Yiwen Wang

Low-loss waveguides in a single-crystal lithium niobate thin film.

We report low-loss channel waveguides in a single-crystal LiNbO(3) thin film achieved using the annealed proton exchange process. The simulation indicated that the mode size of the α phase channel waveguide could be as small as 1.2  μm(2). Waveguides with several different widths were fabricated, and the 4 μm-wide channel waveguide exhibited a mode size of 4.6  μm(2). Its propagation loss was accurately evaluated to be as low as 0.6 dB/cm at 1.55 μm. The single-crystal lattice structure in the LiNbO(3) thin film was preserved by a moderate annealed proton exchange process (5 min of proton exchange at 200°C, followed by 3 h annealing at 350°C), as revealed by measuring the extraordinary refractive index change and x ray rocking curve. A longer proton exchange time followed by stronger annealing would destroy the crystal structure and induce a high loss in the channel waveguides.

Waveguides consisting of single-crystal lithium niobate thin film and oxidized titanium stripe.

Strip-loaded waveguides were fabricated by the direct oxidation of a titanium film based on the single-crystal lithium niobate. The method avoided the surface roughness problems that are normally introduced during dry etching of waveguide sidewalls. Propagation modes of the composite strip waveguide were analyzed by a full-vectorial finite difference method. The minimum dimensions of the propagation modes were calculated to be 0.7 μm(2) and 1.1 μm(2) for quasi-TM mode and quasi-TE mode at 1550 nm when the thickness of the LN layer and TiO(2) strip was 660 nm and 95 nm, respectively. The optical intensity was as high as 93% and was well confined in the LN layer for quasi-TM polarization. In this experiment, the propagation losses for the composite strip waveguide with 6 μm wide TiO(2) were 14 dB/cm for quasi-TM mode and 5.8 dB/cm for quasi-TE mode, respectively. The compact hybrid structures have the potential to be utilized for compact photonic integrated devices.

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

Proton exchanged channel waveguides in x-cut single-crystal lithium niobate thin film could avoid optical leakage loss which existed in the z-cut case. Indicated by simulations, the mechanism and condition of the optical leakage loss were studied. The light energy in the exchanged layer and the mode sizes were calculated to optimize the parameters for fabrication. By a very short time (3 minutes) proton exchange process without anneal, the channel waveguide with 2 μm width and 0.16 μm exchanged depth in the x-cut lithium niobate thin film had a propagation loss as low as 0.2 dB/cm at 1.55 μm. Furthermore, the Y-junctions based on the low-loss waveguide were designed and fabricated. For a Y-junction based on the 3 μm wide channel waveguide with 8000 μm bending radius, the total transmission could reach 85% ~90% and the splitting ratio maintained at a stable level around 1:1. The total length was smaller than 1 mm, much shorter than the conventional Ti-diffused and proton exchanged Y-junctions in bulk lithium niobate.

Amorphous silicon-lithium niobate thin film strip-loaded waveguides

The heterogeneous integration of an amorphous silicon (a-Si) film with a lithium niobate (LN) thin film combines both the mature micro-processing technology of Si and the excellent optical properties of LN. An a-Si thin film was deposited on an LN thin film, and strip-loaded waveguides were designed, fabricated, and characterized. A full-vectorial finite difference method was used to explore the single-mode conditions and appropriate dimensions for the strip-loaded waveguides. The waveguide mode size could be as small as 0.36 μm2. By adjusting the thickness and width of the a-Si loading strip, the distribution of light power could be mainly confined in the LN layer. The maximal light power that could be confined in LN was 91%, which was obtained at an a-Si thickness of 65 nm. A set of waveguides with widths of 2‒7 μm were prepared by inductively coupled plasma (ICP) etching of the a-Si thin film. Following annealing at 300°C in air for 1 hour, light transmission was observed in the waveguide. The 2-μm-wide waveguide showed propagation losses of 20 dB/cm for the quasi-TM (q-TM) mode and 42 dB/cm for the quasi-TE (q-TE) mode at 1550 nm. The root-mean-square (RMS) surface roughness of the a-Si thin film before and after annealing was 1.04 and 0.35 nm, respectively. High-resolution transmission electron microscopy (HRTEM) was performed to investigate the interface morphologies. A well-defined interface was clearly observed, and the structure of the a-Si thin film was proved to be amorphous.

Grating coupler on lithium niobate thin film waveguide with a metal bottom reflector

To improve the coupling efficiency between a single-mode fiber and the waveguide on lithium niobate thin film (LNOI), a fiber-to-chip grating coupler with a metal bottom reflector was designed, fabricated, and characterized. A maximum coupling efficiency of −9.1 dB and −6.9 dB for a grating coupler on LNOI without and with a metal bottom reflector was measured, respectively. Fabrication error sensitivity of etch depth was experimentally investigated and the discrepancy between the simulation and experiment was discussed.

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 ηnor ≈ 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.

Highly-efficient broadband second harmonic generation in compact fiber-integrated thin-film LiNbO 3 nanowaveguides

We report a highly efficient (η ∼ 450 % W^−-1cm^−-2) and broadband (phase matching bandwidth δλ ∼ 100nm) second harmonic generation in a 50μm long fiber-integrated LNOI waveguide. We demonstrate phase matching tunability by small adjustments of microfiber diameter.