THz power is produced from the low-pressure flowing gas cavities, while injecting a grating tuned CO laser. In the near-IR and visible frequency range, the semiconductor diode lasers are predominantly used with appropriate bandgap materials. The quantum-cascade laser (QCL) can also be used to generate terahertz waves. QCL is a unipolar semi- conductor laser which operates differently from other semiconductor diodes and generates THz radiation between 2-5 THz. To have the lasing condition in QCL, the population inversion condition must be satisfied. To achieve this, the thickness of semiconductor layers and bias voltage should be considered carefully. The energy of the system can be improved by reducing the thickness of the semiconductor. A pulsed 5G Wireless Communication System in Healthcare Informatics and continuous wave terahertz is generated using the QCL and the operating fre- quency is based on the quantum well design. Stable and reliable terahertz photons are generated directly from QCL with high output power. The main drawback of QCL is that it can be operated only under cryogenic temperature [Fathololoumi et al. (2012)]. 12.4.2 PLASMA THz generation can be obtained from laser-induced plasma while the high peak power pulsed laser beam is focusing in air or any inert gaseous medium. The sim- plest process for plasma generation is to focus two separate wavelengths simultan- eously in a nonlinear medium to have two color plasma which leads to the terahertz generation. The illuminated laser beam has sufficient intensity to trigger the plasma filament through higher order nonlinear frequency mixing processes. As a result, any two nearby frequencies with high electric field beating each other leads to broad- band THz radiation [Kim (2009)]. For example, the ultrafast Ti: Sapphire laser pulses with 100fs pulse width and 1 mJ pulse energy are sufficient to create THz-plasma radiation. 12.5 THZ SOURCES COMBINING OPTICAL, ELECTRONIC AND MAGNETIC FIELDS THz generation and detection is also possible when the electro