Fabrizia Somma

Spontaneous emission properties of color centers based optical microcavities

Abstract Optical microcavities based on lithium fluoride films treated by low-energy electrons to create visible-emitting F2 color centers have been fabricated, and their radiative properties characterized for the first time. By tuning the photon cavity mode to the maximum of the luminescence band for the F2 centers (∼670 nm), spectral narrowing, peak-intensity enhancement of the emission band as well as a highly directional radiation pattern have been observed comparatively for experiments performed on a half-cavity and a full microcavity. Spontaneous emission decay times have been measured, and a shortening of lifetime by the cavity has been observed.

Luminescence of CsPbBr3-like quantum dots in CsBr single crystals

Abstract Luminescence and decay kinetics of the Pb2+ aggregates in CsBr host crystals were measured in the 4–300 K temperature interval and in 10−10–10−3 time scale. Their emission properties are similar to those of CsPbBr3 bulk crystal showing a subnanosecond free exciton emission in the 520–540 nm spectral region and slower trapped exciton emission in the 530–580 nm spectral region. An efficient energy exchange between the free and trapped exciton states is shown by the temperature dependencies of emission spectra. The quantum size effect is demonstrated in the high energy shift and broadening of the absorption and emission spectra and an estimate of the size of the CsPbBr3-like aggregates is provided. Independent evidence of the presence of the CsPbBr3 and Cs4PbBr6 aggregated phases in the CsBr host was obtained by X-ray structural studies.

Optical and structural properties of ternary nanoaggregates in CsI-PbI2 co-evaporated thin films

Thin films were grown by vacuum co-evaporation of CsI and PbI2 compounds from two independent crucibles. X-ray diffraction spectroscopy revealed formation of CsPbI3 and Cs4 PbI6 nanoaggregates in the thin film samples and gave an estimate of their volume percentage as well. It provided also the grain size estimates, which were compared with direct observation of a thin film surface by atomic force microscopy. The absorption, luminescence and decay kinetics characteristics of thin films grown were measured. The optical characteristics of the mentioned ternary nanoaggregates were determined and compared with those of CsPbX3 and Cs4 PbX6 (X = Cl, Br) bulk structures.

Control of F2 color centers spontaneous emission in LiF thin films inside optical microcavities

Abstract An experimental investigation on spontaneous emission properties of Fabry–Perot microcavities consisting of quarter-wavelength ZnS/Cryolite Bragg mirrors and a lithium fluoride (LiF) thin film colored with low-energy electron beam lithography is reported for the first time. Angular-resolved photoluminescence measurements show the modifications of F2 color centers visible spontaneous emission. The resonator induces a narrowing of the emission spectrum and a related increase in the directionality and intensity of the emission along the cavity axis.

Optical properties of Pb2+-based aggregated phases in CsBr Thin film and single crystal matrices

Abstract Emission characteristics of CsPbBr3 and Cs4PbBr6 aggregates in CsBr bulk and thin film matrices are reported. The emission of the former aggregated phase is peaking about 520–560 nm. It shows small Stokes shift (50 meV) related to narrow free exciton emission line of sub-nanosecond decay times. Quantum size effect was evidenced for the aggregates of 6–7 nm in diameter. The Cs4PbBr6 aggregates show emission peak at 375 nm and overall emission characteristics are similar to those of KBr: Pb, which is explained by very close local arrangement of emission centres—(PbBr6)4- octahedra—in both structures.

Detailed model of the photostimulated realignment ofF FA centres in KCl:Li+

SummaryThe photostimulated reorientation of theFA centres in KCl:Li+ is described by a simple model that takes into account the off-axis angle of the centres and the overlapping of the tails of the two absorption bands. With this model we have interpreted the dependence of the luminescence intensity on the polarization of the exciting beam. The fitting of the experimental data gives an off-axis angle of approximately 5° and an overlapping of the bands larger than that predicted for Gaussian line shapes.RiassuntoLa riorientazione fotostimolata dei centriFA in KCl:Li+ è descritta con un modello semplice che tiene conto dell’angolo di disallineamento e della sovrapposizione delle code delle due bande di assorbimento. Con questo modello abbiamo interpretato la dipendenza dell’intensità di luminescenza dalla polarizzazione della luce di eccitazione. Il fitting dei dati sperimentali dà un angolo di off-axis di circa 5° ed una sovrapposizione delle bande maggiore di quella prevista per curve di tipo gaussiano.РезюмеС помощью простой модели описывается фотостимулированная переориентацияFA центров в KCl:Li+. Эта модель учитывает угловое удаление центров от оси и перекрытие хвостов двух зон поглощения. С помощью этой модели мы интерпретируем зависимость интенсивности люминесценции от поляризации возбуждающего пучка. Подгонка экспериментальных данных дает угловое удаление от оси порядка 5° и большее перекрытие зон, чем было предсказано для гауссовых форм линий.

New luminescence of the FA centre in RbCl:Li+

Type-I or type-II behaviour of the FA centres in alkali halides has always been thought of as a feature dependent on the crystal lattice parameter and on the impurity size. We have found experimental evidence suggesting for the first time that both behaviours are present in RbCl:Li+, where the FA centre at low temperature (T < 15 K) exhibits a luminescence of type-I switching to type-II at higher temperature. This effect is probably related to the possibility for the Li+ ion of being, at high temperature, a mobile and off-centre ion. Both these conditions seem to be necessary for having a type-II behaviour.

Excitons in CsPbX3 (X = Cl, Br, I) ternary nanocrystallites in thin film matrices

Ternary nanophases based on Cs-Pb-X (X = Br, Cl, I) composition, were evidenced in polycrystalline thin films grown by physical vapour deposition. Optical properties of these nanostructures were determined by absorption, emission and decay kinetics measurements. Nanocrystals of CsPbX 3 type were found to be in agreement with reported band-structure calculations (X = Cl, Br); they show optical features of a wide band-gap semiconductor with direct band-to-band transitions. Ternary bromide and chloride phases show, at 10 K, a pronounced fast free Wannier exciton emission (leading decay time < 1 ns) peaking round 537 and 420 nm, respectively, with very small Stokes-shift. On the contrary, the nanophase of CsPbI 3 is characterised by the auto-localised exciton emission around 535 nm, longer decay times in μs time scale and a pronounced Stokes-shift. In all the cases, the optical features are closely similar to the ones of the bulk ternary crystals, which were reported earlier.

Simulation of low-velocity bombardment of asteroid surfaces: regolith formation and reddening of the spectra

Aims. The association of the most abundant population of meteorites, the ordinary chondrites, and the S-type asteroids through the comparison of their reflected spectra in the visible and near infrared spectrum, is still widely debated. Many asteroids show reddened spectra. To date, this reddening has been attributed to the presence of metallic nanoparticles, produced by the vaporization of Fe-bearing silicates or by a shock-induced phase transformation of metallic Fe-Ni alloys. Both the mechanisms are suggested to be related to impacts on asteroid surfaces. We show how sculpting by low-energy impacts can play an important role in the reddening of asteroid surfaces. Methods. We used quartz micro-spheres impacting on ordinary chondrites to simulate the sculpting of airless bodies due to the lowenergy bombardment of micro-meteoroids. Results. The bombarded surfaces show that metals in ordinary chondrites maintain their dimensions while silicates are comminuted. Moreover, the spectra of the simulated surface are redder than traditional powdered meteorites.

Improvement of the optical pumping in the FA(II) colour centre laser

Abstract An increase of about one order of magnitude in the output power of an F A (II) colour centre laser has been obtained by rotating the polarization of a krypton-ion laser pump. The coupling of the pump beam to the colour centre laser cavity has been accomplished by means of an intracavity multilayer dielectric coated beam splitter.