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We briefly review recent results on photoemission spectroscopy based on the deep and vacuum ultraviolet diode pumped solid-state lasers which we have developed. Cascaded second harmonic generation with the nonlinear crystal KBe2BO3F2 (KBBF) is used to generate deep ultraviolet and vacuum ultraviolet laser radiation, which complements traditional incoherent light sources such as gas discharge lamps and synchrotron radiation, and has greatly improved resolution with respect to energy, momentum, and spin of photoemission spectroscopy. Many new functions have been developed with the advantages of high photon energy, narrow linewidth, high photon flux density, and so on. These have led to the observation of various new phenomena and the amassment of new data in the fields of high temperature superconductivity, topological electronics, Fermi semi-metals, and so forth. These laser systems have revived the field of photoemission spectroscopy and provided a new platform in this frontier research field.

The deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystal NaBe2BO3F2 (NBBF) is grown by the spontaneous nucleation method with the dimensions up to 20 × 20 × 1.5 mm3. The crystal structure was determined using single-crystal x-ray diffraction analysis and its space group was defined as C2, which belongs to the biaxial class. The optical properties including the transmittance and the NLO coefficients were first determined. The band structure and density of states of NBBF were also calculated. Based on the calculated refractive indices, the Sellmeier equations were fitted to evaluate its ability for deep-UV laser generation. Our results suggest that NBBF is a viable deep-UV NLO crystal.

A new sodium-rich fluorooxoborate Na3B7O11F2 features a novel large [B14O24F4] ring formed by two corner-shared [B7O13F2] groups, each containing two [B3O7] anionic units linked by a [BO2F2] group. The band gap of the title compound calculated by using first-principles calculations is 7.69 eV (∼161 nm) and the birefringence refractive index is 0.083 at 193 nm. The results show that it is potentially an excellent deep-ultraviolet (DUV) birefringence crystal.

A Hg-based ternary infrared nonlinear optical (NLO) material, Hg2GeSe4, with the defect diamond-like (DL) structure was systematically investigated for the first time. The experimental results show that Hg2GeSe4 exhibits an enhanced second harmonic generation (SHG) response about 2.1 times that of the normal DL selenide AgGaSe2 ( d36 = 33 pm/V) at the particle size of 150-200 μm, as well as good phase-matchable ability. Moreover, theoretical analysis reveals that the nonbonding electrons around Se atoms in the defect DL structure make a dominant contribution to the improvement of the NLO property: d36 = 78.83 pm/V and Δ n = 0.11. This study highlights the promise of electronic engineering strategies and opens new avenues toward the design of new infrared NLO crystals with high performance.

For the explorations of deep ultraviolet (DUV) nonlinear optical (NLO) borates, a type of important optoelectronic material, the (BO3 )3- group has been long regarded as the sole microscopic optically-active unit, and toxic Be-containing raw materials are frequently-adopted. Herein, a new DUV NLO crystal, Cs2 Al2 (B3 O6 )2 O (CABO), was designed and synthesized by simultaneously replacing the (BO3 )3- groups and Be2+ cations for (B3 O6 )3- units and Al3+ cations in Sr2 Be2 (BO3 )2 O, which possesses a favorable structure, through a chemical co-substitution approach. CABO exhibits a considerable DUV NLO capability because of the wide band gap and large birefringence originating from the [Al2 (B3 O6 )2 O]2- double layers. Remarkably, CABO melts congruently and does not contain the toxic beryllium, which is favorable for bulk-size crystal growth and practical applications.

NaSr3Be3B3O9F4 (NSBBF) as a new UV nonlinear optical crystal has aroused great interest in recent years. This study investigates the thermo-physical properties of NSBBF, including thermal expansion, thermal diffusivity, thermal conductivity and specific heat, which are important parameters for applications. The specific heat of NSBBF is comparable to that of CsLiB6O10 (CLBO) and larger than that of β-BaB2O4 (β-BBO), indicating that NSBBF has a very high laser damage threshold. The thermal expansion coefficients of NSBBF are determined as α a = 1.05 × 10−5 K−1 and α c = 1.34 × 10−5 K−1, exhibiting much smaller anisotropy than those of CLBO and β-BBO. The thermal diffusivity and conductivity of NSBBF are also obtained in the temperature region from 323 to 573 K, showing comparable anisotropies to β-BBO. All these results show that NSBBF is suitable for high-power UV laser generation.

Demands for UV birefringent materials are growing dramatically owing to the rapid development of ultraviolet technology. Here, a new family of UV birefringent materials, Ba2Mg(C3N3O3)2 (BMCY) and Ba2Ca(C3N3O3)2 (BCCY), have been successfully discovered. It is the first time that the excellent birefringent properties of cyanurates have been studied. These materials exhibit an extremely large birefringence (Δn = 0.728–0.351 and 0.771–0.346 from 230 nm to 800 nm for BMCY and BCCY, respectively), much larger than that of the commercial UV birefringent crystal α-BaB2O4 (α-BBO) (Δn = 0.12@532 nm). Our study indicates that the impressive optical properties of BMCY and BCCY stem from the strengthened optical anisotropy of the planar (C3N3O3)3− group compared with the isoelectronic (B3O6)3− group. Besides, its congruent-melting properties make it feasible to grow a bulk crystal by the Bridgman–Stockbarger technique. The extraordinary properties of BMCY and BCCY may shed light on a new path to explore birefringent materials for practical application.

A new fluorocarbonate, Na3Zn2(CO3)3F, was synthesized using a subcritical hydrothermal method. Na3Zn2(CO3)3F crystallizes in the space group C2/c with a sandwich-like framework in which the stacked [Zn(CO3)]∞ layers are connected with one another by bridging F atoms and [CO3] groups alternately. Interestingly, each Zn atom is surrounded by one F atom and four O atoms, forming a distorted [ZnO4F] trigonal bipyramid, which is observed for the first time in the carbonate system. Na3Zn2(CO3)3F has high transparency in a wide spectral region ranging from UV to mid IR with a short ultraviolet absorption edge (∼213 nm). First-principles calculations revealed that Na3Zn2(CO3)3F possesses a large birefringence (Δn = 0.11, λ = 589 nm), which is mainly contributed by the coplanar arrangement of [CO3] groups in the ab plane. Na3Zn2(CO3)3F might find applications as a UV birefringence crystal.

Two new polyborates, BaBe2B2O6 and SrBe2B2O6, in a three-dimensional (Be2B2O6)∞ network featuring KBBF-type two-dimensional planes are synthesized. Compared with KBBF, both compounds possess comparable optical birefringence and deep-ultraviolet (deep-UV) cutoff edges and exhibit better bulk growth habits owing to their three-dimensional networks, which make them applicable deep-UV optical materials.

Molecular structural design is a compelling strategy to develop new compounds and optimize the crystal structure by atomic-scale manipulation. Herein, two fluorine–beryllium borates, BaMgBe2(BO3)2F2 and BaCaBe2(BO3)2F2, have been rationally designed to overcome the structural instability problems of Sr2Be2B2O7 (SBBO). When relatively large Ba atoms were introduced, the [Be6B6O15]∞ double layers of SBBO were successfully broken, generating flexible [Be3B3O6F3]∞ single layers. Also, the strategy adopted in this work has many implications in understanding the structural chemistry and designing novel optical functional materials in a beryllium borate system.

The ongoing discoveries and studies of novel topological quantum materials have become an emergent and important field of condensed matter physics. Recently, HfTe5 ignited renewed interest as a candidate of a novel topological material. The single-layer HfTe5 is predicted to be a two-dimensional large band gap topological insulator and can be stacked into a bulk that may host a temperature-driven topological phase transition. Historically, HfTe5 attracted considerable interest for its anomalous transport properties characterized by a peculiar resistivity peak accompanied by a sign reversal carrier type. The origin of the transport anomaly remains under a hot debate. Here we report the first high-resolution laser-based angle-resolved photoemission measurements on the temperature-dependent electronic structure in HfTe5. Our results indicated that a temperature-induced Lifshitz transition occurs in HfTe5, which provides a natural understanding on the origin of the transport anomaly in HfTe5. In addition, our observations suggest that HfTe5 is a weak topological insulator that is located at the phase boundary between weak and strong topological insulators at very low temperature.

Large transparent KBBF crystal up to 70×60×2.5 mm 3 has been successfully grown with a modified KF-B 2 O 3 flux system. The optimal viscosity of the high temperature solution was determined and the effect of B 2 O 3 was discussed. The volatile contents from the growth solution were mainly composed of BeO, BF 3 and KF as determined by TG-MS. Photoluminescence at room temperature was studied for KBBF family crystals at an excited wavelength of 230 nm. The emission peak at 369 nm was ascribed to Zn impurity.

A new alkali tin(II) halide compound, Na3Sn2F6Cl, is synthesized by hydrothermal method. This compound crystallizes trigonally in space group of R-3c (167), and processes a zero-dimensional (0D) structure consisted of Na+ cations, Cl− anions and the isolated [SnF3]- trigonal pyramids in which the stereochemically active 5s2 lone pair electrons are attached to the Sn2+ cations. Interestingly, the [SnF3]− trigonal pyramids are parallel arranged in the a-b plane, while oppositely arranged in line with rotation along the c- axis. Moreover, the energy bandgap, thermal stability and electronic structure of Na3Sn2F6Cl are characterized and the results reveal that this compound has and indirect bandgap of 3.88 eV and is stable under 270 °C.

A new perovskite stoichiometric alkali tin dihalides compound, Cs3Sn3F2Cl7, is synthesized by a hydrothermal method. This compound belongs to the monoclinic space group of P21/c with cell parameters of a = 9.5645(4) A, b = 14.2057(7) A, c = 13.5828(6) A, and β = 93.2450(10)°. Unlike the common perovskites in which octahedra are interconnected to be a three-dimensional network, Cs3Sn3F2Cl7 possesses a zero-dimensional structure consisting of Cs+ cations, isolated [SnCl3]− trigonal pyramids, and dimer structural units [Sn2F2Cl4]2–; the latter microscopic unit is found for the first time. The thermal stability and UV–vis–NIR diffuse reflectance spectroscopy in Cs3Sn3F2Cl7 are measured, and the electronic structure is calculated. Interestingly, the 5s2 lone-pair electrons on Sn2+ cations are stereochemically active, which results in a pretty good photocatalytic activity of the title compound.

Metal sulfides with diamond-like (DL) structures are systematically investigated in this study for nonlinear optical (NLO) applications in the mid-infrared (mid-IR) spectral region. All the selected materials are characterized by a 3D framework connected by various sulfide tetrahedral units, such as [PS4], [SiS4], [GeS4], [GaS4], [InS4], [CdS4], [ZnS4], [LiS4], and [AgS4]. The linear and nonlinear optical properties of these materials are analyzed on the basis of first-principles calculations. It is concluded that it is relatively easy to achieve good balance between the bandgap and the NLO effect. Additionally, moderate birefringence Δn (∼0.03–0.10) is required for practical mid-IR NLO applications. In particular, several metal sulfides with normal DL and defect DL structures, exhibiting superior mid-IR NLO properties, are highlighted. These provide an instructive guide for the design and discovery of new materials with good mid-IR NLO performance.

Spin–orbit interaction entangles the orbitals with the different spins. The spin–orbital-entangled states were discovered in surface states of topological insulators. However, the spin–orbital-entanglement is not specialized in the topological surface states. Here, we show the spin–orbital texture in a surface state of Bi(111) by laser-based spin- and angle-resolved photoelectron spectroscopy (laser-SARPES) and describe three-dimensional spin-rotation effect in photoemission resulting from spin-dependent quantum interference. Our model reveals that, in the spin–orbit-coupled systems, the spins pointing to the mutually opposite directions are independently locked to the orbital symmetries. Furthermore, direct detection of coherent spin phenomena by laser-SARPES enables us to clarify the phase of the dipole transition matrix element responsible for the spin direction in photoexcited states. These results permit the tuning of the spin polarization of optically excited electrons in solids with strong spin–orbit interaction., Spin–orbit coupling produces spin–orbital-entanglement in quasiparticle eigenstates. Here, Yaji et al. present a general description of spin–orbital-entangled states and establish a model for dipole transition based on spin-dependent quantum interference, that permits optical spin control.

A new ultraviolet nonlinear optical crystal, LiSr3Be3B3O9F4 (LSBBF), was obtained by the flux method. LSBBF is isostructural to NaSr3Be3B3O9F4 and inherits its brilliant [Be3B3O12F]10− structural units. It exhibits a considerable second-harmonic generation (SHG) intensity (∼2.2 × KDP) and short ultraviolet cut-off wavelength (∼175 nm). Further theoretical calculations revealed that the shortest SHG phase-matching wavelength of LSBBF is down to ∼245 nm. These results indicated that LSBBF has potential to be used for the 266 nm output of the fourth harmonic of Nd:YAG lasers.

Hydrothermally grown iodate, K8Ce2I18O53, featuring polar 0D [Ce(IO3)8]4- units, has shown the highest visible light driven (VLD) photocatalytic activity in metal iodates reported before. The enhanced VLD photocatalytic property results from a synergistic effect between the narrow band gap and polar 0D [Ce(IO3)8]4- units, as confirmed using first principles calculations.

A series of a novel beryllium-free KBBF family of nonlinear-optical materials AZn2BO3X2 (A = K, Rb and X = Cl; A = Na, K, Rb and X = Br) were successfully synthesized through molecular engineering design, and single crystals of AZn2BO3Cl2 (A = K, Rb) were grown by a spontaneous nucleation technique from self-flux systems. As a representative for the halogen KBBF family of crystals, KZn2BO3Cl2 features the infinite lattice layer [Zn2BO3Cl2]∞ made up of BO3 and ZnO3Cl anionic groups, and the in-layer BO3 groups are completely coplanar and well-aligned. Besides, KZn2BO3Cl2 exhibits high transmittance in the range of 300–2000 nm with a UV-transmission cutoff of around 200 nm according to transmission spectra. The compounds of AZn2BO3Cl2 (A = K, Rb) are both phase-matchable with powder second-harmonic-generation efficiencies of 1.3 and 1.17 times that of KH2PO4 for KZn2BO3Cl2 and RbZn2BO3Cl2, respectively, which are similar to that of KBBF.

In unconventional superconductors, it is generally believed that understanding the physical properties of the normal state is a pre-requisite for understanding the superconductivity mechanism. In conventional superconductors like niobium or lead, the normal state is a Fermi liquid with a well-defined Fermi surface and well-defined quasipartcles along the Fermi surface. Superconductivity is realized in this case by the Fermi surface instability in the superconducting state and the formation and condensation of the electron pairs (Cooper pairing). The high temperature cuprate superconductors, on the other hand, represent another extreme case that superconductivity can be realized in the underdoped region where there is neither well-defined Fermi surface due to the pseudogap formation nor quasiparticles near the antinodal regions in the normal state. Here we report a novel scenario that superconductivity is realized in a system with well-defined Fermi surface but without quasiparticles along the Fermi surface in the normal state. High resolution laser-based angle-resolved photoemission measurements have been performed on an optimally-doped iron-based superconductor (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$. We find that, while sharp superconducting coherence peaks emerge in the superconducting state on the hole-like Fermi surface sheets, no quasiparticle peak is present in the normal state. Its electronic behaviours deviate strongly from a Fermi liquid system. The superconducting gap of such a system exhibits an unusual temperature dependence that it is nearly a constant in the superconducting state and abruptly closes at T$_c$. These observations have provided a new platform to study unconventional superconductivity in a non-Fermi liquid system., 22 pages, 4 figures

The detailed information of the electronic structure is the key for understanding the nature of charge density wave (CDW) order and its relationship with superconducting order in microscopic level. In this paper, we present high resolution laser-based angle-resolved photoemission spectroscopy (ARPES) study on the three-dimensional (3D) hole-like Fermi surface around the Brillouin zone center in a prototypical qusi-one-dimensional CDW and superconducting system ZrTe3. Double Fermi surface sheets are clearly resolved for the 3D hole-like Fermi surface around the zone center. The 3D Fermi surface shows a pronounced shrinking with increasing temperature. In particular, the quasiparticle scattering rate along the 3D Fermi surface experiences an anomaly near the charge density wave transition temperature of ZrTe3 (~63 K). Signature of electron-phonon coupling is observed with a dispersion kink at ~20 meV; the strength of the electron-phonon coupling around the 3D Fermi surface is rather weak. These results indicate that the 3D Fermi surface is also closely connected to the charge-density-wave transition and suggest a more global impact on the entire electronic structure induced by CDW phase transition in ZrTe3.

We report a successful tuning of the hole doping level over a wide range in high temperature superconductor \(\hbox {Bi}_2\hbox {Sr}_2\hbox {CaCu}_2\hbox {O}_{8+\delta }\) (Bi2212) through successive in situ potassium (K) deposition. By taking high resolution angle-resolved photoemission measurements on the Fermi surface and band structure of an overdoped Bi2212 ( \(T_\mathrm{c}=76\) K) at different stages of K deposition, we found that the area of the hole-like Fermi surface around the Brillouin zone corner ( \(\pi \) , \(\pi \) ) shrinks with increasing K deposition. This indicates a continuous hole concentration change from initial \(\sim \) 0.26 to eventual 0.09 after extensive K deposition, a net doping level change of 0.17 that makes it possible to bring Bi2212 from being originally overdoped, to optimally-doped, and eventually becoming heavily underdoped. The electronic behaviors with K deposition are consistent with those of Bi2212 samples with different hole doping levels. These results demonstrate that K deposition is an effective way of in situ controlling the hole concentration in Bi2212. This work opens a good way of studying the doping evolution of electronic structure and establishing the electronic phase diagram in Bi2212 that can be extended to other cuprate superconductors.

A new mercury selenide BaHgSe2 was synthesized. This air-stable compound displays a large nonlinear optical (NLO) response and melts congruently. The structure contains chains of corner-sharing [HgSe3](4-) anions in the form of trigonal planar units, which may serve as a new kind of basic functional group in IR NLO materials to confer large NLO susceptibilities and physicochemical stability. Such trigonal planar units may inspire a path to finding new classes of IR NLO materials of practical utility that are totally different from traditional chalcopyrite materials.

The thermo-physical properties of the nonlinear optical (NLO) crystal K3B6O10Br (KBB) were experimentally investigated, including specific heat, thermal conductivity, coefficient of thermal expansion and refractive index. The specific heat of KBB is lower than that of LiB3O5 and higher than that of other borate NLO crystals, such as β-BaB2O4, CsLiB6O10 and CsB3O5, and KBB manifests a high damage threshold because of its lower temperature gradient during laser pulse irradiation. The thermal expansion coefficients were obtained as α x = 5.09 × 10−6 K−1 and α z = 2.39 × 10−5 K−1, showing weaker anisotropy than those of commonly used NLO crystals. The temperature-dependent Sellmeier dispersion equations of the refractive indices were also obtained, and the phase-matching angles for second harmonic generation (SHG) at temperatures of 313, 343, 373, 403 and 433 K which were calculated from these equations are in good agreement with the experimental values. All results are indicative of the KBB crystal as a novel promising NLO crystal for high power SHG.

Mixed nonlinear optical (NLO) crystals have been widely investigated to precisely optimize their optical properties by controlling the compositions, and have drawn immense attention in the laser science and technology field. Herein, a series of mixed NLO crystals K3B6O10Br1−xClx (x = 0, 0.21, 0.35, 0.47, 0.57, 0.75 and 1) were obtained using spontaneous nucleation and a bulk crystal of K3B6O10Br0.53Cl0.47 with the size 41 × 26 × 15 mm3 was successfully grown by a top seeded solution growth (TSSG) method. The NLO properties of the mixed crystals were studied by combination of second harmonic generation (SHG) tests and first-principles calculations. As a potential mixed crystal for practical application, the optical properties of the K3B6O10Br0.53Cl0.47 crystal including transparency range, refractive indices, the shortest SHG phase-matching wavelength and angle were investigated. The results indicate that the optical performance of the mixed crystals can be effectively adjusted to optimize practical high power laser output by tuning substitution compositions.

A new acentric borate La2Al4.68B8O22 crystal was synthesized by a solid state reaction and the single crystals were successfully grown with B2O3–LiF flux using the top-seeded solution growth method. The structure of the titled crystal features a two-dimensional layered structure composed of [BO4] and [AlO5] groups that are bridged by the skeletal structure of the [LaO6] polyhedron along the c axis. Optical properties including the transmittance spectrum, IR spectrum and refractive indices have been systematically evaluated for the first time.

A new sodium-rich fluorooxoborate Na3B7O11F2 features a novel large [B14O24F4] ring formed by two corner-shared [B7O13F2] groups, each containing two [B3O7] anionic units linked by a [BO2F2] group. The band gap of the title compound calculated by using first-principles calculations is 7.69 eV (∼161 nm) and the birefringence refractive index is 0.083 at 193 nm. The results show that it is potentially an excellent deep-ultraviolet (DUV) birefringence crystal.

The iron-based superconductors are characterized by multiple-orbital physics where all the five Fe 3$d$ orbitals get involved. The multiple-orbital nature gives rise to various novel phenomena like orbital-selective Mott transition, nematicity and orbital fluctuation that provide a new route for realizing superconductivity. The complexity of multiple-orbital also asks to disentangle the relationship between orbital, spin and nematicity, and to identify dominant orbital ingredients that dictate superconductivity. The bulk FeSe superconductor provides an ideal platform to address these issues because of its simple crystal structure and unique coexistence of superconductivity and nematicity. However, the orbital nature of the low energy electronic excitations and its relation to the superconducting gap remain controversial. Here we report direct observation of highly anisotropic Fermi surface and extremely anisotropic superconducting gap in the nematic state of FeSe superconductor by high resolution laser-based angle-resolved photoemission measurements. We find that the low energy excitations of the entire hole pocket at the Brillouin zone center are dominated by the single $d_{xz}$ orbital. The superconducting gap exhibits an anti-correlation relation with the $d_{xz}$ spectral weight near the Fermi level, i.e., the gap size minimum (maximum) corresponds to the maximum (minimum) of the $d_{xz}$ spectral weight along the Fermi surface. These observations provide new insights in understanding the orbital origin of the extremely anisotropic superconducting gap in FeSe superconductor and the relation between nematicity and superconductivity in the iron-based superconductors., 19 pages, 4 figures

For the explorations of deep ultraviolet (DUV) nonlinear optical (NLO) borates, a type of important optoelectronic material, the (BO

The structure of the superconducting gap in unconventional superconductors holds a key to understand the momentum-dependent pairing interactions. In superconducting FeSe, there have been controversial results reporting nodal and nodeless gap structures, raising a fundamental issue of pairing mechanisms of iron-based superconductivity. Here, by utilizing polarization-dependent laser-excited angle-resolved photoemission spectroscopy, we report a detailed momentum dependence of the gap in single- and multi-domain regions of orthorhombic FeSe crystals. We confirm that the superconducting gap has a twofold in-plane anisotropy, associated with the nematicity due to orbital ordering. In twinned regions, we clearly find finite gap minima near the vertices of the major axis of the elliptical zone-centered Fermi surface, indicating a nodeless state. In contrast, the single-domain gap drops steeply to zero in a narrow angle range, evidencing for nascent nodes. Such unusual node lifting in multi-domain regions can be explained by the nematicity-induced time-reversal symmetry breaking near the twin boundaries., The superconducting gap structure of FeSe remains a debated issue. Here, Hashimoto et al. report momentum dependence of the gap in single- and multi-domain regions of orthorhombic FeSe crystals, revealing an unusual node lifting of the gap structure in multi-domain regions.

In this paper, we report a nanosecond (ns) 165 nm deep-ultraviolet (DUV) laser with a maximum average power of 6.8 mW by a homemade cascaded second-harmonic generation (SHG) system.

With a newly developed diffusion-bonded KBe2BO3F2 prism-coupled device and by temperature stabilization of the device, we have achieved an average power of 200 mW at 177.3 nm through the sixth harmonic generation of a picosecond Nd:YAG laser. This is the highest power at 177.3 nm ever generated in any nonlinear optical crystals. A stable average power of 72.2 mW at 177.3 nm was also obtained with a small fluctuation of 3.2 % over a period of 30 min. Moreover, both the angle and temperature acceptances for the 177.3 nm generation were reported for the first time in this paper.

The family of metal thiophosphates is an important but long-ignored compound system of the nonlinear optical (NLO) materials with desirable properties for the mid-infrared (mid-IR) coherent light generation. In the present work, the mid-IR NLO capabilities of metal thiophosphate crystals are systematically investigated based on their structure-property relationship. The linear and nonlinear optical properties of these crystals are predicted and analyzed using the first-principles calculations. In particular, several metal thiophosphate compounds are highlighted to exhibit good mid-IR NLO performances, as supported by the primary experimental results. These candidates would greatly promote the development of the mid-IR NLO functional materials.

Isotropic negative area compressibility, which is very rare, is observed in KBBF and the related mechanism is investigated by combined high-pressure X-ray diffraction (XRD) experiments and first-principles calculations. The strong mechanical anisotropy leads to a large Poisson's ratio and high figure of merit for the acoustic-optics effect, giving KBBF potential applications as smart strain converters and deep-ultraviolet (DUV) acoustic-optic devices.

A widely tunable picosecond midinfrared (MIR) optical parametric amplifier (OPA) with wavelength ranging from 6.4 to 11 $\mu \text{m}$ based on the BaGa4Se7 crystal was demonstrated. The MIR OPA was pumped by a 30 ps 1064-nm Nd:YAG laser and injected by a KTiOPO4 (KTP)-based narrow-bandwidth widely tunable near-infrared seed. The wide spectral generation using type-I ( $\text{o}\,\,\to \,\, \text{e}\,\,+\,\,\text{e}$ ) critically phase-matched parametric amplification in BaGa4 Se7 was investigated. The output energies were measured to be $\sim 38~\mu \text{J}$ at 11 $\mu \text{m}$ and up to $\sim 100~\mu \text{J}$ at 6.4 $\mu \text{m}$ under pumping energy of $\sim 4.1$ mJ. Further, the type-I phase-matching angles of BaGa4Se7 crystal for idler light generation in the range of 6.4–11 $\mu \text{m}$ were experimentally measured and the discrepancy between the experiment and theoretical calculation was discussed.

Deep-UV nonlinear optical crystal RbBe 2 BO 3 F 2 (RBBF) grown by the hydrothermal method was found to have a centrosymmetric structure of R 3 ¯ c by powder X-ray diffraction. The structure of R 3 ¯ c has cell parameters of a = 4.44223(11) A and c = 39.6696(12) A in the lattice where the (BO 3 ) 3 − groups turn 60° along each (Be 2 BO 3 F 2 ) ∞ layer in the a–b plane and it is double size along the c -axis compared with standard R32 structure. As the examined RBBF shows a very low second harmonic generation capability, a stacking fault model was proposed that the hydrothermal RBBF is nonuniform with a mixture of both R32 and R 3 ¯ c structures using the DIFFaX program.

The new UV nonlinear optical crystal NaSr3Be3B3O9F4 with dimensions up to 13 × 14 × 20 mm3 was successfully grown by the top-seeded solution growth method from LiF–NaF–B2O3 flux for the first time. The crystalline morphological faces are mainly (101), while (021) or (32) faces may occur depending on the direction of the seed applied. The crystalline quality of the as-grown crystal was evaluated by the X-ray rocking curve with the full-width at half-maximum being 0.013°. By a minimum deviation method, the refractive indices are measured from 235 to 2325 nm and fitted by the Sellmeier equations. Furthermore, the hardness, density and chemical stability of the crystal were also reported.