Lucila Cescato

Analysis of an active stabilization system for a holographic setup

An active stabilization system for a holographic setup based on detection of phase shift between the interference pattern and a reference hologram is described. Its basic feature is the possibility of operating for 0, pi or +/-pi/2 at will, always in a null-detection mode. The reference hologram may be a previously recorded permanent hologram or a real-time (even reversible) one. The use of the open loop response of the stabilization system is developed for analyzing its performance, which allows closer insight into parameters limiting Its behavior. The effects of different noise sources are analyzed in detail. The real-time effect in a positive resist is successfully employed for operating the stabilization setup for recording an improved grating in this material.

Interference-term real-time measurement for self-stabilized two-wave mixing in photorefractive crystals

We report the real-time direct interference-term measurement for a two-wave-mixing experiment in photorefractive crystals. Knowledge of the interference term may provide information concerning diffraction efficiency, interference pattern-to-recorded hologram phase shift, and optical activity and anisotropic diffraction properties of these materials. This method comprises phase modulation of one of the interfering beams and synchronous detection of the first and second harmonics in the resulting output irradiance modulation. Simultaneous detection of both harmonics enables the measurement to be made even in strongly perturbed conditions, since one harmonic is used for measuring and the other is used for operating Bi(12)TiO(20) are reported.

Photorefractive crystals for the stabilization of the holographic setup.

A photorefractive crystal of the sillenite family in a two-wave mixing experiment is used as a fundamental component in a negative feedback system for stabilizing a holographic setup. The behavior of such a stabilization system is theoretically analyzed, and the advantages and limitations of using a real-time nonpermanent recording material (photorefractive crystal) are discussed. We present experimental results using a Bi12TiO20 crystal, compare actual and predicted performances, and discuss the optimization of relevant parameters for better performance of the whole stabilizing system. Theoretical analysis and experimental results show this system to have interesting perspectives for further development.

Band gap of hexagonal 2D photonic crystals with elliptical holes recorded by interference lithography

Two-dimensional hexagonal photonic crystals can be recorded using the simple superimposition of two interference patterns rotated by 60 masculine. Such process generates high contrast masks, however, it generates elliptical cross section structures instead of cylinders. We study the PBG properties of the experimentally feasible geometries, using this technique and we demonstrate that the effect of this asymmetric shape is a reduction in the PBG map area, for TE polarization, in comparison with cylindrical structures. On the other hand, it appears a PBG for TM polarization.

Bulk photochromism in a tungstate-phosphate glass : A new optical memory material?

In this work, we present a new photochromic tungstate based glass which have both absorption coefficient and refractive index modified under laser exposure. The photosensitive effect is superficial under ultraviolet (UV) irradiation but occurs in the entire volume of the glass under visible irradiation. The effect can be obtained in any specific point inside the volume using an infrared femtosecond laser. In addition, the photosensitive phenomenon can be erased by specific heat treatment. This glass can be useful to substitute actual data storage supports and is a promising material for 3-dimensional (3D) and holographic optical storage.

Developed profile of holographically exposed photoresist gratings

A simulation of the profile of holographically recorded structures in photoresists is performed. In addition to its simplicity this simulation can be used to take into account the effects that arise from exposure, photosensitization, development, and resolution of positive photoresists. We analyzed the effects of isotropy of wet development, nonlinearity of the photoresist response curve, background light, and standing waves produced by reflection at the film-substrate interface by using this simulation, and the results agree with the experimentally recorded profiles.

Phase‐shift measurement in photorefractive holographic recording

A simple direct method for measuring the interference pattern of a light‐to‐recorded hologram phase shift in photorefractive crystals is reported. The method is used for studying the recording in Bi12SiO20 under an externally applied electric field. The results are then compared with theory and used for computing the density of photoelectron traps.

Reflecting polarizing beam splitter

We propose and experimentally demonstrate the use of metal-covered lamellar relief gratings as a polarizing beam splitter operating at a single wavelength near Littrow incidence. We report the characteristics of a grating produced by holography and reactive ion etching that was calculated for operation as beam splitter at lambda = 633 nm (for a He-Ne laser).

SU-8 submicrometric sieves recorded by UV interference lithography

Sieves are promising devices for filtration, separation of particles and drug delivery control because of their uniform pore size distribution and low flow resistance. SU-8 is a negative photoresist type epoxy that is hydrophobic and biocompatible. Thus, it is a good alternative to fabricate micro devices for biological applications. In this paper we show a novel fabrication technique of self-sustained sieves of an SU-8 photoresist, with pore dimensions in the range of hundreds of nanometers, using a combination of UV interference and conventional optical lithographies. The resulting sieves are SU-8 membranes with submicrometric pore sizes and coefficient of variation of 7%, in areas of 1 cm2.

Evolution of Surface Textures on n ‐ InP Samples Etched Photoelectrochemically

The formation and growth of surface textures on (100) n-InP samples etched photoelectrochemically in HCl solution was investigated. This etching produced elongated etch pits, which were analyzed using scanning electron microscopy and angular resolved optical scattering. The evolution of the size and geometry of the microstructures with the etching time is described and discussed. The effect of the surface texture on the sample reflectivity was followed by total integrated scattering and by I-V curve measurements