Abstract-Efficient second-harmonic and terahertz generation from single BiB3O6 crystal using nanosecond and femtosecond lasers

 

Chandan Ghorui, A. M. Rudra, Udit Chatterjee, A. K. Chaudhary,  D. Ganesh, 

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-60-19-5643

The paper reports the efficient UV and terahertz generation from a 1.29 mm thick and Type I, 𝜃=28.9∘$\theta ={28.9}^{\circ }$ cut BiB3O6${\mathrm{B}\mathrm{i}\mathrm{B}}_3{\mathrm{O}}_6$ (bismuth triborate, BIBO) crystal using femtosecond and nanoseconds laser pulses. We have employed 800 nm wavelength pulses of 50 and 140 fs obtained from a Ti:sapphire laser amplifier and oscillators at 1 kHz and 80 MHz repetition rates, respectively. The conversion efficiency of second-harmonic generation (SHG) was ∼50%$\sim 50\mathrm{\%}$ while that obtained for terahertz (THz) generations was of the order of 1.85×10−5%$1.85\times {10}^{-5}\mathrm{\%}$. In addition, LDS-698 dye laser radiation tunable between 650–700 nm was also used as a source for SHG between the 325–350 nm range. The dye laser was pumped by SHG (532 nm) radiation from an electro-optically 𝑄$Q$-switched Nd:YAG laser having a pulse repetition rate of 10 Hz and a pulse width of 10 ns. A conversion efficiency of 4.01% was obtained for generation of UV at 343.5 nm. Finally, we have measured the transmission, refractive index, absorbance, and conductivity properties of BIBO crystal in the THz domain. We also ascertained the coherence length, relative permittivity and reflectivity of the crystal.

© 2021 Optical Society of America

Abstract-Quantification of triglyceride levels in fresh human blood by terahertz time-domain spectroscopy

 

Dan Wang, Yu Zhang, Juan Han, Xiao Li, Xiaofeng Chen, Tianzhu Qiu,  Hua Chen, 

https://www.nature.com/articles/s41598-021-92656-4

We conducted a pilot clinical study to investigate ex vivo fresh human blood from 93 patients with coronary heart disease (CHD). The results indicated that terahertz (THz) time-domain spectroscopy (TDS) can be used to quantify triglyceride (TG) levels in human blood. Based on the TG concentrations and corresponding THz absorption coefficients, the Pearson correlation analysis demonstrated that the THz absorption coefficients have a significant negative linear correlation with TG concentration. Comparisons between the THz measurements at 0.2 THz and an automatic biochemical analyzer were performed using an additional 20 blood samples, and the results confirmed that the relative error was less than 15%. Our ex vivo human blood study indicates that the THz technique can be used to assess blood TG levels in clinical diagnostic practice.

Scientists find simple method to enhance responsivity of terahertz radiation detectors by 3.5 folds

 

https://www.eurekalert.org/pub_releases/2021-06/tpu-sfs062421.php

Scientists of Tomsk Polytechnic University jointly with colleagues from Spanish universities have offered a simple method how to enhance the responsivity of terahertz radiation detectors by 3.5 folds using a small Teflon cube. The 1 mm cube must be put on the surface of the detector without changing the inner design of the detector.

Such detectors are applied, for instance, in a full-body scanner, spectrometer, in medical devices for diagnosing skin cancer, burn injuries, pathological changes in blood. The research findings are published in the Optics Letters academic journal (IF: 3,714; Q1).

Terahertz range lies between microwave and infrared ranges in the electromagnetic spectrum. Waves shorter than 1 mm refer to the terahertz range. Their feature lies in that they are capable to percolate various materials and at the same time, they do not lead to atomic ionization of matter alternatively to X-rays.

"Terahertz radiation detectors are, as a rule, rather compact devices. Nowadays, researchers from different countries are interested in the enhancement of their responsivity and other parameters. The higher responsivity, the weaker signals can be received and more precise measurements can be carried out. Most researchers are trying to solve this problem by changing the design of the detector and the materials it is made from. It is complicated and often very expensive. Meanwhile, our solution is plain to see," Oleg Minin, Professor of the Division for Electronic Engineering of the TPU School of Non-Destructive Testing, one of the authors of the article, says.

In their experiments, the scientists used a microparticle in the form of the Teflon cube, an available dielectric material through which electromagnetic waves of the terahertz range are capable to percolate. The cube was put on the surface of the detector.

"There is a responsive site inside of the detector. The site can be made from various materials but its typical scale is always less than the wavelength. It is the area responsible for trapping electromagnetic waves and transferring them. Due to the form and material, our cube possesses a capability to focalize radiation well, falling on the responsive site of the detector, in the scale limited to or smaller than a diffraction-limited system. The experiments conducted jointly with the Spanish colleagues proved it: the particle focalized the radiation and the emitted radiation fell into the responsive area," Oleg Minin explains.

According to the scientists, the developed method of detector responsivity enhancement without changing its design is applied to almost any detectors of various ranges.

During the experiments, the scientists fixed responsivity enhancement by 11 decibels, which is 3.5 folds higher than the standard parameters of the detector.

The researchers from University of Salamanca (Spain), Polytechnic University of Valencia (Spain), Institute of High-Pressure Physics of the Polish Academy of Sciences (Poland) and Imperial College London (England) took part in the research. The research was conducted with the support of the TPU Competitiveness Enhancement Program.

Abstract-Switchable dual-band and ultra-wideband terahertz wave absorber

 

Yi Chen and Jiu-Sheng Li

Configuration of the proposed absorber, (a) Three-dimensional view, (b) Top view; (c) Middle layer patterns and geometric parameters.

https://www.osapublishing.org/ome/fulltext.cfm?uri=ome-11-7-2197&id=452105

In this paper, we introduced a switchable dual-band and ultra-wideband terahertz wave absorber based on photoconductive silicon combining with vanadium dioxide (VO2). In the terahertz absorber, photoconductive silicon cross array, silicon dioxide layer, vanadium dioxide windmill type array, silicon dioxide dielectric layer, and gold ground plane are placed from the top layer to bottom layer in sequence. When VO2 is in a metallic state and the conductivity of photoconductive silicon is 2.5×10−4 S/m, the designed structure represents an ultra-wideband absorber with an absorption larger than 90% in the range of 3.14∼7.80 THz. As VO2 is in an insulation state and the conductivity of photoconductive silicon becomes 8.0×104 S/m, the designed device acts as two absorption bands, with a terahertz wave absorber with absorption more than 98% at 1.78∼2.90 THz and 7.35∼8.45 THz. The results show that the absorption band (dual-band or ultra-wideband) and absorption intensity (from 2% to 99%) can be switched by changing the phase transition of the VO2 and the conductivity of photoconductive silicon. Furthermore, the proposed device exhibits polarization insensitive and wide incident angles (lager than 70°) for TE- and TM- polarizations incidence.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz rectennas on flexible substrates based on one-dimensional metal-insulator-graphene diodes

 

Andreas Hemmetter,  Xinxin Yang , Zhenxing Wang, Martin Otto, Burkay Uzlu , Marcel Andree , Ullrich Pfeiffer, Andrei Vorobiev, Jan Stake, Max C. Lemme, Daniel Neumaier, 

https://arxiv.org/abs/2106.11679

Flexible energy harvesting devices fabricated in scalable thin-film processes are important components in the field of wearable electronics and the Internet of Things. We present a flexible rectenna based on a one-dimensional junction metal-insulator-graphene diode, which offers low-noise power detection at terahertz (THz) frequencies. The rectennas are fabricated on a flexible polyimide film in a scalable process by photolithography using graphene grown by chemical vapor deposition. A one-dimensional junction area reduces the junction capacitance and enables operation in the D-band (110 - 170 GHz). The rectenna on polyimide shows a maximum voltage responsivity of 80 V/W at 167 GHz in free space measurements and minimum noise equivalent power of 80 pW/Hz−−−√.

Abstract-Enhancing terahertz radiation from femtosecond laser filaments using local gas density modulation

 

Haicheng Xiao, Shengfeng Wang, Yan Peng, Daniel M. Mittleman, Jiayu Zhao, Zuanming Jin, Yiming Zhu, and Songling Zhuang

https://journals.aps.org/pra/accepted/0e07fN3dFc71701f18f40e30527204af2e708c149

We present a method to enhance the terahertz (THz) wave radiation from a femtosecond laser-induced plasma filament by controlling the local gas density within the filament. We develop a theoretical model for THz generation from a laser-induced air plasma filament and the subsequent propagation process, to account for a varying local gas density. By adjusting the local gas density along the filament, the transient current distribution along the filament and the resulting coherent superposition of terahertz waves can be controlled. The location of the gas jet nozzle and the relative phase between multicolor light fields both affect the transient current distribution and thus the strength of the generated THz field. Compared with the conventional terahertz generation by a two-color filament in a homogeneous gas, a three-color filament can realize a increase by 6.12 times in the generated THz pulse energy, with optimized local gas density modulation. Our results suggest that the THz amplification via local gas density modulation can be further improved with well-designed multicolor pulses

Abstract-Terahertz absorber with dynamically switchable dual-broadband based on a hybrid metamaterial with vanadium dioxide and graphene

 

Yan Liu, Rui Huang, Zhengbiao Ouyang, 

(a) Schematic of the proposed absorber based on VO2-graphene metamaterials and the incident light polarization configuration. (b) Top view of the unit cell. (c) Side view of the unit cell.

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-13-20839&id=452098

An absorber based on hybrid metamaterial with vanadium dioxide and graphene has been proposed to achieve dynamically switchable dual-broadband absorption property in the terahertz regime. Due to the phase transition of vanadium dioxide and the electrical tunable property of graphene, the dynamically switchable dual-broadband absorption property is implemented. When the vanadium dioxide is in the metallic phase, the Fermi energy level of graphene is set as zero simultaneously, the high-frequency broadband from 2.05 THz to 4.30 THz can be achieved with the absorptance more than 90%. The tunable absorptance can be realized through thermal control on the conductivity of the vanadium dioxide. The proposed device acts as a low-frequency broadband absorber if the vanadium dioxide is in the insulating phase, for which the Fermi energy level of graphene varies from to 0.1 eV to 0.7 eV. The low-frequency broadband possesses high absorptance which is maintained above 90% from 1.10 THz to 2.30 THz. The absorption intensity can be continuously adjusted from 5.2% to 99.8% by electrically controlling the Fermi energy level of graphene. The absorption window can be further broadened by adjusting the geometrical parameters. Furthermore, the influence of incidence angle on the absorption spectra has been investigated. The proposed absorber has potential applications in the terahertz regime, such as filtering, sensing, cloaking objects, and switches.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Detection of microorganisms using terahertz metamaterials

 

S. J. Park, J. T. Hong, S. J. Choi, H. S. Kim, W. K. Park, S. T. Han, J. Y. Park, S. Lee, D. S. Kim, Y. H. Ahn 

https://www.nature.com/articles/srep04988?proof=t

Microorganisms such as fungi and bacteria cause many human diseases and therefore rapid and accurate identification of these substances is essential for effective treatment and prevention of further infections. In particular, contemporary microbial detection technique is limited by the low detection speed which usually extends over a couple of days. Here we demonstrate that metamaterials operating in the terahertz frequency range shows promising potential for use in fabricating the highly sensitive and selective microbial sensors that are capable of high-speed on-site detection of microorganisms in both ambient and aqueous environments. We were able to detect extremely small amounts of the microorganisms, because their sizes are on the same scale as the micro-gaps of the terahertz metamaterials. The resonant frequency shift of the metamaterials was investigated in terms of the number density and the dielectric constants of the microorganisms, which was successfully interpreted by the change in the effective dielectric constant of a gap area.

Sussex scientists develop ultra-thin terahertz source, paving the way to next generation of communication tech

 

Alice Ingall

http://www.sussex.ac.uk/broadcast/read/55078

Physicists from the University of Sussex have developed an extremely thin, large-area semiconductor surface source of terahertz, composed of just a few atomic layers and compatible with existing electronic platforms.

Terahertz sources emit brief light pulses oscillating at ‘trillion of times per second’. At this scale, they are too fast to be handled by standard electronics, and, until recently, too slow to be handled by optical technologies. This has great significance for the evolution of ultra-fast communication devices above the 300GHz limit – such as that required for 6G mobile phone technology – something that is still fundamentally beyond the limit of current electronics.

Researchers in the Emergent Photonics (EPic) Lab at Sussex, led by the Director of the Emergent Photonics (EPic) Lab Professor Marco Peccianti, are leaders in surface terahertz emission technology having achieved the brightest and thinnest surface semiconductor sources demonstrated so far. The emission region of their new development, a semiconductor source of terahertz, is 10 times thinner than previously achieved, with comparable or even better performances.

The thin layers can be placed on top of existing objects and devices, meaning they are able to place a terahertz source in places that would have been inconceivable otherwise, including everyday object such as a teapot or even a work of art – opening up huge potential for anti-counterfeiting and ‘the internet of things’ - as well as previously incompatible electronics, such as a next generation mobile phone.

Dr Juan S. Totero Gongora, Leverhulme Early Career Fellow at the University of Sussex, said: “From a physics perspective, our results provide a long-sought answer that dates back to the first demonstration of terahertz sources based on two-colour lasers. Semiconductors are widely used in electronic technologies but have remained mostly out of reach for this type of terahertz generation mechanism. Our findings therefore open up a wide range of exciting opportunities for terahertz technologies.”

Dr Luke Peters, Research Fellow of the European Research Council project TIMING at the University of Sussex, said: “The idea of placing terahertz sources in inaccessible places has great scientific appeal but in practice is very challenging. Terahertz radiation can have a superlative role in material science, life science and security. Nevertheless, it is still alien to most of the existing technology, including devices that talk to everyday objects as part of the rapidly expanding ‘internet of things’. This result is a milestone in our route to bring terahertz functions closer to our everyday lives.”

Lying between microwaves and infrared in the electromagnetic spectrum, terahertz waves are a form of radiation highly sought in research and industry. They have a natural ability to reveal the material composition of an object by easily penetrating common materials like paper, clothes and plastic in the same way X-rays do, but without being harmful. Terahertz imaging makes it possible to ‘see’ the molecular composition of objects and distinguish between different materials. Previous developments from Prof Peccianti’s team showcased the potential applications of terahertz cameras, which could be transformative in airport security, and medical scanners – such as those used to detect skin cancers.

One of the biggest challenges faced by scientists working in terahertz technology is that what is commonly accepted as an ‘intense terahertz source’ is faint and bulky when compared with, for example, a light bulb. In many cases, the need for very exotic materials, such as nonlinear crystals, makes them unwieldy and expensive. This requirement poses logistical challenges for integration with other technologies, such as sensors and ultrafast communications.

The Sussex team have overcome these limitations by developing terahertz sources from extremely thin materials (about 25 atomic layers). By illuminating an electronic-grade semiconductor with two different types of lasers light, each oscillating at different frequency or colour, they were able to elicit the emission of short bursts of Terahertz radiation.

This scientific breakthrough has been long-sought by scientists working in the field since the first demonstration of terahertz sources based on two-colour lasers in the early 2000s. Two-colour terahertz sources based on special mixtures of gas, such as nitrogen, argon or krypton, are among the best performing sources available today. Semiconductors, widely used in electronic technologies, have remained mostly out of reach for this type of terahertz generation mechanism.

The research was developed within the framework of the European Research Council project “TIMING”.

The full research paper, titled, ‘All-Optical Two-Color Terahertz Emission from Quasi-2D Nonlinear Surfaces’ is published in the four star journal, Physical Review Letters, and can be read in full here: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.263901   

Unique terahertz microscope can be operated remotely

 

Niels van Hoof. Credit: Eindhoven University of Technology

https://phys.org/news/2021-06-unique-terahertz-microscope-remotely.html

With a wave length of about half a millimeter, terahertz radiation fills the gap between visible light and radio waves. This radiation lends itself very well to the in-depth measurement of the electrical properties of new materials, as doctoral candidate Niels van Hoof has demonstrated. He helped build a unique terahertz microscope that can be operated entirely remotely—handy in a pandemic.

From a scientific perspective, terahertz radiation is an oddball: caught between childhood and adulthood, you could say. Or rather its waves are too short for electrical engineering and too long for physics. In view of this, physicist Niels van Hoof, who carried out his doctoral work in the Surface Photonics group led by Jaime Gómez Rivas (Applied Physics), was also in touch with the group headed up by Professor Marion Matters at Electrical Engineering.

"The two groups even created a spin-off together, TeraNova," he says. "The company is managing the commercial launch of the terahertz microscope that we developed." The cross-pollination between the two blood groups, each with its own jargon, makes the specialist field of terahertz radiation particularly interesting, finds Van Hoof.

Body scanners

Beyond the laboratory, terahertz radiation is known mainly in connection with the body scanners used at airports. Many objects are transparent to terahertz radiation, the doctoral candidate explains. "But metals behave like a perfect mirror for this radiation because they conduct electricity. This makes terahertz radiation highly suitable for detecting weapons."

This sensitivity to electrical conductivity adds another application to the terahertz radiation portfolio: studying materials newly produced in the lab. Think of all kinds of fancy structures like nanowires, which by virtue of their particular form and composition exhibit special electromagnetic properties.

To analyze these new materials, we need to zoom in, as it were, on the object. This can be done using a technique called near-field spectroscopy, a method that has been used successfully in light microscopy for half a century. Here, structures smaller than the wavelength of the light being used are made visible.

Surface

"By applying this technique to terahertz radiation we can detect the electrical fields on the surface of structures that are much smaller than the wavelength of the radiation," explains Van Hoof. "This enables us to achieve a resolution of between three and ten micrometer." In the measurement setup the sample moves past a detector in steps of ten micrometers while being illuminated by pulses of terahertz radiation. "This enables us to measure the local electric field as a function of time. We use this information to understand why the material behaves in a certain way."

Measurements like this are almost impossible to perform with visible light, says the physicist. "In the optical domain you have no choice but to simulate behavior, whereas we can actually measure it. The nice thing about the system is that it is scalable; this means that when working with smaller structures and the correspondingly higher frequencies you can, in principle, expect the same behavior. And so our measurements taken with the terahertz microscope are also relevant to other parts of the electromagnetic spectrum."

Laser pulse

One field of inquiry involved Van Hoof studying a range of materials, including one made of loosely woven silver nanowires. "Cheap, transparent electrodes could possibly be made from this material, for use in, say, flexible plastic solar cells," he explains. "While we can't see any individual nanowires with our microscope, we can determine the relevant electrical properties. I've worked with DIFFER on this; they make these kinds of materials."

As a second field of inquiry he studied the purity of semiconductor material. "You can establish this purity by measuring how long the material remains conductive after you hit it with a short, intense pulse of laser light. The longer the time, the purer the material. This is interesting information for the semiconductor industry. We have devised a way of doing this without the laser pulse damaging the detector. This is so unique that a patent has been granted."

Remote operation

Similarly unique is the fact that the measurement setup built by Van Hoof can be operated entirely remotely—over the internet. As explained in the short film below, this proved very useful during the last phase of his research; after all, this coincided with the lockdowns during the corona pandemic.

Samsung Electronics and University of California Santa Barbara Demonstrate 6G Terahertz Wireless Communication Prototype

 

The demonstration explored the potential of THz spectrum application for 6G wireless communications

https://news.samsung.com/global/samsung-electronics-and-university-of-california-santa-barbara-demonstrate-6g-terahertz-wireless-communication-prototype

Samsung Electronics today announced that the company demonstrated the 6G Terahertz (THz) wireless communication prototype in collaboration with the University of California, Santa Barbara (UCSB).

 

At the recent workshop on Terahertz communications at the IEEE International Conference on Communications (ICC 2021), researchers from Samsung Research, Samsung Research America, and the University of California, Santa Barbara (UCSB) introduced the potential impact that THz could have on next-generation 6G technology, demonstrating an end-to-end 140GHz wireless link using a fully digital beamforming solution.

 

“Samsung has been at the forefront of technological innovation and standardization of 5G and 6G. As we shared in our 6G vision white paper last year, we believe new spectrum opportunities at the THz spectrum will become a driving force of 6G technology. This demonstration can be a major milestone in exploring the feasibility of using the THz spectrum for 6G wireless communications,” said Senior Vice President Sunghyun Choi, an IEEE Fellow and Head of the Advanced Communication Research Center at Samsung Research.

 

The THz band in­cludes an enormous amount of available spectrum, which will enable wideband channels with tens of GHz-wide bandwidth. This could potentially provide a means to meet the 6G requirement of terabits per second data rate. The peak data rate can be 50 times faster than 5G and the over-the-air latency could potentially be reduced to one-tenth. These improvements will enable 6G hyper-connectivity services and ultimate multimedia experience, such as extended reali­ty (XR), high-fidelity mobile hologram, etc.

 16-channel 140GHz phased-array module (middle), dual-channel 140GHz RFICs (left), 128-element antenna array (right)

The end-to-end prototype system the researchers demonstrated consists of a 16-channel phased array transmitter and receiver modules, driven by CMOS (Complementary metal-oxide-semiconductor) RFICs (Radio Frequency Integrated Circuits), and a baseband unit to process signals with 2GHz bandwidth and fast adaptive beamforming. In the over-the-air test, the prototype system achieved real-time throughput of 6.2 Gbps over a 15-meter distance with adaptive beam steering capability at the Terahertz frequency.

 

Samsung and UCSB researchers have been working closely on the THz phased array module development, which is a key to the success of the test. The module requires sophisticated packaging technology to allow research test chips to be used in a large-scale array module. The precise digital beamforming calibration algorithm, developed by Samsung, enables these modules to achieve high beamforming gain.

Samsung researchers: Wonsuk Choi, Shadi Abu-Surra and Gary Xu with the THz proof-of-concept system

“Working together with UCSB, we have been able to overcome many technological challenges and develop this new THz proof-of-concept system to explore 6G use cases and deployment scenarios,” said Senior Vice President Charlie Zhang, an IEEE Fellow and Head of the Standards and Mobility Innovations Team at Samsung Research America. “Samsung and UCSB researchers will continue to push the technological boundaries to bring 6G and THz communication closer to reality.”

Professor Mark Rodwell, University of California, Santa Barbara (UCSB)

UCSB’s group, led by the Electrical and Computer Engineering professor Mark Rodwell, first developed the 140GHz transmitter and receiver RFIC in 2017, as part of a program sponsored by the National Science Foundation (NSF) in the U.S.

 

“We bring our knowledge of advanced mmWave technologies, in particular the THz spectrum above 100GHz, focusing on devices and integrated circuits, while Samsung provides its expertise in wireless systems and cellular networks,” said professor Mark Rodwell, an IEEE Fellow and winner of the IEEE Sarnoff Award and the IEEE Marconi Prize Paper Award.

 

Samsung released a white paper in July 2020 titled “The Next Hyper-Connected Experience for All” outlining the company’s 6G vision, which is to bring the next hyper-connected experience to every corner of life. To accelerate research for 6G, Samsung Research, the advanced R&D hub within Samsung Electronics’ end-product business, founded its Advanced Communications Research Center in May 2019.