Jinzhong Yu

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.

Electromagnetically induced transparency-like effect in a two-bus waveguides coupled microdisk resonator

We observe theoretically and experimentally electromagnetically induced transparency (EIT)-like effect in a single microdisk resonator (MDR) evanescently coupled with two bus waveguides. This structure is modeled using transfer matrix method, and it is revealed that the EIT-like spectrum originates from the coherent interference between two nearby low-order whispering-gallery modes (WGMs) with comparable quality factors. The EIT-like properties have been investigated analytically with respect to coupling efficiency, round-trip power attenuation, as well as phase spacing between two resonances. The resonance spacing and mode coupling are adjustable by varying the effective indices of WGMs and waveguide mode. Consequently, fully integrated MDRs were fabricated in silicon. Resonant modes and coupling efficiency are studied in one-bus waveguide coupled MDRs. Finally, EIT-like resonance is observed in a two-bus waveguides coupled MDR of 3 μm in radius with a quality factor of 4,200 and central transmission larger than 0.65. The experimental results agree with our modeling well and show good internal consistency, confirming that two WGMs coupled in a point-to-point manner are required for EIT-like effect.

Enhanced parametric frequency conversion in a compact silicon-graphene microring resonator

We demonstrate four-wave mixing (FWM) in a 10-μm-radius silicon microring resonator with the assistance of giant nonlinearity of the monolayer graphene. A maximum enhancement of 6.8 dB of conversion efficiency in the silicon-graphene microring (SGM) resonator is observed. A nonlinear propagation model is established and the optical Kerr coefficient of the silicon-graphene hybrid waveguide is three times larger than that of the silicon waveguide.

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.

Dual-band optical filter based on a single microdisk resonator

We propose and experimentally demonstrate a dual-band optical filter based on a single microdisk resonator. An analytical model is built based on the transfer matrix method and is applied to simulate the properties of such a device. Competition and interference of the dual modes in the resonator lead to dual-band filtering with high isolation. As the finite-difference time-domain simulation illustrates, two low-order resonant modes can be effectively triggered by optimizing the waveguide width and spacing gap between the compact resonator and waveguides. In experiment, a double side-coupled microdisk resonator was fabricated on a nanophotonic silicon-on-insulator platform, and dual-band bandpass filtering is realized with an optical isolation higher than 20 dB and an insertion loss lower than 2 dB. The experimental results agree well with our modeling results.

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.

Experimental demonstration of a microdisk resonator filter/buffer utilizing two-mode interference

We have experimentally demonstrated a microdisk-resonator (MDR)-based filter for filtering and buffering applications utilizing two-mode interference. Optical filtering or buffering behavior can be generated by the coherent interference between two orthogonal whispering-gallery modes (WGMs) in a MDR symmetrically coupled to two bus waveguides. MDRs were fabricated using nanofabrication processes on a silicon-on-insulator platform. Multiple resonances of WGMs are excited in a MDR and unique filter characteristics have been observed for two spectrally nearby resonances. The measured results agree well with the theoretical simulation. Bandpass filtering is achieved with a 3 dB bandwidth of 0.66 nm and an in-band ripple of less than 1 dB when the two resonances are detuned, while the filter can function as an optical buffer with an insertion loss of 5.85 dB and a predicted time delay of 15 ps when the two resonances are approximately aligned. Furthermore, the influence of resonance spacing is analyzed on the performance of this filter.