Tm3+-doped cubic potassium yttrium fluoride, KY3F10, is a promising laser crystal for efficient watt-level lasers at ~1.9 ?m because of the relatively easy crystal growth by the Czochralski method, advantageous thermo-optical properties, high available Tm3+ doping levels and very efficient cross-relaxation mechanism. A compact diode-pumped 8 at.% Tm3+:KY3F10 laser generated 1.85 W at 1891 nm with a slope efficiency of 65.2% and a laser threshold of 450 mW. A negative thermal lens was detected in this crystal owing to the negative dn/dT coefficient (-8.9 × 10-6 K-1). Passive Q-switching of the Tm3+:KY3F10 laser by single-walled carbon nanotube saturable absorber was demonstrated yielding 13.2 ?J/490 ns pulses at a repetition rate of 58 kHz.

There is a great interest in the scientific community to perform calorimetry on samples having mass in the nanogram range. A detailed knowledge of the energy (heat) exchange in the fast growing family of micro- and nano-systems could provide valuable information about the chemistry and physics at the nano-scale. The possibility to have an atomically flat thermal probe represents an added value, because it provides the unique opportunity to perform Scanning Probe Microscopy (SPM) together with calorimetry. Here we report the fabrication, characterization, and calibration of atomically flat, single-crystalline gold film thermometers on mica substrate. Gold re-crystallization has been obtained, and successively the thermometer surface has been studied by Low Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM). The thermometer calibration demonstrates a heat exchange coefficient of W/K and a performance about 10 times better than previous sensors based on Si substrates. The experimental setup allows the simultaneous investigation of heat exchange and surface physics on the same sample.

In order to promote problem-based and active learning in the physics laboratory, we designed a flipped classroom focused on the Franck-Hertz experiment. The flipped classroom approach moves course content from the classroom to homework and uses class time for engaging activities and problem solving. This constructivist approach to teaching is an effective means of student-centred collaboration and it can promote active learning, enhance critical thinking and obtain the maximum use of student-faculty time together. We report preliminary results of the flipped classroom approach to a laboratory and how it worked in the context of a small group of students in a physics course.

The extremely wide range of application of high power, ultrashort laser pulses pushed forward the scientific and industrial research in this field during the last decades. Taking advantage of the continuous developments in semiconductor laser technology, diode-pumped Ytterbium-doped solid state lasers operating at 1 ?m emerged as a flexible, efficient and cost-competitive solution in a market previously dominated by more complex and expensive Ti:Sapphire laser systems. Thanks to their high absorption and emission cross section and their good mechanical properties, Yb-doped tungstates received early attention [1] and are presently employed in many market-available products. However, their emission bandwidth is not large enough to easily sustain pulses in the 100-fs range and their relatively low and anisotropic thermal conductivity lead to a limitation in their potential power up-scaling. As a consequence of the increasing demand for higher power and shorter pulse durations, several crystalline hosts exhibiting a favorable combination of good thermo-mechanical properties and much wider emission bandwidth have been recently proposed [2,3] or re-discovered [4].

Lasers based on the Tm3+-ion (Tm) emission near 1.9 ?m are interesting for many medical applications [1], remote sensing (LIDAR), as well as for pumping optical parametric oscillators (OPOs) aimed at frequency down-conversion into the mid-IR. High pulse energy and peak power are advantageous for these applications since they can be achieved at relatively low average power if compared to the CW regime. While high energies have been demonstrated by active Q-switching, passive Q-switching (PQS) is much simpler and cheaper to realize and more importantly, shorter pulses are generated with higher peak power. In recent years, PQS of diode-pumped solid-state (DPSS) lasers by the use of intracavity saturable absorbers (SA), was successfully demonstrated in several CW-pumped, bulk Tm-doped materials. The best results so far, for any DPSS PQS Tm-doped laser, have been achieved, using fluoride hosts - Tm:YLF and Tm:LLF [2], combined with Cr:ZnS SAs. In the present work we investigate the PQS performance of Tm-doped LiGdF4 (GLF). This is a relatively new laser crystal isomorphous to the well-known YLF and LLF, in which the dopant substitutes Gd3+ host ions. Whilst CW and mode-locked operation of Tm:GLF were previously studied, to the best of our knowledge, this is the first report of Q-switching of this laser medium.

Hydrogen adsorption on graphene can be increased by functionalization with Ti. This requires dispersing Ti islands on graphene as small and dense as possible, in order to increase the number of hydrogen adsorption sites per Ti atom. In this report, we investigate the morphology of Ti on nanocrystalline graphene and its hydrogen adsorption by scanning tunneling microscopy and thermal desorption spectroscopy, and compare the results with equivalent measurements on single-crystalline graphene. Nanocrystalline graphene consists of extremely small crystal grains of <5 nm size. Ti atoms preferentially adsorb at the grain boundaries of nanocrystalline graphene and form smaller and denser islands compared to single-crystalline graphene. Surprisingly, however, hydrogen adsorbs less to Ti on nanocrystalline graphene than to Ti on single-crystalline graphene. In particular, hydrogen hardly chemisorbs to 1 ML of Ti on nanocrystalline graphene. This may be attributed to strong bonds between Ti and defects located along the grain boundaries in nanocrystalline graphene. This mechanism might apply to other metals, as well, and therefore our results suggest that when functionalizing graphene by metal atoms for the purpose of hydrogen storage or other chemical reactions, it is important to consider not only the morphology of the resulting surface, but also the influence of graphene on the electronic states of the metal.

Every time a chemical reaction occurs, an energy exchange between reactants and the environment takes place, which is defined as the enthalpy of the reaction. During the last few decades, research has resulted in an increasing number of devices at the micro- or nano-scale. Sensors, catalyzers, and energy storage systems are more and more developed as nano-devices which represent the building blocks for commercial "macroscopic" objects. A general method for the direct evaluation of the energy balance of such systems is not available at present. Calorimetry is a powerful tool to investigate energy exchange, but it usually requires macroscopic sample quantities. Here, we report on the development of an original experimental setup able to detect temperature variations as low as 10 mK in a sample of ~10 ng using a thermometer device having physical dimensions of 5 × 5 mm2. This technique has been utilized to measure the enthalpy release during the adsorption process of H2 on titanium-decorated monolayer graphene. The sensitivity of these thermometers is high enough to detect a hydrogen uptake of ~10-10 moles, corresponding to ~0.2 ng, with an enthalpy release of about 23 ?J. The experimental setup allows, in perspective, scalability to even smaller sizes.

Pr:CaGdAlO4 (Pr:CGA) crystals have been grown and investigated for the first time. A Na+ flux was designed to improve the crystal quality in the Czochralski growth process. The effect of the addition of the Na+ flux on crystal quality has been investigated, and its beneficial impact on the crystallinity, optical homogeneity and thermal properties of the grown crystals has been demonstrated. The optical spectra of Pr:CGA have been measured in order to assess its potential for photonics applications. The intensities of the absorption transitions have been analyzed by means of the Judd-Ofelt approach and the intensity parameters, the branching ratios and the radiative lifetimes of the emitting states have been evaluated. These have been compared with the experimental values obtained from pulsed light measurements. All the properties indicate that the Pr:CGA crystal has great prospects in the applications of visible lasers.

We present a systematical study via scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) on the effect of the exposure of lithium on graphene on silicon carbide (SiC). We have investigated Li deposition both on epitaxial monolayer graphene and on buffer layer surfaces on the Si face of SiC. At room temperature, Li immediately intercalates at the interface between the SiC substrate and the buffer layer and transforms the buffer layer into a quasi-free-standing graphene. This conclusion is substantiated by LEED and STM evidence. We show that intercalation occurs through the SiC step sites or graphene defects. We obtain good quantitative agreement between the number of Li atoms deposited and the number of available Si bonds at the surface of the SiC crystal. Through STM analysis, we are able to determine the interlayer distance induced by Li intercalation at the interface between the SiC substrate and the buffer layer.

Considering their easy growth and doping in bulk crystal growth, and good crystalline quality, ?-Ga2O3 single crystals, a very important wide-bandgap semiconductor, are now also considered to be a promising optical crystal candidate. In this work, a Cr4+:?-Ga2O3 single crystal has been grown successfully by the edge-defined film-fed growth method. The thermal conductivity of Cr4+:?-Ga2O3 has been measured along the a* direction at room temperature obtaining 16.2 W m-1 K-1, and it was much larger than that of the usually used Cr4+-doped crystals, such as Y3Al5O12 (YAG) or YVO4. The Raman spectrum indicated that the cutoff phonon energy of the ?-Ga2O3 crystal was 767.8 cm-1. A passively Q-switched nanosecond pulsed Nd:YAG laser based on a Cr4+:?-Ga2O3 saturable absorber was experimentally demonstrated for the first time to our knowledge and its mechanism is explained by first-principles calculations. By inserting the Cr4+:?-Ga2O3 crystal into the Nd:YAG laser cavity, a Q-switched laser operation was obtained with a maximum average output power of 50 mW. The corresponding pulse repetition rate, pulse width, and pulse energy were determined to be of 421.5 kHz, 235.2 ns and 0.12 ?J, respectively.

Yb:CaLuxGd1-xAlO4 (Yb:CLGA) crystals have been grown and studied for the first time. A more disordered structure than that of the Yb:CaGdAlO4 (Yb:CGA) crystal was designed by introducing Lu3+ ions into the crystal. The absorption spectra at 9.6 K provided clear evidence for the disordered structure of the Yb:CLGA crystal. Moreover, further spectral broadening was achieved compared to that of the Yb:CGA crystal. The crystal quality and spectroscopic properties of the Yb:CLGA crystal with the highest degree of disorder were investigated in detail. Improved ultrafast laser oscillators, in terms of narrower pulse width and higher peak power, are expected with this modified crystal.

We report on a detailed comparative study of the spectroscopic and thermo-optic properties of tetragonal Tm:LiLnF4 (Ln = Y, Gd, and Lu) crystals indicating their suitability for highly-efficient microchip lasers diode-pumped at ~791 nm and operating at ~1.91 ?m. An a-cut 8 at.% Tm:LiYF4 micro-laser generated 3.1 W of linearly polarized output at 1904 nm with a slope efficiency of ? = 72% and a laser threshold of only 0.24 W. The internal loss for this crystal is as low as 0.0011 cm-1. For 8 at.% Tm:LiGdF4 and 12 at.% Tm:LiLuF4 lasers, the output power reached ~2 W and ? was 65% and 52%, respectively. The thermal lens in all Tm:LiLnF4 crystals is weak, positive and low-astigmatic. The potential for the Tm:LiLnF4 lasers to operate beyond ~2 ?m due to a vibronic coupling has been proved. The Tm:LiYF4 vibronic laser generated 375 mW at 2026-2044 nm with ? = 31%. The Tm:LiLnF4 crystals are very promising for passively Q-switched microchip lasers.

Infrared to visible upconversion (UC) is a promising way to enhance the efficiency of silicon based solar cells. In this paper, the spectral conversion and recovery of sub-band gap photons of the solar spectrum, from NIR-IR to the VIS-NIR wavelength region, is investigated in two fluorides hosts doped with trivalent erbium ions (Er3+). The efficiency gain due to upconversion in silicon solar cells is compared for single crystal samples of BaY2F8:Er3+ and LiYF4:Er3+ in a dedicated upconverter solar cell device (UCSCD) with monochromatic excitation in the 1.5 mu m spectral region. The highest external quantum efficiency due to upconversion was found for the UCSCD using the BaY2F8:30 at% Er3+ single crystal, reaching an EQE of 6.8 +/- 0.2% for (1.10 +/- 0.12).10(5) W m(-2) spectral irradiance at 1494 nm. We present a comprehensive spectroscopic study of the crystal samples also taking into account the effects of the different crystal symmetry as well as the different phonon energies. Our findings enable us to explain the higher efficiency of the BaY2F8:Er3+ compared to the LiYF4:Er3+ upconverter in terms of both static and dynamic properties. (C) 2016 Elsevier B.V. All rights reserved.

We report on the growth and laser operation of a fiber-shaped single crystal of Pr3+:LiLuF4 (Pr:LLF) grown by the micro-pulling-down (mu-PD) method. We checked the optical quality of the crystal by measuring its spectroscopic features, and by observing the profile of a laser beam transmitted through it. Using an InGaN laser diode as pump source, we achieved continuous-wave laser emission in the orange, red, and deep red regions, with slope efficiencies comparable with or exceeding previous reports on this material. To the best of our knowledge, this work represents the first observation of visible laser emissions from a crystal grown by the mu-PD technique. (C) 2016 Optical Society of America

A detailed performance comparison of new interesting Yb-doped crystals in the same oscillator setup, with a single-mode fiber-coupled diode laser pump, is reported. We intended to assess the shortest pulses achievable with available SESAM technology, running a fair comparison with laser crystals Yb:KLuW, Yb:SSO, Yb:CALGO, Yb:CALYO and Yb:CaF2, very likely including the most promising choices for the next generation of commercial bulk ultrafast solid-state systems. (C) 2016 Optical Society of America

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Yb:CaYAlO4 has been investigated spectroscopically and compared to better known Yb:CaGdAlO4. It turns out that both materials show very similar spectroscopic parameters relevant to ultrafast lasers design. Employing single-mode fiber-coupled 400-mW laser diode at 976 nm we measured pulses as short as 43 fs, and broad tunability of 40 nm with a simple single-prism setup.

The effects of visible and infrared light on potassium atoms embedded in a nanoporous glass matrix are investigated. Photodesorption by visible light enhances the atomic mobility and causes the formation of metallic nanoparticles. Two different populations of metastable clusters with absorption bands in the near-infrared and infrared are grown as a consequence of illumination. Atoms can move between the two groups through sequences of adsorption/desorption events at the pore surface. Irradiation with infrared light, instead, does not significantly enhance the atomic diffusion inside the pores. However, it induces relevant modifications of the substrate, thus changing its interaction with the assembled clusters. Consequently, infrared light alters the dynamics of the system, affecting also the evolution of non-resonant nanoparticles populations, even after the illumination sequence. These results provide new insights on the photo-induced modifications of the substrate-adsorbate interaction in nano-sized confined systems.

We demonstrate efficient passively Q-switched laser operation, based on Tm³⁺:LiGdF<inf>4</inf> and Cr²⁺:ZnSe, as active medium and saturable absorber, respectively. The polarized emission was centered at 1876 nm, with pulse parameters of 13 ns (duration), 467 ?J (energy), and 37 kW (peak power) at a repetition rate of 350 Hz.

We report on the first laser emission of (Lu<inf>x</inf> Gd<inf>(1-x)</inf>)<inf>3</inf> Ga<inf>5</inf>O<inf>12</inf> doped with Tm³⁺ ions obtaining a slope efficiency of 20% and a maximum output power of 170 mW centered at 2013 nm. Evidences of a multiline emission are shown. Moreover a spectroscopic investigation on six samples of (Lu<inf>x</inf> Gd<inf>(1-x)</inf>)<inf>3</inf> Ga<inf>5</inf>O<inf>12</inf>:Tm³⁺ having different Lu concentrations is presented. In particular the stark sublevels energy of ³H<inf>6</inf> and ³F<inf>4</inf> manifolds have been determined. Spectroscopic data have been utilized to calculate absorption and emission cross sections and gain curves.