side the cavity. Compared to other solid-state components, these tube THz sources produce high powers at lower THz frequencies. These are bulky and larger magnetic bias and voltage power supply is required. To improve the power and efficiency of these devices, special care should be taken on the design of gun, collector, beam transport magnet and the processing circuit. 12.4 THZ SOURCES USING OPTICAL RADIATION Nonlinear crystals such as ZnTe, LiNbO 3' InGaAs, GaP, and the like, are very common terahertz sources that exploit the nonlinear optical properties by optical rectification (OR) and difference frequency generation (DFG) methods when illuminated by ultra- short laser pulses. Semiconductor materials like germanium, silicon, and the like, can also be used to build the THz laser source. Under the condition of semiconductor defected structure, the lasers rely on the population inversion. These lasers provide a better tunability over a wide range of wavelengths. The two down conversion processes, such as, difference frequency generation (DFG) and optical rectification (OR) can be used to generate the coherent terahertz waves with high spectral resolution. A wide terahertz bandwidth can be obtained using these nonlinear processes. The coherent and tunable THz waves can be generated using organic nonlinear crystal, such as, DAST (4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate), DSTMS (4-N, N-dimethylamino-4'-N'-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate), and SO forth. The THz waves can also be efficiently achieved using an annealed ZnGeP, crystal based on the DFG phase matching configuration. The phase matching between the THz and the optical fields is one of its major limitations. Moreover, the thickness of the nonlinear material should be carefully chosen, and high optical power is required to generate high terahertz power. 12.4.1 LASERS Terahertz radiation can be generated directly from various lasers, such as, molecular lasers, semiconductor lasers, and the like. A few tens of milliwatts of 