The feasibility to measure the complex refractive index of a droplet of material is demonstrated from 50-500 GHz with a time-resolved optoelectronic transmissometry method. A train of femtosecond optical pulses illuminate ultrafast photoconductive switch (PCS) generation and detection structures on the substrate at each end of a CPW. The device is first characterized unloaded with no material under test (MUT), and pulse delay and amplitude attenuation are measured with the MUT in a sample well on the CPW. Fourier transforms of the measured values determines a frequency-domain differential transfer function, and the complex refractive index is recovered from an inverted lookup table for the substrate. Validation was performed on a 15nL droplet of glycerol.