High-peak-power long-wave infrared BaGa4Se7 optical parametric oscillator with 6.7-13.9†µm widely tunable range.

A high-peak-power, widely tunable range long-wave infrared optical parametric oscillator (OPO) based on the BaGa4Se7 (BGSe) crystal is demonstrated in this Letter. Pumped by a 1064 nm Nd:YAG laser, a high-peak-power of 0.15 MW was achieved at 9.8 µm with a pulse width of 5.0 ns. At 11.0 µm, a high beam quality of M2x = 4.1 and M2y = 3.3 was achieved. By rotating the BGSe crystal, a broad tuning range of 6.7-13.9 µm was realized. Furthermore, a theoretical analysis was conducted to elucidate the reasons behind the improvement in beam quality in the x-direction as the wavelength of the idler wave increases.

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.

Sub-picosecond tunable mid-infrared light source for driving high-efficiency optical rectification.

Optical rectification (OR) is a popular way to generate coherent terahertz radiation. Here, we develop a sub-picosecond mid-infrared (mid-IR) light source with a tailored wavelength and pulse duration for enhancing the OR efficiency. Numerical simulations for a LiNbO3-based OR with tilted pulse-front excitation are first conducted to determine the optimal parameters of pump wavelength and pulse duration, demonstrating that the OR efficiency pumped by 4-µm sub-picosecond (0.5-0.6 ps) pulses is approximately twice the value with 0.8-µm pump at the same conditions. Guided by the simulation results, we build a BaGa4Se7-based optical parametric chirped-pulse amplification system with 1030-nm thin-disk pump and broadband mid-IR seeds. The output performances of >200-µJ pulse energy, ∼600-fs pulse duration and 1-kHz pulse repetition rate are achieved in a spectral range tunable from 3.5 to 5 µm. The large energy scalability and high parameter tunability make the light source attractive to high-efficiency OR in various materials.

Synthesis, Crystal Structure, and Nonlinear-Optical Properties of a Diamond-Like Chalcogenide Cu2GeS3.

Metal sulfides with diamond-like (DL) structures generally exhibit excellent mid-IR nonlinear-optical (NLO) properties. Here, Cu2GeS3 (CGS) as a member of the DL chalcogenides was synthesized by a high-temperature solid-state method, and the optical properties were carefully studied experimentally and theoretically. The results revealed that CGS has a large second harmonic generation (0.8 × AgGaSe2) and a moderate birefringence of 0.067 at 1064 nm. In addition, the linear and NLO properties of the A2MS3 (A = Cu, Li; M = Ge, Si) series of compounds were evaluated and compared with the help of first-principles calculations.

Structure and Characterization of K2Na3B2P3O13, a New Nonlinear Optical Borophosphate with One-Dimensional Chain Structure and Short Ultraviolet Cutoff Edge.

Nonlinear optical (NLO) crystals, being the primary medium for laser wavelength conversion, are crucial in all-solid-state lasers. Borophosphates offer more structural varieties than pure borates and phosphates, and they have become popular as NLO crystal candidates. Through spontaneous crystallization, we acquired a noncentrosymmetric alkali metal borophosphate crystal material, K2Na3B2P3O13 (KNBPO). KNBPO crystallizes in the orthorhombic Cmc21 space group with the following unit cell parameters: a = 13.9238(18) Å, b = 6.7673(8) Å, c = 12.1298(15) Å, and Z = 4, and its structure is characterized by a fundamental building unit 1∞ [B2P3O13] chain structure made up of bridging oxygen linkages between BO4 and PO4 tetrahedra. KNBPO has a short ultraviolet (UV) cut-off edge (<186 nm), a congruent melting characteristic, good thermal stability, and a moderate second harmonic generation response roughly 0.42 times that of KH2PO4. Theoretical calculations reveal that the optical properties of the compound mainly originate from BO4 and PO4 units. Due to the short UV cut-off edge, KNBPO can be used as a potential NLO material in the UV and even deep UV regions, and it enhances the structural variety of borophosphates, which has a reference value for scholars investigating similar materials.

Mixed Alkali Metal and Alkaline Earth Metal Scandium Borate Birefringence Material with Layered Structure and Short Ultraviolet Cutoff Edge.

Birefringent crystals are essential in the domains of linear and nonlinear optics that need light wave polarization control. Rare earth borate has become a popular study material for ultraviolet (UV) birefringence crystals due to its short cutoff edge in the UV area. RbBaScB6O12, a two-dimensional layered structure compound with the B3O6 group, was effectively synthesized through spontaneous crystallization. The UV cutoff edge of RbBaScB6O12 is shorter than 200 nm, and the experimental birefringence is 0.139 @ 550 nm. Theoretical research indicates that the large birefringence originates from the synergistic impact of the B3O6 group and the ScO6 octahedron. RbBaScB6O12 is an outstanding candidate material for birefringence crystals in the UV and even deep UV regions due to its short UV cutoff edge and significant birefringence.

Structure and Optical Properties of LixAg1-xGaSe2 and LixAg1-xInSe2.

Complete substitution of Li atoms for Ag atoms in AgGaSe2 and AgInSe2 was achieved, resulting in the solid solutions LixAg1-xGaSe2 and LixAg1-xInSe2. The detailed crystal structures were determined by single-crystal X-ray diffraction and solid-state 7Li nuclear magnetic resonance spectroscopy, which confirm that Li atoms occupy unique sites and disorder only with Ag atoms. The tetragonal CuFeS2-type structure (space group I4̅2d) was retained within the entirety of the Ga-containing solid solution LixAg1-xGaSe2, which is noteworthy because the end-member LiGaSe2 normally adopts the orthorhombic ß-NaFeO2-type structure (space group Pna21). These structures are closely related, being superstructures of the cubic sphalerite and hexagonal wurtzite prototypes adopted by diamond-like semiconductors. For the In-containing solid solution LixAg1-xInSe2, the structure transforms from the tetragonal to orthorhombic forms as the Li content increases past x = 0.50. The optical band gaps increase gradually with higher Li content, from 1.8 to 3.4 eV in LixAg1-xGaSe2 and from 1.2 to 2.5 eV in LixAg1-xInSe2, enabling control to desired values, while the second harmonic generation responses become stronger or are similar to those of benchmark infrared nonlinear optical materials such as AgGaS2. All members of these solid solutions remain congruently melting at accessible temperatures between 800 and 900 °C. Electronic structure calculations support the linear trends seen in the optical band gaps and confirm the mostly ionic character present in Li-Se bonds, in contrast to the more covalent character in Ga-Se or In-Se bonds.

Output characteristics of 3-8 µm mid-infrared source based on BaGa4Se7 crystal.

An optical parametric oscillator based on B a G a 4 S e 7 crystal is constructed for use as a 3-8 µm mid-infrared source. The output characteristics of the light source, including wavelength tuning, beam quality, and energy stability, are studied, which are usually concerned differently in spectral analysis, imaging, and laser processing. When the light source operates far from the threshold, the mid-infrared output is poor in beam quality and good in energy stability, and the contradiction should be addressed in practice. The results provide guidance on the selection of operating parameters and performance optimization for the application of the B a G a 4 S e 7 based mid-infrared source in various settings.

The Kurtz-Perry Powder Technique Revisited: A Study of the Effect of Reference Selection on Powder Second-Harmonic Generation Response.

The accurate evaluation of nonlinear optical (NLO) coefficient, the main parameter affecting light conversion efficiency, plays a crucial role in the development of NLO materials. The Kurtz-Perry powder technique can evaluate second-harmonic generation (SHG) intensity in pristine powder form, saving a significant amount of time and energy in the preliminary screening of materials. However, the Kurtz-Perry method has recently been subject to some controversy due to the limitations of the Kurtz-Perry theory and the oversimplified experimental operation. Therefore, it is very meaningful to revisit and develop the Kurtz-Perry technique. In this work, on the basis of introducing the light scattering effect into the original Kurtz-Perry theory, the theoretical expression of second-harmonic generation intensity with respect to band gap and refractive index are analyzed. In addition, the reference-dependent SHG measurements were carried out on polycrystalline LiB3O5 (LBO), AgGaQ2 (Q = S, Se), BaGa4Q7 (Q = S, Se), and ZnGeP2 (ZGP), and the results of SHG response emphasize the importance of using appropriate references to the Kurtz-Perry method. In order to obtain reliable values of nonlinear coefficients, two criteria for selecting a reference compound were proposed: (1) it should possess a band gap close to that of the sample to be measured and (2) it should possess a refractive index close to that of the sample to be measured. This work might shed light on improvements in accuracy that can be made for effective NLO coefficients obtained using the Kurtz-Perry method.

Ultrabroad (3.7-17†µm) tunable femtosecond optical parametric amplifier based on BaGa4Se7 crystal.

We demonstrate the first (to the best of our knowledge) tunable femtosecond (fs) mid-infrared (MIR) optical parametric amplifier (OPA) based on BaGa4Se7 (BGSe) crystal with an ultra-broadband spectral range. Benefiting from the broad transparency range, high nonlinearity, and relatively large bandgap of BGSe, the MIR OPA pumped at 1030 nm with a repetition of 50 kHz has an output spectrum that is tunable across an extremely wide spectral range spanning from 3.7 to 17 µm. The maximum output power of the MIR laser source is measured as 10 mW at a center wavelength of 16 µm, corresponding to a quantum conversion efficiency of 5%. Power scaling is straightforwardly achieved by using a stronger pump in BGSe with an available large aperture size. A pulse width of 290 fs centered at 16 µm is supported by the BGSe OPA. Our experimental result indicates that BGSe crystal could serve as a promising nonlinear crystal for fs MIR generation with an ultra-broadband tuning spectral range via parametric downconversion for applications such as MIR ultrafast spectroscopy.

Second-Harmonic-Generation Effect and Giant Optical Birefringence in the Weyl Material CaAgAs.

Nonlinear-optical (NLO) materials suitable for the "8-14 µm" atmospheric transparent window are in urgent need. Arsenide is one of the most promising material systems for such an application. However, the second-harmonic-generation (SHG) effect of arsenide is difficult to characterize using the conventional powder SHG technique with a 2 µm fundamental laser. To overcome this problem, we focused on a novel arsenide Weyl material, CaAgAs, with a zero band gap and proposed a modified powder SHG measurement method for narrow-band-gap materials. We successfully observed the SHG signal of CaAgAs, which was approximately 0.7 times that of CdGeAs2. Moreover, on the basis of first-principles calculations, the largest SHG coefficient for CaAgAs is equal to 236 pm/V, and the ionic bonds in the [Ca3As13] motif play an indispensable role for the SHG effect because of the distorted Kagome lattice pattern. CaAgAs is predicted to have strong optical anisotropy with a giant optical birefringence of 0.52.

Growth, Structure, and Optical Properties of a Nonlinear Optical Niobium Borate Crystal CsNbOB2O5 with Distorted NbO5 Square Pyramids.

In this paper, a revised structure determination of an already known compound CsNbOB2O5 with new structural insights was performed and the detailed characterization of its optical properties was reported for the first time. CsNbOB2O5 was synthesized by spontaneous crystallization. It crystallizes in the Pmn21 space group, and the unit lattice parameters are a = 7.5220(7) Å, b = 3.9881(4) Å, c = 9.7167(9) Å, and Z = 2. In the structure of CsNbOB2O5, a [NbO5] square pyramid and [B2O5] unit are linked to constitute an infinitely extended two-dimensional ∞2[NbOB2O5] layer via sharing oxygen atoms. Between these two-dimensional layers, there are no covalent bonds perpendicular to their planes based on Mulliken bond order analysis. CsNbOB2O5 has a wide band gap (4.52 eV) and a large second-harmonic-generation (SHG) response (1.2 × KDP) and demonstrates type-I phase-matchable behavior. First-principles simulations reveal that the birefringence is approximately 0.10. Moreover, SHG-weighted charge density analysis shows that the primary source of the nonlinearity of the title compound is the distorted NbO5 square pyramids.

Comparison of a high-power 3.75 µm BaGa4Se7 OPO based on a plane-parallel resonator and an unstable resonator with a Gaussian reflectivity mirror.

Using a homemade Nd:YAG laser with a pulse repetition frequency of 300 Hz as a pump source, we demonstrated a tunable BaGa4Se7 (BGSe) optical parametric oscillator (OPO). Wavelength-tuning ranges of 1.42-1.59 µm and 3.21-4.22 µm were realized, and a maximum average output power of 1.03 W at 3746 nm was achieved in a plane-parallel resonator. To date, this is the highest output power at around 4 µm obtained from a BGSe OPO pumped by a 1 µm laser, to our knowledge. Moreover, the BGSe OPO employing a plano-convex Gaussian reflectivity mirror (GRM) as an output coupler (OC) was demonstrated for the first time, to our knowledge. At the same pump power, the corresponding output power and M2 value of the idler at 3746 nm were 0.86 W and 10.2 (x direction) and 11.6 (y direction), respectively. The results indicate that the beam quality factor M2 of the idler has an over 30% improvement in comparison with the BGSe OPO based on a conventional flat OC of a plane-parallel resonator. Limited by the beam quality of the pump source, there is little room for improvement in the beam quality of the BGSe OPO with a GRM OC, which could be improved in the future by optimizing the pump beam.

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.

Investigation into Structural Variation from 3D to 1D and Strong Second Harmonic Generation of the AHgPS4 (A+ = Na+, K+, Rb+, Cs+) Family.

The continuous exploration of multinary chalcogenide semiconductors has provided a variety of new functional materials. In this paper, four new quaternary chalcogenides AHgPS4 (A+ = Na+, K+, Rb+, Cs+) have been prepared by solid-state syntheses. These findings complement the lack of research on this quaternary system. Influenced by the size effect of cations and the coordination mode of Hg, the four compounds crystallize in four different space groups [NaHgPS4, P4̅n2; KHgPS4, Pnn2; RbHgPS4, P21/n; CsHgPS4, P212121] and show an interesting evolution from a 3D framework structure to a 1D chain structure. Moreover, all of these compounds feature noncentrosymmetric (NCS) structures except for RbHgPS4. The materials exhibit wide band gaps of 2.7 eV < Eg < 3.0 eV. The NCS- related second-harmonic-generation (SHG) property of NaHgPS4 and KHgPS4 was also studied. They display strong powder SHG responses (3.14 × AgGaS2 for NaHgPS4; 4.15 × AgGaS2 for KHgPS4), which indicate their intriguing potential as IR nonlinear-optical materials. Moreover, first-principles theoretical calculations were performed to understand the structure-property relationships of these materials.

Cd7SiAs6, a Nonchalcopyrite Arsenide with a Strong Nonlinear-Optical Response.

A new arsenide, Cd7SiAs6, has been successfully synthesized and characterized. It is the first arsenide that adopts a nonchalcopyrite structure and possesses a strong nonlinear-optical (NLO) response. In the structure, the CdAs3 trigonal planar unit, a kind of π-conjugated planar NLO-active group, was identified for the first time. Furthermore, theoretical calculations reveal that the CdAs3 planar unit contributes more to the NLO effect than the CdAs4 tetrahedron does. The result may provide valuable insights for the future exploration of IR NLO materials, especially for application above 10 µm.

Synthesis and Characterizations of Two Tellurides ß-BaGa2Te4 and Ba5Ga2Ge3Te12 with Flexible Chain Structure.

Demands for IR birefringent materials are increasing due to the rapid developments of IR laser applications. Herein, two new chain tellurides ß-BaGa2Te4 and Ba5Ga2Ge3Te12 have been discovered. ß-BaGa2Te4 crystallizes in the orthorhombic space group Imma (no. 74) with unit cell constants of a = 23.813(3) Å, b = 11.9673(19) Å, and c = 6.7215(9) Å, while Ba5Ga2Ge3Te12 crystallizes in the monoclinic space group P21/c (no. 14) with unit cell constants of a = 13.6540(3) Å, b = 9.6705(2) Å, and c = 23.1134(7) Å. The structure of ß-BaGa2Te4 can be considered to be the antiparallel arrangement of one-dimensional (1D) [GaTe2] chains formed by edge-sharing GaTe4 tetrahedra, which are separated by Ba2+ cations. In the crystal structure of Ba5Ga2Ge3Te12, two kinds of 1D chains, namely chain 1 ∞1[(GaGe)3Te8] and chain 2 ∞1[(GaGe)2Te4], are stacked alternately and put together by the coulomb force with Ba2+ cations. In addition, First-principles calculations indicate that ß-BaGa2Te4 has a large birefringence, ∼0.325 at 2050 nm, derived from the superposition of the polarizabilities of GaTe4 tetrahedra, implying that it has potential as an IR birefringent material. This work may provide some guidance for exploring new IR birefringent crystals.

Na3Bi(IO3)6: An Alkali-Metal Bismuth Iodate with Intriguing One-Dimensional [BiI6O18] Chains and Pressure-Induced Structural Transition.

An alkali-metal bismuth iodate, Na3Bi(IO3)6, was successfully obtained by the hydrothermal method for the first time and contains intriguing one-dimensional [BiI6O18] chains. High-pressure Raman spectra were carried out to investigate the structural transition of Na3Bi(IO3)6. Electronic states and anisotropic optical responses were also investigated by theoretical calculations.

Widely tunable long-wave infrared difference frequency generation with a BaGa4Se7 crystal.

We report an experimental study of long-wave infrared difference frequency generation based on BaGa4Se7 crystal. The sources of two input wavelengths were the fundamental output of a Nd:YAG laser and its second-harmonic pumped ∼1.2µmKTiOPO4 optical parametric oscillator. A wide tuning range of 7.9-17.5 µm (>1.14 octave) was achieved, which reached the upper limit of the BaGa4Se7 transparency region. The spectra and pulse widths, input-output relationship, beam profile, wavelength tolerance, and angular acceptance of the phase-matching were characterized in detail. This presented coherent source can potentially be applied in multiple gas analyses and spectral imaging.

Broadband, few-cycle mid-infrared continuum based on the intra-pulse difference frequency generation with BGSe crystals.

We demonstrate for the first time the generation of octave-spanning mid-infrared using a BGSe nonlinear crystal. A Cr:ZnS laser system delivering 28-fs pulses at a central wavelength of 2.4 µm is used as the pump source, which drives the intra-pulse difference frequency generation inside the BGSe crystal. As a result, a coherent broadband mid-infrared continuum spanning from 6 to 18 µm has been obtained. It shows that the BGSe crystal is a promising material for broadband, few-cycle mid-infrared generation via frequency down conversion with femtosecond pump sources.