Optically pumping of molecular vapours has proved to be the one of the most precious technique for the generation of coherent emission of radiation in the THz (FIR) region of the electromagnetic spectrum. The terahertz lasers are optically pumped by co2 laser. FIR region has been accelerated in 1960's when lasing action in HCN gas was obtained with the discovery of the discharge pumping. However in the year 1970 optically pumped FIR laser was discovered as the footstep towards a valuable coherent FIR source. Optically pumping is very effective and nonstop method to produce specific energy excitation of molecules.
It refers to the electromagnetic waves propagating in the frequency of terahertz range. Terahertz wave lie at the far end of the infrared band and just before the start of the microwave band. The wavelength of terahertz is between 30Âµm (infrared) and 2mm (microwaves). It is commonly termed as sub millimeter radiation.
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Fig.1 Terahertz radiation
The history of coherent THz radiation began with the additional room of millimeter-wave backward oscillators into the THz region and the molecular far-infrared gas lasers. Spectroscopy sensing, imaging and communications are the potential applications of THz. Biology, biomedicine, pharmaceutics, chemical, food, security, astronomy are the new applications in the terahertz radiation. The characteristics of terahertz field need a bottomless knowledge of microwave engineering, optics and photonics, semiconductor physics.
Images generated using terahertz time domain spectroscopy generated a great deal of interest.
Fig.2 Time domain spectroscopy
The generation of time domain spectroscopy produced a brisk growth in the field of terahertz science and technology. Terahertz radiation can easily penetrate clothing, plastics, leather, paper, and packing materials. It can also penetrate fog and clouds which will be easy to support aircraft landing in harsh environment. Terahertz is non ionizing radiation hence it does not cause any damage to the body cells.
Terahertz lasers can be divided into 2 major groups with respect to the pumping method. They are
Discharge pumping method
Optically pumping method 
Optically pumped terahertz laser
Optically pumping is a method in which light is used to pump the electrons from a lower energy level into higher energy level in an atom or a molecule. At the wavelengths of 452, 496 and 541 Âµm, the FIR stimulated emission is obtained from optically pumped methyl fluoride (CH3F). This was first observed by Chang and Bridges in 1970  . Since then optically pumped lasers have developed quickly. Till now, more than 5000 FIR lines have been obtained using optically pumping method whereas other excitation techniques have obtained 500 lines.
Fig.3 schematic representation of an optically pumped terahertz laser
Optical pumping is used to avoid the optical losses linked with the doped materials which gain the capability to deliver the energy exactly where it is needed. The advantage of optical pumping is that the molecule does not dissociate by the pump radiation.
Fig.4 Energy level diagram
Optically pumping method has two states. They are
Ground vibrational state
Excited vibrational state
When an external laser source photon is applied, the molecular gas gets excited from the ground vibrational state to the higher (excited) vibrational state. Population inversion is achieved within the excited vibrational state. The terahertz is lased and emitted as a result of these inversions between the rotational vibration states. Consequently, for the molecules to take part in continuous -wave lasing process, it should get back to the ground vibrational state from the higher state  .
The pump laser should be tuned so that the FIR medium absorption coincided with the pump laser wavelength.
The pump laser cavity length should be adjusted so that a resonator mode falls within the absorption profile to obtain pumping of the FIR medium.
Fig.5. Tuning of pump laser
The vertical thick lines symbolize the longitudinal CO2 laser cavity modes, the broad Gaussian profile is the co2 laser gain profile and the shaded area is the absorption line of the FIR laser active gas.
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Carbon dioxide (CO2) lasers are completely used as a FIR laser pump source. CO2 lasers are powerful emitters of coherent radiation. CO2 lasers are known to be reliable and long-lived. These lasers are the uppermost power continuous wave lasers that are presently available. The radiation at about 90 discrete wavelengths around 10Âµm is emitted by conventional low pressure CO2 lasers. The normal CO2 gas can be substituted for any one of the isotopes such as 13C16O2, 13C18O2, or 14C 18O2 to increase the available laser lines. These isotopes are more expensive, but very useful. With a small modification the CO2 laser tubes can be used for CO or N2O laser operation.
There has been a rapid growth in the performance of CO2 laser over the past few years. The compactness of waveguide design is made better than conventional low pressure design, but when working with FIR lasers, the waveguide design offers a large gain width of 800MHz  . This larger gain width is due to collisional broadening caused by higher gas pressure. The better coincidences between the FIR laser absorption bands and the pump laser wavelength is achieved by large gain width.
The most commonly used FIR laser pump source is CO2-TEA (Transverse Excitation at Atmospheric pressure) since it offers megawatt pulse power and short pulse duration. To tune the pump wavelength to the absorption wavelength, the pump laser requires not only the grating but also some extra fine tuning arrangement. This is usually done by varying the cavity length.
The pump laser resonator should be stable so that the pump wavelength does not change. Most of the electrical input power is converted into heat even in the case of high overall efficiency of CO2 laser. The efficiency is given by Manley-Rowe limit  .
Ïµ = VFIR / 2VIR
Ïµ- Efficiency of converting pump radiation into THz radiation.
VFIR - Frequency of emitted THz photons
VIR - Frequency of pump photons
The heat changes the cavity length which alters the output frequency. The FIR laser becomes very sensitive when pump wavelength changes. So therefore it is very important to keep the CO2 laser resonator wavelength stable by water cooling the discharge tube.
Far infrared laser media
An enormous number of polyatomic molecules have been found to emit coherent radiation when optically pumped by infrared lasers. The number of FIR laser lines and the number of laser molecules are increasing continuously. The first obtained FIR laser line is 496Âµm in CH3F  ,  .
Some of the lasing molecules are
Fig.6. FIR laser media
These FIR lines were obtained under very hard circumstances which may vary from other laser system in a variety of requisites to include pump laser characteristics, purity, detector sensitivity and FIR wave guidance technique. Some laser lines are very weak where in the atmospheric attenuation is pretty high as a point of its absorption in water vapor. Waveguides filled with dry nitrogen can be used to avoid atmospheric absorption. Heavy molecules produce longer FIR wavelengths. For example,
CF2BR which is a heavy molecule pumped by CO2 laser yields the long FIR laser wavelength 2140 Âµm (140 GHz).
Methyl fluoride (CH3F)
It is a symmetric top molecule employed as the active medium in the first optically pumped FIR laser owing to strong IR absorption bands in the 9-10 Âµm regions. Ever since, CH3F has played a very significant role in the growth of FIR lasers. Both experimentally and theoretically this laser proved to be the best understood optically pumped FIR lasers. CH3F is often used in FIR Raman research.
CH3OH has become a favorite FIR laser medium due to following reasons.
Complexity of its FIR molecular spectrum
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Richness of FIR laser lines.
More than 300 FIR laser lines can be obtained using CH3OH. The overlap between the CO-stretch band and the CO2 laser spectrum is outstanding. CH3OH shows great stable electric dipole moments both parallel and orthogonal to axis of symmetry. This implies optical pumping can be used even if the optimum absorption is unreachable from the pump laser.
Fig.7. Energy diagram of 2.5 THz methanol laser
In a weekly asymmetric top molecule so-called methyl alcohol, a portion of molecule interacts with respect to other proportions referred to as one internal degree of freedom. This is effectuated by the potential barrier as a point of three H-atoms in the CH3 group where in the OH-group rotates with respects to the CH3 group. The potential barrier is then driven into torsional vibration  ,  .
Fig.8. Schematic methyl alcohol molecule
CH3OH has 12 vibrational states. Four quantum numbers are required to describe the energy levels belonging to one vibrational state.
Fig.9. Illustration of methanol quantum numbers
n- Torsional quantum number n = 1, 2, 3â€¦.
Ï„- Related to threefold hindering potential Ï„ = 1, 2, 3
J- Total angular momentum
K- It is the component of the J along the axis of symmetry.
Exchanging H by D in the CH3OH molecule, the deuterated species CH3OD, CH2DOH, CHD2OH, CD3OH and CD3OD is obtained which does not shift the CO-stretch band significantly. But, new FIR laser lines do appear since the rotational and vibrational levels are affected  . It is achievable to swap the ordinary 12C carbon atom by the 13C isotope  .
Fig.10.Physical representation of the 2.5 THz lasing process in methanol
Generation and detection of terahertz
There has been many techniques explored to generate terahertz in which photoconductive antennae is chosen to be the most efficient. Photoconductive antennae is also known as the Auston switch. Terahertz radiation has been widely used for THz time domain spectroscopy and imaging. This made the desire to design cheap and compact THz system. The femtosecond lasers have been used to generate coherent terahertz frequency radiation which has driven to develop higher power sources and systems of broader bandwidth  .
Fig.10. Time domain spectroscopy
This technique consists of 120 fs, 830 nm Ti: sapphire laser pumping a cavity tuned optical parametric oscillator (OPO). The output from the OPO used to pump the photoconductive antennas, was tunable from 830 nm to 1500 nm  .
Fig.11. Generation of THz
Photoconductive antennae consist of highly resistive semiconductor thin film. The film is made up of semiconductor like GaAs. The low temperature GaAs (LT- GaAs) is epitaxially grown on semi insulating GaAs (SI- GaAs). The two electric contact pads are placed on the film.
The laser pulse have a photon energy of E= h.v larger than the band gap Eg. When laser pulse hits the thin film the photons get absorbed by the film. The absorbed photon creates an electron-hole pair in the semiconductor film. A voltage V is applied across the contact pads where the photo excited carriers are accelerated by an applied electric field which emits the terahertz radiation due to transient current pulse  .
Fig.12. Detection of THz
The reflected terahertz is then focused on to the Electro-optic crystal. ZnTe is mostly used as the electro-optic crystal. The terahertz beam induces the birefringence in the electro-optic medium. The birefringence modulates the ellipticity of the probe beam which is measured by the quarter wave plate, Wollaston prism and a balanced detector. The quarter wave plate is used to convert the elliptical polarization into circular polarization. The Wollaston prism is used to split the beam into horizontally polarized and vertically polarized beam. The main use of balanced detector is used to overcome the power fluctuations and noise contribution of the laser system. It consists of two photo diodes and an internal differential amplifier which compares the signal of both photodiodes.
The general characteristics of optically pumped lasers have been reviewed in this paper. Optically pumped terahertz laser has come a long way in the past few years. Optically pumped terahertz lasers are a full-grown terahertz source technology which can plug needs in a large range of applications.