Second-harmonic generation in a laterally azo-bridged trimer ferroelectric liquid crystal: Phase matching in the presence of a helicoidal structure

Abstract

Optical second-harmonic generation (SHG) has been studied in a trimer smectic C* liquid crystal especially designed for nonlinear optical applications. The molecule has a long conjugated donor-acceptor unit that transversally links three parallel rod-shaped moieties. A strong SHG signal has been observed at a fundamental wavelength of 1369 nm even in the presence of the spontaneous helicoidal structure of the smectic C* phase.This unusual behavior has been interpreted as due to the existence of a phase matching, in which the wave vector mismatch is compensated by the wave vector of the helix. This point has been confirmed by the study of the SHG intensity versus sample thickness and light polarization characteristics. The main coefficient of the second-order susceptibility tensor of the material has been estimated to be d 22 =28 pm/V.

An Azo-Bridged Ferroelectric Liquid Crystal with Highly Enhanced Second and Third Harmonic Generation

“…The PM condition is attained for p = λ 2 ( n 2 − n 1 ) where n 1 and n 2 are refractive indices for the fundamental and second-harmonic waves, respectively. An in-depth explanation of this phenomenon together with the detailed experimental results will be reported in a separate forthcoming paper . Here we will restrict ourselves to outlining how to obtain information about the d ij coefficients in an experiment with the HPM condition.…”

“…Currently there is no theory dealing with the two problems together. To estimate the d ij size, we adopted an expression deduced for the SHG intensity I 2ω generated by a helical structure, which is supposed to be valid at HPM, normal incidence, and neglecting the absorption at the fundamental wavelength: I 2 ω = C d eff 2 I ω 2 L 2 normale − α L / 2 sinh 2 ( α L / 4 ) false( α L / 4 false) 2 where C is a constant, L the sample thickness, α the absorption coefficient at 2ω, and d eff an effective nonlinear coefficient depending on the input and output polarizations.…”

Liquid Crystalline Properties of 6-(4-Cyanobiphenyl-4’-yloxy)hexyl 4’-[ω-(p-Nitrophenyloxy)alkoxy]-4-biphenylcarboxylates

“…Increasing interest is being shown in the liquid crystalline properties of liquid crystal dimers, trimers, and tetramers, which are the molecules containing two, three, and four mesogenic groups linked by flexible spacers, respectively, because the properties exhibit a remarkable dependence on the length and parity of the flexible spacers [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. That is to say, the liquid crystalline properties of the compounds with odd-membered flexible spacers may be conspicuously different from those with even-membered flexible spacers.…”