We propose and demonstrate a six-hole tapered silicon photonic crystal nanobeam cavity with a theoretical high quality (Q) factor and an ultrasmall mode volume based on machine learning. The crucial element to efficiently obtain high Q factors is to take the prediction result of the designed neural network as the fitness function of the genetic algorithm, whose evolution direction is developing towards higher fitness. Consequently, by combining the neural network and genetic algorithm iteration, an optimized photonic crystal nanobeam cavity with a theoretical Q factor (Q_th) as high as 1.2×10⁸ and an ultrasmall mode volume of 0.32(λ/n)³ is obtained. Leveraging the resonant scattering optical method, the cavity experimental Q factor (Q_exp) is measured as 2.17×10⁶, which is a record high experimental Q factor of silicon photonic crystal nanobeam cavity with maintaining an ultrasmall mode volume of 0.32(λ/n)³ and an ultra-compact device size of 6 μm². Owing to the ultra-high Q factor-to-mode volume ratio, the proposed photonic crystal nanobeam cavities could extremely enhance the interactions between light and matter, which have extensive important applications in low-threshold optical lasers, high-resolution filters and sensors.

We propose and experimentally demonstrate a narrowband notch microwave photonic filter (MPF) with an ultra-high all-optical tuning efficiency based on an optimized high-quality (Q) nanobeam photonic crystal (PhC) cavity. By optimizing the structural parameters, the hole shapes and the waveguide width of the cavity, a silicon nanobeam cavity with a theoretical high-Q factor of 4×10 ⁸ , a small mode volume (V) of 0.32(λ/ n ) ³ and a compact size of 6 μm ² could be obtained. Due to the extremely high Q/V ratio beyond 1×10 ⁹ , the nonlinear effects in the cavity could be efficiently excited, which is beneficial for on-chip low-power signal processing. In the experiment, by adjusting the wavelength of the optical carrier, the central frequency of the notch MPF could be widely tuned from 1 GHz to 40 GHz with maintaining the 3dB-bandwidth around 198 MHz and rejection ratios beyond 50 dB. On the other side, the central frequency of the MPF could be all-optically tuned based on the nonlinear effects in the cavity. Owing to the cavity extremely high Q/V ratio, the MPF frequency can be adjusted with an ultra-high tuning efficiency of 308.8 GHz/mW. To the best of our knowledge, whether the notch MPF narrow 3dB-bandwidth of 198 MHz or the all-optical tuning efficiency of 308.8 GHz/mW is a record value among the reported silicon-cavity-based notch MPFs with such a compact device size. With the dominant advantages of sub-gigahertz narrowband, wide tuning range, ultra-high all-optical tuning efficiency and extremely compact cavity size, the proposed MPFs illustrate competitive performance for on-chip microwave signal processing.

An approach to optimize the sensitivity and the limit of detection of on-chip refractive index sensor is proposed and demonstrated based on sub-wavelength grating racetrack microring resonator and Vernier effect. The sub-wavelength grating waveguide can reduce the structure limitation of the light field, which is beneficial to enhancing the interaction between the photon and analyte. By optimizing the parameters of the sub-wavelength grating racetrack microring resonator, the sensitivity of the sensor could be significantly improved to 664 nm/RIU. Subsequently, capitalizing the Vernier effect, a two cascaded microring-based refractive index sensor is designed. Owing to the Vernier effect, the wavelength spacings among the overlapped peaks could be effectively amplified more than ten times, leading to a high performance. The results demonstrate that an ultra-high sensitivity of 7061 nm/RIU and a low limit of detection of 1.74 × 10 ⁻⁵ RIU. With the advantages of ultra-high sensitivity and low limit of detection, the integrated device has important value in the fields of environmental monitoring and biosensors.

We propose and experimentally demonstrate sub-gigahertz microwave photonic filters (MPFs) with widely tunable bandwidths and ultra-high rejection ratios based on a silicon add-drop Mach–Zehnder interferometers (MZI) coupled ring array. To realize a ring filter with a widely tunable bandwidth and a flexibly adjustable extinction ratio, the two coupling waveguides of the silicon resonator are designed as bending waveguides and are both fabricated with microheaters. Consequently, the effective coupling coefficients between the add-drop ring and the coupling waveguides could be significantly manipulated, resulting in tunable bandwidths and extinction ratios. Capitalizing the designed resonators with broadly adjustable bandwidth, the experimental results show that the 3dB-bandwidth of the MPF could be widely tuned from 0.178 GHz to 22.7 GHz. What's more, owing to the ring flexibly tunable extinction ratios and the complete interference cancellation technology, the maximum MPF rejection ratios could realize beyond 75 dB. To the best of our knowledge, it is the first time to realize the notch MPFs with widely tunable 3dB-bandwidths ranging from sub-gigahertz to tens of gigahertz while maintaining the ultra-high rejection ratios. The proposed scheme paves a way for the notch MPFs with the dominant advantages of narrow bandwidths, wide bandwidth tuning ranges, ultra-high rejection ratios and complementary metal-oxide semiconductor (CMOS)-compatibility, which is competent to process dynamic radio frequency (RF) signals in on-chip microwave systems.

We propose and realize a bandwidth and wavelength-tunable all-optical filter with high tuning efficiency based on double opto-mechanical microring resonators (MRRs) assisted Mach-Zehnder interferometer (MZI) structure, which is beneficial to achieve high extinction ratio and shape factor. As the optical field gradient is largely enhanced in the free-hanging MRRs, the opto-mechanical effect could be excited by low power consumption. By injecting the corresponding low resonance powers, the drop transmission of each MRR could be flexibly tuned based on the opto-mechanical effect. Consequently, the synthetic transmission of the microring-assisted MZI could be efficiently manipulated. The results show that by injecting pump powers of 0.40 mW and 1.84 mW, the bandwidth and wavelength of the all-optical filter could be tuned from 0.17 nm to 0.30 nm, and from 1550.09 nm to 1550.60 nm respectively with maintaining a high extinction ratio of 38 dB and a small passband ripple of 1 dB. Especially, the tuning efficiencies of bandwidth and wavelength could realize up to 0.325 nm/mW and 0.277 nm/mW, respectively. The proposed optical filter has dominant advantages of high tuning efficiencies and extinction ratios, small passband ripple and compact footprint, which has significant applications in on-chip all-optical systems.

We propose and experimentally demonstrate a silicon integrated high-order mode fiber-chip coupler. By utilizing a structure of apodized double-part gratings with mode selective trident, TE ₁ mode can be coupled with low mode crosstalk. Design details and the corresponding simulation results are presented. The coupler is fabricated on silicon integrated platform. Measured results show the loss is 4.68 dB for TE ₁ -LP ₁₁ mode coupling with the mode crosstalk <−23 dB. The proposed coupler could be utilized in the advanced mode division multiplexing (MDM) system.