Channel waveguides and y-junctions in x-cut single-crystal lithium niobate thin film

Reconfigurable AWGR based on LNOI with a tunable central wavelength and bandwidth used in elastic optical networking

Elastic optical networking introduces elasticity and adaptation into the optical domain, which highly depends on reconfigurable optical devices. In this paper, a tunable 4×4 arrayed waveguide grating router based on lithium niobate on insulator is designed. By using the electro-optic effect of lithium niobate, we design electrode regions with specific shapes in the array waveguide region to realize the tuning of the routing wavelength and bandwidth of the third output channel. The designed arrayed waveguide grating router (AWGR) has a dense channel spacing of 0.8 nm, and the minimum insertion loss is 2.3 dB. Experiments show that the tuning range of the central wavelength can reach 3.2 nm, and the 3 dB bandwidth can be expanded from 0.2 to 0.6 nm.

High-responsivity and high-speed black phosphorus photodetectors integrated with proton exchanged thin-film lithium niobate waveguides

We present a high-performance broadband (450-1550 nm) black phosphorus photodetector based on a thin-film lithium niobate waveguide. The waveguides are fabricated by the proton exchange method with flat surfaces, which reduces the stress and deformation of two-dimensional materials. At a wavelength of 1550 nm, the photodetector simultaneously achieves a high responsivity and wide bandwidth, with a responsivity as high as 147 A/W (at an optical power of 17 nW), a 3-dB bandwidth of 0.86 GHz, and a detectivity of 3.04 × 1013 Jones. Our photodetector exhibits one of the highest responsivity values among 2D material-integrated waveguide photodetectors.

Technologically feasible quasi-edge states and topological Bloch oscillation in the synthetic space

The dimensionality of a physical system is one of the major parameters defining its physical properties. The recently introduced concept of synthetic dimension has made it possible to arbitrarily manipulate the system of interest and harness light propagation in different ways. It also facilitates the transformative architecture of system-on-a-chip devices enabling far reaching applications such as optical isolation. In this report, a novel architecture based on dynamically-modulated waveguide arrays with the Su-Schrieffer-Heeger configuration in the spatial dimension is proposed and investigated with an eye on a practical implementation. The propagation of light through the one-dimensional waveguide arrays mimics time evolution of the field in a synthetic two-dimensional lattice. The addition of the effective gauge potential leads to an exotic topologically protected one-way transmission along adjacent boundary. A cosine-shape isolated band, which supports the topological Bloch oscillation in the frequency dimension under the effective constant force, appears and is localized at the spatial boundary being robust against small perturbations. This work paves the way to improved light transmission capabilities under topological protections in both spatial and spectral regimes and provides a novel platform based on a technologically feasible lithium niobate platform for optical computing and communication.

Recent Progress in Lithium Niobate: Optical Damage, Defect Simulation, and On-Chip Devices

Lithium niobate (LN) is one of the most important synthetic crystals. In the past two decades, many breakthroughs have been made in material technology, theoretical understanding, and application of LN crystals. Recent progress in optical damage, defect simulation, and on-chip devices of LN are explored. Optical damage is one of the main obstacles for the practical usage of LN crystals. Recent results reveal that doping with ZrO₂ not only leads to better optical damage resistance in the visible but also improves resistance in the ultraviolet region. It is still awkward to extract defect characteristics and their relationship with the physical properties of LN crystals directly from experimental investigations. Recent simulations provide detailed descriptions of intrinsic defect models, the site occupation of dopants and the variation of energy levels due to extrinsic defects. LN is considered to be one of the most promising platforms for integrated photonics. Benefiting from advances in smart-cut, direct wafer bonding and layer transfer techniques, great progress has been made in the past decade for LNs on insulators. Recent progress on on-chip LN micro-photonic devices and nonlinear optical effects, in particular photorefractive effects, are briefly reviewed.

Design and Optimization of Proton Exchanged Integrated Electro-Optic Modulators in X-Cut Lithium Niobate Thin Film

In this study, we designed, simulated, and optimized proton exchanged integrated Mach-Zehnder modulators in a 0.5-μm-thick x-cut lithium niobate thin film. The single-mode conditions, the mode distributions, and the optical power distribution of the lithium niobate channel waveguides are discussed and compared in this study. The design parameters of the Y-branch and the separation distances between the electrodes were optimized. The relationship between the half-wave voltage length production of the electro-optic modulators and the thickness of the proton exchanged region was studied.

Low-loss waveguides on Y-cut thin film lithium niobate: towards acousto-optic applications

We investigate the dependence of photonic waveguide propagation loss on the thickness of the buried oxide layer in Y-cut lithium niobate on insulator substrate to identify trade-offs between optical losses and electromechanical coupling of surface acoustic wave (SAW) devices for acousto-optic applications. Simulations show that a thicker oxide layer reduces the waveguide loss but lowers the electromechanical coupling coefficient of the SAW device. Optical racetrack resonators with different lengths were fabricated by argon plasma etching to experimentally extract waveguide losses. By increasing the oxide layer thickness from 1 µm to 2 µm, we were able to reduce propagation loss of 2 µm (1 µm) wide waveguide from 1.85 dB/cm (3 dB/cm) to as low as 0.37 dB/cm (0.77 dB/cm). Resonators with a quality factor greater than 1 million were demonstrated as well. An oxide thickness of approximately 1.5 µm is sufficient to significantly reduce propagation loss, due to leakage into the substrate and simultaneously attain good electromechanical coupling in acoustic devices. This work not only provides insights on the design and realization of low-loss photonic waveguides in lithium niobate, but also, most importantly, offers experimental evidence of how the oxide thickness directly impacts losses and guides its selection for the synthesis of high-performance acousto-optic devices in Y-cut lithium niobate on insulator.

Highly efficient broadband second harmonic generation mediated by mode hybridization and nonlinearity patterning in compact fiber-integrated lithium niobate nano-waveguides

The inherent trade-off between efficiency and bandwidth of three-wave mixing processes in χ₂ nonlinear waveguides is the major impediment for scaling down many well-established frequency conversion schemes onto the level of integrated photonic circuit. Here, we show that hybridization between modes of a silica microfiber and a LiNbO₃ nanowaveguide, amalgamated with laminar χ₂ patterning, offers an elegant approach for engineering broadband phase matching and high efficiency of three-wave mixing processes in an ultra-compact and natively fiber-integrated setup. We demonstrate exceptionally high normalized second harmonic generation (SHG) efficiency of up to η_nor ≈ 460% W⁻¹ cm⁻², combined with a large phase matching bandwidth of Δλ ≈ 100 nm (bandwidth-length product of Δλ · L ≈ 5 μm²) near the telecom bands, and extraordinary adjustment flexibility.

Electric-optical property of the proton exchanged phase modulator in single-crystal lithium niobate thin film

The electric-optical property of the proton exchanged phase modulator in an x-cut single-crystal lithium niobate thin film was studied. Proton exchanged waveguides generally suffered from a deteriorated electric-optical coefficient. By introducing a shallow proton exchange layer (thickness = 0.165 μm), most energy of the optical mode was allowed to guide in the untouched single-crystal lithium niobate film, making contribution to the effective electric-optical coefficient as high as 29.5 pm/V, which was very close to that of the bulk lithium niobate (r₃₃ = 31 pm/V). A 12 V voltage applied to the electrodes located on the two sides of the waveguide induced a 0.097 nm shift of the Fabry-Perot resonant peak. Considering the wavelength difference of the neighboring resonant peaks (0.228 nm) and the length of the electrodes (2.3 mm), the voltage-length product was as low as 6.5 V·cm, indicating the efficient electric-optical modulation.