Building a better platform paradigm for terahertz biosensing

On-chip Terahertz (THz) biosensors, which boast accurately tunable resonance frequencies, represent enormous potential as miniature platforms capable of selective, non-invasive, marker-free biomolecule detection. Such integrable devices have applications ranging from food safety and environmental monitoring to virus detection and cancer diagnoses.

However, conventional on-chip THz platforms are limited by small mode-confinement volumes and high sensitivities to manufacturing defects, which can significantly restrict analyte interaction with the electromagnetic field. Furthermore, water absorption at THz frequencies can obscure crucial signals, necessitating laborious drying procedures that can compromise a subject’s bioactivity.

Addressing these limitations, Navaratna et al. developed an innovative on-chip all-Silicon CMOS-compatible photonic topological waveguide-cavity chip and used its evanescent, or out-of-plane field, for biosensing.

“By using the concept of photonic topological insulators, we bestow the chip with an inherent capacity to withstand manufacturing defects and ensure seamless scattering-free propagation of electromagnetic waves, even through sharp corners,” said author Nikhil Navaratna. “The resulting transmission minima of the topological waveguide-cavity chip are sharp, stable, and offer great potential for sensing experiments.

“Additionally, topologically protected electromagnetic waves propagating along the circumference of the cavity offer great flexibility in positioning analytes while increasing the effective regions of interaction for evanescent field-based sensing modalities,” Navartna said.

Indeed, by strategically placing analytes at distances up to 200 µm away from the plane of the chip, the researchers achieved effective sensing, even for aqueous biomolecular solutions. Responsive transmission minima also enabled real-time on-chip leaf hydration monitoring.

“Our study showcases the successful implementation of biosensing on an on-chip THz topological platform,” said Navartna. “We believe that this paves the way for the development of CMOS-compatible on-chip platforms for THz biosensing and THz spectroscopy.”

Source: “On-chip topological THz biosensors,” by Nikhil Navaratna, Yi Ji Tan, Abhishek Kumar, Manoj Gupta, and Ranjan Singh, Applied Physics Letters (2023). The article can be accessed at http://doi.org/10.1063/5.0157357 .