INVESTIGATION OF BRAGG GRATINGS RECORDED IN POLYMER- DISPERSED LIQUID CRYSTALS

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1 Journl of Optoelectronics nd Advnced Mterils Vol., No. 3, Septemer, p INVESTIGATION OF BRAGG GRATINGS RECORDED IN POLYMER- DISPERSED LIQUID CRYSTALS K. Beev *, S. Sinov, T. Angelov, A. G. Petrov Centrl Lortory of Opticl Storge nd Processing of Informtion, BAS, 1113 Sofi, Acd. G. Bonchev Str., l. 11, Bulgri Institute of Solid Stte Physics, BAS, 7 Tsrigrdsko Chusse Blvd, 178 Sofi, Bulgri Brgg grtings, hologrphiclly formed in polymer-dispersed liquid crystls (H-PDLCs) re studied. Mixture of the commercilly ville E7 liquid crystl nd NOA5 polymer in rtion 1:1 re employed. Polymeristion in presence of electric field is produced. A significnt increse of the diffrction efficiency in the cse of field-pplied recording is oserved. The exposure chrcteristics, electric field switching ehviour, polriztion nd Brgg ngle dependences re exmined. (Received July 5, ; ccepted June, ) Keywords: Polymer, Liquid crystl, Hologrphic grting 1. Introduction The incresed ttention pid to hologrphic polymer-dispersed liquid crystls (H-PDLC) during the recent yers is sed on the widespred possiilities tht they provide switchle opticl elements. The low cost nd the different specific chrcteristics otined y vrying the initil components nd curing set-up confirm the interest in tht mteril. Since the first description [1], lrge efforts hve een exerted in order to optimize, to gin etter understnding of the fundmentl properties of H-PDLC nd to pply nd integrte them in different opticl devices. The formtion of the grtings follows the next descried mechnism. Phse seprtion of PDLC occurs during illumintion with sptilly modulted light distriution (interference pttern), inititing counter diffusion of the pre-polymer to the right regions of the interference pttern where polymeriztion tkes plce. The liquid crystl is forced nd set in the drk regions of the opticl field. As result, refrctive index modultion occurs fixing the light distriution. The structure consists of lternting polymer rich nd liquid crystl (LC) rich lyers. An importnt feture of these grtings is the reversile electricl switching of the diffrction efficiency (DE). The ppliction of strong enough electric field ligns the LC inside the droplets in which it is confined llowing the switching of the grting. Dye molecules re dded in the pre-polymer mixture in order to shift the sorption mximum in the visile spectrum. To strt the free-rdicl polymeriztion processes, co-inititor is required to generte rdicls inititing the polymer curing. The thorough H-PDLC review [] with 3 cittions ws pulished in yer. It is importnt to note tht this ttrctive composite recording mteril hs serious high voltge control drwck, connected with the strong nchoring of the nnoscle LC droplets, which is still n unsolved prolem restricting mny prcticl pplictions. For exmple, multiplexed switchle grtings [3, ] nd vrile-wvelength Brgg grting [5] operte t -8 V/µm, tunle Fresnel lenses [] t 1 V/µm. In [7], tunle fce-centered-cuic photonic crystl opertes t 15-5 V/µm. Recently reported, dye lsing using PDLC s ngle-dependnt nrrow spectrl-nd * Corresponding uthor: keev@optics.s.g

2 8 K. Beev, S. Sinov, T. Angelov, A. G. Petrov feedck element [8] requires 15 V/µm. This underlines the importnce of further optimiztion of H- PDLC electro-opticl properties in order to decrese the controlling voltge. In the present work the chrcteristics of Eosine dye-doped H-PDLC re exmined. For the first time, to our est knowledge, H-PDLC grting formtion is performed in the presence of pplied AC voltges. This, to certin extent, is similr to the pplied electric field techniques, used for polymer stilized liquid crystls. The electro-opticl switching chrcteristics, the ngulr sensitivity nd polriztion ehviour of the grtings re studied.. Experimentl results nd discussion The pre-polymer syrup consists of homogeneous mixture of 9% NOA5 (Norlnd Opticl Adhesive 5), 9% E7 liquid crystl,.5% Eosin dye nd 1.5% Triethylmin. The smple is spred etween two indium tin oxide-coted glss sustrtes, seprted with 3µm spcers. The trnsmittnce visile spectr of this recording medium re investigted with Cry 5E spectrophotometer in nm region. Fig. 1- demonstrtes the spectr for the oth polriztions t V electric field. In this cse s- nd p-polriztion correspond to electric field vector prllel nd perpendiculr to the slit, respectively V/µm 1,3 V/µm T [%] 5 3 (Ts - Tp) [%], V/µm 3,5 V/µm λ [νµ] λ [nm] Fig. 1. Spectrl trnsmittnce of the uncured PDLC smple. polriztion chrcteristics; trnsmittnce differences for s- nd p-polriztion t different pplied electric fields. The p-polriztion remins lmost constnt when AC voltge is pplied. Conversely, voltge induced chnges in the trnsmittnce spectr for s-polriztion re oserved. The polriztion difference T s T p is shown in Fig. 1-. The experimentl error is less then.5% [9, 1]. The oserved chnges re due to the dye-molecule reorienttion cused y the LC rottion. The ove descried smples re used for hologrphic grtings recording. Fig. illustrtes the opticl set-up. The PDLC cell is connected to AC genertor working in the rnge of -15 V rms t khz frequency. The em splitter (BS) nd the mirrors M 1, M provide n interference pttern from the Ar + lser irrdition t 51 nm with 15 lines per mm on the smple. The grting spcing Λ is.8 µm. The polriztion of the recording ems is s- (perpendiculr to the incidence plne in this cse). Simultneously, He-Ne lser is pplied for investigtion of the grting formtion nd DE dynmic. With the rottor R the monitoring em polriztion is chnged. In the present work different electric fields E r re pplied during grting formtion in H-PDLC. Different vlues of the diffrction efficiency re otined depending on E r. At E r = V/µm nd low pplied electric field (Fig. 3-, E r =1.3 V/µm) the grting formtion strts lmost immeditely fter the lser illumintion. The DE is low nd even decrese t 1.3 V/µm. Fig. 3- illustrtes the higher field ppliction leding to rpid DE increse comined with induction period ppernce. The threshold exposure (~3 mj/cm ) is reltively low t lest n order of mgnitude smller compred to [11].

3 Investigtion of Brgg grtings recorded in polymer-dispersed liquid crystls 81 Fig.. Opticl set-up sketch: BS em splitter, M 1, M mirrors, R polriztion rottor, PM power meter, AC gen. AC genertor.,,5 V/µm 1,3 V/µm 5, V/µm 3,5 V/µm,,3, 3,1 1, exposure [ mj/cm ] exposure [ mj/cm ] Fig. 3. The diffrction efficiency dependence on the pplied voltge E r during grting formtion. The DE increse could e relted to the mss-trnsfer fcilittion during the grting formtion. The pplied voltges re reltively low compred to the vlues necessry for totl reorienttion of the LC in the pre-polymer syrup. In tht sense only the ulk of the LC droplet is ffected. This looks s lyer to lyer diffusion processes of the mesophse requiring insignificnt force for their initition. We hve investigted the DE ehviour of our Brgg grting, recorded t E r = nd E r = 3.5 V/µm s function of root-men-squre pplied voltge. The mesurement results re shown in Fig.. The otined threshold field is ner to zero for the s-polriztion when E r =3.5 V/µm is pplied during recording (Fig. -). For ll other cses the threshold vlue is ~V/µm. The increse of the pplied field is coupled with electricl shorting [7]. If we extrpolte the switching curves, the DE would decrese to zero t V/µm for E r = nd.5- V/µm for E r = 3.5 V/µm, respectively. It is interesting to note tht ~11V/µm switching field otined in other studies employing similr pre-polymer syrup [1].,5,51,5 E r = V/µm 5 E r = 3.5 V/µm,9,8,7, 3,5,,3, 1 -,5,,5 1, 1,5,,5 3, 3,5 U [V/µm],, 1, 1,8, 3, 3, U [V/µm] Fig.. Diffrction efficiency dependence on the pplied electric field. () E r = ; () E r =3.5 V/µm.

4 8 K. Beev, S. Sinov, T. Angelov, A. G. Petrov The ngulr sensitivity is investigted with s- nd p-polrized lser em t 33 nm with 5% ccurcy. The normlized intensity dependence on the reconstruction ngle is illustrted on Fig. 5. We hve oserved tht the intensity full width t hlf-mximum (FWHM) shows wek polriztion dependence. This result is in greement with similr investigtions [13] showing no differences. The roder FWHM in the present work is proly due to the smll thickness of our Brgg grtings, recorded in reltively thin lyers of 3 µm pre-polymer syrup. It could e clculted tht the grting thickness is ~3 µm for the lowest vlue Q = 1 of the Klein prmeter, Λ =.8 µm, verge refrctive index n = 1.55 nd λ =.51 µm. For Brgg grtings, recorded in 5.3 µm thick emulsion [1], the otined FWHM is ~5., i.e. very close to the otined y us vlue of. 1, s-polriztion p-polriztion,8 Intensity,.u.,,,, ngle, deg Fig. 5. Normlized DE (intensity of the diffrcted em) Brgg dependence for s- nd p- polriztion. The polriztion dependence on the diffrcted intensity I is investigted. The polriztion zimuth θ is chnged y the rottor R (Fig. ) during reconstruction. The theoreticl tretment mde y Gsvik [15] gives the following reltion: I (θ ) / I s = (1 - B) cos θ + B (1) Where the prmeter B is defined s rtio etween diffrcted intensities for p- nd s-polriztion: B=I p /I s.according to Gsvik s theory B=.5, while from Kogelnik [1] B =. In the cse of Brgg grting, recorded in H-PDLC B.5 [1]. From our mesurements B =.3. One possile explntion of the otined results is tht theoreticlly only pure phse grtings re considered. Our recording medium is composite mteril nd for p-polriztion we cn expect higher light scttering. 1,,9 experimentl curve theoreticl cos θ curve I(θ ) / Is,8,7,,5,,3,,1, 8 1 polriztion zimuth θ Fig.. The normlized diffrction efficiency dependence on the polriztion zimuth.

5 Investigtion of Brgg grtings recorded in polymer-dispersed liquid crystls Conclusion In conclusion, we hve demonstrted tht y pplying voltge during the hologrphic grting recording, it is possile to chnge the electric switching chrcteristics of H-PDLC opticl elements long with sustntil DE incresing. In our opinion, it is one of the wys to overcome the high voltge prolem of H-PDLC opticl elements. We shll report soon further studies connected with low frequency voltge tht is pplied on H-PDLC during recording. References [1] R. L. Sutherlnd, V. P. Tondigli, L. V. Ntrjn, T. J. Bunning, Chem. Mter. 5, 1533 (1993). [] T. J. Bunning, R. L. Ntrjn, L. V. Tondigli, R. L. Sutherlnd, Annu. Rev. Mter. Sci. 3, 83(). [3] Ch. Bowely, A. Fontecchio, G. Crwford, J. Lin, L. Li, S. Fris, Appl. Phys. Lett. 7, 53 (). [] J. Qi, M. Sous, A. Fontecchio, G. Crwford, Appl. Phys. Lett. 83, 15 (3). [5] Ch. Bowely, P. Kossyrev, G. Crwford, Appl. Phys. Lett. 79, 9 (1). [] H. Ren, Y. Fn, Sh. Wu, Appl. Phys. Lett. 83, 1515 (3). [7] M. Escuiti, J. Qi, G. Crwford, Opt. Letters 8, 5 (3). [8] D. Lucchett, L. Crinte, O. Frncescngeli, F. Simoni, Appl. Phys. Lett. 8, 837(). [9] I. Konstntinov, Tz. Bev, Sn. Kitov, Appl. Opt. 37 (19), -7 (1998). [1] Tz. Bev, Sn. Kitov, I Konstntinov, Appl. Opt. (1), (1). [11] T. Bunning, L. Ntrjn, V. Tondigli, R. Sutherlnd, Polymer, 3, 1, (1995). [1] R. L. Sutherlnd, V. Tondigli, L. Ntrjn, T. Bunning, W. Adms, Appl. Phys. Lett., 17 (199). [13] R. L. Sutherlnd, L. V. Ntrjn, V. P. Tondigli, T. J. Bunning, Chem. Mter. 5, 1533 (1993). [1] J-H. Yeh, R. Kostuk, Appl. Opt. 3, 315 (1995). [15] K. Gsvik, Optik 39, 7 (1973). [1] H. Kogelnik, Bell Syst. Tech. J. 8, 99 (199).

DIFFRACTION OF LIGHT

DIFFRACTION OF LIGHT The phenomenon of bending of light round the edges of obstcles or nrrow slits nd hence its encrochment into the region of geometricl shdow is known s diffrction. P Frunhofer versus