Th& Solid Films, 210/211 (1992) 153-155 153 Guided-wave frequency-doubling in Langmuir-Blodgett film waveguides Ch. Bosshard, M. Kiipfer, M. Fl6rsheimer and P. Giinter Institute of Quantum Electronics, Nonlinear Optics Laboratory, Swiss Federal Institute of Technology, ETH H6nggerberg, CH-8093 Ziirich, (Switzerland) Abstract We demonstrate phase-matched frequency-doubling in Langmuir-Blodgett film waveguides of 2-docosylamino-5-nitropyridine. Using the Cerenkov-type configuration we can generate light down to a wavelength of 410 nm. Gratings as well as prisms were used to couple the fundamental beam into the waveguide. 1. Introduction Cerenkov-type phase-matched frequency-doubling provides a simple method to generate light in the visible part of the spectrum using near-infrared laser sources [1]. This technique is often more convenient than phase-matching of guided modes where very strict con- ditions on the waveguide thickness are required and where both fundamental and second-harmonic waves have to be guided. In Cerenkov-type phase-matching, however, only the fundamental beam is guided in the nonlinear layer whereas the second-harmonic beam is radiated into the substrate. We report on the applica- tion of this scheme to Langmuir-Blodgett (LB) films of 2-docosylamino-5-nitropyridine (DCANP (CH 3- (CH2)21-NH-CsH3N-NO2)). DCANP is a molecule displaying strong nonlinear optical effects in LB films. Its linear and nonlinear optical properties as well as waveguiding have been described earlier [2-4]. Due to its ability to form multilayers (up to 540 molecular layers) of good optical quality, low loss optical wave- guiding has been obtained. Synthesis and film prepara- tion have been presented elsewhere [3]. 2. Cerenkov-type frequency-doubling For Cerenkov-type second-harmonic generation, the phase-matching condition can be illustrated schemati- cally as shown in Fig. 1 where the process for a conversion using the nonlinear optical coefficient d33 is shown (see e.g. ref. 5). The fundamental mode gener- ates light at the second-harmonic (2co) which is not guided. At point A the frequency-doubled light enters the substrate under an angle 0 and propagates to C. On the way towards point B the fundamental wave gener- ates frequency-doubled light. Constructive interference of the second-harmonic wave generated in the wave- guide region between A and B takes place if to Vv ''~ cos 0 = Vs 2'° or cos 0 = Neff ns2,,, (1) A new wavefront BC of the second-harmonic wave is obtained (Cerenkov radiation). VF" and vf '° are the velocities of the fundamental mode in the film and the second-harmonic wave in the substrate, respectively, and N~r is the effective refractive index of the funda- mental mode in the waveguide. The condition (1) is fulfilled for Vv '° > Vs 2'°. Generally Cerenkov radiation is generated for ns 2'° > N;'~ > ns '° (2) The conversion efficiency for second-harmonic genera- tion qcalc=p2,o/p,o(p,o power of the fundamental guided mode) is given by [6] (we modified the formulas for our case where the substrate was isotropic [7]) p2,,, 8#o3~o2(20~)5Lp '° tg O]c[ 2 rl = p,,, = t~, Wt~ (3) eo and #o are the vacuum permittivity and permeability, respectively, 0 is the Cerenkov angle, flh ---- 4n2-1Neff,,,~ L is the interaction length and W is the width of the mode. The detailed expression for tefr and Icl can found in ref. 7. As was discussed above, the necessary condition for Cerenkov-type phase-matched frequency-doubling is that the effective refractive index of the fundamental mode is smaller than the refractive index of the sub- strate at 2~. This condition can be fulfilled for DCANP deposited on pyrex. Table 1 lists the refractive indices at the fundamental and the second-harmonic wavelengths, the thicknesses t of the films used in our experiments 0040-6090/92/$5.00 (c ~ 1992 -- Elsevier Sequoia. All rights reserved