Potassium tantalate niobate (KTa_1-xNb_xO₃, KTN) crystals exhibit the largest quadratic electro-optic (EO, Kerr) coefficient among known materials. This extraordinary property enables electro-optic modulators to operate at driving voltages below 100 V, fundamentally overcoming the “kV bottleneck” that has historically constrained device miniaturization and energy efficiency in photonic systems. The technological implications are profound: such ultra-low-voltage operation is critical for portable biomedical imaging probes [e.g., endoscopic optical coherence tomography (OCT)], integrated optical communication chips, and compact three-dimensional (3D) sensing receivers where power consumption and footprint dictate feasibility. Beyond the Kerr effect, KTN’s tunable phase transitions and broadband optical transparency (250?5000 nm) offer unprecedented versatility in designing wavelength-agile devices for applications from near-infrared (NIR) telecommunications to mid-infrared (mid-IR) sensing. However, decades of research has revealed persistent barriers to practical adoption. The crystal’s infinite solid-solution behavior between KTaO₃ and KNbO₃ eliminates a congruent melting point, while sequential cubic→tetragonal→orthorhombic phase transitions during cooling induce severe compositional striations, cracking, and defects. Concurrently, challenges in understanding novel physical effects (e.g., space-charge-controlled deflection and ferroelectric domain engineering) have hindered practical applications. This review addresses these gaps by systematizing breakthroughs in crystal growth, property characterization, and device design, and synthesizes transformative advances in overcoming these challenges, establishing a unified framework to harness KTN’s full potential for next-generation photonics.

KTN’s infinite KTaO₃-KNbO₃ solid-solution system and multi-phase transitions demand innovative growth strategies. For Czochralski method (Fig. 3), undercooling isothermal growth technique stabilizes the growth interface by minimizing temperature fluctuations, and double-crucible real-time feeding system continuously supplies stoichiometric material, suppressing compositional segregation. Combined with both technology, device-grade crystal up to 35 mm×37 mm×58 mm with Ta/Nb compositional uniformity better than 10^-5/mm (Fig. 4) is achieved. For hydrothermal method, temperature-difference hydrothermal growth [Fig. 5(b)] enhances homogeneity by minimizing component segregation. Adding H₂O₂ eliminates blue color centers [Fig. 6(c)], though crystal size remains sub-centimeter.

Systematic studies reveal composition-property relationships. Lattice parameter a increases linearly with Nb content x (Table 1). At x=0.37?0.39, KTN achieves optimal dielectric constant (ε_r>50000) near room temperature. The Kerr coefficient can be directly enchanced via the Curie-Weiss law. Cu²⁺/Fe³⁺ doping (0.5%?1% atomic fraction) enhances dielectric response [Fig. 15(d)] and optical homogeneity via trace-impurity-induced dielectric enhancement (e.g., Fe³⁺-oxygen vacancy dipoles polarize microregions, shifting T_C by 60 ℃). Transverse deflection is observed in Cu∶KTN. A compositional gradient generates intrinsic refractivity variations. Coupling this with the Kerr effect under an external electric field produces laser deflection perpendicular to the field direction [Fig. 19(a)], contrasting conventional longitudinal models. Self-powered photoresponse in engineering “head-to-head” charged domain walls (CDWs) creates built-in fields for carrier transport. Cu∶KTN detectors achieve 5.23 mA/W responsivity (four orders higher than that of BaTiO₃) (Fig. 12) and 250?1030 nm spectral coverage [Fig. 13(d)].

Devices based on KTN crystal were well developed. Space-charge-controlled KTN beam deflectors achieve 250 mrad deflection at ±250 V [Fig. 18(b)], 80 times more efficient than LiNbO₃ deflectors.

A maximum modulation contrast model of KTN electro-optic modulators optimizes bias voltage, tripling contrast (0.106 versus 0.03). Room-temperature operation at 395 V half-wave voltage (Fig. 26) is enabled by giant Kerr coefficients (S₁₁=2.2×10^-14 m²/V²), orders of magnitude higher than those of conventional electro-optic crystals. Periodically poled KTN (PPKTN) exhibits 39% second-harmonic generation (SHG) efficiency at 1030→515 nm [Fig. 22(e)] and covers mid-IR (5→2.5 μm) band via quasi-phase matching, filling a critical gap in mid-IR laser sources.

KTN-based devices are poised to enable wide-bandwidth, high-efficiency laser systems for lidar, OCT (Fig. 23), and ultrafast 3D imaging (Fig. 24). KTN-deflector-based swept-source systems scan at 100 kHz rates, resolving strawberry tissue structures at 7 μm axial resolution (100 nm wavelength sweep). Utilizing KTN’s optical isotropy and ultrahigh Kerr coefficient, polarization-modulated 3D lidar achieves 20° field-of-view with 4.4 cm precision at 15 m distance, 60% lower error than DKDP-based systems. KTN large-angle scanners enable synthetic aperture imaging with times wider scan angles than traditional EO crystals [Fig. 24(d)].

KTN crystals have demonstrated transformative potential in high-efficiency deflectors, low-voltage electro-optics, nonlinear photonics, and self-powered photodetection. Critical future directions include the following aspects. 1) Growth challenges: scaling hydrothermal crystals to device sizes and improving temperature stability during growth. 2) Device optimization: developing precise thermal control systems for Kerr-effect-based modulators operating near T_C. 3) High-frequency mechanisms: exploring gigahertz modulation dynamics and field-induced phase transitions. 4) Emerging applications: leveraging scale-free optics and ferroelectric superlattices for quantum optics and integrated photonics. By resolving the dual frontiers of crystal homogeneity and physical understanding, KTN-based technologies are poised to redefine performance ceilings in high-speed optical modulation, energy-autonomous photodetection, and nonlinear frequency conversion.