Terahertz (THz) radiation is an electromagnetic radiation in the frequency range from roughly 0.1 THz to 10 THz. THz frequency is higher than those of radio waves and microwaves, but lower than those of infrared light. The wavelength is in the range of 0.03 mm to 3 mm, and often below 1 mm, giving it another name, sub millimeter radiation. Until very recently, there was a lack of availability of good terahertz sources and suitable detectors for THz radiation, thus, this spectral range was referred as the terahertz gap. It was only in the 1990s that interest in terahertz waves grew strong, and more and more research groups engaged in this area. The fast pace advancement in this field owes largely to the advances in photonics, quantum mechanics. Now there are various powerful solutions both for generation and detection of terahertz waves. THz waves are less harmful than X-rays when used for medical imaging, and when used for spectroscopy they provide information that other waves cannot. THz wave has applications in many different fields, spectroscopy, physics, material science, electrical engineering, chemistry, forensics, biology and medicine and further hold new research potential that are still being discovered. The field receiving the most attention is medical imaging and spectroscopy. These advances strengthen the motivation for further efforts in various areas of THz technology. THz produces a frequency that is both coherent and spectrally broad, so such images can contain far more information than a conventional image formed with a single-frequency source. Although THz frequency can penetrate fabrics and plastics, it is non-ionizing and therefore harmless to living tissue or DNA, making it very valuable for imaging and screening applications. Various THz sources are shown below.
There has been a considerable surge in the research on intense THz sources and their applications. QCLs (Quantum Cascade Lasers) is its one such source based on the inter-sub band transition of semiconductor multiple quantum wells and their oscillation frequency can be controlled by varying the width of the wells regardless of the band gap of the materials. In QCLs, electrons injected into the active layers cascade from one sublevel to another within the well, thus inducing laser oscillation. Before proceeding further, it’s necessary to first understand the concept of quantum wells.
What are Quantum Wells?
Figure above shows the quantum well for the conduction band. The black lines show the potential well due to the changes in conduction band energy between the different materials. The red lines show the allowed energy levels for an electron within the well. The blue lines show the (envelope) wave functions of the electrons for each energy level and the green line shows the Fermi level that indicates how many electrons have been put into the quantum well. An electron’s energy can only take certain values that we call energy levels. A quantum well is one such semiconductor nanostructure , . It is a nanometer thick layer of semiconductor sandwiched between layers of a different but compatible semiconductor. If the semiconductors are chosen correctly then we have created a structure that can trap electrons within this thin layer. In order to create a quantum well, we need to use semiconductors that have compatible lattice constants . For the material family GaAs/AlxGa1-xAs, the lattice constant is almost independent of the aluminum percentage which is one reason that these materials have been exploited so much for creating semiconductor structures.
Quantum wells are important semiconductor devices that are used in many ways. Optical transitions between the conduction and the valence band, are called interband transitions. Optical transitions between the different electron levels within the quantum well, these are the intersubband transitions.
These transitions have a smaller energy gap and so they interact with light in the mid- to far- infrared part of the spectrum.
What is a Quantum Cascade Laser?
Quantum Cascade Laser is a semiconductor laser involving only one type of carrier. It is based on two fundamental quantum phenomena:
· The quantum confinement
· The tunneling
In the QCL transition do not occur between different electronic bands (VB-CB) but on intersubband transitions of a semiconductor material. In QCLs, electrons are injected into the active layers cascaded from one sublevel to another within the well, thus inducing laser oscillation. An electron injected into the gain region undergoes a first transition between the upper two sublevels of a quantum well and a photon is emitted. Then, the electron relaxes to the lowest sub level by a non-radiative transition, before tunneling into the upper level of the next quantum well. The whole process is repeated over a large number of cascaded periods.
Electrons are recycled due to cascaded structure as each injected electron generates N photons (N is the number of stages). QCL is unipolar in operation. In figure below initial and final states have the same curvature. E3-E2 transition emits the Laser (photons). E2-E1 transition emits phonon leading crystal vibration. E2-E1 transition is very fast, and it is made resonant with the optical phonon energy. Emission of photons occurs at the same wavelength, thus increasing the gain.
Initially, QCLs oscillated in the mid-IR range. Unlike conventional lasers, THz QCLs oscillate in the active layers (multiple quantum wells) of metal–metal (or metal–semiconductor) waveguides narrower than the wavelength.
Material Used: InGaAs/InAlAs and GaAs/AlGaAs.
Layer thickness determines emission wavelength
• InGaAs/InAlAs: 3.5 – 24 µm
• GaAs/AlGaAs: Far-IR, THz
Because of Quantum Confinement, the spacing between the sub bands depends on the width of the well, it increases as the well size is decreased. This way, the emission wavelength depends on layer thickness and not on the band gap of the constituent assembly.
· Band Structure
· Building Blocks can be Single QW (Quantum Well), Coupled QWs (Quantum Wells) or Super Lattice
Wavelength Ability: 3.5 to 24 micrometre (AlInAs/GaInAs), 60-160micrometre (AlGaAs/GaAs)
Multi wavelength and ultra-broadband operation
Applications: Trace gas analysis, combustion and medical diagnostics, environmental monitoring, military and law enforcement
Reliability, reproducibility, long term stability
Industrial research and Commercialization
Current Scenario for THz Generation using QCL
Terahertz quantum cascade laser (THz-QCL) is expected as a compact terahertz laser light source which realizes high output power, quite narrow emission line width, and cw (continuous wave) operation. Recent progress and future prospects of THz quantum-cascade lasers. THz-QCLs are studied using GaAs/AlGaAs and GaN/AlGaN semiconductor super lattices.
1. (2013) a team of researchers at TU Wien (Vienna University of Technology) managed to create a new kind of quantum cascade laser with an output of one watt of terahertz radiation, breaking the previous world record of about 0.25 watts. The previous world record for terahertz quantum cascade lasers of almost 250 mill watts held by the Massachusetts Institute of Technology (MIT). The laser of TU Vienna produced one watt of radiation.
2. In 2015, researchers demonstrated 1.9-3.8 THz GaAs/AlGaAs QCLs with double metal waveguide (DMW) structures. They developed a low-frequency higher nature operation QCL (T<160K for 1.9 THz- QCL) by introducing indirect injection scheme design (4-level design) into GaAs/AlGaAs THz-QCLs. Nitride semiconductor is a material having potentials for realizing wide frequency range of QCL, i.e., 3ï1/2'20 THz and 1ï1/28 μm, including an unexplored terahertz frequency range from 5 to 12 THz, as well as realizing room temperature operation of THz-QCL. The merit of using an AlGaN-based semiconductor is that it has much higher longitudinal optical phonon energies (ELO> 90meV) than those of conventional semiconductors (∼ 36 meV). They fabricated high-quality AlGaN/GaN QC stacking layers by introducing a novel growth technique in molecular beam epitaxy (MBE). A GaN/AlGaN QCLs with pure three-level design and obtained the first lasing action of nitride-based QCL from 5.4-7 THz was fabricated. 
3. Innovators at NASA's Glenn Research Center have developed a cutting-edge tunable, multi-frequency controller for a terahertz (THz) quantum cascade laser (QCL) source. The device enables use of the full bandwidth of broadband THz, producing an extensive number of frequency channels. Operating at THz frequencies, QCL emissions deliver higher-resolution imaging than microwaves, and they provide higher-contrast images than X-rays. Glenn's scientists have devised an efficient technique that generates high-resolution tuning over a vast number of usable THz-frequencies, at commercial levels of cost and simplicity. This innovation opens a pathway to vastly expanded use of THz QCL in unprecedented terrestrial applications, including communications, homeland security screening, biomedicine, and quality control. Glenn's innovation is a THz QCL source (range 1 to 5 THz) based on a passive waveguide tuning mechanism. In Glenn’s process, a tunable QCL is coupled to a grating router, which consists of an appropriately configured linear dielectric array. The grating router receives a THz frequency from the QCL and generates a high density of THz frequencies. The output of the grating router enters an on/off switching waveguide controller, which is configured to select one desired THz frequency. This desired frequency is then fed into a waveguide multiplexer, which combines the output ports of the controller into a single signal for transmission. Glenn's novel technology unlocks the potential for THz frequencies to revolutionize sensing and imaging applications across a wide range of industries.
Its applications lie in
· Homeland security screening to detect concealed weapons or explosive
· Biomedical imaging
· Manufacturing, quality control, and process monitoring
· Wireless communications Remote sensing of environmental pollutants in the atmosphere
· Imaging systems within semiconductors Spectroscopy and tomography
4. A report on the Doppler-free saturation spectroscopy of a molecular transition at 3.3 THz based on a quantum-cascade laser and an absorption cell in a collinear pump-probe configuration is presented. A Lamb dip with a sub-Doppler line width of 170 kHz is observed for a rotational transition of HDO. It was found that a certain level of external optical feedback is tolerable as long as the free spectral range of the external cavity is large compared to the width of the absorption line. 
5. High Resolution Terahertz Spectroscopy with Quantum Cascade Lasers
THz quantum cascade lasers (QCLs) are promising sources for implementation into THz spectrometers, in particular at frequencies above 3 THz, which is the least explored portion of the THz region. One application of QCLs in THz spectroscopy is in absorption spectrometers, where they can replace less powerful and somewhat cumbersome sources based on frequency mixing with gas lasers.
6. High-resolution gas phase spectroscopy with a distributed feedback terahertz quantum cascade laser 
Here the authors have implemented a distributed feedback device in a spectrometer for High resolution gas phase spectroscopy. Amplitude as well as frequency modulation Schemes have been realized. The absolute frequency was determined by mixing the radiation from the quantum cascade laser with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519THz2.519THz were measured in order to demonstrate the performance of the spectrometer.
7. High-resolution Terahertz Spectroscopy with Quantum-Cascade Lasers 
The goal of this project consists in the demonstration of a spectrometer for high-resolution laser spectroscopy of semiconductors at THz frequencies based on narrow-line-width quantum-cascade lasers (QCLs). One target is to determine the line width and line shape of impurity transitions in isotope-pure Ge and Si with ultimate accuracy, i.e. without limitation of the spectral resolution by the apparatus function of the spectrometer. A special focus will be the complementary analysis of the lifetimes derived from time-resolved pump-probe techniques and spectrally resolved absorption measurements with the QCL-based spectrometer. The project is realized in collaboration with the Paul-Drude-Institute (PDI) in Berlin, where dedicated QCLs are developed.
 Paul Harrison. Quantum Wells, Wires and Dots: Theoretical and Computational Physics of
Semiconductor Nanostructures. Wiley, 2011.
 Simon M. Sze Kwok K. Ng. Semiconductor Devices: Physics and Technology. Wiley-Blackwell, 3rd edition, 2006.
 High Resolution Terahertz Spectroscopy with Quantum Cascade Lasers, https://link.springer.com/article/10.1007/s10762-013-9973-7
 High-resolution gas phase spectroscopy with a distributed feedback terahertz quantum cascade laser, https://aip.scitation.org/doi/abs/10.1063/1.2335803?journalCode=apl
 High-resolution Terahertz Spectroscopy with Quantum-Cascade Lasers,
 Recent progress and future prospects of THz quantum-cascade lasers