Light switch: Scientists develop method to control nanoscale manipulation in high-powered microscopes

Prof. Jun Takeda (left) and Katsumasa Yoshioka (right).

Credit: Yokohama National University

(a) Schematic of phase-controlled THz-STM. (b) Ultrafast current burst induced by phase-controlled and delay-controlled double THz near-fields. A sinusoidal THz near field produces ultrafast bidirectional current burst between a sample and a nanotip. By precisely tuning the carrier envelope phase (CEP) of THz near field, the direction and the timing of the current burst can be desirably manipulated on the femotosecond timescale. Credit: Yokohama National University

https://www.sciencedaily.com/releases/2018/10/181011112418.htm

Researchers from Japan have taken a step toward faster and more advanced electronics by developing a way to better measure and manipulate conductive materials through scanning tunneling microscopy.

The team published their results in July in Nano Letters, an American Chemical Society journal. Scientists from the University of Tokyo, Yokohama National University, and the Central Research Laboratory of Hamamatsu Photonics contributed to this paper.

Scanning tunneling microscopy (STM) involves placing a conducting tip close to the surface of the conductive material to be imaged. A voltage is applied through the tip to the surface, creating a "tunnel junction" between the two through which electrons travel.

The shape and position of the tip, the voltage strength, and the conductivity and density of the material's surface all come together to provide the scientist with a better understanding of the atomic structure of the material being imaged. With that information, the scientist should be able to change the variables to manipulate the material itself.

Precise manipulation, however, has been a problem -- until now.

The researchers designed a custom terahertz pulse cycle that quickly oscillates between near and far fields within the desired electrical current.

"The characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the nanoscale," said Jun Takeda, a professor in the department of physics in the Graduate School of Engineering at Yokohama National University. "We demonstrated that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy with a phase shifter."

According to Takeda, previous studies in this area assumed that the near and far fields were the same -- spatially and temporally. His team examined the fields closely and not only identified that there was a difference between the two, but realized that the pulse of fast laser could prompt the needed phase shift of the terahertz pulse to switch the current to the near field.

"Our work holds enormous promise for advancing strong-field physics in nano-scale solid state systems, such as the phase change materials used for optical storage media in DVDs and Blu-ray, as well as next-generation ultrafast electronics and microscopies," Takeda said.

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Story Source:

Materials provided by Yokohama National University. Note: Content may be edited for style and length.

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Journal Reference:

1. Katsumasa Yoshioka, Ikufumi Katayama, Yusuke Arashida, Atsuhiko Ban, Yoichi Kawada, Kuniaki Konishi, Hironori Takahashi, Jun Takeda. Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields. Nano Letters, 2018; 18 (8): 5198 DOI: 10.1021/acs.nanolett.8b02161

Abstract-Multi-band terahertz metamaterial absorber for sensing application

Yuan Yao, Shaopeng Li, Lei Zhu, Fengmin Wu, Xunjun He,  Jiuxing JiangYu

https://www.tandfonline.com/doi/abs/10.1080/10584587.2018.1457352?journalCode=ginf20

In this paper, we proposed a multi-band metamaterial absorber for sensing thin film, which consists of tri-layer structure: patterned multi-split electric ring resonators layer, dielectric spacer layer, and continuous metal ground layer. The calculated results demonstrated that the absorber can exhibit three distinctive absorption peaks with narrow bandwidth. As the surface of the absorber was coated with the analyte layer, moreover, the locations of three absorption peaks appear obvious redshift, and the corresponding maximal sensitivities are 0.119 THz/RIU, 0.248 THz/RIU, and 0.662 THz/RIU, respectively. Therefore, the proposed absorber has significant potential applications in detecting and sensing.

Study exposes security vulnerabilities in terahertz data links

InterceptedA new study shows that it's possible to steal data undetected from terahertz wireless links even though those links involved beam transmissions from transmitter to receiver.Mittleman Lab / Brown University

https://news.brown.edu/articles/2018/10/thzsecurity

Terahertz radiation may one day be used in wireless data networks that are many times faster than today's microwave networks. The conventional wisdom in the research community has been that, in addition to greater speed, terahertz data links would also have an inherent immunity to eavesdropping. Unlike microwaves, which travel in wide-angle broadcasts, terahertz waves travel directly from transmitter to receiver in narrow beams. The assumption was that it would be impossible to for an eavesdropper to intercept a terahertz signal without blocking some or all of the beam, which would be easily detected by an intended receiver. But new research finds that a clever eavesdropper can indeed steal terahertz signals undetected. In order for a link to be reliable, the beam's diameter must be slightly larger than the aperture of the receiver. That leaves a sliver of signal available for an attacker to steal without casting a shadow in a receiver. Credit: Mittleman lab / Brown University

Scientists have assumed that future terahertz data links would have an inherent immunity to eavesdropping, but new research shows that’s not necessarily the case.

PROVIDENCE, R.I. [Brown University] — A new study shows that terahertz data links, which may play a role in ultra-high-speed wireless data networks of the future, aren’t as immune to eavesdropping as many researchers have assumed. The research, published in the journal Nature, shows that it is possible for a clever eavesdropper to intercept a signal from a terahertz transmitter without the intrusion being detected at the receiver.

“The conventional wisdom in the terahertz community has been that it’s virtually impossible to spy on a terahertz data link without the attack being noticed,” said Daniel Mittleman, a professor in Brown University’s School of Engineering and a coauthor of the research. “But we show that undetected eavesdropping in the terahertz realm is easier than most people had assumed and that we need to be thinking about security issues as we think about designing network architectures.”

Because of its higher frequency, terahertz radiation can carry up to 100 times more data than the microwaves used in wireless communication today, which makes terahertz an attractive option for use in future wireless networks. Along with enhanced bandwidth, it has also been generally assumed that the way in which high-frequency waves propagate would naturally enhance security. Unlike microwaves, which propagate in wide-angle broadcasts, terahertz waves travel in narrow, very directional beams.

“In microwave communications, an eavesdropper can put an antenna just about anywhere in the broadcast cone and pick up the signal without interfering with the intended receiver,” Mittleman said. “Assuming that the attacker can decode that signal, they can then eavesdrop without being detected. But in terahertz networks, the narrow beams would mean that an eavesdropper would have to place the antenna between the transmitter and receiver. The thought was that there would be no way to do that without blocking some or all of the signal, which would make an eavesdropping attempt easily detectable by the intended receiver.”

Mittleman and colleagues from Brown, Rice University and the University at Buffalo set out to test that notion. They set up a direct line-of-site terahertz data link between a transmitter and receiver, and experimented with devices capable of intercepting signal. They were able show several strategies that could steal signal without being detected — even when the data-carrying beam is very directional, with a cone angle of less than 2 degrees (in contrast to microwave transmission, where the angle is often as large as 120 degrees).  

One set of strategies involves placing objects at the very edge of a beam that is capable of scattering a tiny portion of the beam. In order for a data link to be reliable, the diameter of the beam must be slightly larger than the aperture of the receiver. That leaves a sliver of signal for an attacker to work with without casting a detectable shadow on the receiver.

The researchers showed that a flat piece of metal could redirect a portion of the beam to a secondary receiver operated by an attacker. The researchers were able to acquire a usable signal at the second receiver with no significant loss of power at the primary receiver.

The team showed an even more flexible approach (from the attacker’s perspective) by using a metal cylinder in the beam instead of a flat plate.

“Cylinders have the advantage that they scatter light in all directions, giving an attacker more options in setting up a receiver,” said Josep Jornet, an assistant professor of electrical engineering at Buffalo and a study co-author. “And given the physics of terahertz wave propagation, even a very small cylinder can significantly scatter the signal without blocking the line-of-sight path.”

The researchers went on to demonstrate another type of attack involving a lossless beam splitter that would also be difficult, if not impossible, to detect. The beam splitter placed in front of a transmitter would enable an attacker to steal just enough to be useful, yet not so much that it would set off alarm bells among network administrators.

The bottom line, the researchers say, is that while there are inherent security enhancements associated with terahertz links in comparison with lower frequencies, these security improvements are still far from foolproof.

“Securing wireless transmission from eavesdroppers has been a challenge since the days of Marconi,” said Edward Knightly, professor of electrical and computer engineering at Rice University and a study coauthor. “While terahertz bands take a huge leap in this direction, we unfortunately found that a determined adversary can still be effective in intercepting the signal.”

The research was funded in part by the National Science Foundation, the Army Research Office, the Air Force Office of Scientific Research, and the W. M. Keck Foundation. Other coauthors on the paper were Jianjun Ma, Rabi Shrestha and Jacob Adelberg from Brown University; Chia-Yi Yeh and Edward Knightly from Rice University; and Zahed Hossain from Buffalo.

Abstract-Determination of pesticides in a flour substrate by chemometric methods using terahertz spectroscopy

Binghua Cao,  Hui Li.   Mengbao Fan,  Wei Wang, Mengyun Wang

https://pubs.rsc.org/en/content/articlelanding/2018/ay/c8ay01728j#!divAbstract

Terahertz time-domain spectroscopy (THz-TDS) is utilized as an effective tool for quantitative analysis of imidacloprid and carbendazim in a flour substrate. The partial least squares (PLS), principal component analysis (PCA), support vector machine (SVM) and PCA-SVM methods were used to construct linear and nonlinear regression models to correlate absorption spectra and concentrations of 21 samples based on the whole absorption spectra (0.2–1.4 THz). The multiple spectra baseline correction (MSBC) method which is based on asymmetric least squares smoothing was adopted to correct the slope baselines. The algorithm can eliminate scatter effects on the spectra and improve the signal-to-noise ratio of the THz spectra. The models were optimized by cross-validation, and their performances were evaluated with respect to root mean square error of prediction (RMSEP), correlation coefficient in the prediction set (Rp) and correlation coefficient in the calibration set (Rc). The results show that PLS delivers the best performance with the lowest errors. The optimized PLS models for both imidacloprid and carbendazim are obtained with RMSEP = 0.5439%, Rp = 0.9992 and Rc = 0.9999. Our experimental results also demonstrate that THz-TDS combined with chemometrics can be used for quantitative determination of pesticides in agricultural products.

Abstract-Self-triggered Asynchronous Optical Sampling Terahertz Spectroscopy using a Bidirectional Mode-locked Fiber Laser

R. Dawson Baker, N. Tolga Yardimci, Yi-Hsin Ou, Khanh Kieu, Mona Jarrahi, 

https://www.nature.com/articles/s41598-018-33152-0

We report a self-triggered asynchronous optical sampling terahertz spectroscopy system based on a single bidirectional mode-locked fiber laser and plasmonics-enhanced photoconductive nanoantennas. The fiber laser generates two optical mutually coherent pulse trains with a stable repetition rate difference, enabling time-domain terahertz spectroscopy without using any mechanical delay line, stabilization electronics, or external trigger. The resolved terahertz spectra over a 0.1–2 THz frequency range and a 30-second measurement time show more than a 70-dB dynamic range, revealing water absorption lines matching the HITRAN database, through a light-weight and compact spectroscopy setup.

OT- LUNA Blog-What is HD-FOS and How Does it Deliver New Testing Insight?

http://lunainc.com/hd-fos-deliver-testing-insight/

As the automotive industry continues to develop new lighter weight material systems and move to electrification, new design challenges are created that will require new advanced methods for test, measurement and validation. Fiber optic sensors can reduce time to first measurement and go where strain gages and temperature sensors cannot – in tight bends, on small details and even embedded inside composite materials. In addition, they are small, lightweight and flexible, immune to electromagnetic interference and chemically inert.

High-Definition Fiber Optic Sensing (HD-FOS) provides thousands of millimeter resolution measurements per meter on one fiber optic sensor. Traditional data acquisition (DAQ) systems, on the other hand, employ discrete electrical sensors that are relatively bulky, require multiple wires and are limited to set measurement points.

HD-FOS will help automotive manufacturers speed new technologies to market while lowering the risk associated with the introduction of new materials and processes:

• High definition data helps accurately characterize high strain or thermal gradients that can only be estimated with point sensing and traditional DAQ systems.
• Fiber can reach hard-to-instrument places and at only 150 microns in diameter can be embedded into components without influencing parameters under test.
• Fine mesh finite element models of critical components can be validated with certitude.

At Automotive Testing Expo Oct. 23-25 in Novi, Michigan, Luna is presenting a session, “Distributed High-Definition Strain and Temperature Measurement Delivers New Testing Insight,” which will discuss how more data and more insight provided by HD-FOS results in more complete model verification, enhanced damage detection and development of smart parts.

Learn how high definition data from HD-FOS can fully map the contour of strain or temperature for a structure under test or during manufacturing.

Abstract-Broadband terahertz polarization conversion using metasurfaces (Conference Presentation)

Hou-Tong Chen,  Chun-Chieh Chang,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10756/107560C/Broadband-terahertz-polarization-conversion-using-metasurfaces-Conference-Presentation/10.1117/12.2322496.short

Two-dimensional metamaterials - metasurfaces - offer tremendous opportunities in realizing exotic optical phenomena and functionalities to address the technological challenges encountered in the terahertz frequency regime. By tailoring the resonant response of basic building blocks as well as their mutual interactions, we are able to effectively control of amplitude, phase, and polarization state of terahertz waves. Here we report the realization of highly efficient polarization conversions including: (1) Reflective linear polarization rotation using an array of anisotropic resonators backed with a ground plane; (2) Transmissive linear polarization rotation using an array of anisotropic resonator array sandwiched by two orthogonal gratings; and (3) Reflective linear-to-circular polarization conversion using two cascading arrays of complementary resonators. They operate over a broad bandwidth more than one octave and approaching two octaves in some cases. We further show that the linear polarization rotation is accompanied by a tunable phase discontinuity, which allows us to demonstrate an ultrathin terahertz flat lens enabling diffraction-limited focusing. The broadband linear-to-circular polarization may also find applications including terahertz circular dichroism spectroscopy and excitation of valley polarization in 2D materials.

Abstract-Nanostructured epitaxial graphene for ultra-broadband optoelectronic detectors (Conference Presentation)

Abdel El Fatimy,  Luke St. Marie,  Anindya Nath, Byoung Don Kong, Anthony K. Boyd,  Rachael L. Myers-Ward,  Kevin M. Daniels,  M. Mehdi Jadidi,  Thomas E. Murphy,  D. Kurt Gaskill, Paola Barbara

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10729/1072906/Nanostructured-epitaxial-graphene-for-ultra-broadband-optoelectronic-detectors-Conference-Presentation/10.1117/12.2321313.short

Atomically thin materials like semimetallic graphene and semiconducting transition metal dichalcogenides (TMDs) are an ideal platform for ultra-thin optoelectronic devices due to their direct bandgap (for monolayer thickness) and their considerable light absorption. For devices based on semiconducting TMDs, light detection occurs by optical excitation of charge carriers above the bandgap. For gapless graphene, light absorption causes a large increase in electron temperature, because of its small electronic heat capacity and weak electron-phonon coupling, making it suitable for hot-electron detectors. Here we show that, by nanostructuring graphene into quantum dots, we can exploit quantum confinement to achieve hot-electron bolometric detection. The graphene quantum dots are patterned from epitaxial graphene on SiC, with dot diameter ranging from 30 nm to 700 nm [1]. Nanostructuring greatly increases the temperature dependence of the electrical resistance, yielding detectors with extraordinary performance (responsivities of 1 × 10^(10) V W^(−1) and electrical noise-equivalent power, ∼2 × 10^(−16) W Hz^(−1/2) at 2.5 K). We will discuss how the dynamics of the charge carriers, namely the hot-electron cooling, affects the device operation and its power dependence. These detectors work in a very broad spectral range, from terahertz through telecom to ultraviolet radiation [2], with a design that is easily scalable for detector arrays. [1] El Fatimy, A. et al. , "Epitaxial graphene quantum dots for high-performance terahertz bolometers," Nature Nanotechnology 11, 335-338 (2016). [2] El Fatimy, A. et al. , "Ultra-broadband photodetectors based on epitaxial graphene quantum dots" Nanophotonics (2018).

© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Self-adaptive terahertz spectroscopy from atmospheric vapor based on Hilbert-Huang transform

Huan Liu, Ya-Xian Fan, Lin Li, Hong-Ge Chen, Peng-Fei Wang, and Zhi-Yong Tao

           Fig. 1 Schematic diagram of the optical fiber integrated THz-TDS in transmission mode.
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-21-27279

Absorption lines of atmospheric vapor commonly appear in terahertz (THz) spectra measured in a humid air environment. However, these effects are generally undesirable because they may mask critical spectroscopic information. Here, a self-adaptive method is demonstrated for effectively identifying and eliminating atmospheric vapor noise from THz spectra of an all-fiber THz system with the Hilbert-Huang transform. The THz signal was decomposed into eight components in different time scales called the intrinsic mode functions and the interference of atmospheric vapor was accurately isolated. A series of experiments confirmed the effectiveness and strong self-adaptiveness of the proposed system in vapor noise elimination.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Abstract-Polarization characteristics of terahertz wave generated by differential frequency mixing under exciton excitation condition in a semiconductor quantum well (Conference Presentation)

Osamu Kojima

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10756/1075608/Polarization-characteristics-of-terahertz-wave-generated-by-differential-frequency-mixing/10.1117/12.2319185.short?SSO=1

As continuous wave (CW) terahertz (THz) sources, the differential-frequency-mixing (DFM) has an advantage for the frequency tunability by changing the energy separation of the two lasers. In particular, considering the inhomogeneous width in the quantum confinement systems, use of the exciton lines enables wide frequency tuning. The THz sources with the narrow bandwidth and wide frequency tunability will be applied to the high resolution THz spectroscopy. Recently, we realized the CW-THz wave generation by DFM under the exciton excitation conditions in a GaAs/AlAs multiple quantum well (MQW), which shows the wide frequency tuning range over 18 THz. Therefore, in this work, we report the polarization characteristics of a continuous THz electromagnetic wave generated by DFM due to excitation of two exciton states in the GaAs/AlAs multiple quantum well. As a sample, we used an undoped GaAs/AlAs MQW embedded in a p-i-n structure on a (001) n+-GaAs substrate. The thickness of GaAs and AlAs layer is 7.5 nm. The measurements of the THz wave were carried out at 296 K. As the laser sources, a semiconductor laser and a CW-mode Ti:sapphire laser to change the excitation energy were used. The two beams were focused on the sample surface. Comparing the polarization of the laser beams with that of the THz wave, the conversion process from the laser lights to the THz wave via the exciton states, such as the heavy hole and light hole excitons split by quantum confined effects, will be demonstrated.

© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Article-Formation of nerve impulse and its properties of transport with the terahertz wave along the nerve fiber in the life systems

Pang-Xiao Feng,  Zeng-hong Juan,  Jiang-Ying Rui,  Ni-Yin Jia

https://www.oatext.com/formation-of-nerve-impulse-and-its-properties-of-transport-with-the-terahertz-wave-along-the-nerve-fiber-in-the-life-systems.php#Article

We gave first the formation and properties of the action electro-potential on the nerve-cell membranes, which are directly related to the non-uniformity of distribution of sodium ions and potassium ions between the inner and exterior of cell-membranes as well as their properties of movements under the affection of an electro-information. However, the action electro-potential in essence is only a static impulse, it cannot propagate along the nerve organizations. But, if the nerve organizations are acted by the bio-energy, which could lead to the periodicvariation of these sodium ions and potassium ions in the inner and exterior of the never cell membranes can be varied periodically under the action of bio-energy by virtue of the works of sodium pump and potassium pump on the surface of membrane of cells, then the nerve impulse can propagate along the nerve fiber cell membranes. Our investigations verify that the bio-energy released from the hydrolyses reaction of adenosine phosphate (ATP) molecules in the cells can play the role, it released the bio-energy of 0.42 eV can be transported by the protein molecules to these sodium pump and potassium pump to work, thus the propagation of the nerve impulse can be carried out automatically in living systems. This is just the mechanism of propagation of the nerve impulse along the nerve cell membrane. This means that the energy released from the hydrolyses reaction of ATP molecules controlled the propagation of the nerve impulse. Because ATP molecules are often attached on the protein molecules, where the energy is transported along the protein molecules from the position of generation of hydrolyses reaction to the position used the energy in virtue of transport of the soliton formed by the excitons through the mechanism of self-trapping, where the exciton is a quantum produced by the C=0 stretching (or amide-I) vibrations. We studied and obtained the properties of transport of bio-energy, which is carried by Pang’s soliton, along α-helical protein molecules and found further the lifetimes of Pang’soliton, which is between 0.53×10^-10S 0.65×10^-10S at physiological temperature T=300K. In this lifetime Pang’s soliton can travel over several hundreds of amino acid residues. This implies that Pang ‘s theory is a relevant and correct theory of bio-energy transport, then Pang’s soliton is a real carrier of bio-energy transport in protein molecules. If the bio-energy was transported into the nerve membrane to drive the works of sodium pump and potassium pump, then the transfer of nerve impulse along the nerve membranes can be carried out. So, we can say that there is not the transport of nerve impulse without the works of sodium pump and potassium pump, or the bio-energy. This means that the nerve impulse can be transported along the nerve membrane, only if the bio-energy was provided and was absorbed really by the sodium pump and potassium pump. In order to form a stable propagation of nerve impulse, then its times forming must be shorter than the lifetime of Pang’s soliton or its experimental values, or else, the nerve impulse is not stable and is useless. Thus, we can affirm that the nerve impulse is a terahertz wave. Thus, we affirmed and verified that the nerve impulse can be transport along the nerve systems in the terahertz wave, instead the millimeter wave. We determinate and discuss further its features. This is first time to determinate the terahertz features of transport of the nerve impulse along the nerve fibers in life systems, which will promote great the development of the nerve science 

Abstract-Measurement of Quadratic Terahertz Optical Nonlinearities Using Second-Harmonic Lock-in Detection

Shuai Lin, Shukai Yu, and Diyar Talbayev

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.10.044007

We present a method to measure quadratic terahertz optical nonlinearities in terahertz time-domain spectroscopy. We use a rotating linear polarizer (a polarizing chopper) to modulate the amplitude of the incident terahertz pulse train. We use phase-sensitive lock-in detection at the fundamental and the second harmonic of the modulation frequency to separate the materials’ responses that are linear and quadratic in a terahertz electric field. We demonstrate this method by measuring the quadratic terahertz Kerr effect in the presence of the much stronger linear electro-optic effect in the (110) $\mathrm{GaP}$ crystal. We propose that the method can be used to detect terahertz second-harmonic generation in noncentrosymmetric media in time-domain spectroscopy, with broad potential applications in nonlinear terahertz photonics and related technology.

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Abstract-Optical tuning of dielectric properties of La0.7Sr0.3MnO3/SrTiO3 superlattices in the terahertz range

Honglei Cai, Haoliang Huang, Qiuping Huang, Xiang Hu, Jie Zhang, Xiaofang Zhai,  Yalin Lu

Fig. 2 (a) Schematic diagram of the home-made THz-TDS system. The green arrow indicates the excitation under 532 nm continuous waves. (b) Schematic diagram of the home-made OPTP system. Here, λ/4 and W.P. refer to a quarter-wave plate and a Wollaston prism, respectively.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-6-7842&origin=search

Two (La0.7Sr0.3MnO3)n/(SrTiO3)m superlattices with different superlattice period but the same total thickness were deposited on LaAlO3 substrates by pulsed laser deposition. Dielectric properties of these samples were investigated by means of terahertz time-domain spectroscopy (THz-TDS) under external continuous wave green laser excitation and optical-pump terahertz-probe spectroscopy (OPTP) at room temperature. Experimental results show that the real part of the permittivity for both superlattices increases significantly with increasing green laser pump power, which indicates the decrease of the plasma frequency, along with the increase of the electron scattering rate, soft mode eigenfrequency and oscillator strength in the Drude-Lorentz model. Furthermore, it’s observed that the insulating superlattice exhibits a more significant dielectric tunability than the metallic superlattice. Besides, the carrier lifetime of superlattices is much shorter than the La0.7Sr0.3MnO3 thin film in the OPTP measurements, indicating that the electrons excited in the La0.7Sr0.3MnO3 layers may be trapped by the defects located in the interfaces of La0.7Sr0.3MnO3 and SrTiO3 or the SrTiO3 layers. With the optical field-induced tunability of dielectric properties, (La0.7Sr0.3MnO3)n/(SrTiO3)m superlattices show great potential in the actively tunable devices in the THz range.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Calibration of the field strength of broadband terahertz radiation in air coherent detection technique

Journal of Applied Physics

Hai-Wei Du, Fu Tang, Dong-Yu Zhang, Wang Sheng, Jing-Yi Mao,

https://aip.scitation.org/doi/10.1063/1.5039827

In the air coherent detection of the terahertz (THz) technique, a high voltage signal with a square waveform offers a modulated biased field over the air sensor. In this technique, a temporal THz waveform can be obtained routinely. However, its field strength has to be calibrated separately by other techniques. In this paper, we propose a method to calibrate the field strength of the THz pulse by means of applying a positive square wave instead of an alternating square wave as the modulated biased field. In this method, the ratio of the THz signal to the background signal is used for the calibration of the field strength of the THz pulse. Using this method, both the waveform and the electric field strength of the THz pulse can be easily obtained.

Abstract-Terahertz generation with ballistic photodiodes under pulsed operation

Stefan Malzer, Christian Müller-Landau, Heiko B Weber, Gottfried H Döhler, Stephan Winner,  Peter G Burke, Arthur Gossard,  Sascha Preu

http://iopscience.iop.org/article/10.1088/1361-6641/aae5e4

We investigate high field and ballistic carrier transport in a 1.55 μm photomixing
 device based on pin-diodes by time resolved terahertz (THz) spectroscopy. The device
 consists of 3 stacked In(Al)GaAs pin diodes (n-i-pn-i-p superlattice) attached to a
 broadband logarithmic-periodic antenna. Each pin diode is optimized for exhibiting
 ballistic transport and a reduced transit time roll-off. Ballistic transport signatures
 could be confirmed directly in these experiments. The data are compared with results
 from continuous-wave (CW) experiments and from simulations both supporting our
 theoretical expectations. It is demonstrated that n-i-pn-i-p superlattice photomixers
 are also efficient THz emitters under pulsed operation, showing a maximum THz field
 strength of ~0.5 V/cm (peak to peak) at 30 mW average optical power.

Abstract-Evaluation of semiconductor materials and devices by laser-induced terahertz emissions (Conference Presentation)

Iwao Kawayama

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10756/107560N/Evaluation-of-semiconductor-materials-and-devices-by-laser-induced-terahertz/10.1117/12.2322900.short


I introduce recent studies on evaluation of interface and surface of semiconductor materials and devices with laser-induced terahertz emission spectroscopy and imaging that measure and visualize THz emissions from the materials and devices excited by femtosecond laser pulses. The waveforms of lase-induced THz emissions refrect the dinamics of photoexcited carriers at the area excited by the laser pulses, therefore we can extract various physical properties of the samples using this phenomena in principle. In this study, we have applied this technique to characterize local properties of semiconductor materials and devices such as solar cells, wide-gap semiconductors and Metal-Oxide-Semiconductor (MOS) devices. As a result, we demonstrated that it was possible to evaluate electric polarization, surface potentials, defects, damage, performance deterioration, which were difficult with conventional methods.

© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Enhanced Confinement of Terahertz Surface Plasmon Polaritons in Bulk Dirac Semimetal-Insulator-Metal Waveguides

Yi Su, Qi Lin, Xiang Zhal, Ling-Ling Wang

https://nanoscalereslett.springeropen.com/articles/10.1186/s11671-018-2686-z

A subwavelength terahertz plasmonic waveguide based on bulk Dirac semimetal (BDS)-insulator-metal (BIM) structure is investigated, which indicates that there is an optimized frequency range with the better confinement as well as lower loss. A broadband mode confinement up to λ0/15 with a relatively low loss of 1.0 dB/λ0 can be achieved. We also show that two silicon ribbons introduced into the BIM waveguide can form a dynamically tunable filter tailoring terahertz surface plasmon polaritons in deep-subwavelength scale, which can be further exploited for the design of ultra-compact THz plasmonic devices with dynamical tunability. Our results may also provide potential applications in optical filtering.

Abstract-Independent Manipulating of Orthogonal-Polarization Terahertz Waves Using A Reconfigurable Graphene-Based Metasurface

Li Deng,  Yuanyuan Zhang , Jianfeng Zhu, Meijun Qu, Ling Wang,  Chen Zhang

https://www.mdpi.com/1996-1944/11/10/1817

Viewing the trend of miniaturization and integration in modern electronic device design, a reconfigurable multi-functional graphene-based metasurface is proposed in this paper. By virtue of the reconfigurability of reflection patterns, this metasurface is able to independently manipulate orthogonal linearly polarized terahertz wave. The building blocks of the proposed metasurface are series of graphene-strips-based unit-cells. Each unit-cell consists of two orthogonal graphene strips and a grounded substrate, which has anisotropic responses for each of orthogonal polarizations (x-polarized and y-polarized waves). The reflection phases of both x- and y-polarized waves can be controlled independently through separate electrical tuning. Based on the proposed metasurface, functionalities including beam splitting, beam deflecting, and linear-to-circular polarization converting using a shared aperture are numerically demonstrated and analyzed. Simulation results demonstrate excellent performance, which is consistent with the theorized expectations. This work paves the way for enhancing the miniaturization of modern electronic/optical devices and potentially has important applications in the next-generation information systems for communication, sensing, and imaging.

Abstract-Blind Ghost Imaging

Alba M. Paniagua-Diaz, Ilya Starshynov, Nikos Fayard, Arthur Goetschy, Romain Pierrat, Rémi Carminati, Jacopo Bertolotti

http://export.arxiv.org/abs/1809.10501

Ghost imaging is an unconventional optical imaging technique that reconstructs the shape of an object combining the measurement of two signals: one that interacted with the object, but without any spatial information, the other containing spatial information, but that never interacted with the object. Ghost imaging is a very flexible technique, that has been generalized to the single-photon regime, to the time domain, to infrared and terahertz frequencies, and many more conditions. Here we demonstrate that ghost imaging can be performed without ever knowing the patterns illuminating the object, but using patterns correlated with them, doesn't matter how weakly. As an experimental proof we exploit the recently discovered correlation between the reflected and transmitted light from a scattering layer, and reconstruct the image of an object hidden behind a scattering layer using only the reflected light, which never interacts with the object. This method opens new perspectives for non-invasive imaging behind or within turbid media.

Abstract-Broadband THz radiation via the inverse spin Hall and Rashba-Edelstein effects (Conference Presentation)

Jingbo Qi

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10732/107323I/Broadband-THz-radiation-via-the-inverse-spin-Hall-and-Rashba/10.1117/12.2322834.short

One major challenge in the next generation THz (1012) technology is to develop highly efficient, ultra-broadband and low-cost terahertz emitters with a gapless spectrum. Up-to-date, most broadband and table-top THz emitters are based on the femtosecond laser excitations, taking advantage exclusively of the charge property of the electron. Here, we introduce two novel types of broadband spin-based THz emitters composed of the ferromagnetic metallic heterostructures [1-4], e.g. (Co, Fe)/Pt and Fe/Ag/Bi. We have carried out detailed thickness-dependent experiments in these samples. Such investigations not only enable us to clarify the intrinsic mechanisms behind the THz radiation - the inverse spin Hall effect (ISHE) and the inverse Rashba-Edelstein effect (IREE), but also help to determine the key parameters to optimize the THz emission. The emitted THz wave, with its phase and polarization easily manipulated by changing the film stacking order and the magnetization direction, has an ultra-broadband width (~0.1-20 THz) and strong amplitude (comparable to the conventional nonlinear crystals). We also demonstrate that the THz radiation arising from both the ISHE and IREE can be selectively superimposed with each other. [1] Seifert, T. et al. Efficient metallic spintronic emitters of ultrabroadband terahertz radiation. Nat. Photon. 10, 483 (2016). [2] Yang, D. et al. Powerful and Tunable THz Emitters Based on the Fe/Pt Magnetic Heterostructure. Adv. Opt. Mater. 4, 1944 (2016). [3] Wu, Y. et al. High-performance THz emitters based on ferromagnetic/nonmagnetic heterostructures. Adv. Mater. 29, 1603031 (2017). [4] Zhou. C et al. Broadband terahertz generation via the interface inverse Rashba-Edelstein effect (submitted).