Borriboon Thubthimthong

Asymmetrically and Vertically Coupled Hybrid Si/GaN Microring Resonators for On-Chip Optical Interconnects

A working small-footprint asymmetrically and vertically coupled hybrid Si/GaN microring resonator (HMR) was demonstrated. The HMR of a minimum radius of 20 μm was fabricated in a high-yield (~90%) hybrid nanophotonic platform that allowed interactions between the GaN microring and the underlying Si waveguide circuit. The HMRs spectral response across the telecommunication C- and L-bands was measured. The high extinction ratio of up to 17 dB, the resonance linewidth as narrow as 40 pm, and the intrinsic quality factor of up to 70 000, which was the highest value for GaN microring reported so far, were achieved. The explicit analytical model for the high-index-contrast HMR was developed. Our basic study opened new possibilities for exploring GaN-Si nonlinear phenomena and for developing complex on-chip optical interconnects.

All-optical guided resonance tuning in hybrid GaN/Si microring induced by non-radiatively trapped injected hot electrons.

Advanced Si/III-V nanophotonics that emerged in the last decade has already founded the on-chip optical interconnect technology for future integrated systems on chip. New possibilities of light-on-chip applications beside signal transmissions, such as, all-optical sensing, have been given but small attention. Here, all-optical ultraviolet (UV)-sensitive guided resonance tuning in a hybrid GaN/Si microring resonator (HMR) was studied. Resonance redshifting by free-space UV pumping resulted in a 12 dB guided-mode modulation at the 1560 nm telecommunication wavelength. Investigations by experiments, theory, and simulations indicated the origin of the tuning mechanism from hot-electron heat extraction via defects-assisted non-radiative recombinations and electron-phonon interactions. A photothermal tuning efficiency of 73 pm/mW was attained at a pump power of 850 μW, thank to photothermal energy directly generated in the HMR. The UV-sensitive visible-blind all-optical tuning in the HMR may benefit all-optical UV sensing for the optical data era to come.

Group velocity enhancement for guided lightwave by band-pulling effect in single-mode silicon comb photonic crystal wires

Photonic crystal waveguides (PCWs), which usually exhibit the slow-light effect near the band edge frequency, may in fact be used to induce higher group velocity (vg). The presence of the band edge, due to periodically carving a strip waveguide (SW) into a PCW of a comb-shape dielectric wire, results in the band-pulling effect. The band-pulling effect, which lifts the fundamental photonic band toward the band gap, not only enhances vg by reducing the effective index but also prevents the band from bending toward the light cone of the core material, leading to additional vg enhancement. The maximum PCW vg (0.43c; c = vacuum light speed) found from numerical modeling was higher than SW vg at all widths and thicknesses, except for extremely low thickness below 80 nm. We found by numerical modeling and experiments that up to 75% enhancement at the 1550 nm wavelength may be achieved in a fabricated single-mode PCW (0.38c) compared with an SW (0.21c) of the same width and thickness. The higher vg was confirmed by measuring the PCWs group index using ring resonators. The PCW propagation loss of 2.1 dB mm−1 and the bandwidth-distance product of 100 Gb/s-cm were deduced from experimental results. Our findings seemed encouraging for employing PCWs in optical interconnect applications.

Hybrid Si/GaN microring resonator integrated with Si electrodes for electro-optic tuning

We report here the hybrid Si/GaN microring resonator structure integrated with Si electrodes. Preliminary results on electro-optic tuning of the guided-mode resonance was in the range of 22 pm for the entire applied voltage range of -60 V to 80 V. Up to 9 dB change in the transmission was observed at the maximum applied voltage. The device might be useful for static wavelength-selectable notch filtering of on-chip guided optical signal.

Photonic-crystal-waveguide-assisted directional couplers for hybrid Si/GaN nanophotonics

Photonic-crystal-waveguide-assisted (PCW-assisted) directional coupler fabricated from a hybrid nanophotonic fabrication platform of Si and GaN was studied. The coupler consisted of a Si PCW stacked on top by a GaN ridge waveguide (GaN RdW). High scattered near-infrared intensity at the output ports of the coupler was observed. The maximum coupling found was around 50%. The study may be promising for complex nanophotonic systems such as optical interconnect or optical signal processors.

Fast guided mode of a photonic crystal waveguide

A fast guided mode of a photonic crystal (PC) waveguide was investigated. Dispersion curve of the PC waveguide was calculated and compared to that of a volume-equivalent Si rectangular waveguide. At a wavelength of 1550 nm, the dominant phase velocity and the group velocity of the PC waveguide were higher by 4.6% and 3.4%, respectively. The PC waveguide was incorporated into a Mach-Zehnder interferometer for measuring the guided mode. Normalized measured intensity versus PC waveguide length was found to obey the predicted interference intensity curve which suggested guiding of the mode. Loss associated with the PC waveguide was estimated from the curve to be about -5.6 dB.