A method comprising polarizing and coupling an electromagnetic beam to a first-order transverse electric (TE1) mode with respect to a parallel plate waveguide (PPWG) integrated resonator comprising two plates and a cavity, sending the electromagnetic beam into the PPWG integrated resonator to excite the cavity by the TE1 mode and cause a resonance response, and obtaining wave amplitude data that comprises a resonant frequency, and obtaining the refractive index of fluids filling the cavity via the shift in resonant frequency.