Optical Crystals Research Articles

Ionic organic terahertz crystals: a perspective on design and solid-state phonon absorption

This perspective discusses various design strategies for high-performance ionic organic nonlinear optical crystals, their solid-state molecular phonon vibrations, as well as their influence on THz optical properties and THz applications.

High-pressure plasma etching up to 9 atm toward uniform processing inside narrow grooves of high-precision X-ray crystal optics

We developed a new etching technique using plasma generated at high pressure up to 9 atm. Operating at 9-atm pressure with a 30-μm-diameter wire electrode, we demonstrated the generation of well-ordered plasma at a narrow gap of ∼10 μm between the electrode and workpiece, and realized a high spatial resolution of <40 μm during processing. This technique should allow for the processing of high-precision X-ray crystal optical devices with compact and complex structures, such as a micro channel-cut crystal monochromator with an extremely narrow (sub-100 μm width) groove for realization of Fourier-transform-limited X-ray lasers with high intensity.

Ultra‐Broadband Organic THz Generators: Influence of Substituents on THz Phonon Vibrations in Phenolic Parallel‐Type Cation–Anion Assembly

AbstractOrganic terahertz (THz) crystals consist of polar molecules with various functional substituents, resulting in numerous molecular phonon vibrations in the THz frequency range. However, the effect of incorporating substituents into organic THz crystals on the resulting THz absorption is rather an unexplored area. In this work, by choosing an appropriate model system consisting of two benchmark organic nonlinear optical crystals, exhibiting an identical type of phenolic cation–anion assembly but different substituents, their ultra‐broadband THz wave generation characteristics are demonstrated and investigated. Compared to the unsubstituted crystals, the substituted crystals possess one more methoxy substituent on highly polar cationic chromophores. Their vibrational modes and THz absorption characteristics are investigated theoretically by periodic density functional theory and experimentally by ultra‐broadband THz spectroscopy up to 12 THz. In phenolic parallel‐type cation–anion assembled organic THz crystals, the existence of a methoxy substituent differently influences the molecular vibrations in low and high THz frequency ranges. In the lower THz frequency range, the unsubstituted crystals exhibit additional strong entire molecular vibrations, the so‐called cation‐on‐cation slipping vibrations, which do not exist in the substituted crystals. In contrast, the methoxy substituent leads to additional strong intramolecular vibrations in the higher THz frequency range.

“Three‐in‐One”: A New Hg‐Based Selenide Hg7P2Se12 Exhibiting Wide Infrared Transparency Range and Strong Nonlinear Optical Effect

AbstractThe exploration of high‐performance mid‐ and far‐infrared (IR) nonlinear optical (NLO) crystals is an urgent need. Herein, a new Hg‐based selenide Hg7P2Se12 (HPSe) is designed and synthesized by a rigid and flexible units coupled strategy. It is interesting that in the compound, three types of NLO‐active units, including linear [HgSe2], triangular [HgSe3], and tetrahedral (MSe4) (M = Hg/P) groups, simultaneously exist. It shows a wide transparency range (0.84–22.8 µm), large SHG response (1 × AGS), high laser‐induced damage threshold (≈2 × AGS), and a large birefringence (0.102 @ 2090 nm). Moreover, the compound melts congruently along with good crystal growth habits, and single crystals with a maximum size of ≈7 × 5 × 2 mm3 are obtained. Theoretical analyses confirm that the large NLO response originates from the synergistic effects of Hg–Se units, (PSe4) and nonbonding electrons. The results indicate that HPSe is a promising mid‐ and far‐IR NLO material, and inspire a path to finding new classes of IR NLO materials that are different from traditional chalcopyrite materials by grouping the linear, planar, and tetrahedral units.

Fluorination Strategy Towards Symmetry Breaking of Boron‐centered Tetrahedron for Poly‐fluorinated Optical Crystals

AbstractBorate crystals can be chemically and functionally modified by the fluorination strategy, which encourages the identification of emerging fluorooxoborates with a structure and set of characteristics not seen in any other oxide parents. However, the bulk of fluorooxoborates have been found accidentally, rational methods of synthesis are required, particularly for the infrequently occurring poly‐fluorinated components. Herein, we reported the use of bifluoride salts as a potent source of fluorine to prepare fluorooxoborates that contain rarely tri‐fluorinated [BF3X] (X=O and CH3) tetrahedra and eleven compounds were found. We identified the optical properties of the organofluorinated group [CH3BF3] and their potential for nonlinear optics for the first time. Among these, two non‐centrosymmetric components hold potential for the production of 266 nm harmonic coherent light for nonlinear optics, and more crucially, have the benefit of growing large size single crystals. Our study establishes experimental conditions for the coexistence of the diverse functional groups, enabling the production of poly‐fluorinated optical crystals.

ACu4AsS4 (A = K, Rb): Two infrared nonlinear optical crystals with strong second-harmonic generation responses originating from multiple chromophores

Infrared (IR) nonlinear optical (NLO) crystals are of great significance in modern laser technology, and metal chalcogenides are receiving increasing interest as promising IR NLO candidates. To explore new IR NLO materials with enhanced properties, two quaternary chalcogenides with multiple chromophores, KCu4AsS4 and RbCu4AsS4, were synthesized using the solvothermal method. KCu4AsS4 and RbCu4AsS4 feature 2D [Cu6As2S6]∞ anionic layers with multiple chromophores, and exhibit strong phase-matched second-harmonic generation (SHG) responses (1.8–2.0 × AgGaS2), wide band gaps, suitable birefringence (0.083 and 0.074 @ 2050 nm) and wide infrared transparency. The theoretical calculation results suggest that the strong SHG responses are ascribed to the 2D [Cu6As2S6]∞ anionic layers.

Stacking-Controlled Growth of rBN Crystalline Films with High Nonlinear Optical Conversion Efficiency up to 1.

Nonlinear optical crystals lie at the core of ultrafast laser science and quantum communication technology. The emergence of 2Dmaterials provides a revolutionary potential for nonlinear optical crystals due to their exceptionally high nonlinear coefficients. However, uncontrolled stacking orders generally induce the destructive nonlinear response due to the optical phase deviation in different 2D layers. Therefore, conversion efficiency of 2D nonlinear crystals is typically limited to less than 0.01% (far below the practical criterion of >1%). Here, crystalline films of rhombohedral boron nitride (rBN) with parallel stacked layers are controllably synthesized. This success is realized by the utilization of vicinal FeNi (111) single crystal, where both the unidirectional arrangement of BN grains into a single-crystal monolayer and the continuous precipitation of (B,N) source for thick layers are guaranteed. The preserved in-plane inversion asymmetry in rBN films keeps the in-phase second-harmonic generation field in every layer and leads to a record-high conversion efficiency of 1% in the whole family of 2D materials within the coherence thickness of only 1.6µm. The work provides a route for designing ultrathin nonlinear optical crystals from 2D materials, and will promote the on-demand fabrication of integrated photonic and compact quantum optical devices.

Fluorination Strategy Towards Symmetry Breaking of Boron-centered Tetrahedron for Poly-fluorinated Optical Crystals.

Borate crystals can be chemically and functionally modified by the fluorination strategy, which encourages the identification of emerging fluorooxoborates with a structure and set of characteristics not seen in any other oxide parents. However, the bulk of fluorooxoborates have been found accidentally, rational methods of synthesis are required, particularly for the infrequently occurring poly-fluorinated components. Herein, we reported the use of bifluoride salts as a potent source of fluorine to prepare fluorooxoborates that contain rarely tri-fluorinated [BF3 X] (X=O and CH3 ) tetrahedra and eleven compounds were found. We identified the optical properties of the organofluorinated group [CH3 BF3 ] and their potential for nonlinear optics for the first time. Among these, two non-centrosymmetric components hold potential for the production of 266 nm harmonic coherent light for nonlinear optics, and more crucially, have the benefit of growing large size single crystals. Our study establishes experimental conditions for the coexistence of the diverse functional groups, enabling the production of poly-fluorinated optical crystals.

Cd8(BO3)4SiO4: Metal Cation Inducing the Formation of Isolated [BO3] and [SiO4] Units in Borate Silicate.

The first compound of cadmium-borate silicate Cd8(BO3)4SiO4, crystallizing in space group P42/n (no. 86), has been successfully synthesized by the conventional high-temperature solution method and melts congruently. The zero-dimensional anionic groups of Cd8(BO3)4SiO4 are isolated [BO3] triangles and isolated [SiO4] tetrahedra which are filled in the framework formed by [CdO6] polyhedra. It has a moderate birefringence (Δn = 0.053 at 546 nm), which is measured by experiment and evaluated by first-principles calculations; meanwhile, the source of birefringence is revealed through the response electronic distribution anisotropy method. The UV-vis-NIR diffuse reflectance spectrum indicates that Cd8(BO3)4SiO4 possesses a wide optical transparency range, with a UV cutoff edge at about 254 nm. This work enriches the structure chemistry of borate silicates, and we discussed the possible methods for the exploration and synthesis of novel optical crystals possessing zero-dimensional anionic groups in the borate silicate system.

Preparation and up-conversion luminescence properties of Er: YbAG and Er: YAG single crystals

High quality optical single crystals of cubic Yb2.96Er0.04Al5O12 and Y2.96Er0.04Al5O12 were successfully prepared using the OFZ method. The 980 nm photon absorption intensity by Yb2.96Er0.04Al5O12 crystal is much larger than that in Y2.96Er0.04Al5O12 crystal, the transmission of Yb2.96Er0.04Al5O12 crystal is lower than that of Y2.96Er0.04Al5O12 crystal, because the density of Yb2.96Er0.04Al5O12 single crystal (6.604 g/cm3) is larger than that of Y2.96Er0.04Al5O12 single crystal (4.529 g/cm3). The up-conversion luminescence spectra of the crystals excited by 980 nm laser show several distinct groups of emission peaks at ∼544 and ∼556 nm, as well as ∼650 and ∼667 nm. The peak intensities of the up-conversion luminescence spectrum for the Yb2.96Er0.04Al5O12 crystal were much higher than those for the Y2.96Er0.04Al5O12 crystal. Overall, the results of the present study suggest that Yb2.96Er0.04Al5O12 single crystal may have advantages over Y2.96Er0.04Al5O12 single crystal for luminescence in solid-state lasers.

Ba2Ga8GeS16: new nonlinear optical crystals with high laser-induced damage threshold for parametric down-conversion in mid-IR

Ba2Ga8GeS16: new nonlinear optical crystals with high laser-induced damage threshold for parametric down-conversion in mid-IR

K4ZnV5O15Cl: A Congruent‐Melting Mid‐Infrared Nonlinear Optical Crystal Combined with Strong Second‐Harmonic Generation and High Laser‐Induced Damage Threshold

AbstractIt is extremely challenging to accomplish second‐harmonic generation (SHG) in the whole mid‐infrared (MIR) region for oxide crystals due to their short IR absorption cutoff wavelengths. Herein, a new MIR nonlinear optical (NLO) crystal of K4ZnV5O15Cl (KZVC) is designed and synthesized by the aliovalent substitution of [VIIIO5] with [ZnIIO4Cl] based on the K2(VO)(V2O7) matrix. KZVC features an ordered 2D [ZnV5O15Cl]∞ layered structure comprising [ZnO4Cl] / [VO5] square pyramids and [VO4] tetrahedron as functional units. Importantly, KZVC shows excellently balanced performances of strong SHG responses (12 × KDP @ 1064 nm, 1.1 × AgGaS2 @ 2.05 µm), broad IR transparency range (0.52‒10 µm), congruent‐melting behavior, and highest laser‐induced damage threshold (LIDT) (60 × AgGaS2 @ 1064 nm) among the reported vanadates. Most remarkably, KZVC represents the first example in the oxide‐based IR NLO crystals that can achieve phase matching in a wide wavelength range extending from 1.8 to 10 µm, fully covering the important MIR atmospheric windows (3‒5 µm).

A Ferroelectric Nonlinear Optical Crystal for Deep‐UV Quasi‐Phase‐Matching

AbstractPhase matching is indispensable for high efficiency of second‐order nonlinear optical (NLO) crystals. Thousands of these crystals rely on birefringence phase matching, whereas only a few of them are quasi‐phase matching without the restriction of birefringence. Herein, using oriented seeds inch‐sized LiNH4SO4 (LAS) single crystals are successfully grown. LAS is NLO‐active with a very short deep‐UV absorption edge of 171 nm and high laser damage threshold up to 1.47 GW cm−2 at 1064 nm, 10 ns, but its birefringence is merely 0.0078@532 nm, which is too small to realize birefringence phase matching in deep‐UV region. Fortunately, P−E hysteresis loop, variable‐temperature NLO tests, and ferroelectric domain observations confirm that LAS is ferroelectric. Furthermore, LAS exhibits small refractive index dispersion resulting in a relatively long periodic length of 1.4 µm and its single domain structure can be easily obtained by electrical poling. These attributes make LAS a scarce NLO candidate for deep‐UV quasi‐phase matching. This work highlights the longly overlooked potential of sulfates and provides new opportunities for deep‐UV NLO materials.

A Sharp Improvement of Sulfate's Birefringence Induced by the Synergistic Effect of Heteroleptic and Dimeric Strategies.

As for tetrahedron-based ultraviolet nonlinear optical crystals, it is so difficult to achieve a sufficient birefringence to satisfy the phase-matching condition. Thereover, it is necessary to greatly increase the polarizability anisotropy of tetrahedra. Meanwhile, the tetrahedra should be arranged as uniformly as possible. Based on these ideas, a new strategy that dimerizes the heteroleptic tetrahedra with heteroleptic atoms as bridges is proposed and implemented, resulting in a new compound, Na2S3O6, which crystallizes in a centrosymmetric monoclinic phase (phase-I) and a non-centrosymmetric orthorhombic one (phase-II). In the phase-I, a large birefringence of 0.103 @ 546nm comparable to those of many triangle-based crystals is confirmed experimentally, indicating the ability of [S3O6] group in improving the birefringence. Besides, the phase-II exhibits an improved birefringence of 0.056 @ 546nm and a moderate second harmonic generation response of 1.4×KH2PO4. In particular, the phase-II exhibits a typical phase-matching behavior under the irradiation of 532nm laser. Moreover, the excellent optical properties of [S3O6] group are further verified by theoretical calculations. These results completely illustrate the validity of the design strategy. Therefore, this work lights a new route for exploring novel ultraviolet nonlinear optical crystals.

Synthesis, growth, and optical applications of N, N′-bis (2-aminophenyl) ethane-1,2-diammonium picrate: An organic crystal

Futuristic organic nonlinear optical single AEDP crystal bearing the IUPAC nomenclature of N, N′-Bis(2-Aminoethyl) Ethane-1,2-Diammonium Picrate was grown via slow evaporation approach in aqueous solvent. The single crystal X-ray diffraction studies were employed for resolving lattice parameters of AEDP single crystal. The data regarding triclinic crystalline structure, bonds vibrations and symmetries of molecular vibrations of the prepared single crystal were realized adopting XRD, FT-IR and vibrational measurements techniques respectively. Evidences regarding excellent crystal optical transparency and maximum crystal non-linear optical properties for the prepared AEDP crystal in the range 200–2000 nm were revealed by Optical absorbance spectrum documented in the UV–vis–NIR wavelength range enhances the scope of unearthing and evolving potential nonlinear systems. Thermal stability and nonlinear optical (NLO) nature of AEDP crystal were inspected by applying differential thermal-thermo gravimetric analysis (DTA–TGA) and second harmonic generation test respectively. The promising features of the crystal leads towards application in the design of nonlinear optical devices and light-controlled electrical switches.

Quantum sensitivity of squeezed Schrodinger cat states

Theoretically it is shown that high quantum sensitivity of the squeezed state of the Schrodinger cat relative to the non-squeezed state can be achieved. For realization of squeezing the Schrodinger cat state, a parametric process in a nonlinear optical crystal is used. To visualize the squeezed state of the Schrodinger cat with and without a displacement, the values of the Wigner functions are calculated.

Dose-efficient in vivo X-ray phase contrast imaging at micrometer resolution by Bragg magnifiers

X-ray imaging enables the study of morphodynamic and physiological processes in living organisms. However, the required photon flux increases with the desired spatial resolution and with it the requirements for dose efficiency. We realize full-field imaging at micrometer resolution close to the highest possible dose efficiency. This is achieved by combining propagation-based phase contrast with Bragg crystal optics and a high-Z single-photon-counting detector, all designed for X-ray energies that allow minimal dose for a given image quality. We prove the superior imaging performance compared to conventional systems and, in particular, show a substantial increase in dose efficiency for high spatial frequencies that comprise the relevant high-resolution components of the image. We demonstrate the potential of the technique by a behavioral in vivo study of submillimeter-sized parasitoid chalcid wasps within their host eggs before and during emergence. The findings show that the technique opens up new possibilities for dose-sensitive studies at micrometer resolution, not only in life sciences but also in materials research.

Prediction of Nanoscale Water Meniscus Shape between Deliquescent KDP Crystal Optics and AFM Probe for Water-Dissolution Repairing.

KDP (KH2PO4) crystal optics are the key elements for megajoule laser facilities. Nanoscale surface defects would cause laser-induced damage when the optics are irradiated by a high-fluence laser (over 10 J/cm2). Dip-pen nanolithography (DPN) could be used to repair the nanoscale surface defects in the KDP optics by the water meniscus. The high humidity required for high-efficiency and soft KDP surfaces penetrated by the AFM probe brings challenges for accurately predicting the water meniscus shape to evaluate the effectiveness of the DPN water-dissolution repairing. The multisolutions of the Young-Laplace and Kelvin equations also lead to the wrong water meniscus shape. A theoretical model that takes the high humidity and the penetration of the AFM probes into account is developed. The parametrization Young-Laplace equations are adopted for the zero contact angle of the water films, and the AFM probe is treated as the combination of the cone and sphere for the water meniscus whose size is larger than the AFM tip radius under high humidity. The penetration of the AFM probe is modeled by Hertz theory. Both the water films (3.3 nm thickness at 99% relative humidity) and indentations (1.46 nm depth at 300 nN contact force) are non-negligible for the nanoscale water meniscus between the KDP surface and the AFM probe. Moreover, the rough-fine two-step method is proposed to lock the correct solution of the Young-Laplace and Kelvin equations. The effectiveness of the proposed model is verified by comparison with reported ESEM images and pull-off forces. In addition, the overgrowth dots on the KDP surface are compared with the water meniscus. The linear growth of the water meniscus would cause the linear growth of the overgrowth dot, which proves the proposed model could be used to guide the DPN water-dissolution repairing for the nanoscale surface defects in the KDP optics.

Na2CeF6: A Highly Laser Damage-Tolerant Double Perovskite Type Ce(IV) Fluoride Exhibiting Strong Second-Harmonic Generation Effect.

Perovskite structure compounds are significant candidates for designing new optical function materials due to their structural variability. Here, an inorganic tetravalent cerium fluoride, Na2CeF6, is derived from the perovskite structure through double-site cation co-substitution. Na2CeF6 crystalizes in the non-centrosymmetric space group . Edge-sharing connected NaF9 and CeF9 polyhedra build the whole 3D structure of Na2CeF6. Importantly, it represents the first Ce(IV) fluoride nonlinear optical (NLO) crystal and can produce a strong and phase-matchable second-harmonic generation (SHG) effect of ≈2.1 times that of KH2PO4 (KDP), making it the strongest among non-lone pair electron metal fluoride system. Further, it exhibits a high laser-induced damage threshold (LIDT) of 74.65-76.25MWcm-2, which is over 20 times that of AgGaS2. It also exhibits a wide transparent region (0.5-14.3µm). This work provides a facile route for exploring high-performance halide NLO materials.

Twist Phase Matching in Two-Dimensional Materials.

Optical phase matching involves establishing a proper phase relationship between the fundamental excitation and generated waves to enable efficient optical parametric processes. It is typically achieved through birefringence or periodic polarization. Here, we report that the interlayer twist angle in two-dimensional (2D) materials creates a nonlinear geometric phase that can compensate for the phase mismatch, and the vertical assembly of the 2D layers with a proper twist sequence generates a nontrivial "twist-phase-matching" (twist-PM) regime. The twist-PM model provides superior flexibility in the design of optical crystals, which can be applied for twisted layers with either periodic or random thickness distributions. The designed crystal from the twisted rhombohedral boron nitride films within a thickness of only 3.2 μm is capable of producing a second-harmonic generation with conversion efficiency of ∼8% and facile polarization controllability that is absent in conventional crystals. Our methodology establishes a platform for the rational design and atomic manufacturing of nonlinear optical crystals based on abundant 2D materials.