Claudio Cali

Super-Gaussian reflectivity unstable resonator for excimer lasers

High‐mode‐volume, good optical quality laser beams of 55 mJ, of 19 ns duration and with a brightness of 3×1013 W cm−2 Sr−1, have been extracted from a high‐gain, short‐pulse XeCl laser, with an unstable resonator using a super‐Gaussian reflectivity mirror. The misalignment angle of the output coupler to decrease the normalized output energy by 20% was 3.5 mrad.

Fabrication and characterization of tuned Gaussian mirrors for the visible and the near infrared

A thin-film technique has been developed for vacuum fabrication of mirrors with Gaussian reflectivity profiles. Samples with diameters from 2 to 8 mm and assigned maximum reflectivities for visible or near-IR wavelengths have been made and their optical properties evaluated. By properly choosing both the geometry of the evaporation source and the masking system, one can obtain quasi-Gaussian or super-Gaussian reflectivity profiles.

Nd:YAG laser with multidielectric variable reflectivity output coupler

Abstract A gaussian reflectivity output coupler fabricated by means of a new thin film vacuum deposition technique has been experimentally tested in an unstable Nd:YAG laser resonator. High brightness, diffraction limited beams with flat top profile have been generated. The results are compared with those obtained by unstable and stable resonator configurations with uniform reflectivity mirrors.

Electrochemical Fabrication of Metal/Oxide/Conducting Polymer Junction

After discovery of conducting polymers and the possibility to modify their electrical properties from insulating to metallic like behavior by doping and a careful choice of the processing conditions, a large amount of research effort has been devoted to the theoretical understanding of their solid state properties as well as to exploit the possible application of conducting polymers in many technological fields including large area organic electronics, polymer photovoltaic cell, and sensors. 1-4 Organic thin film transistors appear very promising devices for the development of low cost, flexible, and disposable plastic electronics. In order to reduce the operating voltage it has been suggested in the literature to use mixed inorganic–organic thin film transistors by assembling a structure formed by metal bottom contact/dielectric layer gate/organic semiconductor/top contact source/drain. According to this, a wet electrochemical route appears to be very promising in terms of cost, at least for the preparation of thin thickness 10 nm or thick thickness 10 nm oxide films by anodizing in aqueous electrolytes. Moreover, by taking into account the possibility to grow semiconducting polymers on wide bandgap dielectric oxide Ta2O5 by a photoelectrochemical route, which has been shown recently, 5,6 it seems very appealing to exploit an integral electrochemical route to fabricate advanced inorganic/organic hybrid structure which could be used as a building block for a field effect transistor FET junction. In this paper, we describe and discuss the electrochemical fabrication of a hybrid structure to be used in the production of an inorganic–organic field effect transistor IOFET using 3,4polyethylenedioxythiophene PEDOT as a semiconducting polymer and anodic films grown on the Ti-10 atom % Zr alloy as dielectrics. The choice of the oxide is based on its low dark current value and quite high photocurrent intensity, under monochromatic light, at not too high anodic potential and photon energy, 7 as well as on its high dielectric permittivity 45, according to Ref. 8. The metal/oxide/ polymer junctions are investigated by photocurrent spectroscopy PCS and scanning electron microscopy SEM. Finally, output transistor characteristics are recorded in order to test the performance of the junctions in the IOFET structure. Experimental Ti-10 atom % Zr alloys were prepared by dc magnetron sputtering. Targets consisted of 99.9% zirconium disk, of 100 mm diameter, with an appropriate number of 99.9% titanium disks, of 20 mm diameter, located symmetrically on the erosion region for the preparation of the alloys. Substrates used were glass plates. 8 The composition of the alloy was determined by Rutherford backscattering

Performance of a XeCl laser with super-gaussian reflectivity unstable resonators

Abstract Super-gaussian reflectivity unstable resonators of different magnification factor M have been applied to a high-gain, short-pulse XeCl laser, to investigate to what extent the output laser beam characteristics depend on M . It has been found that high magnification factors are required to get a fast establishment of a diffraction limited mode. However, as M is increased, the cavity feedback and then the super-gaussian mirror peak reflectivity should be also increased to get a good mode control of the laser oscillating radiation.

Erroneous p-type assignment by Hall effect measurements in annealed ZnO films grown on InP substrate

We report on incorrect carrier type identification achieved by Hall effect measurements performed on ZnO films grown by pulsed laser deposition on InP substrates and subsequently annealed for 1 h at 600 °C in air. While Hall measurements, after post-growth annealing, reveal a change in the electrical properties of the films, from n-type to p-type, both photocurrent-based and standard C−V measurements performed on the same samples show no change in the native n-type doping of the ZnO films. A possible interpretation of the two results is reported. In particular, p-type conductivity observed by Hall effect may be ascribed to a highly conductive thin layer formed during the annealing process at the ZnO/InP interface, which dominates the Hall effect measurements and does not influence the photo-electrochemical behavior of ZnO as well as the measured differential capacitance. The conflicting results here reported show that for this kind of samples, Hall effect measurement can be misleading with respect to the ...

Second harmonic generation in InSe

Abstract Experimental evaluation of non-linear susceptibility coefficient d22 for InSe, which is responsible for second harmonic generation in this material at 1.06 and 10.6 μm, is reported and the value compared with GaSe. At the CO2 laser frequency of oscillation, phase matching has been observed and a value of d22 almost equal to d15 for CdSe has been found. The InSe high transparency in the wavelength range 1.1–22 μm, the sizeable value of d22 and the possibility of reaching phase matching condition in the IR range make InSe a potential device for far IR frequency conversion.

Characterization of an unstable gaussian-reflectivity resonator in a XeCl laser

Abstract A gaussian reflectivity resonator of magnification M=5.2 has been used in a high-gain, short-pulse excimer laser. A good optical quality, 20 mm diameter laser beam, of 60 mJ, 18.6 ns long and with a brightness of 2.4x1013 W cm-2 sr-1 has been obtained. By decreasing the active mode volume cross section, higher optical quality laser beams have been obtained.

Warm white LED light by frequency down-conversion of mixed yellow and red Lumogen

This work reports on the benefits and promising opportunities offered by white LED hybrid technology, based on a mixing perylene-based dyes in order to obtain a warm white light for frequency-down conversion. In a standard Ce:YAG-based white LED, the white light appears cold due to the weakness of red wavelength components in the emission spectrum. In order to obtain a warmer white, one possible solution is to add a red phosphor to the yellow one to move the chromatic coordinates properly, though the luminous efficiency drastically decreases due to the increased light absorption of the coating layer. It is generally believed that the low efficiency of warm white LEDs is the main issue today for LED-based lighting. Using photoluminescence of Lumogen® F Yellow 083, a perylene-based polymer dye commercialized by BASF, and adding a small quantity of another perylene-based dye, Lumogen® F Red 305 (BASF), we obtained high-efficiency warm white LEDs by yellow and red conversion from a standard 450 nm GaN/InGaN royal blue LED. Different weight proportions of dyes were dissolved in solutions with equal amounts of poly-methyl-methacrylate (PMMA) in ethyl acetate, then the LEDs were dip-coated in each solution and optically characterized. Record values of 8.03 lm of luminous flux and 116.11 lm/W of optical efficiency were achieved. Finally, the effects of both driving current, and pump wavelength on LED performances – such as chromatic coordinates, correlated color temperature, color rendering index (CRI), and optical efficiency – were investigated.

Use of a Scanning Laser Beam for Thin Film Control and Characterization

A simple and inexpensive method currently used in our laboratory for controlling and characterizing thin films is described. A review of several applications of the method is given, together with a few examples of typical results which can be obtained with our apparatus.