Abstract-Non-plasmonic improvement in photoconductive THz emitters using nano- and micro-structured electrodes

 

Abhishek Singh, Malte Welsch, Stephan Winnerl, Manfred Helm, Harald Schneider, 

Simulation of electric field amplitude ((Ex2+Ey2+Ez2)1/2 in V/m) distribution on a slice passing through the center of the emitter in yz-plane in the photoconductor when 10 V bias is applied to the electrodes. Field lines are drawn to show the direction of the electric field. A white dashed line is drawn at a depth (∼ 1 µm) of penetration depth of 800 nm in GaAs.

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-28-24-35490&id=442489

We investigate here terahertz enhancement effects arising from micrometer and nanometer structured electrode features of photoconductive terahertz emitters. Nanostructured electrode based emitters utilizing the palsmonic effect are currently one of the hottest topics in the research field. We demonstrate here that even in the absence of any plasmonic resonance with the pump pulse, such structures can improve the antenna effect by enhancing the local d.c. electric field near the structure edges. Utilizing this effect in Hilbert-fractal and grating-like designs, enhancement of the THz field of up to a factor of ∼ 2 is observed. We conclude that the cause of this THz emission enhancement in our emitters is different from the earlier reported plasmonic-electrode effect in a similar grating-like structure. In our structure, the proximity of photoexcited carriers to the electrodes and local bias field enhancement close to the metallization cause the enhanced efficiency. Due to the nature of this effect, the THz emission efficiency is almost independent of the pump laser polarization. Compared to the plasmonic effect, these effects work under relaxed device fabrication and operating conditions.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Abstract-Up to 70 THz bandwidth from an implanted Ge photoconductive antenna excited by a femtosecond Er:fibre laser

• Abhishek Singh, 
• Alexej Pashkin,  
• Stephan Winnerl, 
• Malte Welsch, 
• Cornelius Beckh, 
• Philipp Sulzer, 
• Alfred Leitenstorfer, 
• Manfred Helm,  
• Harald Schneider, 

Ultrabroadband THz emission from a Ge:Au antenna pumped at 1100 nm.

https://www.nature.com/articles/s41377-020-0265-4

Phase-stable electromagnetic pulses in the THz frequency range offer several unique capabilities in time-resolved spectroscopy. However, the diversity of their application is limited by the covered spectral bandwidth. In particular, the upper frequency limit of photoconductive emitters - the most widespread technique in THz spectroscopy – reaches only up to 7 THz in the regular transmission mode due to absorption by infrared-active optical phonons. Here, we present ultrabroadband (extending up to 70 THz) THz emission from an Au-implanted Ge emitter that is compatible with mode-locked fibre lasers operating at wavelengths of 1.1 and 1.55 μm with pulse repetition rates of 10 and 20 MHz, respectively. This result opens up the possibility for the development of compact THz photonic devices operating up to multi-THz frequencies that are compatible with Si CMOS technology.

A laser for penetrating waves

An international research team has been able to show that it is relatively easy to generate terahertz waves with an alloy of mercury, cadmium and tellurium. To examine the behavior of the electrons in the material, the physicists use the free-electron laser FELBE at HZDR. Circularly polarized terahertz pulses (orange spiral) excite the electrons (red) from the lowest to the next higher energy level (parabolic shell). The energy gap of these so-called Landau levels can be adjusted with the help of a magnetic field. CREDIT HZDR / Juniks

https://www.eurekalert.org/multimedia/pub/208908.php

Research team develops a new principle to generate terahertz radiation

The "Landau-level laser" is an exciting concept for an unusual radiation source. It has the potential to efficiently generate so-called terahertz waves, which can be used to penetrate materials as well as for future data transmission. So far, however, nearly all attempts to make such a laser reality have failed. An international team of researchers has now taken an important step in the right direction: In the journal Nature Photonics (DOI: 10.1038/s41566-019-0496-1), they describe a material that generates terahertz waves by simply applying an electric current. Physicists from the German research center Helmholtz-Zentrum Dresden-Rossendorf (HZDR) played a significant role in this project.

Like light, terahertz waves are electromagnetic radiation, in a frequency range between microwaves and infrared radiation. Their properties are of great technological and scientific interest, as they allow fundamental researchers to study the oscillations of crystal lattices or the propagation of spin waves. Simultaneously "terahertz waves are of interest for technical applications because they can penetrate numerous substances that are otherwise opaque, such as clothing, plastics and paper," HZDR researcher Stephan Winnerl explains. Terahertz scanners are already used today for airport security checks, detecting whether passengers are concealing dangerous objects under their clothing - without having to resort to harmful X-rays.

Because terahertz waves have a higher frequency than the radio waves we use today, they could also be harnessed for data transmission one day. Current WLAN technology, for instance, operates at frequencies of two to five gigahertz. Since terahertz frequencies are about a thousand times higher, they could transmit images, video, and music much faster, albeit across shorter distances. However, the technology is not yet fully developed. "There has been a lot of progress in recent years," Winnerl reports. "But generating the waves is still a challenge - experts speak of a veritable terahertz gap." A particular issue is the lack of a terahertz laser that is compact, powerful, and tunable at the same time.

Flexible frequencies

Laser light is generated by the electrons in the laser material. According to the quantum effect, energized electrons emit light, but they cannot absorb just any random amount of energy, only certain portions. Accordingly, light is also emitted in portions, in a specific color and as a focused beam. For some time now, experts have set their sights on a specific concept for a terahertz laser: the "Landau-level laser". It is special because it can use a magnetic field to flexibly adjust the electrons' energy levels. These levels, in turn, determine the frequencies that are emitted by the electrons, which makes the laser tunable - a huge advantage for many scientific and technical applications.

There is just one issue: Such a laser does not exist yet. "So far, the problem has been that the electrons pass their energy on to other electrons instead of emitting them as the desired light waves," Winnerl explains. Experts call this physical process the "Auger effect". To their chagrin, this phenomenon also occurs in graphene, a material that they deemed particularly promising for a "Landau-level laser". This two-dimensional form of carbon showed strong Auger scattering in HZDR experiments.

A question of material

The research team therefore tried another material: a heavy metal alloy of mercury, cadmium, and tellurium (HgCdTe) that is used for highly sensitive thermal imaging cameras, among other things. The special feature of this material is that its mercury, cadmium, and tellurium contents can be very precisely chosen, which makes it possible to fine-tune a certain property that experts call the "band gap".

As a result, the material showed properties similar to graphene, but without the issue of strong Auger scattering. "There are subtle differences to graphene that avoid this scattering effect," says Stephan Winnerl. "Put simply, the electrons can't find any other electrons that could absorb the right amount of energy." Therefore, they have no choice but to get rid of their energy in the form that the scientists want: terahertz radiation.

The project was an international team effort: Russian partners had prepared the HgCdTe samples, which the project's lead group in Grenoble then analyzed. One of the pivotal investigations took place in Dresden-Rossendorf: Using the free-electron laser FELBE, experts fired strong terahertz pulses at the sample and were able to observe the electrons' behavior in temporal resolution. The result: "We noticed that the Auger effect that we had observed in graphene had actually disappeared," Winnerl is happy to report.

LED for Terahertz

Lastly, a work group in Montpellier observed that the HgCdTe compound actually emits terahertz waves when electric current is applied. By varying an additional magnetic field of only about 200 millitesla, the experts were able to vary the frequency of the emitted waves in a range of one to two terahertz - a tunable radiation source. "It's not quite a laser yet, but rather like a terahertz LED," Winnerl describes. "But we should be able to extend the concept to a laser, even though it will take some effort." And that's exactly what the French partners want to tackle next.

There is one limiting factor, however: Up to now, the principle has only worked when cooled to very low temperatures, just above absolute zero. "This is certainly a hindrance for everyday applications," Winnerl summarizes. "But for use in research and in certain high-tech systems, we should be able to make it work with this kind of cooling."

###

Publication:

D.B. But, M. Mittendorff, C. Consejo, F. Teppe, N.N. Mikhailov, S.A. Dvoretskii, C. Faugeras, S. Winnerl, M. Helm, W. Knap, M. Potemski, M. Orlita: Suppressed Auger scattering and tunable light emission of Landau-quantized massless Kane electrons, in Nature Photonics, 2019 (DOI: 10.1038/s41566-019-0496-1)

Abstract-Picosecond-scale Terahertz pulse characterization with field-effect transistors

Stefan Regensburger, Stephan Winnerl,  J. Michael Klopf,  Hong Lu, Arthur C. Gossard,  Sascha Preu

https://ieeexplore.ieee.org/document/8662700

Precise real-time detection of Terahertz pulses is a key requirement for characterization of pulsed Terahertz sources and non-destructive testing applications. We experimentally evaluate the speed limits of Terahertz rectification in field-effect transistors using the example of pulses from a free-electron laser. We develop an improved model for the description of these Terahertz pulses and demonstrate its validity experimentally by comparison to spectroscopic data as well as to expectation values calculated from free-electron laser physics. The model in conjunction with the high speed of the detectors permits the detection of an exponential rise time of the pulses as short as 5 ps despite a Gaussian post detection time constant of 11 and 14 ps for a large area and an antenna-coupled detector, respectively. This proves that field-effect transistors are excellent compact, room-temperature Terahertz detectors for applications that require an intermediate frequency bandwidth of several tens of GHz.

Abstract-Gapless broadband terahertz emission from a germanium photoconductive emitter

Abhishek Singh, Alexej Pashkin, Stephan Winnerl, Manfred Helm,  Harald Schneider

https://pubs.acs.org/doi/10.1021/acsphotonics.8b00460

Photoconductive terahertz (THz) emitters have been fulfilling many demands required for table-top THz time-domain spectroscopy up to 3-4 THz. In contrast to the widely used photoconductive materials such as GaAs and InGaAs, Ge is a non-polar semiconductor characterized by a gapless transmission in the THz region due to absence of one-phonon absorption. We present here the realization of a Ge-based photoconductive THz emitter with a smooth broadband spectrum extending up to 13 THz and compare its performance with a GaAs-based analogue. We show that the spectral bandwidth of the Ge emitter is limited mainly by the laser pulse width (65 fs) and, thus, can be potentially extended to even much higher THz frequencies.

Abstract-Terahertz dephasing of Landau level transitions in graphene

Harald Schneider, Jacob C. König-Otto,  Alexej Pashkin, Yongrui Wang, Alexey Belyanin, Manfred Helm,  Stephan Winnerl

http://ieeexplore.ieee.org/document/8066873/

Using degenerate four-wave mixing (DFWM), we have investigated the coherent polarization between the lowest Landau levels in graphene under resonant excitation with narrowband THz pulses. A pronounced DFWM signal is observed and its dependence on THz field strength and magnetic field detuning is explored and compared with theoretical expectations.

Abstract-Plasmonic efficiency enhancement at the anode of strip line photoconductive terahertz emitters

Abhishek Singh, Stephan Winnerl, Jacob C. König-Otto, Daniel R. Stephan, Manfred Helm, and Harald Schneider

https://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=964173647966952561

We investigate strip line photoconductive terahertz (THz) emitters in a regime where both the direct emission of accelerated carriers in the semiconductor and the antenna-mediated emission from the strip line play a significant role. In particular, asymmetric strip line structures are studied. The widths of the two electrodes have been varied from 2 µm to 50 µm. The THz emission efficiency is observed to increase linearly with the width of the anode, which acts here as a plasmonic antenna giving rise to enhanced THz emission. In contrast, the cathode width does not play any significant role on THz emission efficiency.

© 2016 Optical Society of America

Full Article  |  PDF Article

Abstract-Electron dynamics in silicon-germanium terahertz quantum fountain structures

Diego Sabbagh, Johannes Schmidt, Stephan Winnerl, Manfred Helm, Luciana Di Gaspare, Monica De Seta,Michele Virgilio, and Michele Ortolani

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.5b00561

Publication Date (Web): February 1, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.5b00561?journalCode=apchd5

Asymmetric quantum well systems are excellent candidates to realize semiconductor light emitters at far-infrared wavelengths not covered by other gain media. Group-IV semiconductor heterostructures can be grown on silicon substrates and their dipole-active intersubband transitions could be used to generate light from devices integrated with silicon electronic circuits. Here, we have realized an optically pumped emitter structure based on a three-level Ge/Si0.18Ge0.82 asymmetric coupled quantum well design. Optical pumping was performed with a tunable free-electron laser emitting at photon energies of 25 and 41 meV, corresponding to the energies of the first two intersubband transitions 0→1 and 0→2 as measured by Fourier-transform spectroscopy. We have studied with a synchronized terahertz time-domain spectroscopy probe the relaxation dynamics after pumping, and we have interpreted the resulting relaxation times (in the range 60 to 110 ps) in the framework of an out-of-equilibrium model of the intersubband electron-phonon dynamics. The spectral changes in the probe pulse transmitted at pump-probe coincidence were monitored in the range 0.7-2.9 THz for different samples and pump intensity and showed indication of both free carrier absorption increase and bleaching of the 1→2 transition. The quantification from data and models of the free carrier losses and of the bleaching efficiency allowed us to predict the conditions for population inversion and to determine a threshold pump power density for lasing around 500 kW/cm2 in our device. The ensemble of our results shows that optical pumping of germanium quantum wells is a promising route towards silicon-integrated far-infrared emitters.

Abstract-Gouy phase shift of a tightly focused, radially polarized beam

Korbinian J. Kaltnenecker, Jacob C. König-Otto, Martin Mittendorff, Stephan Winnerl, Harald Schneider, Manfred Helm, Hanspeter Helm, Markus Walther, and Bernd M. Fischer

https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-3-1-35&id=335319

Radially polarized beams represent an important member of the family of vector beams, in particular due to the possibility of using them to create strong and tightly focused longitudinal fields, a fundamental property that has been exploited by applications ranging from microscopy to particle acceleration. Since the properties of such a focused beam are intimately related to the Gouy phase shift, proper knowledge of its behavior is crucial. Terahertz microscopic imaging is used to extract the Gouy phase shift of the transverse and longitudinal field components of a tightly focused, radially polarized beam. Since the applied terahertz time-domain approach is capable of mapping the amplitude and phase of an electromagnetic wave in space, we are able to directly trace the evolution of the geometric phase as the wave propagates through the focus. We observe a Gouy phase shift of 2𝜋 for the transverse and of 𝜋 for the longitudinal component. Our experimental procedure is universal and may be applied to determine the geometric phase of other vector beams, such as optical vortices, or even arbitrarily shaped and polarized propagating waves.

© 2016 Optical Society of America

Abstract-Terahertz Nonlinearity in Graphene Plasmons

Mohammad M. Jadidi, Jacob C. König-Otto, Stephan Winnerl, Andrei B. Sushkov, H. Dennis Drew, Thomas E. Murphy, Martin Mittendorff

http://arxiv.org/abs/1512.07508
Sub-wavelength graphene structures support localized plasmonic resonances in the terahertz and mid-infrared spectral regimes. The strong field confinement at the resonant frequency is predicted to significantly enhance the light-graphene interaction, which could enable nonlinear optics at low intensity in atomically thin, sub-wavelength devices. To date, the nonlinear response of graphene plasmons and their energy loss dynamics have not been experimentally studied. We measure and theoretically model the terahertz nonlinear response and energy relaxation dynamics of plasmons in graphene nanoribbons. We employ a THz pump-THz probe technique at the plasmon frequency and observe a strong saturation of plasmon absorption followed by a 10 ps relaxation time. The observed nonlinearity is enhanced by two orders of magnitude compared to unpatterned graphene with no plasmon resonance. We further present a thermal model for the nonlinear plasmonic absorption that supports the experimental results.

Graphene flakes as an ultra-fast stopwatch

The external antenna on the detector captures long-wave infrared and terahertz radiation and funnels it to a graphene flake which is located in the center of the structure. Credit: M. Mittendorff
 
http://phys.org/news/2015-10-graphene-flakes-ultra-fast-stopwatch.html#jCp

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), working with colleagues from the US and Germany, have developed a new optical detector from graphene which reacts very rapidly to incident light of all different wavelengths and even works at room temperature. It is the first time that a single detector has been able to monitor the spectral range from visible light to infrared radiation and right through to terahertz radiation. The HZDR scientists are already using the new graphene detector for the exact synchronization of laser systems.

A tiny flake of graphene on silicon carbide and a futuristic-looking antenna, and there it is - the new graphene detector. Like no other single detector system which has gone before, this comparatively simple and inexpensive construct can cover the enormous spectral range from visible light all the way to terahertz radiation. "In contrast to other semiconductors like silicon or gallium arsenide, graphene can pick up light with a very large range of photon energies and convert it into electric signals. We only needed a broadband antenna and the right substrate to create the ideal conditions," explained Dr. Stephan Winnerl, physicist at the Institute of Ion Beam Physics and Materials Research at the HZDR.

Back in 2013 Martin Mittendorff, who was a PhD student at the HZDR at that time, had developed the precursor to the graphene detector. In his present position as a postdoc at the University of Maryland, he has now perfected it with his Dresden colleagues and with scientists from Marburg, Regensburg and Darmstadt. How it works: the graphene flake and antenna assembly absorbs the rays, thereby transferring the energy of the photons to the electrons in the graphene. These "hot electrons" increase the electrical resistance of the detector and generate rapid electric signals. The detector can register incident light in just 40 picoseconds - these are billionths of a second.

Wide spectral range achieved through silicon carbide substrate

The choice of substrate has now proved a pivotal step in improving the little light trap. "Semiconductor substrates used in the past have always absorbed some wavelengths but silicon carbide remains passive in the spectral range," explained Stephan Winnerl. Then there is also an antenna which acts like a funnel and captures long-wave infrared and terahertz radiation. The scientists have therefore been able to increase the spectral range by a factor of 90 in comparison with the previous model, making the shortest detectable wavelength 1000 times smaller than the longest. By way of comparison, red light, which has the longest wavelength visible to the human eye, is only twice as long as violet light which has the shortest wavelength on the visible spectrum.

This optical universal detector is already being used at the HZDR for the exact synchronization of the two free-electron lasers at the ELBE Center for High-Power Radiation Sources with other lasers. This alignment is particularly important for "pump probe" experiments, as they are called, where researcher take one laser for the excitation of a material ("pump") and then use a second laser with a different wavelength for the measurement ("probe"). The laser pulses must be exactly synchronized for such experiments. So the scientists are using the graphene detector like a stopwatch. It tells them when the laser pulses reach their goal, and the large bandwidth helps to prevent a change of detector from being a potential source of error. Another advantage is that all the measurements can take place at room temperature, obviating the need for the expensive and time-consuming nitrogen or helium cooling processes with other detectors.

 Explore further: How long do electrons live in graphene?

More information: Martin Mittendorff et al. Universal ultrafast detector for short optical pulses based on graphene, Optics Express (2015). DOI: 10.1364/OE.23.028728

Abstract-Terahertz response of patterned epitaxial graphene

Christian Sorger, , Sascha Preu1,2, Johannes Schmidt3, Stephan Winnerl3, Yuliy V Bludov4, Nuno M R Peres4, Mikhail I Vasilevskiy4 and Heiko B Weber1

http://iopscience.iop.org/1367-2630/17/5/053045

heiko.weber@fau.de

1 Department of Physics, Friedrich-Alexander-University Erlangen-Nuremberg (FAU), Staudtstraße 7, D-91058 Erlangen, Germany
2 Department of Electrical Engineering and Information Technology, Technical University Darmstadt, Merckstraße 25, D-64283 Darmstadt, Germany
3 Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, D-01314 Dresden, Germany
4 Department of Physics and Center of Physics, University of Minho, Campus de Gualtar, P-4710-057 Braga, Portugal 

We study the interaction between polarized terahertz (THz) radiation and micro-structured large-area graphene in transmission geometry. In order to efficiently couple the radiation into the two-dimensional material, a lateral periodic patterning of a closed graphene sheet by intercalation doping into stripes is chosen. We observe unequal transmittance of the radiation polarized parallel and perpendicular to the stripes. The relative contrast, partly enhanced by Fabry–Perot oscillations reaches 20%. The effect even increases up to 50% when removing graphene stripes in analogy to a wire grid polarizer. The polarization dependence is analyzed in a large frequency range from <80 GHz to 3 THz, including the plasmon–polariton resonance. The results are in excellent agreement with theoretical calculations based on the electronic energy spectrum of graphene and the electrodynamics of the patterned structure.

Abstract-Plasmonic Superlensing in Doped GaAs

Markus Fehrenbacher , Stephan Winnerl , Harald Schneider , Jonathan Döring , Susanne C. Kehr ,Lukas M. Eng , Yongheng Huo , Oliver G. Schmidt ,Kan Yao , Yongmin Liu , and Manfred Helm

Nano Lett., Just Accepted Manuscript

DOI: 10.1021/nl503996q

Publication Date (Web): January 13, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/nl503996q?journalCode=nalefd

We demonstrate a semiconductor based broadband near-field superlens in the mid-infrared regime. Here, the Drude response of a highly doped n-GaAs layer induces a resonant enhancement of evanescent waves accompanied by a significantly improved spatial resolution at radiation wavelengths around lambda=20um, adjustable by changing the doping concentration. In our experiments, gold stripes below the GaAs superlens are imaged with a lambda/6 sub-wavelength resolution by an apertureless near-field optical microscope utilizing infrared radiation from a free-electron laser. The resonant behavior of the observed superlensing effect is in excellent agreement with simulations based on the Drude-Lorentz model. Our results demonstrate a rather simple superlens implementation for infrared nanospectroscopy.

Determining the life of electrons in graphene for fast electronic and optoelectronic components

(Investigations of graphene were done with the Free Electron Laser at HZDR. Credit: (c) AlexanderAIUS / HZDR)


http://www.physorg.com/news/2011-12-electrons-graphene.html
Together with international colleagues, scientists from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have added another important component towards understanding the material graphene; a material that is currently receiving a lot of attention: They have determined the lifetime of electrons in graphene in lower energy ranges. This is of great relevance for the future development of fast electronic and optoelectronic components. The results were published just recently in the online edition of the journal Physical Review Letters.

After the discovery of graphene had been awarded the Nobel Prize in Physics last year, many research teams around the globe have been seeking to better understand the material's fundamental properties to permit such promising electronic and optoelectronic applications as transistors and rapid detectors for optical data transmission. Graphene – a single carbon layer that has its atoms arranged in a hexagon like a honeycomb – is also very interesting as a transparent electrode material for flat screens and solar cells. According to the HZDR researcher Dr. Stephan Winnerl, graphene might replace the scarce high tech metal indium in this field.
With subsidies from the German Research Foundation's Priority Program "Graphene" and funds from the European Union, Stephan Winnerl and his colleagues at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with scientists from the Technische Universität (TU) Berlin, the Grenoble High Magnetic Field Laboratory, and the Georgia Institute of Technology, USA, managed to determine the "lifetime" of electrons in graphene in lower energy ranges which had not been researched before.
The characteristic behavior of electrons in specific energy ranges typically found in solids is one of the many physical properties in which graphene is fundamentally different from most other materials: Normally, electrons can only adopt specific energy levels (these are referred to as energy bands), but not others (these are referred to as energy gaps). This principle is used, for example, for such optoelectronic components as light emitting diodes which emit light at very specific wavelengths: This releases energy which the electrons set free while "skipping over" energy gaps.
But graphene's behavior differs from other semiconductors: The energy bands touch each other without the appearance of any gaps. Instead of emitting light, graphene is capable of absorbing the radiation of lower energies below the visible spectrum, such as terahertz and infrared light; thus, making it a superb material for detectors.

To be able to develop rapid electronic and optoelectronic components based on graphene, one has to know precisely how long electrons linger at specific energy levels. The examination of such processes, which occur in the picosecond range, i.e. the time scale of one millionth of a millionth second, requires extremely rapid observation methods. The unique feature of the experiments conducted at the Helmholtz-Zentrum in Dresden is the exposure of the graphene samples to light that had longer wavelengths than ever before. This was made possible through the short radiation pulses of the HZDR's Free Electron Laser (FEL). The researchers were, thus, able to study the lifetime of electrons near the contact point of the energy bands which is the unique physical property characteristic of graphene.
The FEL excited the graphene samples with light that had different wavelengths in the infrared range. The researchers discovered that the energy of the light particles exciting the electrons as well as the oscillations of the atomic lattice influence the lifetime of the electrons: If the energy of the light particles is greater than the energy of the lattice oscillations, then the electrons will alter their energy state more rapidly and have a shorter lifetime. Conversely, the electrons will linger longer at a specific energy level if the excitation energy is lower than the energy of the lattice oscillations.
The insights gained from the experiments are substantiated by model calculations from the TU Berlin. These calculations permit a clear assignment of the experimental data to the physical mechanisms in graphene. The researchers have, thus, made a valuable contribution towards a better understanding of the electronic and optical properties of graphene.
More information: Winnerl, S.; Orlita, M.; Plochocka, P; Kossacki, P.; Potemski, M.; Winzer,T.; Malic, E.; Knorr, A.; Sprinkle, M.; Berger, C.; de Heer, W. A.; Schneider, H.; & Helm, M. (2011). Carrier dynamics in epitaxial graphene close to the Dirac point. Physical Review Letters 107, 237401, DOI: 10.1103/PhysRevLett.107.237401
Provided by Helmholtz Association of German Research Centres (news : web)