Laser-induced Damage Threshold Research Articles

High-Repetition-Rate 2.3–2.7 µm Acousto-Optically Tuned Narrow-Line Laser System Comprising Two Master Oscillators and Power Amplifiers Based on Polycrystalline Cr2+:ZnSe with the 2.1 µm Ho3+:YAG Pulsed Pumping

High-average-power narrow-linewidth tunable solid-state lasers in the wavelength region between 2 and 3 μm are attractive light sources for many applications. This paper reports a narrow-linewidth widely tunable laser system based on the polycrystalline Cr2+:ZnSe elements pumped by repetitively pulsed 2.1 µm Ho3+:YAG laser operating at a pulse rate of tens of kilohertz. An advanced procedure of ZnSe element doping and surface improvement was applied to increase the laser-induced damage threshold, which resulted in an increase in the output power of the Cr2+:ZnSe laser system. The high-average-power laser system comprised double master oscillators and power amplifiers: Ho3+:YAG and Cr2+:ZnSe laser oscillators, and Ho3+:YAG and Cr2+:ZnSe power amplifiers. The output wavelength was widely tuned within 2.3–2.7 µm by means of an acousto-optical tunable filter inside a Cr2+:ZnSe master oscillator cavity. The narrow-linewidth operation at the pulse repetition rate of 20–40 kHz in a high-quality beam with an average output power of up to 9.7 W was demonstrated.

Remarkable Second Harmonic Generation Response in (C5H6NO)+(CH3SO3)-: Unraveling the Role of Hydrogen Bond in Thermal Driven Nonlinear Optical Switch.

Heat-activated second harmonic generation (SHG) switching materials are gaining interest for their ability to switch between SHG on and off states, offering potential in optoelectronic applications. The novel nonlinear optical (NLO) switch, (C5H6NO)+(CH3SO3)- (4-hydroxypyridinium methylsulfonate, 4HPMS), is a near-room-temperature thermal driven material with a strong SHG response (3.3 × KDP), making it one of the most potent heat-stimulated NLO switches. It offers excellent contrast of 13 and a high laser-induced damage threshold (2.5 × KDP), with reversibility > 5 cycles. At 73 °C, 4HPMS transitions from the noncentrosymmetric Pna21 room temperature phase (RTP) to the centrosymmetric P21/c phase, caused by the rotation of the (C5H6NO)+ and (CH3SO3)- due to partially thermal breaking of intermolecular hydrogen bonds. The reverse phase change exhibits a large 50 °C thermal hysteresis. Density functional theory (DFT) calculations show that (C5H6NO)+ primarily dictates both the SHG coefficient (dij) and birefringence (▵n(Zeiss) = 0.216 vs ▵n(cal.) = 0.202 at 546 nm; Δn(Immersion) = 0.210 vs ▵n(cal.) = 0.198 at 589.3 nm), while the band gap (Eg) is influenced synergistically by (C5H6NO)+ and (CH3SO3)-. Additionally, 4HPMS-RTP also exhibits mechanochromism upon grinding as well as an aggregation-enhanced emission in a mixture of acetone and water.

Comparative Study of Plasma-Enhanced-Atomic-Layer-Deposited Al2O3/HfO2/SiO2 and HfO2/Al2O3/SiO2 Trilayers for Ultraviolet Laser Applications.

High-performance thin films combining large optical bandgap Al2O3 and high refractive index HfO2 are excellent components for constructing the next generation of laser systems with enhanced output power. However, the growth of low-defect plasma-enhanced-atomic-layer-deposited (PEALD) Al2O3 for high-power laser applications and its combination with HfO2 and SiO2 materials commonly used in high-power laser thin films still face challenges, such as how to minimize defects, especially interface defects. In this work, substrate-layer interface defects in Al2O3 single-layer thin films, layer-layer interface defects in Al2O3-based bilayer and trilayer thin films, and their effects on the laser-induced damage threshold (LIDT) were investigated via capacitance-voltage (C-V) measurements. The experimental results show that by optimizing the deposition parameters, specifically the deposition temperature, precursor exposure time, and plasma oxygen exposure time, Al2O3 thin films with low defect density and high LIDT can be obtained. Two trilayer anti-reflection (AR) thin film structures, Al2O3/HfO2/SiO2 and HfO2/Al2O3/SiO2, were then prepared and compared. The trilayer AR thin film with Al2O3/HfO2/SiO2 structure exhibits a lower interface defect density, better interface bonding performance, and an increase in LIDT by approximately 2.8 times. We believe these results provide guidance for the control of interface defects and the design of thin film structures and will benefit many thin film optics for laser applications.

Double layer silica antireflective films with high strength and rub resistance prepared by sol gel method

The single-layer silica antireflective film with base catalysis prepared by sol gel method is an important part of the high-power laser facility for inertial confinement fusion, while the weak adhesion between the single-layer silica film and the substrate during the preparation process makes it susceptible to be contacted erasure and unable to be used. Double-layer silica antireflective (DLAR) films of different thicknesses were obtained using the base catalysis sol–gel method, in which the upper layer was coated with a relatively dense thin layer, and the performances of the films were characterized. The results showed that the transmittances of the DLAR films with different thicknesses were ˃99.0%, and in which one of the maximum transmittance peaks reached to 99.83% @ 1000 nm. The surface roughness of the DLAR films was < 2.0 nm, and the surfaces of the films were flat. The contact angles between DLAR films and water reached 118° and maintained stable in high humidity environment. The laser induced damage thresholds for different thickness DLAR films (peak transmittances @ 400, 600, 800, 1000 nm) were comparable to device requirements by 1-on-1 testing method, and the DLAR films exhibited high strength and good friction resistance.Graphical abstract

Nb2O5 film optical properties and laser-induced damage by phase-change-driven tuning

Niobium oxide (Nb2O5) is widely used in optical and electrical fields because of its high refractive index, wide band gap, high laser-induced damage threshold (LIDT) and high transparency in the ultraviolet to middle-infrared-wavelength range (0.35–7 μm). In this study, Nb2O5 films with different crystalline phases were successfully prepared on BK7 and Si substrates through electron beam evaporation (EB) and post-annealing. Based on the results of X-ray diffraction (XRD) and Raman spectroscopy analysis, the impact of the post-annealing technique on the structure, and optical and physical properties of Nb2O5 films was thoroughly discussed at the molecular level. The crystal phase of an Nb2O5 film and its molecular bonds considerably affect its properties. The Nb2O5(-12 1 2) film prepared in this study exhibited a higher LIDT (17.72 J/cm2) and a higher RMS (0.447 nm) than the corresponding films prepared in previous studies. Following the post-annealing process, the band gap and refractive index of the prepared films were significantly higher than those of the as-deposited film. Thus, this study has significant guiding implications for Nb2O5 film applications and the development costs of the films.

Correction to “α-Li2ZnGeS4: A Wide-Bandgap Diamond-like Semiconductor with Excellent Balance between Laser-Induced Damage Threshold and Second Harmonic Generation Response”

Correction to “α-Li₂ZnGeS₄: A Wide-Bandgap Diamond-like Semiconductor with Excellent Balance between Laser-Induced Damage Threshold and Second Harmonic Generation Response”

Correction to “Phase Matching, Strong Frequency Doubling, and Outstanding Laser-Induced Damage Threshold in the Biaxial, Quaternary Diamond-like Semiconductor Li4CdSn2S7”

Correction to “Phase Matching, Strong Frequency Doubling, and Outstanding Laser-Induced Damage Threshold in the Biaxial, Quaternary Diamond-like Semiconductor Li₄CdSn₂S₇”

Mechanism of Surface Hydroxylation Acceleration and Laser-Induced Damage Threshold Reduction during Ion Beam Sputtering of Fused Silica

Mechanism of Surface Hydroxylation Acceleration and Laser-Induced Damage Threshold Reduction during Ion Beam Sputtering of Fused Silica

Sr2HgGe2OS6: A Hg-Based Oxychalcogenide Infrared Nonlinear Optical Material Exhibiting Favorable Balance between a Large Band Gap and Strong Second Harmonic Generation Response.

Currently, oxychalcogenides with mixed-anion groups that integrate the property advantages of oxides (wide optical band gap) and chalcogenides [strong second harmonic generation (SHG) response] through chemical substitution engineering have attracted widespread interest and are considered to be important candidates for infrared (IR) nonlinear optical (NLO) materials. Herein, the first Hg-based oxychalcogenide Sr2HgGe2OS6 with mixed anion [GeOS3] units has been successfully synthesized through a spontaneous crystallization method, which exhibits a favorable balance between the strong SHG response (0.7 × AgGaS2) and large optical band gap (2.9 eV). In addition, Sr2HgGe2OS6 shows high laser-induced damage threshold (LIDT, 2.1 × AgGaS2) as well as phase-matching (PM) performance. Theoretical calculations indicate that the Sr2HgGe2OS6 encompasses large birefringence of 0.128@2090 nm (3.3 × AgGaS2) and its SHG density mainly comes from [HgS4] tetrahedra and [GeOS3] units. This work not only demonstrates that Sr2HgGe2OS6 is a promising IR NLO material but also provides new ideas for the exploration of Hg-based oxychalcogenide IR NLO materials.

Optical properties of Ba-impurity defect clusters in KH2PO4 crystals

Potassium dihydrogen phosphate (KDP) is widely used in electro-optical switches and inverter crystals in inertial confinement fusion (ICF). The early studies suggested that defect clusters are one of the main reasons why the laser-induced damage threshold (LIDT) is far lower than the theoretical value. However, it is not clear what type of defect clusters cause this phenomenon. The BaK defect cluster models have been determined by simulation using General Utility Lattice Programs (GULP). We found that BaK+VH and BaK+VK cause a dramatic distortion on H-O bonds with maximum values of 29.5 % and 27.12 %, respectively. By analyzing the configuration coordinate diagram and optical spectra, we believe that both defect clusters can promote ionization through defect-assisted "2+1" photon absorption, which leads to free electrons more easily exceeding the critical value for electron avalanche breakdown. Our study will offer insight into the effects of defect clusters in KDP crystals.

Comparing the laser-induced damage distribution in POFs with raytracing simulations

The understanding of the laser-induced damage behavior in polymer optical materials is of high interest to prevent their damage and to increase the laser damage resistance of optical components. Moreover, compared with optical components made from high-quality glass materials, nanosecond laser-induced damage for wavelengths in the visible and near-infrared (NIR) occurs inside the bulk material of PMMA and not at its surface. This phenomenon complicates the determination of the laser-induced damage threshold (LIDT) in PMMA fibers. Since the bulk material itself determines the LIDT, knowledge of the intensity distribution in the multimode fiber is of utmost importance. Our fibers were irradiated at a wavelength of 532 nm with an ns-pulsed laser system with a 10 Hz repetition rate. To investigate the damage behavior in polymer optical fibers, we applied different imaging and analysis techniques. To our knowledge, those techniques are used here for the first time in order to study damaged polymer materials. With the help of a Nomarski microscope, axial and radial damage distributions within the multimode PMMA fibers were determined and compared with ray-tracing simulations of the intensity distribution within the optical fiber. Moreover, extruded PMMA plates were irradiated with the aim of comparing the damage behavior of materials with different manufacturing. In addition, the planar geometry of the plates allows for a more reliable application of the different measurement methods. Overall, investigations with a thermal imaging camera and EDX analysis indicate that the damage behavior of polymer optical material is thermally driven during the ns-pulsed irradiation. Furthermore, voids are formed during the damaging process within the polymer optical fibers and plates, as indicated by both SEM images and X-ray computed microtomography (µ-CT) investigations. Finally, we investigated the damages in fiber preforms and PMMA plates using two photon-microscopy. By doing this, we detected fluorescence signals from the damaged material, indicating that the damage process leads to a major modification of the polymer.

Delivery of nanosecond laser pulses by multi-mode anti-resonant hollow core fiber at 1 µm wavelength

In this paper we explore the application of low-loss multimode anti-resonant hollow-core fiber (MM-AR-HCF) in the delivery of nanosecond laser pulses at 1 µm wavelength. MM-AR-HCF with large core offers a rich content of low-loss higher-order modes which plays a key role in the efficient coupling and transmission of high-power laser of low beam quality. In the experiment, laser pulses of an average pulse energy of 21.8 mJ with 14.6 ns pulse width (corresponding a peak power of 1.49 MW) are transmitted through MM-AR-HCF of 9.8 m length without damage. 85% transmission efficiency is achieved where the incident laser beam suffers a low beam quality with M2x and M2y of 2.18 and 1.99 respectively. Laser-induced damage threshold (LIDT) of MM-AR-HCF was measured to be 22.6 mJ for 85% transmission efficiency, which is 7 times higher than that for a multimode silica optical fiber with a large core of 200 µm.

Evolution mechanism of subsurface damage during laser machining process of fused silica

The machining-induced subsurface damage (SSD) on fused silica optics would incur damage when irradiated by intense lasers, which severely restricts the service life of fused silica optics. The high absorption of fused silica to 10.6 µm makes it possible to utilize pulsed CO2 laser to remove and characterize SSD by layer-by-layer ablation, which improves its laser-induced damage threshold. However, thermal stress during the laser ablation process may have an impact on SSD, leading to extension. Still, the law of SSD morphology evolution mechanism has not been clearly revealed. In this work, a multi-physics simulated model considering light field modulation is established to reveal the evolution law of radial SSD during the laser layer-by-layer ablation process. Based on the simulation of different characteristic structural parameters, two evolution mechanisms of radial SSD are revealed, and the influence of characteristic structural parameters on SSD is also elaborated. By prefabricating the SSD by femtosecond laser, the measurements of SSD during CO2 laser layer-by-layer ablation experiments are consistent with the simulated results, and three stages of SSD depth variation under two evolution processes are further proposed. The findings of this study provide theoretical guidance for effectively characterizing SSD based on laser layer-by-layer ablation strategies on fused silica optics.

Development of broadband high efficiency Mid-IR gratings for high-energy ultrafast lasers

Broadband high-efficiency diffraction gratings play a crucial role in the pulse stretcher and compressor of high-energy ultrafast lasers. Nevertheless, conventional grating manufacturing techniques, including mechanical ruling and holographic recording, face challenges in creating accurate blazed groove profiles necessary for the fabrication of broadband, high-efficiency mid-infrared gratings. In this work, we utilized combined electron-beam lithography and anisotropic wet etching technology to fabricate nearly perfect blazed grooves, producing high efficiency broadband mid-infrared (IR) grating for 4.3 µm 100 femtosecond laser. Global optimization was performed to achieve a design of &gt; 90% efficiency over spectral range of 3.6 µm – 6.6 µm. Hybrid metal-dielectric coating (Au-Al2O3) is employed and optimized to minimize absorption and to enhance diffraction efficiency and laser-induced damage threshold (LIDT). Prototype gratings undergo testing at a desired application wavelengths of 4.3 µm in a tunable range of 0.2 µm, revealing that the optimal sample achieves a diffraction efficiency of 92%, closely approaching the theoretical value of 94.2%

The first Hg-based oxychalcogenide Sr2HgGe2OS6: Achieving balanced IR nonlinear optical properties through synergistic cation and anion substitution

Recently, newly emerging Hg-based chalcogenides with fascinating non-centrosymmetric (NCS) structures and superior optical properties have received wide attention as candidates for infrared nonlinear optics (IR-NLO). However, simultaneously optimizing three key performance indicators in these materials, namely the second-harmonic generation coefficient (deff), optical energy gap (Eg), and birefringence (Δn), remains a major challenge due to their often-competing structural requirements. Herein, the first Hg-based oxychalcogenide, Sr2HgGe2OS6, was rationally obtained using a synergistic strategy of cation and anion design based on the well-known melilite-type oxide Sr2ZnGe2O7. By substituting Hg2+ for Zn2+ cations and S2− for O2− anions, Sr2HgGe2OS6 features new 2D Cairo pentagonal tiling layers of [HgGe2OS6]4–, separated by charge-balanced Sr2+ cations. Notably, it exhibits well-balanced IR-NLO properties with a substantial deff (1.2 × AgGaS2@2050 nm), wide Eg (3.25 eV) resulting in a favorable laser-induced damage threshold (15.6 × AgGaS2@1064 nm), broad transparent window (up to 14.3 μm), and sufficient calculated Δn (0.147@2050 nm) for phase-matching ability across a wide range. Moreover, detailed theoretical analyses suggest that the simultaneous presence of excellent linear and NLO properties in Sr2HgGe2OS6 can be attributed to the cooperative effect of heteroligand [Ge2OS6] and tetrahedral [HgS4] asymmetric functional motifs. This study offers valuable insights into the development of NCS materials through logical chemical substitution strategies, and the intriguing optical properties of this new oxychalcogenide holds promise for applications in the IR-NLO field.

Design of Pentanary Mixed-Chalcogenides Ag2In2SiS6-xSex (x = 1, 2) Based on the Bucket Effect: Local Structural Difference and High-Performance Nonlinear-Optical Properties Realized by Partial Congener Substitution.

Noncentrosymmetric chalcogenides are promising candidates for infrared nonlinear-optical (NLO) crystals, and exploring high-performance ones is a hot topic and challengeable. Herein, the combination of AgQ4, InQ4, and SiQ4 (Q = S, Se) units with different S/Se ratios resulted in the discovery of the tetrahedral chalcogenides Ag2In2SiS4Se2 (1) and Ag2In2SiS5Se (2). They both crystallize in the monoclinic Cc space group with different local structures. Co-occupied S/Se sites only exist in 2, and the arrangement of [In2SiQ3] six-membered rings builds different helical chains and 3D [(In2SiQ6)2-]n polyanionic frameworks in 1 and 2. They show balanced NLO performances, including phase-matchable moderate NLO responses (0.7 and 0.5 × AGS) and enhanced laser-induced damage thresholds (4.5 and 5.1 × AGS). Theoretical calculations reveal that their NLO responses are predominantly contributed by the AgQ4 and InQ4 units.

A congruent-melting rubidium zinc vanadium oxychloride exhibiting strong mid-infrared second-harmonic generation and high laser-induced damage threshold

A novel mid-infrared (MIR) nonlinear optical (NLO) crystal Rb4ZnV5O15Cl (RZVC) has been designed and sythesized by the aliovalent substitution strategy based on the parent structure Rb2(VO)(V2O7). Modification of square pyramidal VO5 units through introduction of chlorine ion and zinc metal caion significantly enhanced the dipole moments in a unit cell from 1.653 D for Rb2(VO)(V2O7) to 6.559 D for RZVC. Consequently, polycrystalline RZVC exhibits strong SHG responses of 12.5 × KDP @1064 nm and 1.2 × AGS @2 μm. Furthermore, RZVC features a congruent melting property, an ultra-high LIDT value (62 × AGS @1064 nm), and a deep IR transparency cutoff edge (10 μm). Most importantly, RZVC can achieve full-wavelength phase matching in a very wide MIR transparency range of 1.69–10 μm, completely covering the important atmospheric window (3–5 μm) through NLO processes. Because of its many outstanding properties, RZVC can be a highly desirable candidate for practical MIR NLO applications.

Vacancy‐Induced Extraordinary Second Harmonic Generation Response for Diamond‐Like Cu3PS4

AbstractImproving polarizability is an important strategy for designing high‐performance mid‐infrared (mid‐IR) nonlinear optical (NLO) materials. The substitution of equivalent or aliovalent atoms can manipulate the polarizability by adjusting the symmetry of the polyhedron. Herein, the Li+ and Cd2+ are introduced into the Cu3PS4 as the equivalent and aliovalent dopants for the Cu site. As a result, Li+ can significantly improve the bandgap (Eg) of LixCu3−xPS4 from 2.38 to 2.88 eV, leading to a higher laser‐induced damage threshold (LIDT) of 4.9 times than AgGaS2 (AGS) with a comparable second harmonic generation (SHG) response of AGS (26−45 µm). Interestingly, Cd2+ can improve the SHG response and enlarge the Eg simultaneously. As a result, Cd0.4Cu2.2PS4 has a large SHG response of 10 × AGS at 2050 nm (26−45 µm) and a LIDT of 2.6 × AGS. Theoretical calculations reveal that lattice vacancies induced by Cd2+ significantly boost polarizability compared to LixCu3−xPS4 with no vacancy, leading to a strong NLO response.

LaMg6Ga6S16: a chemical stable divalent lanthanide chalcogenide

Divalent lanthanide inorganic compounds can exhibit unique electronic configurations and physicochemical properties, yet their synthesis remains a great challenge because of the weak chemical stability. To the best of our knowledge, although several lanthanide monoxides epitaxial thin films have been reported, there is no chemically stable crystalline divalent lanthanide chalcogenide synthesized up to now. Herein, by using octahedra coupling tetrahedra single/double chains to construct an octahedral crystal field, we synthesized the stable crystalline La(II)-chalcogenide, LaMg6Ga6S16. The nature of the divalent La2+ cations can be identified by X-ray photoelectron spectroscopy, X-ray absorption near-edge structure and electron paramagnetic resonance, while the stability is confirmed by the differential thermal scanning, in-situ variable-temperature powder X-ray diffraction and a series of solid-state reactions. Owing to the particular electronic characteristics of La2+(5d1), LaMg6Ga6S16 displays an ultrabroad-band green emission at 500 nm, which is the inaugural instance of La(II)-based compounds demonstrating luminescent properties. Furthermore, as LaMg6Ga6S16 crystallizes in the non-centrosymmetric space group, P−6, it is the second-harmonic generation (SHG) active, possessing a comparable SHG response with classical AgGaS2. In consideration of its wider band gap (Eg = 3.0 eV) and higher laser-induced damage threshold (5×AgGaS2), LaMg6Ga6S16 is also a promising nonlinear optical material.

Infrared nonlinear optical materials with multiple strongly ionic cations

Infrared nonlinear optical (IR-NLO) crystals with excellent properties are in extensive demand due to their important role in IR laser technology. Currently, it remains a great challenge to obtain IR-NLO materials with both high second harmonic generation (SHG) response and large laser-induced damage thresholds (LIDTs). Some structural design strategies such as ‘structural/functional regions’ have been adopted to develop new high-performance NLO materials. The covalent structural region producing SHG signals has been extensively investigated, whereas the hard cations (alkali, alkaline-earth, and rare-earth metal ions) which are responsible for improving LIDTs, have been relatively neglected. Utilizing the concept of structural/functional regions, we focus on the relation between structural regions and SHG properties in chalcogenides. Combining different kinds of hard cations can change the dimension of structures and affect the stacking of NLO-active groups. Introducing more hard cations and constructing more complex ion regions help to increase the laser damage threshold. Based on the mentioned structural strategies, guidance will be provided for developing high-performance multiple-cation materials for IR NLO applications.