Editorial Feature

Optical Comb Produces Elusive Terahertz Frequency

By harnessing a recently discovered laser state, Scientists from Harvard University in America have been exploring the possibility of generating terahertz frequencies, a frequency which could hold the key to transforming communications and sensing.

The terahertz frequency – which lies between radio waves and infrared light in the electromagnetic spectrum – is very difficult to source. However Researchers from Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) believe they have found a new way to generate terahertz frequencies using an infrared frequency comb in a quantum cascade laser.

Optical combs are widely-used high-precision tools for measuring and detecting different frequencies or colors of light – they’re used for everything from calculating the fingerprints of specific molecules to discovering distant exoplanets. Unlike traditional lasers – which emit just a single frequency – these optical combs emit multiple frequencies at the same time. The equally spaced frequencies resemble the teeth of a comb – hence the name.

SEAS Researchers have developed a new system – dubbed the harmonic frequency comb – which produces a spectrum of teeth tens of thousands of times larger than conventional frequency combs. The large but precise spacing allows these modes of light to beat together to produce incredibly pure terahertz tones.

The discovery of the harmonic state of quantum cascade lasers is surprising from a laser physics point of view. Until recently, it was thought that multimode lasers would normally lase on all the possible frequencies of the cavity. In the harmonic state, many cavity frequencies are skipped. Even more remarkable is that this discovery opens up unforeseen opportunities in unused regions of the electromagnetic spectrum, the terahertz.

Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering

Teeth are separated by a small frequency dictated by the characteristic length of the laser cavity in traditional frequency combs — meaning they sit close together. However the harmonic frequency comb uses a larger multiple of that frequency, meaning they can sit further apart.  The discovered harmonic comb produces a spectrum with an intermodal spacing that is 10 to 100 times larger than that seen in fundamental frequency combs opening up completely new applications in this platform.

With this new comb regime we can bypass the strict limitations set by the cavity length and reach an unprecedented degree of flexibility in the realm of quantum cascade laser frequency combs.

Marco Piccardo, a Postdoctoral Fellow, The Capasso lab

Proving that these largely spaced teeth were in fact equidistant apart was essential to the research, which has been published in Nature Photonics. Scientists employed a reference comb to allow them to study the harmonic frequency comb at a very high resolution.

We show that the lines are equidistant with an uncertainty of only 300 hertz, that quantifies the relative precision of this measurement to five parts per trillion. It is as if one could measure the distance from Earth to the Moon and be off by less than the thickness of a human hair.

Dmitry Kazakov, a Visiting Research Intern, The Capasso group

The harmonic frequency comb operates at room temperature and uses commercial quantum cascade lasers – a positive over most current terahertz generators which use large complex optical systems at very low temperatures to produce terahertz frequencies.

The system is also self-starting, meaning the laser can automatically switch on to this regime when an electrical current is injected into the device.

“This opens up completely new applications for frequency combs, especially in wireless communications,” Capasso said. “We foresee that in the near future this comb regime will enable a new class of chip-scale modem operating at terahertz frequencies, accommodating the ever-increasing consumer demand for high data rate digital communication.”

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