A plasmon resonance sensor (SPR) based on D-shape photonic crystal fiber (PCF) is proposed to realize the simultaneous measurement of refractive index (RI) and temperature. The upper surface of the D-shape PCF is coated with gold film as a plasmon resonance excitation material for the detection of RI. An air hole near the fiber core is coated with gold film and filled with temperature-sensitive liquid ethanol to detect temperature. Results show that the y-polarized peaks supported only shift with RI variation and is unaffected by the temperature floating. Similarly, the x-polarized peak is only influenced by the change of temperature in the external environment. The proposed sensor achieved wavelength sensitivity of 3940nm/RIU with analyte RI between 1.35 and 1.40, and 1.075nm/°C of temperature ranging from 20°C to 60°C. The proposed liquid-filled D-shape photonic crystal fiber will have potential application in solving the problem of cross-sensitivity of temperature and RI.

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.

The integrated photonic technologies benefit from an improved frequency measurement range with compact size relative to the radio frequency counterparts. Nevertheless, the measurement methods are generally limited by a basic trade-off between the measurement range and accuracy. In this work, we break the above limitations and trade-off based on a Mach–Zehnder interferometer (MZI) coupled ring array and a two-step measurement method. The experimental results show that a wide measurement range of 40 GHz and an ultra-low error of 9 MHz could be obtained. Moreover, owing to the ring flexibly tunable bandwidth and resonant wavelength, the measurement range and accuracy could be significantly adjusted. Another frequency measurement range of 20 GHz with a lower error of 4 MHz is also demonstrated. To the best of our knowledge, among the silicon-based measurement schemes, it is a record estimation error with such a wide range. With the dominant advantages of CMOS-compatibility, a broad measurement range, ultra-low estimation errors and adjustable measurement performance, the proposed microwave measurement scheme has many important applications in on-chip microwave systems.