Simultaneous Detection of Relative Humidity and Temperature Based on Silicon On-Chip Cascaded Photonic Crystal Nanobeam Cavities

In this paper, we propose and numerically demonstrate a novel cascaded silicon-on-insulator (SOI) photonic crystal nanobeam cavity (PCNC) dual-parameter sensor for the simultaneous detection of relative humidity (RH) and temperature. The structure consists of two independent PCNCs supporting two different resonant modes: a dielectric-mode and an air-mode, respectively. The dielectric-mode nanobeam cavities (cav1) are covered with SU-8 cladding to increase the sensitivity ratio contrast between RH sensing and temperature sensing. The air-mode nanobeam cavities (cav2) are coated with a water-absorbing polyvinyl-alcohol (PVA) layer that converts the change in RH into a change in refractive index (RI) under different ambient RH levels, thereby inducing a wavelength shift. Due to the positive thermo-optic (TO) coefficient of silicon and the negative TO coefficient of SU-8 cladding, the wavelength responses take the form of a red shift for cav2 and a blue shift for cav1 as the ambient temperature increases. By using 3D finite-difference time-domain (3D-FDTD) simulations, we prove the feasibility of simultaneous sensing by monitoring a single output transmission spectrum and applying the sensor matrix. For cav1, the RH and temperature sensitivities are 0 pm/%RH and −37.9 pm/K, while those of cav2 are −389.2 pm/%RH and 58.6 pm/K. The sensitivity ratios of temperature and RH are −1.5 and 0, respectively, which is the reason for designing two different resonant modes and also implies great potential for realizing dual-parameter sensing detection. In particular, it is also noteworthy that we demonstrate the ability of the dual-parameter sensor to resist external interference by using the dual wavelength matrix method. The maximum RH and temperature detection errors caused by the deviation of resonance wavelength 1 pm are only 0.006% RH and 0.026 K, which indicates that it achieves an excellent anti-interference ability. Furthermore, the structure is very compact, occupying only 32 μm × 4 μm (length × width). Hence, the proposed sensor shows promising prospects for compact lab-on-chip integrated sensor arrays and sensing with multiple parameters.

Photonic crystal nanobeam cavities with lateral fins

We present the design, fabrication, and characterization of suspended arrays of small volume, high quality factor (Q) silicon nitride photonic crystal nanobeam (PCNB) cavities with lateral nanorod fin structures. By controlling the alignment position of the fins with respect to the air holes, the resonance wavelength and Q-factor of the PCNB cavities can be tuned to realize the desired performance. Measured tunable range of 2 × 104 and 10 nm is achieved for Q-factor and resonance wavelength, respectively, with the highest Q-factor measured at 2.5 × 104. Incorporating such nanorod fins into the nanobeam cavity is demonstrated to provide improved mechanical support, thermal transport, and channels of lateral carrier injection for the suspended PCNB. The proposed PCNB cavities with lateral fins are advantageous for energy efficient, ultra-compact lasers, modulators, filters, and sensors.

Tailoring topological edge states with photonic crystal nanobeam cavities

The realization of topological edge states (TESs) in photonic systems has provided unprecedented opportunities for manipulating light in novel manners. The Su–Schrieffer–Heeger (SSH) model has recently gained significant attention and has been exploited in a wide range of photonic platforms to create TESs. We develop a photonic topological insulator strategy based on SSH photonic crystal nanobeam cavities. In contrast to the conventional photonic SSH schemes which are based on alternately tuned coupling strength in one-dimensional lattice, our proposal provides higher flexibility and allows tailoring TESs by manipulating mode coupling in a two-dimensional manner. We reveal that the proposed hole-array based nanobeams in a dielectric membrane can selectively tailor single or double TESs in the telecommunication region by controlling the coupling strength of the adjacent SSH nanobeams in both transverse and axial directions. Our finding provides an additional degree of freedom in exploiting the SSH model for integrated topological photonic devices and functionalities based on the well-established photonic crystal nanobeam cavity platforms.