Design, fabrication and optical characterization of Fabry-Pérot tunable resonator based on microstructured Si and liquid crystal

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1 Design, friction nd opticl chrcteriztion of Fry-Pérot tunle resontor sed on microstructured Si nd liquid crystl Vldimir A. Tolmchev,, Vsily A. Melnikov, Аnn V. Bldychev, Ttin S. Perov* nd Glin I. Fedulov Ioffe Physicl Technicl Institute, Polytechnichesky 26, St.-Petersurg, ussi Deprtment of Electronic nd Electricl Engineering, Trinity College Dulin, Dulin 2, Irelnd ABSTACT The results of simultion of the opticl properties of silicon Fry-Pérot resontor (with liquid crystl filler in the cvity), operted on the shift of the interference nds in the infrred rnge re presented. The possiility of tuning the reflection coefficient from 0 to 0.95 (or trnsmission coefficient from 1 to 5) y chnging the refrctive index y 0.1 in the cvity nd using the stop-nds nd resonnce peks of high order is demonstrted. The prototype Fry-Pérot resontors were fricted y dry nd wet etching of (100)Si nd (110)Si. Some of the resontors were fricted on silicon-on-insultor pltform. A superposition of trnsmission peks with reflection mxim, predicted from clcultions, ws confirmed experimentlly, using infrred microspectroscopy, with temperture vrition from 20 o C to 65 o C nd n pplied electric field from 0V to 10V. Keywords: Fry-Pérot resontor, FTI microspectroscopy, Liquid Crystl, Micromchining, Silicon Photonics, SOI 1. INTODUCTION Silicon is n importnt opticl mteril due to its trnsprency in the Ner Infrred (NI) rnge for oth 1.3 nd 1.55 µm telecommuniction wvelengths s well s in the middle-infrred (MI) nd long-wve infrred (LWI) rnges where the opertionl wvelength of wveguided Si-sed photonic integrted circuits nd optoelectronic integrted circuits cn e extended fr eyond the telecommuniction rnge from 1.55 to 100 μm. The opportunities for Si-sed long-wve infrred photonics re: sensing, communictions nd signl-processing in the 3 5 nd 8 14 µm tmospherictrnsmission windows, missile detection in the 20 µm region, s well s imging, sensing nd communictions in the µm terhertz regime 1. Another importnt dvntge of Si is the mturity of silicon processing in the CMOS electronics industry, resulting in potentilly low cost nd lrge scle mnufcturing cpility 2. Polriztion is one of the importnt properties which provides the ility to control the light propgtion. To use this property, the light needs to e polrized in different mnner. Polriztion modultion is the most common mechnism used in Liquid Crystl (LC) devices for opticl switching, em steering, nd disply pplictions. Electro-opticl devices sed on LCs re extensively implemented in opticl networks. For certin importnt pplictions, LC-sed devices my offer some potentil dvntges: they possess high electro-optic response, non-mechnicl opertion, lower power consumption, esy friction, nd low cost 3. It hs een recently suggested 4 tht comintion of photonic crystls (PC) nd LC cn e used for the friction of ctive devices for opticl communiction. The opticl properties of such devices cn e tuned y mens of temperture or n pplied electric field with the possiility of opertion in the NI, MI nd LWI rnge of wvelengths. *perovt@tcd.ie; phone ; fx ; Photonic Crystl Mterils nd Devices IX, edited y Hernán. Míguez, Sergei G. omnov, Lucio Cludio Andreni, Christin Sessl, Proc. of SPIE Vol. 7713, SPIE CCC code: X/10/$18 doi: / Proc. of SPIE Vol

2 There re numerous pulictions devoted to experimentl investigtions of the electro-tuning effect in LC photonic structures such s LC-opl composite structures, microporous Si-LC structures nd multilyered one-dimensionl (1D) PC with liquid crystl cvity (see references in [5]), which mnipulting with light propgting perpendiculr to the sustrte. The numer of ppers deling with the molding of in-plne light propgtion is reltively smll nd these minly present the results on the thermo-tuning effect in 1D nd 2D PCs. One of the min resons for tht is the difficulty with the ppliction of n electric field to the mtrices sed on electro-conductive mterils. In ef. [6], for exmple, tuning of the defect mode position in the NI frequency rnge y mens of n pplied electric field vried from 10 to 20V ws demonstrted for 2D PC with LC filler in the cvity. In other ppers 7,8 1D PCs with LC filler were investigted nd the tuning of the photonic nd gp (PBG) edge ws otined experimentlly within the rnge of 2 to 10 V pplied electric field, which is in ccordnce with theoreticlly predicted threshold voltge for nemtic LCs 9. The min im of the mjority of the ongoing investigtions, devoted to periodicl Si-LC structures nd sed on oth the 1D PCs nd 2D PCs with high opticl contrst, is to use the PBG mteril's regions of high reflection. For this purpose the utilistion of the lowest PBG is preferle, since this prticulr nd gp for certin design of PCs my provide complete (or qusi-complete) PBG, existing for different ngles nd polristions of incident light. This is the so-clled omni-directionl PBG. In certin cses in 1D PCs the secondry PBGs re lso in use, since, in ccordnce with simultions, they provide the nd gp with resonle width nd qulity of the defect mode, comprle with these prmeters for the lowest PBG. The investigtion of PBGs of high order in rel PCs is further complicted due to the fct tht the imperfections roused during friction (e.g. the roughness nd lck of thickness uniformity of Si sidewlls) ffect the PBGs of high order more strongly. The ltter is one of the resons why the PBGs of high order re rrely used in rel photonic devices. In this pper we nlyse the utilistion of the simplest photonic structure, consisting of only two Si wlls (two mirrors), with the ir gp (resontor cvity) etween them, which cn e infiltrted with LC. This type of photonic structures is well known in physicl optics s the Fry-Pérot (FP) interferometer 10. The operting principle of FP interferometer is sed on receiving the interference nds (fringes) of the light wve propgting through this device. The interference mesurements (i.e. mesurements of the interference nds (IBs) in reflection spectrum) re performed in this study y mens of n externl spectrometer. Since t certin conditions the IBs possess resonnce peks in reflection spectrum, we cll the forementioned device Fry-Pérot resontor (or simply resontor). The prerequisites for the design of FP resontor (FP) re: i) the high reflection of mirrors in cse of Si, ii) potentil for lrge vrition of the devices using micro-structuring of silicon, iii) the cpility to vry the resontor opticl length y infiltrtion of different liquid compounds into the cvity, iv) the ppliction of the resontor in wide rnge of trnsprency of Si (1-15 μm), including LC s filler, nd v) the ility to reorient the LC director y heting or ppliction of electric field of low voltges. Therefore, the ojective of this work is the design nd investigtion of different methods of friction of FP resontor with ility to use the nds of high reflectnce nd the resonnce peks of high order. The empty nd LC infiltrted FPs (with Air nd LC in resontor cvity, correspondingly) were fricted y micro-structuring of (110)Si, using nisotropic etching (AE), s well s of (100)Si, using deep rective ion etching (DIE). The reflection nd trnsmission spectr of the fricted structures were mesured y mens of Fourier Trnsform Infrred (FTI) microspectroscopy in wide infrred rnge nd using Opticl Spectrum Anlyser (OSA) with fire-coupling set up in NI. The perspective trgets of this work re the friction of the tunle polristion element integrted into silicon chip, tunle opticl filter nd modertely fst light modultor. 2. SIMULATION OF FABY-PÉOT ESONATO In order to design the Fry-Pérot resontor it is necessry to select it s prmeters in such wy tht the opticl chrcteristion of the device would e possile fter friction. Technologiclly, the minimum size of the device, tht cn e relised y opticl lithogrphy is, in our cse, of out 1 μm. The rnge of possile spectrl mesurements using the FTI micro-spectrometer is μm. Therefore, for clcultions we cn choose working wvelength of λ 0 =10 μm. In ccordnce with ef. [10], the size of resontor cvity (Fig. 1()) must equl D cv =λ 0 /2N (1) Proc. of SPIE Vol

3 where N is the refrctive index of the cvity. In the cse of the empty cvity with refrctive index N =1 the size (or width) of the resontor cvity is D cv =5 μm. The opticl width of high-refrctive index (OD H ) lyer in this resontor cn then e determined from the following expression OD H = λ 0 /4. (2) Since in our cse we use single Si wll s mirror (see Fig. 1), the geometricl size of the Si wll cn e determined from the expression D H =OD H /N H, (3) where N H is the refrctive index of the H-lyer, nmely Si lyer with N Si =3.42 in the I rnge. Fig.1. Schemtic presenttion of Fry-Pérot resontor, consisting of two Si wlls seprted y () gp nd infiltrted with liquid crystl molecules with different lignment: () verticl plnr, (c) horizontl plnr, (d) isotropic, (e) homeotropic nd (f) qusi-isotropic (or rndom) in the mesophse. Therefore, the width of the Si wlls is D Si =0.731 μm, the width of resontor cvity is D cv =5 μm, nd the otined structure is dpted for working wvelength of λ 0 =10 μm with mximum trnsmission for the resonnce pek nd mximum reflectnce from the Si-wlls (mirrors). The reflection spectrum, clculted y mens of Trnsfer Mtrix Method (TMM) 11, is shown y the dshed line in Fig. 2, which demonstrtes the chrcteristic spectrl pttern of nds with high reflection (with the mxim of t round 0.95). For convenience we will nme these nds s stop-nds (SBs). The clcultions re performed t norml incidence of light nd t the refrctive index of the incoming nd outgoing medium N= Fig. 2. The simultions of the reflection spectr of two idel FP resontors with clculted working () frequency of ν 0 =1000 см -1 or () wvelength of λ 0 =10 μm for i) empty cvity t N cv =1, D cv =5μm (dshed line) nd ii) LC-cvity (solid line) t N cv =1.6, D cv =3.125 μm. Fig. 3 Comprison of two FP resontors, clculted for D cv =5μm with N=1 (dshed line) nd N =1.6 (solid line) in resontor cvity. High vlues re the chrcteristic feture of high opticl contrst periodicl structures, nd it cn e seen tht only two Si wlls in totl support this high vlue of. Figures 2 nd 2 lso demonstrte deep resonnce peks situted exctly Proc. of SPIE Vol

4 in the middle of the SBs nd we chrcterise this resontor s n idel Air-resontor. If we wnt to chnge the prmeters of resontor, there re t lest two wys to do so. One wy is to fricte the resontor with cvities of different geometricl sizes nd shift the resonnce pek to low or high wvelength regions, s ws demonstrted in ef. [12], for exmple. Another wy is to fill the cvity with some compound, like liquid for exmple, with known refrctive index. Since we re going to use the sme working wvelength (λ 0 =10 μm), the geometricl width of the cvity must e decresed y n-times in ccordnce with Eqn. (1). Let us consider liquid crystl with refrctive index N cv =1.6, which will result in vlue of D cv =3.125 μm, nd clculte the spectrum with these new prmeters (see Fig. 2, solid line). As cn e seen from Fig. 2, the reflection spectrum for LC-resontor ppers to e close to the spectrum of the Airresontor. Therefore, for the design of FP resontor oth pproches cn e utilized. However, we must note tht in the second cse, due to the decrese in the opticl contrst of the Si-LC-Si system, the mximum vlues of decrese down to 0.9 s well s the pek t the hlf-height of SB is rodened. This resontor we identify s n idel LC-resontor Fig. 4. The reflection spectr for LC-resontor with D cv =3.125 μm nd N cv =1.6 (dshed line) nd N cv =1 (solid line). Fig. 5. The reflection spectr clculted for idel LCresontor t N cv =1.6 (dshed line) nd fter infiltrtion with compound of N cv =1.5 (solid line). Let us consider now different type of resontor (non-idel) where in the cvity, clculted for the Air-resontor, the LC with N cv =1.6 is infiltrted. As cn e seen from Fig. 3 this pproch results in ppernce of two or three resonnce peks within ll stop-nds of the originl Air-resontor. This result, in fct, ws expected, ecuse y infiltrtion of LC into cvity, the opticl thickness of the cvity increses y 1.6 times, tht provides the condition for oservtion of two or more resonnces. In the first nd the fifth SBs two resonnce peks, red- nd lue-shifted on reltive vlues of Δν/ν (of ~20% nd 40%) with respect to the pek, correspondent to the Air-resontor, re otined. For the third SB the totl overlpping of pek s position for Air nd LC resontors is oserved nd, in ddition to tht, two extr peks re ppered. Therefore, the empty (Air) resontor t ν =5000 cm -1 fter LC infiltrtion demonstrtes the resonnce pek t the sme frequency s well s two dditionl resonnce peks t ν =4500 nd ν =5500 cm -1. We will consider now nother (inverse) pir of resontors, i.e. the idel LC-resontor (Fig. 2, solid line) with hypotheticlly removed LC-filler from the cvity. This mens, tht we clculte the reflection spectrum for the resontor with cvity width estimted for LC (nmely D cv =3.125 μm), ut using N=1 t the sme time (see Fig. 4). It cn e seen from Fig. 4, tht the pek position of LC-resontor t ~1000 cm -1 is situted t the mximum of in the first SB of Air-resontor. A similr sitution is oserved t ν =7000 nd ν =9000 cm -1. Therefore, for the Air-resontor we hve nerly totl reflection (=0.95) t wvelength of 10 μm (1000 cm -1 ), while LC infiltrtion results in totl trnsmission (T=1) t this wvelength. However, the process of LC infiltrtion nd removl is not very fst nd such n pproch is not prcticl for friction of tunle device. For rel devices the utilistion of the opticl medi with the refrctive index vrition under ppliction of the externl forces (temperture or electric field) is more prcticl. In prticulr, the refrctive index modultion of LCs under externl forces could e of order Let us estimte the tuning of the LC-resontor under these conditions, i.e. let us compre the reflection spectr clculted for the idel LC-resontor t N cv =1.6 nd for nonidel one t N cv =1.5 (Fig. 5). Figure 5 shows tht tuning LC from N cv =1.6 to N cv =1.5 leds to the shift of the resonnce pek in the first nd in the second stop-nds. However, the most interesting effect is oserved for the fourth (ν=7000 cm -1 ) nd fifth (ν=9000 cm -1 ) SBs where the totl trnsmission in the resonnce pek in one stte of LC Proc. of SPIE Vol

5 orienttion (Ncv =1.6) replced with zero trnsmission in nother LC stte (Ncv =1.5). This result is similr to the one demonstrted in Fig. 4. Therefore, the clcultions, demonstrted in this Section, shows the cpility of tuning the mximum of reflection from 0 to 0.95 (or T from 1 to 5) y vrition of the refrctive index on ΔNcv in the resontor cvity nd y using the stop-nds of high order. 3. FABICATION AND OPTICAL CHAACTEISATION OF FABY-PÉOT ESONATO BASED ON SILICON AND LIQUID CYSTAL 3.1 Friction of the resontor y microstructuring of Si The FB resontor for infiltrtion of LC ws fricted using opticl lithogrphy oth on (110)Si y mens of nisotropic chemicl etching13 nd on (100)Si using DIE process18. For friction of the pssive FP resontors the conventionl Si wfers were used while tunle LC-FP devices were formed on (110)Si-on-insultor (SOI) wfers. Therml silicon dioxide ws used s msk when etching the wfers. The depth of the grooves ws 50 μm nd 20 μm. The choice of 20 μm depth, in prticulr, ws dictted y the minimum size of the perture of the FTI nd OSA instruments, used for opticl chrcteriztion (see Section 3.2). The exmple of silicon resontor, fricted on (110)Si is depicted in Fig. 6. It ws shown tht nisotropic chemicl etching results in smooth (prcticlly mirror-like) Si sidewlls (Fig. 6). At the sme time, the roughness of Si sidewlls, otined fter DIE, is quite sustntil (see Section 3.4, Fig. 11f). The cvity ws infiltrted with commercil LC E7 (Merck)9 using specilly designed reservoir nd chnnels (Fig. 6). Si electrodes were connected to the outer pds y ttching thin metl wires to the chip contct res with silver pste 8. Fig. 6. SEM imges of Air-resontor, fricted on SOI pltform, using (110)Si, with () reservoir for LC infiltrtion nd ) the cross-section of the resontor edge. Depth of Si-wlls is hsi=20μm. Fig. 7. Schemtic of the opticl setup with OSA for chrcteriztion of LC-resontor in the ner infrred rnge (λ= μm). 3.2 The methods of reflection nd trnsmission spectr mesurements The reflection spectr were first mesured y FTI micro-spectrometer t norml incidence of light in the rnge λ=1.515 μm ( см-1) for the empty resontor nd then mesured gin fter infiltrtion of the resontor cvity with LC E7 (see ef. [14] for more detils). For polrized infrred mesurements it ws ccepted tht the electric vector of incident light ligns long the chnnel of the Si grooves for E-polrized light, while for H-polrized light the electric vector ligns long the depth of the grooves s shown in Fig.1. For chrcteriztion of the structure in the Ner Infrred (NI) rnge λ= μm we used n opticl fier-coupling set-up sed on n OSA. The setup ws developed for the purpose of polrized trnsmission mesurements in the NI rnge (Fig.7). We note tht ll experimentl spectr (reflection nd trnsmission) re demonstrted further in ritrry (or normlized) units for clrity of the results comprison. The normliztion of the dt ws necessry due, in prticulr, to lrge shding effect14 during infrred mesurements of the resontors integrted into the chip (Fig. 6 ). The otined experimentl spectr were fitted using the chosen model for the three-lyer system (Si-Air-Si or Si-LC-Si), which llows us to otin the precise vlues of the thicknesses DSi nd Dcv s well s NLC, served s fitting prmeters. Both thickness vlues, otined from the fitting coincide well with the results estimted from SEM imges. The opticl constnts for Si were tken from ef. [15] ccounting for the dispersion nd sorption of silicon in the NI rnge. The refrctive index of the incoming nd outgoing medium is tken s N=1. Proc. of SPIE Vol

6 3.3 The reflection spectr of Air nd LC resontors, fricted y nisotropic etching of (110)Si. The polrised reflection spectr of the resontor, fricted y nisotropic etching of (110)Si, re discussed in this section. The experimentl, E-polrized reflection spectr of the Air nd LC resontors re shown in Fig. 8. This figure depicts the chrcteristic interference nds spred uniformly throughout the entire rnge of spectr from 650 to 6500 cm -1 ( μm). The fitting of the registered spectr ws performed until the est fitting ws chieved t certin fitting prmeters D Si nd D cv. The spectrum clculted with the est fitting prmeters (D Si =1.2 μm nd D cv =3.4 μm) is shown in Fig. 8 for N cv =1. As cn e seen from Fig. 8 the clculted spectrum shows four chrcteristic resonnce peks, which re lso oserved in the experimentl spectrum with some degrdtion of pek qulity. The ltter is demonstrted s rodening of the resonnce peks s well s decrese of the pek s mplitude. This degrdtion could e cused y few fctors such s the focused light em of the FTI microscope, shding effect, unevenness of the Si wlls nd others. Nevertheless, the position of ll experimentl SB peks is in good greement with clculted spectrum, which confirmed the good qulity of the Si sidewlls nd their correspondence to the desired geometricl prmeters. The infiltrtion of LC into the cvity results in sustntil lue-shift of nerly ll of the IBs, which reflects the decrese in the refrctive index of the resontor cvity. Moreover, the shpe of the resonnce peks ecomes more distinct. As is well known decrese in the opticl contrst t the interfce of two phses diminishes the effect of light scttering, which cn influence the pek qulity nd therey explins some improvement in the resonnce peks qulity. The fitting of the reflection spectr for LC-resontor with the sme prmeters D Si nd D cv, ut t vrition,.u ν, cm ν, cm -1 Fig. 8. The () experimentl nd () clculted reflection spectr for Air (dshed line) nd LC (solid line) resontors. For spectrum, shown in (), N cv =1 (dshed line) nd N LC =1.72 (solid line) for D Si =1.2 μm nd D cv =3.4 μm. The height of Si wll is h Si =50 μm. of N cv =N LC in the rnge from 1.3 to 1.8, leds to reflection spectrum for E-polristion, shown in Fig. 8 (solid line) t N LC =1.53. Quite good greement is oserved gin etween clculted nd experimentl spectr in Fig. 8. A similr pproch ws used for fitting the spectr of the LC-resontor for H-polristion, nd oth spectr (for E nd H polristions) re shown in Fig. 9. Figure 9 revels tht the spectrum for E-polristion for three lowest SBs is lueshifted with respect to tht for H-polristion. Aprt from this, the resonnce peks re oserved on Fig. 9 in mxim of IBs t ~3300 cm -1 nd t ~4200 cm -1 s ws predicted ove (see Figs. 4 nd 5). As ws lredy shown for the E- spectrum, the fitting of H-spectrum demonstrtes very good convergence etween experimentl nd clculted spectr t N LC =1.53. It is known from literture 16, tht LC E7 possesses irefringence of order 0.2 nd chrcterised y two vlues of the refrctive indices in the I rnge, nmely the ordinry refrctive index, N o =1.49 nd extrordinry refrctive index N e =1.69. From the fitting we hve received prcticlly the sme vlue of ΔN=0.19, ut slightly different vlues for N o =1.53 nd N e =1.72. Moreover, the high vlue of ΔN is lso confirmed y the ppernce of the Proc. of SPIE Vol

7 chrcteristic virtionl nds of LC E7 17 t 1497, 1602 nd 2222 cm -1, which re oserved in the E-spectrum nd lmost sent in the H-spectrum. Therefore, we cn conclude tht the spontneous plnr orienttion of the LC director long the chnnel hs occurred, which corresponds to the schemtic, presented in Fig. 1. The experimentl spectr versus wvelengths (the sme ones s presented in Fig. 9 vs. wvenumers) re presented in Fig.10 with extension to the NI rnge μm. A good correspondence etween the positions of nds nd the resonnce peks is oserved in the regions of nd μm, which is in greement with the clculted dt shown in Fig. 10.,.u.,.u Fig. 9. The () experimentl nd () clculted reflection spectr for LC-resontor for E- (solid line) nd H- polristion (dshed line). Clculted spectr with N LC =1.72 (solid line) nd N LC =1.53 (dshed line), D Si =1.2 μm nd D cv =3.4 μm. Fig. 10. The () experimentl nd () clculted reflection spectr in NI rnge vs. wvelengths (from Fig.9). Thus, the resontor with D cv =3.4 μm demonstrtes quite promising chrcteristics s follows from simultions. If it is n LC-resontor, then, in ccordnce with Eqns. (1) - (3), λ 0 =(2 1.6) D cv = 11 μm nd D Si =11/(3.42 4) =0.8 μm. During friction we hve received resontor with D Si =1.2 μm, which is quite close to the clculted vlue. We cn, therefore, conclude tht the LC-resontor with working wvelength of 11 μm (or ~910 cm -1 ) is fricted. The resonnce pek of this FP resontor is seen in experimentl (Fig. 9) nd in clculted (Fig. 9) reflection spectr. In ddition to tht, the corresponding resonnce peks of 2 nd, 3 rd, 4 th nd 5 th SBs re perfectly depicted in the clculted s well s in experimentl spectr of Air nd LC resontors (see Figs. 8 10). 3.4 The reflection spectr of LC resontor fricted on (100)Si y deep rective ion etching. Let us now investigte non-idel resontor with non-optiml clculted prmeters. In contrst to the previously descried FP, this resontor is fricted on (100)Si y DIE process. In this cse, if the depth of the etching is reltively lrge, quite significnt roughness of the Si sidewlls (see Fig. 11e,f) is oserved, which my strongly influence the qulity of the experimentlly registered spectr. Another distinct feture of this resontor is the smller depth (or height, h) of its Si wlls (h Si = 20 μm). The decrese in the height of the Si wlls results in decrese of the perture of the incident light em, which in turn leds to reduction of the signl-to-noise rtio during FTI mesurements. The experimentl nd clculted reflection spectr for Air nd LC resontors re presented in Fig. 11. The interference nds in spectrum re deformed nd demonstrte the reduced mplitude modultion nd so only the prt of the spectrl rnge elow 4000 cm -1 is shown. The fitting of this reduced rnge of spectr ( cm -1 ) for the Air resontor (see Fig. 11) yields the following prmeters of the resontor: D Si =2.6 μm nd D cv =5.4 μm. Therefore, if we use this resontor s resontor clculted for utilistion with LC, then it s working wvelength (the position of the first resonnce pek) will correspond to λ 0 =2 1.6 D cv = 17.3 μm nd the thickness of the Si wlls to D Si = 17.3/(3.42 4)=1.26 μm. Thus, ccording to the clcultions, the otined resontor does not hve optiml prmeters nd, s result, the Proc. of SPIE Vol

8 position nd the qulity of the resonnce peks will e different. Nonetheless, the chrcteristic peks (dshed lines in wvenumer (Figs. 11,c) nd in wvelength (Fig. 11,d) presenttions) t ~1300 cm -1 (7.7μm) nd t ~2500 cm -1 (4 μm) re well identified during the fitting s resonnce peks in the reflection spectrum of the Air resontor. After the infiltrtion of the Air cvity with LC, the N cv chnges nd this leds to chnge in the experimentl nd clculted spectr, denoted y solid lines in Figs.11, nd Figs.11 c,d, respectively. Performing the fitting with known prmeters D Si =2.6 μm nd D cv =5.4 μm, we find the vlue of N cv =1.53. This vlue of N LC provides the shift of spectrl ptterns in such wy tht t the frequency of ~900 cm -1 (11 μm) for the empty resontor hs high vlues, while for LC cvity the vlues of re low. A reverse sitution is oserved t frequency of ~2500 cm -1 (4 μm) where for the empty resontor the low vlues re otined, while for the LC-cvity the vlues re high. Therefore, it ws shown tht n LC resontor cn e designed even with non-optiml prmeters nd despite this it cn possesses reltively good opticl chrcteristics close enough to the clculted vlues of n idel resontor. It ws lso shown tht the roughness of Si sidewlls impirs the qulity of the resontor fricted on Si using the DIE technique.,.u ,.u c d Fig.11. The (,) experimentl reflection H-spectr of empty (dshed line) nd LC (solid line) resontor vs. (,c) wvenumer nd (,d) wvelength. (c,d) Clculted spectr with N=1 (dshed line) nd N LC =1.53 (solid line). SEM imges of the resontor: (e) the top view nd (g) sidewll roughness, otined fter DIE of (100)Si, h Si = 20 μm. 3.5 Tunle resontor operting on the effect of reorienttion of LC director during heting The tunle LC resontor ws fricted on (110)Si y nisotropic etching, in which the tuning of opticl properties ws otined due to the reorienttion of LC molecules (or LC director) s ws predicted in Section 2, Fig. 5. Initilly, the reflection spectrum, of n Air resontor ws mesured nd its prmeters were determined, using fitting procedure s descried ove. The Air resontor prmeters were chosen close to the idel ones, s descried in section 3.3; the prmeters otined during the fitting of the reflection spectr re: D Si =1.2 μm nd D cv =3.6 μm. Then, the cvity ws infiltrted with LC E7 nd the reflection spectr were mesured t room temperture for E- nd H-polriztion of the incident light (see Fig. 12). As cn e seen from Fig. 12, the spectr re shifted t different polristions. The fitting, performed for spectr (Fig. 12), enles the vlue of N LC to e determined from ech of these spectr. The N LC vlue, otined for E-polristion is 1.67, while for H-polristion N LC =1.5. Moreover, s cn e seen from Fig. 12, the chrcteristic virtionl nds of LC E7 t 1497, 1603 nd 2222 сm -1 re lso present in the spectrum only for E- polristion, which confirmed tht LC hs pronounced spontneous orienttion of rod-like molecules long the chnnel. This type of LC orienttion is shown schemticlly in Fig. 1c nd is one of the most common types of lignment, otined in our group during LC-infiltrtion into 1D PCs ir chnnels y cpillry effect. Proc. of SPIE Vol

9 The resontor ws heted up to 65 o C using clirted thermocouple micro-heter nd the reflection spectrum ws registered gin t this temperture (Fig. 12). The vrition of N Si t heting to 65 o C is not lrger thn 01 nd, therefore, cn e neglected in this cse. Using the fitting of the registered reflection spectrum, the vlue of N LC ws estimted to e round N LC =1.54. Two spectrl effects were reveled for spectrum t E-polristion (see the stop-nd in Fig. 12 shown y circle). In prticulr, the resonnce pek, oserved t frequency ~ 5100 cm -1 (t wvelength of 1.94 μm on Fig. 12c), is situted quite close to the mximum of reflection from the heted resontor (thick line). The vlue of =0.25.u. is oserved t this resonnce pek t room temperture, while t 65 o C the mximum of reflection increses up to =0.4.u.. At the sme time, t frequency of ~5300 cm -1 (wvelength 1.94 μm) the reflection is =0.41.u. t room temperture, while t heting of LC to 65 o C nd its trnsition to isotropic phse (see Fig.1d) the resonnce pek t λ=1.9 μm with =0.19.u. is ppered. Thus, the thermo-opticl effect of LC cn e used t two wvelengths. With improved friction technology of this type of FP resontors, the much higher rtio of the reflection vlues (up to 1/0.89) for these two cses cn e chieved (Fig. 12d). Unfortuntely in this type of experiments it is difficult to return to the originl LC orienttion fter cooling the structure down to room temperture. However, this exmple demonstrtes the principle of tuning the resontor for controlling the reltive intensity of the reflected light of certin polriztions. It is lso demonstrtes the possiility of utilizing the oserved opticl effects in SB nd in resonnce peks of high order.,.u ,.u c d Fig. 12. (,c) The experimentl reflection spectr of LC resontor, demonstrted the effect of thermo-tuning, nmely the trnsition from LC mesophse t 20 ºC (dshed line for H- nd thin line for E-polristion) to the isotropic phse t 65 ºC (thick line). (c,d) The frgment of spectr shown ginst wvelength in the region of interest. (,d) Clculted spectr with N LC =1.67 (thin line), N LC =1.54 (thick line) nd N LC =1.5 (dshed line). D cv =3.6 μm, D Si =1.2 μm nd h Si =50 μm. 3.6 Tunle resontor sed on the reorienttion of LC molecules under pplied electric field The resontor ws fricted on SOI pltform with (110)Si device lyer of thickness 20 μm. The design of the resontor includes the chnnels for infiltrtion of the LC, the chnnels for electricl isoltion of one prt of the structure from nother, which will e connected to electrodes of different polrity in order to pply n electric field to reorient the LC molecules from plnr to homeotropic lignment 8 (see Fig.1e). The structure ws fricted y nisotropic etching of the device lyer nd the etching ws stopped t the SiO 2, which results in Si-wll depth of 20 μm (Fig. 6). The mesurements of the reflection spectr of the fricted structure were first performed y FTI micro-spectrometer. By fitting of the registered reflection spectr using TMM the structure prmeters of the fricted resontor were determined s D Si =2.3 μm nd D cv =3.7 μm. FTI mesurements were performed gin fter the infiltrtion of the resontor cvity with LC. The dt nlysis shows tht only smll spectrl shift is reveled etween reflection spectr in H nd E polristions. In ddition to tht, the chrcteristic virtionl nds of LC E7 show up in the spectr for oth polristions. We, therefore, conclude tht the initil lignment of LC molecules is rndom in this cse (see Fig. 1f). The reflection spectr in H-polristion re demonstrted in Figs. 13, s in the sence of the pplied electric field, i.e. t 0V (thin line), nd with n pplied electric field of 10V (thick line). As cn e seen from Figs. 13,, the spectrum t 10V is lue-shifted, which certifies the decrese in the refrctive index of the cvity. At the sme time the Proc. of SPIE Vol

10 intensity of the virtionl nds of LC E7 lso significntly decresed. Both effects enle us to confirm tht good qulity homeotropic lignment is ccomplished t 10V.,.u T,.u ,.u c T Fig. 13 (,) The experimentl reflection H-spectr of LC resontor t 0V (thin line) nd 10V (thick line), demonstrted the electro-tuning effect. (c) Clculted spectr t N LC =1.55 (thick line) t 0V nd N LC = 1.50 (thin line) t 10V. The vlues D Si =2.3μm nd D cv =3.7μm, s well s N LC were otined from fittings. h Si = 20 μm. Fig. 14 ) Electro-tuning effect for the sme smple s in Fig. 13, mesured with OSA-setup in NI rnge. ) Clculted spectr for N LC =1.5 (thin line) t 0V nd N LC =1.45 (thick line) t 10V. Arrows show the resonnce peks in clculted spectr. The fitting of ech spectrum, shown in Fig. 13 c, yields the following vlues for the refrctive index: N LC =1.55 for 0V nd N LC =1.5 for 10V, which results in the reltively smll vlue of ΔN=5. Our previous results 8, otined for electrotuning of the PBGs in the sme rnge of the pplied voltges, demonstrte lrger vlues of ΔN =0.15, correspondent with tuning of the refrctive index vlue from the plnr lignment of LC E7 with N LC =1.67 to homeotropic lignment with N LC =1.52. However, the effect demonstrted in pper 8 ws unrepetle, i.e. the susequent ON/OFF switching of the pplied electric field did not result in the initilly otined shift. We conclude tht in pper 8 the orienttion of the LC molecules fter the pplied electric field ws OFF did not return to the initil stte of lignment, nd we presume tht rndom (or qusi-isotropic) lignment ws formed in the mesophse (see Fig.1f), ecuse the fitting of spectr for oth (H nd E) polristion results in vlue of N LC =1.56. In this pper the reproducile electro-opticl effect with smller shift of SB s edge, resulting in trnsition of the N LC vlue from 1.55 to 1.50, ws oserved. Therefore, we hve received n initil rndom lignment of LC molecules in the mesophse similr to the result otined erlier in pper 8. As fr s the concept of using comintion of different resonnce peks nd corresponding SBs for tuning, we did not confirm this experimentlly for the investigted smple, since the dimensions of the smple were not optimised. Although, sustntil shift of the SBs edges t 1600, 2200, 2900 сm -1 nd other frequencies ws reveled. We, therefore, elieve tht tuning the SBs of high orders could e possile for FP resontor with non-optimised prmeters. Similr investigtions of interference nd shift with pplied electric fields of 0V nd 10V were lso performed on the trnsmission spectr of this smple, using fire coupling set up in comintion with OSA (Fig. 14). As cn e seen from Fig. 14, the spectrum is lue-shifted t the pplied electric field, which indictes decrese in N LC. The fitting of IBs shows, tht for 0V N LC =1.5, while for 10V N LC =1.45. These vlues slightly differentite from N LC otined with FTI micro-spectrometer, ut demonstrte good greement etween clculted nd experimentl dt (Fig. 14). One of the shortcomings is the sence of the resonnce peks in experimentl spectr, which re seen very clerly in the simulted spectr. We elieve tht these resonnce peks, ppering t wvelengths of 1.4 μm nd smller, disppered due to the focused em of the incident light in our experiments or due to the unevenness of Si wlls in resontor. This conclusion Proc. of SPIE Vol

11 is indirectly confirmed y the poor conditions of IBs oserved in FTI spectr in the region elow 3μm (see Fig. 13), since the higher the order of SBs nd the resonnce peks, the stronger is their deformtion. Nevertheless, in this experiment the smll nd repetle vlue of ΔN=5 under pplied electric field of 10V ws otined from mesured trnsmission spectr s well s from mesured spectr shown in Fig CONCLUSION The design of Fry-Pérot resontor with liquid crystl filler in the cvity, operted on shift of the interference nds in the middle-infrred spectr rnge is presented. The ility of tuning the reflection coefficient in nd mximum from 0 to 0.95 (or trnsmission coefficient from 1 to 5) y vrying the refrctive index of the medi in the resontor cvity nd using stop-nds of high order is demonstrted. Fry-Pérot resontor devices were fricted y nisotropic chemicl nd dry etching of (110)Si nd (100)Si, respectively, including SOI sustrtes. Opticl chrcteristion of the resonnce peks is performed with polrized FTI microspectroscopy (in reflection mode in the rnge of μm) nd y using the fire-coupling set-up in conjunction with OSA (in trnsmission mode in the rnge of μm), which enle reflection nd trnsmission spectr to e otined over wide wvelength rnge, nmely from 0.9 to 15 μm. Aprt the interference nds of high reflection (Stop Bnds) nd the resonnce trnsmission peks, the chrcteristic virtionl nds of liquid crystl were lso oserved. The otined totl informtion enles us to determine the geometricl prmeters of the resontor nd to mke conclusion on the lignment of the LC molecules, infiltrted into the cvity. Using the cquired experimentl dt, we were le to confirm the predictions of simultions on mtching the trnsmission peks with reflection mxim in the opticl spectr, including spectr registered during LC reorienttion t heting to 65 o C (the trnsition temperture to the isotropic phse of LC E7). The tuning of the edge of the stop-nds nd the resonnce peks for Fry-Pérot resontor ws otined in reflection nd trnsmission mode of the infrred spectr with n pplied electric field of 10V. ACKNOWLEDGMENTS This work hs een supported y ICSET, Irelnd Postdoctorl Awrd nd ICGEE Progrmme, Science Foundtion Irelnd (NAP-94 Progrmme), Grnts of ussin Foundtion for Bsic eserch, N nd The uthors wish to express their pprecition to E.V. Astrov for useful discussion, M. Lynch for help with OSA mesurements nd Yu. Zhrov for help with smples friction. EFEENCES [1] Soref,. A., Emelett, S. J. nd Buchwld, W.., Silicon wveguided components for the long-wve infrred region, J. Opt. A: Pure Appl. Opt., 8, , [2] Pvesi, L. nd Lockwood, D. J., eds., [Silicon Photonics], Springer, Berlin, 397 (2004). [3] Blinov, L. M. [Electro-opticl nd Mgneto-opticl Properties of Liquid Crystls], Wiley, N.Y., (1984). [4] Busch, K. nd John, S. Liquid-Crystl Photonic-Bnd-Gp Mterils: The Tunle Electromgnetic Vcuum, Phys. ev. Lett., 83(5), (1999). [5] Busch, K., Lölkes, S., Wehrspohn,., Főll, H., eds., [Photonic Crystls. Advnces in Design, Friction, nd Chrcteriztion], Weinheim: Wiley-VCH (2004). [6] Anderson S. P., Hurylu M., Zhng J., nd Fuchet P. M., Hyrid Photonic Crystl Microcvity Switches on SOI, Proc. SPIE 6477, /8 (2007). [7] Tolmchev, V. A., Astrov, E. V., Perov, T. S., Zhrov, J. A, Grudinkin, S. A., Melnikov, V. A. Electrotunle in-plne one-dimensionl photonic structure sed on silicon nd liquid crystl, App. Phys. Lett., 90, , (2007). [8] Tolmchev, V. A., Grudinkin, S. A., Zhrov, J. A., Melnikov, V. A., Astrov, E.V., nd Perov, T.S. Electrotuning of the photonic nd gp in SOI-sed structures infiltrted with liquid crystl, Proc. SPIE 6996, 69961Z-1/9 (2008). Proc. of SPIE Vol

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