The intensity dependent optical properties of 4-dialkylamino-4\u27nitro-stilbene polymer channel waveguides were measured at 1319 nm with a pulse modulated Mach-Zehnder interferometer to be n2 = 0.8 x 10(-13) cm2/W and beta2 \u3c 0.08 cm/GW. This material is promising for all-optical switching at 1319 nm because it satisfies both the W and T figures of merit

Horsthuis, Wiinfried H. G.

Kim, Dug Y.

Möhlmann, Guus R.

Otomo, Akira

Stegeman, George

Sundheimer, Michael

English

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University of Central Florida (UCF): STARS (Showcase of Text, Archives, Research & Scholarship)

University of Central Florida STARS Faculty Bibliography 1990s Faculty Bibliography 1-1-1993 3Rd-Order Nonlinearity Of 4-Dialkylamino-4'Nitro-Stilbene Wave-Guides At 1319 Nm Dug Y. Kim University of Central Florida Michael Sundheimer University of Central Florida Akira Otomo University of Central Florida George Stegeman University of Central Florida Wiinfried H. G. Horsthuis See next page for additional authors Find similar works at: https://stars.library.ucf.edu/facultybib1990 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 1990s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Kim, Dug Y.; Sundheimer, Michael; Otomo, Akira; Stegeman, George; Horsthuis, Wiinfried H. G.; and Möhlmann, Guus R., "3Rd-Order Nonlinearity Of 4-Dialkylamino-4'Nitro-Stilbene Wave-Guides At 1319 Nm" (1993). Faculty Bibliography 1990s. 809. https://stars.library.ucf.edu/facultybib1990/809 Authors Dug Y. Kim, Michael Sundheimer, Akira Otomo, George Stegeman, Wiinfried H. G. Horsthuis, and Guus R. Möhlmann This article is available at STARS: https://stars.library.ucf.edu/facultybib1990/809 Appl. Phys. Lett. 63, 290 (1993); https://doi.org/10.1063/1.110081 63, 290© 1993 American Institute of Physics.Third order nonlinearity of 4-dialkylamino-4’nitro-stilbene waveguidesat 1319 nmCite as: Appl. Phys. Lett. 63, 290 (1993); https://doi.org/10.1063/1.110081Submitted: 11 March 1993 . Accepted: 18 May 1993 . Published Online: 05 August 1998Dug Y. Kim, Michael Sundheimer, Akira Otomo, George Stegeman, Winfried H. G. Horsthuis, and GuusR. MöhlmannARTICLES YOU MAY BE INTERESTED INFabrication of refractive index grating into azo-dye-containing polymer films byirreversible photoinduced bleachingJournal of Applied Physics 87, 3240 (2000); https://doi.org/10.1063/1.372330Fabrication of integrated optical structures in polydiacetylene films by irreversiblephotoinduced bleachingApplied Physics Letters 55, 1161 (1989); https://doi.org/10.1063/1.101684Photochemical laser writing of polymeric optical waveguidesApplied Physics Letters 58, 1955 (1991); https://doi.org/10.1063/1.105030Third order nonlinearity of 4-dialkylamino-4’nitro-stiIbene waveguides at 1319 nm Dug Y. Kim, Michael Sundheimer, Akira Otomo, and George Stegeman CREOL, University of Central Florida, Orlando, Florida 32826 Winfried H, G. Horsthuis AKZO Research Laboratories Arnhem, P. 0. Bo-x 9300, 6800 SB Arnhem, The ?Vetherlands Guus FL M6hlmann AKZO Electronic Products, P. 0. Box 9300, 6800 SB Arnhem. The Netherlands (Received I1 March 1993; accepted for publication 18 May 1993) The intensity dependent optical properties of 4-dialkylamino-4’nitro-stilbene polymer channel waveguides were measured at 1319 nm with a pulse modulated Mach-Zehnder interferometer to be n2=0.8i( lo-‘” cm2/W and & co.08 cm/GW. This material is promising for all-optical switching at 1319 nm because it satisfies both the Wand T figures of merit. DANS (4-dialkylamino-4’nitro-stilbene) moieties in the form of side chain polymers have proven very attrac- tive for nonlinear 0ptics.‘*2 For example, the third order nonlinearities are of interest for all-optical switching.2’” It has already been shown that the nonlinear coefficient n2 (defined by n=n,+nJ) is comparable to that of soluble polydiacetylenes like ~o~~-~BCMU.‘*~ The key question is whether the two switching figures of merit (FOM) W> 1 and T < 1 are satisfied at a communications relevant wave- length such as 1319 nm.3 These FOM, defined in detail later, quantify whether a 2~ nonlinear phase shift is achiev- able over one linear ( W) or nonlinear (T) absorption length. It has in fact. proven difficult to find materials which do satisfy these FOM. Here we report measurements of DANS nonlinearities which show that this material is promising for all-optical phenomena at 1319 nm. The DANS molecule was first developed for its high second order nonlinearity. When poled in channel waveguides, efficient electro-optical switching devices have already been demonstrated at 1319 nm.’ The channel waveguides were fabricated by photobleaching, i.e., by sup- plying photons of sufficient energy to induce cis-trans isomerization which leads to reduced nonlinear activity and refractive index in the bleached region.5 The desired guiding region of high index (channel) is blocked from the photobleaching light. Another method of supplying energy to the cis-traits isomerization process is via the simulta- neous absorption of two or more photons. If there is a significant two photon coefficient at the operating wave- length, usually 13 19 nm, then the device operating charac- teristics will degrade in time. Here, we report an upper limit to the two photon absorption coefficient at 1319 nm. Our experiments were performed on 1.65 cm long sin- gle mode channel waveguides formed by photobleaching spin-coated DANS. The polymer, composed of 35% side chain DANS and 65% of backbone polymer, was spin- coated onto oxidized silicon substrates to form a 1.8 pm thick film. Channel waveguides were formed by exposing the film to UV light through a mask in which only the channel region is blocked. The UV bleached part of DANS film has n = 1.567 while the unbleached part of DANS has a refractive index 1.601. These values were established by calibration measurements on thin films. The estimated depth (based on a series of calibrations of films of different thickness) to which the DANS is bleached is 0.7 pm after 6 h of UV light exposure. These material parameters, along with the mask width, were used in computer program for calculating waveguide modes. We used the Fourier analy- sis method for solving the scalar wave equation, subject to the usual boundary conditions.6 This method shows that the 3 pm width and 1.8 pm thick DANS film supports only a single mode at the wavelength 1319 nm. The calculated effective area, defined by A ef-r = I .I- .I- I E(KY) I *dx dy I * .I- 11 ECx,y) 1 4fLv dy ’ (11 where E(x,y) is the field distribution, is 8.0 pm” with an effective waveguide index N, = 1.570. The nonlinear refractive index was measured by a pulse modulated Mach-Zehnder interferometer as shown in Fig. 1. The prototype version of this interferometer has been described in detail elsewhere.’ A mode-locked Nd:YAG laser operating at 1319 nm provided 90 ps full- Computer PIG. 1. Eaperimental arrangement of pulse modulated Mach-Zdmdcr interferometer for direct measurement of Q. 290 Appl. Phys. Lett. 63 (3), 19 July 1993 0003~6951/93/63(3)/290/3/$6.00 CC- 1993 American institute of Physics 290 1.5 0.5 0 -0.5 -1 i ; 5 s  7 8 Phase scan (sec.) FIG. 2. Measured fringe scan for both high and low intensity pulses. The measured phase difference is 0.57~ rad. width at half-maximum (FWHM) pulses at a 76 MHz repetition rate. An electro-optic pulse slicer which was syn- chronized to the mode locker was used to select one high intensity pulse every 0.5 ms and to attenuate the rest of the mode-locked pulses. The DANS waveguide sample was put in one arm of the interferometer. We put a delay line in the reference arm to make the path length of the two arms equal. Because of the sample nonlinearity, the high inten- sity pulses transmitted through the waveguide propagate through a different optical path length. This will give a phase shifted fringe pattern for the high intensity pulses compared to the low intensity pulses. Two matched detec- tors and boxcar gated averagers were used to sample these two fringe patterns which were scanned at 1 Hz by pushing a wedged plate into the beam path of a reference arm. We were able to scan more than 100 fringes without any de- tectable fringe amplitude change. It should be noted that this interferometer becomes an all optical Mach-Zehnder switching device by itself if the two beams from both arms are well aligned with each other. We tested it without a sample and obtained more than 95% modulation by push- ing the wedge plate into the beam path. In order to avoid any slow thermal refractive index changes, the 3 ns gate of the boxcar used for the low intensity pulses was opened to catch the interference pattern for the low intensity pulse immediately preceding the selected high intensity pulse. We took two sets of fringe scan data, shifted by A$vL, as shown at Fig. 2 for the high and low intensity pulses. Converting these sine wave sets into a data set of A$NL is not trivial because there are phase and amplitude fluctua- tions in the data due to laser noise and vibrations of optical components in the interferometer. As discussed in Ref. 4, to maximize the information we did Hilbert transforma- tions instead of Fourier transformations on each fringe pat- tern4 Then, the phases were compared point by point be- tween the high and low intensity data sets. The standard deviation of the 300 phase differences which constituted a 291 Appl. Phys. Lett., Vol. 63, No. 3, 19 July 1993 data set were used to estimate the measurement error, less than r/50 for most cases. The nonlinear phase change experienced by the wave- guide modes is .a function of the optical beam intensity which varies over the waveguide cross section and pulse duration. The spatial averaging over the waveguide field profile can be taken into account by using the effective area so that Anpeak = n#( O)/A,s, where P( 0) is the peak input power just inside the waveguide. The measured phase shift A#- is already averaged for a given temporal pulse shape when we take data with a gated averager which has much wider gate width than the optical pulse width. We modeled in detail the temporal pulse shape dependence of the inter- ferometer output to simulate the relationship between the measured phase shift and the nonlinear phase shift at the peak intensity of a Gaussian pulse. This modeling shows that A#== 0.753A$Ek within 2% error for phase shifts up to several r in A#I~~ The linear loss due to scattering along the waveguide was measured to be 1.7 dB/cm (0.4 cm-‘) with an IR camera and frame grabber. The nonlinear phase shift ac- cumulated over a length L with loss a present is given by 2%- s L A@$ =/z Anpeak Left Lef= edaz dz. vat 0 Here, LeE is 1.21 cm. The maximum peak power difference of 39.1 W between the high and low intensity pulses gave a 0.5~ phase shift, shown in Fig. 2. This gives A#$ = 0.667r, which in turn gives Anpeak = 3.6 X 10-5.*Using P(0)=0.5e0iV,~J~~E(x,y)~‘dxdywithP(O)=39*5W gives n2= (0.8=tO.2)~~ lo-l3 cm2/W for the polymer with 35% DANS doping. The main source of the uncertainty comes from estimating the intensity inside the waveguide. The nonlinear absorption coefficient & was measured with small modifications to the experimental setup shown in Fig. 1. The interferometer reference arm is replaced by a reference channel for monitoring the input intensity. Since some hysteresis in transmission was observed when the intensity of the cw mode-locked (cw-ML) pulse train was changed with a half-wave plate and a polarizer, we used the pulse slicer again to avoid coupling efficiency changes due to the thermal effects. One high intensity pulse was selected and modulated for every 152 000 low intensity pulses so that the average power through the waveguide was changed very little even though the intensity of the high intensity pulses were changed from the minimum to the maximum. In the presence of linear loss, I(L) =1(O)/ [l +p21(0) L,R].4 When there is no linear absorption the relation between the input intensity and the output inten- sity is given by I(L) =I(O)/[l +&LI(O)], where L= 1.65 cm in our case. We could not observe any change in the throughput with cw-ML laser operation which gave a max- imum intensity of 0.28 GW/cm2 inside the waveguide. Next we used QS-ML (Q-switched mode-locked) opera- tion which gives about a factor of 80 more peak power. With a pulse slicer we selected one of the peak pulses at the center of each QS-ML pulse train. When the intensity of these selected pulses was increased, we destroyed several waveguides at intensities > 1.1 GW/cm2, about one-tenth Kim et a/. 291 0.2- r: I 0.15 “~ SO - j 1 - .’ ““‘---^ ~ I D i . .,. zi C”’ ; ! 5 : *i 0.09. ; ;&.g . . . . . +... i i ..i &a 1 D -3 s  ! 05 I o--$g I ..‘.. + ,” -,,_-.- -0.05, 8 * * I - . . I * s - ; 8 * * i . * . 1 * * t i 0 0.2 0.4 0.6 0.8 1 1.2 Input Intensity ( G W  / cm’ ) FIG. 3. Measured nonlinear transmission change with QS-ML operation of the Nd:YAG laser. of the maximum intensity we could in principle get inside of our waveguide. Typical data are shown in Fig. 3. How- ever, no nonlinear absorption was observed up to the dam- age threshold intensity. This result gives the upper bound of P2 x0.08 cm/GW. Note that our results also give an upper limit to the three photon absorption coefficient & < 0.34 cm3/GW” (in a =/3,1’). We observed no degrada- tion of the sample due to GW/cm* pulses over the duration of the experiments ( - 10 h). The damage observed origi- nated at the cleaved input facets which could probably be improved by better preparation techniques. Our previous experience is that the damage threshold for DANS films at normal incidence is in excess of 5 GW/cm2. We can now evaluate the two nonlinear figures of merit Wand T. In order to have reasonable throughput as well as switching, we need w=An/a/2 > 1.’ For the experimental parameters at the input surface damage threshold inten- sity, Ur> 1.6. However, with better input facets so that the material and not end-face damage threshold can be as- sumed, W increases by an order of magnitude. Even with- out linear loss, two photon absorption can reduce the de- vice throughput and spoil switching. We need the T=2&jt/n2 parameter to be smaller than one.8 We found 0.26 > T. Therefore, both FOM are easily satisfied, making this material a prime candidate for all-optical switching. An intensity of 3.5 GW/cm2 and a peak power of 140 W would be required to switch a nonlinear directional cou- pler, comparable to the best results recently implemented in AlGaAs waveguides below half their band gap.’ In summary, we have measured the third order non- linearities of the DANS side-chain polymer at 1319 nm. The ligures of merit obtained for all-optical switching are very promising. Finally, an upper bound of 0.08 cm/GW was found for the two photon absorption coefficient. This research was supported by AFOSR (91-0339) and NSF (ECS-89 11960). ’ W. H. G. Horsthuis, F. C. J. M. van Veggel, B. H. M. Hams, C. P. J. M. Van Der Vorst, 3. L. P. Heideman, H. W. M&ens, M. van Kheede, and G. R. Mohlmann, Proc. SPIE 1775, 360 ( 1992). *M. B. Marques, G. Assanto, G. I. Stegeman, G. R. Mohlmann, E. W. P. Erdhuisen, and W. Ii. G. Horsthuis, Appl. Phys. Lett. 58, 2613 (1991). ‘G. I. Stegeman and E. M. Wright, J. Opt, Quantum Electron. 22, 95 ( 1990). 4K. B. Rochford, R. Zanoni, G. I. Stegeman, W. Krug, E. Miao, and M. W. Beranek, Appl. Phys. Lett. 58, 13 (1991); IEEE J. Quantum Elec- tron 28, 2044 ( 1992). ‘M. B. J. Diemeer, F. M. M. Suyten, E. S. Trommel, A. McDonach, J. M. Copeland, L. W. Jenneskens, and W. H. G. Horsthuis, Electron. Lett. 26, 379 (1990). 6C. H. Henry and B. & Verbeek, J. Lightwave Technol. 7, 308 ( 1989). ‘G. I. Stegeman, E. M. Wright, N. Finlayson, R. Zanoni, and C. T. Seaton, J. Lightwave Technol. 6, 953 (1988). “V. Mizrahi, F. W. DeLong, and G. I. Stegeman, Opt. Lett. 14, 1 I40 (1989). ‘A. Villeneuve, I. S. Aitchison, C. C. Yang, P. G. 5. Wigley, C. N. Ironside, and G. I. Stegeman, Appl. Phys. Lett. 61, 147 (1992). 292 Appl. Phys. Lett., Vol. 63, No. 3, 19 July 1993 Kim et al. 292 

3Rd-Order Nonlinearity Of 4-Dialkylamino-4\u27Nitro-Stilbene Wave-Guides At 1319 Nm

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3Rd-Order Nonlinearity Of 4-Dialkylamino-4\u27Nitro-Stilbene Wave-Guides At 1319 Nm

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