Terahertz spectrum could be new non-line-of-sight optical communications medium

IMAGE: A directional terahertz wireless link (left) bounces off walls, so that there is no line-of-sight path from the transmitter to the receiver. The inset shows the bit error rate (BER) on a log scale, as a function of the output power of the transmitter. At both 100 GHz and 200 GHz, essentially error free transmission (BER = 10exp-9) can be achieved. A close-up photo (right) shows the transmitter rig used in these measurements, which includes a horn antenna and a Teflon lens to increase the gain of the system. (Image credit: Brown University)

by Gail Overton 

https://www.laserfocusworld.com/articles/2018/10/terahertz-spectrum-could-be-new-non-line-of-sight-optical-communications-medium.html

A new range of frequencies in the terahertz (THz) region of the spectrum may soon be available to support the growing communications demand for more bandwidth. A paper in APL Photonics, from AIP Publishing, demonstrates the feasibility of using THz carrier waves for data transmission in diverse situations and environments, including non-line-of-sight applications where waves bounce off, or are reflected by, walls and other objects. 

Daniel Mittleman of Brown University (Providence, RI), whose group led the study, said, "We're not the first group to study the feasibility of THz wireless links, either indoors or outdoors, but there have been few comprehensive studies." Many researchers in the field have believed that links that rely on indirect, or non-line-of-sight pathways, are impossible. "Our work shows that this isn't necessarily the case," he said.

THz radiation has frequencies higher than 95 gigahertz (GHz), beyond which the US Federal Communications Commission (FCC) has yet to establish service rules. Bandwidth in this region of the spectrum could be available for use in future wireless (free space optical or FSO) technologies, but little is known about power requirements, architectures, hardware or other basic issues for such data carrier waves. 

THz radiation is about 100 times higher in frequency and, thus, higher in photon energy than typical wireless carrier waves like Bluetooth or standard Wi-Fi are. Some have expressed concern about the safety of this type of radiation, but because these waves are not likely to penetrate deeply into tissue, particularly at the powers used in wireless applications, most believe safety will not be an issue. 

Mittleman's group measured data transmission at 100, 200, 300 and 400 GHz using a link with a data transfer rate of 1 gigabit per second in a variety of real-life environments. They set up a THz transmitter that used a frequency multiplier chain to up-convert a modulated base signal to the desired frequency. They also placed a receiver downstream, around various indoor and outdoor obstacles, to detect the pulsed signal. Outdoor measurements were enabled by an experimental license granted by the FCC. 

When the THz signal was pointed directly at the receiver, it produced a line-of-sight measurement. Alternatively, the signal could also be forced to reflect from, or bounce off, objects before detection. These non-line-of-sight experiments used real-life objects, including a painted cinder block, a door, metal foil, and a smooth metal plate, to reflect the signal. 

In a key experiment, the signal source and receiver were placed where they could not see each other. The signal was bounced off an intervening wall twice and easily detected by the receiver. This study demonstrated that, contrary to prior expectations, non-line-of-sight use is possible for this type of carrier wave, and that THz radiation may play a role in future wireless technologies

Semi-OT Team first to detect exciton in metal

Interferogram of the photoelectron counts versus photoelectron energy and time delay between interferometrically scanned pump–probe pulses. Credit: Nature Physics, DOI: 10.1038/nphys2981

 http://phys.org/news/2014-06-team-exciton-metal.html#jCp

University of Pittsburgh researchers have become the first to detect a fundamental particle of light-matter interaction in metals, the exciton. The team will publish its work online June 1 in Nature Physics.

Mankind has used reflection of light from a metal mirror on a daily basis for millennia, but the quantum mechanical magic behind this familiar phenomenon is only now being uncovered.

Physicists describe physical phenomena in terms of interactions between fields and particles, says lead author Hrvoje Petek, Pitt's Richard King Mellon Professor in the Department of Physics and Astronomy within Kenneth P. Dietrich School of Arts and Sciences. When light (an electromagnetic field) reflects from a metal mirror, it shakes the metal's free electrons (the particles), and the consequent acceleration of electrons creates a nearly perfect replica of the incident light (the reflection).

The classical theory of electromagnetism provides a good understanding of inputs and outputs of this process, but a microscopic quantum mechanical description of how the light excites the electrons is lacking.

Petek's team of experimental and theoretical physicists and chemists from the University of Pittsburgh and Institute of Physics in Zagreb, Croatia, report on how light and matter interact at the surface of a silver crystal. They observe, for the first time, an exciton in a metal.

Excitons, particles of light-matter interaction where light photons become transiently entangled with electrons in molecules and semiconductors, are known to be fundamentally important in processes such as plant photosynthesis and optical communications that are the basis for the Internet and cable TV. The optical and electronic properties of metals cause excitons to last no longer than approximately 100 attoseconds (0.1 quadrillionth of a second). Such short lifetimes make it difficult for scientists to study excitons in metals, but it also enables reflected light to be a nearly perfect replica of the incoming light.

Yet, Branko Gumhalter at the Institute of Physics predicted, and Petek and his team experimentally discovered, that the surface electrons of silver crystals can maintain the excitonic state more than 100 times longer than the bulk metal, enabling the excitons in metals to be experimentally captured by a newly developed multidimensional coherent spectroscopic technique.

The ability to detect excitons in metals sheds light on how light is converted to electrical and chemical energy in plants and solar cells, and in the future it may enable metals to function as active elements in optical communications. In other words, it may be possible to control how light is reflected from a metal.

 Explore further: Quasi-particle swap between graphene layers

UC Santa Barbara uses terahertz lasers to increase the speed of processing information

Lucas Reed
Writer

Photo by Ayeyi Aboagye

Recent research in the University of California Santa Barbara physics department made new strides in developing faster processing, using high and low frequency lasers much different than the multicolored ones used at dance parties in Isla Vista.

Along with a team of physicists, Mark Sherwin, professor of physics and director of the Institute of Terahertz Science and Technology, discovered how to use the lasers to increase the speed of processing information, which can lead to quicker forms of communication and higher quality video and audio. The results came as a great surprise to the research group, who previously recorded only a couple of frequencies when mixing lasers. By using a combination of terahertz and infrared lasers, the team documented 11 different frequencies, all represented by a different color. Their research was presented in a paper that is featured in the current issue of the science journal “Nature.”

According to Benjamin Zaks, a sixth-year UCSB doctoral student in physics and co-author of the paper, the process was completed using the free electron laser in Broida Hall. Using a series of magnets and two reflective mirrors, the terahertz laser is generated through the machine; then, an electron beam is shot through a series of magnets that diffracts the laser. When those diffractions occur, radiation is emitted. Most of the radiation is captured between the mirrors; however, a less-reflective mirror is used on one side to allow a small amount of radiation to escape. This radiation is used to create the terahertz laser. When a high-frequency laser like an infrared laser hits a semiconductor metal, it removes an electron causing an electron-hole-pair. When a low-frequency laser like the terahertz laser is introduced, the electron then accelerates and re-collides with the same location where it was removed creating multiple frequencies of light simultaneously.

“This is just the beginning of understanding this phenomenon,” Zaks said. “It has potential to increase the speeds of optical communications and has promising potential in materials’ characteristics.”

Up until Zaks and Sherwin’s research, terahertz lasers were mainly associated with the field of medicine, where they provide a safer method than the use of x-rays in performing full-body scans. Zaks worked with Sherwin for a year and a half to prove that the lasers had additional applications beyond medical testing.

Carl Gwinn, a professor of physics at UCSB for 23 years, believes that the new research could lead to significant changes in technology to come.

“I remember when we were first introduced to the Internet; there was bit-net, then Ethernet connections were developed, then people created optical fibers. This is another step in developing new technologies,” he said.

Alexander J. Pavellas, an electrical engineering graduate student, agrees that the research could mean great strides in optical communications and other forms of technology.

“UCSB is one of the world’s leaders in materials research, as well as transistor and semiconductor laser design,” he said. “If you can send laser pulses down a fiber optic line at multiple frequencies, then each frequency will provide another data channel. More channels mean more data, more data means, among other things, higher quality video/audio can be sent down the line. I think these experiments are a great example of how a new phenomenon in physics can quickly lead to new technologies, and I look forward to seeing what this leads to.”