Marta Lanata

Study of Surface Treatment Effects on the Metal-CdZnTe Interface

The quality of a CdZnTe-based X-ray detector is highly related to the interface between semiconductor and metal contact. One of the factors that increase leakage currents in CdZnTe based X-ray detectors is the presence of a conductive surface layer. In this paper the result of the passivation of the CdZnTe surface by means of an aqueous solution of NH4F/H2O2 is studied by optical ellipsometry and by the current-voltage characteristics of gold contacts deposited on the oxidized surface. Collected data show that leakage currents can be reduced and contact stability improved by the combined use of the passivation layer and a guard ring.

Titania inverse opals for infrared optical applications

Photonic crystals have gathered great importance in recent years. In particular macroporous materials (inverse opals) show interesting properties as photonic crystals. Ordered macroporous titanium dioxide (TiO2) is made using polystyrene spheres as a template. Titania is chosen for its high refractive index (>2.5). Following an already known technique [E.G. Judith, J. Wijnhoven, W.L. Vos, Science 281 (1998) 802; B.T. Holland, C.F. Blanford, A. Stein, Science 281 (1998) 538; B.T. Holland, C.F. Blanford, T. Do, A. Stein, Chem. Mater. 11 (1999) 795] large-scale order in macroporous TiO2 is obtained both using the 778 and the 3190 nm beads as documented by optical microscope and SEM images. These structures would lead to applications in the mid-infrared range.

Enhancement of PMMA nonlinear optical properties by means of a quinoid molecule

Abstract In this paper we present the study of the linear and nonlinear optical properties of organic films based on mixing a polymeric matrix (PMMA) and a highly nonlinear organic material recently synthesised and characterised, namely (5,5 ′ -Bis(3,5-di- tert -butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-5,5 ′ -dihydro-2,2 ′ -bithiophene), with the prospective of reaching a good compromise between good processability and high nonlinear optical (NLO) properties of the solid solution. Linear optical characterisation is performed by means of variable angle spectroscopic ellipsometry in the spectral range between 260 and 1700 nm. Third order NLO properties of the samples have been investigated by third harmonic generation at fundamental wavelength of 1500 nm. Different guest/host concentrations have been studied. Due to the high solubility of the guest into the polymeric matrix, we have been able to reach concentration up to 50% in weight, reaching an enhancement of the NLO properties of PMMA by three order of magnitudes.

Molecules with quinoid ground state: a new class of large molecular optical nonlinearities?

Abstract The extent of π-electron delocalization in polyconjugated organic materials is the basic molecular structural property required for attaining large molecular hyperpolarizabilities (β, γ) and large nonlinear optical (NLO) response (χ(2), χ(3)). When the electronic and the vibrational contributions to hyperpolarizabilities are considered we have shown that for low band-gap molecules at least the longitudinal βe and γe are as large as βv and γv. Since molecular vibrations (frequencies and intensities in IR and Raman) can be derived both experimentally and theoretically by ab initio methods, the vibrational contributions to β and γ can be obtained from either experimental or calculated IR and Raman spectra and the two values compared. New materials with improved NLO properties need to be devised for a successful breakthrough in technology. We have synthesized low band-gap molecules whose ground electronic state is forced to be quinoid (planarity and the consequent π-electron delocalization should yield large γ values) and its first excited state is aromatic. Very little is known on NLO properties of quinoid systems. All our molecules have been characterized by UV-Vis, 1 H -NMR, FT-IR, FT-Raman and their properties theoretically studied by DFT methods. With this work we try to identify the molecular features which optimize the NLO properties and we find that the vibrational contribution to molecular hyperpolarizability γv (measured from spectra and calculated by quantum chemical methods) and the χ(3) value (experimentally determined by third-harmonic generation, THG) of the quinoid structures are extremely large.

Sputtered tellurite glass thin films for planar optical devices

Tellurium oxide is a promising optical material, conjugating transparency in the infrared with high refractive index, low chromatic dispersion with strong nonlinear response, elevated acousto-optic figure of merit and rare earth doping capability. Reactive sputtering is a suitable technique to obtain thin films of pure TeO2 glass, with tunable stoichiometry, without the need for added components to help vitrification. We report on the growth of tellurite thin films, on the linear and non-linear optical characterization and on the process technology to implement high-contrast planar optics.

Heteroepitaxial sputtered Ge on Si (100): Nanostructure and interface morphology

Epitaxial growth of Ge thin films onto (100) silicon by DC-Pulsed Magnetron Sputtering was realized and proved by X-ray and electron diffraction. Transmission and high-resolution electron microscopy across the interface region directly confirmed a high degree of epitaxy and show that planar defects and threading dislocations are the relevant lattice imperfections. Electron microscopy shows that a post-deposition rapid thermal annealing process, up to 673 K, is effective to defect annihilation. The films grow single crystalline, slightly misoriented, below 0.1 degrees. A weak roughness around 0.6 nm, was measured both at the Ge-Si interface and at the film surface. The Ge films grown onto n-type Si show the rectifying electrical behaviour typical of p-type semiconductors.

Sputtered Ge-Si heteroepitaxial thin films for photodetection in third window

DC-pulsed magnetron sputtering (PMS) allows to produce heteroepitaxial p-type Germanium thin films on 6rdquo Silicon wafers. Integrated p-n photodiodes, based on DC-PMS deposited Ge/Si heterojunctions, feature flat responsivity over the whole third communication window.

New developments in photochromic materials showing large change in the refractive index

Organic photochromic materials have been studied as active materials in optical devices since they show a reversible change of color in the visible region and appreciable reversible changes of the refractive index (Δn) in the NIR. The latter peculiar property can be suitably exploited for the realization of holographic optical elements (HOEs) and in particular volume phase holographic gratings (VPHGs). Photochromic polyester based on diarylethene alcohol was synthesized and characterized both in solution and in thin film. The optical properties of the films were good enough for making optical devices and the modulation of the refractive index (measured by spectral ellipsometry) was large at 1.5 micron and it became even larger at shorter wavelength where the material is still transparent. Such photochromic material is a good candidate for making rewritable efficient HOEs.