Fano resonance arises from the interference of a localized discrete state coupled to the continuum states, which has become an indispensable probe in physical and chemical sciences. Compared with plenty of studies in the artificial meta-structures, Fano resonances in single crystals were rarely reported. Herein, we performed a comprehensive study on Fano resonances of KTa1-xNbxO3 (KTN) ferroelectric single crystals using temperature dependent Raman spectroscopy. The Fano asymmetric q factors were fitted for KTN crystals with various Nb concentrations. We found that the q factors were strongly correlated to Curie temperature Tc and ferroelectric polarization arrangement in different phase regions. In addition, the regulation of dopant transition metals on the Fano effect was also investigated in Cu:KTN and Fe:KTN crystals.

It has always been a hot topic to design an orderly mesoscopic structure in functional materials to tailor the macroscopic properties or realize new functions. The existence of domains in ferroelectric materials has been proven to affect the macroscopic properties, being actively studied in nonlinear optical conversion and piezoelectric effects. However, the high‐efficiency photoelectric conversion capability of ferroelectric crystals has not yet been explored. Here, the authors study the orderly arrangement of ferroelectric order in KTa1−xNbxO3 (KTN) perovskite crystals, and design the “head‐to‐head” domains by tuning the Curie temperature Tc, thereby generating abundant charged domain walls and robust conductive channels for electrons and holes. An ultrahigh ultraviolet photoresponsivity is achieved in the KTN crystal under zero bias voltage, being about four orders magnitude higher than that of the well‐known ferroelectric materials. The substantial improvement can be attributed to the judiciously designed ferroelectric order, as demonstrated by the conductive atomic force microscopy. In addition, KTN detector exhibits high stability and reliability after high‐temperature and fatigue treatment. KTN crystal features giant photoresponsivity, high electric–optical coefficient, and large χ(2) nonlinearity concurrently, indicating its great potential for application of all‐optical devices on photonic chips.

coefficients, which have been utilized for obtaining the generation of broadband light at new frequencies and its control. [1,2] In parametric conversion processes, phase matching (PM) condition is the prerequisite for highly efficient energy conversion, corresponding to the conservation of momentum in the nonlinear optical interaction. In order to overcome the limitations of traditional angular PM by optical birefringence, several applicable PM strategies have been proposed, comprising quasi-phase-matching (QPM), [3,4] Fresnel phase-matching (FPM), [5] additional periodic phase (APP), [6] random quasi-phasematching (RQPM), [7] etc. Among them, periodically poled engineered crystals, especially lithium niobate waveguides on insulators (LNOI), [8][9][10] have been extensively developed as a unique platform for integrated photonic applications on quantum information, [11] smart chips, [12] remote communications, [13] to name just a few examples. [14][15][16] The periodically poling technique is mainly applied in ferroelectric materials, thus producing designed domain inversion and reciprocal lattice vectors (RLVs) to compensate for the phase mismatch. Compared with angular birefringent phase-matching (BPM), the QPM technique has two apparent advantages. [10] First, it can utilize the largest nonlinear tensor element d ij . For example, the effective d 33 (27-34 pm V −1 ) in periodically poled LiNbO 3 (PPLN) is three times larger than d 31 (4.5 pm V −1 ) that is utilized in the BPM technique. Second, in principle, QPM allows efficient nonlinear interactions at any frequencies in the transparent range of the ferroelectric materials, including inaccessible wavelength by the BPM technique. Therefore, finding new QPM ferroelectric crystals with large d ij and wide transparency simultaneously is an urgent but a challenging task.KTa 1-x Nb x O 3 (KTN) perovskite is a mixed crystal of KTaO 3 and KNbO 3 . It is a typical displacement-type ferroelectric crystal with a tunable Curie temperature T c by adjusting Ta/Nb ratio. [17] In as-grown KTN crystals, both 90° and 180° domains are randomly arranged throughout the crystal, thus providing rich RLVs to support three-dimensional QPM conditions. [18][19][20][21][22] However, these natural random domains are difficult to Since the first proposal of quasi-phase-matching (QPM) in 1962, finding new nonlinear photonic crystals has been an attractive topic in nonlinear optics fields. However, combining high conversion efficiency and wide operation range concurrently into one single crystal remains a great challenge. Herein, a periodically poled KTa 0.51 Nb 0.49 O 3 (PPKTN) is fabricated as a one-dimensional nonlinear photonic crystal for the first time, to the authors' best knowledge. Benefitting from the large quadratic nonlinear coefficient, the QPM second harmonic generation (SHG) at 1030 nm is realized with a high conversion efficiency of up to 39%. More importantly, KTN crystal possesses wider midinfrared spectral transparency (0.4-8.0 µm) than commercialized LiNbO 3 and KTiOPO ...