Terahertz imaging with dynamic aperture

A system for using terahertz (THz) radiation to produce an image of an object, the system comprising:

• a laser source providing an optical pump pulse, an optical probe pulse having a polarization ellipticity, and an optical gating pulse;
• a delay stage providing a variable delay time between the optical pump pulse and the optical probe pulse;
• a delay stage providing a variable delay time between the optical gating pulse and the optical pump pulse;
• a chopper turning the optical gating pulse on and off;
• a THz emitter for emitting a beam of THz radiation when activated by the optical pump pulse;
• a layer of semiconductive material comprising (a) a part of the object, or (b) a discrete layer positioned between the object and the THz radiation;
• optics focusing the THz beam on the layer of semiconductive material;
• a lens focusing the optical gating pulse on the layer of semiconductive material so that the gate pulse illuminates a gate pulse focal spot on the layer of semiconductive material, the gating pulse focal spot having a diameter effective to cause measurable modulation in transmission of the THz beam through the layer of semiconductive material when the gating pulse is on as compared to when the gating pulse is off, creating alternating modulated THz beams which illuminate the object;
• a THz receiver, positioned to receive the alternating modulated THz beams reflected from or transmitted through the object, modulating the optical probe pulse with the alternating modulated THz beams to create corresponding modulation in the optical probe pulse polarization ellipticity;
• a polarizer converting the modulation in the optical probe pulse polarization ellipticity to intensity modulation in the optical probe pulse;
• a photodetector converting the intensity modulated optical output pulses to electronic information;
• and
• a processor receiving the electronic information and producing an image of the object from the electronic information.

A method for using THz radiation to generate an image of an object, the method comprising the steps of:

• (a) providing an optical pump pulse, an optical probe pulse having a polarization ellipticity, and an optical gating pulse with a variable delay time between the optical pump pulse and the optical probe pulse;
• (b) activating a THz emitter with the optical pump pulse to emit a beam of THz radiation;
• (c) chopping the optical gating pulse on and off;
• (d) providing a layer of semiconductive material that is either (i) part of the object, or (ii) a discrete layer placed between the object and the THz beam;
• (e) focusing the optical gating pulse and the THz beam on the layer of semiconductive material so that the gating pulse illuminates a gating pulse focal spot on the layer of semiconductive material, the gating pulse focal spot having a diameter effective to cause measurable modulation in transmission of the THz beam through the layer of semiconductive material when the gating pulse is on as compared to when the gating pulse is off, creating alternating modulated THz beams while illuminate the object;
• (f) modulating the optical probe pulse with the alternating modulated THz beams in a THz receiver, positioned to receive the alternating modulated THz beams reflected from or transmitted through the object, to create corresponding modulation in the optical probe pulse polarization ellipticity;
• (g) converting the modulation in the optical probe pulse polarization ellipticity to intensity modulation in the optical probe pulse;
• (h) detecting the intensity modulation in the optical probe pulse and converting the intensity modulation to electronic information;
• and
• (i) receiving and processing the electronic information to produce the image of the object.