Bridging the EM Void
Engineers from Prof. Mikhail Belkin’s group at The University of Texas at Austin in collaboration with Prof. Markus Amann’s group at the Technical University of Munich have demonstrated the first broadly- tunable electrically-pumped semiconductor source of coherent terahertz radiation (or T-rays) that operates at room-temperature. The findings were published in the June 17th issue of Nature Communications and presented as an invited talk at the leading optoelectronics conference CLEO-QELS in June 2013. Graduate students Karun Vijayraghavan and Yifan Jiang in Dr. Belkin’s group spearheaded the research.
The terahertz (THz) frequency range is a narrow portion of the electromagnetic spectrum that bridges the void between high-frequency electronics and infrared optoelectronics. Typically this range is referred to as the “THz gap” due to the lack of sources, detectors, and components. One of the goals in THz research is the development of practical sources that could easily be integrated in real-world applications. At the moment, they are bulky, complex and cost-prohibitive – more suitable for a research laboratory than a commercial setting. If compact, mass-producible sources capable of 1 to 5 THz frequency generation are developed, it may open up new avenues for technology to aid our everyday lives.
Dr. Belkin’s group has been working on addressing this challenge by developing room-temperature, electrically-pumped semiconductor THz sources using quantum cascade lasers (QCLs). QCLs are a class of compact, semiconductor lasers that are made up of hundreds of repetitions of quantum wells and barriers. Their simple operation and materials used in their construction is virtually identical to diode lasers found in laser pointers or telecommunication systems. Therefore they have a similar capacity to be mass-produced in foundries. While QCLs can operate at room-temperature with high power in the mid-infrared frequency range, they are still limited to cryogenic temperatures when operating at THz frequencies.
“THz QCLs still require cryogenic cooling to operate and this complicates the issue of a practical source. Unfortunately the progress to improve the temperature performance has come to a virtual standstill. We pursue an alternative approach based on using nonlinear frequency-mixing inside of mid-infrared QCLs to generate terahertz radiation at room-temperature” explains Dr. Belkin.
The particular method his group uses is the nonlinear optical process of difference-frequency generation in mid-infrared QCLs to generate THz radiation at room-temperature. The active region of a THz QCL source is quantum-engineered to provide lasing at two mid-infrared frequencies, ω1 and ω, and simultaneously to have giant optical nonlinearity. Upon applying a proper voltage bias to the structure, THz radiation is produced by difference-frequency generation inside of the laser cavity at frequency ωTHz =ω1 –ω2.
The approach was pioneered by Dr. Belkin to produce the first room-temperature THz emission from a QCL in 2008; however, the power output was only a few hundreds of nanowatts, too small for practical applications and the devices only operated over at a fixed frequency.
Extensive work has since been carried out to improve the power output and bandwidth of THz DFG- QCLs. Last year, Belkin’s group developed a novel waveguide geometry designed to provide so-called Cherenkov terahertz difference-frequency emission in which T-rays are emitted at an angle to the laser waveguide. This approach allows for efficient terahertz radiation extraction and boosts power by orders of magnitude. It also represented a breakthrough in extending the device bandwidth to operate over the entire 1 THz to 5 THz frequency spectrum. Adapting this waveguide geometry to current designs has resulted in the first-time demonstration of sources that can be broadly tuned from 1.7 THz to 5.25 THz with narrow-linewidth (
Dr. Belkin comments, “This is a first-of-its kind source that covers such a large bandwidth of the THz spectrum from a single chip. And with QCL technology, we’re able to shrink the system to a size no larger than your standard iPad. So now we can really start thinking about portable, handheld THz systems. ”
The group envisions that THz DFG-QCL technology may one day be used in mundane daily activities while quietly help improve the overall safety and quality of our lives. All it takes is an imagination and improving the technology enthuses Karun, “Imagine this type of source used in postal offices to non- destructively scan packages for hazardous substances that have recently been mentioned in the news, such as anthrax or ricin. Another attractive option is developing a small-scale, non-invasive dental or skin imaging system that could be used for medical diagnostics.”
Moving forward, work needs to be done in improving the THz output power to the milliwatt-level required for most applications. “There is still a lot of room for growth”, says Dr. Belkin. “Redesigned active regions and optimizing waveguide geometry and THz extraction efficiency from our chips may increase the THz output power by up to two orders of magnitude. Improved processing and packaging will certainly lower the energy foot-print of devices. But we really believe our approach could be a breakthrough moment for THz source commercializing.”
The research was supported by the Defense Advanced Research Projects Agency, the National Science Foundation, and the Norman Hackerman Advanced Research Program from the state of Texas.
Figure 1: Image of the tunable terahertz QCL sources
(a) An image of THz-DFG QCL laser chip on a US dime. The chip contains 10 broadly-tunable terahertz sources. (b) Image of the chip mounted on the copper heat sink in a widely-tunable T-ray system. A single laser is approximately 2 mm long and 0.025 mm wide. One of the lasers is connected to the contact pad (seen on the top) by thin gold wires. Diffraction grating (seen in the back) is used to tune terahertz output from the laser.
Figure 2: Overview of the tunable T-ray system.
Overview of the tunable T-ray system. Rotating diffraction grating (yellow) is used to tune the terahertz frequency output from a laser (positioned on top of a copper heatsink on the left). The prototype system occupies a 6 inch x 6 inch breadboard, but can be significantly compressed using miniaturized optical and optomechanical components.