NONLINEAR OPTICAL CHARACTERIZATION OF ORGANIC MATERIALS. M.J. Soileau, T.H. Wei, M. Sheik-bahae, D.J. Hagan, Martine Sence, and E.W.

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1 Mol. Cryst. Liq. Cryst., 1991, Vol. 27, pp Reprints available diretly from the publisher Photoopying permitted by liense only 1991 Gordon and Breah Siene Publishers S.A. Printed in the United States of Ameria NONLINAR OPTICAL CHARACTRIZATION OF ORGANIC MATRIALS M.J. Soileau, T.H. Wei, M. Sheik-bahae, D.J. Hagan, Martine Sene, and.w. Van Stryland CROL, Center for Researh in letro-optis and Lasers Physis and letrial ngineering Departments University of Central Florida, Orlando, Florida We report a sensitive single beam tehnique for measuring both the nonlinear refrative index and nonlinear absorption oeffiient for a wide variety of materials. We desribe the experiment and present a brief analysis inluding ases where nonlinear refration is aompanied by nonlinear absorption. In these experiments the transmittane of a sample is measured through a finite aperture in the far-field as the sample is moved along the propagation path () of a foused Gaussian beam. The sign and magnitude of the nonlinear refration are easily dedued from suh a transmittane urve (Z-san). mploying this tehnique a sensitivity of better than.>./3 wavefront distortion has been ahieved using pioseond frequeny doubled Nd:YAG laser pulses. In ases where nonlinear refration is aompanied by nonlinear absorption, it is possible to separately evaluate the nonlinear refration as well as the nonlinear absorption by performing a seond Z-san with the aperture removed. We demonstrate this method for a solution of hloro-aluminum-phthaloyanine at 532 nm where exited state absorption is present and the nonlinear refration is positive. We have reently developed a sensitive single beam tehnique for measuring both nonlinear refration and nonlinear absorption. 1 We refer to this tehnique as a Z-san. This method is rapidly gaining use for measuring eletroni nonlinearities (eg. n ) 2 and nonlinear absorption (eg. two-photon absorption oeffiients p or exited-state ross setions o) in materials from semiondutors to glasses to organis. We review this tehnique and the analysis of Z-san data to show how nonlinear refration an be separated from nonlinear absorption. We then apply this tehnique to a solution of hloro-aluminum-phthaloyanine (CAP) dissolved in methanol whih we have previously used for passive opallimiting.3 APRTUR 2 Fig.! The Z-san experimental apparatus in whih the ratio D2/Dl is reorded as a funtion of the sample position. 97

2 98 M.J. SOILAU T AL. Using a Gaussian laser beam in a tight fous limiting geometry, we measure the transmittane of a nonlinear medium through a finite aperture plaed in the far field as a funtion of the sample position () measured with respet to the foal plane as shown in Fig. 1. If only nonlinear refration is present the following example qualitatively explains how suh a trae (Z-san) yields n 2 First assume n 2 <. As the sample is moved toward fous the inreased irradiane leads to a negative tensing effet whih tends to ollimate the beam, thus. inreasing the aperture transmittane. With the sample on the + side of fous, the negative lensing effet tends to augment diffration and the aperture transmittane is redued. The approximate null at :=O is analogous to plaing a thin lens at fous whih results in a minimal far field pattern hange. For still larger + the irradiane is redued and the transmittane returns to the original linear value. A positive nonlinearity results in the opposite effet, ie. lowered transmittane for the sample at negative and enhaned transmittane for positive. We refer to the transmittane hange from peak to valley as ATp-v Theoretial Z-san urves for both positive and negative nonlinear refration are shown in Fig. 2 for a peak on axis, time averaged phase distortion.a (phase delay or advane) of A/25. For example, for an instantaneous positive nonlinearity this phase distortion is given by; n 2 t.w o- - " t.nl = " 1IL = " 2...!1. 2 L (I) where L is the sample length, I the irradiane (MKS) and the field (CGS), and 1 and n 2 are related by n 2 (esu)=(nof4>')1(m 2 /W), where (m/se) is the speed of light in vauum :; s 1. "- --- h.<fl = ±.25. " ' +.96 ' -.92 '----'----'----..L------' -6-3 Z/ZO 3 6 Fig. 2 Theoretial output of a Z-san for a) negative n 3 and b) for a positive n 3 We an define an easily measurable quantity t. T p-v as the differene between the normalied peak (maximum) and valley (minimum) tmnsmittanes. The variation of this quantity as a funtion of A, as alulated for various aperture sies is found to be almost linearly dependent on.6. Based on a numerial fitting, the following relationship an be used to determine All from the Z-san to within a ±2% auray; t.tp-v "'.46(1-S) 25 lt.w l for lt.w l;;,., (2) where S is the transmittane of the aperture in the linear regime. If our experimental apparatus and data aquisition systems are apable of resolving transmission hanges AT_p-v of,;j%, we will be able to measure phase hanges orresponding to less than /25 wavefront distortion. If the aperture is removed (ie. S=I in q. 2) the Z-san is no longer sensitive to nonlinear refration. A Z-san then results in no signal unless nonlinear absorption is present in whih ase a symmetrial urve about the foal position is obtained. If both

3 NONLINAR OPTICAL CHARACTRIZATION OF ORGANIC MATRIALS 99 nonlinear refration and nonlinear absorption are present simultaneously, an analysis of the "open" aperture (S= l) and "losed" aperture (S<l) experiments an be used to separately determine the nonlinear refration and nonlinear absorption. The separation and evaluation proess is simple: divide the "losed" aperture normalied Z-san by the one with open aperture. The result is a new Z-san where T p-v agrees to -within ±1% of that obtained from a purely refrative Z-san. This division proess will give a faithful value for n 1 as long as the nonlinear absorption is not dominant In pratie we have found this method to work quite well provided the urve obtained by division looks like Fig. 2 (ie. appears antisymmetri). We have heked it by numerially alulating the results of Z-sans inluding both nonlinearities. 1 We define {! as the two-photon absorption (2PA) oeffient and u as the exited state absorption (SA) oeffiient. For small nonlinear absorption (ie. Tp-v<O.l) the following approximation an be used to determine p or u from the open aperture Z-san; Tp-v"' -&l (1-R)L.rr, for 2PA, <1 Tp-v"'- 4 hw F (1-R)L.rr, for SA, where Lerf=(l-e 1 L)ja, with a the linear absorption oeffiient." 4 5 Here fo(w/m") is the peak on axis irradiane assuming a temporally arid spatially Gaussian shaped pulse, and F (J/m1) is the on axis fluene assuming a Gaussian spatial beam. In the ase of CAP we found that the nonlinear absorption was due to SA and not 2PA. We deterotined this by monitoring the nonlinear absorption for different pulsewidths of 29 ps and 61 ps (FWHM). The same fluene for the different pulsewidths gave the same nonlinear absorption as expeted from q. 3b for exited state absorption. quation 3a for 2PA predits that the same irradiane would give the same absorption. 1.2 u I-.9 " ID o.75 - (3a) (3b) Fig. 3. Open aperture Z-san on CAP at 3.18,J and theoretial fitting (solid line) with a=l.sxi-17 m1 at a onentration of I.Oxi-3 moles/liter. The open aperture Z-San, along with a numerial fit, is shown in Fig. 3 for an input energy of 3.2,J at 532 nm using "'27 ps (FWHM) pulses. Note that for Fig. 3 the T>O.l and q. 3b is not appliable.' The parameters used are w =28 JJm (HW1/e'M), R=.5, L=.2 em, a=l.42 m-1 (or 139 m-1 per mole per liter) as deterotined from the 68% measured linear transmittane for a onentration of l.2xl- 3 moles per liter. This numerial fit gives a value for u of ad.8xl-t7m1.

4 1 M.J. SOILAU T AL. 1.1 w ' u.95 f-.8 " w.65 / ""'\ 9, d' )' ox I) xxt!lx ""'>< Fig. 4. Open aperture Z-san (ross) and losed aperture Z-san (square) results for CAP presented as the normalied transmittane versus position [mm]. Figure 4 shows both the open and losed aperture (S=.4) Z-san results for CAP for an input energy of 4.8 p.j. Figure 5 shows the results of dividing the losed aperture Z-san by the open aperture Z-san, thus, giving the nonlinear refrative ontribution. The value obtained for n 3 is, n 3 2.lxi- 12 esu. We found, however, that this nonlinear refration is assoiated with the real exitation of the singlet state and is, therefore, also fluene dependent. Thus, this norilinear refration is not a true x( 3 ) effet but is assoiated with a sequential x( 1 l:x( 1 ) proess w u 1.15 "' f- 1. " w.85 " '-"' Fig. 5 The results of the division of the urves in Fig. 4 (ie. losed aperture results divided by open aperture results). In onlusion we have demonstrated a simple sensitive single beam tehnique for measuring both nonlinear absorption and nonlinear refration. The sign of the nonlinear refration is also obtained. We give simple relations that allow the refrative index to be obtained diretly from the Z-san data without resorting to omputer fits. We have applied

5 NONLINAR OPTICAL CHARACTRIZATION OF ORGANIC MATRIALS 11 this tehnique to several materials displaying a variety of. nonlinearities on different time sales. Here we have presented data on hloro-aluminum-phthaloyanine that gives the exited state absorption ross setion and the nonlinear refrative index, all for.53 #ffi pioseond pulses. It is expeted that this method will be a valuable tool for experimenters searhing for highly nonlinear materials. ACKNOWLDGMNT We gratefully aknowledge the support of the Army Researh Offie, the National Siene Foundation grant #CS , the support of DARPA/CNVO and the Florida High Tehnology and Industry Counil. We also thank Dan Coulter and J Perry of the Jet Propulsion Laboratory for providing samples and their expertise on materials. I. M. Sheik-bahae, A.A. Said, and.w. Van Stryland, "High Sensitivity, Single Beam n, Measurements", Opt. Lett. 14, (1989). 2. M. Sheik-bahae, A.A. Said, T.H. Wei, D.J. Hagan, and.w. Van Stryland, "Sensitive Measurement of Optial Nonlinearities Using a Single Beam", Jour. Quan. let., Q 26, (199). 3. D.R. Coulter, V.M. Miskowsky, J.W. Perry, T.H. Wei,.W. Van Stryland, and D.J. Hagan, Proeedings of SPI, Materials for Optial Swithes, Isolators, and Limiters, 115, (1989). 4..W. Van Stryland, H. Vanhereele, M.A. Woodall, M.J. Soileau, A.L Smirl, S. Guha, and T.F. Boggess "Two-Photon Absorption, Nonlinear Refration. and Optial Limiting in Sentiondutors", Opt. ng. 24, (1985). 5. T.F. Boggess, S.C. Moss, l.w. Boyd, and A.L. Smirl, Opt. Lett. 9, 291 (1984).

Supplementary information for: All-optical signal processing using dynamic Brillouin gratings

Supplementary information for: All-optial signal proessing using dynami Brillouin gratings Maro Santagiustina, Sanghoon Chin 2, Niolay Primerov 2, Leonora Ursini, Lu Thévena 2 Department of Information