Fig. 1: Concept of utilizing photonic crystal to detect multiphase materials.

a Lab-scale experimental configuration with the transmitting and receiving antenna on opposite sides of the photonic crystal (PC). Both antennas are connected to a network analyzer (see methods). The PC is placed on top of a plastic rod that is connected to a rotational stage. b An illustration of the photonic crystal with multitude of holes arranged in a square lattice configuration; In this case the holes are filled with air. Here \(a\) is the lattice constant. c An illustration of the same photonic crystal structure with a proportion of the holes filled with water (blue) and the remaining holes filled with oil (yellow). d Computed photonic band structure of the square lattice (first four eigenmodes are shown, sequentially as red, green, yellow and blue curves). Here k is the wave vector and \(a\) is the lattice constant. The symmetry points in the first Brillouin zone are shown on the x-axis, (Γ, X, and M) and the y-axis is the frequency \(f\) normalized by the characteristic frequency \({f}_{c}\). e, f Color contour plots of the microwave transmission measurements through the structures shown in b and c, respectively. The x-axis is incident angle, with zero defined as the X symmetry point in d or equivalently vertical direction of b. Increasing incident angles indicates rotating in clock-wise direction. The y-axis is microwave frequency. The color in e and f corresponds to the microwave transmission coefficient as indicated in the accompanying scale bar.