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Second-harmonic generation (SHG), linear electro-optic effect (LEO) and electric-field induced second-harmonic generation (EFISH) are nonlinear optical processes with important applications in optoelectronics and photovoltaics. SHG and LEO are second-order nonlinear optical processes described by second-order susceptibility. Instead, EFISH is a third-order nonlinear optical process described by third-order susceptibility. LEO and EFISH are only observed in the presence of a static electric field. These nonlinear processes are very sensitive to the symmetry of the systems. In particular, LEO is usually observed through a change in the dielectric properties of the material while EFISH can be used to generate a "second harmonic" response in centrosymmetric material. In this work, we present a first-principle formalism to calculate second- and third-order susceptibility for LEO and EFISH. LEO is studied for GaAs semiconductor and compared with the dielectric properties of this material. We also present how it is possible for LEO to include the ionic contribution to the second-order macroscopic susceptibility. Concerning EFISH we present for the first time the theory we developed in the framework of TDDFT to calculate this nonlinear optical process. Our approach permits to obtain an expression for EFISH which does not contain the mathematical divergences in the frequency-dependent second-order susceptibility that caused until now many difficulties for numerical calculations. [ABSTRACT FROM AUTHOR]

Nonlinear optical (NLO) crystal, simultaneously exhibits second-harmonic generation response and linear electric-optical (EO) susceptibility, is of great significance for optoelectronics due to its frequency extension technique and optical modulation effect. In this paper, the growth system of K 3 B 6 O 10 Br (KBB) crystal in the green-flux K 2 MoO 4 –KBF 4 was systematically investigated to get rid of the non-intrinsic absorption in the ultraviolet wavelength range, which is beneficial for high-power applications. Bulk KBB NLO crystals were successfully grown by the top-seeded solution growth method from different fluxes for comparison. Moreover, the electro-optical (EO) property of KBB crystal was explored, and the effective EO coefficient γ 22 along transverse modulation was determined to be 1.10 p.m./V. • The growth system of K 3 B 6 O 10 Br (KBB) crystal in the green-flux K 2 MoO 4 –KBF 4 was systematically investigated. • The absorption band ranging from 220 to 280 nm disappeared in the KBB crystal grown from green-flux. • The effective EO coefficient γ 22 along transverse modulation was determined to be 1.10 p.m./V. [ABSTRACT FROM AUTHOR]

Tunable optical frequency combs offer a flexible solution for specific applications such as dual-comb spectroscopy, optical communications and microwave photonics, delivering improved precision, compatibility, and performance. However, previously, there has been a trade-off between reconfigurability and system simplicity in comb generation. Here, we present a fast-switched repetition rate frequency comb system that utilizes an electro-optic modulation time-lens technique with a high third-order nonlinear AlGaAsOI waveguide. Only one stage of modulator is used in the time-lens system which significantly reduces the complexity of the overall system. Our system allows for tuning of the center wavelength from 1542 nm to 1556 nm, as well as independent adjustment of the repetition rates from 18 GHz to 26.5 GHz, enabling fast-switching capabilities. Additionally, our system exhibits a high pump-to-comb conversion efficiency of up to 67.9%. It also demonstrates robustness to temperature changes and environmental instability. All the involved devices can be integrated onto a single chip, making this comb suitable for various applications. Tunable optical frequency combs are a flexible solution for applications in optics, but they are typically limited in reconfigurability or simplicity of the We agree to use the draft summary by the editor. setups. The authors present a frequency comb platform exploiting electrooptic modulation and nonlinear AlGaAs-on-insulator waveguide, ensuring reconfigurability and fast-switched repetition rates. [ABSTRACT FROM AUTHOR]

Free-space optical applications benefit greatly from actively reconfigurable metasurfaces. While static metasurfaces have demonstrated remarkable capabilities in manipulating optical wave characteristics, their fixed electromagnetic response faces a challenge for active device demands. This work presents an all-dielectric metasurface of lithium niobate with high Q-factor Fano resonances for enhanced electro-optic modulation. The resonance mechanism is investigated through multipole decomposition and near-field analysis, revealing that toroidal dipole contributions are dominant in all found three resonances. The strongest resonance arises from symmetry-protected bound states in the continuum (SP-BIC), and it is excited in an asymmetric metasurface by introducing a perturbation in the symmetric nanohole clusters, to enable polarization-independent operation. We numerically achieve a tuning efficiency of 0.65 nm/V and π-phase retardation driven by a bias of 0.53 V, due to the strong field confinement of quasi-BIC mode. Our work provides a scheme for compact, high-performance, and electrically tunable metasurfaces that can help in the development of spatial optical switches as well as tunable micro-lenses and displays. • Multiple toroidal resonance system of lithium niobate metasurface. • High-Q factor Fano resonance of symmetry-protected bound states in the continuum (SP-BIC). • Polarization-insensitive SP-BIC for the symmetry structure and dual-BIC modes from the anisotropic material. • An effective method to tuning the resonances and the underlying physical mechanism. [ABSTRACT FROM AUTHOR]

• Switching the internal state machine from FPGA is proposed to adjust the operating bias-point control of Mach-Zehnder electro-optic modulator, which indicates the proposed method can realize automatic bias-point control in real time without circuit, feedback or algorithm to compensate the drift. • A pulsed laser source of Mach-Zehnder electro-optic modulator based on the FPGA is established experimentally to demonstrate the modulation depth and the extinction ratio of pulse are improved by 40.50% and 32.85 dB, separately. • The sensing performances of ring-down cavity are improved including the initial curve rise of ring-down signals and the distortion of ring-down waveform. The good linearity, repeatability, stability and accuracy of temperature sensing system are investigated experimentally at the continuous temperature variation from −30℃ to 30℃. Mach-Zehnder electro-optic modulator as the essential component of high-speed modulation in the optical fiber communication and sensing system, its stability control of operating bias-point plays a key role in determining the transmission signal quality. Based on the output characteristic principle of Mach-Zehnder electro-optic modulator, researches on its bias-point drifting for causing the reduction mechanism of modulation depth and extinction ratio are analyzed. Switching its internal state machine from FPGA is proposed to automatically adjust the bias voltage in real time realizing the operating bias-point control of Mach-Zehnder electro-optic modulator, which is studied through the theoretical investigation. A pulsed laser with the temperature sensing system of fiber Bragg grating ring-down cavity based on FPGA is established experimentally to demonstrate its effectiveness. The experimental results show the modulation depth and the extinction ratio of pulsed laser are improved by 40.45% and 32.85 dB, respectively. Output characteristic curve of the sensing system is optimized, which includes the phenomenon of initial curve rise of ring-down signals and the distortion of ring-down waveform are eliminated. The good linearity, repeatability, stability and accuracy of sensing system are investigated at the continuous variation temperature from −30℃ to 30℃. [ABSTRACT FROM AUTHOR]