We demonstrate surface emitted second harmonic generation due to mixing of counter propagating waves in in-plane poled, low loss, channel waveguides of 4-dimethylamino-4\u27-nitrostilbene (DANS) side-chain polymers. Single film DANS side-chain polymer waveguides yield high conversion efficiency devices

Bosshard, Christian

Horsthuis, Wiinfried H. G.

Mittler-Neher, Silvia

Möhlmann, Gustaaf R.

Otomo, Akira

Stegeman, George I.

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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 2Nd-Harmonic Generation By Counter Propagating Beams In 4-Dimethylamino-4'-Nitrostilbene Side-Chain Polymer Channel Wave-Guides Akira Otomo University of Central Florida Silvia Mittler-Neher University of Central Florida Christian Bosshard University of Central Florida George I. 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 y u 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 Otomo, Akira; Mittler-Neher, Silvia; Bosshard, Christian; Stegeman, George I.; Horsthuis, Wiinfried H. G.; and Möhlmann, Gustaaf R., "2Nd-Harmonic Generation By Counter Propagating Beams In 4-Dimethylamino-4'-Nitrostilbene Side-Chain Polymer Channel Wave-Guides" (1993). Faculty Bibliography 1990s. 872. https://stars.library.ucf.edu/facultybib1990/872 Authors Akira Otomo, Silvia Mittler-Neher, Christian Bosshard, George I. Stegeman, Wiinfried H. G. Horsthuis, and Gustaaf R. Möhlmann This article is available at STARS: https://stars.library.ucf.edu/facultybib1990/872 Appl. Phys. Lett. 63, 3405 (1993); https://doi.org/10.1063/1.110155 63, 3405© 1993 American Institute of Physics.Second harmonic generation bycounter propagating beams in 4-dimethylamino-4’-nitrostilbene side-chainpolymer channel waveguidesCite as: Appl. Phys. Lett. 63, 3405 (1993); https://doi.org/10.1063/1.110155Submitted: 21 June 1993 . Accepted: 05 October 1993 . Published Online: 04 June 1998Akira Otomo, Silvia Mittler-Neher, Christian Bosshard, George I. Stegeman, Winnfried H. G.Horsthuis, and Gustaaf R. MöhlmannARTICLES YOU MAY BE INTERESTED INEnhanced second-harmonic generation by counter-propagating guided optical wavesApplied Physics Letters 38, 759 (1981); https://doi.org/10.1063/1.92152Nonlinear AlGaAs waveguide for the generation of counterpropagating twin photons inthe telecom rangeJournal of Applied Physics 98, 063103 (2005); https://doi.org/10.1063/1.2058197Wavelength dependence of 4-dimethylamino- -nitrostilbene polymer thin filmphotodegradationApplied Physics Letters 73, 912 (1998); https://doi.org/10.1063/1.122035Second BPaPmQnic generaaiom by csglnter propagatimg beams in 4dmethyla -&nitrostilb me side-chain polymer ehammel waveguides Akin3 Btonm, Silvia Mitiler=Nefier,a~ Cl?ri&m Bossh~rd, znd George 1. Stegemzn Cmtcr filr Research in Elects-0ptic.s and Lasers, Uniwrsity of Central Florida, Orlando, Florida 32825 Winnfried i-4. 6. Horsthuis and Gustaaf R. MBhlmann AKZO Research hahnratnricr ArPzhmt, Cnrpinrate Research, Applied Physim Department, Amhem, The Netherh.mds U&xeived 21 June 1993; accepted for publication 5 October 1993) We demonstrdte surface emitted second harmonic generation due to mixing of counter propagating waves in in-plane poled! low loss, channel waveguides of 4=dimethylamino= 4’=nitrostilbene (DANS ) side-chain polymers. _ Single film DANS side-chain polymer waveguides yield high conversion efEeiency devices. The mixing of counter propagating guided waves to generate second harmonic (SEE) normal to the surface was initially demonstrated in Ti in-diffused LiNb03 waveguides over a decade ago.’ Several applications were also demonstrated or proposed, including picosecond sig- nal processing,’ picosecond optical t,ransient digitizers,3 and spectrometers.” The device potential of this phename- non for ultrafast signal processing, wavelength division de- multiplexing, etc., has attracted attention recentZy.5 In the last few years, several works in GaAs based waveguides were reported ‘-M and the conversion efliciency was im- proved drama&ally by using a form of quasi-phase match- ing in multilayered structum. On the other hand, organic polymers have also been shown to have great potential for second-order nonlinear optical applications because they have large nonresonant nonlinearities and ease of fabrica- tion. In this letter we report the first observation of this effect in polymer waveguides made from a single film of ~imsthy~amino=4’=nitr~~stilbent (DANS) side-chain polymer. The measured nonlinear cross-section coefficient is comparable to that for single film &As waveguides, in good agreement with our calculations. DANS side-chain polymers are one of the most highly developed poled polymers for applications to electro- optical switching devices. Their relatively high glass tmn- sition temperature prot<des long-terna stability of the poled state at room temperature and large poling field strengths provide nonlinear coefficients d as high as 2or3 pm/V at a resonant wavelength (3, = 1.064 pm).‘* Furthermore, struc.tures with lateral confinement can be made by pho- tobleaching” which allowed us to fabricate low loss chan- nel waveguides. I3 The device etEciency for counter propagating mixing SHG is given by the nonlinear cross section -4”’ which is d&mj ap,‘-l’ lh2w) =ANLI, (o)L(cu), (11 where I, (TV) is the line intensity in W/m of the guided waves at the fundamental frequency, i.e., the power per nJCurrent n&-Ire%: Max-Plmck-Institute t?ir Polymerforschung, Acker- maml wcg 10, tmo h?fainz, Germmy. unit width of the guided wave wave front. Also, P(2w) is the intensity of the second harmonic light generated nor- mal to the waveguide, in W/m’. Analysis of the field gen- erated by the nonlinear polarization shows that the non- linear cross section is strongly dependent on the superposition integral S( m ) ketween the guided mode profile of the fundamental beams and the generated second harmonic field: ANL=~~kg(2W)?Z,(2W)IC141S( co) 12, (21 with = d&‘>f;(.(x’l s(=0)= J-, llf(201] Xexp[ikc(20>n,(2co)x’]dxf, (31 where k,(2~j =2&c, and n,(&) is the refractive index of the cover and r~~(2.w) is the complex refractive index of the film. Furthermore, fY(x’) is the guided mode transverse distribution and C is the normalization constant. of the guided mode for the fundamental bemn4 Obviously, the properties of the nonlinear cross section are directly related to the properties of the superposition integral which needs to be optimized. We first examine numerically the trade-off’s in S( ,X ) for the DANS waveguide parameters. The magnitude of the overlap integral oscillates with increasing film thick- ness L Since the guided mode profile fY(x’) is broadened with increasing thickness and the refractive index np(2u>) includes absorption, tho envelope of this oscillation de- creases with increasing thickness when normalized to input power. The important tendencies are clearly shown in Fig. 1 based on typical DANS parameters [n/w) = 1.63, nf(2w) = 1.81. Because the peak of x+x’) moves towards the cover-film interface when q( ti) IS large, approaching the value of nf(o), the influence of n,(o) on ANL [Fig. l(a)] indirectly demonstrates the influence of the guided mode profile =f,(x’). The three curves correspond to the first three smallest film thicknesses for which the osciliat- ing X4 NL is maximum. The first and largest peak value is obtained for n,(w) = 1.49. The corresponding nf(2w) de- pendence is evaluated with an air cladding and a fused 3405 kppl. Phys. L&t. 63 (25), 20 December 1993 0003-6951/93/63(25)/3405/3/$6.00 0 1993 American Institute of Physics 3405 c2c4 nf FIG. 1. Variation of iiNL with refractive index; (a) The dependence on the refractive index of the cladding at 1= 1064 nm with the DANS poly- mer film and the fused silica substrate. (b) The dependence on the re- fractive index of the film at A = 532 nm. silica substrate [Fig. l(b)]. The nonlinear cross section ANL at the film thickness of 1 pm is estimated from the envelope of the oscillating decay for comparison. Since the oscillating period is inversely proportional to nf(2u), the integral within each period dramatically decreases for large refractive indices of the film. In this experiment, the cover polymer which has n,(o) = 1.55 was used and Fig. 1 (b) indicates large potential of this device. A schematic of the cross section of the fabricated DANS polymer waveguide is shown in Fig. 2. DANS side- chain polymer films were made by spin coating onto fused silica substrates which have two coupling gratings, one at each end, and poling electrodes on the glass surface. The period of the grating was 0.5 pm and the spacing of the poling electrodes was 20 pm. The film thickness of 0.83 +0.04 pm was determined from calibrated data on the spinning rate and the solid concentration in the spinning solution. A 4-pm-wide channel waveguide with parabolic tapered mode couplers on both sidesi was constructed by photobleaching used to reduce the film index outside the channel region. In this process the film is exposed through a mask to collimated light from a Kg-arc lamp (A =436 3406 Appl. Phys. Lett., Vol. 63, No. 25, 20 December 1993 Cover Polymer FIG. 2. Schematic cross section of the waveguide. nm) for 4 h. The effective device length was 1 cm. A 15pm-thick cover layer was spin coated on top of the DANS polymer in order to prevent discharge through the air during poling and to create a better guided mode profile for efficient surface emitting SHG. Since a large lateral component (in-plane, along the )-’ axis in Fig. 2) of the second-order nonlinearity is required for efficient conver- sion, plane parallel poling was performed at just below the glass transition temperature of Tg= 142 “C. Although the applied electric field strength was only 50 V/pm for the sample investigated for SHG, the highest field which can be reached was determined to be as large as 200 V/pm. The nonlinear coefficient dzr was measured at 1.064 pm (on resonance) after poling with 50 V/pm and a value of 26 pm/V was determined. This value is lower by a factor of 2 compared to the value derived from the measurement of the electro-optical coefficient for poling with the same value of field normal to film’i This is reasonable because the field lines in our geometry are curved, resulting in field being used to align molecules normal as well as parallel to the surface. The surface emitted SHG power was implemented by coupling la)-ps Q-switched mode locked pulses at 1064 nm into both sides of the waveguide. A pulse was split into two identical halves and the overlap of these two pulses in the waveguide was optimized by changing the optical path length of one arm. The coupling efficiency of the grating couplers was determined by measuring the transmission change and the total coupling efficiency into the channel waveguide was estimated as 10% for each of the two cou- plers, including the tapered mode transition 10~s.‘~ The SHG power was detected in the direction normal to the surfaces by a photomultiplier tube through a lens. Since the generated light has a large divergence angle along the p axis, the power collected by the lens was only 30% of the total generated power, including reflection losses. The mea- sured SHG power was 17 m W  for a 32 W  input power which yields 0.64 GW/cm’ of peak intensity in the channel waveguide. The nonlinear cross section was evaluated as 8.5 X 10-s W-t by assuming a Gaussian shape for the pulses in the pulse train. This value is larger than that for a single layer GaAs film and the generated second har- monic light was easily seen by eye under room light.5 The thickness dependence of the magnitude of the non- Otomo et al. 3406 t ) 0-7 ; .- I@“! -7 B ‘Y ),p _ $ - l’y’” I im-m-p*- . !IJ”‘f 0.0 1 .o 2.0 thickness (urn) FIG. 3. Thickness dependence of the nonlinear cross section for DANS side-chain polymer waveguides with a cladding refractive index of 1.55 and a fused silica substrate. linear cross section was calculated for the DANS side- chain polymer and is shown in Fig. 3. The measured value of the nonlinear coefiicient dz2 for the 50 V/,um poling field was used, and absorption and multiple reflection were in- cluded in the calculation. The calculated value 1.45 x lo- * W-’ at a thickness of 0.83 /lrn agrees with the measured value. The maximum ethciency would be obtained at a film thickness of 0.47 [lrn with /INL= 5.0~ IO-* W-‘. A simple extrapolation to our largest poling voltages to data of 200 V/pm gives XX 1O-7 W-r, the comparable to that of a multiple layer GaAs film.” Such an efficient single layer device should make it possible to operate over a large fre- quency range, and to demonstrate new applications such as spectrometers, etc. In principle, extending the device to multiple layer films with alternate films of zero activity should lead to even larger efficiencies. In summary, counter propagating mixing SHG was demonstrated with polymer channel waveguides for the first time. High efficiency was observed in the poled DANS side-chain polymer. The reasonable agreement observed between the experimental values and theory supports the promise for even higher efficiencies with optimized devices. The authors acknowledge the support of the Air Force Office of Scientific Research (AFOSR-91-0339). We also thank J. Ross for preparing the coupling gratings and the poling electrodes. ‘R. Normandin and G. I. 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Chaten- oud, Appi. Phys. Lett. 62, 118 ( 1993). “W. H. G. Horsthuis, G. R. Mohlmann, and H. W. Mertens, Digest of the Conference on Nonlinear Guided-Wave Phenomena (GSA, Cam- bridge, England, 1991), paper MDl, pp. 7&73. “M. D. J. Diemeer, F. M. M. Suyten, E. S. Trammel, A. McDonach, J. M. Copeland, L. W. Jenneskens, and W. H. G. Horsthuis, Electron. Lett. 26, 379 ( 1990). ‘“A. Otomo, S. M.-Neher, G. I. Stegeman, W. II. G. Horsthuis, and G. R. Miihlmann, Electron. L&t. 29, 129 ( 19939. “P. 5. Vella, R. Normandin, and G. 1. Stegeman, Appl. Phys. L&t. 38, 759 (1981). 3407 Appt. Phys. Lett., Vol. 63, No. 25, 20 December 1993 Otomo et al. 3407 

2Nd-Harmonic Generation By Counter Propagating Beams In 4-Dimethylamino-4\u27-Nitrostilbene Side-Chain Polymer Channel Wave-Guides

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2Nd-Harmonic Generation By Counter Propagating Beams In 4-Dimethylamino-4\u27-Nitrostilbene Side-Chain Polymer Channel Wave-Guides

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