Compared to the conventional strip waveguide microring resonators, subwavelength grating (SWG) waveguide microring resonators have better sensitivity and lower detection limit due to the enhanced photon-analyte interaction. As sensors, especially biosensors, are usually used in absorptive ambient environment, it is very challenging to further improve the detection limit of the SWG ring resonator by simply increasing the sensitivity. The high sensitivity resulted from larger mode-analyte overlap also brings significant absorption loss, which deteriorates the quality factor of the resonator. To explore the potential of the SWG ring resonator, we theoretically and experimentally optimize an ultrasensitive transverse magnetic mode SWG racetrack resonator to obtain maximum quality factor and thus lowest detection limit. A quality factor of 9800 around 1550 nm and sensitivity of 429.7 ± 0.4nm/RIU in water environment are achieved. It corresponds to a detection limit (λ/S·Q) of 3.71 × 10⁻⁴ RIU, which marks a reduction of 32.5% compared to the best value reported for SWG microring sensors.

All-optical switches have been considered as a promising solution to overcome the fundamental speed limit of the current electronic switches. However, the lack of a suitable third-order nonlinear material greatly hinders the development of this technology. Here we report the observation of ultrahigh third-order nonlinearity about 0.45 cm²/GW in graphene oxide thin films at the telecommunication wavelength region, which is four orders of magnitude higher than that of single crystalline silicon. Besides, graphene oxide is water soluble and thus easy to process due to the existence of oxygen containing groups. These unique properties can potentially significantly advance the performance of all-optical switches.

We investigate the waveguide dispersion of subwavelength structures, and propose that the waveguide dispersion can be reduced by reducing the period of subwavelength structures. A 3 dB bandwidth increment of 20% has been observed by introducing this concept into previously demonstrated grating couplers. To fully exploit the bandwidth merits of the structures, gratings with interleaved subwavelength structures were designed and fabricated. Two typical types of interleaving geometries have been investigated. Both demonstrated a 1 dB bandwidth $\sim 70\mathrm{nm}$, a 3 dB bandwidth $\sim 117\mathrm{nm}$, and a peak efficiency $\sim -5.1\mathrm{dB}$ at 1570 nm for transverse-electric polarized light. The simulation confirms that the dispersion engineering adds an extra 12 nm to the 1 dB bandwidth.

In this paper, we present on-chip optical true time delay lines based on slow light one-dimensional (1D) fishbone photonic crystal waveguides (FPCWs). The structural slow light is generated by modulating the index guided optical mode with periodically arranged sidewalls along the propagation direction. Due to the reduced mode overlap with the rough etched surface, the propagation loss of the 1D FPCW is significantly reduced compared to the two-dimensional photonic crystal waveguide. A delay time of 65 ps/mm is observed experimentally.

We present a high yield and high sensitivity on-chip biosensing system by combining subwavelength grating coupling and high sensitivity photonic crystal microcavity side coupled to a photonic crystal waveguide. 80% yield of working devices was experimentally demonstrated for sensitivity engineered L13 photonic crystal microcavities and 70% for L21 photonic crystal microcavities. Subwavelength grating couplers significantly improved the quality of the output transmission spectrum. By engineering the optical loss rate from the cavity to the waveguide, we experimentally detected 1 pM (67 pg/ml) and 50 femto-molar (3.35 pg/ml) concentration of avidin binding to biotin in phosphate buffered saline for L21 and L55 PC microcavities respectively, which represents the highest sensitivity versus other chip-based optical biosensors.