A fluid sensor based on a sub-terahertz photonic crystal waveguide

We present two-dimensional photonic-crystal waveguides for fluid-sensing applications in the sub-terahertz range. The structures are produced using a standard machining processes and are characterized in the frequency range from 67 to 110 GHz using a vector network analyzer. The photonic crystal consists of an air-hole array drilled into a high-density polyethylene block. A waveguide is introduced by reducing the diameter of the holes in one row. The holes can be loaded with liquid samples. For all structures we observe photonic band gaps between 97 and 109 GHz. While the pure photonic crystal shows the deepest stop band (28 dB), its depth is reduced by 5 dB when inserting a waveguiding structure. The depth of the photonic band gap is further reduced by several decibels depending on the refractive index of the liquid that is inserted. With this type of fluid sensor we can clearly distinguish between cyclohexane and tetrachloromethane with refractive indices of 1.42 and 1.51, respectively. The results are in good agreement with theoretical calculations based on the 2D finite-difference time-domain (FDTD) method.