In recent decades, the terahertz (THz) frequency range, defined roughly between 100 GHz and 10 THz, has attracted significant interest. THz waves offer several advantages, including non-ionizing radiation, penetrating dielectrics, relatively short wavelengths, and specific absorption spectra for many substances. These characteristics make THz techniques a promising candidate for biosensing applications. A THz biosensor is capable of utilizing the interaction between electromagnetic (EM) fields and target analytes, which are dielectric materials, and converting biological reactions into readable electrical signals. This provides a rapid and label-free solution for pathogen detection. However, the majority of THz biosensors face limitations due to a low quality factor (Q-factor) or weak interaction with immobilized analytes, leading to a low figure of merits (FOM). Recently, photonic crystals (PhC), which originate from optics, have gathered significant attention for THz communication and biosensing applications. Of particular interest is the potential of PhC resonators for the detection of thin-film analytes. This is due to the advantages that they offer, including an ultra-high Q-factor, enhanced interaction, mature fabrication technology, and high design flexibility.

This work presents a dual-channel PhC chip with high sensitivity and high temperature robustness, designed for thin-film sensing. The PhC chip comprises a splitter, a combiner, and two PhC slot resonators. The PhC slot resonators feature a slot that enhances the interaction. The Q-factor is optimized by tuning the dimensional parameters and displacing the holes surrounding the slot. One of the resonators serves as a reference without the presence of analytes, while the other acts as a sensing resonator that is loaded with analytes. Consequently, the difference in their resonant frequencies remains constant with varying temperature. To prove the principle, sub-THz PhC sensors are fabricated using laser-cutting technology and characterized with NaCl as the analyte. The PhC chips exhibit a Q-factor of up to 4600 at about 92 GHz and a considerably higher FOM than other THz biosensors, even though their operating frequency is relatively low. Moreover, the resonance difference remains almost constant with varying temperatures.

The majority of current detection methods are only capable of identifying pathogen following infection, which present a significant challenge in controlling the rapid spread of airborne pathogens. To effectively counter the transmission of pathogens, it is critical to develop methods for the early detection of pathogens in the air before they cause infection. Such methods must be capable of reading potential exposure information over large areas. Nevertheless, it is a significant challenge to achieve a long reading range and a high sensitivity simultaneously in a low-cost and compact sensor. In this context, the THz biosensor offers significant advantages. (i) Wireless communication is based on the same physical principle as the sensor, namely EM waves. The absence of active circuitry and battery in passive EM sensors results in a reduction of complexity and cost. (ii) The maximum reading range increases with increasing wavelength, while the sensitivity increases with decreasing wavelength. The THz frequency range offers a satisfactory compromise between sensitivity and reading range. Among the various types of sensors, PhC sensors offer both high sensitivity and long-range readout capabilities.

The second significant contribution of this work is the development of a remote sensor based on the PhC for a wireless sensor network. The remote sensor comprises a PhC slot resonator for thin-film sensing and a dielectric rod antenna (DRA) for wireless radiation. The maximum reading range is analyzed using the radar equation and temporal coupled mode theory. Sensors for the W-band are fabricated using a 3D printing technique for rapid prototyping. The fabricated sensors of different dimensions are characterized using a wireless reflection measurement setup. The reflection parameters are gated in time domain to extract resonant frequency from reflection clutters. The sensors have demonstrated a reading range of up to 0.9 m and an acceptance angle of 90° in both the elevation and azimuth planes. Furthermore, the sensors have exhibited a high remote sensitivity to a model protein at a distance of 0.5 m.

In summary, the proposed sub-THz biosensors based on PhC demonstrate a range of functionalities and high potential for various pathogen detection scenarios. In future works, the PhC biosensors can be further equipped with multiple channels, microfluidics, immobilization, and a compact radar reader to develop a commercial product.