Alain Haché

Discrete surface solitons.

It is theoretically shown that discrete nonlinear surface waves are possible in waveguide lattices. These self-trapped states are located at the edge of the array and can exist only above a certain power threshold. The excitation characteristics and stability properties of these surface waves are systematically investigated.

Self-assembling three-dimensional colloidal photonic crystal structure with high crystalline quality

High-quality colloidal crystal multilayers were fabricated from aqueous solutions by the vertical deposition method. The effect of the evaporation temperature on the crystalline quality of colloidal crystals was carried out. It is found that with the increase of the evaporation temperature, the colloidal crystal shows an increasing tendency towards equilibrium face-centered-cubic phase, and the resulted sample also shows few dislocations and vacancies when the balance in the processes of nucleus formation, particle transport, and crystallization can be kept. However, with the further increase of the evaporation temperature (above 55 °C), a vast amount of defects appear in the crystal because the fast water evaporation rate, which results in a fast crystal growth rate, will spoil the balance. Optical measurements correspond well to the microstructure results.

Ultrafast all-optical switching in a silicon-based photonic crystal

We study the effect of two-photon absorption and Kerr nonlinearity on the optical properties of a one-dimensional photonic crystal made with amorphous silicon and SiO2. A stop band appearing near 1.5 μm is monitored with a weak probe beam and modulated by changes in the refractive index caused by a pump pulse at 1.71 μm with 18 GW/cm2 peak intensity. Nonlinear optical characterization of the sample using Z-scan points out to two-photon absorption as the main contributor to free carrier excitation in silicon at that power level. Modulation in the transmittance near the band edge is found to be dominated by the optical Kerr effect within the pulse overlap (∼400 fs) whereas free carrier index changes are observed for 12 ps.

Long-range superluminal pulse propagation in a coaxial photonic crystal

We study the propagation of brief electric pulses along a coaxial line having a spatially periodic impedance. The periodicity causes anomalous dispersion and the appearance of a stop band in transmission near 10 MHz. Group velocities of up to three times the speed of light are observed in that spectral region, in accordance with calculations based on an effective index theory.

Thermal conduction in thin films measured by optical surface thermal lensing

Thermal conduction across thin films is measured optically by using the surface thermal lensing effect. Pump-probe laser measurements combined with numerical modeling are used to study thermal conduction in a variety of materials as thin as 10 nm. The method is relatively simple, robust, rapid, and offers an alternative to current techniques. Thermal conductivity in gold films is found to drop from 300 to 100 W/Km when the film thickness is reduced from 2000 to 100 nm. Results for silver, tin and aluminum films are also presented and compared with results from other studies.

Reflective thermal lensing and optical measurement of thermal diffusivity in liquids

Using a pump-probe laser technique, we analyze the relaxation dynamics of a thermal lens generated at the surface of metal films deposited on a glass substrate. When the film is in contact with a liquid and the film is periodically heated with an optical source, the dynamics of the thermal lens is found to be closely related to the thermal properties of its environment. As a result, reflective thermal lensing can be used to measure the thermal diffusivity of a variety of liquids placed in contact with the film to an accuracy better than 2%. Experimental results point to applications to the study of transient effects in fluids and immiscible liquids, and detection of trace of contaminants in solvents.

Nonlinear coaxial photonic crystal

We demonstrate that large-scale photonic crystals can be used to simulate nonlinear optical effects occurring in real photonic crystals. A crystal made of coaxial segments with periodic impedance is used to create a stop band in transmission near 10 MHz. When a semiconductor rectifying diode is added to the crystal, a nonlinear mode of propagation appears within the forbidden band gap. It originates from a breaking of symmetry and an intensity-dependent attenuation similar to that encountered in saturable absorbers. Experimental results agree well with a theory based on a simple coupled-mass model with nonlinear resistive force.

A comparative study of inverted-opal titania photonic crystals made from polymer and silica colloidal crystal templates

Photonic crystals with an inverted-opal structure using polymer and silica colloidal crystal templates were prepared and compared. We show that the behaviors of the template during the removal process and heat treatment are determinant factors on the crystal formation. While both templates result in ordered macroporous structures, the optical quality in each case is quite different. The removal of the polymer template by sintering causes a large shrinkage of the inverted framework and produces a high density of cracks in the sample. With a silica template, sintering actually improves the quality of the inverted structure by enhancing the template’s mechanical stability, helping increase the filling fraction, and consolidating the titania framework. The role of the other important factors such as preheating and multiple infiltrations is also investigated.

Enhanced photochromism in nanostructured molybdenum trioxide films

We present evidence of enhancement of photochromism in nanostructured thin films of molybdenum oxide fabricated by glancing angle deposition. The strong correlation of coloration response with the internal surface area of the films provides evidence of the importance of nanostructuring on the photochromic effect and the vital role played by the availability of water in the photochromic mechanism.

Optically tunable hollow Gaussian beams with thin metal films

We report the formation of optically tunable, smooth hollow beams by reflection of a TEM00 Gaussian beam off a metal thin film. Hollow (doughnutlike) beams (HBs) with controllable profiles are created by a phase distortion at the surface. Two regimes of operation are observed: Below a certain power threshold, the HB formation is reversible and optically tunable on a time scale of milliseconds; above that threshold, alterations on the film surface make the effect permanent. Optical control of the HB shape is demonstrated by tuning the power of a second beam. High stability in the radial intensity profile and the possibility of adjusting the spatial distribution and aspect ratios make this technique promising for applications such as atom trapping and manipulation of Bose-Einstein condensates.