Electro-optically tunable optical delay on a lithium niobate photonic chip.

An approach for continuous tuning of on-chip optical delay with a microring resonator is proposed and demonstrated. By introducing an electro-optically tunable waveguide coupler, the bus waveguide to the resonance coupling can be effectively tuned from the under-coupling regime to the over-coupling regime. The optical delay is experimentally characterized by measuring the relative phase shift between lasers and shows a large dynamic range of delay from -600 to 600 ps and an efficient tuning of delay from -430 to -180 ps and from 40 to 240 ps by only a 5 V voltage.

Numerical analysis of on-chip acousto-optic modulators for visible wavelengths.

On-chip acousto-optic modulators that operate at an optical wavelength of 780 nm and a microwave frequency of 6.835 GHz are proposed. The modulators are based on a lithium-niobate-on-sapphire platform and efficiently excite surface acoustic waves and exhibit strong interactions with tightly confined optical modes in waveguides. In particular, a high-efficiency phase modulator and single-sideband mode converter are designed. We found that for both microwave and optical wavelengths below 1 µm, the interactions at the cross-sections of photonic waveguides are sensitive to the waveguide width and are significantly different from those in previous studies. Our designed devices have small footprints and high efficiencies, making them suitable for controlling rubidium atoms and realizing hybrid photonic-atomic chips. Furthermore, our devices have the potential to extend the acousto-optic modulators to other visible wavelengths for other atom transitions and for visible light applications, including imaging and sensing.

Synthetic five-wave mixing in an integrated microcavity for visible-telecom entanglement generation.

Nonlinear optics processes lie at the heart of photonics and quantum optics for their indispensable role in light sources and information processing. During the past decades, the three- and four-wave mixing (χ(2) and χ(3)) effects have been extensively studied, especially in the micro-/nano-structures by which the photon-photon interaction strength is greatly enhanced. So far, the high-order nonlinearity beyond the χ(3) has rarely been studied in dielectric materials due to their weak intrinsic nonlinear susceptibility, even in high-quality microcavities. Here, an effective five-wave mixing process (χ(4)) is synthesized by incorporating χ(2) and χ(3) processes in a single microcavity. The coherence of the synthetic χ(4) is verified by generating time-energy entangled visible-telecom photon pairs, which requires only one drive laser at the telecom waveband. The photon-pair generation rate from the synthetic process shows an estimated enhancement factor over 500 times upon intrinsic five-wave mixing. Our work demonstrates a universal approach of nonlinear synthesis via photonic structure engineering at the mesoscopic scale rather than material engineering, and thus opens a new avenue for realizing high-order optical nonlinearities and exploring functional photonic devices.

Efficient Frequency Conversion in a Degenerate χ

Microresonators on a photonic chip could enhance nonlinear optics effects and thus are promising for realizing scalable high-efficiency frequency conversion devices. However, fulfilling phase matching conditions among multiple wavelengths remains a significant challenge. Here, we present a feasible scheme for degenerate sum-frequency conversion that only requires the two-mode phase matching condition. When the drive and the signal are both near resonance to the same telecom mode, an on-chip photon-number conversion efficiency up to 42% is achieved, showing a broad tuning bandwidth over 250 GHz. Furthermore, cascaded Pockels and Kerr nonlinear optical effects are observed, enabling the parametric amplification of the optical signal to distinct wavelengths in a single device. The scheme demonstrated in this Letter provides an alternative approach to realizing high-efficiency frequency conversion and is promising for future studies on communications, atom clocks, sensing, and imaging.

All-optical thermal control for second-harmonic generation in an integrated microcavity.

Nonlinear optical effects in integrated microcavities have been studied extensively with the advantages of strong light-matter interaction, great scalability, and stability due to the small mode volume. However, the pump lasers stimulating nonlinear effects impose obstacles for practical applications, since the material absorption causes thermal resonance drift and instability. Here we experimentally demonstrate an all-optical control of the thermal behavior in optical microcavities for tunable doubly-resonant second-harmonic (SH) generation on an integrated photonic chip. Through an auxiliary control laser, the temperature of a selected microring can be efficiently changed, thus allowing precise frequency tuning of the doubly-resonant wavelength while eliminating the distortion of the lineshape induced by the thermo-optic effect. Although the phase-matching conditions will limit the tuning range of 55GHz, the technique is still potential to achieve a larger tuning range in combination with temperature regulation. Additionally, this approach has the advantage of quick reconfiguration, showing a fast modulation rate up to about 256 kHz. The theoretical model behind our experimental scheme is universal and applicable to other microcavity-enhanced nonlinear optical processes, and our work paves the way for controlling and utilizing the thermal effect in the applications of microcavities.