Azzurra Volpi

Novel approach for solid state cryocoolers.

Laser cooling in solids is based on anti-Stokes luminescence, via the annihilation of lattice phonons needed to compensate the energy of emitted photons, higher than absorbed ones. Usually the anti-Stokes process is obtained using a rare-earth active ion, like Yb. In this work we demonstrate a novel approach for optical cooling based not only to Yb anti-Stokes cycle but also to virtuous energy-transfer processes from the active ion, obtaining an increase of the cooling efficiency of a single crystal LiYF(4) (YLF) doped Yb at 5at.% with a controlled co-doping of 0.0016% Thulium ions. A model for efficiency enhancement based on Yb-Tm energy transfer is also suggested.

Influence of other rare earth ions on the optical refrigeration efficiency in Yb:YLF crystals

We investigated the effect of rare earth impurities on the cooling efficiency of Yb³⁺:LiYF₄ (Yb:YLF). The refrigeration performance of two single crystals, doped with 5%-at. Yb and with identical history but with different amount of contaminations, have been compared by measuring the cooling efficiency curves. Spectroscopic and elemental analyses of the samples have been carried out to identify the contaminants, to quantify their concentrations and to understand their effect on the cooling efficiencies. A model of energy transfer processes between Yb and other rare earth ions is suggested, identifying Erbium and Holmium as elements that produce a detrimental effect on the cooling performance.

Effect of impurities on cooling efficiency in fluoride crystals

We present a characterization of the optical refrigeration properties of two 5% Ytterbium-doped YLF crystals. Measurements showed different cooling efficiencies for the samples, despite the same concentration of dopant. We carried out a spectroscopic study on the crystals devoted to the identification of foreign contaminants inside them. These searches determined the presence of Erbium and Holmium impurities in both the cooling samples. We attributed the reduction of the cooling efficiency in one of the crystals to an increased amount of these contaminants.

Investigation of Yb-doped LiLuF4 single crystals for optical cooling

Abstract. Optical cooling of solids, relying on annihilation of lattice phonons via anti-Stokes fluorescence, is an emerging technology that is rapidly advancing. The development of high-quality Yb-doped fluoride single crystals definitely led to cryogenic and sub-100-K operations, and the potential for further improvements has not been exhausted by far. Among fluorides, so far the best results have been achieved with Yb-doped LiYF4 (YLF) single crystals, with a record cooling to 91 K of a stand-alone YLF:10%Yb. We report on preliminary investigation of optical cooling of an LiLuF4 (LLF) single crystal, an isomorph of YLF where yttrium is replaced by lutetium. Different samples of 5% Yb-doped LLF single crystals have been grown and optically characterized. Optical cooling was observed by exciting the Yb transition in single-pass at 1025 nm and the cooling efficiency curve has been measured detecting the heating/cooling temperature change as a function of pumping laser frequency.

Crystal growth, spectroscopic characterization, and sub-100 femtosecond mode-locked operation of a Yb:LiLuF_4 laser

We present a comprehensive growth, spectroscopic, and laser performance investigation of a Yb:LiLuF4 crystal, including CW laser experiments, and, for the first time to the best of our knowledge, mode-locked laser operation. Pumping with two 400 mW, polarization-combined, single-mode fiber-coupled laser diodes, we demonstrated efficient CW emission with 140 mW maximum output power and up to 59% slope efficiency. Using a semiconductor saturable absorber mirror for inducing the mode-locked regime and a pair of fused silica prisms for intracavity group delay dispersion compensation, we obtained soliton mode-locking pulses with 90 fs duration for the emission with polarization parallel to the crystal c-axis and 113 fs for the orthogonal polarization, with average power exceeding 30 mW in both cases at about a 90 MHz repetition rate.

Spectroscopy and laser test emission in Tm3+?:?BaYLuF8 single crystal

A novel laser material BaYLuF8 (BYLF), doped with 12?at% of Tm3+, has been grown and optically investigated, in order to evaluate its potential performances as a 2??m laser. The BYLF crystal is interesting mainly because indications are that the mixed crystal would be sturdier than BaY2F8 (BYF). The addition of lutetium would improve the thermo-mechanical properties of the host. Absorption, fluorescence and lifetime measurements have been performed in the temperature range 10?300?K focusing on the 3H4 and 3F4 manifolds, those involved in the laser scheme at 2??m. The Stark sublevels structure of Tm3+ up to the 1D2 manifold has been figured out. Diode-pumped CW laser emission at 2??m has been achieved obtaining a slope efficiency of about 28% with respect to the absorbed power, by pumping along the Z-axis. A maximum output power of 240?mW was achieved by pumping along the favourable Y-axis, with an incident power of about 800?mW.

Fluoride crystals: materials for near-infrared solid state lasers

In this work we present an overview of the best 2μm laser results obtained in Tm-doped fluoride hosts LiYF4(YLF), LiLuF4 (LLF) and BaY2F8 (BYF) and we report on the growth, spectroscopy and first laser test emission of a novel mixed material BaYLuF8 (BYLF), interesting as a variant of BYF material with a partial substitution of Y3+ ions by Lu3+. The novel host is interesting mainly because indications are that the mixed crystal would be sturdier than BYF. The addition of Lutetium would improve the thermo-mechanical properties going into the direction of high power applications, as suggest from works on YLF and its isomorph LLF. A detailed description of Czochralski growth of fluoride laser materials is provided, focusing on the growth parameters of the novel BYLF:Tm3+12% material grown. With regard of spectroscopy analysis, we report on the results obtained with BYLF host. Detailed absorption, fluorescence and lifetime measurements have been performed focusing on the 3H4 and 3F4 manifolds, the pumping and upper laser level. Moreover diode pumped CW laser emission at 2 μm has been achieved in BYLF: Tm3+12% sample obtaining a slope efficiency of about 28% with respect to the absorbed power.

Bridgman growth of fluoride crystals for radiation-balanced lasers (Conference Presentation)

Radiation-balanced lasers (RBL) combine solid-state optical refrigeration and lasing in one material to enable a net zero thermal load that allows for favorable scaling to high laser powers. A high-performance RBL material, therefore, has to first qualify as a high-performance laser-cooling material. This necessitates exquisite material purity in order to achieve the required near-unity external quantum efficiency and low background absorption. Solvent extraction, ion exchange, and electrochemical treatment of aqueous solutions or melts are some of the techniques available for the purification of starting materials used in the growth of RBL crystals. Scaling these methods to the 100s of gram scale needed for traditional Czochralski crystal growth while maintaining parts-per-billion level impurity concentrations however has proven challenging in several past efforts. In contrast, we have previously shown solvent extraction and electrochemical treatment to be effective on the several gram scale. This creates a need for exploring alternative methods for growing optical-cooling-grade fluoride crystals on the small scale. We will present results on growing Yb-doped YLiF4 (YLF) and LuLiF4 (LLF) single crystals using the vertical Bridgman method. The external quantum efficiency and background absorption of these samples will be reported and discussed in the context of RBL.

Investigation of temperature dependence of quantum efficiency and parasitic absorption in rare-earth doped crystals (Conference Presentation)

Optical refrigeration of rare-earth doped crystals has exceptional qualities that can be used for building a compact and vibration-free all-solid-state optical cooler. Estimating the lowest achievable temperature and cooling power of such a device requires accurate measurements of external quantum efficiency, mean fluorescence wavelength, and parasitic absorption. Here we discuss temperature dependent measurements of these parameters for a high quality Yb:YLF sample by performing a LITMoS test (Laser Induced Temperature Modulation Spectrum) combined with contact-free differential luminescence thermometry. These measurements are challenging at low temperatures, but by integrating these two methods, we can perform LITMoS test at any temperature.

87 fs pulse generation in a diode-pumped SESAM mode-locked Yb:YLF laser

We present for the first time sub-100-fs pulse generation in a SESAM mode-locked, Yb:YLF laser pumped with low-power single-mode fiber-coupled lasers at 976 nm. Almost Fourier transform limited 87-fs pulses at 1052 nm were obtained.