-based HCG e GaN-based H method. The b le indicates the e parameter a is ed and investi easured to be observed indic ing characteri ely. Finally, th ode pattern. Si laser printer, h HCG SELs ha urces in the ne G SEL when it HCG SEL. The

blue dashed line e forth order ban

s the grating pe tigated at room 0.56 mJ/cm2 cating the Fa istics such as t he specific su ince GaN-bas high density o ave the great ear future. was above e DOP and e represents nd. (b) The riod. m temperature and the lasing ano resonanc the divergence urface emitting sed SELs hav optical storage t potential fo e. g e e g e e, or

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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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