## Saturday, October 31, 2015

### Abstract-Broadband, wide-angle, low-scattering terahertz wave by a flexible 2-bit coding metasurface

Xin Yan, Lanju Liang, Jing Yang, Weiwei Liu, Xin Ding, Degang Xu, Yating Zhang, Tiejun Cui, and Jianquan Yao
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-23-22-29128

Abstract: Expanding bandwidths and arbitrary control of technology remain key issues in the field of electromagnetic waves, especially in terahertz (THz) wave. In this paper, we propose a novel method to achieve broadband low-scattering THz characteristics with wide-angle and polarization independence by a 2-bit flexible and nonabsorptive coding metasurface. The coding metasurface is composed of four digital elements based on double cross metallic line for “00”, “01”, “10”, and “11.” The reflection phase difference of neighboring elements is about 90° over a broad THz frequency band and wide incident angles. The low scattering coefficients below –10 dB were achieved over a wide frequency band from 0.8 THz to 1.5 THz when the incident angle is less than 50° by coding the four elements sequences. This superior property is maintained when the flexible coding metasurface is wrapped around a metallic cylinder with different dimensions. These results present a novel method to control THz waves freely and demonstrate significant scientific value in practical applications.
© 2015 Optical Society of America

### Abstract-Terahertz metasurfaces with a high refractive index enhanced by the strong nearest neighbor coupling

Siyu Tan, Fengping Yan, Leena Singh, Wei Cao, Ningning Xu, Xiang Hu, Ranjan Singh, Mingwei Wang, and Weili Zhang

The realization of high refractive index is of significant interest in optical imaging with enhanced resolution. Strongly coupled subwavelength resonators were proposed and demonstrated at both optical and terahertz frequencies to enhance the refractive index due to large induced dipole moment in meta-atoms. Here, we report an alternative design for flexible free-standing terahertz metasurface in the strong coupling regime where we experimentally achieve a peak refractive index value of 14.36. We also investigate the impact of the nearest neighbor coupling in the form of frequency tuning and enhancement of the peak refractive index. We provide an analytical circuit model to explain the impact of geometrical parameters and coupling on the effective refractive index of the metasurface. The proposed meta-atom structure enables tailoring of the peak refractive index based on nearest neighbor coupling and this property offers tremendous design flexibility for transformation optics and other index-gradient devices at terahertz frequencies.
© 2015 Optical Society of America

## Friday, October 30, 2015

### Abstract-Voltage adjusting characteristics in terahertz transmission through Fabry-Pérot-based metamaterials

OA

Metallic electric split-ring resonators (SRRs) with featured size in micrometer scale, which are connected by thin metal wires, are patterned to form a periodically distributed planar array. The arrayed metallic SRRs are fabricated on an n-doped gallium arsenide (n-GaAs) layer grown directly over a semi-insulating gallium arsenide (SI-GaAs) wafer. The patterned metal microstructures and n-GaAs layer construct a Schottky diode, which can support an external voltage applied to modify the device properties. The developed architectures present typical functional metamaterial characters, and thus is proposed to reveal voltage adjusting characteristics in the transmission of terahertz waves at normal incidence. We also demonstrate the terahertz transmission characteristics of the voltage controlled Fabry-Pérot-based metamaterial device, which is composed of arrayed metallic SRRs. To date, many metamaterialsdeveloped in earlier works have been used to regulate the transmission amplitude or phase at specific frequencies in terahertz wavelength range, which are mainly dominated by the inductance-capacitance () resonance mechanism. However, in our work, the external voltage controlled metamaterial device is developed, and the extraordinary transmission regulation characteristics based on both the Fabry-Pérot (FP) resonance and relatively weak surface plasmon polariton (SPP) resonance in 0.025-1.5 THz range, are presented. Our research therefore shows a potential application of the dual-mode-resonance-based metamaterial for improving terahertz transmission regulation.

### Abstract-Molecular dynamics simulations of conformation and chain length dependent terahertz spectra of alanine polypeptides

DOI:
10.1080/08927022.2015.1059429

Terahertz absorption spectra of alanine polypeptides in water were simulated with classical molecular dynamics at 310 K. Vibrational modes and oscillator strengths were calculated based on a quasi-harmonic approximation. Absorption spectra of Alan (n = 5, 15, 30) with different chain lengths and Ala15 in coiled and helical conformations were studied in 10–40 cm− 1 bandwidth. Simulation results indicated both the chain length and the conformation have significant influences on THz spectra of alanine polypeptides. With the increase of chain length, the average THz absorption intensity increases. Compared with the helical Ala15 polypeptide, the THz spectra of coiled one shows stronger absorption peaks. These results were explained from different numbers of hydrogen bonds formed between polypeptides and the surrounding water molecules.

### Abstract-Optical field terahertz amplitude modulation by graphene nanoribbons

Hong Zhang,*a    Xinlu Chengc and  Angel Rubiodef
*
Corresponding authors
a
College of Physical Science and Technology, Sichuan University, Chengdu 610065, China
E-mail: hongzhang@scu.edu.cn
b
Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba 305-8568, Japan
c
Key Laboratory of High Energy Density Physics and Technology of Ministry of Education; Sichuan University, Chengdu, China
d
Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany
e
Nano-Bio Spectroscopy group, Universidad del País Vasco CFM CSIC-UPV/EHU-MPC DIPC, 20018 San Sebastian, Spain
f
European Theoretical Spectroscopy Facility (ETSF)

DOI: 10.1039/C5NR05889A

In this study, first-principles time-dependent density functional theory calculations were used to demonstrate the possibility to modulate the amplitude of the optical electric field (E-field) near a semiconducting graphene nanoribbon. A significant enhancement of the optical E-field was observed 3.34 Å above the graphene nanoribbon sheet, with an amplitude modulation of approximately 100 fs, which corresponds to a frequency of 10 THz. In general, a six-fold E-field enhancement could be obtained, which means that the power of the obtained THz is about 36 times that of incident UV light. We suggest the use of semiconducting graphene nanoribbons for converting visible and UV light into a THz signal.

## Thursday, October 29, 2015

### OT-Picometrix Division of Luna Announces $7 Million Order Large quantity of High-Speed Optical Receivers sold to a leader in the telecommunications industry http://finance.yahoo.com/news/picometrix-division-luna-announces-7-203300502.html Luna Innovations Incorporated (LUNA) today announced that its Picometrix division recently received a$7 million purchase order for its industry-leading 100G integrated coherent receivers (ICR). The ICR is critical to enable high-speed optical communication in long haul networks.
The Picometrix division of Luna manufactures and markets the fastest, most sensitive, most temperature-stable, and broadest-wavelength high-speed optical receivers (HSOR) available for both the Transmission and Test & Measurement markets.
“This order reinforces the value of our high-speed optical receiver technology to the telecommunications industry, and therefore we are pleased with the growth potential that it demonstrates for this area of our business,” said My Chung, president and chief executive officer of Luna. “Considering the continued level of deployment of new fiber optic networks, including 100G networks in Asia and North America, creating significant demand for our high-speed optical receiver and detector products, I believe we will see continued growth of this product line. Furthermore, this is strong evidence of the value that can be realized from our merger with Advanced Photonix, Inc. earlier this year.”
Luna Innovations Incorporated (www.lunainc.com) develops, manufactures and markets fiber optic sensing, test and measurement products and is focused on bringing new and innovative technology solutions to measure, monitor, protect and improve critical processes in the aerospace, automotive, energy, composite, telecommunications and defense industries. Following its merger with Advanced Photonix, Inc. (API), the company also packages optoelectronic semiconductors into high-speed optical receivers (HSOR products), custom optoelectronic subsystems (Optosolutions products) and Terahertz (THz) instrumentation. Luna is organized into two business segments, which work closely together to turn ideas into products: a Technology Development segment and a Products and Licensing segment. Luna's business model is designed to accelerate the process of bringing new and innovative technologies to market.
Forward-Looking Statements
The statements in this release that are not historical facts constitute “forward-looking statements” made pursuant to the safe harbor provision of the Private Securities Litigation Reform Act of 1995 that involve risks and uncertainties. These statements include the company's expectations regarding the company’s future financial performance, operating results and future growth of the company’s business, greater capabilities following the completion of the merger with API, potential demand for the company's high speed optical receiver and detector products, and potential for improved financial performance. Management cautions the reader that these forward-looking statements are only predictions and are subject to a number of both known and unknown risks and uncertainties, and actual results, performance, and/or achievements of the company may differ materially from the future results, performance, and/or achievements expressed or implied by these forward-looking statements as a result of a number of factors. These factors include, without limitation, failure of demand for the company’s products and services to meet expectations, integration or other operational issues related to the merger, technological challenges and those risks and uncertainties set forth in the company’s periodic reports and other filings with the Securities and Exchange Commission. Such filings are available at the SEC’s website at www.sec.gov and at the company’s website at www.lunainc.com. The statements made in this release are based on information available to the company as of the date of this release and Luna undertakes no obligation to update any of the forward-looking statements after the date of this release.
Contact:
Luna Innovations Incorporated
Dale Messick, CFO
1-540-769-8400

### View from… IRMMW-THz: Strength in diversity

NATURE PHOTONICS | NEWS AND VIEWS

Noriaki Horiuchi

Next-generation wireless communication, high-harmonic generation of sub-cycle pulses and ultrafast probing of the excitation dynamics of materials were all topics of discussion at this year’s IRMMW-THz conference in Hong Kong.

Although opportunities in imaging and spectroscopy are often highlighted as being the most important applications and drivers for developments in terahertz (THz) science, another topic that is highly promising is THz wireless communications. This was one of the messages to emerge from the 40th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), held over 23–28 August at the Chinese University of Hong Kong.
The ever-increasing demand for higher data rates in wireless communications means that carrier waves with ever higher frequencies will need to be utilized. Indeed, to meet predictions that data rates will need to scale to 100 Gbits s?1 within ten years may well necessitate a carrier frequency above 275 GHz.
“We now have many enabling technologies thanks to the recent progress of semiconductor devices and integrated circuits operating at THz frequencies,” Nagatsuma of Osaka University told Nature Photonics. “In addition to the data rate, other expected advantages of THz communications over microwave communications are low power consumption and smaller transceiver size, particularly coming from a reduction in the antenna size,” he added.

 Group photograph in the lecture room of the Chinese University ofHong Kong, where IRMMW-THz 2015 was held over 23–28 August. More than 600 researchers from 41 countries gathered.
He says that scientists are turning to the development of photonic, rather than electronic, devices for THz communications because it is easier to achieve higher data rates using photonic components. “In addition, photonics-based systems might be deployed in the future convergence of fibre optic and wireless communications networks,” commented Nagatsuma. He believes that ultrawideband amplifiers and antennas are the most crucial components needed to make full use of the bandwidth. “Even for photonics-based systems, amplifiers are necessary to boost the output power in the transmitter and to increase the sensitivity in the receiver,” he stressed.
THz communication devices will require innovation in integration and packaging to be practical. Guillermo Carpintero of Universidad Carlos III de Madrid in Spain described how he and his co-workers are tackling this challenge and have developed integrated photonics-based sources of millimetre and THz waves. “Although we tried to use available generic integration-platform building blocks, there is no building block for Bragg mirrors,” said Carpintero. As a result, the team developed the concept of integrated multimode interference reflector mirrors for mode-locked lasers. The optical spectrum of the optical heterodyne source based on the mode-locked photonic integrated circuit around 1,560 nm showed a carrier wave frequency of 90 GHz. The team have used this on-chip optical heterodyne source to perform broadband wireless data transmission.
In terms of fundamental science, according to Xi-Cheng Zhang, the program chair of the IRMMW-THz 2015 program committee, from the University of Rochester in the USA, many of the invited and contributed talks were on the topics of THz metamaterials and THz nanophotonics.
Rupert Huber of the University of Regensburg, Germany talked about research enabled by sub-cycle THz pulses. The first half of his talk was devoted to the study of excitons in monolayer metal dichalcogenides — a class of 2D materials that is currently receiving a high level of interest. Unlike the well-known 2D material graphene, single atomic layers of transition metal dichalcogenides feature direct energy gaps in the optical range, which makes them promising for constructing thin optoelectronic devices. The optical properties of the material are dominated by excitons whose behaviour and dynamics have been challenging to unravel. Huber’s research used ultrashort, ultrabroadband THz pulses to probe the atom-like internal Lyman transition between the 1s and 2p orbitals of the excitons and to directly measure the transition energy, oscillator strength, density and dynamics. However, the THz response of a photoexcited exciton sheet in a single atomic layer is exceptionally weak. So, his team had to improve the sensitivity of their THz detectors to trace tiny signal changes.
The ultrabroadband THz pulses revealed important details about the 1s–2p resonance, including accurate transition energies, oscillator strengths, densities and linewidths. Unlike interband photoluminescence and absorption spectroscopy, the THz probes are sensitive to all 1s excitons, regardless of their momentum, making it possible to draw a comprehensive picture of the exciton dynamics. Remarkably, the observed decay dynamics indicate an ultrafast radiative annihilation of small-momentum excitons within 150 fs, whereas Auger recombination prevails for optically dark states. “The results also suggest internal exciton transitions as a new degree of freedom for quantum control, optoelectronics and valleytronics of dichalcogenide monolayers,” explained Huber.
The second half of Huber’s talk was dedicated to high-harmonic (HH) generation from a bulk GaSe crystal subjected to intense THz pulses. The recent discovery of HH generation in solids has sparked hope for the development of compact solid-state attosecond sources and lightwave-driven electronics. Huber has been studying THz HH generation in the time domain with sub-cycle temporal resolution to explore the microscopic electron dynamics involved. The low carrier frequency of THz pulses offers the possibility to attain specifically large ponderomotive energies, which should — at least in principle — open a way to push the HH cut-off frequency to new records.
“The sub-cycle temporal structure of HH pulses emitted from bulk GaSe reflects a new quality of strong-field excitations,” he said. In contrast to established atomic sources, the source emits HH radiation as a sequence of sub-cycle bursts. He showed that these features hallmarked a novel non-perturbative quantum interference involving electrons from multiple valence bands. “The results identify key mechanisms for future solid-state attosecond sources and next-generation lightwave electronics and inspire sub-cycle quantum control based on strong-field interference,” he told Nature Photonics.
Frank Hegmann of the University of Alberta in Canada talked about imaging ultrafast dynamics of a single InAs nanodot on GaAs with THz scanning tunnelling microscopy (STM). In THz STM, free-space-propagating THz pulses with picosecond duration are antenna-coupled to the tip of a scanning tunnelling microscope, producing a transient rectified tunnel current signal that depends on the shape of the current–voltage (I–V) curve of the tunnel junction. Excitations of the sample may affect local electric fields and the local density of states, which can modify the I–V response. The THz-STM, which is sensitive to the local I–V response of the tunnel junction, can provide information on the transient dynamics of excitations on surfaces with 0.5 ps time resolution and 2 nm spatial resolution.
Hegmann is now working on upgrading the system. “We are currently developing THz STM for operation in ultrahigh vacuum with the goal of imaging ultrafast dynamics on surfaces with atomic resolution,” he mentioned. The physical factors that limit the spatial resolution of THz STM are similar in nature to those for conventional STM, such as tip quality. However, high-THz fields may produce tunnel currents over a larger area from the tip, which further limits spatial resolution. The temporal resolution of THz STM is limited primarily by the duration of the THz pulse itself, but is also affected by the nature of the antenna coupling to the junction (tip material and geometry, and THz incident angle). Ultimately, the temporal resolution is limited by the tunnel time, which can be fast (~1 fs).
As the absorption coefficient of THz waves in water is millions of times higher than that for visible light, water is usually viewed as a nuisance in THz spectroscopy. Nevertheless, Martina Havenith of Ruhr-University of Bochum highlighted the importance of THz absorption spectroscopy in water.
“The majority of chemical reactions and virtually all biological processes take place in a liquid-state environment,” she explained to the audience. She pointed out that THz absorption spectroscopy does have some benefits. Firstly, THz waves cover the low-frequency modes of water, which are associated with collective modes. Thus they can directly probe the collective hydrogen bond network dynamics. Secondly, THz waves are sensitive to the picosecond motions associated with opening and closing of hydrogen bonds. Thirdly, the THz spectrum of a protein is sensitive to mutation and depends on the surface charge and flexibility of the protein.
She has used a p-type Ge laser with a high average power to implement THz absorption spectroscopy in water layers 50–100 ?m thick. She says that during the experiments it is important to keep the humidity around the sample cell less than 10% and the temperature in the sample cell fixed within ±0.5 °C. As a result, a precision of less than 0.2% was achieved for the difference in THz absorption coefficient. The THz absorption spectroscopy results supported her theoretical hypothesis that proteins influence not only single water molecules at the protein surface but also the hydration dynamics in their surroundings up to a distance of 10–15 Å, that is, 3–5 hydration shells. “We pioneered kinetic THz absorption during enzymatic catalysis. A gradient of water motions towards functional sites of proteins (recognition sites) is observed — the so-called hydration funnel,” she explained.
Matthew Swithenbank of the University of Leeds in the UK is also studying THz spectroscopy in water, but with different technology — using on-chip THz time-domain spectroscopy. The approach combines a microfluidic channel and a planar Goubau line (PGL) that is formed by a single rectangular-shaped conducting wire lying on a flat dielectric substrate. However, it was a far from easy task, because microfluidic channels introduce impedance-mismatched interfaces at locations where the channel crossed the transmission line. These interfaces generated THz-frequency reflections in the time-domain, which at best complicated, and at worst prevented, subsequent analysis.
To avoid this problem, he fabricated a PGL on a 50-?m-thick polyimide film and positioned a microfluidic channel on the underside of the film for through-substrate measurement of liquid samples. This allowed total coverage of the transmission line sensing region (thereby eliminating time-domain reflections). He demonstrated that this technique was sufficiently sensitive to discriminate easily between alcohols in a homologous series that differed only by a single CH2 group. “We are now poised to explore a range of solvent-based biochemical systems,” he told Nature Photonics.

The next IRMMW-THz will be held in Copenhagen, Denmark over 26–30 September 2016.
Nature Photonics 9, 714–716 (2015) doi:10.1038/nphoton.2015.210