We demonstrate second-harmonic generation due to counterpropagating beams in planar waveguides of 2-docosylamino-5-nitropyridine (DCANP). The DCANP molecules were deposited by Langmuir-Blodgett techniques and have a preferred alignment within the substrate plane. Four-layer waveguide structures were used to optimize the trade-off between propagation loss and efficient surface-emitted green light

Bosshard, Ch.

Flörsheimer, M.

Günter, P.

Küpfer, M.

Otomo, A.

Stegeman, G. I.

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

University of Central Florida STARS Faculty Bibliography 1990s Faculty Bibliography 1-1-1994 Surface-Emitted Green Light Generated In Langmuir-Blodgett-Film Wave-Guides Ch. Bosshard University of Central Florida A. Otomo University of Central Florida G. I. Stegeman University of Central Florida M. Küpfer M. Flörsheimer 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 Bosshard, Ch.; Otomo, A.; Stegeman, G. I.; Küpfer, M.; Flörsheimer, M.; and Günter, P., "Surface-Emitted Green Light Generated In Langmuir-Blodgett-Film Wave-Guides" (1994). Faculty Bibliography 1990s. 2935. https://stars.library.ucf.edu/facultybib1990/2935 Authors Ch. Bosshard, A. Otomo, G. I. Stegeman, M. Küpfer, M. Flörsheimer, and P. Günter This article is available at STARS: https://stars.library.ucf.edu/facultybib1990/2935 Appl. Phys. Lett. 64, 2076 (1994); https://doi.org/10.1063/1.111688 64, 2076© 1994 American Institute of Physics.Surface-emitted green light generated inLangmuir–Blodgett film waveguidesCite as: Appl. Phys. Lett. 64, 2076 (1994); https://doi.org/10.1063/1.111688Submitted: 21 October 1993 . Accepted: 16 February 1994 . Published Online: 04 June 1998Ch. Bosshard, A. Otomo, G. I. Stegeman, M. Küpfer, M. Flörsheimer, and P. GünterSurface-emitted green light generated in Langmuir-Blodgett film waveguides Ch. Bosshard,a) A. Otomo, and G. I. Stegeman Center for Research in Electra-Optics and Lasers, University of Central Florida, Orlando, Florida 32826 M. Kiipfer, M. Flijrsheimer, and P. Giinter Nonlinear Optics Laboratory, Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH HGnggerberg, CH-8093 Ziirich., Switzerland (Received 21 October 1993; accepted for publication 16 February 1994) We demonstrate second-harmonic generation due to counterpropagating beams in planar waveguides of 2-docosylamino-5nitropyridine (DCANP). The DCANP molecules were deposited by Langmuir-Blodgett techniques and have a preferred alignment within the substrate plane. Four-layer waveguide structures were used to optimize the trade-off between propagation loss and efficient surface-emitted green light. Langmuir-Blodgett (LB) films have many attractive fea- tures for applications to nonlinear optics. Since they can be deposited one monolayer at a time with a net second-order susceptibility, precise film thickness (and therefore effective index in waveguides) control for second-harmonic genera- tion (SHG) is possible.ld6 However, until recently the wave- guide propagation losses have been too high for the efficient SHG needed for efficient blue sources. The development of low loss LB guiding films has now led to the demonstration of efficient collinear SHG.te6 With the flexibility of the deposition technique it is possible to reverse the direction of the nonlinearity within a film. This should ultimately allow the large diagonal elements of the nonlinear susceptibility tensor to be used for efficient SHG with fundamental and doubled guided waves with different mode numbers. SHG normal to the waveguide surfaces by the mixing of oppositely propagating guided waves has features and appli- cations different from the usual copropagating SHG, e.g., wavelength demultiplexing, picosecond signal processing, etc.7-11 Recently the work in this field has concentrated on GaAs-based waveguide structures.11-16 Due to their large nonlinear optical coefficients organic materials provide a promising alternative to semiconductor devices. Efficient surface emission of SHG has already been demonstrated in poled polymers of 4-dimethylamino-4’-nitrostilbene side- chain channel waveguides.r7 In this letter we report the fhst experiments on an alternative approach to using organic ma- terials, planar waveguides made from a LB film, 2-docosylamino-5-nitropyridine (DCANP).1-5 LB films offer new opportunities for increasing the effi- ciency of this process. Since the output signal only traverses a few optical wavelengths of the sample, the large resonant nonlinearities of organics can be used. Furthermore, although this interaction is not subject to the stringent limitations of wave vector matching encountered in copropagating SHG, wave vector matching in the waveguide in the direction nor- mal to the surface can considerably enhance the efficiency. In semiconductors this has been achieved by using molecular akurrent address: Nonlinear Optics Laboratory, Institute of Quantum Elec- tronics, Swiss Federal Institute of Technology, ETH Hijnggerberg, CH- 8093 Zlirich, Switzerland. beam epitaxy growth techniques to modulate the magnitude of the nonlinearity.” Using LB films desired thicknesses can be fabricated to an accuracy of about 4 nm and the direction of the nonlinearity can be periodically reversed.t8 In the cur- rent case we do not reverse the nonlinearify but do demon- strate a four-layer geometry which includes an inactive layer to improve the propagation loss characteristics of the wave&ides. The linear and nonlinear optical properties of DCANP are well established and both Cerenkov and phase-matched, copropagating SHG using modal conversion in waveguide configuration have been reported.“-5 The LB film fabrication procedure of DCANP induces a preferred alignment of the molecules along the dipping direction and films which are stable over years. This leads to a net dipole moment within the substrate plane and to a Y-type “herringbonelike” struc- ture [Fig. l(a)]. The intensity for SHG by counterpropagating beams can be described by ~(2w)=AmI,(~)Z-(~) where I,(w) are the line intensities (in W/m) of the fundamental beams and ANL is the nonlinear cross section.7 Here Am= ~,ok~(2o)n,(2w)IC1~1S(oo)1~ . (1) Am is strongly dependent on the superposition integral S(m)= I +m MY ‘>f2(Y ‘1 exp[iko(2w)a,(20)y’]dy’ , --co +-wJ) (2) where k,(2wj=2&, and n,(2w) and nf(2w) are the com- plex refractive indices of the cover and the film, respectively. f(y ‘) is the guided mode profile and C is the power normal- ization constant of the guided mode. Figure 2 shows the theoretical nonlinear cross section versus thickness for three- and four-layer geometries of DCANP. The substrate is either a high-index glass (AF45) or Pyrex. For the four-layer case an optically inactive Ti02/Si02 film was used. The following parameters were utilized for the calculation: wavelength h=1064 nm, nonlinear optical coefficient d,,=7.8 pm/V (A,,=375 nm), thickness of linear TiOz/Si02 layer t ,=145 run, refractive indices at frequency w: n,=1.75, n,=1.516 (or ns=1.4667 for Pyrex), nDCANP=1.573, refrac- 2076 Appl. Phys. Lett. 64 (i6), i8 April 1994 0003-6951/94/64(16)12078/3/$6.00 Q 1994 American institute of Physics one bilayer I ra) \ = N”2~~-C22H45 air tLB tLlN I TiO$SiOp substrate Cb) FIG. 1. (a) Orientation of the DCANP molecules in the LB film. The dip- Y ping procedure induces the noncentrosymmetric Y-type structure. The ar- rows indicate the direction of the dipoles. (b) Sample geometry and sche- matic diagram of the counterpropagation experiment. tive indices at frequency 20: n,=l.85, ns=1.531 (or ns=1.4743 for Pyrex), nDc,,=l.624. Typically ANL is larger for emission towards the substrate than towards the air due to the asymmetry of the electric fields in the waveguide. In our experiments we used the four-layer structures mainly for two reasons. First, the attenuation losses of the waveguide can be strongly decreased for LB films as has been demonstrated in Ref. 4. Second, fewer layers of 1.5x 108 ANL D/v-‘] 1.0 x lad 5.0x 10.9 0 0 200 400 600 800 1000 thickness LB film [nm] FIG. 2. The nonlinear cross section Am vs film thickness for three- and four-layer waveguide geometries of DCANP. The lines represent the calcu- lated values, and the (W) is the measured point. The solid curve is for the four-layer structure. The dotted and dashed curves represent the three-layer structure with Pyrex and AF45 as the substrate, respectively. Appl. Phys. Lett., Vol. 64, No. 16, 18 April 1994 DCANP molecules have to be deposited to obtain similar nonlinear cross sections as for usual three-layer structures. This is clear since optimum conversion efficiencies are ob- tained for film thicknesses which are separated by Xa/[2n(20j]. For a TiO,/SiO, film of thickness tLIN= 145 nm, the first peak in Am accessible above waveguide cutoff for the fundamental occurs for a LB thickness of only 230 nm. Note, however, that this first peak for a single layer film may not always be the largest since the input field confinement may be small if it occurs near cutoff. Commercially available TiOa/SiOa waveguides (sub- strate glass AF45) from AS1 AG (Zurich, Switzerland) were used into which gratings with a period of 1000 mm-’ were fabricated by ion milling. Subsequently the surface was cleaned and the LB film was deposited. The exact prepara- tion procedure is described in Ref. 3. Figure l(b) shows the sample geometry together with a schematic diagram of the counterpropagating SHG process. For our experiment a sample with a linear Iayer thickness of t,=145 run and a LB film thickness of t Ln=230 nm (corresponding to 104 monolayers) was used. First the attenuation of the waveguide was measured by imaging the streak of the guided wave with a camera (Elec- trophysics Micronviewer Model 7290) and by subsequent analysis of the streak. We obtained a value of about 5.5 dB/cm for our sampIe. Note that the loss would have been about 2.5 times larger (and the signal more than 2 times smaller) for operation on the first peak of a guiding film made solely from DCANP. 100 ps pulses from a &-switched (500 Hz) and mode- locked (76 MHz) Nd:YAG laser (Quantronix 416) were split in half and then coupled into both sides of the waveguide to excite TEa modes. The overlap in the waveguide was opti- mized by changing the optical delay in one arm. The cou- pling gratings were separated by 5.5 mm thus defining the interaction length of the device. The fundamental beams were focused on the grating using cylindrical lenses in order to optimize the coupling. Coupling efficiencies of 11% and 8% were estimated. The peak power of the fundamental in the waveguide was 74 W (average power of 70 ~Wj yielding a peak input line intensity of 3.46 kW/m. The generated second-harmonic light could be observed perpendicular to the sample surface easily by the eye in room light conditions. The measured second-harmonic power showed the expected quadratic behavior on the intensity of the fundamental. We measured a maximum average power of P,,(20)=6 nW cor- responding to a peak power of P,(2o) of 12.6 mW. The full divergence angle of the green light was of the order of 0.036 rad. Therefore it could be expected that all the light gener- ated was incident on our detector. From P(w) and P(2w) the nonlinear cross section Am could be calculated. Our measured value of 1.3 X 10m8 W-’ is in good agreement with the theoretical value of ANL=1.4X 10e8 W-’ in Fig. 2 which also shows the measured point. The signals measured here are adequate for the projected applications. However it is useful to project what the ulti- mate signal limits might be. In the LB film case, the super- position integral can be optimized by constructing LB layers with periodically reversed nonlinearities and optimized layer Bosshard et al. 2077 thicknesses, increasing the cross section by about one order of magnitude. Since the generated second harmonic only needs to propagate in the nonlinear medium for a short dis- tance (of the order of 600 nm for the periodically reversed layer case), working with resonantly enhanced nonlinearities is feasible.17 Assuming operation at the communications wavelength of 1300 mn with different molecules with reso- nant nonlinearities, values of d@) in excess of 300 pm/V are feasible and Am-+2X 10e4 W-‘, including enhancements due to periodically reversed layers.19*20 Using the expression P(2w)=AmP2(o)LIW where L and W are the sample length and channel width, respectively, P(20)/ P”(w) -+200%/W for a l-cm-long guide and a channel 1 pm wide. These values could even be competitive with copropa- gating devices, without their tight tolerances on phase match- ing. In conclusion we have demonstrated efficient SHG due to mixing of counterpropagating beams in planar waveguides of a LB film for the fhst time. We have measured large nonlinear cross sections for single films of DCANP. Our four-layer structure (linear/nonlinear) waveguide allowed an advantageous trade-off between loss and conversion effi- ciency. We thank J. Ross for fabricating the coupling gratings and U. Stalder for preparing the LB films. ‘G. Decher, B. Tieke, Ch. Bosshard, and P. Giinter, J. Chem. Sot. Chem. Commun. 1988, 933; G. Decher, B. Tieke, Ch. Bosshard, and P. Giinter, Ferroelectrics 91, 193 (1989). *Ch. Bosshard, M. Kiipfer, P. Giinter, C Pasquier, S. Zahir, and M. Seifert, Appl: Phys. Lett. 56, 1204 (1990). ‘Ch. Bosshard, M. Fiorsheimer, M. Kiipfer, and P. Giinter, Opt. Commun. 85, 247 (1991). ‘M. Fiijrsheimer, M. Kiipfer, Ch. Bosshard, H. Looser, and P. Giinter, Adv. Mat. 1992, 795. “Ch. Bosshard, M. Kiipfer, M. Fiijrsheimer, and P. Giinter, Mol. Cryst. Liq. Sci. Technol. Sec. B: Nonlin. Opt. 3, 215 (1992). “K. Clays, N. J. Armstrong, and T. L. Penner, J. Opt. Sot. Am. B 10, 886 (1993). 7R Normandin and G. I. Stegeman, Opt. Lett. 4, 58 (1979). “R. Normandin and G. I. Stegeman, Appl. Phys. L&t. 36, 253 (1980). ‘R. Normandin and G. I. Stegeman, Appl. Phys. Lett. 40, 759 (1982). r”G. I. Stegeman, J. J. Burke, and C T. Seaton, in Optical Engineering: Integrated Optical Circuits and Components, edited by L D. Hutcheson (Mar& Dekker, New York, 1987), Vol. 13. ” R. Normandin, S. Lctoumeau, F. Chatenoud, and R. L. Williams, J. Quan- tum Electron. QE-27, 1520 (1991); H. Dai, S. Janz, R. Normandin, R. L. Williams, and M. Dion, Electron. Lett. 28, 1540 (1992). “D. Vakhshoori and S. Wang, Appl. Phys. Lett. 53,347 (1988). 13D. Vakhshoori, J. Walker, S. Dijaili, S. Wang, and J. S. Smith, Appl. Phys. L&t. 55, 1164 (1989). 14R Normandin, R. L. Williams, and F. Chatenoud, Electron. Lett. 26, 2088 (1990). “D. Vakhshoori, R J. Fischer, M. Hong, D. L. Sivco, G. J. Zydzik, G. N. S. Chu, and A. Y. Cho, Appl. Phys. Lett. 59, 896 (1991). r6R. Norrnandin, H. Dai, S. Janz, A. Delage, J. Brown, and F. Chatenoud, Appl. Phys. Lett. 62, 118 (1993). t7A. Otomo, S. Mittler-Neher, Ch. Bosshard, G. Stegeman, W. H. G. Horst- huis, and G. R. Mijhlmann, Appl. Phys. Lett. 63, 3405 (1993). “P. J. Velia, R. Normandin, and G. I. Stegeman, Appl. Phys. Lett. 38, 759 (1981). r9B. Lawrence, M. SC. thesis, Department of Electrical Engineering, MIT, 1992. “W. H. Steier, Y. Shi, L. Yu, M. Chen, and L. R. Dalton, Proc. SPIE 1775, 379 (1992). 2078 Appl. Phys. Lett., Vol. 64, No. 16, 18 April 1994 Bosshard et al. 

Surface-Emitted Green Light Generated In Langmuir-Blodgett-Film Wave-Guides

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Surface-Emitted Green Light Generated In Langmuir-Blodgett-Film Wave-Guides

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