Phase‐matched optical second‐harmonic generation in Langmuir–Blodgett film waveguides by mode conversion

Abstract

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 ...

Phase-Matched Second-Harmonic Generation from Five-Layer Langmuir-Blodgett Film Waveguides with Large Fabrication Tolerance and Overlap Integral for Mode Conversion

“…The measured conversion efficiency was normalized and compared with other experiments. The efficiency of modal conversion for DCANP (2x10 -2 %/W) is higher than our data (1.9x10 -4 %/W) [16]. The relatively low efficiency of our device is ascribed to small d 33 of DONPU.…”

“…The orientation of charge transfer is defined by both tilt (θ) and azimuthal angle (φ). In the case of DCANP average θ and φ were reported as 63 and 27 , respectively [16], whereas θ and φ of our DONPU film are estimated to be 64 and 47 , respectively. Although DONPU possesses small χ (2) it can be deposited up to several hundreds layers without deforming the structures.…”

“…Therefore, DONPU becomes a good candidate to confirm the large dimension tolerance of five-layer waveguide. The modal dispersion PM has been already applied to inorganic materials [5], organic single crystals [17], poled polymers [18][19][20][21] as well as Langmuir-Blodgett (LB) films [16][17][18][19][20][21][22]. In this paper we define the full width of half maximum (FWHM) as a fabrication tolerance of waveguide dimensions which one can obtain 50% of maximum SHG intensity.…”

Phase-matched second harmonic generation and nonlinear phase shift in a Langmuir-Blodgett film waveguide

“…It was found that DCANP is an amphiphile which combines high second-order hyperpolarisability with the ability to form LB multi-layers thick enough to allow wave-guiding. Therefore, in such DCANP-films the principle of modal dispersion 17,19,20,24,28 can be used to reach efficient SHG. The present work reports about SHG and non-linear phase-shift in DCANP wave-guides having a special supra-molecular architecture.…”

“…In the present work a low-molecular amphiphile was used for wave-guide preparation: 2-docosylamino-5-nitropyridine (DCANP). The non-linear optical properties of this material have been investigated previously [22][23][24][25][26][27] . It was found that DCANP is an amphiphile which combines high second-order hyperpolarisability with the ability to form LB multi-layers thick enough to allow wave-guiding.…”