Stefano Bigotta

Anti-Stokes luminescence cooling of Tm3+ doped BaY2F8.

We report laser-induced cooling with thulium-doped BaY2F8 single crystals grown using the Czochralski technique. The spectroscopic characterization of the crystals has been used to evaluate the laser cooling performance of the samples. Cooling by 3 degrees below ambient temperature is obtained in a single-pass geometry with 4.4 Watts of pump laser power at lambda = 1855 nm.

Spectroscopic and laser cooling results on Yb3+-doped BaY2F8 single crystal

Anti-Stokes cooling has been observed in an Yb3+-doped BaY2F8 single crystal. Single crystals have been grown by the Czochralski technique. The absorption spectra and the emission properties have been measured at room temperature and at 10K. The energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and separated from the vibronic substructure. The intrinsic decay time of the F5∕22 level has been measured taking care of avoiding the effect of multiple reabsorption processes. The theoretical and experimental cooling efficiencies of Yb:BaY2F8 are evaluated and compared with respect to those of the most frequently investigated materials for laser cooling. A temperature drop of almost 4K was measured by pumping the crystal with 3W of laser radiation at ∼1025nm in single pass configuration with a cooling efficiency of ∼3%.Anti-Stokes cooling has been observed in an Yb3+-doped BaY2F8 single crystal. Single crystals have been grown by the Czochralski technique. The absorption spectra and the emission properties have been measured at room temperature and at 10K. The energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and separated from the vibronic substructure. The intrinsic decay time of the F5∕22 level has been measured taking care of avoiding the effect of multiple reabsorption processes. The theoretical and experimental cooling efficiencies of Yb:BaY2F8 are evaluated and compared with respect to those of the most frequently investigated materials for laser cooling. A temperature drop of almost 4K was measured by pumping the crystal with 3W of laser radiation at ∼1025nm in single pass configuration with a cooling efficiency of ∼3%.

Study of the visible spectra of Ca3Sc2Ge3O12 garnet crystals doped with Ce3+ or Pr3+

Abstract We have measured the absorption, the visible emission spectra and the lifetimes of Ca3Sc2Ge3O12 single crystals activated with Ce3+ or Pr3+. The crystals were grown by the ‘flux growth’ method. The optical spectra of the Ce3+-doped crystal consist of broadbands as a consequence of the strong electron–phonon coupling in the 2D(5d1) excited level, the emission band being located in the blue-green region with maximum at about 480 nm. The spectroscopic properties of CaSGG:Pr are affected by inhomogeneous broadening arising from the presence of non-equivalent optical centres. The spontaneous transition probabilities, the branching ratios and the radiative lifetimes have been calculated in the framework of the Judd–Ofelt Theory and compared with the experimental results for the 3P0 and 1D2 levels. The emission cross-sections of the transitions originating from the 3P0 level were calculated and compared with those reported for other Pr3+-doped crystals of technological interest.

Q-switched resonantly diode-pumped Er3+:YAG laser with fiberlike geometry

Crystalline fiberlike diode-pumped laser systems offer a convenient way to obtain compact, robust, and efficient operation whenever long active media are required. One of these cases is the diode-pumped Er(3+):YAG laser emitting at 1.6 microm, where long crystals are needed to avoid the upconversion processes. By resonantly pumping the (4)I(13/2) level around 1.53 microm, and by confining the pump radiation into the crystal by total internal reflection, a maximum output power of 14.5 W in cw mode is reached when pumped with approximately 40 W of absorbed power. In Q-switching mode, pulse energies of more than 8 mJ were obtained at a repetition rate of 90 Hz, limited by coating damage.

Cooling of Yb:YLF using cavity enhanced resonant absorption

Using a cavity resonant absorption scheme we demonstrate record laser cooling for the rare-earth doped crystalline solid Yb:YLF. A temperature drop of nearly 70 degrees is obtained with respect to the ambient. Our preliminary results indicate that minimum achievable temperature in this material/sample is 170 K, as measured using a modified differential luminescence thermometry technique. This indicates outstanding potential for Yb:YLF as a cryogenic laser cooler material.

Demonstration of an optical cryocooler

We present the first observation of cryogenic operation in an all-solid-state refrigerator. A temperature drop of ~150 K is demonstrated in a 0.2 cm3 rare-earth doped fluoride crystal (Yb:YLF) using anti-Stokes fluorescence, at a record cooling power of 110 mW. Lowest electronic transition within Yb3+ Stark manifold along with cavity enhanced absorption and thermal-load management were key in achieving this operation. We show that temperatures down to 100 K are achievable in this arrangement given sufficient absorbed laser power at 1020 nm.

Optical spectroscopy of Ho3+ and Tm3+ in Ca3Sc2Ge3O12 crystals

Abstract Single crystals of the Ca 3 Sc 2 Ge 3 O 12 (CaSGG) garnet activated with Ho 3+ and Tm 3+ have been grown by the ‘flux growth’ technique. The absorption spectra, the visible emission properties and the decay curves of the excited states have been measured at 10 and 298 K. The room temperature absorption spectra have been analyzed in the framework of the Judd–Ofelt theory and the intensity parameters have been evaluated. The spontaneous transition probabilities, the branching ratios and the radiative lifetimes have been determined for the most important emitting states using the calculated intensity parameters, and the results have been compared with the experimental data in order to estimate the efficiency of the non-radiative processes.

Single fluoride crystals as materials for laser cooling applications

We report the successful growth and the laser cooling results of Yb3+-doped single fluoride crystals. By investigating the mechanical and thermal properties of Yb-doped BaY2F8 and LiYF4 crystals and using the spectroscopic data we collected from our samples, the theoretical and experimental cooling efficiency of fluoride crystals are evaluated and compared with respect to those of ZBLAN. Two different methods, a thermal camera and a fluorescence intensity ratio technique, have been used to monitor the temperature change of the samples. The temperature change is clearly exponential, as expected from theory, and the temperature drops are 6.3 K and 4 K for Yb:LiYF4 and Yb:BaY2F8 respectively in single-pass configuration, corresponding to a cooling efficiency of about 2% and 3%. This last value is slightly larger than that observed in Yb-ZBLAN in similar experimental condition.

Novel non-planar ring cavity for enhanced beam quality in high-pulse-energy optical parametric oscillators

A novel non-planar ring cavity is presented. It is shown that by using a fractional image rotation, i.e. a rotation whose angle cannot be expressed as 2π/n, where n is a small integer, the number of modes that can oscillate in the cavity is greatly reduced. This Fractional Image Rotation Enhancement (FIRE) cavity can thus be used to increase the beam quality in cases of low transversal mode discrimination, such as lasers with large pump and beam size and high-pulse-energy optical parametric oscillators (OPOs), especially when compactness and efficiency are key parameters. The experimental results obtained with a FIRE OPO for mid-IR (3–5 μm) generation based on a ZnGeP2 crystal pumped by a Ho3+:LiLuF4 MOPA system at 2.05 μm are compared to those realized with the same crystal and pump arrangement using a standard Rotated Image Singly-Resonant Twisted RectAngle (RISTRA) cavity. An increase of the beam quality from M2 of 2.08–2.29 to M2 of 1.88–1.99 is reached at 20 mJ pulse energy when the FIRE cavity is used.

Laser cooling of a semiconductor load to 165 K

We demonstrate cooling of a 2 micron thick GaAs/InGaP double-heterostructure to 165 K by means of an optical refrigerator. Cooler is comprised of Yb3+-doped YLF crystal, pumped by 9 Watt near E4-E5 Stark manifold transition