All-solid-state 3-µm single-frequency Er:CaF2 laser with a stable and switchable wavelength.

We have demonstrated a 3-µm all-solid-state single-frequency laser with a stable center frequency and a switchable wavelength using the intra-cavity Fabry-Perot etalon method. Experimentally, the central wavelengths of the laser for the single-longitudinal mode are 2728 and 2794 nm, with maximum output powers of 268 and 440 mW, respectively. The corresponding single-longitudinal mode linewidths are 25 and 11 MHz. In particular, the central wavelengths of the single-longitudinal mode laser remain almost constant as the incident pump power increases. To the best of our knowledge, this study represents the first instance of using a laser diode to directly pump Er:CaF2 block single crystals for single-frequency lasers in the 3 µm region. Additionally, it achieves the highest output power of a 3-µm all-solid-state single-longitudinal mode.

Sub-50-fs Kerr-lens mode-locked Yb-doped fluoride laser with 44% optical efficiency.

Yb-doped fluoride has been demonstrated to be potential crystals for application in efficient ultrafast lasers. However, the trade-off between the shorter pulses with higher efficiencies is a challenge. In this work, using Y b,G d:C a S r F 2 crystal, we report on a sub-50-fs Kerr-lens mode-locked oscillator with an optical efficiency up to 44%. Pumped by a 976-nm diffraction-limited fiber laser and using chirped mirrors combined with prism pairs for the dispersion compensation, a pulse as short as 46 fs was obtained with 620-mW output power, corresponding to an optical efficiency more than 40%. Stable Kerr-lens mode-locking with RMS of output power lower than 0.3% and beam quality factors M 2<1.14 were achieved. Moreover, a maximum output power of 780 mW was obtained in continuous-wave operation with 55.3% optical efficiency. To the best of our knowledge, the results in this work represent the shortest pulses generated from Yb-doped fluoride lasers as well as the highest optical efficiencies ever reported in sub-100 fs Yb bulk lasers.

Diode-pumped mode-locked Yb:Sc2SiO5 laser generating 38 fs pulses.

We report on sub-40 fs pulse generation from a Yb:Sc2SiO5 laser pumped by a spatially single-mode fiber-coupled laser diode at 976 nm. A maximum output power of 545 mW was obtained at 1062.6 nm in the continuous-wave regime, corresponding to a slope efficiency of 64% and a laser threshold of 143 mW. A continuous wavelength tuning across 80 nm (1030 -1110 nm) was also achieved. Implementing a SESAM for starting and stabilizing the mode-locked operation, the Yb:Sc2SiO5 laser delivered soliton pulses as short as 38 fs at 1069.5 nm with an average output power of 76 mW at a pulse repetition rate of ∼79.8 MHz. The maximum output power was scaled to 216 mW for slightly longer pulses of 42 fs, which corresponded to a peak power of 56.6 kW and an optical efficiency of 22.7%. To the best of our knowledge, these results represent the shortest pulses ever achieved with any Yb3+-doped rare-earth oxyorthosilicate crystal.

Diode-pumped SESAM mode-locked Tm:Sc2SiO5 laser.

We report a diode-pumped passively mode-locked Tm:Sc2SiO5 (Tm:SSO) laser for the first time, to the best of our knowledge. The stable continuous-wave (CW) mode-locking is achieved with a semiconductor saturable absorber mirror (SESAM). Operating at the eye-safe wavelength of 1967.7 nm, the pulsed laser delivers a pulse duration of 16.5 ps with an average output power of 207 mW. At a fundamental repetition frequency of 81 MHz, the signal-to-noise ratio is as high as 70 dB. These results demonstrate the great potential of Tm:SSO crystal for ultrashort pulse generation.

Sub-60-fs ultralow threshold and efficient Kerr-lens mode-locked Yb,Gd:CaSrF2 laser.

In this Letter, using a Yb,Gd:CaSrF2 co-doping mixed crystal, an ultra-low threshold and efficient Kerr-lens mode-locked femtosecond oscillator is realized with a mode-locking threshold as low as 150 mW. With a 200-mW pump power, the shortest pulses are obtained with a pulse duration of 57 fs. A maximum mode-locked output power of 185 mW is observed under a 500-mW pump power, corresponding to an optical-to-optical efficiency of up to 37%. To the best of our knowledge, the 150-mW threshold is the lowest pump power to realize Kerr-lens mode-locking operation in Yb-doped bulk lasers. Furthermore, an optical efficiency of 37% is the highest efficiency in Yb-doped fluoride bulk lasers to date. Our results provide a new basis for high-efficiency and low-threshold Yb-doped ultrafast bulk lasers.

Laser-diode-pumped Tm:SrF2 single crystal for high efficiency CW laser operation at ∼2 µm.

A Tm:SrF2 single crystal is grown by a modified temperature gradient technique (TGT). A study of the fluorescence characteristics and laser performance is carried out. Under laser-diode (LD) end-pumping, a continuous-wave (CW) laser is demonstrated. A slope efficiency of up to 81.8% is first achieved in the Tm-doped single crystals, which is extremely close to the photon quantum efficiency of 2.00. Such a high photon quantum efficiency is caused by the effective cross-relaxation process between Tm3+ ions. A maximum output power of 4.082 W is also obtained. The laser performance indicates that Tm:SrF2 is a promising candidate for highly efficient ∼2 µm lasers.

Indium Tin Oxide Nanowire Arrays as a Saturable Absorber for Mid-Infrared Er:Ca0.8Sr0.2F2 Laser.

We demonstrated a passively Q-switched Er:Ca0.8Sr0.2F2 laser with indium tin oxide nanowire arrays as an optical modulator in the mid-infrared region. In the Q-switched regime, the maximum output power of 58 mW with a slope efficiency of 18.3% was acquired. Meanwhile, the minimum pulse duration and highest repetition rate of the stable pulse trains were 490 ns and 17.09 kHz, corresponding to single pulse energy of 3.4 µJ and peak power of 6.93 W, respectively. To the best of our knowledge it was the first time that indium tin oxide nanowire arrays were employed as a saturable absorber to make pulse lasers carried out at 2.8 µm. The experimental data show that indium tin oxide nanowire arrays can be employed as a competitive candidate for saturable absorber in the field of mid-infrared solid-state lasers.

Tb,Y:SrF2 crystal for efficient laser operation in the visible spectral region.

A Tb,Y:SrF2 crystal with high optical quality is grown using the temperature gradient technique. The spectroscopic and laser properties of the crystal in the visible spectral region are studied. A fluorescence lifetime of 5.6 ms is measured from the crystal, which is beneficial for laser operation with a low threshold. A continuous-wave Tb,Y:SrF2 laser delivers an output power of 259 mW at 545 nm, with a slope efficiency of 35.2%. To the best of the authors' knowledge, this is the first report on a Tb-doped SrF2 laser and represents the highest output power for visible alkaline-earth fluoride lasers. The limitations for power scaling are discussed.

Tailoring local coordination structure of the Er3+ ions for tuning the up-conversion multicolor luminescence.

The regulation of the local structure around Er3+ ions is an important channel for adjusting the characteristic of up-conversion luminescence. In this paper, the cubic-phased Er3+:CaF2 crystals with different Er3+ doping concentrations were fabricated with temperature gradient technique (TGT) method and the effect of the local coordination structure of the Er3+ ions on its luminescence performance was investigated. The local coordination structure of Er3+ ions was simulated by density functional theory. The computational results show that clusters evolve from low order to high order with the increase of Er3+ ion doping concentration. In this evolution process, the local structure transforms from cubic structure to the co-existence of cubic and lower symmetric square anti-prism structures. Meanwhile, the distance between Er3+ ions in the cluster decreased first and then increased slightly, and in dimers and trimers this distance reached the minimum. Under 980 nm excitation, with the increase of Er3+ ion concentration, the intensity ratios of the red and green emissions of Er3+:CaF2 first increased from 0.61 to 42.03 and then decreased to 12.11. The corresponding up-conversion luminescence gamut was adjusted from monochrome green to red to red-yellow. This work provides a new thread for realizing upconversion multicolor luminescence by regulating the clusters of rare earth ions.

Irreversible accumulated SERS behavior of the molecule-linked silver and silver-doped titanium dioxide hybrid system.

In recent years, surface-enhanced Raman scattering (SERS) of a molecule/metal-semiconductor hybrid system has attracted considerable interest and regarded as the synergetic contribution of the electromagnetic and chemical enhancements from the incorporation of noble metal into semiconductor nanomaterials. However, the underlying mechanism is still to be revealed in detail. Herein, we report an irreversible accumulated SERS behavior induced by near-infrared (NIR) light irradiating on a 4-mercaptobenzoic acid linked with silver and silver-doped titanium dioxide (4MBA/Ag/Ag-doped TiO2) hybrid system. With increasing irradiation time, the SERS intensity of 4MBA shows an irreversible exponential increase, and the Raman signal of the Ag/Ag-doped TiO2 substrate displays an exponential decrease. A microscopic understanding of the time-dependent SERS behavior is derived based on the microanalysis of the Ag/Ag-doped TiO2 nanostructure and the molecular dynamics, which is attributed to three factors: (1) higher crystallinity of Ag/Ag-doped TiO2 substrate; (2) photo-induced charge transfer; (3) charge-induced molecular reorientation.

High-efficiency ∼2 µm CW laser operation of LD-pumped Tm3+:CaF2 single-crystal fibers.

The Tm:CaF2 single-crystal fibers were successfully grown by modified temperature gradient technique method. The J-O intensity parameters, spontaneous radiative transition rates, radiative lifetimes and fluorescence branching ratios of Tm3+ were calculated with Judd-Ofelt theory. A systematic study of the fluorescence characteristics has been carried out. Simulated emission cross-sections of the 3F4 → 3H6 transition were calculated to be 6.68×10-21 cm-2 and 4.65×10-21cm-2 for crystal doped with 3 at.% and 4 at.% Tm3+. The 64.4% slope efficiency with output power of 2.23W was achieved in 3 at.% Tm:CaF2 single-crystal fiber. The slope efficiency decreased to 44.5% and maximum output power decreased to 1.65W in 4 at.% Tm:CaF2 single-crystal fiber. Obtained results show that Tm-doped CaF2 single-crystal fibers are promising materials for IR laser action generation.

Continuous-wave and Q-switched Nd:BGSO lasers based on bismuth nanosheets absorber.

This Nd:BG1-xSxO (Nd:BGSO) crystal was grown using the micro-pulling-down method, and the continuous-wave laser operation of this crystal was demonstrated for the first time, to the best of our knowledge. The maximum output power of 1.038 W was obtained under the absorbed pump power of 3.01 W, which corresponds to a slope efficiency of 31.3%. Bismuth nanosheets were first employed as saturable absorbers to generate a passively Q-switched Nd:BGSO laser. Stable Q-switched pulses with the shortest pulse width of 376.5 ns and the maximum repetition rate of 136.6 kHz were achieved at the absorbed pump power of 3.01 W. The largest pulse energy and highest peak power achieved were 0.94 µJ and 2.48 W, respectively.

Active Q-switching operation of slab Ho:SYSO laser wing-pumped by fiber coupled laser diodes.

In this paper, we demonstrate the continuous-wave and acousto-optical Q-switched performance of diode-pumped slab Ho:(Sc0.5Y0.5)2SiO5 (Ho:SYSO) laser at 2.1 µm for the first time. Two 1.91-µm laser diodes were used to pump the Ho:SYSO crystal. With a wing-pumping structure, at absorbed pump power of 44.7 W, the continuous wave slab Ho:SYSO laser produced 20.7 W maximum output power at 2097.9 nm, resulting in a slope efficiency of 53.1% with respect to the absorbed pump power. In the Q-switched regime, the slab Ho:SYSO laser produced up to 3.4 mJ pulse energy with 20 ns minimum pulse width at pulse repetition frequency of 5 kHz, corresponding to a peak power of 170 kW.

Efficient Ho:(Sc0.5Y0.5)2SiO5 laser at 2.1 µm in-band pumped by Tm fiber laser.

In this paper, we demonstrate a new 2.1-µm Ho:(Sc0.5Y0.5)2SiO5 (Ho:SYSO) laser at room temperature. The absorption and emission spectra of Ho:SYSO crystal were studied at temperature of 300 K. The strongest absorption peak of 5I7 level of Ho ions in SYSO crystal is located at 1943 nm with cross section of 0.79 × 10-20 cm2. The maximum emission cross section of 1.74 × 10-20 cm2 is located at 2032 nm. A 1940.3-nm narrow-linewidth Tm fiber was used to pump the Ho:SYSO crystal. At absorbed pump power of 20.4 W, the continuous wave Ho:SYSO laser yielded 10.3 W maximum output power at 2097.67 nm and 54.7% slope efficiency respect with absorbed pump power. In addition, we have estimated the beam quality factor (M2) of Ho:SYSO laser to be 1.7 near maximum output level.

Color-tunable visible photoluminescence of Eu:CaF2 single crystals: variations of valence state and local lattice environment of Eu ions.

In this article, Eu-activated CaF2 single crystals were synthesized by Bridgman-Stockbarge method. The dependence of photoluminescence properties of Eu: CaF2 crystals in UV-Vis regions on EuF3 doping concentrations were investigated. While the EuF3 doping concentration is increased from 0.6% to 6.0%, the CIE (Commission Internationale de L'Eclairage) color coordinates of Eu: CaF2 crystals can be tuned from (0.28, 0.12) to (0.60, 0.38), corresponding to the luminescence color from blue to orange. XPS (X-ray Photoelectron Spectroscopy) measurements indicated that Eu2+ and Eu3+ ions both existed in the crystals. With EuF3 doping concentration increasing, the proportion of Eu3+ ions increase from 16.73% to 39.00%, while that of Eu2+ ions decrease from 83.27% to 61.00%. Moreover, the integrated intensity ratio (R) of the 614 nm to 593 nm of Eu3+ ions increase from 0.38 to 0.44, indicating the local lattice environment symmetry of Eu3+ ions become lower with higher EuF3 doping concentrations. Furthermore, the CIE chromaticity coordinates of Eu: CaF2 crystals greatly depend on the excitation wavelength. The warm white-light emission has been realized in 0.6%Eu: CaF2 crystal when the excitation wavelength is around 322 nm.

1886-nm mode-locked and wavelength tunable Tm-doped CaF2 lasers.

A 3 at.% Tm-doped CaF2 crystal was grown successfully and used as a gain medium. A high-efficiency continuous-wave laser and wide tuning performance were demonstrated in this crystal, and passive mode-locking laser operation was realized for the first time, to the best of our knowledge. For continuous-wave operation, the maximum output power was 2.71 W with a high slope efficiency of 70.3%. Using a quartz birefringent filter, a continuous tuning range of approximately 190 nm was observed in the tuning mode. In addition, a passively mode-locked Tm3+:CaF2 laser emitting at 1886.8 nm was achieved with a semiconductor saturable absorber mirror. At an absorbed pump power of 2.65 W, a pulse duration with tens of picoseconds was obtained at a pulse repetition rate of 96.35 MHz.

Wavelength-locked continuous-wave and Q-switched Ho:CaF2 laser at 2100.5 nm.

In this paper, we demonstrated a linewidth-narrowed continuous-wave (CW) and acousto-optical Q-switched Ho:CaF2 laser for the first time. With a volume Bragg grating, a maximum CW output power of 6.94 W at 2100.5 nm, and slope efficiency of 57.9%, an FWHM linewidth of 0.31 nm was obtained. When absorbed pump power was 13.2 W, the maximum average output powers of 6.08 W, 5.9 W, and 5.71 W were achieved in a Q-switched Ho:CaF2 laser under pulse repetition frequencies of 10 kHz, 5 kHz, and 3 kHz, corresponding to the slope efficiencies of 51.2%, 49.6%, and 48.5%, respectively. The minimum pulse width of 54 ns was achieved at pulse repetition frequency of 3 kHz.

Tunable and passively Q-switched laser operation of Nd,Lu:CaF2 disordered crystal.

We report the first demonstration, to the best of our knowledge, of tunable and Q-switched lasers based on diode-pumped Nd,Lu:CaF2 disordered crystal. A continuous tuning range of approximately 32 nm (1047-1079 nm) was obtained for tunable operations. A passively Q-switched laser was successfully achieved using a Cr4+:YAG as a saturable absorber. The shortest pulse width of 275 ns was obtained with a repetition rate of 1.85 kHz and corresponding peak power and single pulse energy of 189 W and 51.9 µJ, respectively.

High-efficiency 2 µm continuous-wave laser in laser diode-pumped Tm3+, La3+: CaF2 single crystal.

A Tm3+, La3+: CaF2 single crystal was grown, and its spectral properties and laser performance were investigated. Under diode end-pumping, a continuous-wave laser has been demonstrated. The slope efficiency of up to 67.8% was first achieved in the co-doping crystal, which is close to twice the theoretical quantum efficiency. A maximum output power of 4.269 W and a broad tunable range of 192 nm were also obtained, indicating this system is a promising candidate for highly efficient ∼2 µm lasers.

Efficient continuous-wave and passive Q-switched mode-locked Er3+ : CaF2-SrF2 lasers in the mid-infrared region.

An Er3+-doped CaF2-SrF2 mixed crystal was grown using the temperature gradient technique, and its laser characteristics were studied. In a compact linear cavity, a continuous-wave output power of 712 mW was obtained with the highest slope efficiency of 41.4%. Using a semiconductor saturable absorber mirror, a passive Q-switched mode-locked Er3+:CaF2-SrF2 laser emitting at 2729.5 nm was demonstrated for the first time, to the best of our knowledge. The maximum average output power of 125 mW was obtained at an absorbed pump power of 1.81 W, and the repetition rate of the Q-switched envelope was 4.17 kHz. The mode-locked pulses in the Q-switched pulse envelope had a repetition rate of 136.3 MHz, and its duration was estimated to be approximately 1.78 ns. These results indicate that the Er3+-doped CaF2-SrF2 mixed crystal is promising for the development of an ultrafast laser in the mid-infrared regime.