Harvard Researchers Claim “Most Compact Terahertz Laser Ever Designed”

February 07, 2022 by Darshil Patel

In a recent announcement, Harvard researchers unveiled a compact tunable terahertz laser to be used in imaging, security, and wireless communications.

Researchers from Harvard University claim to have developed the most compact terahertz laser ever designed.

The team achieved this feat by accessing terahertz frequencies that are usually out of reach in the electromagnetic spectrum. This laser operates at room temperature and outperforms other terahertz sources in terms of range—creating 120 individual frequencies between 0.25–1.3 THz. The researchers are optimistic that this laser can be safely used in applications like drug detection, high-capacity optical wireless links, medical imaging, and airport security. 


Research setup of Harvard terahertz laser

The researchers used a gold-plated silicon wafer to reflect a small portion of the pump beam into the reference gas cell. Meanwhile, the rest of the beam entered into the terahertz cavity. Image used courtesy of Harvard University

What obstacles did the Harvard team overcome to develop this compact technology?


The Challenges of Terahertz Technologies

Terahertz radiation ranges from 0.1 THz to 10 THz frequencies, translating to wavelengths between 30 µm and 1 mm. While terahertz technologies have great potential, the wavelengths of THz waves pose challenges for commercialization.

It is hard to confine THz waves within a laser medium to build a THz semiconductor laser. Also, there are not many materials with a bandgap in the THz range. The ones that offer such a bandgap are hard to fabricate and are not very robust. Moreover, in semiconductors, photons are absorbed by mechanical vibrations in the crystal lattice. In gallium arsenide (GaAs), for instance, this absorption occurs at around 9 THz, which makes it very difficult to generate a THz radiation source with GaAs.

Beyond the challenge of finding THz sources, THz detectors are also difficult to implement. Most receivers available today operate under cryogenic temperatures to suppress thermal background.


How Silicon Photonics Taps Into Terahertz

Researchers are now trying to find reliable sources and detectors of THz radiation. One fruitful avenue is silicon photonics. In silicon photonics-based applications, silicon is patterned with sub-micrometer precision, allowing components to operate at high frequencies. The light propagation in silicon devices is dictated by many non-linear effects, which are necessary to allow light to interact with light. This interaction makes silicon photonics useful in various radiation-emitting sources. 

Researchers are also trying to circumvent the issues related to the wavelengths of THz waves through bandgap engineering. Quantum cascade lasers (QCLs) are one example. The radiation in QCLs is created by transitions between the sub-bands within the conduction band of semiconductor material. Once an electron undergoes an inter-sub-band transition, it emits a photon and tunnels to the next sub-band to emit another photon. This kind of cascade operation allows QCLs to generate THz radiation with an output power of more than 1 W. As such, the bandgap of the material is almost irrelevant in this case. 


Example of various silicon photonics applications

Example of various silicon photonics applications. Image used courtesy of MDPI

However, QCLs are limited by their operation at cryogenic temperatures because thermal vibrations can attenuate the laser by redistributing electrons between the tightly-bound states. Another challenge with QCLs is that at THz frequencies, the wavelengths are in the range of hundreds of micrometers, which would require QCLs to be made thicker than the limit of the material deposition technology.

Against so many oppositions, how did Harvard engineers modify their approach to QLCs to bring a miniaturized terahertz laser to life? 


Harvard Develops a Compact, Room-temp Terahertz Laser

Researchers from Harvard's School of Engineering and Applied Sciences recently reported on a high-performance quantum cascade laser-pumped molecular laser (QPML)—tunable over 1 THz. They demonstrated a compact QPML with a small cavity, the radiation spanning 0.25–1.3 THz. The research is published in the APL Photonics journal.


Schematic of the QCL-pumped molecular laser

Schematic of the QCL-pumped molecular laser. Image used courtesy of Harvard University


The above figure depicts the schematic of a QPML system. The QCL-generated beam passes through the front mirror and excites the molecular gain medium inside the laser cavity. The rotation transition in the gain medium is resonated by adjusting the back mirror. As a result, the THz radiation is generated from the front mirror, which is then detected by a Schottky diode or another receiver.

The tunability of the cavity length by the back mirror allows for any molecular gain medium. However, finding an ideal gain medium that provides both power and tunability is a challenge. The Harvard researchers introduced methyl fluoride (CH3F) as a molecular gain medium because it is good at absorbing and emitting THz radiation. Thus, it increases the efficiency and tunability of the laser.

The team at Harvard claims the compact continuous-wave THz laser will be applicable in various applications in radio astronomy, skin and breast cancer imaging, security, wireless communications, and aerospace industries. Lead researcher Paul Chevalier noted that shrinking the device to less than a cubic foot will allow the team to use this frequency range for even more use cases, spanning short-range communications and short-range radar to other biomedical imaging.