Danping Li

High-quality-factor photonic crystal ring resonator

A design for enhancing the quality (Q) factor of a photonic crystal ring resonator (PCRR) is introduced. The highest Q factor based on simulations is 121,000. The analysis of momentum space distributions of the electric field profile for PCRR resonance shows that a high Q factor of a PCRR is attributed to the reduction of tangential k-vector component inside the leaky region. A high Q factor of 75,200 is experimentally demonstrated for a modified PCRR on a silicon-on-insulator wafer. The high-Q-factor PCRR demonstrated here will be beneficial for channel drop filters, lasers, sensors, and other applications.

Ultralow Power Nonlinear Response in an Si Photonic Crystal Nanocavity

Optical nonlinear response and bistability behavior are observed in a fabricated silicon photonic crystal three defect-long (L3) nanocavity. The L3 cavity, which is coupled with a photonic crystal waveguide, has a quality factor of 60 000. Optical nonlinear response of the L3 cavity is observed at 4.65- μW input power, and the threshold power for optical bistability in the L3 cavity is 26.1 μW, which are the lowest values for silicon L3 cavities. A nonlinear coupled mode model is established to analyze nanocavity characteristics systematically. Numerical simulation results indicate that the ultralow power nonlinearity is due to the high Q factor and large thermal resistance of the nanocavity.

Enhanced 1524-nm Emission From Ge Quantum Dots in a Modified Photonic Crystal L3 Cavity

Light emitters based on Ge quantum dots embedded in modified photonic crystal three defect-long (L3) cavities are fabricated and characterized. Several sharp resonant luminescence peaks dominate the photoluminescence (PL) spectrum at room temperature. The strongest resonant luminescence peak is obtained at 1524 nm. The enhancement factor is 110, and the corresponding Purcell factor is estimated to be 6.7. The large enhancement is due to high Purcell factor and high collection efficiency of modified L3 cavity verified by far-field patterns. The intrinsic Q factor measured from crossed-polarized resonant scattering is much higher than the Q factor measured from PL, indicating that the Q factors measured from PL are inaccurate due to free-carrier absorption of the photogenerated carriers.

High Confinement Factor Ridge Slot Waveguide for Optical Sensing

A ridge slot waveguide on 220 nm silicon-on-insulator platform is proposed and theoretically investigated. The confinement factor of the ridge slot waveguide is much higher than the fully etched slot waveguide in the case of air-cladding. The sensitivity of sensors based on ridge slot waveguides is higher than that of the fully etched slot waveguides when the cladding refractive index is in the range of 1.0-1.18.

Enhanced light emission from Ge quantum dots in photonic crystal ring resonator

Light emitter based on Ge quantum dots embedded in photonic crystal ring resonator is designed and fabricated. Six sharp resonant peaks dominate the photoluminescence (PL) spectrum ranging from 1500 to 1600 nm at room temperature. The light emission enhancement is due to Purcell effect and high collection efficiency of the PCRR verified by calculated far-field patterns. The Purcell factor of the PCRR is estimated from enhancement factor and increased collection efficiency. The linewidth of the emission of a single Ge quantum dot is estimated from the Purcell factor.

Compact Grating Coupler for 700-nm Silicon Nitride Strip Waveguides

700-nm-thick silicon nitride-on-insulator (SNOI) is widely used for nonlinear applications due to its broadband nearly zero flattened dispersion. In this paper, we present a grating coupler for the TE mode of strip waveguides on 700-nm-thick SNOI. Focusing grating structure and inverse taper are combined to reduce the footprint to 70.2 μm × 19.7 μm. The peak coupling efficiency is -3.7 dB and the 1-dB bandwidth is 54 nm. The fabrication process is CMOS-compatible with only one additional etching step required.

Single-Mode Emission From Ge Quantum Dots in Photonic Crystal Nanobeam Cavity

A freestanding one-dimensional photonic crystal nanobeam cavity embedded with Ge self-assembled quantum dots is designed and fabricated on a silicon-on-insulator substrate. Only one photoluminescence peak is observed in the wavelength range of 1000 to 1600 nm at room temperature. The emission peak dominates the photoluminescence spectrum over an almost flat and weak background emission, which indicates single-mode emission is realized in the proposed light emitter.

Tuning of resonance spacing over whole free spectral range based on Autler-Townes splitting in a single microring resonator.

In this paper, a single microring resonator structure formed by incorporating a reflectivity-tunable loop mirror is demonstrated for the tuning of resonance spacing. Autler-Townes splitting in the resonator is utilized to tune the spacing between two adjacent resonances by controlling the strength of coupling between the two counter-propagating degenerate modes in the microring resonator. A theoretical model based on the transfer matrix method is built to analyze the device. The theoretical analysis indicates that the resonance spacing can be tuned from zero to one free spectral range (FSR). In experiment, by integrating metallic microheater, the tuning of resonance spacing in the range of the whole FSR (1.17 nm) is achieved within 9.82 mW heating power dissipation. The device has potential for applications in reconfigurable optical filtering and microwave photonics.

Slow-Light Dispersion in One-Dimensional Photonic Crystal Racetrack Ring Resonator

A one-dimensional photonic crystal racetrack ring resonator coupling with an optimized bus waveguide on a silicon-on-insulator wafer is proposed and demonstrated. The nonuniform free spectral range is realized in the resonator. A maximum group index of 37.6 and a slowdown factor of 11 are obtained. The nonuniform free spectral range and large group index are attributed to the slow-light effect near the band edge of the one-dimensional photonic crystal. A three-dimensional finite-difference time-domain method is performed to simulate the resonator. The simulation results show good agreement with the experimental results. The photonic crystal racetrack ring resonator demonstrated here will be beneficial for channel drop filters, wavelength-division multiplexing, lasers, and other applications.

Low Power and Large Modulation Depth Optical Bistability in an Si Photonic Crystal L3 Cavity

Optical bistability with low power and large modulation depth is observed in the silicon 2D photonic crystal 3 defect-long (L3) cavity coupling with a photonic crystal waveguide experimentally. The cavity-waveguide resonator system operates under the critical coupling condition. The triangular line shape of the cavity resonance and the hysteresis loop of the system is observed. The switching contrast is 7.7 dB and the modulation depth is 0.70 at the switching power of -12.1 dBm. A nonlinear coupled mode model is established to analyze the behavior of the cavity, and numerical simulation results are in good agreement with the experiment results.