Abstract-Grating-Graphene Metamaterial as a Platform for Terahertz Nonlinear Photonics

 

Jan-Christoph Deinert, David Alcaraz Iranzo, Raúl Pérez, Xiaoyu Jia, Hassan A. Hafez, Igor Ilyakov, Nilesh Awari, Min Chen, Mohammed Bawatna, Alexey N. Ponomaryov, Semyon Germanskiy, Mischa Bonn, Frank H.L. Koppens, Dmitry Turchinovich, Michael Gensch, Sergey Kovalev,  Klaas-Jan Tielrooij 

https://pubs.acs.org/doi/full/10.1021/acsnano.0c08106

Nonlinear optics is an increasingly important field for scientific and technological applications, owing to its relevance and potential for optical and optoelectronic technologies. Currently, there is an active search for suitable nonlinear material systems with efficient conversion and a small material footprint. Ideally, the material system should allow for chip integration and room-temperature operation. Two-dimensional materials are highly interesting in this regard. Particularly promising is graphene, which has demonstrated an exceptionally large nonlinearity in the terahertz regime. Yet, the light–matter interaction length in two-dimensional materials is inherently minimal, thus limiting the overall nonlinear optical conversion efficiency. Here, we overcome this challenge using a metamaterial platform that combines graphene with a photonic grating structure providing field enhancement. We measure terahertz third-harmonic generation in this metamaterial and obtain an effective third-order nonlinear susceptibility with a magnitude as large as 3 × 10–8 m2/V2, or 21 esu, for a fundamental frequency of 0.7 THz. This nonlinearity is 50 times larger than what we obtain for graphene without grating. Such an enhancement corresponds to a third-harmonic signal with an intensity that is 3 orders of magnitude larger due to the grating. Moreover, we demonstrate a field conversion efficiency for the third harmonic of up to ∼1% using a moderate field strength of ∼30 kV/cm. Finally, we show that harmonics beyond the third are enhanced even more strongly, allowing us to observe signatures of up to the ninth harmonic. Grating-graphene metamaterials thus constitute an outstanding platform for commercially viable, CMOS-compatible, room-temperature, chip-integrated, THz nonlinear conversion applications.

Abstract-Ultrafast terahertz magnetometry

Wentao Zhang, Pablo Maldonado, Zuanming Jin, Tom S. Seifert, Jacek Arabski, Guy Schmerber, Eric Beaurepaire, Mischa Bonn, Tobias Kampfrath, Peter M. Oppeneer, Dmitry Turchinovich

https://www.nature.com/articles/s41467-020-17935-6

A material’s magnetic state and its dynamics are of great fundamental research interest and are also at the core of a wide plethora of modern technologies. However, reliable access to magnetization dynamics in materials and devices on the technologically relevant ultrafast timescale, and under realistic device-operation conditions, remains a challenge. Here, we demonstrate a method of ultrafast terahertz (THz) magnetometry, which gives direct access to the (sub-)picosecond magnetization dynamics even in encapsulated materials or devices in a contact-free fashion, in a fully calibrated manner, and under ambient conditions. As a showcase for this powerful method, we measure the ultrafast magnetization dynamics in a laser-excited encapsulated iron film. Our measurements reveal and disentangle distinct contributions originating from (i) incoherent hot-magnon-driven magnetization quenching and (ii) coherent acoustically-driven modulation of the exchange interaction in iron, paving the way to technologies utilizing ultrafast heat-free control of magnetism. High sensitivity and relative ease of experimental arrangement highlight the promise of ultrafast THz magnetometry for both fundamental studies and the technological applications of magnetism.

Graphene hits the right note at high frequencies

Fig. 1. Graphene converts electronic signals with frequencies in the gigahertz range extremely efficiently into signals with several times higher frequency. (Image credit: Juniks/HZDR.).

Cordelia Sealy
https://www.materialstoday.com/carbon/news/graphene-hits-the-right-note-at-high-frequencies/

Graphene holds the potential to deliver a new generation of ultrafast electronic devices. Current silicon technology can achieve clock rates – a measure of how fast devices can switch – of several hundred gigahertz (GHz). Graphene could achieve clock rates up to a thousand times faster, propelling electronics into the terahertz (THz) range. But, until now, graphene’s ability to convert oscillating electromagnetic signals into higher frequency modes has been just a theoretical prediction.

Now researchers from the Helmholtz Zentrum Dresden Rossendorf (HZDR) and University of Duisburg-Essen (UDE), in collaboration with the director of the Max Planck Institute for Polymer Research (MPI-P) Mischa Bonn and other researchers, have shown that graphene can covert high frequency gigahertz signals into the terahertz range [Hafez et al., Nature (2018), https://doi.org/ 10.1038/s41586-018-0508-1].

“We have been able to provide the first direct proof of frequency multiplication from gigahertz to terahertz in a graphene monolayer and to generate electronic signals in the terahertz range with remarkable efficiency,” explain Michael Gensch of HZDR and Dmitry Turchinovich of UDE.

Using the novel superconducting accelerator TELBE terahertz radiation source at HZDR’s ELBE Center for High-Power Radiation Sources, the researchers bombarded chemical vapor deposition (CVD)-produced graphene with electromagnetic pulses in the frequency range 300–680 GHz. As previous theoretical calculations have predicted, the results show that graphene is able to convert these pulses into signals with three, five, or seven times the initial frequency, reaching the terahertz range (Fig. 1).

“We were not only able to demonstrate a long-predicted effect in graphene experimentally for the first time, but also to understand it quantitatively at the same time,” points out Turchinovich.

By doping the graphene, the researchers created a high proportion of free electrons or a so-called Fermi liquid. When an external oscillating field excites these free electrons, rather like a normal liquid, they heat up and share their energy with surrounding electrons. The hot electrons form a vapor-like state, just like an evaporating liquid. When the hot Fermi vapor phase cools, it returns to its liquid form extremely quickly. The transition back and forth between these vapor and liquid phases in graphene induces a corresponding change in its conductivity. This very rapid oscillation in conductivity drives the frequency multiplication effect.

“In theory, [this] should allow clock rates up to a thousand times faster than today’s silicon-based electronics,” say Gensch and Turchinovich.

The conversion efficiency of graphene is at least 7–18 orders of magnitude more efficient than other electronic materials, the researchers point out. Since the effect has been demonstrated with mass-produced CVD graphene, they believe there are no real obstacles to overcome other than the engineering challenge of integrating graphene into circuits.

“Our discovery is groundbreaking,” says Bonn. “We have demonstrated that carbon-based electronics can operate extremely efficiently at ultrafast rates. Ultrafast hybrid components made of graphene and traditional semiconductors are also now conceivable.”

Nathalie Vermeulen, professor in the Brussels Photonics group (B-PHOT) at Vrije Universiteit Brussel (VUB) in Belgium, agrees that the work is a major breakthrough.

“The nonlinear-optical physics of graphene is an insufficiently understood field, with experimental results often differing from theoretical predictions,” she says. “These new insights, however, shine new light on the nonlinear-optical behavior of graphene in the terahertz regime.”

The researchers’ experimental findings are clearly supported by corresponding theory, Vermeulen adds, which is very convincing.

“It is not often that major advances in fundamental scientific understanding and practical applications go hand in hand, but I believe it is the case here,” she says. “The demonstration of such efficient high-harmonic terahertz generation at room temperature is very powerful and paves the way for concrete application possibilities.”

The advance could extend the functionality of graphene transistors into high-frequency optoelectronic applications and opens up the possibility of similar behavior in other two-dimensional Dirac materials. Marc Dignam of Queen’s University in Canada is also positive about the technological innovations that the demonstration of monolayer graphene’s nonlinear response to terahertz fields could open up.

“The experiments are performed at room temperature in air and, given the relatively short scattering time, it is evident that harmonic generation will occur for relatively moderate field amplitudes, even in samples that are not particularly pristine,” he points out. “This indicates that such harmonic generation could find its way into future devices, once higher-efficiency guiding structures, such as waveguides, are employed.”

He believes that the key to the success of the work is the low-noise, multi-cycle terahertz source (TELBE) used by the researchers. However, Dignam is less convinced by the team’s theoretical explanation of graphene’s nonlinear response. No doubt these exciting results will spur further microscopic theoretical investigations examining carrier dynamics in graphene in more detail.

This article was originally published in Nano Today 23 (2018) 2-3. 

Abstract-Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions

Hassan A. Hafez, Sergey Kovalev, Jan-Christoph Deinert, Zoltán Mics, Bertram Green, Nilesh Awari, Min Chen, Semyon Germanskiy, Ulf Lehnert, Jochen Teichert, Zhe Wang, Klaas-Jan Tielrooij, Zhaoyang Liu, Zongping Chen, Akimitsu Narita, Klaus Müllen, Mischa Bonn, Michael Gensch,  Dmitry Turchinovich

https://www.nature.com/articles/s41586-018-0508-1

Multiple optical harmonic generation—the multiplication of photon energy as a result of nonlinear interaction between light and matter—is a key technology in modern electronics and optoelectronics, because it allows the conversion of optical or electronic signals into signals with much higher frequency, and the generation of frequency combs. Owing to the unique electronic band structure of graphene, which features massless Dirac fermions, it has been repeatedly predicted that optical harmonic generation in graphene should be particularly efficient at the technologically important terahertz frequencies. However, these predictions have yet to be confirmed experimentally under technologically relevant operation conditions. Here we report the generation of terahertz harmonics up to the seventh order in single-layer graphene at room temperature and under ambient conditions, driven by terahertz fields of only tens of kilovolts per centimetre, and with field conversion efficiencies in excess of 10−3, 10−4 and 10−5 for the third, fifth and seventh terahertz harmonics, respectively. These conversion efficiencies are remarkably high, given that the electromagnetic interaction occurs in a single atomic layer. The key to such extremely efficient generation of terahertz high harmonics in graphene is the collective thermal response of its background Dirac electrons to the driving terahertz fields. The terahertz harmonics, generated via hot Dirac fermion dynamics, were observed directly in the time domain as electromagnetic field oscillations at these newly synthesized higher frequencies. The effective nonlinear optical coefficients of graphene for the third, fifth and seventh harmonics exceed the respective nonlinear coefficients of typical solids by 7–18 orders of magnitude. Our results provide a direct pathway to highly efficient terahertz frequency synthesis using the present generation of graphene electronics, which operate at much lower fundamental frequencies of only a few hundreds of gigahertz.

Graphene enables clock rates in the terahertz range

Graphene converts electronic signals with frequencies in the gigahertz range extremely efficiently into signals with several times higher frequency. Credit: Juniks/HZDR

 https://phys.org/news/2018-09-graphene-enables-clock-terahertz-range.html#jCp

Graphene—an ultrathin material consisting of a single layer of interlinked carbon atoms—is considered a promising candidate for the nanoelectronics of the future. In theory, it should allow clock rates up to a thousand times faster than today's silicon-based electronics. Scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) and the University of Duisburg-Essen (UDE), in cooperation with the Max Planck Institute for Polymer Research (MPI-P), have now shown for the first time that graphene can actually convert electronic signals with frequencies in the gigahertz range—which correspond to today's clock rates—extremely efficiently into signals with several times higher frequency. The researchers present their results in the scientific journal Nature.

"We have now been able to provide the first direct proof of frequency multiplication from gigahertz to terahertz in a graphene monolayer and to generate electronic signals in the terahertz range with remarkable efficiency," explains Dr. Michael Gensch, whose group conducts research on ultrafast physics and operates the novel TELBE terahertz radiation source at the HZDR. And not only that—their 

cooperation partners led by Prof. Dmitry Turchinovich, experimental physicist at the University of Duisburg-Essen (UDE), have succeeded in describing the measurements quantitatively well using a simple model based on fundamental physical principles of thermodynamics.Today's silicon-based electronic components operate at clock rates of several hundred gigahertz (GHz), that is, they are switching several billion times per second. The electronics industry is currently trying to access the terahertz (THz) range, i.e., up to thousand times faster clock rates. A promising material and potential successor to silicon could be graphene, which has a high electrical conductivity and is compatible with all existing electronic technologies. In particular, theory has long predicted that graphene could be a very efficient "nonlinear" electronic material, i.e., a material that can very efficiently convert an applied oscillating electromagnetic field into fields with a much higher frequency. However, all experimental efforts to prove this effect in graphene over the past ten years have not been successful.

 

With this breakthrough, the researchers are paving the way for ultrafast graphene-based nanoelectronics: "We were not only able to experimentally demonstrate a long-predicted effect in graphene for the first time, but also to understand it quantitatively well at the same time," emphasizes Prof. Dmitry Turchinovich. "In my laboratory we have been investigating the basic physical mechanisms of the electronic nonlinearity of graphene already for several years. However, our light sources were not sufficient to actually detect and quantify the frequency multiplication clean and clear. For this, we needed experimental capabilities which are currently only available at the TELBE facility."

The long-awaited experimental proof of extremely efficient terahertz high harmonics generation in graphene has succeeded with the help of a trick: The researchers used graphene that contains many free electrons, which come from the interaction of graphene with the substrate onto which it is deposited, as well as with the ambient air. If these mobile electrons are excited by an oscillating electric field, they share their energy very quickly with the other electrons in graphene, which then react much like a heated fluid: From an electronic "liquid", figuratively speaking, an electronic "vapor" forms within the graphene. The change from the "liquid" to the "vapor" phase occurs within trillionths of a second and causes particularly rapid and strong changes in the conductivity of graphene. This is the key effect leading to efficient frequency multiplication.

The scientists used electromagnetic pulses from the TELBE facility with frequencies between 300 and 680 gigahertz and converted them in the graphene into electromagnetic pulses with three, five and seven times the initial frequency, i.e. up-converted them into the terahertz frequency range. "The nonlinear coefficients describing the efficiency of the generation of this third, fifth and seventh harmonic frequency were exceptionally high," explains Turchinovich. "Graphene is thus possibly the electronic material with the strongest nonlinearity known to date. The good agreement of the measured values with our thermodynamic model suggests that we will also be able to use it to predict the properties of ultrahigh-speed nanoelectronic devices made of graphene." Prof. Mischa Bonn, Director of the MPI-P, who was also involved in this work, emphasizes: "Our discovery is groundbreaking. We have demonstrated that carbon-based electronics can operate extremely efficiently at ultrafast rates. Ultrafast hybrid components made of graphene and traditional semiconductors are also conceivable."

The experiment was performed using the novel, superconducting-accelerator-based TELBE terahertz radiation source at the ELBE Center for High-Power Radiation Sources at the HZDR. Its hundred times higher pulse rate compared to typical laser-based terahertz sources made the measurement accuracy required for the investigation of graphene possible in the first place. A data processing method developed as part of the EU project EUCALL allows the researchers to actually use the measurement data taken with each of the 100,000 light pulses per second. "For us there is no bad data," says Gensch. "Since we can measure every single pulse, we gain orders of magnitude in measurement accuracy. In terms of measurement technology, we are at the limit of what is currently feasible." The first authors of the article are the two young scientists Hassan A. Hafez (UDE/MPI-P) and Sergey Kovalev (HZDR).

 Explore further: On the way to breaking the terahertz barrier for graphene nanoelectronics

More information: Hassan A. Hafez et al, Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions, Nature (2018). DOI: 10.1038/s41586-018-0508-1

A boost for graphene-based light detectors: Photoexcited graphene puzzle solved

Light detection and control lies at the heart of many modern device applications, such as the camera you have in your phone. Using graphene as a light-sensitive material for light detectors can offer significant improvements with respect to materials being used now. For example, graphene can detect light of almost any color, and it gives an extremely fast electronic response within one millionth of a millionth of a second. Thus, in order to properly design graphene-based light detectors it is crucial to understand the processes that take place inside the graphene after it absorbs light

Schematic representation of the ultrafast optical pump - terahertz probe experiment, where the optical pump induces electron heating and the terahertz pulse is sensitive to the conductivity of graphene directly after this heating process, which occurs on a timescale faster than a millionth of a millionth of a second.

Credit: Illustration: Fabien Vialla/ICFO

https://www.sciencedaily.com/releases/2018/05/180511150442.htm

Light detection and control lies at the heart of many modern device applications, such as the camera you have in your phone. Using graphene as a light-sensitive material for light detectors can offer significant improvements with respect to materials being used nowadays. For example, graphene can detect light of almost any colour, and it gives an extremely fast electronic response within one millionth of a millionth of a second. Thus, in order to properly design graphene-based light detectors it is crucial to understand the processes that take place inside the graphene after it absorbs light.

A team of European scientists including ICFO from Barcelona (Spain), IIT from Genova (Italy), the University of Exeter from Exeter (UK) and Johannes Gutenberg University from Mainz (Germany), have now succeeded in understanding these processes. Published recently in Science Advances, their work gives a thorough explanation of why, in some cases, the graphene conductivity increases after light absorption and in other cases, it decreases. The researchers show that this behaviour correlates with the way in which energy from absorbed light flows to the graphene electrons: After light is absorbed by the graphene, the processes through which graphene electrons heat up happen extremely fast and with a very high efficiency.

For highly doped graphene (where many free electrons are present), ultrafast electron heating leads to carriers with elevated energy -- hot carriers -- which, in turn, leads to a decrease in conductivity. Interestingly enough, for weakly doped graphene (where not so many free electrons are present), electron heating leads to the creation of additional free electrons, and therefore an increase in conductivity. These additional carriers are the direct result of the gapless nature of graphene -- in gapped materials, electron heating does not lead to additional free carriers.

This simple scenario of light-induced electron heating in graphene can explain many observed effects. Aside from describing the conductive properties of the material after light absorption, it can explain carrier multiplication, where -- under specific conditions -- one absorbed light particle (photon) can indirectly generate more than one additional free electron, and thus create an efficient photoresponse within a device.

The results of the paper, in particular, understanding electron heating processes accurately, will definitely mean a great boost in the design and development of graphene-based light detection technology.

This work was funded by the E.C. under Graphene Flagship, as well as a Mineco Young Investigator grant.

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Story Source:

Materials provided by ICFO-The Institute of Photonic Sciences. Note: Content may be edited for style and length.

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Journal Reference:

1. Andrea Tomadin, Sam M. Hornett, Hai I. Wang, Evgeny M. Alexeev, Andrea Candini, Camilla Coletti, Dmitry Turchinovich, Mathias Kläui, Mischa Bonn, Frank H. L. Koppens, Euan Hendry, Marco Polini, Klaas-Jan Tielrooij. The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies. Science Advances, 2018; 4 (5): eaar5313 DOI: 10.1126/sciadv.aar5313

Abstract-Coupling between intra- and intermolecular motions in liquid water revealed by two-dimensional terahertz-infrared-visible spectroscopy

Maksim Grechko, Taisuke Hasegawa, Francesco D’Angelo, Hironobu Ito, Dmitry Turchinovich, Yuki Nagata, Mischa Bonn

https://www.nature.com/articles/s41467-018-03303-y

The interaction between intramolecular and intermolecular degrees of freedom in liquid water underlies fundamental chemical and physical phenomena such as energy dissipation and proton transfer. Yet, it has been challenging to elucidate the coupling between these different types of modes. Here, we report on the direct observation and quantification of the coupling between intermolecular and intramolecular coordinates using two-dimensional, ultra-broadband, terahertz-infrared-visible (2D TIRV) spectroscopy and molecular dynamics calculations. Our study reveals strong coupling of the O-H stretch vibration, independent of the degree of delocalization of this high-frequency mode, to low-frequency intermolecular motions over a wide frequency range from 50 to 250 cm−1, corresponding to both the intermolecular hydrogen bond bending (≈ 60 cm−1) and stretching (≈ 180 cm−1) modes. Our results provide mechanistic insights into the coupling of the O-H stretch vibration to collective, delocalized intermolecular modes.

Abstract- Accurate terahertz spectroscopy of supported thin films by precise substrate thickness correction

Keno L. Krewer, Zoltan Mics, Jacek Arabski, Guy Schmerber, Eric Beaurepaire, Mischa Bonn, and Dmitry Turchinovich

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-43-3-447

We present a new approach for accurate terahertz time-domain spectroscopy of thin films deposited on dielectric substrates. Our approach relies on the simultaneous measurement of film and substrate, allowing for 15 nm—precise determination of the thickness variation between the sample and reference. Our approach allows for unprecedentedly accurate determination of the terahertz conductivity of the thin film. We demonstrate our approach on a 10 nm thin iron film deposited on a 500 μm MgO substrate. We determine the Drude momentum relaxation time in iron to within 0.15 fs uncertainty.

© 2018 Optical Society of America

Abstract-Electron transfer from bi-isonicotinic acid emerges upon photodegradation of N3 sensitized TiO2 electrodes

Melike Karakus, Wen Zhang, Hans Joachim Räder, Mischa Bonn, Enrique Cánovas,

http://pubsdc3.acs.org/doi/10.1021/acsami.7b08986

The long-term stability of dye sensitized solar cells (DSSCs) is determined to a large extent by the photodegradation of their sensitizers. Understanding the mechanism of light induced decomposition of dyes sensitizing a mesoporous oxide matrix may therefore contribute to solutions to increase the life span of DSSCs. Here, we investigate, using ultrafast terahertz photoconductivity measurements, the evolution of interfacial electron transfer (ET) dynamics in N3 dye-sensitized mesoporous TiO2 electrodes upon dye photodegradation. Under inert environment, interfacial ET dynamics do not change over time, indicating that the dye is stable and photodegradation is absent; the associated ET dynamics are characterized by a sub-100 fs rise of the photo-conductivity, followed by long-lived (>>1 ns) electrons in the oxide electrode. When the N3-TiO2 sample is exposed to air under identical illumination conditions, dye photodegradation is evident from the disappearance of the optical absorption associated with the dye. Remarkably, approximately half of the sub-100 fs ET is observed to still occur, but is followed by very rapid (~10 ps) electron-hole recombination. Laser desorption/ionization mass spectrometry, attenuated total reflection FTIR and terahertz photoconductivity analyses reveal that the photo-degraded ET signal originates from the N3 dye photodegradation product as dcbpy (4,4’- dicarboxylic acid - 2,2’-bipyridine), which remains bonded to the TiO2 surface via either bidentate chelation or bridging type geometry.

Abstract-Efficient formation of excitons in a dense electron-hole plasma at room temperature

Andreas Hangleiter, Zuanming Jin, Marina Gerhard, Dimitry Kalincev, Torsten Langer, Heiko Bremers, Uwe Rossow, Martin Koch, Mischa Bonn, and Dmitry Turchinovich

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.92.241305

Room-temperature electronic properties of semiconductors, especially in the case of higher charge densities, are commonly discussed in terms of single-particle excitations – free electrons and holes. Many-particle effects, such as the formation of excitons (Coulomb-bound electron-hole pairs), are usually seen as low-temperature and low-density phenomena. In this paper, using ultrafast terahertz and photoluminescence measurements, the authors find that under certain conditions typical for wide-band-gap semiconductors, the radiative excitons can be efficiently formed at high charge density and at room temperature. This effect is believed to contribute to the extraordinarily high quantum efficiency of group III nitride light emitters.

 

Commonly, excitons in semiconductors are regarded as a low-temperature, low carrier density phenomenon, becoming unstable as the temperature and carrier density increase. Contrary to the common expectation, our ultrafast conductivity and luminescence measurements in GaInN/GaN quantum wells reveal a highly efficient formation of radiative excitons from a high-density electron-hole plasma at room temperature, and provide a quantitative measure of the exciton fraction to reach more than 40% at a total carrier population as high as                                                                         

∼10cm−21013cm−2Driven by the mass action of electrons and holes, this effect is believed to contribute to the extraordinarily high quantum efficiency of group-III nitride light emitters.

Abstract-Accessing the fundamentals of magnetotransport in metals with terahertz probes

• Zuanming Jin,
• Alexander Tkach,
• Frederick Casper,
• Victor Spetter,
• Hubert Grimm,
• Andy Thomas,
• Tobias Kampfrath,
• Mischa Bonn,
• Mathias Kläui
• & Dmitry Turchinovich

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3384.html

Spin-dependent conduction in metals underlies all modern magnetic memory technologies, such as giant magnetoresistance (GMR). The charge current in ferromagnetic transition metals is carried by two non-mixing populations of sp-band Fermi-level electrons: one of majority-spin and one of minority-spin. These electrons experience spin-dependent momentum scattering with localized electrons, which originate from the spin-split d-band. The direct observation of magnetotransport under such fundamental conditions, however, requires magnetotransport measurements on the same timescale as the electron momentum scattering, which takes place in the sub-100 fs regime. Using terahertz electromagnetic probes, we directly observe the magnetotransport in a metallic system under the fundamental conditions, and determine the spin-dependent densities and momentum scattering times of conduction electrons. We show that traditional measurements significantly underestimate the spin asymmetry in electron scattering, a key parameter responsible for effects such as GMR. Furthermore, we demonstrate the possibility of magnetic modulation of terahertz waves, along with heat- and contact-free GMR readout using ultrafast terahertz signals.

On the way to breaking the terahertz barrier for graphene nanoelectronics

Interaction of the terahertz field with graphene leads to efficient electron heating, which in turn strongly changes graphene conductivity. Credit: © Zoltan Mics / MPIP

 http://phys.org/news/2015-07-terahertz-barrier-graphene-nanoelectronics.html#jCp

A team of scientists at the Max Planck Institute for Polymer Research (MPI-P) discovered that electrical conduction in graphene on the picosecond timescale - a picosecond being one thousandth of one billionth of a second - is governed by the same basic laws that describe the thermal properties of gases. This much simpler thermodynamic approach to the electrical conduction in graphene will allow scientists and engineers not only to better understand but also to improve the performance of graphene-based nanoelectronic devices.

The researchers found that the energy of ultrafast electrical currents passing through graphene is very efficiently converted into electron heat, making graphene electrons behave just like a hot gas. "The heat is distributed evenly over all electrons. And the rise in electronic temperature, caused by the passing currents, in turn has a strong effect on the electrical conduction of graphene" explains Professor Mischa Bonn, Director at the MPI-P. The study, entitled "Thermodynamic picture of ultrafast charge transport in graphene", has recently been published in Nature Communications.

Graphene - a single sheet of carbon atoms - is known to be a very good electrical conductor. As a result, graphene finds a multitude of applications in modern nanoelectronics. They range from highly efficient detectors for optical and wireless communications to transistors operating at very high speeds. A constantly increasing demand for telecommunication bandwidth requires an ever faster operation of electronic devices, pushing their response times to be as short as a picosecond. "The results of this study will help improve the performance of graphene-basednanoelectronic devices such as ultra-high speed transistors and photodetectors" says Professor Dmitry Turchinovich, who led the research at the MPI-P. In particular they show the way for breaking the terahertz operation speed barrier - i.e. one thousand billions of oscillations per second - for graphene transistors.

 Explore further: On the edge of graphene

More information: "Thermodynamic picture of ultrafast charge transport in graphene", Nature Communications, 2015. 

Ultrafast Terahertz Spectroscopy Enables Fundamental Magnetotransport Measurements in Metals

Difference in conduction by electrons with opposite spins in ferromagnetic metals can be precisely resolved using terahertz waves. (Credit: MPI-P)

By Stuart Milne

http://www.azom.com/news.aspx?newsID=44124

Researchers at the Max Planck Institute for Polymer Research (MPI-P) and the Johannes Gutenberg University (JGU) have developed a new ultrafast terahertz spectroscopy that yields direct insight into the building block of modern magnetic memories.

A huge amount of information can be stored in the existing magnetic memories including hard drives that can be found in nearly every computer. This is due to the presence of small, nanoscale magnetic sensors called spin-valves that support memory readout. These spin-valves operate based on the giant magnetoresistance (GMR) effect developed by Albert Fert and Peter Grünberg, who were awarded with a Nobel Prize in Physics in 2007.

The GMR effect is derived from Sir Nevill F. Mott’s principles of electrical conduction in ferromagnetic metals, proposed in 1936. According to Mott, the scattering of the conduction electrons in ferromagnetic metals is based on the electron’s microscopic magnetic moment, called the spin. This means that electrons with single spin orientation have a less scattering effect, and hence, more conductive than the electrons with reverse spin orientation. The amplification of spin-asymmetry in electron conduction is achieved on combining the thin films of ferromagnetic and nonmagnetic metals to produce a spin-valve. When the electrical resistivity of the spin-valve becomes responsive to the magnetic field, a GMR effect is produced.

Despite its significance in magnetic memories and several other technologies, the Mott spin-dependent conductivity is quite difficult to observe directly. In fact, it is possible to directly and unambiguously determine the basic factors of Mott conduction such as spin-dependent electron scattering time and spin-dependent electron density only if the measurement of the metal conductivity is on the same ultrafast timescale at which electron scattering takes place (i.e. sub-100fs). This shows that the need for the ultrafast timescale of measurement has prevented the fundamental magnetotransport observation in metals for a long time.

The new research has overcome the long time challenge with the introduction of ultrafastterahertz spectroscopy (1THz = 10¹²Hz, i.e. one thousand billion oscillations per second).

By studying the interaction of THz electromagnetic waves - which oscillate about as fast as the electrons in metal scatter their momentum - with a spin-valve, we could directly measure for the first time the fundamental parameters of Mott conduction. In particular, we found that the traditional measurements performed on the slower timescales significantly underestimate the spin-asymmetry in electron scattering which is responsible for the magnetic sensor operation.

Dmitry Turchinovich, project leader at the MPI-P.

The research team led by Dmitry Turchinovich (MPI-P) and Mathias Kläui (JGU) included Zuanming Jin, Alexander Tkach, Frederick Casper, Victor Spetter, Hubert Grimm, Andy Thomas, Tobias Kampfrath, and Mischa Bonn.

The research work was contributed by the Fritz Haber Institute of the Max Planck Society and Sensitec GmbH. This work delivers an innovative and versatile solution, ultrafast THz spectroscopy for spintronics investigations, paving way for a new research platform called terahertz spintronics.

The findings of the work were published in the Nature Physics journal.

References

• Fundamental observation of spin-controlled electrical conduction in metals

Abstract-Terahertz Depolarization Effects in Colloidal TiO2 Films Reveal Particle Morphology

Soeren Alkaersig Jensen , Klaas-Jan Tielrooij , Euan Hendry , Mischa Bonn , Ivan Rychetský , and Hynek Soeren 

J.Phys.Chem. C, Just Accepted Manuscript

DOI: 10.1021/jp406897y

Publication Date (Web): December 10, 2013

Copyright © 2013 American Chemical Society
http://pubs.acs.org/doi/abs/10.1021/jp406897y

Films of colloidal TiO2 nanoparticles are widely used in photovoltaic and photocatalytic applications, and the nature of electrical conductivity in such materials is therefore of both fundamental and practical interest. The conductive properties of colloid TiO2 films depend strongly on their morphology and deviate greatly from the properties of the bulk material. We report ultrafast photoconductivity studies of films consisting of sintered TiO2 particles of very different sizes performed using time-resolved Terahertz spectroscopy. Remarkably, identical photoconductivity spectra are observed for films of particles with diameters of tens and hundreds of nm respectively. The independence of photoconductivity on particle size directly demonstrates that the terahertz photoconductive response of colloidal TiO2 films is not affected by carrier backscattering at particle boundaries as has previously been concluded, but rather by depolarization fields resulting from the spatial inhomogeneities in the dielectric function inherent to these types of films. Modelling of the influence of depolarization fields on the terahertz conductivity allows us to explain the measured data and gain insights into the morphology of the film. Specifically, we show that the observed photoconductivity spectra reflect percolated pathways in the colloidal TiO2 nanoparticles films, through which charge carrier diffusion can occur over macroscopic length scales.