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We report intracavity Bragg scattering induced by photorefractive (PR) effect in high-Q lithium niobate (LN) ring resonators at cryogenic temperatures. We show that, when a cavity mode is strongly excited, the PR effect imprints a long-lived periodic space-charge field. This residual field in turn creates a refractive index modulation pattern that dramatically enhances the back scattering of an incoming probe light, and results in selective and reconfigurable mode splittings. This PR-induced Bragg scattering effect, despite being undesired for many applications, could be utilized to enable optically programmable photonic components.
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Prospective integrated quantum optical technologies will combine nonlinear optics and components requiring cryogenic operating temperatures. Despite the prevalence of integrated platforms exploiting $chi^{(2)}$-nonlinearities for quantum optics, for example used for quantum state generation and frequency conversion, their material properties at low temperatures are largely unstudied. Here, we demonstrate the first second harmonic generation in a fiber-coupled lithium niobate waveguide at temperatures down to 4.4K. We observe a reproducible shift in the phase-matched pump wavelength within the telecom band, in addition to transient discontinuities while temperature cycling. Our results establish lithium niobate as a versatile nonlinear photonic integration platform compatible with cryogenic quantum technologies.
We investigate the performance of microwave-frequency phononic crystal resonators fabricated on thin-film lithium niobate for integration with superconducting quantum circuits. For different design geometries at millikelvin temperatures, we achieve mechanical internal quality factors $Q_i$ above $10^5 - 10^6$ at high microwave drive power, corresponding to $5times10^6$ phonons inside the resonator. By sweeping the defect size of resonators with identical mirror cell designs, we are able to indirectly observe signatures of the complete phononic bandgap via the resonators internal quality factors. Examination of quality factors temperature dependence shows how superconducting and two-level system (TLS) loss channels impact device performance. Finally, we observe an anomalous low-temperature frequency shift consistent with resonant TLS decay and find that material choice can help to mitigate these losses.
Many technologies in quantum photonics require cryogenic conditions to operate. However, the underlying platform behind active components such as switches, modulators and phase shifters must be compatible with these operating conditions. To address this, we demonstrate an electro-optic polarisation converter for 1550nm light at 0.8K in titanium in-diffused lithium niobate waveguides. To do so, we exploit the electro-optic properties of lithium niobate to convert between orthogonal polarisation modes with a fiber-to-fiber transmission >43%. We achieve a modulation depth of 23.6 +/-3.3dB and a conversion voltage-length product of 28.8 V cm. This enables the combination of cryogenic photonics and active components on a single integration platform.
Lithium niobate is a multi-functional material, which has been regarded as one of the most promising platform for the multi-purpose optical components and photonic circuits. Targeting at the miniature optical components and systems, lithium niobate microstructures with feature sizes of several to hundreds of micrometers have been demonstrated, such as waveguides, photonic crystals, micro-cavities, and modulators, et al. In this paper, we presented subwavelength nanograting metasurfaces fabricated in a crystalline lithium niobate film, which hold the possibilities towards further shrinking the footprint of the photonic devices with new optical functionalities. Due to the collective lattice interactions between isolated ridge resonances, distinct transmission spectral resonances were observed, which could be tunable by varying the structural parameters. Furthermore, our metasurfaces are capable to show high efficiency transmission structural colors as a result of structural resonances and intrinsic high transparency of lithium niobate in visible spectral range. Our results would pave the way for the new types of ultracompact photonic devices based on lithium niobate.
The absence of the single-photon nonlinearity has been a major roadblock in developing quantum photonic circuits at optical frequencies. In this paper, we demonstrate a periodically-poled thin film lithium niobate microring resonator (PPLNMR) that reaches 5,000,000%/W second harmonic conversion efficiency---almost 20-fold enhancement over the state-of-the-art---by accessing its largest $chi^{(2)}$ tensor component $d_{33}$ via quasi-phase matching. The corresponding single photon coupling rate $g/2pi$ is estimated to be 1.2 MHz, which is an important milestone as it approaches the dissipation rate $kappa/2pi$ of best available lithium niobate microresonators developed in the community. Using a figure of merit defined as $g/kappa$, our devices reach a single photon nonlinearity approaching 1%. We show that, by further scaling of the device, it is possible to improve the single photon nonlinearity to a regime where photon-blockade effect can be manifested.
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