Improving fourth harmonic generation performance by elevating the operation temperature of ADP crystal.

In current inertial confinement fusion (ICF) facilities, potassium dihydrogen phosphate (KH2PO4, KDP) type crystals are the only nonlinear optical (NLO) materials that can satisfy the aperture requirement of the ICF laser driver. Ammonium dihydrogen phosphate (NH4H2PO4, ADP) crystal is a typical isomer of KDP crystal, with a large nonlinear optical coefficient, high ultraviolet transmittance, and large growth sizes, which is an important deep ultraviolet (UV) NLO material. In this paper, we investigated the effect of ADP temperature on its fourth-harmonic-generation (FHG) performance. When the temperature of the ADP crystal was elevated to 48.9 °C, the 90° phase-matched FHG of the 1064 nm laser was realized. Compared with the 79° phase-matched FHG at room temperature (23.0 °C), the output energy at 266 nm, conversion efficiency, angular acceptance, and laser-induced damage threshold (LIDT) increased 113%, 71%, 623%, 19.6%, respectively. It shows that elevating ADP temperature is an efficient method to improve its deep UV frequency conversion properties, which may also be available to other NLO crystals. This discovery provides a very valuable technology for the future development of UV, deep UV lasers in ICF facilities.

A corrugated epsilon-nearzero saturable absorber for a high-performance 1.3 µm solid-state bulk laser.

Epsilon-near-zero (ENZ) materials with vanishing permittivity exhibit unprecedented optical nonlinearity within subwavelength propagation lengths in the ENZ region, making them promising photoelectric materials that have achieved exciting results in ultrafast pulse laser modulations. In this study, we fabricated a novel saturable absorber (SA) based on a corrugated indium tin oxide (CITO) film with a symmetrical geometry using a low-cost self-assembly process. The strong saturable absorption of the CITO film triggered by the ENZ effect at normal incidence was comparable to that of the planar indium tin oxide (ITO) film at an optimal 60° incidence (TM polarization) at 1340 nm. In addition, the strong nonlinear optical properties of the CITO film were not limited by the incident angle and polarization state of the pump laser over a wide range of 0-20°. Benefiting from the excellent saturable absorption of CITO-based SA at normal incidence, a Q-switching operation with CITO-based SA at 1.34 µm was achieved in a Nd:YVO4 solid-state laser system, obtaining pulses of a duration of 85.6 ns, which was one order of magnitude narrower than that of the planar ITO-based SA. This study presents a new strategy for developing high-performance ENZ-based SAs and ultrafast lasers.

High performance 1.9†µm passively Q-switched bulk laser with germanene as a saturable absorber.

Germanene is an analog of graphene, and its independent novel low-bending honeycomb structure gives outstanding advantages such as environmental stability and significant low-frequency optical absorbance. In this paper, the few-layer germanene was successfully prepared by the liquid phase exfoliation method. The saturable absorption characteristics of germanene in the infrared waveband were detected by the open-aperture Z-scan method. With germanene as a saturable absorber, a high-performance passively Q-switched bulk laser was realized at 1.9 µm. The shortest pulse width of 60.5 ns was obtained from continuous-wave pumping, corresponding to a single pulse energy of 6.7 µJ and peak power of 110 W. By utilizing the pulse pumping style with a repletion rate of 10 Hz, the single pulse energy and peak power increased to 45.8 µJ and 328 W, respectively, which exceeded all two-dimensional SA materials reported before. This research manifests that germanene is an excellent SA material for mid-infrared solid-state lasers.

Progress on deuterated potassium dihydrogen phosphate (DKDP) crystals for high power laser system application.

In this review, we introduce the progress in the growth of large-aperture DKDP crystals and some aspects of crystal quality including determination of deuterium content, homogeneity of deuterium distribution, residual strains, nonlinear absorption, and laser-induced damage resistance for its application in high power laser system. Large-aperture high-quality DKDP crystal with deuteration level of 70% has been successfully grown by the traditional method, which can fabricate the large single-crystal optics with the size exceeding 400 mm. Neutron diffraction technique is an efficient method to research the deuterium content and 3D residual strains in single crystals. More efforts have been paid in the processes of purity of raw materials, continuous filtration technology, thermal annealing and laser conditioning for increasing the laser-induced damage threshold (LIDT) and these processes enable the currently grown crystals to meet the specifications of the laser system for inertial confinement fusion (ICF), although the laser damage mechanism and laser conditioning mechanism are still not well understood. The advancements on growth of large-aperture high-quality DKDP crystal would support the development of ICF in China.

Bi3TeBO9: A Promising Mid-Infrared Nonlinear Optical Crystal with a Large Laser Damage Threshold.

Mid-infrared (mid-IR) nonlinear optical (NLO) crystals are key materials in the field of laser technology. Nevertheless, the applications of these significant optical materials are always limited by their low laser damage threshold (LDT). Here, an oxide mid-IR NLO crystal, Bi3TeBO9, with a large LDT was discovered. The centimeter-sized Bi3TeBO9 single crystal was successfully grown by the top-seeded solution growth (TSSG) method. The Bi3TeBO9 features a high LDT of 450 MW/cm2 (∼15 × AgGaS2) and the widest transparent range (0.35-7.10 µm) among known borate crystals. The second-harmonic generation (SHG) effect is about 1.6 times that of KDP. First-principles calculations and structural analysis show that the optical characteristics of Bi3TeBO9 are mainly affected by distorted [BiO6] groups. Furthermore, its thermal characteristics including specific heat, thermal diffusivity, thermal conductivity, and thermal expansion were also measured.

Broadband second-harmonic-generation in GdCOB crystals.

Broadband second-harmonic-generation (SHG) in GdCOB crystals was demonstrated for the first time. Theoretical calculation and experiments for the type-I frequency doubling of GdCOB crystal was performed. The result revealed that the spectral retracing point of phase-matching angle was at around 1.65 µm. For broadband fundamental laser source tuning in the range of 1.55-1.7 µm, efficient SHG was realized, the highest conversion efficiency was 56%, and the output bandwidth reached 16 nm.

Recalibration of the nonlinear optical coefficients of BaGa4Se7 crystal using second-harmonic-generation method.

Nonlinear optical coefficients are important parameters for nonlinear optical crystals. Based on the previously disclosed BaGa4Se7 crystal coefficients, the optimum phase-matching direction with the largest effective nonlinear optical coefficient appears in the second octant (90∘<θ<180∘, 0∘<ϕ<90∘). In this study, a recalibration was performed using the second-harmonic-generation method, and it was confirmed that the optimum phase-matching direction of the BaGa4Se7 crystal is located in its XZ principal plane (ϕ=0∘). Therefore, this study will serve as a good reference for future applications of this excellent crystal.

Tm:YAG single-crystal fiber laser.

In this Letter, to the best of our knowledge, we present the first thulium (Tm) single-crystal fiber (SCF) laser with free-space propagation of the laser beam only. The SCF is equipped with diffusion-bonded end caps of undoped YAG for better thermal management and enhancement of pump guiding. By utilizing mode matching and pump guiding in different SCF parts, an output power of 9.1 W is achieved at ∼2.02µm with a slope efficiency of 49.4%. This straightforward approach, which is also simple to realize and is based on combining the advantages of fiber-geometry structure and crystalline properties of Tm:YAG, is expected to be useful for 2 µm amplification stages in different time formats as well.

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.

Remarkable temperature-dependent second-harmonic-generation performance of a YCOB crystal.

For the first time, the temperature stability of second-harmonic-generation (SHG) is reported for the entire space of a YCa4O(BO3)3 (YCOB) crystal for a temperature range of -10 - 520 °C. Both theoretical calculations and experimental data indicate an optimum phase-matching (PM) direction of (θ = 149.2°, ϕ = 0°), which is located in the XZ principle plane (90° < θ < 180°). A special regression phenomenon of the PM angle was found in this direction, which further increased the SHG output at high temperature (> 200 °C). As a result, for SHG of the Nd:YAG laser, the measured temperature bandwidth of a YCOB crystal cut along the optimum PM direction is larger than 490 °C·cm. As demonstrated in this study, among all nonlinear optical crystals, this cut-type is currently the best choice when temperature-insensitive SHG is required.

2D graphdiyne: an excellent ultraviolet nonlinear absorption material.

With an sp2-hybridized carbon atom structure, graphene is recognized as a nonlinear absorption (NLA) material, which has motivated scientists to explore new allotropes of carbon. Different from graphene, graphdiyne (GDY) consists of sp- and sp2-hybridized carbon atoms. An sp-hybridized carbon-carbon triple bond structure will bring in novel nonlinear optical properties, which are different from other allotropes of carbon. In this study, we investigated the broadband NLA properties (ultraviolet-infrared waveband) of GDY nanosheets, exfoliated using a liquid-phase exfoliation (LPE) method. The short ultraviolet cut-off wavelength (around 200 nm-220 nm) forebodes the potential application of GDY as an ultraviolet optical material. The outstanding NLA resulting in an ultraviolet waveband attests that the GDY nanosheets are veritable ultraviolet NLA materials, which have potential applications in ultraviolet optics. Our study broadens the application scopes of nanomaterials.

High-energy 2 µm pulsed vortex beam excitation from a Q-switched Tm:LuYAG laser.

We report on the first, to the best of our knowledge, direct generation of pulsed vortex beams at 2 µm from a ${ Q}$Q-switched Tm:LuYAG laser. High-energy Laguerre-Gaussian (${{\rm LG}_{0,l}}$LG0,l) pulsed laser beams with well-defined handedness are selectively excited through spatially matched pump gain distribution and asymmetric cavity loss without using any intracavity handedness-selective optical elements. Pulse energies of 1.48 mJ for the ${{\rm LG}_{0, + 1}}$LG0,+1 mode and 1.51 mJ for the ${{\rm LG}_{0, - 1}}$LG0,-1 mode, respectively, are achieved at a repetition rate of 500 Hz. The pulsed laser beams with helical wavefronts are potentially useful for studying orbital angular momentum transformation dynamics, generation of mid-IR vortex beams, and nanostructuring of organic materials.

2.7 µm optical vortex beam directly generated from an Er:Y2O3 ceramic laser.

We demonstrate, to the best of our knowledge, the first direct vortex beam generation in the 3 µm spectral region by employing an Er:Y2O3 ceramic laser. Controllable handedness with high purity is achieved by introducing asymmetric cavity loss and reducing the number of longitudinal modes. The average orbital angular momentum of the produced scalar vortex beam is quantitatively evaluated to be 0.95h for the LG0,+1 mode and -0.94h for the LG0,-1 mode. The corresponding optical spectrum is centered at 2710.8 and 2710.5 nm, respectively.

Broadband nonlinear absorption properties of two-dimensional hexagonal tellurene nanosheets.

Two dimensional (2D) Group-VI Te, tellurene, was successfully exfoliated using a liquid phase exfoliation (LPE) method. The prepared tellurene nanosheets possessed a thickness of 4.3-4.6 nm and the lateral dimension ranged from hundreds of nanometers to several microns. The broadband nonlinear absorption properties were explored for the first time (as we know) using a z-scan method with the laser photon energy in the range of 0.73-2.76 eV, corresponding to the near-infrared-visible waveband. Tellurene nanosheets exhibited excellent broadband saturated absorption and optical limiting behaviors. The low saturable intensity and the large modulation depth for saturated absorption with low energy photon excitation highlight the superiority of the infrared band as a saturable absorber. In addition, with large energy excitation, tellurene manifested an apparent two photon absorption behavior in the visible band, thus it can be used as an optical limiting material. By adopting the mode-locking technique, this high-quality saturable absorber can be applied in all-solid-state or fiber lasers to generate ultra-short and ultra-high peak power laser pulses. Meanwhile, tellurene as an optical limiting material can protect the sensitive optical devices and human eyes. So, our work not only demonstrates that tellurene is a promising broadband nonlinear optical material, but also implies its application prospects in optics.

Temperature bandwidth of second-harmonic-generation in GdCOB crystal.

The temperature bandwidth of the second harmonic generation (SHG) phase-matching process was investigated for the GdCa4O(BO3)3 (GdCOB) crystal. GdCOB exhibits a much broader temperature bandwidth in comparison with many familiar nonlinear optical (NLO) crystals. For a fundamental wave of 1,064 nm, the maximum temperature bandwidth appeared at (θ=135°, ϕ=47.3°), as predicted by the theoretical calculation and demonstrated by the SHG experiments. The GdCOB crystal is a good candidate for NLO frequency conversions under extreme temperatures.

Comparison of hydrogen vacancies in KDP and ADP crystals: a combination of density functional theory calculations and experiment.

The hydrogen vacancy (VH) is the most common point defect that may lead to optical damage of potassium dihydrogen phosphate (KDP) and its analog ammonium dihydrogen phosphate (ADP), further limiting their practical application in high-power laser systems. In this work, we have grown KDP and ADP crystals by using a rapid growth method, and investigated the physical origin of the different stability of VH as well as the defect-induced electronic structure and optical absorption in KDP and ADP crystals. The inclusion of van der Waals correction to density functional theory calculations is found to have little influence on VH energetics of KDP whereas it largely reduces the charge transition level ε(+/-) of VH by >2 eV in ADP. It is found that hydrogen vacancies mainly contribute to the redshift of the measured absorption edges of both KDP and ADP crystals. Owing to the varied lattice environments and locations, the VH defects exhibit different stability, and electronic and optical properties in KDP and ADP crystals. Notably, the extra optical absorption caused by the positively-charged VH in KDP could be largely reduced by decreasing the defect concentration, whereas ADP exhibits defect-location dependence - the optical damage center of the VH in the NH4+ group could not be eliminated because of electron capture of its neighboring N atoms. The calculation results help us to better understand the origin of laser damage in KDP and ADP crystals.

Tin diselenide-based saturable absorbers for eye-safe pulse lasers.

Eye-safe pulse lasers have attracted increasing attention due to their potential wide application in many fields. However, optical modulators with excellent nonlinear optical absorption properties in the range of 1.4-2.1 µm are still very scarce. In this study, tin diselenide (SnSe2), a newly-developed 2D layered semiconductor material with facile processability and low cost, was investigated. The nonlinear optical response of SnSe2 was investigated using the open aperture Z-scan method at 1500 and 1800 nm. Using SnSe2 as the saturable absorber, a passive Q-switched solid-state laser was realized at 1.3 and 1.9 µm for the first time. This study proved SnSe2 to be an effective optical modulating material for the eye-safe waveband.

Graphene and SESAM mode-locked Yb:CNGS lasers with self-frequency doubling properties.

We report on mode-locking of an Yb:Ca3NbGa3Si2O14 laser, which is pumped by a fiber-coupled single-mode laser diode. The shortest pulse duration obtained with a semiconductor saturable absorber mirror is 52 fs, with 75 mW of average output power. Sub-60 fs operation tunable between 1055 and 1074 nm is achieved by employing semiconductor absorbers with different characteristics. We also demonstrate passive mode-locking results with transmissive graphene saturable absorber, reaching an 85 fs pulse duration with 23 mW output power. Moreover, we present the non-phase-matched self-frequency doubling properties of this non-centrosymmetric crystal in the femtosecond regime.

Liquid-Phase Exfoliated Silicon Nanosheets: Saturable Absorber for Solid-State Lasers.

As a newly-developed two-dimensional (2D) material of group-IVA, few-layer silicon (Si) nanosheets were prepared by the liquid phase exfoliation (LPE) method. Its non-linear saturable adsorption property was investigated by 532 and 1064 nm nanosecond lasers. Using Si nanosheets as the saturable absorber (SA), passive Q-switched all-solid-state lasers were demonstrated for the first time. For different laser emissions of Nd3+ at 0.9, 1.06, and 1.34 µm, the narrowest Q-switched pulse widths were 200.2, 103.7, and 110.4 ns, corresponding to the highest peak powers of 2.76, 2.15, and 1.26 W. The results provide a promising SA for solid-state pulsed lasers and broaden the potential application range of Si nanosheets in ultrafast photonics and optoelectronics.

Efficient laser operations of unprocessed thin plate of Nd:YPO4 crystal.

The laser properties of Nd:YPO4 crystal were demonstrated for the first time. For a 1.2 at.% doped Nd:YPO4 crystal, the absorption cross-section at 803 nm, stimulated emission cross-section at 1063 nm, and fluorescence lifetime was measured to be 8.1 × 10-20 cm2, 1.6 × 10-19 cm2, 156 µs, respectively. With an as-grown 0.6 mm thin slice which was unpolished and uncoated, efficient diode-pumped continue-wave (CW) laser operations were realized at 1.06 and 1.3 µm wavebands. The 1063 nm output power reached 2.16 W when the absorbed pump power was 4.07 W, corresponding to an optical-to-optical efficiency of 53%, and a slope efficiency of 56.4%. The 1.3 µm laser output exhibited the simultaneous operations of dual-wavelengths, i.e. 1338 and 1347 nm. The maximum output power was 800 mW at an absorbed pump power of 3.08 W, giving an optical-to-optical efficiency of 26% and a slope efficiency of 28.2%.