Diode-pumped Tm:YAP laser operating at 2.3†µm with enhanced performance through cascade lasing.

The laser diode (LD)-pumped Tm:YAP (a-cut, 3.5 at.%) laser generated a maximum ∼2.3 µm continuous wave (CW) laser output power of ∼3 W. The higher output power benefited from the positive effect of the cascade lasing (simultaneously operating on the 3H4 → 3H5 and 3F4 → 3H6 Tm3+ transition). It was the highest CW laser output power amongst the LD/Ti:Sapphire-CW-pumped ∼2.3 µm Tm3+-doped lasers reported so far. Under the cascade laser operation, the slope efficiency of the ∼2.3 µm laser emission versus the absorbed pump power increased from 13.0% to 21.4%.

Six-watt diode-pumped Tm:GdVO4 laser at 2.29†µm.

A compact Tm:GdVO4 laser pumped by a 794 nm laser diode generated 6.09 W at 2.29 µm (3H4 → 3H5 Tm3+ transition) with a high slope efficiency of 30.8% and linear laser polarization (π). The polarized spectroscopic properties of Tm3+ in GdVO4 were also revised. The peak stimulated-emission cross section of Tm3+ is 2.97 × 10-20 cm2 at 2280 nm, corresponding to an emission bandwidth of 42 nm for π-polarized light.

Diode-pumped efficient high-power cascade Tm:GdVO4 laser simultaneously operating at ∼2†µm and ∼2.3 µm.

The laser diode (LD)-pumped efficient high-power cascade Tm:GdVO4 laser simultaneously operating on the 3F4 → 3H6 (at ∼2 µm) and 3H4 → 3H5 (at ∼2.3 µm) Tm3+ transition was first reported in this paper. The cascade Tm:GdVO4 laser generated a maximum total continuous-wave (CW) laser output power of 8.42 W with a slope efficiency of 40%, out of which the maximum ∼2.3 µm CW laser output power was 2.88 W with a slope efficiency of 14%. To our knowledge, 2.88 W is the highest CW laser output power amongst the LD-CW-pumped ∼2.3 µm Tm3+-doped lasers reported so far.

Tm:CALGO lasers at 2.32†µm: cascade lasing and upconversion pumping.

We report on the first laser operation of a disordered Tm:CaGdAlO4 crystal on the 3H4 → 3H5 transition. Under direct pumping at 0.79 µm, it generates 264 mW at 2.32 µm with a slope efficiency of 13.9% and 22.5% vs. incident and absorbed pump power, respectively, and a linear polarization (σ). Two strategies to overcome the bottleneck effect of the metastable 3F4 Tm3+ state leading to the ground-state bleaching are exploited: cascade lasing on the 3H4 → 3H5 and 3F4 → 3H6 transitions and dual-wavelength pumping at 0.79 and 1.05 µm combining the direct and upconversion pumping schemes. The cascade Tm-laser generates a maximum output power of 585 mW at 1.77 µm (3F4 → 3H6) and 2.32 µm (3H4 → 3H5) with a higher slope efficiency of 28.3% and a lower laser threshold of 1.43 W, out of which 332 mW are achieved at 2.32 µm. Under dual-wavelength pumping, further power scaling to 357 mW at at 2.32 µm is observed at the expense of increased laser threshold. To support the upconversion pumping experiment, excited-state absorption spectra of Tm3+ ions for the 3F4 → 3F2,3 and 3F4 → 3H4 transitions are measured for polarized light. Tm3+ ions in CaGdAlO4 exhibit broadband emission at 2.3 - 2.5 µm making this crystal promising for ultrashort pulse generation.

Optical absorption of bismuthene with a single vacancy: first-principle calculations.

The exceptional mechanical, electronic, topological, and optical properties, make bismuthene an ideal candidate for various applications in ultrafast saturation absorption and spintronics. Despite the extensive research efforts devoted to synthesizing this material, the introduction of defects, which can significantly affect its properties, remains a substantial obstacle. In this study, we investigate the transition dipole moment and joint density of states of bismuthene with/without single vacancy defect via energy band theory and interband transition theory. It is demonstrated that the existence of the single defect enhances the dipole transition and joint density of states at lower photon energies, ultimately resulting in an additional absorption peak in the absorption spectrum. Our results suggest that the manipulation of defects in bismuthene has enormous potential for improving the optoelectronic properties of this material.

Spectroscopy and 2.8†µm laser operation of disordered Er:CLNGG crystals.

We report on the first, to the best of our knowledge, laser operation on the 4I11/2 → 4I13/2 transition of erbium-doped disordered calcium lithium niobium gallium garnet (CLNGG) crystals with broadband mid-infrared emission properties. A 41.4 at.% Er:CLNGG continuous-wave laser generated 292 mW at 2.80 µm with 23.3% slope efficiency and a laser threshold of 209 mW. Er3+ ions in CLNGG feature inhomogeneously broadened spectral bands (σSE = 1.79 × 10-21 cm2 at 2.79 µm; emission bandwidth, 27.5 nm), a large luminescence branching ratio for the 4I11/2 → 4I13/2 transition of 17.9%, and a favorable ratio of the 4I11/2 and 4I13/2 lifetimes, exhibiting values of 0.34 ms and 1.17 ms (for 41.4 at.% Er3+), respectively.

Third-order optical nonlinearity and passively Q-switching operation with BiOCl nanosheets at 1.3†µm.

In this contribution, we measured the third-order nonlinear optical response of bismuth oxychloride (BiOCl) nanosheets with the open-aperture (OA) and the closed-aperture (CA) Z-scan techniques with a variable excitation intensity at 1.34 µm. The effective nonlinear absorption coefficient ßeff and the nonlinear refractive index n2 of the prepared BiOCl nanosheets with abundant oxygen vacancies were obtained under the excitation intensity. The third-order nonlinear optical susceptibility |χ(3)| was 1.64 × 10-9 esu. The nonlinear optical features of BiOCl enabled it as a superb saturable absorber for pulse laser generation. As a consequence, we demonstrated the first passively Q-switched Nd:GdVO4 laser with the BiOCl saturable absorber, producing a shortest pulse duration of 543 ns and a highest repetition rate of 227 kHz, leading to a maximum pulse energy of 74 nJ. Our findings show that BiOCl nanosheets with oxygen vacancies have large nonlinear optical sensitivities and can be exploited to generate optical pulses.

Large-scale few-layered MoS2 as a saturable absorber for Q-switching operation at 2.3†µm.

In this Letter, the fabrication of large-scale (50.8 mm in diameter) few-layered MoS2 with physical vapor deposition on sapphire is described. Open-aperture Z-scan technology with a home-made excitation source at 2275 nm was applied to explore its nonlinear saturable absorption properties. The as-grown few-layered MoS2 membrane possessed a modulation depth of 17% and a saturable intensity of 1.185 MW cm-2. As a consequence, the deposited MoS2 membrane was exploited as a saturable absorber to realize a passively Q-switched Tm:YAP laser for the first time, to the best of our knowledge. Pulses as short as 316 ns were generated with a repetition rate of 228 kHz, corresponding to a peak power of 5.53 W. Results confirmed that the two-dimensional layered MoS2 could be beneficial for mid-infrared photonic applications.

Soliton mode-locked Yb:Ca3Gd2(BO3)4 laser.

We report on a soliton mode-locked Yb:Ca3Gd2(BO3)4 laser at ∼1.06 µm stabilized by a semiconductor saturable absorber mirror. Pumping with a single-transverse mode, fiber-coupled laser diode at 976 nm, the Yb:Ca3Gd2(BO3)4 laser delivers soliton pulses as short as 39 fs at a central wavelength of 1059.2 nm with an average output power of 70 mW and a pulse repetition rate of ∼67.3 MHz.

Polarized spectroscopy and SESAM mode-locking of Tm,Ho:CALGO.

A Tm,Ho:CALGO laser passively mode-locked by a GaSb-based SESAM generated pulses as short as 52 fs at a central wavelength of 2015 nm with a broad spectral bandwidth of 82 nm (full width at half maximum) owing to the combined gain profiles of both dopants for σ-polarized light. The average output power reached 376 mW at a repetition rate of 85.65 MHz. In the continuous-wave regime, the laser was power scaled up to 1.01 W at 2080.6 nm with a slope efficiency of 32.0%, a laser threshold of 155 mW and π-polarized emission. Polarized spectroscopic properties of Ho3+ ions in singly doped and codoped CALGO crystals were revisited to explain the observed laser performance.

Compact diode-pumped continuous wave and passively Q switched Tm:YAG laser at 2.33†µm.

Compact diode-pumped continuous wave (CW) and passively Q switched Tm:YAG lasers operating on the 3H4 → 3H5 transition are demonstrated. Using a 3.5-at.% Tm:YAG crystal, a maximum CW output power of 1.49 W is achieved at 2330 nm with a slope efficiency of 10.1%. The first Q switched operation of the mid-infrared Tm:YAG laser around 2.3 µm is realized with a few-atomic-layer MoS2 saturable absorber. Pulses as short as 150 ns are generated at a repetition rate of 190 kHz, corresponding to a pulse energy of 1.07 µJ. Tm:YAG is an attractive material for diode-pumped CW and pulsed mid-infrared lasers emitting around 2.3 µm.

Watt-level diode-pumped Tm:YVO4 laser at 2.3 µm.

In this Letter, a watt-level laser diode (LD)-pumped ∼2.3-µm (on the 3H4→3H5 quasi-four-level transition) laser is reported based on a 1.5 at.% a-cut Tm:YVO4 crystal. The maximum continuous wave (CW) output power obtained is 1.89 W and 1.11 W with the maximum slope efficiency of 13.6% and 7.3% (versus the absorbed pump power) for the 1% and 0.5% transmittance of the output coupler, respectively. To the best of our knowledge, the CW output power of 1.89 W we obtained is the highest CW output power amongst the LD-pumped ∼2.3-µm Tm3+-doped lasers.

SWCNT-SA mode-locked Tm,Ho:LCLNGG laser.

Sub-100 fs pulse generation from a passively mode-locked Tm,Ho-codoped cubic multicomponent disordered garnet laser at ∼2 µm is demonstrated. A single-walled carbon nanotube saturable absorber is implemented to initiate and stabilize the soliton mode-locking. The Tm,Ho:LCLNGG (lanthanum calcium lithium niobium gallium garnet) laser generated pulses as short as 63 fs at a central wavelength of 2072.7 nm with an average output power of 63 mW at a pulse repetition rate of ∼102.5 MHz. Higher average output power of 121 mW was obtained at the expense of longer pulse duration (96 fs) at 2067.6 nm using higher output coupling. To the best of our knowledge, this is the first report on mode-locked operation of the Tm,Ho:LCLNGG crystal.

Sub-100 fs mode-locked Tm:CLTGG laser.

We report on the first sub-100 fs mode-locked laser operation of a Tm3+-doped disordered calcium lithium tantalum gallium garnet (Tm:CLTGG) crystal. Soliton mode-locking was initiated and stabilized by a transmission-type single-walled carbon nanotube saturable absorber. Pulses as short as 69 fs were achieved at a central wavelength of 2010.4 nm with an average power of 28 mW at a pulse repetition rate of ∼87.7 MHz. In the sub-100 fs regime, the maximum average output power amounted to 103 mW.

Kerr-lens mode-locked Tm-doped sesquioxide ceramic laser.

Kerr-lens mode-locked solid-state laser operation at ∼2µm is investigated. Using a Tm3+-doped (Lu,Sc)2O3 "mixed" sesquioxide ceramic as a gain medium, pulses as short as 58 fs are generated at ∼2081nm via soft-aperture Kerr-lens mode locking. The average output power amounts to 220 mW at a pulse repetition rate of 84.8 MHz. The emitted spectrum at the long-wavelength wing extends to >2.2µm which is attributed to vibronic transitions of the Tm3+ ions. The latter is found to be essential for generating pulses with durations in the 50 fs range.

Sub-50 fs pulse generation from a SESAM mode-locked Tm,Ho-codoped calcium aluminate laser.

We report on sub-50 fs pulse generation from a passively mode-locked (ML) Tm,Ho-codoped crystalline laser operating in a 2 µm spectral region. A ${\rm Tm},{\rm Ho}{:}{\rm Ca}({\rm Gd},{\rm Lu}){{\rm AlO}_4}$ laser delivers pulses as short as 46 fs at 2033 nm with an average power of 121 mW at a pulse repetition rate of ${\sim}{78}\;{\rm MHz}$ employing a semiconductor saturable absorber mirror as a saturable absorber. To the best of our knowledge, this result represents the shortest pulses ever generated from a Tm- and/or Ho-based solid-state laser. Polarization switching in the anisotropic gain material is observed in the ML regime without any polarization selection elements which is essential for the shortest pulses.

SESAM mode-locked Tm:LuYO3 ceramic laser generating 54-fs pulses at 2048 nm.

Mode-locked laser operation near 2.05 µm based on a mixed sesquioxide Tm:LuYO3 ceramic is demonstrated. Continuous-wave and wavelength-tunable operation is also investigated. Employing a GaSb-based semiconductor saturable absorber mirror as a saturable absorber, a maximum average output power of 133 mW is obtained for a pulse duration of 59 fs. Pulses as short as 54 fs, i.e., eight optical cycles are generated at a repetition rate of ∼78MHz with an average output power of 51 mW. To the best of our knowledge, this result represents the shortest pulse duration ever achieved from Tm-based solid-state mode-locked lasers.

Single-walled carbon-nanotube saturable absorber assisted Kerr-lens mode-locked Tm:MgWO4 laser.

We demonstrate sub-100-fs Kerr-lens mode-locking of a Tm:MgWO4 laser emitting at ∼2µm assisted by a single-walled carbon-nanotube saturable absorber. A maximum average output power of 100 mW is achieved with pulse duration of 89 fs at a pulse repetition rate of ∼86MHz. The shortest pulse duration derived from frequency-resolved optical gating amounts to 76 fs at 2037 nm, corresponding to nearly bandwidth-limited pulses. To the best of our knowledge, these are the shortest pulses generated from any Tm-doped tungstate crystal and the first report on saturable absorber assisted Kerr-lens mode-locking of a Tm laser at ∼2µm.

Passively Q-switched Tm:CaLu0.1Gd0.9AlO4 laser at 2†µm with hematite nanosheets as the saturable absorber.

We report the first passive Q-switching operation at 1.95 µm utilizing the disordered Tm:CaLu0.1Gd0.9AlO4 (Tm:CLGA) crystal and the hematite (α-Fe2O3) nanosheets as the saturable absorber. The nonlinear saturable absorption properties of the hematite nanosheets were investigated by the conventional Z-scan technology. The modulation depth of 14.3% with the low saturation intensity of 205 kW/cm2 was obtained, indicating that the hematite could be a suitable saturable absorber for the mid-infrared pulse generation. Using the disordered Tm:CLGA crystal as the gain medium, the passive Q-switching operation could be realized with the hematite nanosheets as the saturable absorber, producing the shortest pulse duration of 402 ns with a repetition rate of 76 kHz. The experimental results convinced us that the hematite nanosheets could be of great interest in the optical pulse generation in the mid-infrared region.

Saturable absorption characteristics of Bi2Se3 in a 2 µm Q-switching bulk laser.

We investigate the saturable absorption properties of Bi2Se3 in a bulk laser operating at 2 µm wavelength region. The Bi2Se3 saturable absorber (SA) is prepared with the liquid-phase exfoliation method, which gives a saturable input flux of 4.3 mJ/cm2, a modulation depth of ∼10%, and a non-saturable absorption of 10.2%. With the Bi2Se3 saturable absorber, a passive Q-witching Tm:YAG ceramic laser is realized with a shortest pulse duration of 355 ns, a single pulse energy of 6.76 µJ and peak power of 19 W. We believe that this is the first report on Bi2Se3 Q-switched 2 µm bulk laser.