Angela Piegari

On the super-Gaussian unstable resonators for high-gain short-pulse laser media

High-mode-volume, high-energy (70-mJ), high-brightness (1.6*10/sup 14/ W cm/sup -2/ Sr/sup -1/) laser beams have been extracted from a high-gain short-pulse XeCl laser with a super-Gaussian unstable resonator. These results are obtained by increasing the cavity magnification factor to 6.6 and the output mirror peak reflectivity to 78%. The output beam divergence is seen to decrease from an initial value of approximately 0.2 mrad down to nearly diffraction limited after 6 ns for the short gain build-up time of the active medium. >

Variable narrow-band transmission filters for spectrometry from space.2. Fabrication process

The optical components described here are variable narrow-band transmission filters, where the transmittance peak varies with the position along the surface of the filter itself. They allow the construction of ultracompact and low-weight spectrometers for space applications. The theoretical behavior of graded filters has been already investigated by the authors, for imaging spectrometry of the Earth surface. The application of graded filters to miniaturized instruments for planetary missions (Mercury) is considered. Experimental results on the fabrication of small-dimension variable transmission filters operating over a wide spectrum, from visible to near infrared, are reported.

Optical coatings with variable reflectance for laser mirrors.

Design procedures and fabrication techniques of variable reflectance dielectric mirrors for laser output couplers are proposed. The design criteria, based on the use of only one shaped layer in a multilayer structure, yield any peak value and shape of the reflectance profile. The deposition technique of the variable thickness layer consists of a sputtering process with a suitable mask interposed between the target and substrate. Results of a simple model that correlates the mirror parameters to mask diameter and position are presented. A few devices for a Nd:YAG laser have been proposed, characterized, and successfully tested in a laser resonator.

Variable narrowband transmission filters with a wide rejection band for spectrometry.

Variable narrowband transmission filters are useful for the development of compact spectrometers. For this purpose the filter should be directly coupled to the detector and the wavelength of the transmission peak should move in one surface direction over a length of a few millimeters. To obtain both a wide measurement spectrum and high accuracy, the ratio of extreme operating wavelengths is required to be greater than 2:1 and the width of the transmission band narrower than 10 nm. A metal-dielectric variable transmission filter, with an operating range of 400-1000 nm, is proposed. The method for obtaining variable transmission filters, with dimensions of a few millimeters, is described.

Laser optical coatings produced by ion beam assisted deposition

Abstract A general requirement of optical coatings for lasers is low absorption combined with good mechanical behaviour. Ion beam assisted deposition is recognized to be a technique that gives high density and durable coatings but frequently with unacceptably high absorption. In this work the influence of ion assistance on optical and mechanical properties of films of interest for CO 2 laser and excimer laser coatings (MgF 2 , BaF 2 , Y 2 O 3 , ZrO 2 , ZnSe) has been investigated. Ion assisted deposited fluorides exhibit a large improvement in hardness and density and at the same time their absorption is low at the wavelengths of interest. The mechanical properties of the other materials are also improved by the ion assistance and, for oxide films, the absorption is lowered by using an oxygen ion beam. Antireflection and partially reflecting coatings for pulsed XeCl (308 nm) and CO 2 (10.6μm) lasers have been produced and tested in the operating conditions of the laser resonators where they will be employed.

Radio frequency sputtered Al:ZnO-Ag transparent conductor: A plasmonic nanostructure with enhanced optical and electrical properties

Optimization of metal-based transparent conductors (MTCs) made of silver and aluminium-doped zinc oxide (AZO) prepared by radio-frequency (r.f.) sputtering has been carried out through tuning of metal film properties. The influence of morphology and related plasmonic features of AZO/Ag/AZO MTCs on their optical and electrical performance is demonstrated and it is shown that the nominal thickness of the silver layer itself is not the most crucial value determining the MTC performance. The MTC performance has been optimized by a search of deposition conditions ensuring fractal-type metal layer formation up to a certain coalescence state that enables full gaining from silver optical properties, including its plasmonic features. For 150 W- and 200 W-deposited silver, MTCs with maximum transmittance as high as 83.6% have been obtained. These coatings have a figure of merit as good as 0.01 Ω−1 and a remarkably wide spectral transparency region: transmittance higher than 70% down to 1200 nm for 200W-samples. Modelling of the MTC coatings is proposed additionally, based on variable angle spectroscopic ellipsometric measurements, which takes into account the variation of the optical properties of silver when deposited in various conditions and embedded in a semiconductor stack.

Analysis of inhomogeneous thin films by spectrophotometric measurements

Abstract A method for the determination of optical parameters of inhomogeneous thin films is proposed. It allows the calculation of the refractive index profile throughout the layer and is not limited to linear profiles. From transmittance and reflectance measurements approximate values for the mean refractive index, its gradient and the film absorption are worked out and are used as initial conditions for a refining process based on a random search. As a result of this process the index profile is obtained at different wavelengths and that allows the calculation of the index mean value and its inhomogeneity with their dispersion. Inhomogeneous dielectric films have been analysed with satisfactory results.

Behavior of optical thin-film materials and coatings under proton and gamma irradiation

Optical materials and coatings are exposed to the flux of energetic particles when used in either space applications or nuclear energy plants. The study of their behavior in such an environment is important to avoid failure of the optical components during their operation. The optical performance of several thin-film materials ((HfO2, Ta2O5, Nb2O5, TiO2, SiO2) and coatings, under irradiation with high-dose gamma rays (5.8 MGy) and exposure to low-energy (60 keV) protons, has been investigated. Some variations of optical properties have been detected in silicon oxide after irradiation, while the other materials are stable in such conditions.

Laser mirrors with variable reflected intensity and uniform phase shift: design process.

Optical coatings with circularly symmetric graded reflectance are used as laser mirrors in unstable resonators. A proper design of such coatings permits any maximum central reflectance to be obtained along with a null external reflectance. Different design approaches are discussed, and an optimized design that gives negligible distortion of the reflected and transmitted wave front is proposed. Coatings with super-Gaussian and step reflectance profiles are examined as two different solutions for improving laser beam quality.

Laser damage studies on MgF2 thin films

The results of laser damage studies performed at 248 nm (KrF excimer laser) on MgF2 thin films deposited by different techniques (electron-beam evaporation, thermal boat evaporation, and ion-beam sputtering) on fused silica and CaF2 substrates are presented. We find that the films deposited on CaF2 substrates by the electron-beam evaporation technique present the highest damage threshold fluence (9 J/cm2). The photoacoustic (PA) beam deflection technique was employed, in addition to microscopical inspection, to determine laser damage fluences. We confirm, by scanning electron microscopy analysis of the damaged spots, the capability of the PA technique to provide information on the mechanisms leading to damage. The dependence of both laser damage fluence and damage morphology on the film deposition technique, as well as on the film substrate, is discussed.