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1.
Materials (Basel) ; 13(20)2020 Oct 19.
Artigo em Inglês | MEDLINE | ID: mdl-33086548

##### RESUMO

The demand for tunable visible laser sources with high power and high beam quality, for application ranging from metrology to remote sensing, is constantly increasing. In this work, we report on the details of crystal growth, via the Czochralski method, and laser characterization of a Pr-doped Ba(Y1-xLux)2F8 (BYLF) single crystal, which is a promising candidate for fulfilling these requirements, both in terms of tunability and high-power capabilities. We measured for the first time the laser tunability curve in the deep red region obtaining a continuous range of 17 nm. The laser emission of the three main Pr3+ lines in the visible (orange, red, and deep red) was tested under increased pump power with respect to previous studies on this material, demonstrating output powers of more than 360 mW and no thermal rolloff, up to 1.9 W of absorbed power.

2.
Opt Lett ; 45(15): 4104-4107, 2020 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-32735234

##### RESUMO

We report, for the first time to our knowledge, tunable continuous-wave laser action in the Tm3+:BaY2F8 (BYF) crystal near 2.3 µm. In the experiments, a BYF crystal doped with 3 at. % thulium was end pumped with a narrow-linewidth, tunable Ti3+:sapphire laser with up to 920 mW of incident power. Lasing was achieved for the two pump polarizations of E//x and E//y. The best power performance was obtained in the case of E//x, double-end pumping, where 100 mW of output power was obtained at 2290 nm with 920 mW of pump power and 1% output coupler. The laser could be continuously tuned from 2233 to 2385 nm. Excitation spectra for E//x and E//y pumping were measured in the 760-810 nm range, and the optimum pumping wavelength was determined to be 779 nm for E//x. By using the lifetime and lasing threshold data, the stimulated emission cross section at 2290 nm was further determined to be (0.66±0.06)×10-24m2.

3.
Opt Express ; 28(10): 14476-14489, 2020 May 11.
Artigo em Inglês | MEDLINE | ID: mdl-32403487

##### RESUMO

In this work, we present the cooling efficiency of a LiYF4 (lithium yttrium fluoride,YLF) sample co-doped with 10at.% ytterbium (Yb3+) and 0.0040at.% thulium (Tm3+). For the first time at room temperature, the cooling efficiency of a sample overcomes the 4%-barrier, and it must be compared with the best values reported in literature, about 3%, obtained with a YLF:10at.%Yb sample. We also investigated the frequency behaviour of energy transfer mechanisms between Yb and Tm ions in order to have a better understanding of the contribution of phonons to the cooling cycle. These mechanisms can explain the cooling efficiency enhancement in the co-doped system.

4.
Opt Lett ; 45(3): 656-659, 2020 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-32004276

##### RESUMO

We report experimental demonstration of graphene mode-locked operation of ${{\rm Tm}^{3 + }}\!:\!{{\rm YLiF}_4}$Tm3+:YLiF4 (YLF) and ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm3+:KY3F10 (KYF) lasers near 2.3 µm. To scale up the intracavity pulse energy, the cavity was extended, and double-end pumping was employed with a continuous-wave, tunable ${{\rm Ti}^{3 + }}\!:\!{\rm sapphire}$Ti3+:sapphire laser delivering up to 1 W near 780 nm. The extended ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm3+:KYF laser cavity was purged with dry nitrogen to eliminate pulsing instabilities due to atmospheric absorption lines, but this was not needed in the case of the ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm3+:YLF laser. Once initiated by graphene, stable uninterrupted mode-locked operation could be maintained with both lasers. With the extended cavity ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm3+:YLF laser, 921 fs pulses were generated at a repetition rate of 17.2 MHz at 2304 nm. 739 fs pulses were obtained at the repetition rate of 54 MHz from the ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm3+:KYF laser at 2340 nm. The corresponding pulse energy and peak power were 2.4 nJ and 2.6 kW for the ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm3+:YLF laser, and 1.2 nJ and 1.6 kW for the ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm3+:KYF laser. We foresee that it should be possible to generate shorter pulses at higher pump levels.

5.
Opt Lett ; 45(4): 931-934, 2020 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-32058509

##### RESUMO

We report efficient lasing of the isotropic ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm3+:KY3F10 crystal near 2.3 µm via upconversion pumping with a 1064 nm ytterbium fiber laser as the pump source. When pumped at 1064 nm, an x-cavity ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm3+:KY3F10 laser operated at the free-running wavelength of 2344 nm. Lasing was obtained with output couplers having transmissions in the range of 1-3%, and as high as 124 mW of continuous-wave (cw) output power was generated with 604 mW of absorbed pump power by using the 3% output coupler. Broadly tunable cw lasing could be obtained in the 2268-2373 nm wavelength range. An analysis of the experimental power efficiency data shows that nearly all of the absorbed pump photons were converted to 2.3 µm laser output after accounting for the quantum defect of the laser transition and resonator losses. We expect that higher lasing efficiency should be possible by using longer crystals to increase the pump absorption.

6.
Opt Express ; 27(23): 33217-33232, 2019 Nov 11.
Artigo em Inglês | MEDLINE | ID: mdl-31878395

##### RESUMO

We explore the arguably most fundamental aspect of energy-transfer upconversion (ETU), namely the dependence of upconversion luminescence from a higher-energy level, following ETU excitation from a metastable lower-energy level, on direct luminescence from that metastable level. We investigate ETU among neighboring Nd3+ ions in single crystals of GdVO4 and LaSc3(BO3)4 with different doping concentrations by measuring, after short-pulse laser excitation with different pump energies, the infrared luminescence decay from the metastable 4F3/2 level and the yellow upconversion luminescence decay from the 4G7/2 level. We observe a highly super-quadratic dependence of upconversion on direct luminescence intensity. We conclude that the commonly assumed quadratic law of ETU, as proposed by Grant's model and frequently employed in rate-equation simulations, is inadequate to the description of ETU processes. Whereas Zubenko's model, which considers a finite migration rate, provides significantly better fits to the experimental luminescence-decay curves, also this model cannot accurately reproduce the measured decay curves, partly because it does not take the non-homogeneous distribution of active ions into account.

7.
Opt Express ; 27(21): 29710-29718, 2019 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-31684228

##### RESUMO

Optical cooling of a YLF:Yb single crystal to 87 K, well below the minimum achievable temperature predicted from existing theory, has been observed. This discrepancy between theory and data has motivated us to revisit the current model of optical refrigeration, in particular the critical role of parasitic background absorption. Challenging experiments that measured the cooling efficiency as a function of temperature reveal that the background absorption coefficient decreases with temperature, resulting in a significant enhancement of the cooling efficiency at cryogenic temperatures. These discoveries emphasize the high sensitivity of optical cooling to impurity-mediated processes and show the necessity of formulating a cooling model that includes the temperature dependence of the background absorption. To properly characterize the cooling properties of any sample, it is necessary to measure its low-temperature performance.

8.
Opt Lett ; 44(15): 3665-3668, 2019 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-31368938

##### RESUMO

We demonstrate the Kerr-lens mode-locked Pr3+:LuLiF4 (Pr:LLF) laser pumped by a blue laser diode (LD). By theoretical calculation of the group velocity dispersion in the laser gain, the compensation was employed for the realization of the continuous-wave mode-locked laser at the wavelength of 604 nm with the pulse width of 1.1 ps which, to the best of our knowledge, is the shortest pulse width in the Pr3+ ion doped crystal lasers pumped with LDs. It can be believed that the present Pr:LLF laser should provide some inspiration for the development of the blue LD pumped visible lasers, especially in the mode-locking laser operation.

9.
Opt Lett ; 44(17): 4095-4098, 2019 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-31465337

##### RESUMO

We present, to the best of our knowledge, the first demonstration of a single crystal fiber solid-state amplifier based on a birefringent Yb3+-doped thin crystal grown by the micro-pulling-down method. We measured a small signal gain >30 in a four-passes Yb:LiLuF4 amplifier pumping with a 120-W maximum power fiber-coupled laser diode. At an absorbed pump power of 80 W, a maximum output power of 8.4 W was obtained seeding the 42-mm-long, 2%Yb3+-doped single crystal fiber sample with 0.8 W at 1021 nm. Even at the maximum incident pump power, the amplified laser beam polarization and spatial quality were excellent (M2=1.15×1.06).

10.
Opt Lett ; 44(13): 3242-3245, 2019 Jul 01.
Artigo em Inglês | MEDLINE | ID: mdl-31259931

##### RESUMO

We report continuous-wave lasing demonstration of a Tm3+:KY3F10 crystalline gain medium at 2343 nm. A narrow-linewidth, tunable, continuous-wave Ti3+:sapphire laser was used to end pump a Tm3+:KY3F10 crystal with thulium doping of 8 at. %. With 1 W of pump power, the resonator with a 1% output coupler generated output powers of 31 and 122 mW in single-pumping and double-pumping configurations, respectively. Excitation efficiency of the laser was investigated for pump wavelengths in the 765-806 nm range. Four pump bands were identified in this wavelength range, with the two most efficient pump bands centered at 773 and 778 nm. The output of the laser could be tuned smoothly and continuously over a spectral width of 125 nm from 2260 to 2385 nm. The lifetime of the H43 level was further measured to be 16 µs, and the emission cross section was determined based on lasing threshold measurements.

11.
Opt Lett ; 44(6): 1419-1422, 2019 Mar 15.
Artigo em Inglês | MEDLINE | ID: mdl-30874668

##### RESUMO

We report the complete characterization of various cooling-grade Tm-doped crystals including, to the best of our knowledge, the first demonstration of optical refrigeration in Tm:YLF crystals. Room temperature laser cooling efficiencies of 1% and 2% (mol) Tm:YLF and 1% Tm:BYF crystals at different excitation polarizations are measured, and their external quantum efficiency and background absorption are extracted. By performing detailed low-temperature spectroscopic analysis of the samples, global minimum achievable temperatures of 160 to 110 K are estimated. The potential of Tm-doped crystals to realize mid-IR optical cryocoolers and radiation balanced lasers (RBLs) in the eye-safe region of the spectrum is discussed, and a promising two-tone RBL in a tandem structure of Tm:YLF and Ho:YLF crystals is proposed.

12.
Opt Lett ; 44(3): 594-597, 2019 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-30702687

##### RESUMO

Optical clocks based on strontium (Sr) atoms and ions are currently used to provide precise standards for measuring time. These devices require several narrow-band visible laser sources at specific wavelengths to operate. We report on the diode-pumped continuous-wave laser operation at 674, 688, 689, 698, and 707 nm of a single crystal of Pr3+:LiGdF4 (Pr:GLF). To the best of our knowledge, Pr:GLF is the only active medium suitable for solid-state laser operation at all the wavelengths of interest for Sr-based clocks in the deep-red region. We also investigated the spectral tunability of this source, achieving an uninterrupted tuning range of 17 nm, between 697 and 714 nm.

13.
Appl Opt ; 57(9): 2223-2226, 2018 Mar 20.
Artigo em Inglês | MEDLINE | ID: mdl-29604016

##### RESUMO

Fiber rods of 10% doped Yb:YLF were fabricated with the micro-pulling-down technique and characterized. The crystal a axis was oriented along the rod length, allowing polarized emission with the largest cross section available in the c direction. Laser experiments showed that these fiber samples perform similarly to crystals grown by the standard Czochralski method. Intrinsic slope efficiency of â¼50% was measured in both cases, with small comparable intracavity losses, proving the good quality of the fiber material.

14.
Opt Express ; 26(5): 5368-5380, 2018 Mar 05.
Artigo em Inglês | MEDLINE | ID: mdl-29529740

##### RESUMO

We report on the growth, spectroscopy, and laser operation of monocrystalline Tm3+:SrF2. Spectroscopic investigations confirmed the presence of broad absorption and emission bands caused by inequivalent doped sites, introduced by charge compensation effects which also caused the clusterization of doping ions in the lattice. We obtained continuous-wave laser emission at about 2 µm, with efficiencies comparable with other Tm-doped crystals. We also achieved an uninterrupted tuning range of 180 nm between 1.8 and 2 µm. This characterization indicates that SrF2 enhances the cooperative mechanisms between Tm ions, helping to obtain remarkable laser performances at low doping concentrations.

15.
Sci Rep ; 7(1): 15168, 2017 11 09.
Artigo em Inglês | MEDLINE | ID: mdl-29123171

##### RESUMO

We present a detection scheme to search for QCD axion dark matter, that is based on a direct interaction between axions and electrons explicitly predicted by DFSZ axion models. The local axion dark matter field shall drive transitions between Zeeman-split atomic levels separated by the axion rest mass energy m a c 2. Axion-related excitations are then detected with an upconversion scheme involving a pump laser that converts the absorbed axion energy (~hundreds of µeV) to visible or infrared photons, where single photon detection is an established technique. The proposed scheme involves rare-earth ions doped into solid-state crystalline materials, and the optical transitions take place between energy levels of 4f N electron configuration. Beyond discussing theoretical aspects and requirements to achieve a cosmologically relevant sensitivity, especially in terms of spectroscopic material properties, we experimentally investigate backgrounds due to the pump laser at temperatures in the range 1.9 - 4.2 K. Our results rule out excitation of the upper Zeeman component of the ground state by laser-related heating effects, and are of some help in optimizing activated material parameters to suppress the multiphonon-assisted Stokes fluorescence.

16.
Opt Lett ; 42(3): 547-550, 2017 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-28146524

##### RESUMO

Visible lasers are a fascinating regime, and their significance is illustrated by the 2014 Noble prizes in physics and chemistry. With the development of blue laser diodes (LDs), the LD-pumped solid-state visible lasers become a burgeoning direction today. Constrained by the scarce visible optical modulators, the solid-state ultrafast visible lasers are rarely realized. Based on the bandgap structure and optoelectronic properties of atomic-layer MoS2, it can be proposed that MoS2 has the potential as a visible optical modulator. Here, by originally revealing layer-dependent nonlinear absorption of the atomic-layer MoS2 in the visible range, broadband atomic-layer MoS2 optical modulators for the visible ultrafast pulse generation are developed and selected based on the proposed design criteria for novel two-dimensional (2D) optical modulators. By applying the selected MoS2 optical modulators in the solid-state praseodymium lasers, broadband mode-locked ultrafast lasers from 522 to 639 nm are originally realized. We believe that this Letter should promote the development of visible ultrafast photonics and further applications of 2D optoelectronic materials.

17.
Opt Lett ; 41(23): 5555-5558, 2016 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-27906237

##### RESUMO

We report on, to the best of our knowledge, the first growth, spectroscopy, and laser operation of monocrystalline Ba(Y0.8Lu0.2)2F8 doped with 1.25 at.% of Pr3+ ions (Pr:BYLF). We studied the spectroscopic properties of this material, particularly in comparison with its isomorph Pr3+: BaY2F8 (Pr:BYF). Under diode-pumped excitation, we achieved continuous-wave laser operation in the orange, red, and deep red regions in the three possible crystal orientations. We obtained considerably improved laser performances and remarkably low passive losses, in comparison with Pr:BYF crystals and other fluorides, indicating that Pr:BYLF is an excellent host material in the visible region.

18.
Opt Express ; 24(11): 11782-92, 2016 May 30.
Artigo em Inglês | MEDLINE | ID: mdl-27410103

##### RESUMO

A detailed performance comparison of new interesting Yb-doped crystals in the same oscillator setup, with 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.

19.
Appl Opt ; 55(16): 4414-7, 2016 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-27411196

##### RESUMO

A Yb:YLF crystal has been investigated in a femtosecond oscillator pumped by two 400 mW single-mode fiber-coupled diodes emitting at 976 nm and mode locked with a semiconductor saturable absorber mirror. Almost Fourier transform-limited pulses with durations of 87 and 107 fs were demonstrated for extraordinary and ordinary polarizations, respectively. This is, to the best of our knowledge, the first demonstration of sub-100 fs pulses with Yb:YLF, and it proves the potential for ultrashort pulse generation and amplification with this material.

20.
Opt Lett ; 41(12): 2692-5, 2016 Jun 15.
Artigo em Inglês | MEDLINE | ID: mdl-27304265

##### RESUMO

We demonstrate efficient laser-diode pumped multi-gigahertz (GHz) self-mode-locked praseodymium (Pr3+) visible lasers with broadband spectra from green to deep red for the first time to our knowledge. With a Pr3+-doped GdLiF4 crystal, stable self-mode-locked visible pulsed lasers at the wavelengths of 522 nm, 607 nm, 639 nm, and 720 nm have been obtained with the repetition rates of 2.8 GHz, 3.1 GHz, 3.1 GHz, and 3.0 GHz, respectively. The maximum output power was 612 mW with the slope efficiency of 46.9% at 639 nm. The mode-locking mechanism was theoretically analyzed. The stable second-harmonic mode-locking with doubled repetition frequency was also realized based on the Fabry-Perot effect formed in the laser cavity. In addition, we find that the polarization directions were turned with lasing wavelengths. This work may provide a new way for generating efficient ultrafast pulses with high- and changeable-repetition rates in the visible range.

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