Xavier Hutsebaut

Simulations and experiments on self-focusing conditions in nematic liquid-crystal planar cells

Owing to the nonlinear effect of optical field-induced director reorientation, self-focusing of an optical beam can occur in nematic liquid crystals and an almost diffraction-compensated propagation can be observed with milliwatts of light power and propagation lengths of several millimeters. This opens the way for applications in all-optical signal handling and reconfigurable optical interconnections. Self-focusing of an optical beam in nematic liquid-crystal cells has been studied experimentally and by means of numerical simulation. The relationships between bias voltage, cell thickness and required optical power have been examined, thus allowing the determination of the most favorable conditions for soliton-like beam propagation.

Measurement of the self-induced waveguide of a solitonlike optical beam in a nematic liquid crystal

Using phase-measurement interferometry, we observe the waveguide induced by a solitonlike optical beam sustained by the molecular reorientation-induced optical nonlinearity of a nematic liquid crystal in planar configuration. Our purpose in the study is to characterize the nonlocality of the optical response of nematic liquid crystals. A good agreement is obtained between the experiment and a full (2+1)-dimensional numerical simulation of the nonlinear optical beam propagation in the cell.

Time dependence of soliton formation in planar cells of nematic liquid crystals

Spatial optical solitons can be observed in bulk nematic-liquid-crystal cells with an entrance window and a bias voltage applied over the cell to enhance the optical nonlinear effect of laser-induced molecular reorientation. The soliton-induced waveguide is observed in transmission by use of a second light source and crossed polarizers. This setup allows us to investigate the time dependence of the molecular reorientation that sustains the soliton-like beam propagation. Results from a numerical simulation are in good agreement with the experimental results, which confirms the validity of our model.

Induced modulation instability and recurrence in nematic liquid crystals

We study induced modulation instability in a nematic liquid crystal cell. Two broad elliptical beams along one direction are launched into the cell. The two beams have slightly different angle in order to create a sinusoidally varying intensity at the entrance of the cell. In this way, the gain of perturbations with different spatial frequency is investigated. The evolution of the optical pattern, for certain conditions, shows a recurrence of the signal. We believe that this is the manifestation of the Fermi-Pasta- Ulam recurrence and to the best of our knowledge, the first experimental observation of this phenomenon in the spatial optical domain. Numerical simulations show a good agreement with the experimental findings.

Observation of out-coupling of a nematicon

In this work we present the observation of spatial optical solitons in liquid crystal cells by recording the diffraction pattern of the out-coupled beam on a distant screen. Simultaneously, the light propagation is observed via scattering measurements. The most important observation is displacement of the beam on the screen due to the transverse undulation inside the cell. This undulation is caused by the anisotropic walk-off of the beam. The displacement is in good agreement with the values of the undulation earlier reported.

One-dimensional simulation of field-induced director reorientation and lateral light propagation in liquid crystals

Spatial solitons in liquid crystals can be observed with milliwatts of light power due to the nonlocal saturable nonlinear effect of field-induced director reorientation. In novel generations of all-optical switching circuits and optical networks spatial solitons show some promising possibilities. Director orientation in an LC-layer is simulated for a dc-voltage over the layer and an optical field injected lateral to the layer. Due to a torque induced by the electric field the molecules will tilt and the index of refraction in the layer will rise. The simulation of the soliton behavior is based on the Euler-Lagrange variational formula for the distortion free energy of the liquid crystal. The raise in the refractive index causes a self-focusing effect. When the self-focusing balances the diffraction, a spatial soliton can occur. A BPM-algorithm is used to simulate the light propagation in the layer. Without a dc-field, a large optical field for soliton propagation is required to reach the threshold to initiate the molecular reorientation of the liquid crystal molecules. A dc-field can be used to overcome the Fréederickz-transition so that a lower optical field is required. The relations between optical field profiles, dc-voltage and layer thickness which enable soliton propagation are discussed.

Nonlinear wave guiding in nematic liquid crystals

Waveguiding in liquid crystals can be achieved by controlling the molecular orientation by means of external fields or by shaping the geometry of the substrates that contain the liquid crystal material. The creation of these waveguides in liquid crystals can also be achieved by using the optical nonlinear properties of the material. For a sufficient optical power (in the order of a few mW), the beam can induce its own optical waveguide. This is a selfinduced waveguide and the resulting beam is referred to as a soliton beam. In the last few years, the properties of these soliton beams have been studied thoroughly, revealing some interesting phenomena. In this article, simulations are reported on two common configurations in which solitons have been generated experimentally. The soliton beam, for certain configurations, displays an undulative behavior inside the cell, which may be used for large angle steering of the optical beam.