Sub-60-fs mode-locked Tm,Ho:CaYLuAlO4 laser at 2.05†µm.

Tm,Ho:CaYLuAlO4 (Tm,Ho:CALYLO) crystal has wide emission spectra both for π-polarization and σ-polarization, showing significant potential for the generation of ultrashort pulses. Here, a widely tunable and passively mode-locked laser operation based on Tm,Ho:CALYLO crystal under two polarizations was demonstrated for what we believe to be the first time ever. For π-polarization, a maximum output power of 1.52 W and a tuning range of 255.3 nm were achieved in the continuous wave (CW) regime. In the mode-locked regime, a pulse duration of 68 fs and an average output power of 228 mW were achieved upon GaSb-based semiconductor saturable absorber mirror (SESAM). As for σ-polarization, a broader tuning range of 267.1 nm was realized, leading to the shorter pulse duration of 58 fs at 79.7 MHz repetition rate.

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.

Spectroscopy and efficient laser performances of a Tm,Ho:CaY0.9Lu0.1AlO4 crystal.

We report on the spectral properties and laser performances of a novel, to the best of our knowledge, Tm,Ho:CaY0.9Lu0.1AlO4 (Tm,Ho:CYLA) crystal. The polarized absorption spectra, luminescence spectra, and fluorescence lifetime are systematically investigated, presenting a broad and smooth luminescence band. Furthermore, a maximum continuous wave (CW) laser output power of 0.51 W at 2092 nm is obtained under an absorbed pump power of 2.89 W, corresponding to a slope efficiency of 20.4%. The beam quality factors (M2) are measured to be 1.04 in both the x and y axes. A tuning range of 123.4 nm, from 2017.8 nm to 2141.2 nm, is achieved in the CW regime by using a birefringent filter (BF). A stable passively Q switched Tm,Ho:CYLA laser employing Cr2+:ZnSe as a saturable absorber (SA) is realized for the first time, delivering the shortest pulse width of 560 ns with a transmittance of 1%. The results indicate that the Tm,Ho:CYLA crystal is an excellent laser medium for generating high-efficiency laser at ∼2 µm and has a potential in ultrafast laser generation.

44-fs pulse generation at 2.05 µm from a SESAM mode-locked Tm:GdScO3 laser.

Pulses as short as 44 fs (6 optical cycles) with a spectral width of 120 nm are generated from a mode-locked solid-state laser near 2 µm employing an orthorhombic Tm:GdScO3 perovskite crystal. The average power amounts to 188 mW at a repetition rate of ∼77.6 MHz. The strong inhomogeneous line broadening in GdScO3 suggests optimum conditions for few-optical-cycle pulse generation of rare-earth ion doped GdScO3 bulk lasers.

GaInSn liquid nanospheres as a saturable absorber for Q-switched pulse generation at 639 nm.

Liquid metals, which possess the superiority of low cost, shape-reconfigurability, and excellent optoelectronic properties, have been applied in various fields, such as flexible electronics, superconductivity, and coolants. In this paper, high-quality GaInSn liquid nanospheres synthesized by the ultrasonic method are applied for nonlinear optics and laser switches. The saturation absorption property derived from localized surface plasmon resonance at 639 nm is studied based on the open-aperture Z-scan technique, exhibiting a modulation depth of ∼35.5% and a saturation fluence of ∼21.75 mJ/cm2, respectively. The as-prepared GaInSn liquid nanospheres are also successfully utilized as a saturable absorber to achieve a stable Q-switched Pr:YLF laser at 639 nm. The output pulse width can reach ∼280 ns with a pulse repetition rate of ∼174.8 kHz. Our results suggest that GaInSn liquid nanospheres are a candidate material for generating visible laser pulses, which is of great interest for potential applications in visible nonlinear optics.

SESAM mode-locked Tm:Y2O3 ceramic laser.

We demonstrate a widely tunable and passively mode-locked Tm:Y2O3 ceramic laser in-band pumped by a 1627-nm Raman fiber laser. A tuning range of 318 nm, from 1833 to 2151 nm, is obtained in the continuous-wave regime. The SESAM mode-locked laser produces Fourier-transform-limited pulses as short as 75 fs at ∼ 2.06 µm with an average output power of 0.26 W at 86.3 MHz. For longer pulse durations of 178 fs, an average power of 0.59 W is achieved with a laser efficiency of 29%. This is, to the best of our knowledge, the first mode-locked Tm:Y2O3 laser in the femtosecond regime. The spectroscopic properties and laser performance confirm that Tm:Y2O3 transparent ceramics are a promising gain material for ultrafast lasers at 2 µm.

Watt-level femtosecond Tm-doped "mixed" sesquioxide ceramic laser in-band pumped by a Raman fiber laser at 1627 nm.

We report on a semiconductor saturable absorber mirror mode-locked Tm:(Lu,Sc)2O3 ceramic laser in-band pumped by a Raman fiber laser at 1627 nm. The nonlinear refractive index (n2) of the Tm:(Lu,Sc)2O3 ceramic has been measured to be 4.66 × 10-20 m2/W at 2000 nm. An average output power up to 1.02 W at 2060 nm is achieved for transform-limited 280-fs pulses at a repetition rate of 86.5 MHz, giving an optical efficiency with respect to the absorbed pump power of 36.4%. Pulses as short as 66 fs at 2076 nm are produced at the expense of output power (0.3 W), corresponding to a spectral bandwidth of 69 nm. The present work reveals the potential of Tm3+-doped sesquioxide transparent ceramics for power scaling of femtosecond mode-locked bulk lasers emitting in the 2-µm spectral range.

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.

Nonlinear optical properties and passively Q-switched laser application of a layered molybdenum carbide at 639 nm.

Molybdenum carbide (Mo2C) exhibits enormous potential applications in various optoelectronic and photonic fields due to its remarkably electrical and optical characteristics. Here, we fabricate a high-quality Mo2C film by the radio frequency magnetron sputtering deposition method. The nonlinear optical response and ultrafast dynamics are thoroughly studied based on open-aperture Z-scan and nondegenerate pump-probe experimental measurements. The open-aperture Z-scan experimental result exhibits a modulation depth of 8.5% and a saturation fluence of 0.28 mJ/cm2. Simultaneously, the relaxation time constant is fitted by a biexponential decay function, showing an ultrafast intraband carrier recovery time of 0.58 ps at 530 nm. Consequently, by employing the Mo2C film as a saturable absorber (SA), stable Q-switched Pr:YLF laser pulses with the shortest pulse width of 160 ns are generated at 639 nm. Our experimental results demonstrate excellent nonlinear optical properties of the layered Mo2C in the visible region and will further advance their potential applications in visible nonlinear optics.

Tunable and Passively Mode-Locking Nd0.01:Gd0.89La0.1NbO4 Picosecond Laser.

A high-quality Nd0.01:Gd0.89La0.1NbO4 (Nd:GLNO) crystal is grown by the Czochralski method, demonstrating wide absorption and fluorescence spectra and advantage for producing ultrafast laser pulses. In this paper, the tunable and passively mode-locking Nd:GLNO lasers are characterized for the first time. The tuning coverage is 34.87 nm ranging from 1058.05 to 1092.92 nm with a maximum output power of 4.6 W at 1065.29 nm. A stable continuous-wave (CW) passively mode-locking Nd:GLNO laser is achieved at 1065.26 nm, delivering a pulse width of 9.1 ps and a maximum CW mode-locking output power of 0.27 W.

Few-layer TiSe2 as a saturable absorber for nanosecond pulse generation in 2.95 µm bulk laser.

1T-phase titanium diselenide (1T-TiSe2), a model two-dimensional (2D) transition metal dichalcogenide, has attracted much attention due to its intriguing electrical and optical properties. In this work, a 1T-TiSe2-based high-quality large-area saturable absorber (SA) (1T-TiSe2-SA) was successfully fabricated with the liquid-phase exfoliation method. With the as-prepared 1T-TiSe2-SA, a stable, passively Q-switched laser operating at 2.95 µm was first realized. Under an absorbed pump power of 3.35 W, the maximum average output power was 130 mW with a slope efficiency of 5%. A pulse width of 160.5 ns was obtained, which is the shortest among 3.0 µm passively Q-switched lasers ever achieved with 2D materials as SAs, to the best of our knowledge. The results indicate that 1T-TiSe2 is a promising alternative as a nonlinear optical modulator for short-pulse laser generation near the 3.0 µm mid-infrared region.

Tunable plasmon-induced transparency in H-shaped Dirac semimetal metamaterial.

We present a numerical and theoretical study on the realization of tunable plasmon-induced transparency (PIT) effect at terahertz frequencies in Dirac semimetal (known as "three-dimensional graphene") metamaterials. Simulations reveal that the PIT effect is generated by an electric field transferred from the central strip to side strips due to the structural symmetry breaking. The most prominent feature is that the plasmonic resonance in Dirac semimetals can be actively tuned by changing the Fermi energy and an ultrahigh group delay of about 6.81 ps is obtained in our proposed design. Our study can provide guidance for various terahertz devices in practical applications.

Two-photon saturable absorption properties and laser Q-switch application of carbon quantum dots.

In this Letter, high-quality carbon quantum dots (C-QDs) with an average size of 15 nm are synthesized by using a solvothermal method. A C-QD saturable absorber mirror (SAM) was prepared, characterized, and employed as an ultrafast optical switch successfully in a 1.0 µm solid-state laser. The saturable absorption effect (at 1 µm) far away from the linear absorption band of the C-QDs could be attributed to two-photon saturable absorption, which has a native characteristic of wavelength selectivity. The modulation depth (ΔT) and saturable energy intensity (ϕs) of the C-QD-SA was measured to be about 4% and 15.34 W/mm2, respectively. By using this SA, a Q-switched Nd:GdVO4 laser at 1 µm were first realized with the shortest pulse width of 66.8 ns and a maximum repetition rate of 1.13 MHz, respectively. The results indicate that C-QDs may found to be a decent carbon SA material for the high-repetition-rate pulsed laser applications.

Femtosecond pulse generation with an a-cut Nd:CaYAlO4 disordered crystal.

We experimentally demonstrated a diode-pumped 587 fs ultrafast laser by using an a-cut Nd:CaYAlO4 crystal. Pumped by an 808 nm fiber-coupled laser diode, a stable continuous-wave mode-locked ultrafast laser was achieved with a semiconductor saturable absorber. The ultrafast pulses had a repetition rate of 75 MHz at the center wavelength of 1080.8 nm. A maximum average output power of the mode-locked laser reached 375 mW delivering a slope efficiency of 9%.

Efficient graphene Q switching and mode locking of 1.34 µm neodymium lasers.

We demonstrate that few-layered graphene sheets used as a saturable absorber can provide efficient Q-switching and mode-locking modulation in 1.34 µm Nd:GdVO(4) bulk lasers. The minimum Q-switched pulses were 450 ns for 260 mW average power, 43 kHz repetition rate, and 2.5 µJ pulse energy. For the mode-locked laser, an average power of 1.29 W was achieved with 11 ps pulse duration and 13 nJ pulse energy. To our knowledge, this average power is the highest yet obtained from a graphene mode-locked laser, and the corresponding optical-optical efficiency of 23% is the best result among 1.3 µm neodymium mode-locked lasers. The quality factor M(2) of the Q-switched beam was 1.4 and 1.6 in the horizontal and longitudinal planes, respectively, and the M(2) of the mode-locked beam reached 1.1 and 1.0. These results clearly indicate the advantages of few-layered graphene as a saturable absorber.

Diode-pumped passively mode-locked Nd:YAG laser at 1338 nm with a semiconductor saturable absorber mirror.

We demonstrate a diode-end-pumped passively mode-locked 1338 nm Nd:YAG laser with a semiconductor saturable absorber mirror. At the absorbed pump power of 8.89 W, an average output power of 1.12 W was obtained with a slope efficiency of 14%. The pulse width was 22.4 ps with a repetition rate of 63.9 MHz, corresponding to a peak power of 782 W. In addition, the bandwidth of the mode-locking spectrum is as narrow as 20.44 GHz, which shows the potential application in long-distance ranging and fiber information transmission because of the low dispersion of these ultrashort pulses.

Diode-end-pumped passively Q-switched 1.33 microm Nd:Gd3AlxGa(5-x)O12 laser with V3+: YAG saturable absorber.

We demonstrated the 1.33 microm laser performance with Nd:Gd(3)Al(x)Ga(5-x)O(12) (x = 0.94) (Nd:GAGG) laser crystals for the first time. Continuous-wave (cw) output power of 2.45 W was obtained with the optical-optical conversion efficiency of 21.8% and slope efficiency of 23.3%. In the passive Q-switching regime, the highest output power, the shortest pulse width, largest pulse energy and highest peak power were achieved to be 326 mW 18.2 ns, 36.3 microJ and 2.0 kW, respectively, with V(3+):YAG crystal as the saturable absorber.

The visible nonlinear optical properties and passively Q-switched laser application of a layered PtSe2 material.

Platinum diselenide (PtSe2), a type-II Dirac semi-metal material, is a potential saturable absorber (SA) to generate visible pulsed lasers due to its prominent optoelectronic properties. A high quality PtSe2 thin film was fabricated by combining magnetron sputtering with chemical vapor deposition methods; then, its microtopography was characterized by atomic force microscopy. The saturable absorption properties and electron relaxation time in the visible region were measured via open-aperture Z-scan and femtosecond pump probe technology, respectively. The modulation depth and saturable intensity are 39.9% and 256.6 GW cm-2, respectively. The relaxation time constants were fitted to be τ1 = (1.405 ± 0.024) ps, τ2 = (99.03 ± 0.01) ps, and τ3 = (2.032 ± 0.27) ns. The as-prepared PtSe2 thin film was experimentally applied as a novel SA to achieve a stable passively Q-switched (PQS) Pr:YLF visible laser. A shortest pulse width of 91.8 ns with a maximum repetition rate of 297.6 kHz was obtained. These results demonstrate that the PtSe2 thin film has promising applications in generating visible pulsed lasers.

The energy band structure analysis and 2 µm Q-switched laser application of layered rhenium diselenide.

Layered rhenium diselenide (ReSe2) has triggered strong interest because of its outstanding optical and electrical properties. In this paper, we prepared a high-quality multilayer ReSe2 saturable absorber with a liquid-phase exfoliation method and characterized its saturable absorption properties around 2 µm. During the Q-switching regime, a maximum average output power of 1.7 W was obtained. A shortest pulse width of 925.8 ns was measured and the corresponding single pulse energy and peak power were 17.6 µJ and 19.0 W, respectively. The results indicate that layered ReSe2 is a promising alternative as a nonlinear optical modulator near the 2 µm region.