Surface-normal electro-optic-polymer modulator with silicon subwavelength grating

Transcription

1 This articl has bn accptd and publishd on J-STAGE in advanc of copditing. Contnt is final as prsntd. IEICE Elctronics Eprss, Vol.* No.*,*-* Surfac-normal lctro-optic-polmr modulator with silicon subwavlngth grating Yuji Kosugi a), Toshiki Yamada 2, Akira Otomo 2, Yoshiaki Nakano and Takuo Tanmura Dpartmnt of Elctrical Enginring and Information Sstms, School of Enginring, Th Univrsit of Toko 7-3- Hongo, Bunko-ku, Toko , Japan 2 National Institut of Information and Communications Tchnolog Iwaoka, Nishi-ku, Kob , Japan a) kosugi@hotaka.t.u-toko.ac.jp Abstract: W propos novl silicon-basd surfac-normal optical modulator using lctro-optic (EO) polmr. Th EO polmr is mbddd insid a thin silicon subwavlngth grating lar, which is usd as both th intrdigitatd lctrods for ffctiv poling of th EO polmr, and as high-q rsonant structur for th incidnt light to nabl fficint modulation. W numricall dmonstrat -db intnsit modulation at 55-nm wavlngth undr a rfractiv ind chang of onl 3.8-4, corrsponding to th driving voltag blow V. Total-rflctanc phas modulator is also dmonstratd b adding a backsid rflctor. With inhrntl high-spd rspons ovr svral tns of GH, scalabilit to dns 2-D arra intgratd with CMOS drivr circuitr, and rlativl as and low-cost fabrication without pitaial procss, th proposd dvic ma find vrsatil applications in optical intrconncts, fr-spac optical communications, imaging and snsing. Kwords: surfac-normal modulator, spatial light modulator, lctro-optic polmr, high-contrast grating Classification: Intgratd optolctronics Rfrncs IEICE 26 DOI:.587/l Rcivd Jun 5, 26 Accptd Jul 9, 26 Publicid August 8, 26 [] A. Kirk, t al.: Dsign and implmntation of a modulator-basd fr-spac optical backplan for multiprocssor applications, Appl. Opt., 42 (23) 2465 (DOI:.364/AO ). [2] R. M. Audt, t al.: Surfac-normal G/G asmmtric Fabr Prot optical modulators fabricatd on silicon substrats, J. Lightwav Tchnol., 3 (23)

2 3995 (DOI:.9/JLT ). [3] W. S. Rabinovich, t al.: Fr-spac optical communications rsarch and dmonstrations at th U.S. Naval Rsarch Laborator, Appl. Opt., 54 (25) F89 (DOI:.7/ ). [4] A. P. Mosk, t al.: Controlling wavs in spac and tim for imaging and focusing in compl mdia, Natur Photon., 6 (22) 283 (DOI:.38/nphoton.22.88). [5] C. Maurr, t al.: Tailoring of arbitrar optical vctor bams, Nw J. Phs., 9 (27) 78 (DOI:.88/ /9/3/78). [6] W. Yang, t al.: High spd optical phasd arra using high contrast grating all-pass filtrs, Opt. Eprss 22 (24) 238 (DOI:.364/OE ). [7] Y. Kanamori, t al.: Fabrication of transmission color filtrs using silicon subwavlngth gratings on quart substrats, IEEE Photon. Tchnol. Ltt., 8 (26) 226 (DOI:.9/LPT ). [8] C. J. Chang-Hasnain and W. Yang: High contrast gratings for intgratd optolctronics, Adv. Opt. Photon., 4 (22) 379 (DOI:.364/AOP.4.379). [9] L. R. Dalton, t al.: Elctric fild pold organic lctro-optic matrials: Stat of th art and futur prospcts, Chm. Rv., (2) 25 (DOI:.2/cr9429). [] X. Piao, t al.: Nonlinar Optical d-chain Polmrs Post-Functionalid with High-β Chromophors Ehibiting Larg Elctro-Optic Proprt, J. Polm. Sci. A. Polm Chm., 49 (2) 47 (DOI:.2/pola.244). [] T. Yamada and A. Otomo: Usfulnss of transmission llipsomtric mthod for valuation of lctro-optic matrials, IEICE Trans. Elctron., E98-C (25) 43 (DOI:.587/transl.E98.C.43). [2] R. Palmr, t al.: High-spd, low driv-voltag silicon-organic hbrid modulator basd on a binar-chromophor lctro-optic matrial, J. Lightwav Tchnol., 32 (24) 2726 (DOI:.9/JLT ). [3] Y. Hori, t al.: Guidd rsonanc rflctiv phas shiftrs, Proc. SPIE 9372 (25) 9372W (DOI:.7/ ). [4] High Contrast Grating Solvr Packag, Univ. of California, Brkl: Introduction Surfac-normal intgratd optical modulators that oprat on optical bams orintd prpndicular to th chip surfac rathr than in-plan wavguid configuration hav attractd considrabl attntion in divrs fild of optics. Having inhrnt compatibilit with dns 2-D arra intgration, th hav found wid rang of potntial applications, including high-capacit optical intrconncts [,2], fr-spac optical communications [3], dnamic bam forming and adaptiv optics [4,5]. Surfac-normal modulators basd on liquid crstals and micro-lctro-mchanical sstms (MEMS) ar rlativl matur; but thir oprating spd is tpicall in a rang from kh up to MH [4-6]. On th othr hand, high-spd surfac-normal intnsit modulators with bandwidth bond GH rang hav bn dmonstratd b using multipl-quantum-wll (MQW) smiconductor matrials [-3]. Ths dvics, howvr, rquir complicatd 2

3 (a) EO polmr Incidnt E Rflctd V pol V mod O 2 (b) Incidnt E Rflctd Transmittd E pol EO polmr E mod O 2 Transmittd Fig.. (a) Schmatic illustration of th surfac-normal modulator using EO polmr mbddd insid silicon subwavlngth grating. (b) Cross-sctional viw of th dvic structur with th dfinitions of gomtric paramtrs. pitaial structurs and ar inhrntl absorptiv, which maks thm unsuitabl for som applications. In this papr, w propos a novl silicon-basd surfac-normal optical modulator using lctro-optic (EO) polmr. B mbdding EO polmr insid 856-nm-thick silicon () subwavlngth high-contrast grating (HCG) lar, w numricall dmonstrat fficint intnsit and phas modulation at 55-nm wavlngth with a rquird driving voltag blow V. Owing to th high-spd Pockls ffct of th EO polmr, th can potntiall b opratd at svral tns of GH or fastr with th modulation bandwidth onl limitd b th RC constant. Morovr, th proposd modulator can b radil scald to a dns 2-D arra with intgratd CMOS drivr circuitr, making it attractiv for optical intrconncts and imaging applications. 2 Proposd surfac-normal modulator: Structur and principl Th schmatic of th proposd optical modulator is dpictd in Fig.. It consists of a thin (tpicall lss than m) subwavlngth grating formd on top of a dilctric (O 2 ) lar. B stting th priod of grating to b shortr than th wavlngth of light, diffraction of incidnt light is prohibitd. Morovr, du to th larg contrast in rfractiv ind btwn th grating and surrounding matrials, it works as HCG, which can, with a carful dsign of gomtrical paramtrs, oprat ithr as a high-q rsonator or broadband rflctor [6-8]. In ordr to achiv lctro-optic tuning of th rsonant proprt, w coat th ntir grating with EO polmr [9-2]. Th grating lar is pattrnd in a comb shap as shown in Fig. and lowl dopd so that it can b usd as intrdigitatd 3

4 E Incidnt Rflctd w b w g E pol EO polmr E mod O 2 t p t g t d Mtal Fig. 2. Th structur of total-rflctanc high-spd phas modulator with a mtallic rflctor. lctrods for appling th poling voltag V pol. During th poling procss, thrfor, EO polmr is priodicall pold as shown b th grn arrows (E pol ) in Fig., inducing an lctro-optic cofficint r 33 with opposit signs insid th adjacnt gaps. Hr, not that 3 in r 33 corrsponds to th ais of th coordinat shown in Fig.. Th rfractiv ind of th EO polmr for transvrs-magntic (TM) light (having lctric fild orintd prpndicular to th grating as shown in Fig. ) can thn b modulatd via th Pockls ffct b appling a modulating voltag V mod. Th chang in rfractiv ind is prssd as 3 n nr33emod, () 2 whr n is th rfractiv ind of th EO polmr without modulation and E mod (= V mod /w g, with w g bing th width of th grating gap) is th modulating lctric fild along th -ais. B using th sam intrdigitatd lctrods for modulation, w can appl lctric fild E mod as shown b rd arrows in Fig.. nc th factor r 33 E mod in Eq. () has idntical sign and magnitud insid all gaps, th rfractiv ind of EO polmr btwn th grating bars changs uniforml upon modulation. As a rsult, w can modif th rsonant proprt of HCG and modulat both th rflctanc and transmittanc of TM-polarid light nar th rsonant wavlngth. nc th chang of rfractiv ind of EO polmr originats from lctronic polariation, w can achiv inhrntl high-spd modulation [2], with th bandwidth onl limitd b th RC dla of th lctrods. Whil th dvic shown in Fig. oprats as an intnsit modulator, w can also rali a total-rflctanc high-spd phas modulator b adding a mtallic mirror with narl prfct rflctivit as shown in Fig. 2 [3]. Whn th optical absorption insid th HCG is ngligibl, w can modulat onl th phas of th rflctd light without changing its intnsit. With dns 2-D arra intgration, such a dvic should b usful in dnamic bam shaping and adaptiv optics, with th oprating spd much fastr than that of convntional spatial light modulators [4,5]. 3 Numrical rsults Th transmittanc and rflctanc charactristics of th proposd dvic ar calculatd for TM-polarid light b using th ignmod-pansion-basd 4

5 Magnifid viw at A A B Fig. 3. Th rlationship btwn th thicknss t g of th grating and th rflctivit spctrum whn w b = w g. Th makr A and B indicat th rgims whr HCG works as high-q rsonator and broadband rflctor, rspctivl. Th nlargd viw of th plot nar rgim A is shown in th inst. (a) c n ta c fl.5 R n EO =.59 n EO =.6 n EO =.6 (b) Wavlngth (nm) ) B (d c n ta c fl R = 55 nm n EO Fig. 4. (a) Rflctanc charactristic with n EO varid from.59 to.6 with a stp of.2. (b) Intnsit modulation charactristic of rflctd light at 55 nm (TM mod, Λ = 588nm, w b = w g = 294nm, t g = 856nm). analtical mthod [8,4]. Assuming 55-nm wavlngth rang, w st th rfractiv indics of and O 2 to b n = 3.48 and n O2 =.44, rspctivl. W assum th rfractiv ind of EO polmr bfor appling modulation to b n = Surfac-normal intnsit modulator W first anal th rflctanc proprt of th dvic shown in Fig. without 5

6 (a) n EO =.59 n EO =.6 n EO =.6 Rflctanc.5 Phas (rad) Wavlngth (nm) (b) Rflctanc /2 - /2 Phas (rad) - = 55 nm Fig. 5. (a) Th intnsit and phas charactristics of th rflctd light with n EO varid from.59 to.6 with a stp of.2. (b) Intnsit and phas modulation charactristic of rflctd light at 55 nm (TM mod, Λ = 588nm, w b = w g = 294nm, t g = 856nm). n EO modulation. For simplicit, w considr a cas of w b = w g, whr w b and w g ar th width of th grating bars and th gaps btwn thm, rspctivl. W also assum th thicknsss of th top EO polmr and bottom O 2 lars, t p and t d, to b infinit. Such assumption is quivalnt to th cas whr w hav idal anti-rflction coating on th both sids. Fig. 3 shows th calculatd spctral proprt of rflctanc as a function of normalid grating thicknss t g /Λ and wavlngth λ/λ, whr Λ is th grating priod, t g is th thicknss of th grating lar, and λ is th wavlngth. Not that Fig. 3 onl dpnds on th normalid thicknss and wavlngth, and rmains valid at arbitrar wavlngth rang as long as w scal ach dimnsion accordingl. Whn λ/ > n O2, th grating priod bcoms subwavlngth, so that diffraction is supprssd. In particular, whn λ/ is in th rang from.44 to around 2.83, a charactristic chckrboard-lik pattrn mrgs in Fig. 3 as a rsult of optical intrfrnc btwn two guiding mods insid th HCG lar [8]. W s from Fig. 3 that dpnding on t g /Λ, HCG oprats as a high-q rsonator (for ampl, rgim A in Fig. 3) or a rflctiv mirror with broad spctral bandwidth (rgim B in Fig. 3). B using th high-q rsonator rgim, w can dmonstrat surfac-normal intnsit modulator. For this purpos, rgim A in Fig. 3 is slctd bcaus t g is smallst among othr high-q rsonant conditions, allviating th fabrication 6

7 difficult to mbd EO polmr insid th grating gaps. B stting th rsonanc wavlngth to b 55 nm, othr paramtrs ar dtrmind automaticall from Fig. 3 as Λ = 588 nm, w b = w g = Λ/2 = 294 nm, and t g = 856 nm. W assum that th rfractiv ind of EO polmr is modulatd uniforml onl insid th rgions btwn bars as w appl th modulation voltag V mod. Fig. 4(a) shows th calculatd rflctanc for various valus of n EO. W s a sharp rsonant pak with a Q factor largr than 5,. Th rsonant wavlngth shifts continuousl from nm to 55.9 nm as n EO changs from.59 to.6. As an ampl cas, if w fi th signal wavlngth to 55 nm, th intnsit of rflctd light can b modulatd b varing n EO as shown in Fig. 4(b). W s that -db intnsit modulation can b obtaind with Δn of onl Insrting Δn = and w g = 294 nm to Eq. () and assuming a tpical thrmall stabl EO polmr with r 33 = pm/v [9-], w stimat that th rquird driving voltag can b as small as V mod =.27 V. Morovr, if w could mplo mor rcnt EO polmrs with nhancd r 33 cding 2 pm/v [2], th driving voltag could b rducd furthr. Dpnding on application, w could also modulat th intnsit of th transmittd light. Th modulation proprt of th transmittd light is complmntar to that shown in Fig Total-rflctanc phas modulator As discussd in Sction 2, w can also rali all-pass phas modulator b adding a highl rflctiv mirror as shown in Fig. 2. milar to th cas in Fig. 4, w assum Λ = 588 nm, w b = w g = 294 nm, and t g = 856 nm, whil w st th O 2 lar thicknss to b t d = 8 nm and w assum a silvr mirror with th compl rfractiv ind of n mtal = i. Th intnsit and phas charactristics of th rflctd light ar shown in Fig. 5(a). W s that th optical phas of rflctd light changs sharpl nar th rsonanc wavlngth, whil its intnsit is kpt almost constant with th rflctivit abov.97. Whn th rsonant wavlngth is modulatd b appling voltag V mod, fficint phas modulation is obtaind at 55-nm wavlngth as shown in Fig. 5(b). Rquird rfractiv ind modulation to achiv shift is Δn = 5.4-4, which corrspond to V mod =.39 V if w assum r 33 = pm/v. With dns 2-D arra intgration capabilit, th dmonstratd phas modulator should b usful in high-spd bam shaping and adaptiv optics. 4 Wavlngth tunabilit b tilting incidnt angl Whil fficint intnsit/phas modulation is possibl b th proposd surfac-normal modulator, such modulation is obtaind onl at th rsonant wavlngth of HCG, which svrl limits th oprating wavlngth rang of th dvic. To solv this constraint, w dmonstrat hr that th rsonant wavlngth could b prcisl tund b adjusting th incidnt angl of light. Fig. 6(a) shows th schmatic of th dvic usd undr tiltd incidnt light having a wav vctor of (k, k, k ). Whn th incidnt light is tiltd b an angl φ 2 2 within -plan, k bcoms nonro and k dcrass to k k k, whr k is 7

8 (a) k, E EO polmr Rflctd k Incidnt (k = ) O 2 Transmittd (b) (c) In/Out f f Fig. 6. (a) Schmatic of th dvic usd undr a tiltd incidnt angl for wavlngth tuning. (b, c) Practical tuning schms that could b mplod in ithr transmitting (b) and rflcting (c) configurations. ) 56 m (n th 55 g n l 54 v 3nm a W53 t n a n 52 o 8.6 dg s R Incidnt Angl, (dg) Fig. 7. Rsonant wavlngth with incrasing incidnt angl (TM mod, Λ = 588nm, w b = w g = 294nm, t g = 856nm) th wav numbr. In this cas, w obtain ssntiall similar rsonant proprt of HCG (sinc k = ), whras th quivalnt thicknss t g dcrass du to rducd k. As a rsult, w s from Fig. 3 that w can tun th rsonant wavlngth b adjusting φ. Fig. 7 shows how th rsonant wavlngth changs as w incras th incidnt angl φ. W s that th oprating wavlngth of th modulator can b tund ovr 3 nm (52-55 nm) with an incidnt angl rang of onl 8.6 dg. In practic, incidnt angl can b adjustd b simpl rotating th dvic as shown in Fig. 6(b) for th transmitting configuration, or b shifting th position of th input and output port at th Fourir plan as shown in Fig. 6(c) for th rflcting configuration. 8

9 5 Conclusion W hav proposd and numricall dmonstratd a novl tp of surfac-normal optical modulator using a thin (< m) silicon subwavlngth grating coatd with EO polmr. Th silicon grating was mplod for two purposs: () to induc rsonant guidd mods for th incidnt light and (2) to priodicall pol th EO polmr with opposit sign insid th adjacnt gaps. With a rfractiv ind modulation of lss than -3 applid to th EO polmr insid th gaps, w numricall dmonstratd fficint intnsit modulation with tinction ratio ovr db as wll as total-rflctanc phas modulator, both at 55-nm wavlngth. Assuming a tpicall availabl EO polmr with r 33 = pm/v, w stimat that th driving voltag for modulation to b blow V. Finall, th oprating wavlngth could b tund prcisl b adjusting th incidnt angl of light, which would gratl nhanc th practicabilit of th dvic. With inhrntl high-spd modulation rspons ovr svral tns of GH, potntial scalabilit to dns 2-D arra intgratd with CMOS drivr circuitr, and rlativl as and low-cost fabrication without pitaial procss, th proposd dvics ma b usful in wid rangs of applications, including optical intrconncts, fr-spac optical communications, and high-spd optical bam shaping for imaging and snsing. 9