Thursday, October 29, 2015

View from… IRMMW-THz: Strength in diversity


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 of
Hong 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

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