Combining photoluminescence and positron annihilation studies of hydrothermally grown ZnO crystals with stoichiometry varied by controlled annealing enabled us to clarify the origin of green luminescence. It was found that green luminescence in ZnO has multiple origins and consists of a band at 2.3(1) eV due to recombination of electrons of the conduction band by zinc vacancy acceptors coupled with hydrogen and a band at 2.47(2) eV related to oxygen vacancies. The as-grown ZnO crystals contain zinc vacancies associated with hydrogen and exhibit a green luminescence at 2.3(1) eV. Annealing in Zn vapor removed zinc vacancies and introduced oxygen vacancies. This led to disappearance of the green luminescence band at 2.3(1) eV and appearance of a green emission at higher energy of 2.47(2) eV. Moreover, the color of the crystal was changed from colorless to dark red. In contrast, annealing of the as-grown crystal in Cd vapor did not remove zinc vacancies and did not cause any significant change of green luminescence nor change in coloration.

The aim of this work is to produce 2D plasmonic and diffractive structures in Ag films with sharp features for which both a deeper understanding of laser induced transformation upon modulated laser intensity and a correlation between structural and optical properties are required. We compare results obtained by exposing silver films to an excimer laser operating at 193 nm whose intensity is either modulated or homogeneous. In all cases, one laser exposure is enough to break the film into nanoparticles (NPs). The use of the modulated beam intensity leads to diffractive 2D patterns that are formed by rectangular regions of untransformed material surrounded by transformed regions covered by NPs. The former have sharp edges that are consistent with the absence of significant mass transport that is discussed in terms of the thermal gradient induced. The latter contain NPs whose diameter increases as the initial film effective thickness increases. The surface plasmons associated with the NPs in the transformed regions dominate the reflectivity spectrum and the 2D array formed by the untransformed regions is responsible for the diffractive properties. Evidence for spinodal dewetting is only observed in our case for the steep gradient conditions achieved at the border of the homogeneously irradiated regions.

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.

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.