Revealing the nature of low-temperature photoluminescence peaks by laser treatment in van der Waals epitaxially grown WS2 monolayers.

Monolayers of transition metal dichalcogenides (TMD) are promising materials for optoelectronics devices. However, one of the challenges is to fabricate large-scale growth of high quality TMD monolayers with the desired properties in order to expand their use in potential applications. Here, we demonstrate large-scale tungsten disulfide (WS2) monolayers grown by van der Waals Epitaxy (VdWE). We show that, in addition to the large structural uniformity and homogeneity of these samples, their optical properties are very sensitive to laser irradiation. We observe a time instability in the photoluminescence (PL) emission at low temperatures in the scale of seconds to minutes. Interestingly, this change of the PL spectra with time, which is due to laser induced carrier doping, is employed to successfully distinguish the emission of two negatively charged bright excitons. Furthermore, we also detect blinking sharp bound exciton emissions which are usually attractive for single photon sources. Our findings contribute to a deeper understanding of this complex carrier dynamics induced by laser irradiation which is very important for future optoelectronic devices based on large scale TMD monolayers.

Structural and spectral characterisation of Er3+ and Nd3+ doped Ga-La-S-Se glasses.

In this work, the spectroscopy of Er3+ and Nd3+ doped Se-GLS glasses was studied. A structural comparison between doped and non-doped samples was done to assess the differences between the glasses. For this comparison, Raman spectroscopy and thermal analysis were employed. The spectral properties of the samples were studied in order to identify the mechanisms responsible for quenching the fluorescence lifetime of the dopants. In particular, cross-relaxation and concentration quenching were observed in Nd3+ doped samples, whilst co-operative upconversion, radiation trapping and concentration quenching were observed in Er3+ doped samples. The results obtained demonstrated the fundamental role of the phonon energy in the mechanism of fluorescence. The low phonon energy of chalcogenides decreased the rate of non-radiative processes promoting co-operative upconversion. This effect could be exploited to design new lasers and sensitizers for solar energy harvesters.

Characterization of supercooled liquid Ge2Sb2Te5 and its crystallization by ultrafast-heating calorimetry.

Differential scanning calorimetry (DSC) is widely used to study the stability of amorphous solids, characterizing the kinetics of crystallization close to the glass-transition temperature T(g). We apply ultrafast DSC to the phase-change material Ge(2)Sb(2)Te(5) (GST) and show that if the range of heating rates is extended to more than 10(4) K s(-1), the analysis can cover a wider temperature range, up to the point where the crystal growth rate approaches its maximum. The growth rates that can be characterized are some four orders of magnitude higher than in conventional DSC, reaching values relevant for the application of GST as a data-storage medium. The kinetic coefficient for crystal growth has a strongly non-Arrhenius temperature dependence, revealing that supercooled liquid GST has a high fragility. Near T(g) there is evidence for decoupling of the crystal-growth kinetics from viscous flow, matching the behaviour for a fragile liquid suggested by studies on oxide and organic systems.

Spectroscopic properties of Nd 3 + and Pr 3 + in gallate glasses with low phonon energies.

Absorption and fluorescence spectra of Nd3 +were measured in potassium tantalum gallate, lead bismuth gallate (PBG), fluorozirconate (ZBLAN), and Ge-Ga-S glasses. A Judd -Ofelt analysis was performed to determine the spontaneous emission probability and stimulated emission cross section of the4 F3/2 ?4 I11/2 transition of Nd3 +. Raman spectra were studied to clarify the maximum phonon energies of the glasses. The fluorescence of the1 G4?3 H5 transition of Pr3 + in a dehydrated PBG glass was observed for the first time to our knowledge. The PBG glass has a higher quantum efficiency than that of ZBLAN glass based on the Judd -Ofelt analysis.

Properties of dysprosium-doped gallium lanthanum sulfide fiber amplifiers operating at 1.3 microm.

In light of recent progress in the fabrication of gallium lanthanum sulfide (GaLaS) fibers, we have modeled the performance of dysprosium-doped GaLaS fiber amplifiers operating at 1.3 microm . Based on experimental data, we find that the incorporation of a codopant (terbium) in the fiber core significantly shortens the optimum amplifier length from >30 m to approximately 3 m . Such a device may be practical, given the fiber losses currently achieved in GaLaS fibers.

Fabrication and spectroscopy of erbium doped gallium lanthanum sulphide glass fibres for mid-infrared laser applications.

Gallium lanthanum sulphide based glasses are proposed as high quality hosts for rare-earth doped, mid-infrared fibre lasers, that would offer compact and rugged sources for gas sensing, atmospheric transmission, and medical applications. The infrared emission spectroscopy of erbium doped glasses and fibres shows the potential of this glass host for the above applications. Mid-infrared transitions at 2.0, 2.75, 3.6, and 4.5mm have been detected and characterized.

Spectroscopic data of the 1.8-, 2.9-, and 4.3-microm transitions in dysprosium-doped gallium lanthanum sulfide glass.

Infrared emission at 1.8, 2.9, and 4.3 microm is measured in dysprosium-doped gallium lanthanum sulfide (Ga:La:S) glass excited at 815 nm. Emission cross sections were calculated by Judd-Ofelt analysis, the Füchtbauer- Ladenburg equation, and the theory of McCumber. The sigmatau value for the 4.3-microm transition is ~4000 times larger in the Ga:La:S glass than in a dysprosium-doped LiYF(4) crystal, which has lased on this transition. The large sigmatau value and the recently reported ability of Ga:La:S glass to be fabricated into fiber form show the potential for an efficient, low-threshold mid-infrared fiber laser. The f luorescence peak at 4.3 microm coincides with the fundamental absorption of atmospheric carbon dioxide, making the glass a potential laser source for gas-sensing applications.

Solution deposited optical waveguide lens.

The development of a solution deposited optical waveguide lens, whose focusing effect is produced by an effective index gradient with a hyperbolic secant variation, is described. The lenses are fabricated by a microcontrolled dip coating procedure from colloidal SiO(2):TiO(2) solutions. Both the design and construction of the lens, along with the analytical and experimental results of the focusing properties, are described. The best lenses had speeds of ~f/10 and focal spots ~1.2 times the diffraction limit at apertures of 2.0 mm.

Fabrication of tapers and lenslike waveguides by a microcontrolled dip coating procedure.

A technique for the fabrication of tapered and lenslike waveguides from solution-deposited thin films is described. Using a microprocessor controlled dipping arm, substrates are withdrawn from a solution with a carefully controlled and varying velocity. Inthisway optical waveguides with regions of varying thickness are deposited. Following the drying and baking of the films, desired structures are obtained in hard inorganic optical waveguides of good optical quality. With refined profile control, we propose to fabricate other optical waveguide components, for example, thin film lenses with this method.

Generalized dispersion properties of a four-layer thin-film waveguide.

A general analytic expression is derived, which describes the dispersion properties of a four-layer thin-film optical waveguide. The formula describes the variation of the effective refractive index with respect to any physical parameter with which the refractive index of any layer or the thickness of the guiding layers may vary. Normalized parameters are used, and curves are presented which describe the contributions of the individual layers and waveguide geometry to the total dispersion. Uses of the formula are illustrated for the cases of temperature and wavelength dispersions. Results show that considerable control of the effective index of refraction is possible. Application to the control of chromatic dispersion in a thin-film Luneburg lens is also demonstrated.