1. A method of imaging a sample, comprising: a) irradiating the sample to be imaged with pulsed electro-magnetic radiation with a plurality of frequencies in the range of 50 GHz to 84 THz; b) subdividing an area of the sample into a two dimensional array of pixels, and detecting radiation from each pixel over a plurality of frequencies; and c) generating an image of the area of the sample from the radiation detected in step (b) by calculating the difference between the temporal positions of a maxima and a minima of the radiation detected in step (b) for each pixel.