Thermally-Induced Nonlinear Spatial Shaping of Infrared Femtosecond Pulses in Nematic Liquid Crystals

Liquid crystals (LC) offer numerous prospects in photonics, for instance, nematics LC-based electro-optical devices enables tunable phase-shifting of optical and THz radiations, or phase-sensing. Indium-Tin-Oxide (ITO) is one of the most employed electrode coating for this applications, thanks to its good transmission in the visible part of the optical spectrum. However, extension towards infrared wavelengths finds some difficulties, such as ITO strong absorption and nonlinear behavior around 1.55μιη. Although some optimized versions of ITO are available in this spectral range, with a remaining weak absorption might have a significant impact on the LC optical answer. We demonstrate that an infrared femtosecond oscillator focalized up to 7.6 kW/cm
₂
, undergoes strong spatial self-phase modulation in a 180μm thick E7 nematic mixture cell, due to partial laser light absorption (20%) in the yet optimized ITO coating. The heat absorption in the ITO layer is transferred to the nematic layer and a well-confined thermal gradient is established due to the different heat transfer coefficients. Large and nonlinear sensitivity of the thennotropic LC with the temperature generates a typical spatial self-phase modulation pattern in thin media (i.e. multiple-ring pattern) when the average power density is high enough and dephasing is larger than 2π. The number of rings (N) is a function of the index gradient and, therefore, increases with the power density, meanwhile the polarization sets the shape of the rings and the sign of the non-linearity (Figure a, b). From the number of rings we can estimate the introduced dephasing Δφ, so the refractive index variation. Figure c), e) represents the measured An as a function of the average power (c) and as a function of the position with respect to the focus. Close to the focus, the phase transition between nematic and isotropic phases (T
_c
= 331K for E7) is reached. We can also recover ΔT inside the LC across the laser spot from Δn (Figure d, f) and verify that the anisotropy of the heat transfer coefficients (two times lower in the ordinary axe) makes that T
₀
>T
_e
. The stability of the non-linear pattern is better than π/5 for an overall of phase shift of 30π. The refractive index variration is temporally stable and the spectral distribution in the rings is found unchanged, therefore the thermal gradient is stable and well-confined with a spatial resolution measured below 140 μm. The usual spatial nonlinear effects achieved with fs pulses are often generated with bulk media, so self-focusing or defocusing effects limit spatial phase shift. Here, no modulation of the temporal phase occurs, and thus the spectro-temporal characteristics of the pulse are not affected.