## Friday, September 18, 2020

### Resonant tunneling diode oscillators for terahertz-wave detection

 Photograph showing the terahertz detector chip based on a resonant tunneling diode oscillator (inset) being used to detect terahertz radiation. Credit: RIKEN Center for Advanced Photonics
https://www.blogger.com/blog/post/edit/124073320791841682/493314866453444811

A semiconductor device that is promising for both generating and detecting terahertz radiation has been demonstrated by physicists at RIKEN. This may aid the development of high-performance integrated solutions for terahertz imaging and sensing applications as well as for high-speed, next-generation wireless communications systems.

Terahertz radiation is  with frequencies ranging between 0.1 and 10 . It falls between microwaves and  on the electromagnetic spectrum. This range has been dubbed the terahertz gap because it has been underutilized in applications compared to other regions of the spectrum, which have been extensively used in many applications.

One reason why terahertz radiation has not been used much is that it has been traditionally difficult to generate and detect terahertz radiation. But recent years have seen many advances in this area, and terahertz radiation is gathering interest for imaging for  and medical purposes as well as for wireless communications systems that use terahertz waves instead of microwaves as the information carrier.

While  known as resonant tunneling diode (RTD) oscillators have been used as terahertz emitters for many years, Yuma Takida and Hiroaki Minamide from the RIKEN Center for Advanced Photonics have now shown that they can also detect  at room temperature.

"Our result demonstrates that terahertz RTD oscillators can be used as sensitive detectors of terahertz waves," says Takida. "This promises to accelerate the development of integrated oscillator and detector single chips, which will pave the way toward real-world terahertz applications."

The RIKEN pair, who collaborated with Safumi Suzuki and Masahiro Asada from Tokyo Institute of Technology, fabricated an RTD that can operate in two detection modes. One of these modes was especially sensitive at detecting terahertz waves, with a performance rivaling that of diode-based detectors.

"RTDs have several  over other detectors," notes Takida. "These advantages include a wider dynamic range owing to resistance to high input power and a higher sensitivity at room temperature. Furthermore, we have shown that a single RTD device can be used as both an oscillator and a detector at terahertz frequencies."

Takida says that the growing demand for terahertz technology and progress in semiconductor technology made the work possible.

The team anticipates that optimizing the design will allow devices to be fabricated that operate anywhere in the 0.1–2 terahertz region. Future work will focus on improving the sensitivity of their RTD detector and demonstrating integrated solutions for broadband heterodyne mixing at terahertz frequencies.

## Tuesday, September 15, 2020

### Shining a light on disordered and fractal systems

https://phys.org/news/2020-09-disordered-fractal.html

A University of Tsukuba research team uses terahertz-frequency light to probe the unusual behavior of disordered systems to discover that the anonymously large vibrations in lysozyme can be explained by its glassy and fractal nature

Tsukuba, Japan—Researchers led by the University of Tsukuba studied the vibrational modes of an intrinsically disordered protein to understand its anomalously strong response at low frequencies. This work may lead to improvements in our knowledge of materials that lack long-range order, which may influence industrial glass manufacturing.

Glassy materials have many surprising properties. Not quite a solid or a liquid, glasses are made of atoms that are frozen in a disordered, non-crystalline state. Over a century ago, physicist Peter Debye proposed a formula for understanding the possible vibrational modes of solids. While mostly successful, this theory does not explain the surprisingly universal vibrations that can be excited in disordered materials—like glass—by electromagnetic radiation in the terahertz range. This deviation has been seen often enough to gets its own name, the "boson peak," but its origin remains unclear.

Now, researchers at the University of Tsukuba have conducted a series of experiments to investigate the physics behind the boson peak using the protein lysozyme.

"This protein has an intrinsically disordered and  structure," first author of the study Professor Tatsuya Mori says. "We believe that it makes sense to consider the entire system as a single supramolecule."

Fractals, which are mathematical structures that exhibit self-similarity over a wide range of scales, are common in nature. Think of trees: they appear similar whether you zoom out to look at the branches, as well as when you come close to inspect the twigs. Fractals have the surprising ability to be described by a non-integer number of dimensions. That is, an object with a fractal dimension of 1.5 is halfway between a two-dimensional and a three-dimensional object, which means that its mass increases with its size to the 1.5 power.

On the basis of the results of terahertz spectroscopy, the mass  of the lysozyme molecules was found to be around 2.75. This value was also determined to be related to the absorption coefficient of the material.

"The findings suggest that the fractal properties originate from the self-similarity of the structure of the amino acids of the lysozyme proteins," Professor Mori says. "This research may hold the key to resolving a long-standing puzzle regarding disordered and fractal materials, which can lead to more efficient production of glass or fractal structures."

The work is published in Physical Review E as "Detection of boson peak and fractal dynamics of disordered systems using terahertz spectroscopy."

## Monday, September 14, 2020

### Abstract-Generalized Kramers–Kronig receiver for coherent terahertz communications

T. Harter, C. Füllner, J. N. Kemal, S. Ummethala, J. L. Steinmann, M. Brosi, J. L. Hesler, E. Bründermann, A.-S. Müller, W. Freude, S. Randel, C. Koos

https://www.nature.com/articles/s41566-020-0675-0

Modern communication systems rely on efficient quadrature amplitude modulation formats that encode information on both the amplitude and phase of an electromagnetic carrier. Coherent detection of such signals typically requires complex receivers that contain a continuous-wave local oscillator as a phase reference and a mixer circuit for spectral down-conversion. In optical communications, the so-called Kramers–Kronig scheme has been demonstrated to simplify the receiver, reducing the hardware to a single photodiode1,2,3. In this approach, a local-oscillator tone is transmitted along with the signal, and the amplitude and phase of the complex signal envelope are digitally reconstructed from the photocurrent by exploiting their Kramers–Kronig-type relation4,5,6. Here, we transfer the Kramers–Kronig scheme to high-speed wireless communications at terahertz carrier frequencies. To this end, we generalize the approach to account for non-quadratic receiver characteristics and employ a Schottky-barrier diode as a nonlinear receiver element. Using 16-state quadrature amplitude modulation, we transmit a net data rate of 115 Gbit s−1 at a carrier frequency of 0.3 THz over a distance of 110 m.

## Friday, September 11, 2020

### Abstract-Repeatability of material parameter extraction of liquids from transmission terahertz time-domain measurements

Jan Ornik, Jannik Lehr, Marco Reuter, David Jahn, Felipe Beltran-Mejia, Jan C. Balzer, Thomas Kleine-Ostmann, and Martin Koch
 Experimental setup composed of a pair of photoconductive antennas, a delay line and a femtosecond fiber laser emitting at 780 nm. The laser beam forms the optical path as depicted by the red lines. The blue region represents the THz radiation that impinges one of the five cuvettes.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-19-28178

Recently, many research groups worldwide have reported on the THz properties of liquids. Often these parameters, i.e., refractive index and absorption coefficient, are determined using liquids in cuvettes and terahertz time-domain spectroscopy. Here, we discuss the measurement process and determine how repeatable such measurements and the data extraction are using rapeseed oil as a sample. We address system stability, cuvette positioning, cuvette cleaning and cuvette assembly as sources affecting the repeatability. The results show that system stability and cuvette assembly are the most prominent factors limiting the repeatability of the THz measurements. These findings suggest that a single cuvette with precise positioning and thorough cleaning of the cuvette delivers the best discrimination among different liquid samples. Furthermore, when using a single cuvette and measurement systems of similar stability, the repeatability calculated based on several consecutive measurements is a good estimate to tell whether samples can be discriminated.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

## Thursday, September 10, 2020

### Terahertz receiver for 6G wireless communications

 Future mobile network: Small radio cells (orange) are connected by wireless high-speed terahertz links (green). Credit: IPQ, KIT / Nature Photonics
by Monika Landgraf

https://phys.org/news/2020-09-terahertz-6g-wireless.html

Future wireless networks of the 6th generation (6G) will consist of a multitude of small radio cells that need to be connected by broadband communication links. In this context, wireless transmission at THz frequencies represents a particularly attractive and flexible solution. Researchers at Karlsruhe Institute of Technology (KIT) have now developed a novel concept for low-cost terahertz receivers that consist of a single diode in combination with a dedicated signal processing technique. In a proof-of-concept experiment, the team demonstrated transmission at a data rate of 115 Gbit/s and a carrier frequency of 0.3 THz over a distance of 110 meters. The results are reported in Nature Photonics.

5G will be followed by 6G: The sixth generation of mobile communications promises even higher data rates, shorter latency, and strongly increased densities of terminal devices, while exploiting Artificial Intelligence (AI) to control devices or autonomous vehicles in the Internet-of-Things era. "To simultaneously serve as many users as possible and to transmit data at utmost speed, future wireless networks will consist of a large number of small radio cells," explains Professor Christian Koos, who works on 6G technologies at KIT together with his colleague Professor Sebastian Randel. In these radio cells, distances are short such that  can be transmitted with minimum energy consumption and low electromagnetic immission. The associated base stations will be compact and can easily be mounted to building facades or street lights.
To form a powerful and flexible network, these base stations need to be connected by high-speed wireless links that offer data rates of tens or even hundreds of gigabits per second (Gbit/s). This may be accomplished by terahertz carrier waves, which occupy the frequency range between microwaves and infrared light waves. However, terahertz receivers are still rather complex and expensive and often represent the bandwidht bottleneck of the entire link. In cooperation with Virginia Diodes (VDI) in Charlottesville, U.S., researchers of KIT's Institute of Photonics and Quantum Electronics (IPQ), Institute of Microstructure Technology (IMT), and Institute for Beam Physics and Technology (IBPT) have now demonstrated a particularly simple inexpensive receiver for terahertz signals. The concept is presented in Nature Photonics.
Highest Data Rate Demonstrated So Far for Wireless THz Communications over More Than 100 Meters
"At its core, the receiver consists a single diode, which rectifies the terahertz signal," says Dr. Tobias Harter, who carried out the demonstration together with his colleague Christoph Füllner in the framework of his doctoral thesis. The diode is a so-called Schottky barrier diode, that offers large bandwidth and that is used as an envelope detector to recover the amplitude of the terahertz signal. Correct decoding of the data, however, additionally requires the time-dependent phase of the terahertz wave that is usually lost during rectification.
To overcome this problem, researchers use digital signal processing techniques in combination with a special class of data signals, for which the phase can be reconstructed from the amplitude via the so-called Kramers-Kronig relations. The Kramers-Kronig relation describe a mathematical relationship between the real part and the imaginary part of an analytic signal. Using their receiver concept, the scientists achieved a transmission rate of 115 Gbit/s at a carrier frequency of 0.3 THz over a distance of 110 m.
"This is the highest data rate so far demonstrated for wireless terahertz transmission over more than 100 m," Füllner says. The  receiver developed by KIT stands out due to its technical simplicity and lends itself to cost-efficient mass production.

## Wednesday, September 9, 2020

### Abstract-Exact frequency and phase control of a terahertz laser

Reshma A. Mohandas, Lalitha Ponnampalam, Lianhe Li, Paul Dean, Alwyn J. Seeds, Edmund H. Linfield, A. Giles Davies, and Joshua R. Freeman

 Schematic diagram of the experimental arrangement. EDFA, erbium doped fibre amplifier; Tx, photomixer emitter; Rx1 and Rx2, photomixer receivers; PLL, phase lock loop; Δ𝜑$\mathrm{\Delta }\phi$, variable delay line. Electrical connections are shown in black, optical fiber and IR connections in red, and terahertz connections in green.

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-9-1143

The accuracy of high-resolution spectroscopy depends critically on the stability, frequency control, and traceability available from laser sources. In this work, we report exact tunable frequency synthesis and phase control of a terahertz laser. The terahertz laser is locked by a terahertz injection phase lock loop for the first time, with the terahertz signal generated by heterodyning selected lines from an all-fiber infrared frequency comb generator in an ultrafast photodetector. The comb line frequency separation is exactly determined by a Global Positioning System-locked microwave frequency synthesizer, providing traceability of the terahertz laser frequency to primary standards. The locking technique reduced the heterodyne linewidth of the terahertz laser to a measurement instrument-limited linewidth of $<1\phantom{\rule{thickmathspace}{0ex}}\mathrm{H}\mathrm{z}$, robust against short- and long-term environmental fluctuations. The terahertz laser frequency can be tuned in increments determined only by the microwave synthesizer resolution, and the phase of the laser, relative to the reference, is independently and precisely controlled within a range $±0.3\pi$. These findings are expected to enable applications in phase-resolved high-precision terahertz gas spectroscopy and radiometry.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.