1. A method of imaging a sample, the method comprising: irradiating a plurality of points of a surface of the sample with a pulse of electromagnetic radiation, said pulse having a plurality of frequencies in the range from 25 GHz to 100 THz; detecting the amplitude of the radiation having a plurality of frequencies in the range from 25 GHz to 100 THz reflected from or transmitted by each point of the sample as a function of delay time, wherein the delay time is the time that it takes the radiation to travel through a region of the sample; generating a first two-dimensional image of the points of the sample at a first interface of the sample using an amplitude of the radiation detected for a delay time corresponding to the first interface; and generating a second two-dimensional image of the points of the sample at a second interface of the sample using an amplitude of the radiation detected for a delay time corresponding to the second interface; obtaining a depth calibration based on a refractive index; determining variation in thickness of the layer of the sample between the first interface and the second interface from the delay time corresponding to the first interface and the delay time corresponding to the second interface.