Generation of harmonics and supercontinuum in nematic liquid crystals

Abstract

Nonlinear optical properties of nematic liquid crystals (NLC) have been investigated. A technique for efficient laser frequency conversion in a microscopic NLC volume deposited on an optical fibre end face is experimentally demonstrated. An efficient design of a compact NLC-based IR frequency converter with a fibre input and achromatic collimator is proposed and implemented. Simultaneous generation of the second and third harmonics is obtained for the first time under pumping NLC by a 1.56-mm femtosecond fibre laser. The second-harmonic generation efficiency is measured to be about 1 %, while the efficiency of third-harmonic generation is several tenths of percent. A strong polarisation dependence of the third-harmonic generation efficiency is revealed. When pumping NLC by a cw laser, generation of spectral supercontinua (covering the visible and near-IR spectral ranges) is observed. The nonlinear effects revealed can be due to the light-induced change in the orientational order in liquid crystals, which breaks the initial symmetry and leads to formation of disclination structures. The NLC optical nonlinearity is believed to be of mixed orientationalelectronic nature as a whole.

Femtosecond Green Light Generation Using a MgO-Doped Periodically Poled Lithium Niobate Crystal Pumped by a Yb-Doped Fiber Laser

“…Indeed, there have been various wavelength-conversion methods, such as thirdharmonic generation in nematic liquid crystals [10], supercontinuum generation in fused silica [11], self-frequency doubling lasers [12] and optical sum-frequency from dual-wavelength lasers [13]. For reliable and efficient green femtosecond laser generation, the conventional route applies frequency doubling from femtosecond lasers around 1.0 µm.…”

Harmonic generation in a nematic liquid crystal

“…These factors also include influences that can be combined with one term-selfaction, which was first demonstrated in [2]. In laser irradiation, self-actions [2,[6][7][8][9], which deform the NLC and change the symmetry in the bulk, may include the light-induced Fredericks transition [10,11], thermo-mechanical [12], and thermo-orientation phenomena [13]. As a result, the generation of even harmonics seems to be divided into two consecutive stages: first, bulk deformation occurs, as a rule, leading to changes in the uniform direction of the optical axis (director) of the NLC in space and removing central symmetry, then nonlinear generation, as in solid crystals, occurs even harmonics.…”

“…Nonlinear optical phenomena in liquid crystals which are associated with the bulk conversion of the radiance frequency (harmonic generation, sum-frequency and differencefrequency generation) are phenomena that may eventually be of great applied value [6,7,14]. The features of the frequency conversion in LC mostly include the high efficiency of the process, which can reach several percent [7][8][9] at thicknesses of 10-100 microns. Such efficiencies, combined with small sizes, allow to create fairly simple and small-sized converters or optical parametric generators [9] to expand the frequency range of laser systems with relatively small (∼100 mW) average power.…”

Fiber-to-fiber nonlinear coupling via a nematic liquid crystal

“…The listed features make NLCs an attractive optical medium for research into nonlinear photonics and the development of novel optic devices [7], including fibercoupled ones. For instance, a miniature fiber-coupled NLCbased laser frequency converter was demonstrated recently [8]. It had a fiber input and a free-space output.…”