Alberto Di Lieto

Efficient broadly tunable continuous-wave Cr 2+ :ZnSe laser

An efficient continuous-wave Cr2+-doped ZnSe laser pumped by a Co:MgF2 laser is experimentally demonstrated. In a single-pass pump scheme we observed up to 520 mW at ∼2500 nm in 0.4-nm narrow-band operation, with 52% incident-power slope efficiency, and a tuning range between 2180 and 2800 nm. In the multipass pump scheme we also observed and analyzed the effect of dual Q-switching laser action at 1.75 and 2.5 µm in the Co:MgF2–Cr:ZnSe coupled-cavity oscillator. Finally, we report the measurement of the passive losses and of the ground-state absorption at the lasing wavelength.

Optical refrigeration to 119 K, below National Institute of Standards and Technology cryogenic temperature

We report on bulk optical refrigeration of Yb:YLF crystal to a temperature of ~124 K, starting from the ambient. This is achieved by pumping the E4-E5 Stark multiplet transition at ~1020 nm. A lower temperature of 119±1 K (~-154C) with available cooling power of 18 mW is attained when the temperature of the surrounding crystal is reduced to 210 K. This result is within only a few degrees of the minimum achievable temperature of our crystal and signifies the bulk solid-state laser cooling below the National Institute of Standards and Technology (NIST)-defined cryogenic temperature of 123 K.

Local Laser Cooling of Yb:YLF to 110 K

Minimum achievable temperature of ~110 K is measured in a 5% doped Yb:YLF crystal at λ = 1020 nm, corresponding to E4-E5 resonance of Stark manifold. This measurement is in excellent agreement with the laser cooling model and was made possible by employing a novel and sensitive implementation of differential luminescence thermometry using balanced photo-detectors.

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%.

High-efficiency diode-pumped Tm:GdLiF 4 laser at 1.9 μm

Continuous-wave laser action of a Tm-doped GdLiF(4) (GLF) crystal pumped by a laser diode is reported at room temperature. A comparative analysis of laser performance using GLF crystals with doping concentrations of 8 at. % and 12 at. % of Tm(3+) has been carried out. A maximum output power of 1.47 W with 57% slope efficiency and a wide tunability range from 1826 to 2054 nm have been obtained at 8% Tm doping level.

Passive mode locking of an in-band-pumped Ho:YLiF 4 laser at 2.06 μm

We demonstrate the passive mode-locking operation of an in-band-pumped Ho:YLiF(4) laser at 2.06 μm using a semiconductor saturable absorber mirror based on InGaAsSb quantum wells. A transform-limited pulse train with minimum duration of 1.1 ps and average power of 0.58 W has been obtained at a repetition frequency of 122 MHz. A maximum output power of 1.7 W has been generated with a corresponding pulse duration of 1.9 ps.

Atomic-layer molybdenum sulfide optical modulator for visible coherent light

Coherent light sources in the visible range are playing important roles in our daily life and modern technology, since about 50% of the capability of the our human brains is devoted to processing visual information. Visible lasers can be achieved by nonlinear optical process of infrared lasers and direct lasing of gain materials, and the latter has advantages in the aspects of compactness, efficiency, simplicity, etc. However, due to lack of visible optical modulators, the directly generated visible lasers with only a gain material are constrained in continuous-wave operation. Here, we demonstrated the fabrication of a visible optical modulator and pulsed visible lasers based on atomic-layer molybdenum sulfide (MoS2), a ultrathin two-dimensional material with about 9–10 layers. By employing the nonlinear absorption of the modulator, the pulsed orange, red and deep red lasers were directly generated. Besides, the present atomic-layer MoS2 optical modulator has broadband modulating properties and advantages in the simple preparation process. The present results experimentally verify the theoretical prediction for the low-dimensional optoelectronic modulating devices in the visible wavelength region and may open an attractive avenue for removing a stumbling block for the further development of pulsed visible lasers.

1.6-W self-referenced frequency comb at 2.06 μm using a Ho:YLF multipass amplifier

A high-power optical frequency comb at 2.06 μm has been generated using a Ho:YLF multipass amplifier seeded by the long wavelength supercontinuum tail of an octave-spanning Er:fiber comb source. The Ho:YLF amplifier showed a net gain larger than 30 dB from 2048 to 2068 nm, allowing the generation of a 20 nm bandwidth comb with a mode spacing of 100 MHz and a power per mode ranging from 20 to 370 μW. In the time domain, the amplified comb corresponds to a pulse train with 1.6 W total power and 508 fs transform-limited pulse duration. Using a self-referencing f-2f interferometer and a phase-locking loop, spectral narrowing of the offset frequency down to less than 17 Hz has been achieved.

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.

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.