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Terahertz waves and acoustic modulation: A datalink “Odd Couple”?

Electromagnetic energy in the terahertz region of the spectrum – nominally 300 GHz/0.3 THz to 10 THz, although some consider it to start at 100 GHz/0.1 THz – presents a technical dilemma. On one hand, the enormous width of this span means it has the potential to support extremely high data rates as well as some fascinating sensing and scanning instrumentation. On the other hand, terahertz waves exist in a sort of “EM desert” between the RF spectrum and the optical world. Still, the lure of terahertz waves is quite strong. It’s largely unexplored and unrestricted spectrum, which adds to its appeal.

For data link purposes the RF spectrum, at tens of GHz, can be managed with leading-edge solid-state and vacuum-tube devices supported by specialized coaxial cables and waveguides. The optical world can be managed with many different lasers, LEDs, and photodetectors. But generating terahertz waves is hard, often accomplished using a quantum cascade laser (QCL), and modulating terahertz waves for use as data links is even harder. (The two References at the end, both from IEEE Spectrum, present very different perspectives the viability of terahertz-band applications.)

That may be changing, though. A recent project and paper by a collaborative team from the University of Leeds and the University of Nottingham (both in the UK) not only provided insight on the generation of terahertz waves, but also demonstrated how they employed acoustic, non-EM wave energy to modulate those waves.

It might seem that there is no possibility of a relationship between acoustic eaves and terahertz waves. Not only do they differ in frequency by many orders of magnitude, but terahertz waves are a form of electromagnetic energy and can exist in a vacuum while acoustic waves are not EM and need a tangible medium in which to propagate. Yet the team was able to cleverly use the acoustic-wave energy to affect the terahertz waves from the laser.

In greatly simplified form, what happens is this: An electron passes through the optical component of the quantum cascade laser and, in doing so, it goes through a series of ‘quantum wells.’ At these wells, the electron’s energy level decreases and a photon pulse is emitted, with one electron capable of causing the emission of multiple photons as a steady, continuous-wave (CW) stream.

To implement modulation in the experimental setup, shown in Figure 1, the researchers focused acoustic-wave energy to vibrate the quantum wells inside the quantum cascade laser, which is mounted on the cold finger of an optical cryostat at an operating temperature of 15K. Here, “acoustic wave energy” does not mean they pointed a loudspeaker at the laser source. Instead, they generated the acoustic waves using the impact of a pulse from another laser to induce an optically generated, picosecond-wide, strain pulse on an aluminum-film acoustic transducer. This strain pulse, in turn, caused that film to expand and contract, thus sending a mechanical wave through the quantum cascade laser, and this mechanical wave established the modulation.

Figure 1 This experimental arrangement (a) measures the optical and electronic perturbation of a THz quantum cascade laser (QCL) device by laser-generated picosecond acoustic pulse. A schematic diagram of the QCL device structure (b) shows the transmitted (solid red line) and reflected (dashed red line) strain pulses.

Professor Tony Kent, a team leader, described the experiment by saying, “essentially, what we did was use the acoustic wave to shake the intricate electronic states inside the quantum cascade laser. We could then see that its terahertz light output was being altered by the acoustic wave. This result opens a new area for physics and engineering to come together in the exploration of the interaction of terahertz sound and light waves, which could have real technological applications.”

Before you think “problem solved,” it is important to note that the maximum modulation depth they achieved was relatively low, around 6%. That’s’ a long way from the tens of percent of modulation depth needed for a viable communications link, Nonetheless, operating in the terahertz band holds the promise of data rates approaching 100 gigabits/second, which is a truly impressive (and maybe scary) possibility.

Whether this advance is an early step on a research path with more success ahead, or further results will be stalled by major barriers – well, it’s too early to say. But it is an interesting case of how two totally disparate manifestations of wave energy – here EM energy in the terahertz region, and high-frequency, pulsed acoustic energy – can work together for an interesting and perhaps otherwise unobtainable result.

Their readable paper “High-speed modulation of a terahertz quantum cascade laser by coherent acoustic phonon pulses” was published in Nature Communications (the pdf version is here). Interestingly, with most of these academic papers, the HTML version also has a link to Supplementary Information. That type of file usually has extra details on the configuration, BOM, materials used and their preparations, issues faced and overcome, details of the test arrangement, and more.

However, this paper’s Supplementary Information was quite different: whether by accident or deliberately, it has none of those aspects. Instead, it has 13 pages of comments, suggestions, and questions from the peer reviewers of this paper, along with the paper authors’ responses. Reading those was quite fascinating!

Have you ever been involved in a project where two very different technologies, which seemed at first to have little to do with each other, were actually joined in a complementary way to accomplish a larger goal?

References

IEEE Spectrum, “Wireless Industry’s Newest Gambit: Terahertz Communication Bands

IEEE Spectrum, “The Truth About Terahertz

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