Opt Express. 2023 Mar 13;31(6):10947-10954. doi: 10.1364/OE.477547.
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.
利用在目标分子特征吸收频率处共振的光子结构来增强吸收并提高许多光谱区域的灵敏度，是一种被广泛采用的方法。不幸的是，结构制造对精确光谱匹配的要求带来了巨大的挑战，而使用外部手段（如电门控）对给定结构进行共振的主动调谐则显著增加了系统的复杂性。在这项工作中，我们建议通过利用准导模来规避这个问题，这些准导模具有超高 Q 因子和在大工作带宽内的波矢相关共振。这些模式在一个变形的光子晶格中得到支持，其带结构由于能带折叠效应而在光线线以上形成。通过在硅片波导上使用复合光栅结构来实现对纳米级α-乳糖薄膜的检测，阐明并举例说明了这种在太赫兹传感中的优势和灵活性。通过使用具有在正常入射时失谐共振的有缺陷结构，演示了在改变入射角时漏波共振与 529.2GHz 的α-乳糖吸收频率之间的光谱匹配。基于共振处的透射率对α-乳糖厚度的高度依赖性，我们的结果表明，有可能通过有效感应厚度小至 0.5nm 来实现对α-乳糖的选择性检测。
