Terahertz (THz) frequency range with various interesting applications is not easily accessible. Over the past two decades, intense research has resulted in developing compact THz sources to be utilized in THz spectroscopy and imaging systems. THz sources can be divided into two major categories: electronic and photonic sources. THz electronic sources that are widely used include electron beam and solid-state sources, and frequency multipliers. The common THz photonic sources include THz semiconductor and gas lasers and THz optoelectronic sources.
Electron beam sources like Gyrotrons, free electron lasers, and backward wave oscillators (BWO) generate relatively high-power signals at the THz frequency range. Yet, the complete system of a BWO is heavy and bulky and needs high bias voltage and usually a water-cooling system. Gyrotrons with 1 MW power at 140 GHz have been successfully developed. Solid state sources like Gunn or Schottky diodes provide high output levels (mW range) up to a few hundreds of GHz, but they are not very efficient in the submillimeter range. Frequency multipliers with the capability of providing power levels up to microwatt have been reported; however, the maximum achievable frequency with reasonable THz power is limited.
Quantum cascade lasers (QCLs) are the most promising electrically pumped semiconductor lasers for generation of THz radiation in the range of 1 − 5 THz. Nevertheless, their room-temperature operation is still a challenging issue.
Optoelectronic THz generation includes indirect methods, where near-infrared laser light generates photocurrent in a semiconductor which consequently generates THz radiation. Nonlinear optical conversion is one of the approaches to generate THz radiation. Due to low values of nonlinear susceptibility, this technique requires laser intensities of several hundreds of watts per square centimeter. Hence, its continous-wave (cw) operation is rather inefficient such that it is generally utilized for pulsed-THz generation.
THz photoconductive antennas (THz-PCAs) are widely used to generate THz broadband pulses and THz narrowband cw signals. In cw mode, two cw laser beams, with their frequency difference in the THz range, combined either inside an optical fiber or properly overlapped in space, are mixed in a photo-absorbing medium (photomixer) and generates a beat frequency signal. Using this method, THz signals with frequency linewidth as low as a few KHz can be generated. The frequency of the generated THz signal can be tuned by tuning the wavelength of the laser lights. THz photomixers are promising for utilization in portable, compact, and low-cost THz imaging and spectroscopy systems.
Broadband THz pulses can be also generated by exciting THz-PCAs with a femto-second short laser pulse. Pulsed THz radiation offers a higher bandwidth with frequency content extended up to around 5 THz and allows for fast measurements while cw THz systems offer a high resolution down to a few MHz. Depending on the application, cw or pulsed PCA-based systems can be efficiently utilized to address certain measurement requirements.