High Laser-induced Damage Threshold Research Articles

Chalcohalides: A Rising Type of Second‐Order Nonlinear Optical Materials

AbstractIn the past few decades, many new infrared (IR) second‐order nonlinear optical (NLO) materials have been prepared by various synthetic methods to satisfy the growing market demand. However, few ones have the high potentials to be applicable since their comprehensive NLO properties cannot meet the standards, which include a series of indicators like large NLO effects, high laser‐induced damage thresholds (LIDTs), phase matchability, high thermal stability, good growth habit, and so on. Considering the available IR NLO materials’ disadvantages and the market requirements, it is necessary to continuously explore new high‐performance ones. Hitherto, lots of chalcogenides, halides, and oxides have been explored as IR NLO materials. Recently, researchers have found that the combination of two or more structural units into one structure may achieve good NLO properties, as evidenced by the fact that diverse materials with multiple anions are receiving growing interest. Specifically, chalcohalides are highly promising as they may combine the advantages of chalcogenides and halides,viz. large NLO coefficients and wide IR transparency of the former, and large band gaps and high LIDTs of the latter. To address the status of chalcohalides, this review focuses on the newest progresses on this topic (mainly from the last five years), including ternary chalcohalides, quaternary chalcohalides, salt‐inclusion chalcohalides, and rare‐earth chalcohalides. Here we summarize and analyze their compositions, crystal structures, NLO properties, and discuss the structure‐property relationship. Finally, the development prospects for NLO chalcohalides are given.

Ag[SC(NH2)2]2F: A Laser Damage-Tolerant Infrared Nonlinear Optical Material

Mild hydrothermal reactions of π-conjugated SC(NH2)2, Ag(NO3), and NaF afforded a new prospective infrared (IR) nonlinear optical (NLO) crystal, namely, Ag[SC(NH2)2]2F. It shows a 3D open framework consisting of corner-sharing AgS4 tetrahedra with six-member ring tunnels that are filled by organic groups of the SC(NH2)2 molecules. Ag[SC(NH2)2]2F maintains 80% transmittance in the spectral region of 0.32–5.91 μm and features large second harmonic generation (SHG) responses with phase-matching at both 1064 nm (15.8 × KH2PO4) and 2.05 μm (0.7 × AgGaS2). It also exhibits a large band gap of 4.19 eV, resulting in a high laser-induced damage threshold (LIDT) of 9.3 times that of AgGaS2. Theoretical calculation revealed that SC(NH2)2, AgS4 tetrahedra, and F– anions all give the positive contribution to SHG effects. Ag[SC(NH2)2]2F overcomes the small band gap and low LIDT of tetrahedral-based chalcogenides while maintaining a large SHG response. This work gives a promising facile design strategy for IR NLO materials.

Characterizations and Nonlinear-Optical Properties of Pentanary Transition-Metal Oxysulfide Sr2CoGe2OS6.

As one type of material containing multiple anions, oxysulfides can combine the advantages of oxides and sulfides and are deeply studied as nonlinear-optical (NLO) materials. Herein, a new melilite-type pentanary oxysulfide Sr2CoGe2OS6 is studied. It crystallizes in the noncentrosymmetric tetragonal space group P4̅21m, and its structure features GeOS3 and CoS4 tetrahedra-built {[CoGe2OS6]4-}∞ layers. Its powder sample exhibits a moderate phase-matchable NLO response and a high laser-induced damage threshold. The NLO response is mainly determined by CoS4 tetrahedra according to the theoretical calculation results. This work indicates that transition-metal oxysulfides can also be considered as potential infrared NLO materials.

Oxychalcogenides as Promising Ultraviolet Nonlinear Optical Candidates: Experimental and Theoretical Studies of AEGeOS2 (AE = Sr and Ba).

Oxychalcogenides have gained widespread attention as promising infrared nonlinear optical (IR-NLO) candidates. However, high-performance oxychalcogenides have rarely been reported in the ultraviolet (UV) region owing to the low energy gaps (Eg < 4.0 eV). Herein, two non-centrosymmetric (NCS) oxychalcogenides with one-dimensional (1D) chain structures and wide Eg (>4.3 eV), namely, AEGeOS2 (AE = Sr and Ba), have been discovered by combined experiments and theory calculations as a new source of UV-NLO materials. Significantly, they exhibit excellent comprehensive performance comparable to the commercial UV-NLO material KH2PO4 (KDP), including large phase-matching ranges (>380 nm), sufficient second harmonic generation intensities (0.7-1.1 × KDP), high laser-induced damage thresholds (1.2 × KDP), wide transparent regions (0.26-12.2 μm), and good thermal stability (up to 1100 K). Moreover, systematic structure-activity relationship analysis illustrates that the 1D homochiral helical [GeOS2]2- chains composed of heteroanionic [GeS2O2] units make major contribution to the desirable UV-NLO performance. This work makes the two compounds shine out as new energy in the UV-NLO field and offers a new perspective for the exploration of structure-driven functional oxychalcogenides.

Optical thin film coated organic nonlinear crystal for efficient terahertz wave generation

In the process of terahertz (THz) wave generation via optical rectification of infrared femtosecond pulses in a non-linear optical crystal, the power of terahertz wave is directly proportional to the square of the optical pump power. Therefore, high power terahertz wave can be generated using a high power femtosecond laser provided that the crystal has both high laser induced damage threshold and optical non-linear coefficient. However, a significant amount of pump power is lost in this process due to the Fresnel’s reflection at the air-crystal boundary. In this paper, we numerically and experimentally demonstrate that the coat of optical thin film called Cytop on the 4-N, N-dimethylamino-4’-N’-methyl-stilbazolium tosylate (DAST) crystal effectively reduces the reflection loss of pump power, thereby increasing the THz wave emission efficiency of the DAST crystal. We found that the average power of THz wave emitted by the thin film coated crystal is about 28% higher than the THz power emitted by the uncoated crystal when an equal amount of laser power is used. The thin film coated DAST crystals can be used not only in terahertz measurement systems but also in optical devices such as modulators and waveguides.

(Na0.74 Ag1.26 )BaSnS4 : A New AgGaS2 -Type Nonlinear Optical Sulfide with a Wide Band Gap and High Laser Induced Damage Threshold.

The development of high-power solid-state lasers is in urgent need of infrared (IR) nonlinear optical (NLO) materials with wide band gaps and high laser-induced damage thresholds (LIDTs). Herein, a new compressed chalcopyrite-like IR NLO crystal (Na0.74 Ag1.26 )BaSnS4 was successfully synthesized using a facile high-temperature solid-state method. Its structure can be considered as a variant of chalcopyrite AgGaS2 (AGS)-type ones. It features a three-dimensional framework constructed by corner-sharing {[(Na/Ag)S4 ]7- }∞ layers and isolated SnS4 tetrahedra with negative cavities occupied by counter ion Ba2+ . (Na0.74 Ag1.26 )BaSnS4 exhibits phase-matchable moderate SHG response (0.31 × AGS), wide band gap (3.70 eV), and high LIDT (6.44 × AGS). Theoretical calculations reveal that the NLO response of (Na0.74 Ag1.26 )BaSnS4 is mainly originated from the synergetic effects of AgS4 and SnS4 tetrahedra, and the inclusion of alkaline and alkaline earth metals is responsible for the wide band gap and high LIDT. Moreover, the discovery of this chalcopyrite-like compound will provide a feasible design strategy for the exploration of new promising IR NLO materials.

Effects of the Processing Technology of CVD-ZnSe, Cr2+:ZnSe, and Fe2+:ZnSe Polycrystalline Optical Elements on the Damage Threshold Induced by a Repetitively Pulsed Laser at 2.1 µm

Polycrystalline zinc selenide (ZnSe) and Cr2+ or Fe2+ doped ZnSe are key optical elements in mid-infrared laser systems. The laser-induced damage of the optical elements is the limiting factor for increasing the power and pulse energy of the lasers. In the present work, the optical damage of the ZnSe, Cr2+:ZnSe, and Fe2+:ZnSe samples induced by a repetitively pulsed Ho3+:YAG laser at 2091 nm was studied. The probability of the optical damage and the laser-induced damage threshold (LIDT) were determined for the samples manufactured using different processing techniques. The highest LIDT was found in ZnSe samples annealed in an argon atmosphere. It was also found that the samples annealed in a zinc atmosphere or with hot isostatic pressing resulted in a decrease in the LIDT. The Cr2+-doped ZnSe had the lowest LIDT at 2.1 µm compared to Fe2+-doped or undoped ZnSe. The LIDT fluence of all tested ZnSe samples decreased with the increase in the pulse repetition rate and the exposure duration. The results obtained may be used to improve the treatment procedures of ZnSe, Cr2+:ZnSe, and Fe2+:ZnSe polycrystals to further increase their LIDT.

Eu2 P2 S6 : The First Rare-Earth Chalcogenophosphate Exhibiting Large Second-Harmonic Generation Response and High Laser-Induced Damage Threshold.

Metal chalcogenophosphates are receiving increasing interest, specifically as promising infrared nonlinear optical (NLO) candidates. Here, a rare-earth chalcogenophosphate Eu2 P2 S6 crystallizing in the monoclinic noncentrosymmetric space group Pn was synthesized using a high-temperature solid-state method. Its structure features isolated [P2 S6 ]4- dimer, and two types of EuS8 bicapped triangular prisms. Eu2 P2 S6 exhibits a phase-matchable second-harmonic generation (SHG) response ≈0.9×AgGaS2 @2.1 μm, and high laser-induced damage threshold of 3.4×AgGaS2 , representing the first rare-earth NLO chalcogenophosphate. The theoretical calculation result suggests that the SHG response is ascribed to the synergetic contribution of [P2 S6 ]4- dimers and EuS8 bicapped triangular prisms. This work provides not only a promising high-performance infrared NLO material, but also opens the avenue for exploring rare-earth chalcogenophosphates as potential IR NLO materials.

The Development Progress of Surface Structure Diffraction Gratings: From Manufacturing Technology to Spectroscopic Applications

The high-precision diffraction grating is an important chromatic dispersion component that has been widely used in many fields, including laser beam combining, chirped pulse compression, spectroscopy, among others. In this paper, we review the development status of reflection and transmission gratings with high diffraction efficiency and high laser-induced damage thresholds, such as metal-film and multilayer-dielectric-film gratings. Then, we review the basic principles and most recent stages in the development of manufacturing techniques, such as mechanical scribing, holographic exposure, electron-beam lithography, and nanoimprinting.

NaBaBS3: A Promising Infrared Functional Material with Large Birefringence Induced by π-Conjugated [BS3] Units

The first thioborate in the Na–Ba–B–S system NaBaBS3 has been synthesized successfully, together with a new phase of LiBaBS3 (β-phase). NaBaBS3 shows great potential to be an infrared (IR) birefringence (Δn) material due to its large Δn = 0.243, wide band gap (3.96 eV), and high laser-induced damage threshold (5 × AGS). The results of structure–property relationship based on the response electron distribution anisotropy and real space atom cutting methods illustrate that the large birefringence is induced from the π-conjugated [BS3] units (about 90% contribution). Moreover, a series of thioborates (with [BS3] as the only anion group) were investigated about the birefringence, and the results indicate that [BS3] could be regarded as a good gene for designing IR birefringent materials.

SrZnGeS4 : A Dual-Waveband Nonlinear Optical Material with a Transparency Spanning UV/Vis and Far-IR Spectral Regions.

Non-linear optical chalcogenides with a wide band gap (Eg ) and excellent NLO properties are key materials for highly desirable multiwaveband tunable optical parametric oscillators (OPOs). We exploit the "electronic structure engineer bucket effect" to develop a novel dual-waveband SrZnGeS4 with an ultrawide transparency window. It exhibits an asymmetric Fdd2 structure that consists of layers formed by corner-sharing [ZnGeS6 ] dimers. SrZnGeS4 is transparent from 0.30 to 23.6 μm, spanning the UV-, vis-, mid- and far-IR spectral regions and has the widest Eg (3.63 eV) in the AeMII MIV Q4 family to date. It exhibits phase matching, high SHG intensities (e.g., 11.0×KDP and 17.5×AGS under λinc =1450 and 950 nm, respectively), and a very high laser-induced damage threshold (35×AGS). These results not only suggest bright prospects for high-power laser applications but may also enable applications of the multiwaveband OPO system from the UV-visible to far-IR regions.

Second-Harmonic Generation-Positive Na2Ga2SiS6with a Broad Band Gap and a High Laser Damage Threshold

The development of high-power solid-state lasers is in urgent need of new infrared nonlinear optical (IR NLO) materials with a wide band gap and a high laser-induced damage threshold. A new infrared nonlinear optical material Na2Ga2SiS6 has been synthesized for the first time, crystallizing in the Fdd2 (no. 43) noncentrosymmetric space group. Its three-dimensional tunnel framework consists of two typical NLO active motifs [GaS4] and [SiS4], with Na+ cations located inside the tunnels. Na2Ga2SiS6 exhibits comprehensive optical properties, namely, a wide transmission range, a high laser-induced damage threshold (10 × AgGaS2), a type-I phase-matching second-harmonic generation response (0.2 × AgGaS2), and especially a wide band gap (3.93 eV), which is the largest in the A2MIII2MIVQ6 (A = alkali metals; MIII = IIIA elements; MIV = IVA elements; Q = S and Se) family. Therefore, Na2Ga2SiS6 does not produce two-photon absorption under a 1064 nm laser pump and could be used in high-energy laser systems, which makes Na2Ga2SiS6 a promising candidate for high-energy IR NLO applications.

Controllable shearing chain-thickening polishing process for machining of barium borate

The functionality of nonlinear optics made from barium borate is significantly determined by its surface integrity. Nevertheless, it has been a challenge to control surface integrity precisely to prevent weak deliquescence. Here a novel finishing process - the Controllable Shearing Chain-Thickening Polishing (CS-CTP) - is proposed which provides a controllable discontinuous shearing effecting chain-thickening in a water-free slurry. Precise shearing control is applied to use the chain-thickening rheology for the material removal mechanism. Optimized conditions of the CS-CTP process enable to efficiently achieve ultra-smooth barium borate surfaces with ultra-low subsurface damage and high laser-induced damage threshold.

The Combination of Structure Prediction and Experiment for the Exploration of Alkali-Earth Metal-Contained Chalcopyrite-Like IR Nonlinear Optical Material.

Design and fabrication of new infrared (IR) nonlinear optical (NLO) materials with balanced properties are urgently needed since commercial chalcopyrite‐like (CL) NLO crystals are suffering from their intrinsic drawbacks. Herein, the first defect‐CL (DCL) alkali‐earth metal (AEM) selenide IR NLO material, DCL‐MgGa2Se4, has been rationally designed and fabricated by a structure prediction and experiment combined strategy. The introduction of AEM tetrahedral unit MgSe4 effectively widens the band gap of DCL compounds. The title compound exhibits a wide band gap of 2.96 eV, resulting in a high laser induced damage threshold (LIDT) of ≈3.0 × AgGaS2 (AGS). Furthermore, the compound shows a suitable second harmonic generation (SHG) response (≈0.9 × AGS) with a type‐I phase‐matching (PM) behavior and a wide IR transparent range. The results indicate that DCL‐MgGa2Se4 is a promising mid‐to‐far IR NLO material and give some insights into the design of new CL compound with outstanding IR NLO properties based on the AEM tetrahedra and the structure predication and experiment combined strategy.

Effect of annealing on properties and performance of HfO2/SiO2 optical coatings for UV-applications.

The field of ultraviolet (UV)-laser applications is currently experiencing rapid growth in the semiconductor processing, laser micromachining and biomedical markets. Key enablers for these technologies are optical coatings used to manipulate and guide laser beams in a targeted manner. As laser power, laser fluence and pulse frequencies increase, the demands on the physical properties of the coating materials become more stringent. Ion beam sputtering is a technique that allows producing optical coatings with the low losses required in these applications. In this study, we investigate the influence of ion beam sputtering (IBS) parameters on the optical properties of HfO2 and SiO2 single layers as well as the impact of annealing duration at 475 °C for anti-reflective (AR) and highly reflective (HR) optical coatings at 355 nm. For HfO2 sputtered from a metal target the O2 flow during the coating process is a key parameter to reduce absorption. SiO2 single layers exhibit improved transmission in the UV-range as the ion beam energy for the sputtering process is reduced. Furthermore, a complex behavior for film stress, absorption, surface roughness and coating structure was unraveled as a function of annealing duration for AR- and HR-coatings at 355 nm. The reflectance of the HR-mirror after optimized annealing exceeded 99.94% at 355 nm and a high laser induced damage threshold (LIDT) of 6.9 J/cm2 was measured after 2 hours of annealing. For the AR-coating a LIDT-value of 15.7 J/cm2 was observed after 12 hours of annealing.

Plasma-Enhanced Atomic Layer Deposition of HfO2 with Substrate Biasing: Thin Films for High-Reflective Mirrors.

Tuning ion energies in plasma-enhanced atomic layer deposition (PEALD) processes enables fine control over the material properties of functional coatings. The growth, structural, mechanical, and optical properties of HfO2 thin films are presented in detail toward photonic applications. The influence of the film thickness and bias value on the properties of HfO2 thin films deposited at 100 °C using tetrakis(dimethylamino)hafnium (TDMAH) and oxygen plasma using substrate biasing is systematically analyzed. The HfO2 films deposited without a substrate bias show an amorphous microstructure with a low density, low refractive index, high incorporation of residual hydroxyl (OH) content, and high residual tensile stress. The material properties of HfO2 films significantly improved at a low bias voltage due to the interaction with oxygen ions accelerated to the film. Such HfO2 films have a higher density, higher refractive index, and lower residual OH incorporation than films without bias. The mechanical stress becomes compressive depending on the bias values. Further increasing the ion energies by applying a larger substrate bias results in a decrease of the film density, refractive index, and a higher residual OH incorporation as well as crystalline inclusions. The comparable material properties of the HfO2 films have been reported using tris(dimethylamino)cyclopentadienyl hafnium (TDMACpH) in a different apparatus, indicating that this approach can be transferred to various systems and is highly versatile. Finally, the substrate biasing technique has been introduced to deposit stress-compensated, crack- and delamination-free high-reflective (HR) mirrors at 355 and 532 nm wavelengths using HfO2 and SiO2 as high and low refractive index materials, respectively. Such mirrors could not be obtained without the substrate biasing during the deposition because of the high tensile stress of HfO2, leading to cracks in thick multilayer systems. An HR mirror for 532 nm wavelength shows a high reflectance of 99.93%, a residual transmittance of ∼530 ppm, and a low absorption of ∼11 ppm, as well as low scattering losses of ∼4 ppm, high laser-induced damage threshold, low mechanical stress, and high environmental stability.

Phosphogermanate Crystal: A New Ultraviolet-Infrared Nonlinear Optical Crystal with Excellent Optical Performances.

The phase matching ability is a key factor for nonlinear optical crystals to realize coherent output. Herein, a new design strategy combining ultraviolet and infrared functional groups into a ferroelectric was put forward. Thus, a phosphogermanate crystal, KGeOPO4, was designed and studied. It exhibits a wide transparency window (0.22-9.70 μm), a strong second harmonic generation response (5× KH2PO4), a high laser-induced damage threshold (1.61 GW/cm2), and the typical ferroelectricity (coercive field ∼ 9.8 kV/cm and remnant polarization ∼7.6 μC/cm2). In the infrared region, it could realize coherent output by the birefringence phase matching method, while it could generate ultraviolet coherent lights by the quasi-phase matching technique. Therefore, this work designs a promising ultraviolet-infrared nonlinear optical crystal and provides a new perspective for exploring nonlinear optical crystals.

Chiral Hybrid Copper(I) Halides for High Efficiency Second Harmonic Generation with a Broadband Transparency Window

AbstractChiral hybrid organic–inorganic metal halides (HOMHs) with intrinsic noncentrosymmetry have shown great promise for applications in second‐order nonlinear optics (NLO). However, established chiral HOMHs often suffer from their relatively small band gaps, which lead to negative impacts on transparent window and laser‐induced damage thresholds (LDT). Here, we have synthesized two chiral HOMHs based on CuI halides, namely (R‐/S‐MBA)CuBr2, which feature well‐balanced NLO performances with a highly efficient SHG response, outstanding optical transparency, and high LDT. The effective second‐order NLO coefficient of (R‐MBA)CuBr2 has been determined to be ≈24.7 pm V−1, which is two orders of magnitude higher than that of their CuII counterparts. This work shows the promising potential of CuI‐based chiral HOMHs for nonlinear photonic applications in wide wavelength regions.

Chiral Hybrid Copper(I) Halides for High Efficiency Second Harmonic Generation with a Broadband Transparency Window.

Chiral hybrid organic-inorganic metal halides (HOMHs) with intrinsic noncentrosymmetry have shown great promise for applications in second-order nonlinear optics (NLO). However, established chiral HOMHs often suffer from their relatively small band gaps, which lead to negative impacts on transparent window and laser-induced damage thresholds (LDT). Here, we have synthesized two chiral HOMHs based on CuI halides, namely (R-/S-MBA)CuBr2 , which feature well-balanced NLO performances with a highly efficient SHG response, outstanding optical transparency, and high LDT. The effective second-order NLO coefficient of (R-MBA)CuBr2 has been determined to be ≈24.7 pm V-1 , which is two orders of magnitude higher than that of their CuII counterparts. This work shows the promising potential of CuI -based chiral HOMHs for nonlinear photonic applications in wide wavelength regions.

The Influence of Ion Beam Bombardment on the Properties of High Laser-Induced Damage Threshold HfO2 Thin Films

HfO2 thin films were deposited on BK-7 glass substrates using an electron beam evaporation deposition (EBD) technique and then post-treated with argon and oxygen ions at an ion energy ranging from 800 to 1200 eV. The optical properties, laser damage resistance, and surface morphology of the thin films exposed to Ar ions and O2 ions at various energies were studied. It was found that the two ion post-treatment methods after deposition were effective for improving the LIDT of HfO2 thin films, but the mechanism for the improvement differs. The dense thin films highly resistant to laser damage can be obtained through Ar ion post-treatment at a certain ion energy. The laser-induced damage threshold (LIDT) of thin films after O2 ion post-treatment was higher in comparison to those irradiated with Ar ion at the same ion energy.