A super cover illustration highlights superconductivity research

https://www.uab.edu/news/research/item/10945-a-super-cover-illustration-highlights-superconductivity-research

Ilias Perakis, Ph.D.

by Jeff Hansen

For his fifth paper published in a Nature Research journal since 2015, Ilias Perakis, Ph.D., had a bonus that many researchers yearn for. The front cover of Nature Photonics featured his illustration of terahertz-driven superconductivity, the topic of his research paper inside.

The image shows a wavy orange-red line — a laser pulse with a frequency of a thousand-billion cycles per second — hitting a target material made of niobium and tin. Inside the material, the shape is distorted and breaks the symmetry by accelerating the electrons in the preferred direction of the electric field. With this comes an amazing change — momentary superconductivity with a disappearing quantum energy gap. 

“Normally, every electron in a material behaves independently of each other,” said Perakis, professor and chair of the University of Alabama at Birmingham Department of Physics, in the UAB College of Arts and Sciences. “Our applied pulse accelerates the electrons in one direction, into a new superconducting state with zero resistivity, where the electrons behave as a whole.”

The cover image shows the content of the study, “Lightwave-driven gapless superconductivity and forbidden quantum beats by terahertz symmetry breaking.” Perakis provided the physics theory that underlies experiments done by colleagues at Iowa State University, the United States Department of Energy’s Ames Laboratory and the University of Wisconsin-Madison. Corresponding author is Jigang Wang, Ph.D., Iowa State University.

This group was the first to show this technique as a tool to tune the quantum mechanical state of a material. Those terahertz pulses of laser light can both control the quantum state and sense the change in the quantum state.

Why an interest in such research? The dream of quantum computing, new machines that can operate at speeds vastly faster than supercomputers. Making such devices is a challenge.

“A quantum computer needs three things,” Perakis said. “Good material, good sensors of the quantum state and a good tool to manipulate the quantum state. We need to be able to change the quantum state in a controlled way.”

For the Nature Photonics cover, Perakis commissioned the Ella Maru Studio of South Carolina. “We want to combine the arts with the science,” Perakis said, “so that we can understand the concept. Reasoning in the quantum world requires imagination, and art helps you with that.” Ella Maru specializes in scientific design and animation.

Perakis was named chair of UAB Physics in 2015. He says the department’s five undergraduate major concentrations place an emphasis on excellence and on using critical thinking and systematic analysis to understand complex phenomena and solve today’s interdisciplinary scientific problems.  

“We personalize education to graduate a diverse group of students with well-developed complex skills and hands-on research experiences, who are well-connected to industry and well-prepared to serve a fast-changing and technology-driven global society,” Perakis said.

Abstract-Ultra-broadband photodetector based on three-dimensional graphene

Yifan Li,  Yating Zhang,  Zhiliang Chen,  Tengteng Li, Qingyan Li,  Hongliang Zhao,  Jie Li,  Yu Yu,  Lufan Jin, Jianquan Yao

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11184/111840Q/Ultra-broadband-photodetector-based-on-three-dimensional-graphene/10.1117/12.2538065.short

Graphene is a hot material for photodetectors due to its high carrier mobility, superior electronic and optical properties. However, the low optical absorption (2.3%) of graphene results in a low photoresponsivity, which limits its wide application in photodetection field. Three-dimensional (3D) graphene with connection carbon nanomaterials is expected to possess better optical and electrical properties than single-layer graphene. In this paper, we studied an ultra-broadband photodetector based on 3D graphene and investigated the different photoresponse with three kinds of 3D graphene including the 3D reduced oxide graphene foam (rGOF), the 3D Nickel (Ni) skeleton graphene foam (GF) and the 3D removal of nickel graphene foam (RNi GF). Obvious photocurrents and ultra-broadband absorption from ultraviolet (UV) spectrum to terahertz (THz) region can be measure in the three 3D GF. A high photoresponsivity of 50 mA W^-1 and a fast time response of 100 ms have been achieved. Particularly, the 3D RNi GF presents the highest absorption coefficient of 200 cm^-1 at THz region. The results reveal 3D graphene a good candidate for broadband photodetectors.

© (2019) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Time-domain terahertz compressive imaging

Luca Zanotto

https://arxiv.org/abs/1911.09545

We present an implementation of the single-pixel imaging approach into a terahertz (THz) time-domain spectroscopy (TDS) system. We demonstrate the indirect coherent reconstruction of THz temporal waveforms at each spatial position of an object, without the need of mechanical raster-scanning. First, we exploit such temporal information to realize (far-field) time-of-flight images. In addition, as a proof of concept, we apply a typical compressive sensing algorithm to demonstrate image reconstruction with less than 50% of the total required measurements. Finally, the access to frequency domain is also demonstrated by reconstructing spectral images of an object featuring an absorption line in the THz range. The combination of single-pixel imaging with compressive sensing algorithms allows to reduce both complexity and acquisition time of current THz-TDS imaging systems.

Abstract-Terahertz hyper-Raman time-domain spectroscopy of gallium selenide and its application in terahertz detection

Applied Physics Letters

Sen Mou, Andrea Rubano, Domenico Paparo

Experimental setup: BS, beam splitter; L, lens; CH, triggered chopper; DL, delay line; PL, air plasma; F, BG39 colored filter; MC, monochromator; PM, photomultiplier tube; Si, silicon plate.

https://aip.scitation.org/doi/abs/10.1063/1.5115986

We report the observation of Terahertz (THz) hyper-Raman generation in a gallium selenide crystal. This nonlinear optical process derives from the four- and five-wave-mixing of femtosecond optical pulses and intense, subps, broadband terahertz pulses. The wavelength spectrum of the resulting signal displays two pronounced frequency sidebands close to the optical second-harmonic central frequency 2ωL$2{\omega }_L$, where ωL is the optical central frequency of the fundamental beam. The two sidebands develop around the central frequency at the (anti-) Stokes side of ωs,a=2ωL∓ωT${\omega }_{s,a}=2{\omega }_L\mp {\omega }_T$, where ωT is the THz central frequency. This nonlinear optical process is used for the coherent detection of intense and broadband terahertz waves. The proposed technique shows a good linear response of up to 90 kV/cm and a better efficiency in detecting the lowest terahertz frequencies, as compared to the standard electro-optic sampling performed in two different nonlinear crystals.

The authors acknowledge funding from the “Ministero Istruzione Università e Ricerca” and “Consiglio Nazionale delle Ricerche.”

Abstract-Terahertz Time-Domain Reconstruction of Coating Microstratigraphy on Gilded Surfaces

Ilaria Cacciari, Daniele Ciofini, Hubert Baija, Salvatore Siano,

https://www.mdpi.com/1996-1944/12/23/3822

Here, a systematic study in order to assess the potential of THz time domain reflectometry for measuring the thicknesses of overpaint layers applied on original gilded surfaces was carried out. The work is part of a thorough characterization campaign, which is going on at the Rijksmuseum for addressing the conservation problems of a set of 19th century gilded picture frames on which heavy coatings were applied in previous undocumented restoration interventions. To perform such non-invasive thickness measurements, an analytical protocol based on Gaussian fits of the THz pulse-echo temporal profiles was optimized through the preparation of suitable technical samples and the comparison with direct thickness measurements. Finally, the methodology was validated by characterizing the microstratigraphy of an original sculptural element from a gilded picture frame in the Rijksmuseum collection. The results achieved show the effectiveness of the present approach in revealing multi-layered dielectric microstructures with a spatial resolution of about 30 µm when using a spectral range up to 1.5 THz.

Abstract-Monitoring the Progress of Lactic Acid Fermentation in Yogurt Manufacturing Using Terahertz Time-Domain–Attenuated Total-Reflection Spectroscopy

Koichiro Akiyama, Kazuki Horita, Tomoaki Sakamoto, Hiroshi Satozono, Hironori Takahashi, Yukihiro Goda,

https://link.springer.com/article/10.1007/s10762-019-00642-9

Lactic acid fermentation in yogurt manufacturing can be monitored using terahertz (THz)-attenuated total-reflection (ATR) spectroscopy. Yogurt manufacturing was performed on an ATR prism. The THz absorption coefficient and pH were measured for the entire 1000 min of the fermentation process. The absorption spectra were similar to the spectrum of water at the THz range. Temporal changes in the absorption coefficient at 0.4, 1.0, and 1.6 THz all decreased during the fermentation process, with two inflection points. The absolute value of the change in temporal absorption was greater at high frequencies than at low frequencies. However, the normalized absorption coefficient was larger at 0.4 THz. Because temporal changes in absorption corresponded with temporal changes in pH, the absorption changes appeared to be caused by the decomposition of the milk ingredients during the lactic acid fermentation. THz measurements can therefore be applied to the nondestructive monitoring of lactic acid fermentation in yogurt manufacturing.

ESA see-through security in worldwide service

Start-up company ThruVision, with the help of ESA's Technology Transfer Programme Office, transferred space-based ESA developed terahertz imaging technology into their security device T5000 which makes it possible to screen people for concealed objects without them even noticing.

http://www.spacedaily.com/reports/ESA_see_through_security_in_worldwide_service_999.html

The odds are high that you have already interacted with one of ESA's most-far-reaching inventions without realising it. Terahertz security cameras - currently employed in 18 countries including many major airports and the LA Metro - scan passengers for concealed weapons or contraband in their clothing from up to 10 metres away, operating on an entirely passive, non-invasive basis.

There is no need to step into a dedicated machine, as with X-ray scanners - you just walk on by. No radiation is emitted by the imager; instead it works by detecting the extremely high-frequency 'terahertz' waves emitted by anything that is warm, including the human body, and highlighting anything blocking these emissions. The scanning resolution is low enough that no anatomical details are revealed.

"Our imagers are looking at the thermal contrast between the temperature of the body and any items concealed around the body," explains Colin Evans, CEO of the Thruvision company, licensing the terahertz imaging technology from co-patent holders ESA and the Rutherford Appleton Laboratory in the UK.

"Because they are passive sensors, emitting no harmful energy, then their operation is covered by standard CCTV legislation. It's like looking at someone with night vision goggles, except at higher frequency."

The technology dates back to the start of this century, and an ambitious ESA-RAL research effort. This was the first of the ESA-led 'StarTiger' projects, gathering together top experts at a single site to work on a single, top priority project.

"Terahertz and sub-millimetre waves are used in space quite often," explains Peter de Maagt, head of ESA's Antennas and Sub-mm Waves Section, and co-author of the patent on the ESA side. "For instance to track early galactic evolution, or to perform temperature and humidity sounding for weather forecasting and atmospheric research.

"But the devices back then were really bulky, built up from unintegrated blocks, and multi-frequency real-time imaging was still a problem. What we did was to completely integrate everything into a very small credit-card-like unit.

"The whole basis of StarTiger is that, at the end, we have to produce working hardware, not just a report. So we targeted a camera capable of photographing the human hand in terahertz waves. We harnessed micro-machining technology used for mobile phone technology to make it work, achieving several technical world firsts in the process."

Dr Chris Mann led the work from the RAL side, and is the other patent co-author: "Every single element in the detector had to be hand-crafted using milling machines. We had to redesign all the optics because we weren't radio-astronomers looking at light years distant but wanting to image something just a few metres away.

"The challenge really captured people's imagination, and this was where the StarTiger approach came into its own, because we achieved in a few months what otherwise might have taken years."

Dr Mann went on to establish Thruvision, today serving as the company's Chief Scientific Advisor. He holds the same role in another company, SubTeraNDT, applying the same ESA-RAL patent for oil and gas infrastructure inspection.

"It has been a long process, but today Thruvision is well-established," adds Colin Evans. "Based over the road from RAL, our core sensor is manufactured here. And we've worked hard to build up a full supply chain for our extremely high-precision components, which operate at 250 Ghz, way above standard.

"We're very proud of our pioneering ESA and RAL technological heritage, and that absolutely cuts the mustard on the world stage - giving us a very strong pedigree."

The terahertz technology is part of ESA's intellectual property rights portfolio, consisting of around 450 patents on space innovations, available for licensing by European companies for both space and terrestrial applications.

China Unicom Gets Cracking on 6G Terahertz

Robert Clark, contributing editor,

https://www.lightreading.com/asia-pacific/china-unicom-gets-cracking-on-6g-terahertz-/d/d-id/755846?_mc=RSS_LR_EDT

China Unicom has begun research on Terahertz communication, one of the core technologies for 6G.

The operator has set up a research group, the Millimeter Wave Terahertz Joint Innovation Center, in partnership with 20 other companies and research organizations.

The research partnership would build several demonstration projects and develop a dozen or more standards, according to Feng Yi, head of the China Unicom Network Research Institute 5G Innovation Center. He did not say who the research partners were.

"Terahertz communication is the key technology to realize 6G," Feng told a seminar on the topic in Beijing last week.

"The 6G vision includes deep integration with AI technology; peak rates that reach terabit speeds; new multiple access and coding technologies; integrated air-sea communication and multi-scale communication from macro scale to micro," he said.

Unicom's disclosures follow the formal launch by two government ministries of China's 6G program on November 3.

The Chinese aren't the only people working on 6G, but Unicom is the first operator to sketch out its plans.

It draws on industry views set out by Finland's University of Oulu in a white paper created following an industry conference earlier this year.

The paper said a priority would be achieving 1 Tbit/s per user throughput, most likely through efficient utilization of Terahertz spectrum.

With the industry currently grappling with 5G millimeter wave at 26GHz and above, the leap to Terahertz is a big one.

Feng said one likely use for Terahertz-based 6G would be traditional connectivity. Its peak rate of 1 Tbit/s or above meant it could it deploy as a fiber substitute in areas where fixed-line access is uneconomical.

But he cited two use cases quite different from wireless as we know it.

One is inter-satellite or integrated air-space communication, taking advantage of the Terahertz wave's ability to transmit in outer space and to travel long distances at very low power.

"It might be deployed on platforms such as satellites, drones and airships, and act as a wireless relay device to realize air-ground integrated communication," he said.

Another is nano or micro-scale communication for IoT.

Using the extremely short Terahertz wavelength, the nano-scale Terahertz transceiver and antenna can be used for on-chip communication, wearable or other small devices to achieve high data speeds over a very short range.

He said some of the challenges in developing Terahertz tech included materials technology, chip integration and packaging, miniaturization, real-time and space division multiplexing and power consumption.

Abstract-Controllable broadband asymmetric transmission of terahertz wave based on Dirac semimetals

Linlin Dai, Yuping Zhang, John F. O’Hara, and Huiyun Zhang

 (a) PCR of linearly polarized wave, (b) the current densities in the top and bottom layers of the x-polarized wave at 1.389 THz and 1.668 THz, respectively.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-24-35784

We present a dynamic metamaterial based on Dirac semimetals and capable of realizing broadband and tunable asymmetric transmission in the terahertz region. The Dirac semimetal resonators have a chiral structure patterned with double-T resonators that results in partial polarization conversion of waves incident upon the material, leading to asymmetric transmission across a wide frequency range. We show how the gradual shift of the semimetal Fermi energy permits a method of control over the asymmetric total transmission.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Laser combo opens up futuristic terahertz technology

Experimental setup showing the system components and highlighting the path followed by the quantum cascade laser light (red) and terahertz radiation (blue). (Courtesy: Arman Amirzhan, Harvard SEAS)

https://physicsworld.com/a/laser-combo-opens-up-futuristic-terahertz-technology/

Researchers have created a new terahertz radiation emitter with highly-sought-after frequency adjustment capability. The compact source could enable the development of futuristic communications, security, biomedical and astronomical imaging systems.

The high bandwidth, high resolution, long-range sensing and ability to visualize objects through materials, makes terahertz electromagnetic frequencies much-coveted. However, the costliness, bulk, inefficiencies and lack of tunability of traditional terahertz emitters has stymied these promising avenues. This new combined laser terahertz source, product of a collaboration between researchers at Harvard, the US Army, MIT and Duke University, paves the way for future technologies, from T-ray imaging in airports and space observatories, to ultrahigh-capacity wireless connections.

“Existing sources have limited tunability, not more than 15-20% of the main frequency, so it’s fair to say that terahertz is underutilized,” explains co-senior author Federico Capasso from Harvard University. “Our laser opens up this spectral region, and in my opinion, will have revolutionary impact.”

The team has now described the theoretical proof and demonstration of this widely tunable and compact terahertz laser system (Science 10.1126/science.aay8683).

Perfect partnership

Capasso is no stranger to laser technology. He invented a compact tunable semiconductor laser, the quantum cascade laser (QCL), which is used commercially for chemical sensing and trace gas analysis. The QCL emits mid-infrared light, the spectral region where most gases have their characteristic absorption fingerprints, to detect low concentrations of molecules.

But it wasn’t until a conference in 2017 when Capasso met Henry Everitt, senior technologist with the US Army and adjunct professor at Duke University, that the idea to apply the widely tunable QCL to a laser with terahertz ability, formed.

Everitt, alongside Steven Johnson’s group at MIT, theoretically calculated that terahertz waves could be emitted with high efficiency from gas molecules held within cavities much smaller than those currently used on the optically pumped far-infrared (OPFIR) laser – one of the earliest sources of terahertz radiation. Like all traditional terahertz sources, the OPFIR was inefficient with limited tunability. But, guided by the theoretical calculations, Capasso’s team were able to use the QCL to dramatically increase the terahertz tuning range of a nitrous oxide (laughing gas) OPFIR laser.

“The same laser is now widely tunable – it’s a fantastic marriage between two existing lasers,” says Capasso.

Universal use

In initial experiments with the shoe-boxed sized QCL pumped molecular laser – QPML – the researchers demonstrated that the terahertz output could be tuned to produce 29 direct lasing transitions between 0.251 and 0.955 THz.

Artistic view of the QCL pumped terahertz laser showing the QCL beam (red) and the terahertz beam (blue) along with rotating molecules inside the cavity. The figure shows emission spectra of each gas. The QCL is continuously tunable and the terahertz laser emits at discrete frequencies and different ranges depending upon the gas used in the laser. (Courtesy: Arman Amirzhan, Harvard SEAS)

It was Johnson and Everitt’s theoretical models that highlighted nitrous oxide as a strongly polar gas with predicted terahertz release in the QPML. Similarly, a whole menu of other gas molecules have been predicted for terahertz generation at different frequencies and tuning ranges. Using this menu, it should be possible to select a gas laser appropriate for almost any application.

“This is a universal concept, because it can be applied to other gases,” says Capasso. “We haven’t quite reached one terahertz, so next thing is to try a carbon monoxide laser and go up to a few terahertz, which is very exciting for applications!”

Both Capasso and Everitt are particularly keen to use their laser to look skywards and sensitively identify unknown spectral features in the terahertz region. The team is developing higher power terahertz QPMLs for astronomical observations, while also eagerly working towards other commercial applications.

Abstract-Terahertz synthetic aperture in-line holography with intensity correction and sparsity autofocusing reconstruction

Zeyu Li, Ruijiao Zou, Weipeng Kong, Xuemin Wang, Qinghua Deng, Qiang Yan, Yu Qin, Weidong Wu, and Xun Zhou

Synthetic aperture hologram with intensity correction for a dragonfly forewing. (a) Nine normalized sub-holograms with intensity correction. (b) Synthetic aperture hologram composed of (a). (c) Synthetic aperture hologram without intensity correction. (d) Optical image of the dragonfly forewing sample. (e) Amplitude distribution reconstructed from (b) with 20 iterations. (f) Amplitude distribution reconstructed from (c) with 20 iterations. The effect of non-uniform intensity on reconstruction can be seen from the parts marked by the white and blue dotted circles.

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-7-12-1391

We demonstrate high-resolution and high-quality terahertz (THz) in-line digital holography based on the synthetic aperture method. The setup is built on a self-developed THz quantum cascade laser, and a lateral resolution better than 70 μm (∼𝜆$\sim \lambda$) is achieved at 4.3 THz. To correct intensity differences between sub-holograms before aperture stitching, a practical algorithm with global optimization is proposed. To address the twin-image problem for in-line holography, a sparsity-based phase retrieval algorithm is applied to perform the high-quality reconstruction. Furthermore, a new autofocusing criterion termed “reconstruction objective function” is introduced to obtain the best in-focus reconstruction distance, so the autofocusing procedure and the reconstruction are unified within the same framework. Both simulation and experiment prove its accuracy and robustness. Note that all the methods proposed here can be applied to other wavebands as well. We demonstrate the success of this THz synthetic aperture in-line holography on biological and semiconductor samples, showing its potential applications in bioimaging and materials analysis.

© 2019 Chinese Laser Press

Abstract-The Effect of Humidity and Temperature on Dielectric Fibre–Bound THz Transmission

M. Taherkhani, R. A. Sadeghzadeh, J. Taiber, J. Ornik, M. Koch

https://link.springer.com/article/10.1007%2Fs10762-019-00644-7

We experimentally study the impact of humidity and temperature on the transmission of 120-GHz waves through a solid core fibre of polypropylene without cladding. We find that the transmission is greatly affected by both, an increase in the water vapour density and in the temperature. We attribute this to the growth of a water film around the fibre which attenuates the evanescent field, and to an increasing absorption of the material with increasing temperature.

Abstract-Directly Observe Charge Injection of Graphene in the Graphene/WSe2 heterostructure by Optical-pump THz-probe Spectroscopy

 Liangliang Zhang, Zefeng Chen, Rui Zhang, Yong Tan, Tong Wu, Mostafa Shalaby, Rui Xie, Jianbin Xu

https://pubs.acs.org/doi/pdf/10.1021/acsami.9b13996

Charge transfer across the interface and interlayer coupling in graphene van der Waals heterostructure, which is constructed by graphene and semiconducting transition metal dichalcogenides (TMDCs), is critical for their electronic and optoelectronic applications. Photo-induced charge injection from TMDC to graphene has been studied in several heterostructure photodetector. However, the response time significantly varies among different reports, ranging from microseconds to milliseconds. In this work, using graphene/WSe2 heterostructure as an example, we directly observe the carriers density change of graphene by time-resolved optical-pump terahertz (THz)-probe spectroscopy and show ultrafast picosecond photoresponse of graphene. In the absence of photoexcitation, THz time-domain spectroscopic measurements show that WSe2 can transfer holes to graphene and pull down the Fermi level of graphene. After excitation by the ultrafast laser pulse, the transient THz response shows a rapid (∼0.35 ps) increase in the graphene conductivity mainly due to the holes injection from the WSe2 into graphene. Unlike previous reports on band bend as the guidance mechanism for charge transfer, our results show that the relevant mechanism is band offset across the atomically sharp interface.

Abstract-Optomechanical response with nanometer resolution in the self-mixing signal of a terahertz quantum cascade laser

Andrea Ottomaniello, James Keeley, Pierluigi Rubino, Lianhe Li, Marco Cecchini, Edmund H. Linfield, A. Giles Davies, Paul Dean, Alessandro Pitanti, Alessandro Tredicucci,

(a) Sketch of the two configurations of the SM apparatus. (b) Calculated Δ𝑁$\mathrm{\Delta }N$ (blue curve), and measured 𝑉SM$V_{\mathrm{S}\mathrm{M}}$ (red points) as a function of Δ𝐿$\mathrm{\Delta }L$ using configuration 1

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-44-23-5663

Owing to their intrinsic stability against optical feedback (OF), quantum cascade lasers (QCLs) represent a uniquely versatile source to further improve self-mixing interferometry at mid-infrared and terahertz (THz) frequencies. Here, we show the feasibility of detecting with nanometer precision, the deeply subwavelength (<𝜆/6000$<\lambda /6000$) mechanical vibrations of a suspended Si3N4${\mathrm{S}\mathrm{i}}_3{\mathrm{N}}_4$ membrane used as the external element of a THz QCL feedback interferometer. Besides representing an extension of the applicability of vibrometric characterization at THz frequencies, our system can be exploited for the realization of optomechanical applications, such as dynamical switching between different OF regimes and a still-lacking THz master-slave configuration.

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.