The first laser radio transmitter

This device uses a frequency comb laser to emit and modulate microwaves wirelessly. The laser uses different frequencies of light beating together to generate microwave radiation. The researchers used this phenomenon to send a song wirelessly to a receiver.

Credit: Image courtesy of Marco Piccardo/Harvard SEAS

https://www.sciencedaily.com/releases/2019/04/190425104238.htm

You've never heard Dean Martin like this.

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences transmitted a recording of Martin's classic "Volare" wirelessly via a semiconductor laser -- the first time a laser has been used as a radio frequency transmitter.

In a paper published in the Proceedings of the National Academy of Sciences, the researchers demonstrated a laser that can emit microwaves wirelessly, modulate them, and receive external radio frequency signals.

"The research opens the door to new types of hybrid electronic-photonic devices and is the first step toward ultra-high-speed Wi-Fi," said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, at SEAS and senior author of the study.

This research builds on previous work from the Capasso Lab. In 2017, the researchers discovered that an infrared frequency comb in a quantum cascade laser could be used to generate terahertz frequencies, the submillimeter wavelengths of the electromagnetic spectrum that could move data hundreds of times faster than today's wireless platforms. In 2018, the team found that quantum cascade laser frequency combs could also act as integrated transmitters or receivers to efficiently encode information.

Now, the researchers have figured out a way to extract and transmit wireless signals from laser frequency combs.

Unlike conventional lasers, which emit a single frequency of light, laser frequency combs emit multiple frequencies simultaneously, evenly spaced to resemble the teeth of a comb. In 2018, the researchers discovered that inside the laser, the different frequencies of light beat together to generate microwave radiation. The light inside the cavity of the laser caused electrons to oscillate at microwave frequencies -- which are within the communications spectrum.

"If you want to use this device for Wi-Fi, you need to be able to put useful information in the microwave signals and extract that information from the device," said Marco Piccardo, a postdoctoral fellow at SEAS and first author of the paper.

The first thing the new device needed to transmit microwave signals was an antenna. So, the researchers etched a gap into the top electrode of the device, creating a dipole antenna (like the rabbit ears on the top of an old TV). Next, they modulated the frequency comb to encode information on the microwave radiation created by the beating light of the comb. Then, using the antenna, the microwaves are radiated out from the device, containing the encoded information. The radio signal is received by a horn antenna, filtered and sent to a computer.

The researchers also demonstrated that the laser radio could receive signals. The team was able to remotely control the behavior of the laser using microwave signals from another device.

"This all-in-one, integrated device holds great promise for wireless communication," said Piccardo. "While the dream of terahertz wireless communication is still a ways away, this research provides a clear roadmap showing how to get there."

The Harvard Office of Technology Development has protected the intellectual property relating to this project and is exploring commercialization opportunities.

This research was co-authored by Michele Tamagnone, Benedikt Schwarz, Paul Chevalier, Noah A. Rubin, Yongrui Wang, Christine A. Wang, Michael K. Connors, Daniel McNulty and Alexey Belyanin. It was supported in part by the National Science Foundation.

Abstract-Time-dependent population inversion gratings in laser frequency combs

Marco Piccardo, Dmitry Kazakov, Noah A. Rubin, Paul Chevalier, Yongrui Wang, Feng Xie, Kevin Lascola, Alexey Belyanin, and Federico Capasso

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-4-475

In standing-wave lasers, spatial hole burning induces a static grating of the population inversion, enabling multimode operation with several independent lasing modes. In the presence of a mode-locking mechanism, these modes may become correlated, giving origin to a frequency comb. Quantum cascade lasers, owing to their ultrafast gain dynamics, are ideally suited to achieve comb operation. Here we experimentally demonstrate that the modes of a quantum cascade laser frequency comb coherently beat to produce time-dependent population inversion gratings, which spatially modulate the current in the device at frequencies equal to the mode separation and its higher harmonics. This phenomenon allows the laser to serve as a phased collection of microwave local oscillators and is utilized to demonstrate quadrature amplitude modulation, a staple of modern communications. These findings may provide for a new class of integrated transmitters, potentially extending from the microwave to the low terahertz band.

 © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz dephasing of Landau level transitions in graphene

Harald Schneider, Jacob C. König-Otto,  Alexej Pashkin, Yongrui Wang, Alexey Belyanin, Manfred Helm,  Stephan Winnerl

http://ieeexplore.ieee.org/document/8066873/

Using degenerate four-wave mixing (DFWM), we have investigated the coherent polarization between the lowest Landau levels in graphene under resonant excitation with narrowband THz pulses. A pronounced DFWM signal is observed and its dependence on THz field strength and magnetic field detuning is explored and compared with theoretical expectations.

Abstract-Magnetopolaritons in Weyl semimetals in a strong magnetic field

Zhongqu Long, Yongrui Wang, Maria Erukhimova, Mikhail Tokman, Alexey Belyanin

https://journals.aps.org/prl/accepted/db079YeeU431ec6360d15d55d3caf290cc659514f

Exotic topological and transport properties of Weyl semimetals generated a lot of excitement in the condensed matter community. Here we show that Weyl semimetals in a strong magnetic field are highly unusual optical materials. The propagation of electromagnetic waves is affected by an interplay between plasmonic response of chiral Weyl fermions and extreme anisotropy induced by a magnetic field. The resulting magneto-polaritons possess a number of peculiar properties, such as hyperbolic dispersion, photonic stop bands, coupling-induced transparency, and broadband polarization conversion. These effects can be used for optical spectroscopy of these materials including detection of the chiral anomaly, or for broadband terahertz/infrared applications.

Bridging the terahertz gap

Optical frequency combs generated in quantum cascade lasers. The discovered harmonic comb regime produces a spectrum with an intermodal spacing that is 10 to 100 times larger than that observed in fundamental frequency combs (right) enabling completely new applications in this platform. Both types of frequency combs can be generated using the same type of device. (Image courtesy of Jared Sisler/Havard SEAS)

Optical frequency comb offers a convenient way to generate elusive terahertz frequencies

By Leah Burrows

https://www.seas.harvard.edu/news/2017/10/bridging-terahertz-gap

Optical frequency combs are widely-used, high-precision tools for measuring and detecting different frequencies — a.k.a. colors — of light. Unlike conventional lasers, which emit a single frequency, these lasers emit multiple frequencies simultaneously. The equally spaced frequencies resemble the teeth of a comb. Optical frequency combs are used for everything from measuring the fingerprints of specific molecules to detecting distant exoplanets.

Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) are exploring the possibility of using an infrared frequency comb to generate elusive terahertz frequencies. These frequencies — which lie in the electromagnetic spectrum between radio waves and infrared light — have long promised to transform communications and sensing but are very challenging to source. By harnessing a recently discovered laser state, SEAS researchers have discovered an infrared frequency comb in a quantum cascade laser that offers a new way to generate terahertz frequencies.

Dubbed a harmonic frequency comb, this new system produces a spectrum of teeth with spacing tens of times larger than traditional frequency combs. The large but precise spacing allows these modes of light to beat together to produce extremely pure terahertz tones.

The research is described in Nature Photonics.

“The discovery of the harmonic state of quantum cascade lasers is surprising from a laser physics point of view."Tweet This

“The discovery of the harmonic state of quantum cascade lasers is surprising from a laser physics point of view,” said Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering and senior author of the paper. “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.”

In traditional frequency combs, teeth are separated by a small frequency dictated by the characteristic length of the laser cavity — meaning teeth sit close together. The harmonic frequency comb, however, can use a larger multiple of that frequency.    

“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,” said Marco Piccardo, a postdoctoral fellow in the Capasso lab and co-first author of the paper.

Key to the research was proving that these largely-spaced teeth were indeed equidistant. Using another reference comb, the team was able to study the harmonic frequency comb spectrum at 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,” said Dmitry Kazakov, a visiting research intern in the Capasso group and co-first author of the paper. “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.”

Most current terahertz generators use large, complex optical systems operating at near zero-temperatures to produce terahertz frequencies. The harmonic frequency comb operates at room temperature, uses commercial quantum cascade lasers, and is self-starting, meaning that the laser can automatically switch to this regime when 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.“

This paper was co-authored by Yongrui Wang, Paul Chevalier, Tobias Mansuripur, Feng Xie, Kevin Lascola and Alexey Belyanin. It was supported in part by the DARPA SCOUT program and the National Science Foundation.