Significance The mid-infrared (MIR) spectral region, particularly the 3-5 & micro;m atmospheric window, is of great importance in applications such as laser medicine, environmental monitoring, infrared countermeasures, and molecular spectroscopy. Nonlinear optical (NLO) crystals, which enable frequency conversion via processes including second harmonic generation (SHG), optical parametric oscillation (OPO), and difference frequency generation (DFG), are indispensable materials for generating coherent MIR radiation. Although classical crystals such as KDP, KTP, BBO, and LBO have been widely used in the visible to near-infrared range, their utility in the MIR region is limited by strong multi-phonon absorption and insufficient infrared transmission. Commercially available MIR NLO crystals, such as AgGaS2 and ZnGeP2, suffer from low laser-induced damage thresholds (LIDT), poor physicochemical stability, and demanding growth conditions (e.g., oxygen-and moisture-free environments). Therefore, developing oxide-based NLO crystals that integrate wide infrared transparency, strong nonlinearity, high LIDT, and favorable growth characteristics has become a critical research direction in photonic materials. Progress This review systematically summarizes recent advances in oxide NLO crystals for the MIR region, classifying them into four main systems: traditional infrared oxides, lanthanum gallium silicate-type compounds, compounds containing cations with second-order Jahn-Teller (SOJT) effects, and systems incorporating other active cations. Traditional oxide crystals such as LiNbO3 (LN), LiTaO3 (LT), KNbO3, KTiOPO4 (KTP), and KTiOAsO4 (KTA) exhibit excellent NLO coefficients and moderate LIDT but are limited by infrared absorption edges around 4-5 & micro;m due to strong M-O bonds. Recent work on periodically poled MgO-doped LN (PPMgLN) has enabled efficient OPO/DFG processes, yielding watt-level output around 3-4 & micro;m with high beam quality. Lanthanum gallium silicates (e. g., LGS, LGN, and LGT) possess a flexible structure with multiple cationic sites, allowing compositional tuning to balance band gap, SHG response, birefringence, and infrared cut-off. These crystals can be grown in large sizes by the Czochralski method and have been applied in electro-optic Q-switching and mode-locked lasers. Compounds with SOJT-active cations are particularly promising for enhancing NLO performance. Materials containing d0 transition metals (e.g., Mo6+, V-5+,V- W6+, and Ti4+) or lone-pair cations (e.g., Pb2+, Bi3+, Te4+, and I5+) often exhibit large structural distortions and strong SHG responses. Examples include KLTG and RLTG, Pb-based oxyhalides, Li2ZrTeO6, and tellurite molybdates/tungstates, which show extended infrared transparency up to 7-10 & micro;m, SHG efficiencies comparable to or exceeding those of benchmark crystals, and improved LIDT. Other cationic systems, such as those combining d10 ions (e.g., Zn2+ and Cd2+ ) with vanadate groups (e.g., Rb3ZnV4O12Br), illustrate strategies for band-gap engineering and polar alignment to achieve balanced NLO performance. Key material design strategies are discussed, including: extending the infrared cut-off by introducing heavy atoms and weakening chemical bonds; enhancing SHG coefficients through SOJT-driven distortions and optimal arrangement of NLO-active units; balancing phase-matching requirements and walk-off effects via birefringence control; and employing quasi-phase matching (QPM) and non-critical phase matching (NCPM) to overcome limitations of birefringent phase matching. Conclusions and Prospects Oxide NLO crystals offer distinct advantages in crystal growth feasibility, environmental stability, and damage resistance over chalcogenide counterparts. However, achieving simultaneous optimization of wide infrared transmission (especially beyond 5 & micro;m), large NLO coefficients, high LIDT, and suitable birefringence remains challenging due to inherent trade-offs among these properties. Future research should increasingly integrate first-principles calculations, high-throughput screening, and machine learning to accelerate the discovery of new compositions and crystal structures. Furthermore, advancing crystal growth techniques for difficult-to-synthesize systems and developing hybrid materials that combine oxide stability with extended infrared transparency will be essential for realizing practical high-performance MIR NLO devices. Collaborative efforts in computational design, synthetic exploration, and device integration are expected to drive the development of next-generation oxide-based infrared photonic materials.

Abstract A novel gallium-based nonlinear optical (NLO) crystal without the toxic Be element, K3Sr3Li2Ga4B6O20F, was successfully realized using a chemical substitution strategy from Sr2Be2B2O7 (SBBO), in which Sr2+ and Be2+ cations are cosubstituted by K+ and Sr2+ and Li+ and Ga3+, respectively. In the structure, BO3, GaO4, (Ga/Li)O4, GaO3F, and LiO3F units are connected by shared oxygen atoms to form two distinct types of single layers, [LiGa2B3O10F] and [(Ga/Li)3B3O11]. Two adjacent [LiGa2B3O10F] single layers are coupled through bridging oxygen atoms and fluorine atoms to generate the [Li2Ga4B6O20F] double layer, and two identical [(Ga/Li)3B3O11] single layers are interconnected by sharing apical oxygen atoms to produce the [(Ga/Li)6B6O21] double layer. These two distinct types of double layers are alternately stacked perpendicular to the ab plane, forming a 3D crystal network. The powder second-harmonic-generation efficiency of the title crystal at a fundamental wavelength of 1064 nm is 1/4 of that of KH2PO4. It exhibits a broad band gap of 5.9 eV and moderate birefringence (0.073 @ 532 nm). Simulation results reveal that the shortest phase-matching threshold is 232 nm, which can be utilized for Nd-based laser fourth-harmonic-generation output.

Infrared (IR) nonlinear optical (NLO) materials have long been sought, but their development has been hindered by conflicting microstructural requirements, particularly the challenge of simultaneously achieving a strong second-harmonic generation (SHG) response, sufficient birefringence, and a wide bandgap. We herein propose a "rigid-flexible cooperative bond-engineering" strategy that precisely modulates the band structure and module configuration by rationally assembling d 10 cations and alkali metal cations to construct a unique rigid-flexible hybrid bonding framework. Guided by this concept, a series of novel IR NLO materials A2Li4-xAgx(PS4)2 and A2Li4-xAgx(AsS4)2 (A = Na, K) were successfully designed and synthesized. Experimental and computational studies reveal that the flexible components of the hybrid bonds drive the formation of distorted motifs and endow them with high field responsiveness, while the rigid components raise the conduction band minimum to a higher energy. As a result, the title compounds have achieved a rare balance between strong SHG response, large birefringence, and relatively wide bandgap. This work not only provides a series of promising IR NLO materials but also opens a new avenue for the customized design of novel optoelectronic materials.

In Li 3 GaTe 4 O 11 , high framework density and small void space confine small Li + cations, enabling pronounced steric effects that regulate Te 4+ lone-pair orientations. Small cations act as “structural mediators” in high-dimensional tellurites.

Owing to the outstanding photoluminescent (PL) characteristics including the broadband emission and high PL quantum yield, metal halides with robust crystal structures have promising applications in information anticounterfeiting and encryption, optical temperature sensing, optoelectronic devices, etc. However, their intrinsic luminescent thermal quenching severely limits their practical applications at high temperatures. Herein, a novel three-dimensional all-inorganic metal halide CsCd5Cl11 with the distorted octahedra was successfully synthesized. Upon doping with Sb3+, the CsCd5Cl11:Sb3+ halides exhibit antithermal quenching orange PL over the temperature range of 260-440 K. Their integrated PL intensity at 440 K can reach approximately 610% and 320% of those at 260 and 300 K, respectively. Structure and thermoluminescence analysis indicates that the luminescent antithermal quenching originates from the intrinsic structural rigidity of the octahedra in the structure and the trapped excitons absorbing thermal energy to escape from the shallow traps, thereby compensating for the thermally induced nonradiative transition losses of excitons. Moreover, the CsCd5Cl11:Sb3+ underlying PL luminescence mechanism was systematically elucidated by first-principles calculations. Enlightened by the adaptable optical properties, the platform of the developed halides for high-temperature visualization and anticounterfeiting is demonstrated. This work not only presents a new halide material with antithermal quenching but also broadens versatile applications in optoelectronic fields.

In Li3GaTe4O11, asymmetric TeOn (n = 3, 4) polyhedra and GaO6 octahedra form a three-dimensional (3D) tunnel framework, with Li+ cations occupying the sixteen-membered channels. The high framework density (12.1) and small void space (3.75%) of Li3GaTe4O11 collectively demonstrate that Li+ cations are spatially confined within the tunnels by the dense, compact framework. This confinement amplifies their steric influence, thereby enabling effective regulation of TeOn (n = 3, 4) with large local dipole moments. The real-space projections of the valence band maximum (VBM) and conduction band minimum (CBM) also reveal that small interstitial Li+ cations can act as "structural mediators" to tune the arrangement of TeOn (n = 3, 4) units, thereby influencing the framework of Li3GaTe4O11. This work opens a new avenue for exploring non-centrosymmetric (NCS) materials through resident cation engineering.

In recent years, relaxor ferroelectric single crystals (1-x)Pb(Mg1/3Nb2/3)O-3-xPbTiO(3) (PMN-xPT) have been widely investigated and attracted attention due to their excellent piezoelectric and electromechanical properties. Yet, their internal microdomain structure results in low transmittance, which limits their application in optical field. In this paper, 1% mol Sm-doped 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO(3) (Sm-PMN-30PT) crystals were successfully grown by a modified Bridgman technique. Sm-3(+) induces nanoscale structural disorder in the crystal, significantly enhancing contribution of polar nanoregions (PNRs) to macroscopic properties. Piezoelectric, ferroelectric, and electro-optic performances were enhanced. Electromechanical coupling coefficients k(33) and k(t) reach 66% and 48%, respectively. Piezoelectric coefficient d(33) value of Sm-PMN-30PT crystal is 5800-6100 pC/N with 55 Hz test frequency. After poled along [110] direction, transmittance along [001] direction reaches 56%. Effective electro-optic coefficient gamma(c) is 240 pm/V at room temperature, and it can reach 1293 pm/V near the Curie temperature (109 degrees C). These results show that Sm-PMN-30PT single crystals could be ideal materials for piezoelectric and electro-optic devices.

Tunable fluorescence emission endows luminescent materials with immense applications in optoelectronic devices and information security. Herein, broad-wavelength photoluminescence (PL) tuning from yellow to white to blue is realized in a zero-dimensional (0D) K4CdCl6 halide via a facile Rb/Sn codoping strategy. Interestingly, K4CdCl6 and isostructural Rb4CdCl6 halides exhibit yellow and blue PL emissions via the homovalent doping of Sn2+, respectively. The relative intensity ratio of yellow and blue emission bands can be effectively regulated in K4CdCl6:Sn2+ halides by Rb+ alloying; accordingly, the continuous luminescence color tuning covering yellow, white, and blue spectral regions was successfully achieved. Density functional theory calculations further verify that the doping of Sn2+ leads to the appearance of an impurity energy level within the band gap, accelerates electronic transitions, and ultimately enhances PL emission efficiency. On the basis of this rationally designed tunable luminescent system, the three-level color-coded intelligent password devices, combining multimode optical anticounterfeiting and digital information encryption, were developed. This work not only develops a reliable ion doping strategy for luminescence modulation in the metal halides but also expands their diversified application prospects in advanced optoelectronic fields.

In recent years, metal halide X-ray detectors have attracted considerable attention owing to their high sensitivity, suitable band gap, and facile solution processability. Compared with polycrystalline thin films, single-crystal scintillators possess lower defect state density, higher carrier mobility-lifetime product, and superior uniformity and stability due to the absence of grain boundaries, which is conducive to enhancing the performance of X-ray detectors. In this work, a prismatic RbCdCl3:Mn2+ single crystal with dimensions of 12 & times; 11 & times; 11 mm(3) was successfully grown using an aqueous solution method, and its optical and luminescent performances were investigated. The crystal features a wide optical transmission window in the visible and infrared wavelength regions, with a short ultraviolet absorption edge at 229 nm. Under excitation with 241 nm light, the RbCdCl3:Mn2+ crystal emits a broad red fluorescence band centered at 625 nm, exhibiting a photoluminescence (PL) decay lifetime of 21.38 ms and a photoluminescence quantum yield (PLQY) of 22.45%. This emission originates from the T-4(1)(G) -> (6)A(1)(S) electron transitions of Mn2+ ions. Notably, under X-ray excitation, the RbCdCl3:Mn2+ crystal shows a red emission centered at 571 nm, with a full width at half maximum of 68 nm, which is considerably narrower than that of the Bi4Ge3O12 (BGO) crystal (154 nm). The measured light yield of the RbCdCl3:Mn2+ crystal is approximately 10,400 photons/MeV, corresponding to 1.3 times that of the commercial BGO crystal. Moreover, the RbCdCl3:Mn2+ wafer exhibits an outstanding X-ray imaging spatial resolution of 18 lp/mm. These exceptional performance characteristics demonstrate that the RbCdCl3:Mn2+ crystal possesses considerable promises for X-Ray detector applications.

X-ray detectors are extensively utilized in industrial inspection, cosmic physics research, medical diagnosis, and imaging. In this study, we have explored the potential application of Rb4Li2TiOGe4O12 (RLTG) crystals as a promising X-ray detection material and investigated their anisotropic detection performances. Under a 200 V bias voltage and 40 keV irradiation conditions, the Z-direction RLTG detector exhibits a sensitivity of 39.0 mu C Gy cm-2, which is higher than that of the X-direction RLTG detector (33.3 mu C Gy cm-2). Furthermore, the detection limit of the Z-direction RLTG-based detector (0.927 nGy s-1) is notably lower than that of the X-direction detector (3.63 nGy s-1). This work examines the anisotropic X-ray responses of RLTG crystals, demonstrating that RLTG is a potential X-ray detection material whose performance can be enhanced by selecting a specific orientation.

The anisotropic X-ray responses of RLTG were systematically investigated based on X - and Z -cut RLTG detectors. The ultralow detection limit of 0.927 nGy s −1 for the Z -cut RLTG detector was a record value among all reported X-ray detectors.

LiB3O5 (LBO), as an important nonlinear optical crystal, has been widely utilized in the field of solid-state laser frequency conversion. In this study, the batch growth of large-sized LBO crystals at 4 kg level was achieved from the high temperature solution method using a home-made scale electric resistance furnace. The as-grown LBO has a high optical homogeneity of 5 & times; 10-6. The laser-induced damage threshold of large aperture LBO crystal with the dimensions of 50 & times; 50 & times; 5 mm3 was up to 60.89 J/cm2 at 355 nm.

K3B6O10Br (KBB) is a novel nonlinear optical (NLO) crystal for 355 nm ultraviolet (UV) laser generation. The second-order NLO properties of KBB crystals have been widely studied, however, their third-order NLO properties have rarely been investigated. The third-order NLO properties of crystals determine the ability of the laser output for frequency conversion. In this work, the nonlinear absorption coefficient (β) and nonlinear refractive index (n2) of KBB were investigated by Z-scan technique under different intensities at 532 and 355 nm. We observed that β increased with the increasing laser intensity at both 532 and 355 nm, suggesting that KBB is mainly associated with defect-state-assisted sequential two-photon absorption (TPA) process. The value of n2 > 0 indicates that KBB is a self-focusing crystal, and n2 increases with increasing laser intensity. The correlation of shorter wavelengths with higher β and n2 values confirms the strong third-order nonlinear response of the KBB crystal in the UV band. Moreover, the transient absorption spectra demonstrated that KBB underwent excited-state absorption (ESA) under laser excitation. Building upon these findings, an energy-level model of defect-state-assisted sequential TPA in KBB is proposed.

Tunable fluorescence emission endows luminescent materials with immense applications in optoelectronic devices and information security. Herein, broad-wavelength photoluminescence (PL) tuning from yellow to white to blue is realized in a zero-dimensional (0D) K4CdCl6 halide via a facile Rb/Sn codoping strategy. Interestingly, K4CdCl6 and isostructural Rb4CdCl6 halides exhibit yellow and blue PL emissions via the homovalent doping of Sn2+, respectively. The relative intensity ratio of yellow and blue emission bands can be effectively regulated in K4CdCl6:Sn2+ halides by Rb+ alloying; accordingly, the continuous luminescence color tuning covering yellow, white, and blue spectral regions was successfully achieved. Density functional theory calculations further verify that the doping of Sn2+ leads to the appearance of an impurity energy level within the band gap, accelerates electronic transitions, and ultimately enhances PL emission efficiency. On the basis of this rationally designed tunable luminescent system, the three-level color-coded intelligent password devices, combining multimode optical anticounterfeiting and digital information encryption, were developed. This work not only develops a reliable ion doping strategy for luminescence modulation in the metal halides but also expands their diversified application prospects in advanced optoelectronic fields.

ABSTRACT In recent years, relaxor ferroelectric single crystals (1‐x)Pb(Mg 1/3 Nb 2/3 )O 3 ‐xPbTiO 3 (PMN‐xPT) have been widely investigated and attracted attention due to their excellent piezoelectric and electromechanical properties. Yet, their internal microdomain structure results in low transmittance, which limits their application in optical field. In this paper, 1% mol Sm‐doped 0.7Pb(Mg 1/3 Nb 2/3 )‐0.3PbTiO 3 (Sm‐PMN‐30PT) crystals were successfully grown by a modified Bridgman technique. Sm 3 + induces nanoscale structural disorder in the crystal, significantly enhancing contribution of polar nanoregions (PNRs) to macroscopic properties. Piezoelectric, ferroelectric, and electro‐optic performances were enhanced. Electromechanical coupling coefficients k 33 and k t reach 66% and 48%, respectively. Piezoelectric coefficient d 33 value of Sm‐PMN‐30PT crystal is 5800–6100 pC/N with 55 Hz test frequency. After poled along [110] direction, transmittance along [001] direction reaches 56%. Effective electro‐optic coefficient γ c is 240 pm/V at room temperature, and it can reach 1293 pm/V near the Curie temperature (109°C). These results show that Sm‐PMN‐30PT single crystals could be ideal materials for piezoelectric and electro‐optic devices.

Copper(I) halides are promising candidates for advanced optoelectronics, such as white-light-emitting diodes (WLEDs), scintillators, and photodetectors. Designing and synthesizing low-dimensional hybrid copper halides with blue-light excitation remains an enormous challenge. Herein, we have prepared two one-dimensional (1D) hybrid Cu(I)-based metal iodides, namely, (C6H7N)CuI and (C6H8N)CuI2 single crystals, by deliberately adjusting the ratio of the reactants 4-methylpyridine (4-MePy) and CuI. (C6H7N)CuI crystals are nonluminous, while (C6H8N)CuI2 crystals exhibit an unusual yellow emission with a broad excitation band in the range of 260-500 nm Furthermore, a prototypical WLED is fabricated by combining a commercial 430 nm blue chip and (C6H8N)CuI2 phosphor, which exhibits a correlated color temperature (CCT) of 5645 K and a CIE color coordinate of (0.329, 0.334), thus demonstrating potential application for white lighting. Remarkably, (C6H8N)CuI2 crystals emit red fluorescence upon adsorption of methanol but fail for all other alcohols. Notably, the initial yellow luminescence can be recovered upon volatilization of methanol from the crystals, thus achieving a reversible photoluminescence switching. This work not only presents an important reference of blue-light-excitable hybrid Cu(I)-based metal iodide phosphor for WLEDs but also provides an intriguing fluorescence sensor for the reversible detection of methanol.

The pursuit of second-order nonlinear optical (NLO) materials for mid-infrared (mid-IR, 3–5 μm) applications has been a significant research frontier due to their potential in optoelectronic technologies. In this review, we focus on noncentrosymmetric (NCS) molybdenum/tungsten tellurites incorporating d0 transition metal cations (Mo6+/W6+) and Te4+ cation with stereo-chemically active lone electron pair, which are prone to the second-order Jahn-Teller (SOJT) distortions. These compounds exhibit exceptional NLO effects, high optical transmittance in the 3–5 μm range, moderate birefringence, and ease of crystal growth in ambient conditions. Furthermore, they possess wider band gaps compared to conventional mid-IR NLO materials like metal phosphides and chalcogenides, enhancing their laser damage resistances. We categorize the notable NCS molybdenum/tungsten tellurites into nine series based on their cationic types, concentrating on ternary and quaternary systems. The review outlines their synthesis methods, crystal structures, growth techniques, and physical properties, with an emphasis on the relationship between SOJT-active units, NCS structures, and NLO performances. This work provides a clear perspective on the understanding of these materials and aims to accelerate the exploration of high-performance mid-IR NLO crystals to meet the increasing technological demands.

The fluorescence intensity ratio (FIR) technique has emerged as a pivotal approach in non-contact optical thermometry, offering substantial application potential. In this study, Y3Al5O12 (YAG) phosphors co-activated with Ce3+ and Cr3+ ions were successfully synthesized through a high-temperature solid-state reaction method. Spectroscopic analysis revealed efficient energy transfer between the dopant ions, manifesting as distinct luminescence intensity variations compared to their single-doped counterparts. Under 450 nm excitation, the system demonstrates prominently yellow-green emission from Ce3+ ions and well-resolved red emission from Cr3+ ions, ensuring excellent spectral discrimination. Capitalizing on the differential thermal response between the two luminescent centers, the developed system achieves remarkable temperature sensing performance. The FIR method yields a maximum relative sensitivity of 0.845 % K-1, maintaining sensitivity above 0.6 % K-1 throughout the 340-580 K range, coupled with exceptional temperature resolution below 0.03 K. Furthermore, we have engineered a novel FIR thermometry system incorporating 3D-printed spectral modules and fiber-optic phosphor encapsulation. The integrated system demonstrates superior sensing capabilities, reaching sensitivity values of 1.23 % K-1 in Cr3+ lifetime mode and 4.64 % K-1 in FIR mode. These results indicate that Ce/Cr codoped YAG has exceptional potential in multimodal optical thermometry applications.

Mid-infrared (MIR) nonlinear optical (NLO) crystals have long been pursued, yet the realization of a balance among key performance metrics remains a formidable challenge. Herein, we propose a band-orientation co-anchoring strategy to precisely regulate the band structures and module spatial arrangement for the rational design of high-performance MIR NLO crystals. Leveraging the deep-ultraviolet NLO KBe 2 BO 3 F 2 (KBBF) as a template, a novel MIR NLO Rb 3 ZnV 4 O 12 Br (RZVB) crystal was successfully obtained through multimodule substitution. Importantly, the tailored tetrahedral hybridization mode effectively anchors the conduction band minimum—dominated by d 0 cations—at a higher energy level and suppresses d–d transitions. Concurrently, structural confinement within the KBBF-derived lattice enforces the optimal alignment of distorted tetrahedra. Furthermore, RZVB displays an unprecedented second harmonic generation (SHG) enhancement mechanism, arising from a unique cross-module electron transfer. Consequently, RZVB exhibits superior linear and NLO performances, including a wide bandgap (3.25 eV), high laser threshold damage (1.07 GW/cm 2 @1064 nm), broad transmission window (0.382–7.6 µm), moderate birefringence (0.06@589.3 nm) and the strongest SHG response (7.7 × KDP@1064 nm and 1.45 × AGS@2.09 µm) among vanadates with bandgap exceeding 3 eV. This work presents a high-performance MIR NLO crystal and establishes a bottom-up, broadly applicable design paradigm for the tailored development of next-generation crystalline materials.