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Phase-matched second harmonic light generation by guided mode conversion has been observed for the first time in nonlinear optically active Langmuir-Blodgett (LB) films. The advantage of using a material with an overall in-plane polarization is pointed out.Second harmonic generation (SHG) using guided wave configurations is very attractive since high light powers can be confined over long propagation distances. Hence, considerably higher conversion efficiencies can be expected than with bulk samples.['] A main advantage of the LB filmpreparation technique is the possibility to control the waveguide thickness to about one bilayer['I (bilayer thickness -4 nm). This meets the waveguide dimension tolerances very well which must be controlled within & 5 nm in order to ensure phase-matching of the fundamental and SH waves over the full waveguide length.r31As nonlinear optical material we use 2-docosylamino-5-nitropyridine (DCANP). The synthesis, the optimum conditions for LB film transfer and the linear and nonlinear optical properties have been described el~ewhere.[~-~] As in the case of almost all the nonlinear optical organic materials developed so far, the nonlinear response (hyperpolarizability) of the DCANP molecules is due to an intramolecular charge-transfer group: a polarizable K electron system (pyridine ring) substituted by an electron donor (amino group) and an electron acceptor (nitro group). Using a simple approximation the chromophore can be described as a dipole oscillator with its ground state and transition dipole moment along the charge-transfer axis. ['] For large macroscopic nonlinear optical susceptibilities it is essential that the second harmonic wavelets radiated from the ensemble of oscillators interfere constructively. Unfortunately, the highly polar charge-transfer molecules tend to form centrosymmetric bulk crystals where the ground-state dipoles and hence the emitted SH wavelets cancel each other. The LB multifilm preparation, however, starts from a monolayer of the molecules on a water surface. Corresponding to the symmetry break on the interface the polar molecules usually form a film with a nonvanishing overall normal polarization. With special preparation techniques and suitable molecules it is possible to deposit monolayer by monolayer ["I so that the normal polarizations of all the layers add up.191 So far, almost all the efforts to find new LB materials for efficient SHG have concentrated on this approach.However, for frequency doubling in a waveguide configuration, the predetermined configuration for LB materials, just this polarization (normal polarization) is not optimal. In contrast, DCANP is a material where the molecular dipoles can be aligned preferentially along the dipping direction during the LB filmThe orientation of the chargetransfer axis 5 for a single molecule is sketched in Figure 1 a together with the coordinate system used. The z axis is the dipping direction. The orientation of 5 is defined by two angles 6 and 4 with average absolute values of I@/ = 63" and 141 ...

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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.

The linear and nonlinear optical properties of well-ordered Y-type Langmuir–Blodgett (LB) multilayers of 2-docosylamino-5-nitropyridine (DCANP) have been investigated. The nonlinear optical susceptibilities have been determined for the wavelengths λ=1064 nm [d33=(7.8±1) pm/V, d31=(2.0±0.5) pm/V] and λ=1318 nm [d33=(5.6±1) pm/V]. In first waveguiding experiments carried out TE0 and TM0 modes propagating over more than 20 mm could be excited. Coupling experiments allowed the determination of the dispersion of the refractive index n3 (n632.8 nm3 =1.598). Guided wave attenuation coefficients as low as 12 dB/cm (at the wavelength λ=632.8 nm) are reported.