Fig. 1. Application fields of KTN series crystals

Fig. 2. Phase diagram and phase transition region of KTN series crystals. (a) Phase diagram; (b) phase transition region

Fig. 3. Optimization process of KTN crystal growth by Czochralski method. (a) Undercooling isothermal (sublevel) growth technique; (b) lattice cell and striation structure of KTN crystal; (c) schematic of double-crucible real-time feeding system^[26-27]

Fig. 4. Device level samples of as-grown KTN crystals with different compositions, different sizes, and different doped ions

Fig. 5. Schematic diagram of autoclaves. (a) Cooling hydrothermal method; (b) temperature-difference hydrothermal method^[33]

Fig. 6. Spontaneous nucleation KTN crystals obtained by cooling hydrothermal method. (a) KTN crystals obtained at temperature of 700 ℃ and cooling rate of 5 ℃/h; (b) KTN crystals with blue color centers; (c) KTN crystals without blue color centers after adding H₂O₂^[33]

Fig. 7. KTN crystal grown by temperature-difference hydrothermal method^[33]

Fig. 8. Crystalline characterization of KTN series crystals. (a) XRD patterns for KTaO₃, KTa_0.75Nb_0.25O₃, KTa_0.63Nb_0.37O₃, and KTa_0.55Nb_0.45O₃^[38]; (b) HXRD patterns for KTaO₃, KTa_0.91Nb_0.09O₃, KTa_0.81Nb_0.19O₃, KTa_0.75Nb_0.25O₃, and KTa_0.63Nb_0.37O₃^[26]; (c) EPMA results for Nb fluctuations of (001) and (100) faces for KTa_0.63Nb_0.37O₃ crystal^[26]; (d) AFM image showing surface roughness and striations for KTa_0.63Nb_0.37O₃ crystal^[26]

Fig. 9. Linear optical properties of KTN series crystals. (a) Reflective index dispersion curves of KTa_0.63Nb_0.37O₃ and Cu∶KTa_0.63Nb_0.37O₃ single crystals^[46]; (b) transmittance spectra of KTa_0.63Nb_0.37O₃ and Cu∶KTa_0.63Nb_0.37O₃ single crystals^[46]; (c) transmittance spectra of KTa_1-xNb_xO₃ (x=0, 0.25, 0.37, and 0.45) series crystals^[38]; (d) optical absorption coefficient spectra of KTa_1-xNb_xO₃ (x=0, 0.25, 0.37, and 0.45) series crystals^[38]

Fig. 10. Photocurrent circuit test. (a) Schematic diagram; (b) matching photocurrent spectra of KTa_0.65Nb_0.35O₃ crystal with different applied electric fields^[47]

Fig. 11. CDWs in multidomain ferroelectric crystal. (a) Electrostatic potential energy with 90° CDWs, where upper graph is schematic of ferroelectric arrangement containing “head-to-head” and “tail-to-tail” 90° domain walls, and bottom graph shows calculated electrostatic potential energy in 1×1×6 supercell tetragonal perovskite ferroelectric crystal; (b) illustration of electron/hole “ferroelectric highways” along CDWs (see yellow and blue regions), where yellow and blue regions represent gathering place of electrons and holes, respectively, red and blue arrows denote different polarization directions, and polarization changes with bound charges at CDWs (labeled by + and -)^[28]

Fig. 12. Comparison of self-powered photoresponsivity^[28] of BaTiO₃ (BTO)^[48], KH₂PO₄ (KDP)^[49], [KNbO₃]_1-x[BaNi_1/2Nb_1/2O_3-δ]_x (KBNNO)^[50], and Fe∶KTa_0.41Nb_0.59O₃ (Fe∶KTN59)^[51]

Fig. 13. Performance of Cu∶KTN detector. (a) Wavelength-dependent response time of Cu∶KTN detector at zero bias voltage^[52]; (b) wavelength-dependent responsivity of Cu∶KTN detector at zero bias voltage, with absorption spectrum plotted in logarithmic form^[52]; (c) comparison of self-powered responsivity of BTO^[53], KDP^[54], KBNNO^[55], Fe∶KTa_0.41Nb_0.59O₃ (Fe∶KTN)^[56], BA₂CsPb₂Br₇ (BACPB)^[57], EA₄Pb₃Cl₁₀ (EAPC)^[58], Bi_0.5Na_0.5TiO₃ (BNT)^[59], KTa_0.59Nb_0.41O₃ (KTN)^[28], and Cu∶KTa_0.56Nb_0.44O₃ (Cu∶KTN) ^[52] for ultraviolet light; (d) comparison of photoelectric response range of self-powered detectors, including KTN^[28],KDP^[54],Bi₄Ti₃O₁₂^[60],BaTiO₃^[53],Fe∶BaTiO₃^[61],Fe∶KTN^[56],AgNbO₃^[62],BiFeO₃^[63],KBNNO^[55], and Cu∶KTN^[52]

Fig. 14. Curie-Weiss characteristics of KTa_0.61Nb_0.39O₃ crystal^[64]

Fig. 15. Dielectric properties of KTN series crystals. (a) Relative permittivity ɛ_r of KTN crystals in various compositions plotted as function of temperature^[66]; (b) relative permittivity ɛ_r of KTa_0.63Nb_0.37O₃ crystal with temperature, plotted in blue and red lines for cooling and heating processes, respectively, where different phases are specified and separated by vertical dotted lines^[68]; (c) relative dielectric constant of KTa_0.65Nb_0.35O₃ as measured by Yagi^[3]; (d) relative dielectric constant of Cu∶KTa_0.65Nb_0.35O₃ with different doping densities measured by our research group^[27]

Fig. 16. Kerr coefficients measurements. (a) Single-beam Senarmont compensation method for measuring S₁₁‒S₁₂^[25]; (b) Mach-Zehnder interferometer operating system for measuring S₁₂^[25]; (c) optical path difference method for measuring S₄₄^[41]; (d) Kerr coefficient dependent on temperature of KTa_0.65Nb_0.35O₃^[67]; (e) frequency response characteristics for Kerr coefficient of KTa_0.65Nb_0.35O₃^[72]

Fig. 17. Performance comparison of common laser deflectors. (a) Traditional light deflector device containing mechanical components (large deflection angle, slow response speed); (b) linear EO crystal deflector (high voltage, small deflection angle); (c) KTN Kerr deflector (low voltage, large deflection angle); (d) comparison of parameters among mechanical, acousto-optic, linear EO crystal, and KTN Kerr deflectors^[76]

Fig. 18. KTN deflector based on space-charge-controlled EO effect. (a) KTN beam deflector and its mode of operation^[77]; (b) advantages of KTN beam scanner^[78-79]; (c) comparison between KTN beam scanner and conventional EO beam scanner^[78-79]; (d) structure of KTN two-dimensional convergence lenses^[78-79]; (e) experimental setup for evaluating varifocal performance^[78-79]

Fig. 19. Abnormal laser deflection phenomenon based on interactions of EO effect and graded refractivity effect in Cu∶KTN crystal. (a) Schematic and physical mechanism of abnormal laser deflection; (b) integrated EO modulation technology of Cu∶KTN crystal^[27]

Fig. 20. KTN EO modulation measurements. (a) EO modulation system based on PSA structure of KTN crystal^[37,81]; (b) theoretical curve calculated and simulated by effective Kerr coefficient^[37,81]; (c) light intensity modulation curve when sinusoidal voltage of 0‒900 V is applied^[37,81]; (d) configuration of KTN Kerr modulator^[82]; (e) optical signal versus applied sawtooth voltage, where “first”, “second”, “third”, and “fourth” represent order corresponding to middle position of optical signal, and magenta dashed lines indicate optimal bias voltages^[82]

Fig. 21. Crystal structure and ferroelectric domain distribution of KTN. (a) Crystal structure of tetragonal KTN; (b) polarizing microscope image of KTN crystal along x-y plane (scale bar is 25 μm), where red dashed line represents 90° domain wall, green dashed line represents 180° domain wall, and blue arrows represent spontaneous polarization direction; (c) diagram of natural Rubik’s cube-like domain structures in KTN crystal; (d) structural model of ferroelectric domain (out-of-plane) in KTN x-y plane, where × and • represent inside and outside polarization, respectively; (e) structural model of ferroelectric domain (in-plane) in KTN x-y plane; (f),(g) vertical piezoresponse force microscope (PFM) amplitude image (f) and lateral PFM phase image (g) of KTN x-y plane (scale bar is 2 μm)^[94]

Fig. 22. SHG from PPKTN sample. (a) Schematic of experimental setup; (b) photograph of SHG green laser; (c) spectrum of FW and SH; (d) polarization dependence of FW and SH; (e) quadratic power dependence of SH on FW^[95]

Fig. 23. 3D-OCT system based on high-speed KTN deflector. (a) High-speed 3D-OCT system; (b) photograph of entire strawberry; (c) details of portion of strawberry; (d) x-z cross-sectional 3D-OCT image; (e) x-y cross-sectional 3D-OCT image^[99]

Fig. 24. Practical application exploration based on KTN EO modulation technique. (a) Laser 3D imaging system based on photoelectric mixing technology of KTN crystals^[88]; (b) superlattice light switch using Cu∶KTN crystal^[87]; (c) femtosecond laser microscopic imaging method based on KTN EO lens; (d) Fresnel telescope full-aperture synthetic imaging lidar^[100]

Fig. 25. Experimental system of KTN crystal-based polarization-modulated 3D lidar^[101]

Fig. 26. Polarization-modulated images and 3D images of three flat boards located at 14, 15, and 16 m. 3D images were taken under field-of-view (FOV) of 20° and maximum unambiguous range of 60 m. (a), (b) Polarization-modulated images obtained with KTN; (c), (d) 3D images obtained with KTN and DKDP^[101]

Fig. 27. Application prospects and market evaluation for KTN crystals. (a) Comparison of various beam scanning techniques and KTN beam scanner; (b) capacity evaluation for KTaO₃ solid immersion lens; (c) applications mapped against resolution and frequency for KTN devices; (d) market forecast and advantages of KTN crystals with various application fields^[3,102-103]