A Nanoscale injection technique enables higher operating temperatures and a variety of potential applications.
Light’s versatility is awesome – it inspires painters, it regulates auto traffic and it guides a moonlit stroll on the beach.
Visible light waves occupy just one portion of the electromagnetic spectrum. Radio waves, with their relatively low frequency, let the world communicate wirelessly. Higher-frequency infrared rays enable telecommunications, the Internet and consumer electronics. Farther up the spectrum, X-rays are used for medical imaging and baggage screening.
One largely unexploited region of the electromagnetic spectrum, says Sushil Kumar, is the narrow “gap” between the radio and infrared bands that is occupied by terahertz (THz) waves.
In the presence of THz light, says Kumar, assistant professor of electrical and computer engineering, some large molecules yield specific spectral signatures, like the lines of a barcode.
This sensitivity opens up a variety of potential applications. THz technology can be used to sense trace amounts of drugs or explosives concealed in clothing or packages. It can detect hidden metal objects without harming the body, and it can decode light emanating from the outer universe to reveal clues to the formation of stars and galaxies.
Other applications include the remote sensing of the earth’s atmosphere, medical imaging and disease diagnosis, and quality control in drug manufacturing. And scientists have found that THz radiation can be used to diagnose skin cancer with advantages over X-rays.
One large hurdle stands in the way of these advances, says Kumar – the lack of a commercially viable technique to generate narrowband and high-power THz radiation.
Traditionally high-power THz radiation was produced by bulky, expensive lasers fueled by a molecular gas such as methane. In 2001, scientists in Italy invented the first THz semiconductor quantum-cascade laser (QCL). QCLs are attractive because of their size – they are as tiny as the diode in a laser pointer. But the lasers must be cooled to temperatures as low as that of liquid nitrogen (320 degrees below zero Fahrenheit) before they emit powerful THz radiation.
Kumar has collaborated with researchers at MIT and Sandia National Laboratories to make a QCL that emits THz radiation at higher operating temperatures than ever before. The group, whose work is supported by NASA and NSF, reported their achievement recently in Nature Physics.
Using nanoscale technology, Kumar’s group assembled a super lattice of 1,500 alternating layers, placing atoms of two semiconducting materials – gallium-arsenide and aluminum gallium-arsenide – in periodic arrangements. Because each layer in the sandwich must be of an extremely precise thickness, it takes one day to “grow” a wafer. An electric current applied to the sandwich causes electrons to travel, or cascade, within its structure, according to the rules of quantum mechanics, and generate light along the way.
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.