Mittleman Wins Humboldt Award

https://www.brown.edu/academics/engineering/news/2018-04/mittleman-wins-humboldt-award

Daniel Mittleman, professor of engineering who studies frequencies in the terahertz range, has received an Alexander von Humboldt Foundation Research Award for 2018.

Brown Professor of Engineering Daniel Mittleman is a 2018 recipient of the Alexander von Humboldt Foundation Research Award. The foundation grants up to 100 such awards each year to researchers from around the world to support collaborative projects with scientists and researchers in Germany. The awards are granted to researchers “whose fundamental discoveries, new theories, or insights have had a significant impact on their own discipline and who are expected to continue producing cutting-edge achievements in the future.”

Mittleman was recognized for his work on the science and technology of terahertz radiation. The award will support ongoing research and enable new partnerships involving devices for terahertz wireless communications and non-linear terahertz spectroscopy of materials. He will collaborate with Professor Martin Koch at the University of Marburg, a collaboration that began many years ago.

“I am honored to receive this prestigious award from the Humboldt Foundation,” Mittleman said. It will provide me with the opportunity to extend my long-standing research collaboration with Professor Koch, as well as to build new collaborations with other groups in Germany who are doing cutting-edge research in terahertz science and technology. Many of my current research interests overlap strongly with ongoing work in Germany, including not only my group’s work in terahertz wireless systems, but also terahertz high-field science and terahertz nano-spectroscopy.”

Mittleman received his B.S. in physics from the Massachusetts Institute of Technology in 1988, and his M.S. in 1990 and Ph.D. in 1994, both from the University of California, Berkeley, under the direction of Dr. Charles Shank. He then joined AT&T Bell Laboratories as a post-doctoral member of the technical staff, working first for Dr. Richard Freeman on a terawatt laser system, and then for Dr. Martin Nuss on terahertz spectroscopy and imaging.

Mittleman joined the Electrical and Computer Engineering Department at Rice University in September 1996, before moving to the School of Engineering at Brown University in 2015. He is a Fellow of the Optical Society of America, the American Physical Society, and the Institute of Electrical and Electronics Engineers, and is currently serving a three-year term as Chair of the International Society for Infrared Millimeter and Terahertz Waves.

Brown Professor of Engineering Sharvan Kumar was a 2015 recipient of the award.

On the crest of the wave: Electronics on a time scale shorter than a cycle of light

An intense lightwave drives ultrafast electronic motion in a bulk crystal. A novel quantum interference creates free electrons and causes the emission of ultrashort high-harmonic light bursts. Credit: B. Baxley /parttowhole.com

 http://phys.org/news/2015-07-crest-electronics-scale-shorter.html#jCp

Physicists from Regensburg and Marburg, Germany have succeeded in taking a slow-motion movie of speeding electrons in a solid driven by a strong light wave. In the process, they have unraveled a novel quantum phenomenon, which is reported in the recent edition of Nature.

The advent of ever faster electronics featuring clock rates up to the multiple-gigahertz range has revolutionized our day-to-day life. Researchers and engineers all over the world have racked their brains about one central question: Is there a fundamental limit for the speed of electronics? Indeed, all electronic circuits rely on charge motion controlled by electric fields. Future high-speed electronics would, therefore, benefit immensely from bias fields that switch faster than state-of-the-art electronic clocks. The solution to this challenge may be surprisingly straightforward: One could try to employ the fastest alternating electric field available in nature – a light wave.

The team of researchers from Germany has now directly observed the electrons' motion in a semiconductor driven by a strong light pulse in the terahertz spectral region. The pioneering experiment carried out in Rupert Huber's group at the University of Regensburg enabled the first simultaneous clocking measurement of extremely broadband radiation sent out by the accelerated electrons, so-called high-order harmonics, and the driving light wave. It turns out that the harmonics are emitted in ultrashort light bursts which have now been characterized with a temporal resolution of approximately one femtosecond – the millionth of a billionth part of a second. In combination with numerical simulations performed in the groups of Mackillo Kira and Stephan W. Koch at the University of Marburg, this study provides unprecedented insights into the quantum world of a solid.

The results shed light onto a surprising behavior of the crystal electrons: During an extremely short timespan after excitation, the strong light field drives an electron simultaneously along multiple paths instead of one only. This strange scenario is possible in the quantum world where particles can behave like waves. As an indisputable quantum wave aspect, the electrons were shown to interfere constructively (destructively) only at the positive (negative) crests of the driving field, massively reshaping the temporal emission of the harmonics. While such quantum effects are often fragile and usually become observable only in extremely gentle fields the newly discovered phenomenon is qualitatively different because it is robust, producing pronounced interference contrast especially for extremely strong driving fields.

The breakthrough reveals the temporal structure of high-harmonics from a solid for the first time and thus helps the development of new sources of ever shorter light pulses. Moreover, this discovery opens new perspectives for modern high-speed electronics and sets an important milestone on the way towards light-wave-driven electronics.

 Explore further: Lightwave electronics at sharp metal tips

More information: M. Hohenleutner, F. Langer, O. Schubert, M. Knorr, U. Huttner, S. W. Koch, M.Kira und R. Huber, Real-time observation of interfering crystal electrons in high-harmonic generation, Nature (2015), DOI: 10.1038/nature14652

Researchers observe new charge transport phenomenon

Illustration of the transport phenomenon
http://phys.org/news/2015-04-phenomenon.html

Researchers of Aalto University in Finland and the German University of Marburg have collaborated in the study of the movement of charges over interfaces of semiconductor materials. The group noticed a new kind of transport phenomenon for charges. In the phenomenon, a pair formed by a negative electron and a positive charge moves onto an interface, after which its 'message' is passed on to the other side of the interface, where it is carried on by a similar pair. The new theoretical result opens up interesting prospects for carrying out logical operations in electronics.

In addition to microelectronics, transport phenomena of charges are in a key role in many biological processes, such as photosynthesis," explains Professor Ilkka Tittonen from Aalto University.

A unique observation

In the tunnelling phenomenon a particle can, with certain likelihood, penetrate the thin interface between materials, even if it would be seen as impossible according to classical physics. The newly discovered phenomenon is not based on the tunnelling of individual charges, but rather on the dynamics of a pair made of an electron and a positive charge that is connected to it electrically. This bound pair composed of an electron and a positively charged hole is called an exciton.

"The observation is quite unique. Finally, an optical pulse functioning on a terahertz frequency brings information or the so-called correlation of the electron hole pair from one side of the interface to the other, without any tunnelling of the exciton itself. No phenomenon that would be fully equivalent to this has been found previously in the field of modern physics," Tittonen explains.

The phenomenon combines semiconductor and terahertz techniques and it allows a new kind of logical operation in microelectronics. The group believes that it will be possible, on the basis of the phenomenon, to design new kinds of processors which function partly through optics and partly through electricity.

The group published its observations in the publication Physical Review Letters on 16 March 2015.

The study took place at Aalto University School of Electrical Engineering in Finland, primarily by Osmo Vänskä, under the supervision of Professor Ilkka Tittonen and Professors Mackillo Kira and Stephan Koch.  Finnish funding for the study has come from Aalto University and the Academy of Finland. Professor Mackillo Kira serves as a visiting professor at Aalto University regularly every year.

 Explore further: New nanowire structure absorbs light efficiently

More information: "Coherent Terahertz Control of Vertical Transport in Semiconductor Heterostructures" Phys. Rev. Lett. 114, 116802. dx.doi.org/10.1103/PhysRevLett.114.116802

Challenges Not Insurmountable for Terahertz Spectroscopy

Marie Freebody, Contributing Editor, marie.freebody@photonics.com

Cost, size and power must be addressed before terahertz spectroscopy can broaden its commercial acceptance – but steps are being taken. 
http://www.photonics.com/Article.aspx?AID=56212
Today, terahertz spectroscopy shows promise for some fascinating applications, from detecting illicit drugs and counterfeit pharmaceuticals to measuring the water content of plants and identifying hazardous trace gases. One reason terahertz has gained so much traction for these tasks comes down to the nature of its radiation. Terahertz is ideally placed between IR and microwave radiation, giving it a unique imaging power that crucially lies in the nondestructive region of the spectrum. 

For detecting illicit drugs, for example, time-domain spectroscopy (TDS) systems are ideally suited. Pharmaceutical substances – and in general, most nonpolar organic solids – show fairly broad absorption peaks at terahertz frequencies. These chemical fingerprints are well detected with a time-domain terahertz system. 

Other techniques such as Raman or IR spectroscopy also can identify most of the same substances, of course, but they require direct optical access to the sample. 

“By contrast, one of the key strengths of working at terahertz frequencies is that the terahertz beam penetrates materials such as paper, cardboard or plastics. This opens the possibility to detect chemical agents or illicit drugs even through parcels or sealed mail envelopes,” said Dr. Anselm Deninger, product manager of terahertz technologies at Toptica Photonics AG. “The main advantages of time-domain techniques are their broad bandwidth and the relatively short data-acquisition time: A full spectrum is recorded within subseconds to several 10 s, depending on the number of averages.” 

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Performance of Toptica’s CW terahertz spectrometer: The dynamic range is 80 dB at 500 GHz and still >65 dB at 1.0 THz. The dips are absorption lines of water vapor. Courtesy of © Toptica Photonics AG.

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Drug analysis is not just restricted to the illegal kind; polymorphic forms can also be identified. Various forms of the same chemical compound can exhibit different pharmaceutical properties, which are vital for drug manufacturers to remove from the production line. Different conformations or crystal packing of the same molecules can lead to different optical properties in the terahertz frequency regime, making them ideally suited for spectroscopic detection.

Water: Friend or foe?

At first encounter, water appears to be the enemy for terahertz radiation. Water is a very strong absorber at these frequencies; water vapor has distinct absorption lines more or less throughout the entire terahertz frequency range, and liquid water does not show any sharp peaks but causes broadband attenuation.

Medical research scientists find this a common problem when applying terahertz technology, because anything that takes place more than some hundred microns under the surface of the skin is impossible to reach due to strong water absorption of the terahertz signal.

But turn this around and measure instead the water content of a specimen, and you transform an enemy into a friend. By using the absorbing properties of water as contrast medium for imaging, you can potentially resolve fine differences in water content across a sample.

Terahertz for optimizing irrigation

Professor Martin Koch’s group at University of Marburg in Germany is taking this idea and running with it: The group has been measuring the water status of plants using terahertz TDS.

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Terahertz transmission image of a fresh plant leaf 24 hours after cutting: The dried leaf shows only little remaining water content. (Leaf images are for Dr. Rafal Wilk’s dissertation, available athttp://www.amazon.de/Switchable-THz-Reflectors-Rafal-Wilk/dp/3867273154, ISBN-13: 978-3867273152; Wilk, who completed this Ph.D. thesis under professor Martin Koch, is a leader of the terahertz business unit at Menlo Systems). Courtesy of Menlo Systems.

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The water content in vegetative tissues is of high importance to plant scientists, as it is a good indication of general crop health. Until recently, one of the only ways to evaluate this parameter was to compare the weight of fresh, dry leaves. This does not allow for instantaneous or continuous monitoring of the water content in live tissues.

Terahertz TDS offers a noncontact, nondestructive alternative. “Here, the strong absorption of terahertz radiation by water becomes an advantage, as small changes in water content of a leaf can be detected very sensitively,” said Ralf Gente, a Ph.D. student in Koch’s group. “In future experiments, we will investigate the behavior of agricultural crops like corn, wheat or soy under drought stress and re-irrigation.”

Spectroscopy through smoke

Black smoke is opaque for visible light – there is simply too much scattering. But because terahertz light has a longer wavelength (roughly a factor of 1000), it experiences much less scattering, opening up the possibility of detecting toxic gases released by a fire – in a chemical plant, for example.

“A drawback of traditional techniques – chemical, optical or acoustic sensors – is their limited versatility: A chemical sensor, for instance, requires a dedicated receptor tailored to the gas species to be measured,” Deninger said. “However, the sensor usually saturates after being activated once, and then has to be replaced. If you want to detect several hazardous gases, you may end up with a whole battery of different sensors.”

On the other hand, a single terahertz spectrometer can identify a broad range of substances. It can even do so remotely, if the laser light is guided via optical fibers to the location of interest, transmitting the terahertz beam to exactly where it is needed.

“A CW terahertz source is the instrument of choice here – first, because it is much easier to transmit CW laser light through a long stretch of fiber and, second, because the spectral resolution is just so good,” Deninger said. “Our own CW terahertz systems achieve single-megahertz resolution, which helps you to identify trace amounts of gases unambiguously.”

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Next-generation CW terahertz spectroscopy system: The black cylinders in the foreground are fiber-coupled terahertz antennas, the box on the right-hand side is the laser head, and the “flatpack” box on the left comprises the driver electronics and data-acquisition unit. Courtesy of © Toptica Photonics AG.

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Overcoming the challenges 

Despite the far-reaching potential of terahertz spectroscopy, adoption by the mass market has not yet materialized. But what is stopping industry from turning its back on the established imaging and inspection tools such as Raman spectroscopy, IR and x-ray imaging and replacing them with the versatile, nondestructive terahertz alternative?

Some limits are simply determined by the laws of physics: Atmospheric absorption – especially due to water vapor – limits the transmission of terahertz radiation in air to a few meters at most.

Terahertz transmission image of a fresh plant leaf recorded within 8 hours after cutting. Dark-blue color represents areas of high water content. Courtesy of Menlo Systems.

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“There are different approaches to achieve broadband terahertz operation for spectroscopic applications. Optoelectronic systems have the advantage to be compact and easy to use. Robust and reliable femtosecond lasers allow pulsed terahertz operation where a broad spectral range is available for the measurement at once,” said Dr. Patrizia Krok, who handles international sales of terahertz systems at Menlo Systems GmbH. “Highest reproducibility of the laser parameters such as in our Figure 9 T-Light laser are the key to high-performance THz-TDS systems.”

Other challenges include the need for more compact systems at higher powers. To overcome this, engineering homework and sufficient funding are required.

“We admittedly have some sort of ‘hen and egg’ problem here,” Deninger said. “The price for a fully fledged CW terahertz system is on the US$50,000 - 100,000 level. This is still prohibitively high for broadband industrial use. On the other hand, it is generally agreed that prices will drop significantly and systems will become more mature, if only the sales volume grows large enough.”

Toptica uses optoelectronic terahertz generation techniques in which GaAs- or InGaAs-based antennas convert NIR laser light into terahertz radiation. This works, said Deninger, but the industry still expects smaller, lighter-weight control electronics.

“I think the industry expects more compact systems, more terahertz power, a higher dynamic range, a lower price and ideally a ‘push-button’ level of complexity,” he said. “So our next step is to shrink the size of our CW-terahertz system and make it more efficient at the same time.”

Another challenge is accessing the gap between around 4 THz (time-domain spectrometers) and around 12 THz (Fourier transform IR spectroscopy, or FTIR). Dr. Mira Naftaly and colleagues at the National Physical Laboratory in Teddington, England, are currently developing a terahertz optical parametric oscillator that will operate at 6 to 12 THz. This could enable widespread testing of a greater variety of materials and chemicals in industrial labs as well as in research; it would be comparable to Raman spectroscopy or FTIR.

While the list of hurdles is significant, most in the industry believe that the next five to 10 years will be very exciting for the field.

On-resonance field enhancement factor (a) and terahertz transmission spectra (c) of terahertz resonator pictured in (b). Micron-sized gaps in metamaterial structures can be used to spatially localize terahertz radiation to regions smaller than the wavelength and improve the contrast for biological sensing applications. Courtesy of Lake Shore Cryotronics.

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“The evolution of Raman spectroscopy could be a helpful road map for thinking about the future of terahertz spectroscopy,” said Dr. David Daughton, applications scientist at Lake Shore Cryotronics Inc. in Westerville, Ohio. Lake Shore is a sensors, measurement and control specialist for a variety of research establishments. “The Raman technique is nearly a century old; a generation ago, Raman spectroscopy was isolated to well-equipped laser laboratories, and only in the past decade or so has the emergence of cost-effective diode lasers and filter technology enabled broad adoption of turnkey systems.”

What’s next for terahertz

With the availability of higher-power terahertz sources, materials that partially absorb terahertz radiation or thicker objects can be investigated. For example, Menlo Systems’ fiber-coupled terahertz antennas Tera15-FC already provide high power in their systems, and one of the UK’s terahertz companies, TeraView Ltd., claims to be developing a number of high-power sources that are both cost-effective and usable at ambient temperatures. These systems should be commercially available in the next two to three years.

Among the key developments for bio and medical applications, Daughton believes, are the metal and semiconductor structures that bring spectroscopy down to biological-length scales. To have sufficient sensitivity for bacterial detection, for example, semiconductor antennas can be used to couple free-space terahertz and direct the radiation to a 10-µm gap (which is filled with the specimen).

Also on the horizon is terahertz quasi-TDS, which uses a cheap multimode laser diode to generate terahertz radiation instead of a femtosecond laser. The technique promises to open up a new range of practical applications by massively reducing the cost, while at the same time allowing for much smaller systems that consume less power.

New device electrically steers and focuses terahertz waves

http://www.rdmag.com/news/2013/01/new-device-electrically-steers-and-focuses-terahertz-waves

Yasuaki Monnai (right) and Kristian Altmann with the device mounted to the characterization setup at Marburg University. Photo: Bastian Reitemeier, University of MarburgIn a close collaboration, researchers from the University of Marburg/Germany and of the University of Tokyo/Japan have demonstrated a device which allows for an electric and flexible focusing and steering of terahertz (THz) waves.

The ability to redirect and focus THz beams will be of particular importance for THz communication systems, which will work with directed links between emitters and receivers. Yet, the position of THz emitters and/or receivers or the distance between them are likely to change from time to time, as we move with a laptop or other mobile devices freely in a room. Moreover, walking persons or moving objects might block the link. Hence, it is crucial to have the ability to redirect THz beams or to vary their divergence. Other application fields include remote sensing and the inspection of industrial goods.

The device was developed by Yasuaki Monnai in the group of Prof. Hiroyuki Shinoda at the University of Tokyo. It is based on a sub-wavelength array of metal cantilevers which can be micromechanically actuated by electrostatic forces such that tunable gratings of different periodicity can be created.  Tuning the grating pattern allows for a shaping of the wavefront of the diffracted radiation and, hence, to vary the direction of the THz beams. Furthermore, the divergence of the THz beam can be controlled.

The characterization experiments have been performed by Kristian Altmann and Yasuaki Monnai in the group of Prof. Martin Koch at the University of Marburg. In the first proof of concept, the steerable range at 0.3 THz exceeded an angle of 40 degrees. The accomplished beam directions and the field profiles agree well with theoretical expectations.

Terahertz beam steering and variable focusing using programmable diffraction gratings

Source: University of Marburg

Polymer-based lens project tackles terahertz and millimeter-wave applications

http://www.laserfocusworld.com/articles/2011/09/polymer-thz-lenses.html

Marburg, Germany--A research project that aims for the development of new polymer-based lenses for terahertz and sub-millimeter-wave applications is being pursued by the experimental semiconductor physics group of the University of Marburg in cooperation with the South German Plastics Center (SKZ; Wurzburg, Germany). "We anticipate that these polymer-based lenses will provide an improved imaging quality, additional functionality as well as low material and production costs," says professor Martin Koch, head of the Marburg research group.

The increasing technological maturity of terahertz and microwave systems during the last two decades means that innovative technology such as low-cost plastic optics must advance beyond purely scientific interest towards industrial applications. The researchers say that terahertz technology will have a plethora of applications including industrial quality control, security checks, radio astronomy, and wireless communications.

The goal of the project is the development of terahertz lenses based on new mixtures between polymers and additives such as titanium dioxide or alumina powder. "By mixing the polymers with additives their refractive index will be increased. Simultaneously the material mixture should be highly transparent to terahertz waves," explains Koch.

The new terahertz lenses are expected to outperform lenses made of pure base polymers and they could replace more expensive lenses made from pure silicon. The project is funded by the Association of Industrial Research Organizations "Otto von Guericke" (AiF) within the program "Industrial Community Research" (IGF) on behalf of the German Federal Ministry of Economics.

SOURCE: University of Marburg; www.uni-marburg.de/fb13/forschung/experimentelle-halbleiterphysik/agkoch/ag-koch?set_language=en