The refractive index (RI) of biological tissues is a fundamental material parameter that characterizes how light interacts with tissues, making accurate measurement of RI crucial for biomedical diagnostics and environmental monitoring. A Janus sensor (JBS) is designed in this paper, and the photonic spin Hall effect (PSHE) is used to detect subtle changes in RI in biological tissues. The asymmetric arrangement of the dielectric layers breaks spatial parity symmetry, resulting in significantly different PSHE displacements during the forward and backward propagation of electromagnetic waves, thereby realizing the Janus effect. The designed JBS can detect the RI range of 1.3~1.55 RIU when electromagnetic waves are incident along the +z-axis, with a sensitivity of 96.29°/refractive index unit (RIU). In the reverse direction, blood glucose concentrations are identified by the JBS, achieving a sensitivity of 18.30°/RIU. Detecting different RI range from forward and backward scales not only overcomes the limitation that single-scale sensors can only detect a single RI range, but also provides new insights and applications for optical biological detection through high-sensitivity, label-free and non-contact detection.

In this paper, a kind of metastructure–photonic crystal (MPC) with multi-frequency asymmetric absorption–transmission properties is proposed. It is composed of various dielectric layers arranged in a periodically tilting pattern. When electromagnetic waves (EMWs) enter from the opposite direction, MPC shows an obvious asymmetry. EMWs are absorbed at 13.71 GHz, 14.37 GHz, and 17.10 GHz in forward incidence, with maximum absorptions of 0.919, 0.917, and 0.956, respectively. In the case of backward incidence, transmission above 0.877 is achieved. Additionally, the MPC is utilized for refractive index (RI) sensing, allowing for wide RI range detection. The refractive index unit is denoted as RIU. The RI detection range is 1.4~3.0, with the corresponding absorption peak variation range being 17.054~17.194 GHz, and a sensitivity of 86 MHz/RIU. By adjusting the number of MPC cycles and tilt angle, the sensing performance and operating frequency band can be tailored to meet various operational requirements. This MPC-based RI sensor is simple to fabricate and has the potential to be used in the development of high-performance and compact sensing devices.

Identifying brain-tissue types holds significant research value in the biomedical field of non-contact brain-tissue measurement applications. In this paper, a layered metastructure is proposed, and the second harmonic generation (SHG) in a multilayer metastructure is derived using the transfer matrix method. With the SHG conversion efficiency (CE) as the measurement signal, the refractive index ranges that can be distinguished are 1.23~1.31 refractive index unit (RIU) and 1.38~1.44 RIU, with sensitivities of 0.8597 RIU⁻¹ and 1.2967 RIU⁻¹, respectively. It can distinguish various brain tissues, including gray matter, white matter, and low-grade glioma, achieving the function of a second harmonic mode sensor (SHMS). Furthermore, temperature has a significant impact on the SHG CE, which can be used to define the switch signal indicating whether the SHMS is functioning properly. When the temperature range is 291.4~307.9 Kelvin (K), the temperature switch is in the “open” state, and the optimal SHG CE is higher than 0.298%, indicating that the SHMS is in the working state. For other temperature ranges, the SHG CE will decrease significantly, indicating that the temperature switch is in the “off” state, and the SHMS is not working. By stimulating temperature and using the response of SHG CE, the temperature-switch function is achieved, providing a new approach for temperature-controlled second harmonic detection.

In this letter, an innovative silicon-based metastructure (SBMS) is proposed for second harmonic generation (SHG) to enable the detection of refractive index (RI) and coronavirus disease 2019 (COVID-19) solution concentrations (SC). Utilizing the transfer matrix method (TMM) for analysis and computation, RI sensing is achieved, unveiling the sensitivity of SBMS towards variations in COVID-19 SC. Subsequently, a model correlating COVID-19 SC with the frequency of the second harmonic wave (SHW) transmission peak (TP) is established. Specifically, SBMS demonstrates a RI sensing detection range from 1.1 to 1.4, with a sensitivity of up to 16.946 THz/RIU and an optimal quality factor of $1.66 \times 10 ^{\mathbf {5}}$ . Additionally, the relevant values for COVID-19 SC detection are 0 mM to 525 mM (where M is molarity, and 1 M =1 mol/L), 0.71225 THz/M, and $1.66 \times 10 ^{\mathbf {5}}$ . The SBMS not only provides an important theoretical foundation for the development of novel virus detection technologies but also expands new possibilities for the application of SBMS in the biomedical field.

A novel hyperstructure (HS) based on periodically poled strontium barium niobate (SBN) is proposed for second harmonic generation, showcasing its multi-functionality in multi-window and multi-biomass sensing applications. Through meticulously designed HS, distinct measurement windows are achieved in both the fundamental wave (FW) and the second harmonic wave (SHW). Within the FW window, a refractive index (RI) range of 1.35~1.4 is measured, enabling the differentiation of various stages of malaria by detecting the RI of red blood cells. The SHW window facilitates RI measurements in the range of 1.53~1.56, allowing the discrimination between healthy and malignant cells by detecting the RI of cell nuclei. Furthermore, the HS proposed enables the measurements of the thickness of air film, with measurement ranges of 9 μm~9.5 μm for the FW and 9.5 μm~10.1 μm for the SHW. The multi-functionality of this HS demonstrates extensive potential applications in medical diagnostics and beyond. Additionally, it presents an approach for real-time, and label-free detection in the field of biomedical research.