L. F. Avila

Design and fabrication of two-dimensional hexagonal photonic crystals with a linear waveguide in erbium doped GeO2-Bi2O3-PbO-TiO2 glasses

In this work, we designed and recorded two-dimensional Hexagonal Photonic Crystals (2D-HPC) layers, with a linear waveguide, in erbium doped GeO2-Bi2O3-PbO-TiO2 glassy films, by combining the techniques of holographic recording and femtosecond (fs) laser micromachining. The 2D-HPC is recorded holographically in a photoresist film coated on a glass substrate by exposing the sample to the same interference pattern twice and rotating the sample of 60° between the exposures. After the development a two dimensional hexagonal array of photoresist columns remain on the glass substrate. The recording of the waveguide is made by a fs laser micromachining system focused at sample surface. The laser spot produces the ablation of the photoresist columns generating a defect line in the periodic hexagonal array. After the recording of the photoresist template, the erbium doped GeO2-Bi2O3-PbO-TiO2 film is evaporated on the photoresist and finally the photoresist template is removed using acetone. The design of the geometrical parameters of the 2D-HPC is performed by calculation of the dispersion mode curves of the photonic crystal using a 2D finite element method. The proper geometrical parameters depend on both the refractive index of the glass film and thickness. Such parameters as well as the period of the 2D-HPC have been defined in order to obtain a photonic band gap in the region of erbium luminescence band. In such condition the erbium luminescence will propagate only through the waveguide.

Self diffraction holographic techniques for investigation of photosensitive materials

Holographic techniques are powerful tools to study photosensitive materials due to the high sensitivity of diffraction measurement and the ability to detect dynamic gratings. The self diffraction technique consists in to project an interference fringe pattern into the photosensitive material and to measure, in real time, the self-diffraction of the interfering beams, at the grating generated in the photosensitive material. Besides the higher sensitivity, such measurement allows to measure simultaneously and separately the phase and the amplitude grating contributions, as well as thin or thick gratings. In order to demonstrate potentiality of this technique we measured the kinetic constant of the photo-reaction in positive photoresists (AZ types) and negative SU-8 photoresist, as well as the maximum values of the refractive index and of the absorption coefficient modulations induced in these materials at different wavelengths of exposure. The same measurements were performed in SB based chalcogenide glasses in order to evaluate the potential of such materials to be used as optical data storage devices.

Measurement of phase and amplitude modulations in Sb-Based Films

In this work we studied the changes of the optical constants of films in the binary system Sb2O3-Sb2S3 induced by light in the VIS-UV. The measurements were performed before and after homogeneous irradiation of the films to a Hg lamp and in real time during the holographic exposure of the samples (at 458nm). Changes of the absorption coefficient (amplitude grating) and refractive index (phase grating) were measured simultaneously using the self-diffraction using the holographic setup. Besides the films presented a strong photodarkening effect under homogeneous irradiation, the samples holographically exposed presented only refractive index modulations. None amplitude modulation was measured in real time for spatial frequencies of about 1000 l/mm.

Kinetic of Photoacid Generation in SU8 Negative Photoresists

We measure the kinetic constant of the photoacid generation in SU8 photoresists using two different methods. The measurements, performed under homogeneous exposures and holographic exposures, show a good accordance for the kinetic constant values.

Photoresist absorption effect in holographic recording

In this paper we compare the photoresist profile of holographic gratings recorded using two different wavelengths λ = 458 nm and λ = 407 nm. We show that the use of the wavelength λ = 458 nm allows the recording of very deep structures (aspect ratios of >3) because for photoresist films up to 1 μm of thickness the absorption is negligible. By the other side for the wavelength λ = 407 nm the absorption coefficient increases by a factor 5. This effect reduces the inclination of lateral walls of the structures as well as disturbs our fringe locker system that uses the residual real time modulation of the refractive index of the photoresist to monitor the fringe perturbation.