WIRELESS COMMUNICATIONS
electromagnetic spectrum and atomic radiation sources,” adds research director Shahraam Afshar.
“This opens up new doors of development in a wide range of nanotechnologies and quantum technologies such as quantum signal processing.”
SPIN CURRENT GENERATOR Meanwhile, German physicists at the Technische Universität Kaiserslautern (TUK) have developed a new method for generating terahertz waves. They use a quantum magnetic current flow, a so-called spin current, in magnetic metal nanostructures. Spin current is analogous to the electric current, in which electrical charges, namely electrons, flow. “A spin describes the intrinsic angular momentum of a quasiparticle, such as an electron,” says Associate Professor Evangelos Papaioannou. “It forms the basis for all magnetic phenomena. Simply said, an electron rotates left or right around its axis like a spinning top.” A special nanostructure has been developed by the
research team of Papaioannou for the application of the technique. “It consists of a metal bilayer of magnetic iron and non-magnetic platinum,” as the physicist describes the structure. “These are very thin layers that are only a few nanometres thick.” To generate the terahertz waves, the researchers
use a femtosecond laser that emits extremely short laser pulses. When the laser pulses hit the nanostructure they stimulate the electrons in the iron film, creating a spin current. The atomic nuclei of platinum deflect electrons with a left- and right- handed spin in opposite directions, which leads to the transformation of the spin current into an ultrafast transient charge current, which is then the source of terahertz waves. As a feature of the setup, a small silicon lens is
attached to the structure to concentrate the waves. In this way, the Terahertz waves could be forwarded easily and efficiently in future applications.
TERAHERTZ COMB GENERATOR Optical frequency “combs” are widely-used as high- precision tools for measuring and detecting different frequencies (colours) 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. Researchers at the Harvard John A Paulson
School of Engineering and Applied Sciences (SEAS) are 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
16 /// Environmental Engineering /// March 2018
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 discovery of the harmonic state of quantum
cascade lasers is surprising from a laser physics point of view,” says Federico Capasso, the Robert L Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering. “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.” 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,” adds Capasso. “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.” EE
❱❱ To read more on advanced telecoms technology, scan the QR code or visit
http://goo.gl/RyRNjH
❱❱ Diagram showing how spin current generation of Terahertz waves works, top; Australian sub-wavelength fibre emitter; above; and Associate Professor Evangelos Papaioannou at the Technische Universität Kaiserslautern (TUK), below
IMAGE: ANDREY E MIROSHNICHENKO
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