EXPLANATION OF THE ASYMMETRIC RESPONSE OF A DYE DOPED NEMATIC SAMPLE IN A LASER FIELD

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1 Romanan Repors n Physcs, Vol. 56, No. 4, P , 004 EXPLANATION OF THE ASYMMETRIC RESPONSE OF A DYE DOPED NEMATIC SAMPLE IN A LASER FIELD NICOLETA ESEANU 1, I. PALARIE, CORNELIA UNCHESELU 3 1 Deparmen of Physcs, ''Polehnca'' Unversy of Buchares, 313 Splaul Independene, Buchares Faculy of Physcs, Unversy of Craova, 13 A. I. Cuza Sr., Craova 3 Deparmen of Physcs, Techncal Unversy of Cvl Engneerng of Buchares, 1-14 Ave. Lacul Te, Buchares ncoleaeseanu@yahoo.com (Receved July, 004) Absrac. We presen a model for he asymmerc opcal response of a sandwch ype dye doped nemac cell n a pump-probe expermen. The dfferen nal anchorng srengh on he wo polymer surfaces and he absorpon of he Ar + laser beam explan he me dependen roang angle of he nemac drecor. Key words: lqud crysals, dyes, molecular reorenaon, asymmerc response. Explanaon - asymmerc response of dye doped nemacs INTRODUCTION Dye doped nemac lqud crysals (DDNLCs) exhb enhanced orenaonal nonlneary, up o hree orders of magnude larger han n pure lqud crysals [1]. For hese reason new opporunes of applcaon appear such as opcal sorage and mage processng. The explanaon proposed by Janossy [] s relaed o he formaon of measable saes n he absorpon process: an exced elecronc srucure (sngle or rple) or a measable confguraon of he nuclear posons (lke he cs-confguraon of an azo-dye). A dye molecule n he measable sae neracs wh he surroundng hos NLC molecules dfferenly han a

2 750 Ncolea Esanu, I. Palare, Cornela Uncheselu dye molecule n he ground sae. The change of neracon modfes he collecve behavor of NLC and hus causes large opcal effecs. A surface drven reorenaon effec as a resul of he lgh acon on he bulk of a lgh-sensve NLC-azo-dye mxure was repored by Voloschenko and co-workers [3]. The explanaon was he adsorpon of dye phooransformed molecules ono he algnng phoopolymer surface. Sysemac sudes abou he dffracon effcency of a LC-dye mxure revealed he basc role of lgh-nduced adsorpon and desorpon of he dye molecules ono he boundary surfaces (SINE - Surface-Induced Nonlnear Effec) [4-6]. Recenly we repored a sudy of he algnng dye effec on he nemac drecor for varous values of he excng Ar + laser power and we have shown he presence of a hreshold effec [7]. In order o clarfy he role of surface srucure we nvesgaed he dye algnng effec when he excng Ar + laser beam s mpngng hrough he soropc surface (case 1) or hrough he rubbed one (case ). A sgnfcan asymmery has been observed [8]. In hs paper we derve he roang angle of he nemac drecor on he soropc surface as funcon of he rradaon me n hese wo cases. By akng no accoun he dfferen nal anchorng srengh and he aenuaon of he Ar + laser beam (due o dye molecules) we explan he asymmerc opcal response. We conclude ha he rradaon of he sample hrough he soropc surface generaes a fas roang of he nemac drecor. EXPERIMENTAL RESULTS The expermens were realzed usng a sandard sandwch glass cell flled wh a mxure of NLC 4'-n-penyl-4-cyanobphenyl (5CB, clearng pon T = 35. C) and azo-dye mehyl red (MR) as a dopan (wegh concenraon 1%). Ther srucural formulas are gven n Ref. [7]. Mehyl red s a phoosensve dye [9]; s nrnsc phoochemcal processes are: rans-cs somerzaon and bleachng under he llumnaon n he absorpon band. The vsble absorpon specrum of 5CB+MR s repored n Ref. [3]. The hckness of he cell was approxmaely 19 µm (obaned by Mylar spacers). The nner surfaces of he glasses composng he cell were covered wh a hn polymerc flm (for deals, see Ref. [7]). One of he surfaces was

3 Response of a dye doped nemac sample 751 rubbed n one drecon provded a srong unaxal anchorng of NLC molecules on he surface. Ths orened surface (also named passve surface, S pas ) mposed he planar orenaon of he nemac drecor for he whole cell n he nal sae. The oher surface (conrol surface, S con ) s quassoropc. The expermenal seup s shown n Fg. 1. The cell was placed normally o he excng Ar + laser beam (wavelengh λ = nm; power P exc = 15 mw). The polarzaon of he exced beam was esablshed by he polaryzer P, he elecrc vecor beng horzonal. The probe He-Ne laser beam (wavelengh λ = 633 nm; power P es = 1 mw) was focused by a lens L n he rradaed area. Fg. 1 - Expermenal se-up The wo laser beams mus be superposed n he same area of he NLC cell bu he esng one mus be smaller. The polarzaon of he He-Ne beam was vercal and he cell was placed wh he rubbng drecon parallel wh he elecrc vecor of he probe beam. Afer havng passed hrough he cell he probe beam was devaed by a beam-spler (BS) and wen hrough he analyzer A. The ransmsson drecon of he analyzer was horzonal. The probe beam nensy measuremen was performed by an opcal fber (OF) conneced wh an Ocean Opcs specromeer S000 (S) and a compuer. We have recorded he nensy of he ransmed probe beam as a funcon of he rradang me n boh cases (Fg. ). As he excng laser radaon eners he rubbed polymer surface he nensy of ransmed probe beam ncreases slowly as a funcon of he rradaon me and s

4 75 Ncolea Esanu, I. Palare, Cornela Uncheselu values are smaller han hose from case when he excng laser beam eners he quas-soropc surface. Fg. - Transmed nensy versus rradaon me. EXPLANATION OF THE ASYMMETRIC RESPONSE The asymmerc response, namely he dfferences beween expermenal opcal sgnal when he excng laser radaon s mpngng hrough he quas-soropc surface or hrough he orened one, could be explaned by he dfferen nemac drecor confguraons durng rradaon. In case 1 he reorenaon of NLC molecules occurs n he NLC bulk from he quas-soropc surface o he orened one. The resul s a wsed confguraon of he nemac drecor from he laser - nduced orenaon o he orenaon mposed by he easy axs of he rubbed polymerc flm. The & lnearly polarzed He-Ne probe beam havng he elecrc vecor Ees parallel o he rubbng drecon of he orened surface s mpngng hrough one. Le I he nensy of he He-Ne laser beam. The elecrc vecor follows he nemac drecor hrough he whole cell (Maugun lm [10] s fulflled n our expermens) and roaes n he plane of he LC cell wh an angle θ 1 (passng hrough he reorened DDNLC cell). Afer he reflecon on BS ~ hs laser beam has a new drecon of polarzaon makng he angle θ 1 wh he vercal drecon. The relaon gves hs angle:

5 Response of a dye doped nemac sample 753 ~ r gθ1 = gθ1 (1) r Here r and r are he Fresnel coeffcens correspondng o he ransversal elecrc and ransversal magnec cases. The nensy of he refleced beam s: I R θ ) = I ( r sn θ + r cos ) () ( 1 1 θ1 The analyzer A (wh a horzonal drecon of ransmsson) allows only he horzonal componen of he refleced wave o pass: ~ I ( θ1) = I R ( θ1)sn θ1 (3) By usng he Eqs. (1-3) we oban he ransmed nensy by he analyzer a he rradaon momen : I ) = I r sn θ ( ) (4) r1( 1 In case, as he Ar + laser beam ener he rubbed surface, a new local order of he nemac molecules appears, n planes parallel o he surfaces. Due o he presence of he dye molecules, he NLC molecules end o oren hemselves parallel o he Ar + laser beam polarzaon. Sarng he rradaon, he nemac order mposed by he bulk drecor s no he same on boh surfaces; he anchorng energy s larger on he rubbed one hus prevenng a sgnfcan angular gldng of he nemac drecor n hs plane. On he oher surface he anchorng energy does no mpose an easy axs (degeneracy) and he nemac drecor follows he bulk orenaon. So, hs s agan a wsed nemac confguraon as n case 1 bu he probe beam mpnges he surface a a ceran angle θ beween he polarzaon drecon (vercal) of He-Ne laser beam and he nemac drecor on he quassoropc surface. Le I he nensy of he He-Ne laser beam, lnearly polarzed a vercal drecon, ncden on he cell hrough he quas-soropc surface. We denoe by θ ( ) he roang angle of he nemac drecor on hs surface a rradaon me. A he enrance n he sample, he He-Ne laser beam can be decomposed n wo waves: he exraordnary wave (e. w.), polarzed parallel wh he nemac drecor on hs enran surface, havng he

6 754 Ncolea Esanu, I. Palare, Cornela Uncheselu nensy I cos θ ( ) and he ordnary wave (o. w.), polarzed orhogonal on he local nemac drecor, havng he nensy I sn θ ( ). A he ex from he sample (a he rubbed polymer surface), he e. w. s vercally polarzed whle he o. w. s horzonally polarzed. Afer he reflecon on BS he e. w. s blocked off by he analyzer (s ransmsson drecon s horzonal). The o. w. s refleced on BS, s nensy becomes I sn ( r θ ) and passes hrough he analyzer whou aenuaon. The ransmed nensy n hs case s: I ) = I r sn θ ( ) (5) r ( We remark ha he relaons (4) and (5) have he same form. The opcal fber receves only a fracon k < 1 from he nensy ransmed by he analyzer. I also receves he dark sgnal I dark, whch s obaned by swchng off he He-Ne laser beam. The nensy I r () receved by he opcal fber a he rradaon me, n boh cases, has he followng form: I ( ) = ki r sn θ ( ) + I (6) r dark The presence of he facor ki n he relaon (6) mposes a supplemenary measuremen o be made before rradang he sample. Now he He-Ne laser beam propagaes hrough he sample as an exraordnary wave. Afer he reflecon on BS, s nensy becomes I r. The recorded nensy I γ (when he analyzer s roang by he angle γ wh respec o he horzonal drecon) s: I = ki sn γ + (7) γ r I dark By usng he Eqs. (6) and (7), he me dependen roang angle θ () can be calculaed: sn θ ( ) r I r ( ) I dark sn γ (8) r Iγ I = dark

7 Response of a dye doped nemac sample 755 The measuremens were performed, for boh cases, under he same condons (power of he Ar + laser beam, γ, I γ, I dark ), bu he nensy of he ransmed probe beam a rradaon momen n case s smaller han ha n case 1, I r ( ) < ( ). The relaon (8) allows he compuaon of I r 1 θ ( ) and θ ( ) (see Fg.3). 1 Fg. 3 - Roang angles θ ( ) and θ ( ) versus rradaon me. 1 We conclude ha he roang angle of he nemac drecor whn he quas-soropc surface n he case when he excng laser beam eners hs surface s greaer han he correspondng value obaned by rradang he rubbed surface, θ 1( ) > θ ( ). The Ar + laser beam s aenuaed durng s propagaon hrough he cell flled wh 5CB+MR due o he MR molecules, whch absorb hs radaon. The anchorng energy on he rubbed surface s large prevenng a roaon of he nemac drecor n hs plane. On he quas-soropc surface, he anchorng energy does no mpose an easy axs and he nemac drecor roaes under laser rradaon. In case 1, he nensy of Ar + laser beam has a maxmum on he quassoropc surface (enrance surface) and a mnmum on he rubbed surface (ex surface). In case, he maxmum nensy of he pump beam s on he rubbed surface (whch s he enrance surface n hs case) and he mnmum nensy s on he quas-soropc surface. The dfferen values of he pump beam nensy on he quas-soropc surface n hese wo cases are responsble for he asymmerc response of sample.

8 756 Ncolea Esanu, I. Palare, Cornela Uncheselu CONCLUSIONS The values of he ransmed probe lgh nensy passng hrough a DDNLC cell n case (he excng laser beam eners he orened surface) are smaller han hose from case 1 (he excng laser beam eners he quassoropc surface) and he me dependences are dfferen (asymmerc response). As a consequence he calculaed values of he roang angle of he nemac drecor whn he quas-soropc surface show an asymmery oo. The explanaon of hs effec s based on: dfferen nemac drecor confguraon durng rradaon, absorpon of he Ar + laser beam n he DDNLC sample and dfferen anchorng srengh on he wo surfaces. Acknowledgemens. The auhors acknowledge fruful dscussons wh Professor Anca-Luza Alexe-Ionescu and Professor Andre Th. Ionescu. REFERENCES 1. I. JANOSSY, A. D. LLOYD, Mol. Crys. Lq. Crys. 03, 77 (1991). I. JANOSSY, Phys. Rev. E 49, 957 (1994); I. JANOSSY, J. Nonln. Op. Phys. & Ma. 8, 361 (1999); I. JANOSSY, elecronc-l C Comm. (004) (hp:// 3. D. VOLOSHCHENKO, A. KHYZHHYAK, Y. REZNIKOV, V. RESHETNYAK, Jpn. J. Appl. Phys. 34, 566 (1995) 4. F. SIMONI, L. LUCCHETTI, D. E. LUCCHETTA, O. FRANCESCANGELI, Op. Express 9, 85 (001) 5. L. LUCCHETTI, D. E. LUCCHETTA, O. FRANCESCANGELI, F. SIMONI, Mol. Crys. Lq. Crys. 375, 641 (00). 6. L. LUCCHETTI, M. DI FABRIZIO, O. FRANCESCANGELI, F. SIMONI, J. Nonln. Op. Phys. & Ma. 11, 13 (00); dem, Op. Comm. 33, 417 (004). 7. N. ESEANU, C. UNCHESELU, I. PALARIE, B. UMANSKI, Romanan Repors n Physcs 55, no. 3, 417 (003) (hp://alpha1.nfm.ro/rrp/003_55_3/eseanu) 8. N. ESEANU, C. DASCALU, I. PALARIE, C. UNCHESELU, U.P.B. Sc. Bull., Seres A, Vol. 65, no. 1-4, 151 (003)

9 Response of a dye doped nemac sample H. RAU, n Phoochemsry and Phoophyscs, ed. F. J. Rabeck (CRC, Boca Raon, 1990), Vol.. Chap P. G. DE GENNES, J. PROST, The Physcs of Lqud Crysals (Clarendon Press, Oxford, 1993) pp