Lithium triborate (LBO): A review of its properties and applications

Laser damage properties of LiB3O5 crystal surface under UV laser irradiation

LiB3O5 (LBO) crystal has a very high bulk laser damage threshold. Laser damage often occurs on the surfaces with a large number of processing defects during application. In this paper, the surface laser damage threshold, damage growth threshold, and damage growth curve of LBO crystal and fused silica under the same processing process have been comparatively studied by using a 355 nm pulsed laser. The surface laser damage performance of LBO crystal has been comprehensive evaluated. The results show that the laser damage threshold and damage growth threshold of LBO are about twice that of fused silica, and the damage growth coefficient is about 0.7 times that of fused silica. The detection and analysis of impurity defects and photothermal weak absorption defects show that the subsurface defects of LBO crystal are less than that of fused silica. Laser damage morphologies show that the damage process is related to strongly bonded chemical structure and anisotropic physical characteristics of LBO crystal. These characteristics together determine the high threshold damage performance of LBO crystal. The results of this study are of great guidance for the application of LBO crystal in high-power laser systems.

Below-threshold harmonic generation in gas-jets for Th-229 nuclear spectroscopy

The generation of below-threshold harmonics in gas-jets constitutes a promising path towards optical frequency combs in the vacuum ultra-violet (VUV) spectral range. Of particular interest is the 150 nm range, which can be exploited to probe the nuclear isomeric transition of the Thorium-229 isotope. Using widely available high-power, high-repetition-rate Ytterbium-based laser sources, VUV frequency combs can be generated through the process of below-threshold harmonic generation, in particular 7^th harmonic generation of 1030 nm. Knowledge about the achievable efficiencies of the harmonic generation process is crucial for the development of suitable VUV sources. In this work, we measure the total output pulse energies and conversion efficiencies of below-threshold harmonics in gas-jets in a phase-mismatched generation scheme using Argon and Krypton as nonlinear media. Using a 220 fs, 1030 nm source, we reach a maximum conversion efficiency of 1.1 × 10^-5 for the 7^th harmonic (147 nm) and 0.78 × 10^-4 for the 5^th harmonic (206 nm). In addition, we characterize the 3^rd harmonic of a 178 fs, 515 nm source with a maximum efficiency of 0.3%.

Design of an Optical Parametric Oscillator using a BBO partial cylinder for a continuous tunability between 0.4 μm and 0.9 μm

This work describes the different steps of the design of a cylindric Optical Parametric Oscillator. It is based on a BBO nonlinear crystal shaped as a partial cylinder to be pumped by a commercial micro-laser at 0.355 μm for an energetic and sub-nanosecond emission continuously tunable between 0.4 μm and 0.9 μm.

Efficient second-harmonic generation of a high-energy, femtosecond laser pulse in a lithium triborate crystal

We demonstrate the highest efficiency (∼80%) second harmonic generation of joule level, 27 fs, high-contrast pulses in a type-I lithium triborate (LBO) crystal. In comparison, potassium dihydrogen phosphate gives a maximum efficiency of 26%. LBO thus offers high-intensity (>10^18-19W/cm²), ultra-high contrast femtosecond pulses, which have great potential for high energy density science and applications, particularly with nanostructured targets.

600 W green and 300 W UV light generated from an eight-beam, sub-nanosecond fiber laser system

We describe a sub-nanosecond pulse laser system that delivers high-average-power green and ultraviolet (UV) light. This system includes a front end, two modules each of which has four photonic crystal fiber main amplifiers, two sets of coherent beam combiners with efficiencies of 83 and 90%, and two sets of harmonic converters. A single beam was ultimately produced through polarization-beam combining. The maximum average output power was 955 W at the fundamental wavelength (1040 nm) with a 10 MHz repetition rate and 285 ps pulse duration. The M-square of the UV was 1.3. The average power of each of its harmonics is 600 W in green (520 nm) and 300 W in UV (347 nm). The nonlinear crystals were longitudinally cooled by Peltier devices for efficient harmonic conversion.

Temperature-dependent birefringence of lithium triborate, LBO in the THz regime

Optical properties of lithium triborate (LBO) in the terahertz regime (0.2-2 THz) were characterized using broadband terahertz time-domain spectroscopy. The frequency dependence of refractive index and absorption coefficient of the LBO crystal was experimentally investigated over the temperature range of 77-297 K, which the experimental results indicated that LBO has very low optical absorption coefficient at terahertz frequencies especially for the beam polarization along the crystal's principal dielectric axis X. Moreover, a giant birefringence was observed, and the refractive index difference between the axis X and Z gradually decreased with decreasing temperature, which is attributed to the behavior of the TO phonon modes of B₁ and B₂ symmetries at low frequencies at different temperatures. As potential applications, LBO can be exemplarily used as terahertz wave shapers, beam splitters, terahertz wave plates, circular polarizers and other polarization devices.

Research on the mechanical stability of high laser resistant coatings on lithium triborate crystal

The thermomechanical property of the hafnium/silica antireflection (AR) coatings on lithium triborate (LBO) crystal was investigated by simulation and experiment. From the analysis of the stress and fracture toughness, it was found that the crack originated due to the high tensile stress in hafnium coating. Then we proposed the approaches of decreasing the deposition temperature and substituting the hafnium layers with alumina to improve the mechanical stability of AR coatings on LBO crystals, and cracks were effectively suppressed. The laser damage threshold of different coatings on LBO crystal was tested, and it illustrated that the alumina/silica coatings possess better laser resistance than hafnium/silica AR coatings deposited in low deposition temperature.

Structural studies of a Li2O·4B2O3 melt by high-temperature Raman spectroscopy and density functional theory

High-temperature Raman spectroscopy and density functional theory have been employed to study the Li₂O·4B₂O₃ melt structure; B₃O₄Ø₂ and B₃O₃Ø₃ six-membered rings were found to be the main anion groups present in the melt.

Synthesis of single-crystal low-loss LiB3O5 nanowire and its optical properties

Optical-quality single-crystal LiB₃O₅ (LBO) nanowires are synthesized for the first time using a sol–gel method. The LBO nanowires possess diameters ranging from 200 to 800 nm and lengths of up to 200 μm, and exhibit excellent uniformity, smooth surfaces, and good mechanical properties. A typical propagating loss of 0.038 dB/μm at 532 nm is obtained for a 620 nm-diameter nanowire. This is a decrease of one order of magnitude compared with that of a β-BaB₂O₄ (BBO) nanowire with similar diameter, which makes the LBO nanowire a promising candidate to construct miniaturized nonlinear photonic devices.

High average power third harmonic generation at 355 nm with K3B6O10Br crystal

We demonstrate a nanosecond 355 nm laser by third harmonic generation of an Nd: YAG laser with a 4 × 4 × 13.3 mm³ type I cut K₃B₆O₁₀Br crystal for the first time. The maximum average power of 19.3 W is achieved with the corresponding conversion efficiency of 16.3% and the maximum conversion efficiency of 18.3% is recorded when the 355 nm average power is 15.8W. In addition, several nonlinear optical parameters including the angle and temperature bandwidths at third harmonic generation are also investigated.

High power burst-mode optical parametric amplifier with arbitrary pulse selection

We present results from a unique burst-mode femtosecond non-collinear optical parametric amplifier (NOPA) under development for the optical - x-ray pump-probe experiments at the European X-Ray Free-Electron Laser Facility. The NOPA operates at a burst rate of 10 Hz, a duty cycle of 2.5% and an intra-burst repetition rate of up to 4.5 MHz, producing high fidelity 15 fs pulses at a center wavelength of 810 nm. Using dispersive amplification filtering of the super-continuum seed pulses allows for selectable pulse duration up to 75 fs, combined with a tuning range in excess of 100 nm whilst remaining nearly transform limited. At an intra-burst rate of 188 kHz the single pulse energy from two sequential NOPA stages reached 180 µJ, corresponding to an average power of 34W during the burst. Acousto- and electro-optic switching techniques enable the generation of transient free bursts of required length and the selection of arbitrary pulse sequences inside the burst.

High-power picosecond 355 nm laser based on La₂CaB₁₀O₁₉ crystal

Third harmonic generation experiments were performed on a type-I phase-matching La2CaB10O19 crystal cut at θ=49.4° and φ=0.0° with dimensions of 4.0  mm×4.0  mm×17.6  mm. A 1064 nm laser with a maximum average power of 35.2 W was employed as the fundamental light source, which has a pulse width of 10 picoseconds and a pulse repetition rate of 80 MHz. A type-I noncritical phase-matching LBO crystal was used to generate 532 nm lasers. By investigating a series of focusing lens combinations, a picosecond 355 nm laser of 5.3 W was obtained, which is the highest power of picosecond 355 nm laser based on a La2CaB10O19 crystal so far. The total conversion efficiency from 1064 to 355 nm was up to 15.1%.

Temperature-insensitive frequency tripling for generating high-average power UV lasers

Aimed for generating high-average power ultraviolet (UV) lasers via third-harmonic generation (THG) consisting of frequency doubling and tripling stages, we numerically and experimentally demonstrate a novel frequency tripling scheme capable of supporting temperature-insensitive phase-matching (PM). Two cascaded tripling crystals, with opposite signs of the temperature derivation of phase-mismatch, are proposed and theoretically studied for improving the temperature-acceptance of PM. The proof-of-principle tripling experiment using two crystals of LBO and BBO shows that the temperature acceptance can be ~1.5 times larger than that of using a single tripling crystal. In addition, the phase shift caused by air dispersion, along with its influence on the temperature-insensitive PM, are also discussed. To illustrate the potential applications of proposed two-crystal tripling design in the high-average-power regime, full numerical simulations for the tripling process, are implemented based on the realistic crystals. The demonstrated two-crystal tripling scheme may provide a promising route to high-average-power THG in the UV region.

Multiple pulse nanosecond laser induced damage study in LiB3O5 crystals

Multiple pulse nanosecond laser induced damage in the bulk of LiB3O5 (LBO) crystals was investigated at 1064 nm, 532 nm and 355 nm. Scanning electron microscopy of cleaved damage sites confirmed the presence of different zones that have already been reported in the case of KH2PO4 (KDP). Multi pulse measurements reveal a strong decrease of the damage threshold with increasing pulse number at 1064 nm (fatigue effect). A weaker fatigue effect was observed at 532 nm and no fatigue effect was found at 355 nm. This observation is best explained by an inherently statistical light matter interaction generating laser induced damage. Finally, a polarization dependent damage threshold anisotropy was evidenced at all three wavelengths, being strongest at 1064 nm. The results indicate the importance of Li+ vacancy stabilized color centers for the damage mechanism.