CHAPTER - I. Nonlinear Optics: Materials and Importance

Transcription

1 CHAPTR - I Nonlnar Opcs: Marals and Imporanc. Inroducon Nonlnar opcal marals play a pvoal rol n h fuur voluon of nonlnar opcs and s mpac n chnology and ndusral applcaons ar xclln. Ths chapr provds a gnral rvw on h fld of nonlnar opcs (NLO and nonlnar opcal marals. Nonlnar Opcs (NLO ook s brh n 875, wh h commncmn of h obsrvaons on a quadrac lcrc fld nducd chang n h rfracv ndx of CS by Krr, known as Krr ffc []. Ths was followd by h obsrvaon of a smlar bu lnar lcrc fld ffc n quarz known as h Pockls ffc []. Howvr, n h fld of opcs, nonlnar ffcs bcam a subjc of nrs only afr h nvnon of h lasr. Afr h nvnon of lasr, many classcal xprmns n NLO such as xprmns on Scond Harmonc Gnraon (SHG by Frankn al, [] n 96, Sum Frquncy Gnraon (SFG by Bass al [3] n 96 and opcal rcfcaon by Bass al [4] n 96 wr prformd. Snc hn, NLO had bcom a rapdly growng fld n Physcs. Nonlnars ar found vrywhr n opcal applcaons and a prsn, nonlnar opcal proprs of many opcal marals wh sgnfcan rlvanc o chnologcal and opcal applcaons hav bn found [5]. Vbraonal spcroscopc chnqus combnd wh quanum chmcal compuaons hav rcnly bn usd as an ffcv ool for h nvsgaon of h srucural faurs rsponsbl for h nonlnar opcal proprs. In h prsn scnaro, whr a lo of mphass s dvod o h growh and srucural lucdaon of nonlnar opcal marals, hs hss prsns a dald nvsgaon of h nonlnar opcal proprs of cran marals usng h vbraonal spcroscopc chnqus add by horcal mhods. Charg ransfr

2 nracons whn h molculs s a rqurmn for h molcul o b nonlnarly acv whch ar nvsgad hrough chnqus of Fourr Transform Infrard (FT- IR and Fourr Transform Raman add by Dnsy Funconal Thory (DFT compuaons. Th am of h prsn work s o opmz gomry, lucda srucur-propry rlaonshp, analyz scond-ordr nonlnar bhavor and valua srucural and lcronc ffcs of h chosn marals. Th nvsgaon nally sars wh h possbly of obanng h srucural nformaon on h chosn marals. Furhrmor, h nvsgaons ncompass h confrmaon of nonlnar opcal acvy of h chosn marals by hyprpolarzably calculaons and Scond Harmonc Gnraon (SHG masurmns.. Rsarch Movaon Currnly, rsarch on organc nonlnar opcal (NLO marals s srongly aggravad du o h dmand of hghr daa ras n prospc opcal communcaon chnologs. Organc conjugad molculs ar spcal canddas wh nonlnar opcal proprs bcaus of hr hgh scond and hrd ordr rsponss [6]. Th π- conjugad sysms havng a donor and an accpor show larg NLO rsponss whch nhanc h sudy of hs marals. Alld wh h dlocalzd π-lcrons, hy xhb larg and ulra-fas NLO rsponss and as such h srucural and lcronc ffcs on h nonlnar opcal marals provd a wd scop for addonal xpanson of h cd ambon... Movaon o Op Organc Marals wh Nonlnar Opcal Applcaons In currn rsarch, nonlnar opcs has mrgd as h mos aracv fld of sudy du o s applcaons n aras such as opcal modulaon, opcal swchng, frquncy shfng and opcal daa sorag for h dvlopmn of chnologs n lcommuncaons and nformaon procssng and as such, nonlnar opcal

3 procsss ar of vas scnfc ponal [7-5]. Marals xhbng nonlnar opcal proprs form h bass of nonlnar opcs and so h dvlopmn of marals wh sgnfcan nonlnar opcal proprs plays an mporan rol n h prsn scnaro. Dsgn of marals possssng scond ordr NLO ffcs rls on h rlaonshp bwn molcular srucur and propry. For h dsgn of h SHG marals h facors ha hav o b akn no accoun ar: ( h maral should b polarzabl ( hr should b asymmrc charg dsrbuon ( should conan a π- conjugad lcron pahway [6]. Morovr h lcronc ffcs such as Hybrdzaon, Backdonaon, Inducon and Hyprconjugaon nfluncs h srucur, racvy and proprs of molcul. Basd on hs characrscs, has bn dcdd o op nonlnar opcal marals wh NLO applcaons for h rsarch purpos... Movaon for h Chosn xprmnal and Thorcal Sudy Boh h xprmnal and horcal chnqus ar ncssary for h accura masurmn of h proprs of NLO marals. Furhrmor, for h sysmac analyss of h proprs of hs NLO marals, dsgnng of opmzaon srags ar also mporan and hs can b nhancd by undrsandng h rlaonshp bwn lcronc and gomrc srucur. In hs rgard, quanum chmcal compuaons whch ar parcularly usful for h drmnaon of molcular proprs and nrpraon of xprmnal daa hos ar naccssbl xprmnally has o b carrd ou. Kpng hs n mnd, ffors wr mad o grow fv nonlnar opcally acv crysals by slow vaporaon chnqu and wr characrzd usng FT-IR, FT-Raman and UV-vsbl spcral chnqus and nrprd by quanum chmcal compuaons usng Gaussan 09 program packag. 3

4 .3 Nonlnar Opcs (NLO Nonlnar Opcs s a branch of opcs ha dals wh h bhavor of lgh n nonlnar mda. I dals wh h nracons of appld lcromagnc flds n varous marals o gnra nw lcromagnc flds alrd n phas, frquncy, amplud or ohr physcal proprs [7]. Th fld of nonlnar opcs fnds a larg nrscon wh horcal and xprmnal physcs, chmsry and ngnrng and has applcaons n all physcal and lf scncs. Nonlnar opcal procsss nclud procsss such as harmonc gnraon, sum and dffrnc frquncy gnraon, nnsy dpndnc of h complx rfracv ndx, lgh-by-lgh scarng and smulad lgh scarng. Ths procsss lad o h applcaons of opcal swchng, opcal powr lmng, mag manpulaon, and mag procssng [0]. For h dsgn and opraon of a wd rang of ponal dvcs nvolvng lgh, undrsandng h concps of nonlnar opcs ar ssnal..3. Concps of Nonlnar Opcs Bfor h advn of lasrs, ransparn opcal marals wr assumd o b ransparn and wr unaffcd by lgh ravlng hrough hm. Th hgh powr of lasr bams mad possbl for h frs m o obsrv, ha, h prsnc of lgh can affc h opcal proprs of h mdum [8]. Accordng o classcal hory of Raman ffc, whn a molcul s pu n a sac lcrc fld, h posvly chargd nucl s aracd by h ngav pol of h fld and h ngavly chargd lcrons ar aracd owards h posv pol of h fld. Ths sparaon of charg cnrs causs an nducd dpol momn o b sup n h molcul and h molcul s sad o b polarzd. Th nducd dpol momn pr un volum s calld polarzaon P and for a lnar, soropc maral polarzaon s lnarly rlad o h srngh of h fld as 4

5 5 P (. whr χ s h lnar suscpbly of a collcon of molculs whch s rlad o h dlcrc consan and rfracv ndx of h maral. Whn a larg lcrc fld s ncdn upon an ansoropc maral, h polarzaon may b xpandd n a Taylor srs o oban P. (. l k j jkl k j jk j j P 3 (.3 whr s h scond ordr nonlnar suscpbly and 3 s h hrd ordr nonlnar suscpbly [9]. Th prsn work mphaszs mor only on h scond ordr nonlnar ffcs. For xampl, consdrng a scond-ordr nonlnar opcal procss, h nracon of h opcal lcrc fld a frquncy ω and amplud ω can b wrn as ( cos( ( (.4 whr 0 s a sac lcrc fld n h mdum wh a nonzro χ. Th scond ordr polarzaon fld n hs mdum s 0 ( ] cos( [ ( ( P = 0 = = ] cos( cos( [ 0 0 (.5 Th rsulng polarzaon fld conans componns oscllang a varous frquncs. In quaon (.5, h frs rm oscllas a ω and dpnds only on h prsnc of h opcal lcrc fld and no h sac fld. Ths ffc s known as

6 Scond Harmonc Gnraon (SHG or frquncy-doublng. Th scond rm oscllas a ω and causs varaon n h rfracv ndx of h mdum and hs ffc s calld h lnar lcro-opc ffc. Th hrd rm s known as opcal rcfcaon and s a convrson of h oscllang lcrc fld o a sac lcrc fld. Ths phnomna ar du o h mxng of lcrc flds of varous frquncs n a nonlnar mdum [0]. For a maral o gnra scond-ordr nonlnar opcal ffc, should possss, a noncnrosymmrc srucur along h drcon of h lcrc fld. Consdr a maral wh nvrson cnr such ha s symmrc n all drcons. If an lcrc fld wh m-dpndn magnud ( cos (.6 s ncdn upon such a mdum, h magnud of h rsulan scond-ordr polarzaon fld wll b P (.7 Du o nvrson symmry, ( P (.8 P (.9 quaons (.8 and (.9 ar ru only whn h polarzaon fld s zro [0]. I ndcas χ s zro for cnrosymmrc mda and only a maral havng noncnrosymmrc srucur wll gnra a nonlnar opcal rspons..4 Imporanc of Nonlnar Opcal Marals Nonlnar opcal marals hav applcaons n aras such as opcal modulaon, opcal swchng, frquncy shfng and opcal daa sorag for h dvlopmn of chnologs n lcommuncaons and nformaon procssng and 6

7 as such hy hav rcvd much anon n rcn yars [0,]. In h las dcad, xnsv rsarch has provd ha, organc crysals ofn possss a hghr dgr of opcal nonlnary han hr norganc counrpars [,3]. Hnc nowadays, organc nonlnar opcal marals hav bn nvsgad xnsvly [4,5]. Mor ovr organc marals hav h advanag of hgh nonlnary, srucural dvrsy or flxbly and rapd rspons n lcro-opc ffc. Anohr advanag of organc marals s ha hr proprs can b opmzd by modfyng h molcular srucur usng molcular ngnrng and chmcal synhss [6]. From quaons (.8 and (.9 s clar ha, for a maral o hav srong scond-ordr NLO proprs, mus crysallz n a noncnrosymmrc srucur o hav a nonzro χ. Anohr rqurmn s ha h maral should hav a larg molcular hyprpolarzably (β. Th scond-ordr molcular nonlnary can b nhancd by larg dlocalzd π-lcron sysms wh srong donor and accpor groups [7-9]. Snc h xsnc of larg molcular hyprpolarzably s h bass of srong Scond Harmonc Gnraon (SHG, organc molculs wh long conjugaon sysms ha usually xhb larg β valus ar cranly candda molculs for NLO marals [30]. Th larg valu of h frs hyprpolarzably, β, s assocad wh h nramolcular charg ransfr rsulng from an lcron cloud movmn hrough a π conjugad framwork from lcron donor o lcron accpor groups. Wh an ncras n conjugaon lngh bwn h donor and h accpor, h magnuds of molcular polarzably and hyprpolarzably coffcns ar found o ncras suprlnarly. Larg molcular hyprpolarzabls rqur hghly polarzabl lcrons (o b abl o rspond o an lcrc fld and asymmry on h molcular lvl. Larg polarzably s achvd hrough 7

8 conjugaon, whr π-lcron bonds bwn unsaurad aoms n organc compounds ar dlocalzd and asly movd by lcrc flds [30]. Undr h prurbaon of an xrnal lcrc fld, h conjugad π- lcron sysm provds a pahway for h nr lngh of conjugaon. Fconalzaon of boh nds of h π conjugad sysm wh appropra lcron donor and accpor groups can ncras h asymmrc lcronc dsrbuon n hr or boh h ground and xcd sas, hus ladng o an ncrasd opcal non-lnary [7]..5 Rvw of Lraur Th dsgn of ffcn organc marals for applcaons n nonlnar opcal ffc s basd on asymmrc polarzaon, nducd by lcron donor and lcron accpor groups on hr sd of h molcul a appropra posons n h molcular sysms. Du o hr commrcal mporanc n h flds of opcal communcaon, sgnal procssng, snsng and nsrumnaon, nonlnar opcal marals capabl of producng scond harmonc gnraon, hav bn sudd [3,3]. In ordr o undrsand h mcroscopc orgn of nonlnar bhavor of organc NLO marals, consdrabl horcal and xprmnal nvsgaons hav also bn mad [33,34]. Ths ffors hav brough s frus n appld aspcs of nonlnar opcs. To oban nformaon on h bhavor of normal mods, h ffc of varous yps of nrmolcular forcs and h naur of hydrogn bondng on hs nonlnar opcal marals, IR and Raman spcroscopy has bn usd. Informaon abou lcronc ransons can b achvd by h analyss of UV-vsbl spcrum [35]. Snc aoms or molculs absorb UV-vsbl radaon a dffrn wavlngh, spcroscopy s ofn usd n physcal and analycal chmsry for h dnfcaon of subsancs hrough h spcrum md from or absorbd by hm. 8

9 For h compuaon of molcular srucur, vbraonal wavnumbrs and nrgs of molculs, ab no communy has accpd Dnsy funconal hory (DFT sudy as a popular pos-hf (Harr Fock approach [36] bcaus of s ffcncy and accuracy wh rspc o h valuaon of a numbr of molcular proprs. DFT calculaons ar also usful for ganng nsgh no h nonlnar opcal proprs of conjugad molculs. In a srs of rcnly prformd ovrlay calculaons, a forc fld for som conjugad molculs has bn consrucd [37-4] and full normal coordna analyss has bn prformd by mans of h MOLVIB program wrn by T. Sundus [4,43]. Ths ncluds ransformaon of h DFT forc fld o naural nrnal coordnas and scalng of h quadrac forc fld accordng o Pulay s mhod [44,45] by a s of scal facors rfnd n a las-squar procdur whch hlps o achv br agrmn bwn h calculad and obsrvd frquncs. Basd on hs nformaons, n h prsn work, h dnsy funconal hr-paramr hybrd modl (DFT/B3LYP a h 6-3G(d, cc-pvdz and cc-pvz bass s lvls hav bn adopd o calcula h proprs of h grown molculs and h normal mods ar drvd by mployng MOLVIB program. Cnnamc acd, a drvav of phnylalann s xnsvly sudd bcaus of s vry spcfc srucur. In h cnnamc acd molcul, h carboxylc group s sparad from h aromac rng by a doubl bond. I causs conjugaon bwn h C=C bond and h π-lcron sysm. In ordr o sudy h xsnc of h hydrogn bonds n dmr, h polarzd IR spcra of cnnamc acd has bn sudd by H.T. Flakus and M. Jabojska [46]. Tan-Jy Hsh al [47] had sudd h molcular srucur of cnnamc acd usng h B3LYP/6-3G(d. Th ffc of alkal mals (L,Na,K,Rb and Cs on h lcronc srucur of cnnamc acd has bn sudd by mans of FT-IR, FT-Raman, Nuclar Magnc Rsonanc (NMR and quanum 9

10 mchancal calculaons [48]. From h analyss mad by Bna Johy al [49] on hyl-3-(3,4-dhydroxyphnyl--propnoa, h charg ransfr nracons hrough h π-conjugad brdg has bn undrsood. Th nflunc of lcronc ffc on h C-H srchng vbraons can also b dnfd. Phhala basd crysals possss hgh pzolcrc coffcns wh acousoopcal nracon [50,5]. Thy ar ofn usd as dffracng crysals bcaus hy ar rlavly nxpnsv, asly clavd, and suffcnly bndabl for a compac spcrograph. Th growh, opcal, hrmal and mchancal proprs of ammonum acd phhala crysal has bn rpord by A. Arun Kumar al [5]. Th nhancmn of h scond harmonc gnraon ffcncy of Sodum hydrogn phhala sngl crysals whn doppd wh znc has bn sudd by N Gol al [53]. S. Sudhahar al hav sudd h ffc of Sm + rar arh on on h srucural, hrmal, mchancal and opcal proprs of Poassum Hydrogn Phhala sngl crysals [54]. Th nramolcular charg ransfr, vbraonal spcroscopc and h Z- scan suds of nonlnar opcal maral Sodum acd phhala hmhydra has bn sudd by D. Sajan al [55]. Th nracon bwn lon par oxygn and h σ* orbal of O-H confrms h xsnc of nrmolcular O-H O hydrogn bondng whch s rvald by h Naural Bond Orbal (NBO analyss. Th hghr dgr of conjugaon bwn h subsun groups has also bn dnfd. Among many organc compounds, chalcon drvavs hav xclln nonlnar opcal proprs. Thy show prfrnc o crysallz as noncnrosymmrc srucurs. Du o hs fac hy hav bn h objcv of svral xprmnal and horcal suds. Synhss, growh, opcal (UV vs NIR absorpon and hrmal (TGA/DTA characrzaon of 3-Br-4 -mhoxychalcon hav bn carrd ou by P.S. Pal al [56]. V. Shgar al [57] hav rpord h 0

11 growh and characrzaon of an ffcn π-conjugad ponal push pull NLO chromophor, -(4-mhoxyphnyl-3-(3,4-dmhoxyphnyl--propn--on. Jos P. Abraham al [58] hav carrd ou an nvsgaon on h nramolcular charg ransfr of hs molcul by usng IR and Raman spcroscopy and Dnsy Funconal Thory (DFT compuaons. Th lcronc ffcs such as nducon and backdonaon on h mhyl hydrogn aoms hav bn dnfd from h vbraonal analyss. FT-IR, FT-Raman and UV-vsbl spcral and h hrmal analyss of -(4- Amnophnyl-3-(3,4-dmhoxyphnyl-prop--n--on hav bn carrd ou by Lynn Josph al [59]. Th nramolcular charg ransfr, dlocalzaon and h conjugaon ffcs ar dscussd and h hrmal bhavor has bn characrzd by DSC and TGA/DTA mhod. Pcrc acd forms sabl pcras wh varous organc molculs hrough π- bondng or hydrogn bondng. S.R. Thlagavah al [60] hav grown Glycnum Pcra Mono Glycn crysal and masurd s Scond Harmonc Gnraon (SHG ffcncy. Alhough hs crysal crysallzs n h cnrosymmrc spac group, xhb SHG ffcncy. Th horcal and vbraonal spcra of 8- hydroxyqunolnum pcra hav bn sudd by A. Basŏglu al [6]. A nonlnar opcal crysal from h amnoacd famly, l-thronnum pcra has bn grown by S. Naarajan al [6] and s srucural, spcroscopc and nonlnar opcal suds hav bn carrd ou. Th prsnc of N-H..O hydrogn bondng has also bn obsrvd. Vbraonal spcral suds and nonlnar opcal proprs of L-lucn L- lucnum pcra wh h hlp of Dnsy Funconal Thory (DFT has bn carrd ou by Samh Gudara al [63]. Th xsnc of nrmolcular hydrogn bondng on hs sysm has also bn dscussd.

12 π-brdgd donor-accpor-donor sysms ar canddas for hr nonlnar opcal proprs. K.J. Akrman and O.Q. Munro [64] hav carrd ou h X-ray crysallographc and dnsy funconal hory sudy of (3Z-4-(5-hyl-sulfonyl-- hy-droxy-anlno pn-3-n--on and (3Z-4-(5-r-buyl--hy-droxy-anlno pn- 3-n--on. Thy hav lucdad h prsnc of nr and nramolcular hydrogn bondng on hs compounds. A. Collas and F. Blockhuys [65] hav grown (-- (,4,6-Trmhoxyphnylpn--n-3-on compound and carrd ou X-ray dffracon suds. 4-(5-Bromo--hydroxyphnyl bu-3-n--on crysals hav bn grown by A. Zonouz al [66]. From h analyss has bn sablshd ha h molcular srucur and horcal calculaons suppor h hypohss, ha, h conjugaon n a molcul s h drmnng facor for h poson of h nol H aom. Sfan llngr al [67] hav prpard a famly of mul-hrocycl donor accpor donor lchlc conjugad olgomrs dsgnd for wo-phoon absorpon and msson n h nar-nfrard (nar-ir and nvsgad h rlaonshp bwn hr spcral, srucural, and lcrochmcal proprs.