(a) A schematic diagram of the fabricated GC-HMM based sensor device and an SEM image of the 2D subwavelength Au diffraction grating on top the HMM. (b) A photograph of the sensor device. Scale bar = 10 mm. (c) Calculated real parts of effective permittivity for an HMM consisting of eight pairs of gold/Al₂O₃ layers determined using effective media theory. (d) Excited BPP modes of the GC-HMM at different angles of incidence. (e–f) Standard sensor calibration tests, injecting different weight percentages of glycerol (0.1–0.5 % w/v), using wavelength interrogation (e) and angular interrogation (f).