Demonstration of 4H-silicon carbide on an aluminum nitride integrated photonic platform.

The existing silicon-carbide-on-insulator photonic platform utilizes a thin layer of silicon dioxide under silicon carbide (SiC) to provide optical confinement and mode isolation. Here, we replace the underneath silicon dioxide layer with 1-µm-thick aluminum nitride and demonstrate a 4H-silicon-carbide-on-aluminum-nitride integrated photonic platform for the first time to our knowledge. Efficient grating couplers, low-loss waveguides, and compact microring resonators with intrinsic quality factors up to 210,000 are fabricated. In addition, by undercutting the aluminum nitride layer, the intrinsic quality factor of the silicon carbide microring is improved by nearly one order of magnitude (1.8 million). Finally, an optical pump-probe method is developed to measure the thermal conductivity of the aluminum nitride layer, which is estimated to be over 30 times of that of silicon dioxide.

Highly efficient single-layer graphene electro-absorption modulator.

We propose a single-layer graphene electro-absorption modulator (EAM) with an angled waveguide sidewall. By utilizing the hybridization of the TM0 mode and the TE1 mode in the waveguide, the light-graphene interaction is enhanced. A modulation depth of 0.124 dB/µm and a figure of merit up to 25 are obtained at 1550 nm. Moreover, we show that the longitudinal electric field plays a significant role in making the optical absorption efficient, which indicates that modulation depth can be increased by enhancing the longitudinal electric field in modulators. This provides a promising solution for the future design of graphene optical modulators.

Investigation of the electro-optic effect in high-Q 4H-SiC microresonators.

Silicon carbide (SiC) recently emerged as a promising photonic and quantum material owing to its unique material properties. In this work, we carried out an exploratory investigation of the Pockels effect in high-quality-factor (high-Q) 4H-SiC microresonators and demonstrated gigahertz-level electro-optic modulation for the first time. The extracted Pockels coefficients show certain variations among 4H-SiC wafers from different manufacturers, with the magnitudes of r 13 and r 33 estimated to be in the range of (0.3-0.7) pm/V and (0-0.03) pm/V, respectively.

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.

Novel on chip rotation detection based on the acousto-optic effect in surface acoustic wave gyroscopes.

An Acousto-Optic Gyroscope (AOG) consisting of a photonic integrated device embedded into two inherently matched piezoelectric surface acoustic wave (SAW) resonators sharing the same acoustic cavity is presented. This constitutes the first demonstration of a micromachined strain-based optomechanical gyroscope that uses the effective index of the optical waveguide due to the acousto-optic effect rather than conventional displacement sensing. The theoretical analysis comparing various photonic phase sensing techniques is presented and verified experimentally for the cases based on a Mach-Zehnder interferometer, as well as a racetrack resonator. This first prototype integrates acoustic and photonic components on the same lithium niobate on insulator (LNOI) substrate and constitutes the first proof of concept demonstration of the AOG. This approach enables the development of a new class of micromachined gyroscopes that combines the advantages of both conventional microscale vibrating gyroscopes and optical gyroscopes.

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.

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.

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.

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.

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.

A sensitive protein-based sensor for quantifying histone acetylation levels.

H3K14ac (acetylation of lysine 14 of histone H3) is one of the most important epigenetic modifications in cells. Aberrant changes in H3K14ac are commonly found in various types of cancers and neurological disorders. Current detection approaches for histone modifications, however, require either tedious sample pre-treatments or lack the quantitative accuracy required for biochemical and biomedical applications. In this study, we engineered a protein sensor using the amino acid sequences derived from the bromodomain of human polybromo-1 (PB1). The protein sensor was conjugated to a fluorescent dye for sensitive detection of H3K14ac. Different detection conditions, such as additive concentrations and probe concentrations, were optimally selected by balancing signal strength (I(Rel)) and signal-to-noise ratio (SNR). The protein sensor was verified using histone H3 peptides containing different H3K14 acetylation levels. The detection signal was found to be linearly dependent on acetylation levels of H3K14 ranging from 5% to 100%. The designed platform can be used for screening epigienetic drugs regulating H3K14 acetylation levels as well as monitoring H3K14 acetylation level of circulating nucleosomes for disease progression.

Waveguides in single-crystal lithium niobate thin film by proton exchange.

The proton exchanged (PE) planar and channel waveguides in a 500 nm thick single-crystal lithium niobate thin film (lithium niobate on insulator, LNOI) were studied. The mature PE technique and strong confinement of light in the LN single-crystal thin film were used. The single mode and cut-off conditions of the channel waveguides were obtained by finite difference simulation. The results showed that the single mode channel waveguide would form if the width of the PE region was between 0.75 µm and 2.1 µm in the ß(4) phase. The channel waveguide in LNOI had a much smaller mode size than that in the bulk material due to the high-refractive-index contrast. The mode size reached as small as 0.6 µm(2). in simulation. In the experiment, the refractive index and phase transition after PE in LNOI were analyzed using the prism coupling method and X-ray diffraction. Three different width waveguides (5 µm, 7 µm and 11 µm) were optically characterized. Near-field intensity distribution showed that their mode sizes were 3.3 µm(2).,5 µm(2). and 7 µm(2). The propagation losses were evaluated to be about 16 dB/cm, 12 dB/cm and 11 dB/cm, respectively. The results indicate that PE is a promising method for building more complicated photonic integrated circuits in single-crystal LN thin film.

Photonic crystal slab fabricated on the platform of lithium niobate-on-insulator.

We report on a photonic crystal slab patterned on a 690 nm thick LiNbO3 thin film bonded to SiO2 on lithium niobate substrate. The transmission spectrum is measured and a broad and clear photonic bandgap ranging from 1335 to 1535 nm with a maximum extinction ratio of more than 20 dB is observed. The bandgap is simulated by plane wave expansion and 3D finite-difference time-domain methods. Such a deep and broad bandgap structure can be used to form high-performance photonic devices and circuits on the platform of lithium niobate-on-insulator.