M. Lugarà

First order Raman scattering in GaN

Abstract Experimental evidence of A1 and E1 longitudinal optical modes in Raman spectra of GaN is reported for the first time. The static dielectric constant and the effective charge are also calculated, showing the ionic character of GaN.

Stimulated photoluminescence of CdTe

Abstract Emission spectra of CdTe at low temperature under high intensity optical pumping by means of a tunable dye laser are reported. We show that the optical gain process is enhanced when a high exciton density is achieved by means of quasi-resonant pumping. The mechanism responsible for the amplification process is found to be the well-known exciton-exciton scattering.

Optical gain in GaSe due to exciton-exciton recombination☆

Abstract Measurements of the optical gain spectrum in high quality GaSe are reported. The mechanism for light amplification has been investigated, giving evidence that the exciton-exciton scattering process is responsible for stimulated emission.

Electron-hole plasma generation in gallium nitride

Abstract The electron-hole plasma has been studied in both epitaxial and needle GaN. The threshold and the stability of the plasma are also discussed. The observed different behaviour of the samples we studied is interpreted in terms of growth technique.

Stimulated emission of GaN under high one and two quantum excitation

Abstract Stimulated emission at 3.435 eV in GaN at 80 K is reported. This emission is attributed to a cooperative excitonic process and is observed only in undoped samples. One and two photon optical pumping has been used and optical gain of about 103 cm-1 has been measured.

Photoelectromagnetic properties of HgI2

The photoelectromagnetic effect and photoconductivity measurements at room temperature in HgI2 single crystals are reported for the first time. The sublinear dependence of these effects on the incident light intensity suggests the occurrence of trapping effects; this is taken into account by modifying the already developed calculations and introducing an ’’effective lifetime’’ τ* and an ’’effective diffusion length’’ L*. These two parameters are related to the electron and hole lifetimes. τ* and L* are calculated by the experimental data and they are dependent on the incident light intensity with a −1/3 power dependence; at an incident photon flux of 1015 photon cm−2 sec−1 we found τ*=9×10−8 sec and L*=6×10−5 cm.

Resonance Raman scattering in HfSe2 and HfS2

The energies of IR active A2u modes in both HfSe2 and HfS2 are not known, since it is experimentally difficult to perform IR spectroscopy measurements for E || c. The experimental results of our first and second order resonance Raman measurements show that A2u(TO) = 180cm−1 and A2u(LO) = 295cm−1 in HfS2; in HfSe2 we obtain A2u(TO) = 120cm−1 and A2u(LO) = 175cm−1. Our results are consistent with those theoretically calculated by means of the polarizable ion model.

Stimulated emission and optical gain spectrum in highly excited CdSe

The stimulated emission from CdSe at 80 °K under nitrogen or dye laser excitation has been studied. Its dependence on the pumping laser wavelength, on the excitation intensity, and on the excited length of the sample has been investigated. Optical gain spectrum measurements confirm the attribution of the light amplification process to two different radiative mechanisms involving, respectively, the exciton‐exciton interaction and the electron‐hole plasma recombination, for a photoinjected electron‐hole density nc?1017 pairs/cm3.

Photoelectromagnetic effect in lead iodide

Abstract The photoelectromagnetic effect and the photoconductivity in PbI 2 layer compound have been studied by using a double modulation technique. The experimental results allow to estimate the lifetimes and the diffusion lengths of the majority and minority carriers.

Excitonic emission in highly excited lead iodine

Abstract Excitonic luminescence in PbI2 has been investigated in the temperature range between 77 and 300°K under high excitation by a nitrogen laser. A single emission band is observed which, at low pumping, is centered at the free exciton energy. When the excitation intensity is increased, the band broadens and shifts towards lower energies. These facts are interpreted on the basis of an exciton-electron interaction.