Improvement of terahertz imaging with a dynamic subtraction technique.

By use of dynamic subtraction it is feasible to adopt phase-sensitive detection with a CCD camera to reduce long-term optical background drift. We report on a two-order improvement of the signal-to-noise ratio. The improved system is used to image terahertz field distribution generated by an optically rectified electro-optic crystal with a modulation depth as small as 10(-4). We also introduce a modified detection geometry that realizes near-field imaging capability with greatly improved spatial resolution.