TeraHertz beams of ultrafast pulses of far infrared (λ=40–1000μm, v = 0.3x1012− 7.5×1012Hz) radiation generated via the induced dipole of photogenerated charge within a strong electric field in semiconductors are an emerging spectroscopic technique which incorporates ultrafast optical pulse generation, optoelectronics, and far infrared techniques. Recent results obtained using the trap enhanced field (TEF) effect in the generation of THz beams demonstrate the extended frequency range of these sources and show their importance to time resolved infrared spectroscopy.
The generation of collimated THz radiation within semi-insulating materials is dramatically improved by the extremely large field enhancement near the anode of an electrically biased metal/semi-insulator/metal structure. Our experimental results for semiinsulating GaAs establish an operational regime in which the applied potential is confined to a small region near the anode resulting from a space charge region which exists due to a dramatic change in the number of ionized EL2 traps. The effect, contrary to that observed in trap free or doped materials, is enhanced by optical injection of carriers near the anode, and can be exploited for the efficient generation of ultrafast THz radiation.
Spectroscopy using THz beams allows both the static and dynamic properties, including refractive index, absorption, and photoconductivity, of materials and structures to be accurately measured. The energy range and time resolution of freely propagating subpsec THz pulses have not been previously available.