Highly sensitive terahertz fingerprint sensing based on the quasi-guided modes in a distorted photonic lattice.

Using photonic structures resonating at the characteristic absorption frequency of the target molecules is a widely-adopted approach to enhance the absorption and improve the sensitivity in many spectral regions. Unfortunately, the requirement of accurate spectral matching poses a big challenge for the structure fabrication, while active tuning of the resonance for a given structure using external means like the electric gating significantly complicates the system. In this work, we propose to circumvent the problem by making use of quasi-guided modes which feature both ultra-high Q factors and wavevector-dependent resonances over a large operating bandwidth. These modes are supported in a distorted photonic lattice, whose band structure is formed above the light line due to the band-folding effect. The advantage and flexibility of this scheme in terahertz sensing are elucidated and exemplified by using a compound grating structure on a silicon slab waveguide to achieve the detection of a nanometer scale α-lactose film. The spectral matching between the leaky resonance and the α-lactose absorption frequency at 529.2 GHz by changing the incident angle is demonstrated using a flawed structure which exhibits a detuned resonance at normal incidence. Based on the high dependence of the transmittance at the resonance on the thickness of α-lactose, our results show it is possible to achieve an exclusive detection of α-lactose with the effective sensing of thickness as small as 0.5 nm.