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Previous groups had used a technique called tunneling injection to generate THz waves with a QCL. Kumar’s is the first to use a scattering-assisted injection. This technique utilizes phonons, or thermal vibrations in the semiconductor lattice, to obtain the optical gain, or amplification in the sandwich of semiconductor layers, necessary to generate a laser.
“What we have done,” says Kumar, “is to change the way the electrons move through the semiconductor super lattice so that the cascade occurs in a more robust manner with increasing temperature of the semiconductor material.”
“With tunneling injection, if the structure is not cryogenically cooled, the electrons fizzle out and fail to generate light. With scattering-assisted injection, we obtain optical gain even at relatively high temperatures.”
A critical aspect to his group’s success, says Kumar, is the ability to exploit a QCL's “tunability.”
The frequency of light generated in a material is determined by the spacing of energy levels at the molecular level. The spacing of a QCL’s energy levels can be tuned by controlling the thickness of its alternating semiconductor layers so the laser emits THz radiation.
Proper tuning, says Kumar, is achieved by injecting electrons into the correct energy level of the semiconductor layers. Electrons hop from one level to another in the layered semiconductor to generate power in the form of THz photons.
To raise QCLs’ operating temperature, Kumar's group has harnessed the “relaxation process.” Electrons tend to dissipate energy in the form of lattice vibrations at higher temperatures, called “non-radiative relaxation,” which is typically detrimental to laser operation.
Kumar’s group used this natural phenomenon in a controlled manner to inject electrons into the correct energy levels. This scattering-assisted injection technique is less sensitive to the thermal energy of electrons and remains efficient at high temperatures.
“This tremendous achievement is very promising for the future of THz laser technologies,” says Alessandro Tredicucci, research director at the National Research Council of Italy and inventor of the first THz QCL. “It shows that the power of quantum design has yet to be fully tapped and it encourages people to look for new materials and structures whose relaxation times can be slowed down.” Kumar’s group developed a 1.8-THz laser at -166 degrees Fahrenheit (163 Kelvin) compared to the previous best result, which was achieved at -260 degrees F., or 110 K, for a laser operating at similar frequencies. More importantly, the group's solid state laser was the first whose thermal electron energy significantly exceeded the energy of its photons.
“The injection technique utilizes phonons and enables electrons to be hotter than the photon energy and still generate sufficient optical gain to generate a laser,” says Kumar. “The energy of the photons in solid state lasers had not previously been exceeded by the thermal energy of the electrons, with the exception of a specific class of lasers that utilize very high magnetic fields for their operation.”
The QCLs are not yet operating at room temperature, says Kumar, but further improvements could make it possible to generate THz light by cooling the lasers with a relatively inexpensive and commercially available thermoelectric cooler. “This would go a long way toward making QCLs usable in medical imaging, quality control, and security and sensing applications among other yet unexplored applications.” Kumar, who joined the faculty in 2010, says his work will benefit from Lehigh’s world-class research facilities. These include a state-of-the-art clean room in the Center for Optical Technologies, and unmatched scanning transmission electron microscopes in the Center for Advanced Materials and Nanotechnology, which enable precise characterization of the QCL’s sandwich structure.