Thin-film lithium niobate (TFLN) waveguides have emerged as a pivotal platform for on-chip spontaneous parametric down-conversion (SPDC), serving as a crucible for the generation of entangled photon pairs. The periodic poling of TFLN, while capable of generating high-efficiency SPDC, demands intricate fabrication processes that can be onerous in terms of scalability and manufacturability. In this work, we introduce a novel approach to the generation of entangled photon pairs via SPDC within TFLN waveguides, harnessing the principles of modal phase-matching (MPM). To address the challenge of efficiently exciting pump light typically in a higher-order mode, we have engineered a mode converter that couples two asymmetrically dimensioned waveguides. This converter adeptly transforms the fundamental mode into a higher-order mode, demonstrating a conversion loss of 1.55 dB at 785 nm with a 3 dB bandwidth exceeding 30 nm. Subsequently, we have showcased the device’s capabilities by characterizing the pair generation rate (PGR), coincidences-to-accidentals ratio (CAR), and spectral profile of the entangled photon source. Our findings present a simplified and versatile method for the on-chip generation of entangled photon sources, which may pave the way for the application in the realms of quantum information processing and communication technologies.

This work proposes an all-optical on-chip nonlinear activation function unit for multi-wavelength computing applications. Our proposed unit utilizes the unique properties of silicon, specifically the two-photon absorption and thermo-optic effects, to achieve nonlinearity essential for optical neural networks. We designed the parameters of the corresponding unit structure through simulation, fabricated and tested the actual structure experimentally, and obtained a CELU-like response curve. In practical classification tasks, we evaluated its performance in various classification tasks through numerical simulation and achieved a recognition accuracy of 98.84% on the MNIST dataset. This work addresses a critical gap in multi-wavelength systems and contributes to the development of more powerful and efficient optical computing platforms.

Tunable and low-power microcavities are essential for large-scale photonic integrated circuits. Thermal tuning, a convenient and stable tuning method, has been widely adopted in optical neural networks and quantum information processing. Recently, graphene thermal tuning has been demonstrated to be a power-efficient technique, as it does not require thick spacers to prevent light absorption. In this paper, a silicon-based on-chip Fano resonator with graphene nanoheaters is proposed and fabricated. This novel Fano structure is achieved by introducing a scattering block, and it can be easily fabricated in large quantities. Experimental results demonstrate that the resonator has the characteristics of a high quality factor (∼31,000) and low state-switching power (∼1 mW). The temporal responses of the microcavity exhibit qualified modulation speed with 9.8 μs rise time and 16.6 μs fall time. The thermal imaging and Raman spectroscopy of graphene at different biases were also measured to intuitively show that the tuning is derived from the joule heating effect of graphene. This work provides an alternative for future large-scale tunable and low-power-consumption optical networks, and has potential applications in optical filters and switches.

We present a systematic analysis of the quality (Q) factors and polarization characteristics of unidirectional guided resonances (UGRs) in a symmetry-broken silicon-on-lithium niobate (Si-on-LN) photonic crystal (PhC) slab. Through numerical investigations, we demonstrate ultra-high Q-factor UGRs at the Γ-point, enabled by controlled structural asymmetry. By optimizing the asymmetry parameters, the Q-factor of UGRs can be flexibly tuned across a wide range of 10³ to 10⁶. Furthermore, leveraging the polarization sensitivity and all-pass phase-shift properties of UGRs, we achieve broadband high-efficiency linear-to-circular polarization conversion. The proposed device exhibits exceptional stability in conversion efficiency (>99.5%) over a 20.5 nm wavelength span, 1534.2–1554.7 nm. This work advances the fundamental understanding of UGRs while offering promising applications in PhC-based surface-emitting lasers, optical phased-array radars, and circular dichroism spectroscopy.

Thin film lithium niobate, as a highly promising integrated photonics platform, has emerged as an ideal material platform for wavelength conversion owing to its exceptional second-order nonlinear characteristics. Here, we present the first demonstration of concurrent phase-matchings for multi-wavelength conversion in coupled thin film lithium niobate waveguides without poling technique. In the coupled dual waveguide system, employing modal phase-matching, we validate three effective phase-matching conditions for second harmonic generation, arising from additional momentum compensation due to the coupling of waveguides. Simultaneously, we confirm that phase-matching wavelengths can be tuned by adjusting the gap between waveguides. The concurrent phase-matchings in coupled dual waveguide systems can be readily extended to other nonlinear optical processes, such as difference frequency generation and spontaneous parametric down conversion. Our work contributes to advancing the exploration of efficient on-chip nonlinear optical processes within the realms of nanophotonics and quantum optics.