3.2 Solids and Nanostructures: Electrons, Spins, and Phonons
Project coordinator(s): C. von Korff Schmising, M. Wörner
Recent Highlights


Highlights in Topics


"Progress in Nonlinear Nano-Optics"

Recently it was reported that the eBook edition of our book "Progress in Nonlinear Nano-Optics", (Editors: M. Sakabe, C. Lienau, R. Grunwald, Springer-Verlag, Heidelberg, 2014) was among the 25% most downloaded eBooks in the relevant Springer Book Collection with 7,435 chapter downloads in the year 2015. Contact: R. Grunwald

28 April 2016:

Quantum Swing - a pendulum that moves forward and backwards at the same time

18 February 2016:

Amplification of Sound Waves at Extreme Frequencies

28th September 2015:

Hot means slow: Electron plasma oscillations tuned down with light

Oscillations of an optically heated electron plasma depend sensitively on the plasma temperature. Ultrafast heating and cooling of a plasma in the semiconductor zinc oxide (ZnO) leads to pronounced shifts of plasma frequency, holding a strong potential for novel switching applications in optoelectronics.

A plasma is a special state of matter in which a large number of electrons form are negatively charged cloud of particles which is separated from a positively charged background of ions. Plasma exists in many systems including hot stars, the ionosphere and other ionized gases, as well as solid state materials. The electric forces between electrons and ions allow for generating periodic spatial motions of the electron cloud relative to the ions, so called plasma oscillations or plasmons. Recently, plasmons in metals and semiconductors have received strong interest. They display peculiar optical properties and hold strong potential for applications in high-speed optoelectronics and optical microscopy with sub-wavelength spatial resolution.

A basic and interesting question is: Can one manipulate plasma oscillations with light and, in particular, modify their frequency? This would allow for switching the electric and optical properties for a short period of time, changes most helpful for novel optoelectronic devices. In the current issue of Physical Review Letters [115, 147401 (2015)], a joint research team from the Max-Born-Institute and Humboldt University in Berlin demonstrates a novel concept for ultrafast plasmon switching in the semiconductor ZnO (Movie). In their experiments, the researchers investigated plasma oscillations in a 100 nanometer thick crystalline ZnO layer containing a high density of approximately 1020 free electrons per cubic centimeter. Plasma oscillations are excited by an infrared pulse of 150 fs duration (1 fs = 10-15 s) and the frequency shift of the infrared plasmon absorption band is measured with a second delayed and much weaker probe pulse. The shift of the absorption band allows for extracting the momentary plasma frequency as a function of time (Fig. 1). The experiments give direct evidence of a transient shift of plasma oscillations to lower frequency. The strong frequency reduction by 20% lasts for only 400 fs after which the original plasma frequency is restored. Over the period of the experiment, the electron density remains unchanged.

The physical origin of the frequency reduction lies in the transient heating of the electron plasma by the infrared excitation pulse. The electrons reach a peak temperature of ≈3300 K and populate a very wide range of the conduction band of ZnO (Fig. 2). In this range, the average electron mass is higher than in the initial state and, thus, the plasma frequency is reduced. The hot electrons transfer most of their thermal excess energy to the crystal lattice within some 400 fs. As a result, both the average electron mass and the plasma frequency return to their original values. All experimental observations are in excellent agreement with theoretical calculations.

TybPressAbb2 Fig. 1: Experimentally observed time-dependent shift of the plasma frequency in a thin ZnO layer. Left: 3D-plot of the absorption change as a function of the probe frequency and time delay between pump and probe pulses. Right: concept of a transient difference spectrum. The cold plasma (blue) shows an absorption peak at the plasma frequency of the cold electron gas. The pump pulse heats the plasma resulting in a red-shift of the plasmon resonance (red). In the time-resolved experiments we measured the so called difference spectrum, i.e., the absorption of the hot plasma minus that of the cold plasma (black).
TybPressAbb1 Fig. 2: The conduction band of ZnO shows a non-parabolic band structure, i.e., the electron energy as a function of the electron momentum follows a hyperbola rather than a parabola. As a result electrons at the conduction band minimum are quite light (low energy, small mass) compared to the much heavier electrons (large mass) at high energies. A cold plasma (left) contains essentially light electrons whereas a hot plasma (right) contains many heavy electrons at high energies.
Movie Animation: Right: plasma oscillations in a thin ZnO layer. Negatively charged electrons (blue clouds) oscillate collectively versus the positively charged ions (red dots). Left: Such a plasma oscillation resembles strongly a classical pendulum, i.e., a massive ball hanging on a elastic spring. (i) For negative times t < 0 (time counter upper left), the oscillation frequency is quite high due to the small mass of the electrons. (ii) During the period 0 < t < 100 fs fs the pump pulse heats the electron plasma (lighters below, temperature display upper right) resulting in an elevated mass of the electrons in ZnO (right) or increased mass in the pendulum (left). (iii) For t >100 fs the probe pulse measures the plasma oscillation frequency again, now showing a distinctly slower motion.

Original Publication: Physical Review Letters 115, 147401
Ultrafast Nonlinear Response of Bulk Plasmons in Highly Doped ZnO Layers

Tobias Tyborski, Sascha Kalusniak, Sergey Sadofev, Fritz Henneberger, Michael Woerner, and Thomas Elsaesser


Dr. Michael Woerner Tel. 030 6392 1470
Prof. Dr. Thomas Elsaesser Tel. 030 6392 1400

AC/DC for terahertz waves - rectification with picosecond clock rates

09th April 2014

Researchers at the Max-Born-Institute in Berlin, Germany discover an ultrafast rectifier for terahertz radiation. In the unit cells of a lithium niobate crystal alternating currents (AC) with a frequency 1000 times higher than that of modern computer systems are transformed into a direct current (DC), thereby generating simultaneously a series of overtones of the terahertz radiation.

When the guitarist Angus Young of the Australian hard rock band AC/DC touches the strings of his electric guitar, a strongly distorted sound rings out from the loudspeaker. The origin of the electronically generated overtones is the rectifying effect in the electronic tubes of the guitar amplifier. In the simplest case an (A)lternating (C)urrent generates a (D)irect (C)urrent, an effect which finds its application in telecommunications at much higher radio or mobile phone frequencies. From a physics point of view the highly interesting question arises: up to which cut-off frequencies can one generate directed currents (DC) and which microscopic mechanisms underlie them?

For the generation of a direct current out of alternating currents the material used must feature a preferred direction. This condition is fulfilled by ferroelectric crystals, in which the spatial separation of positively and negatively charged ions is connected to a static electric polarization. Most ferroelectrics are electric insulators, i.e., low electric fields cannot cause any detectable electric currents in the material. A drastic change of this behavior is observed if one applies for a short period an extremely high electric field in the range of several 100.000 volts per centimeter. At such field strengths, bound electrons, the so called valence electrons can be freed for a short period by means of the quantum mechanical tunneling process leading in turn to a current through the crystal.

Now, researchers at the Max-Born-Institute in Berlin, Germany investigated the properties of such a current for the first time and report their results in the current issue of the journal Physical Review Letters 112.146602 (2014)). Using ultrashort, intense terahertz pulses (1 Terahertz = 1012 Hz, period of a field oscillation 1 picosecond=10-12 seconds) they applied an AC field to a thin lithium niobate (LiNbO3) crystal which causes an electric current in the material. The properties of this current were studied in detail by measuring and analyzing the electric field radiated by the accelerated electrons. Besides an oscillating current with the frequency of the applied terahertz field (2 THz) and several overtones of the latter, the researchers observed the signature of a directed current (DC) along the c-axis the preferred direction of the ferroelectric LiNbO3 crystal.

The rectified current along the ferroelectric c-axis has its origin in the interplay of quantum mechanical tunneling of electrons between the valence and several conduction bands of the LiNbO3 crystal and the deceleration of electrons by friction processes. The tunneling process generates free electrons which in absence of friction would spatially oscillate in time with the applied terahertz field. The friction destroys this oscillatory motion, a mechanism called decoherence. Due to the asymmetry of the tunneling barrier along the ferroelectric c-axis decoherence results in a spatially asymmetric transport, i.e., the tunneling barrier lets pass more electrons from right to left than from left to right. This mechanism is operative within each unit cell of the crystal, i.e., on a sub-nanometer length scale, and causes the rectification of the terahertz field. The effect can be exploited at even higher frequencies, offering novel interesting applications in high frequency electronics.

Original article:
C. Somma, K. Reimann, C. Flytzanis, T. Elsaesser, und M. Woerner:
High-Field Terahertz Bulk Photovoltaic Effect in Lithium Niobate
Physical Review Letters 112.146602 (2014)

Dr. Martin Hempel receives the Adlershof Dissertation Prize 2013

14th February 2014

On February 13, the Adlershof Dissertation Award was granted to Dr. Martin Hempel of the Max-Born-Institute (MBI). In competition with two other nominees and according to the jury's unanimous opinion, he presented best his dissertation work "Defect Mechanism in Diode Lasers at High Optical Output Power: The Catastrophic Optical Damage".

As a student, Martin Hempel was fascinated by research on novel semiconductor diode lasers. As a PhD student at MBI, he investigated the ultimate performance limits of these universal devices. His research was focused on the so-called Catastrophic Optical Damage (COD), a defect mechanism that affects semiconductor diode lasers at very high optical intensities. A volume of ∼1 µm3 of the laser crystal is heated up to ∼1600°C by re-absorbed laser light. This temperature jump takes place in just 1 ns. Subsequently, the COD defect expands in the laser cavity with a velocity of about 90 km/h. The entire diode laser is degraded after a few µs. The final COD defect volume is approx. 1000 times larger than the initial COD starting point. Hempel was able to resolve the spatio-temporal dynamics of the COD process by a combination of methods. His results provide new insight into the physical mechanisms behind the COD. Based on this knowledge, it is possible to identify the root causes for device failures in an efficient and fast manner. In close cooperation with manufacturers of diode lasers this makes it possible to increase output power and reliability of the devices.


About the prize


Each year since 2002, IGAFA, Humboldt University and WISTA-MANAGEMENT GmbH jointly award the Adlershof Dissertation Prize to young scientists for PhD research which was conducted at one of the research institutions in Adlershof and got an excellent grade of the scientific quality of their work. Three selected nominees present their dissertation work in form of short talks to a general audience. The jury then makes the decision which of the finalists not only conducts excellent research but also explains complex topics enthusiastically and in simple terms.

Figure 1 (click to enlarge)  

COD defect pattern in the active layer of a diode laser (grey shaded area). The thermal signature of the COD defect front was mapped temporally resolved by a thermocamera. This was done by monitoring the side of the laser cavity (along z axis) and the front facet (along x) in parallel. The center of gravity of these signals, seen form two sides of the device, gives the position of the defect front (black dots). Thermography is demonstrated to be a tool to resolve the motion of the COD defect front temporally and spatially at once. In the upper part, thermal images are shown, record at different times while looking on the front facet. The corresponding positions in the x-z-diagram are marked. The constant signal amplitude indicates a constant temperature during the entire COD defect growth. Evaluating the temporal and spatial data gives a velocity of ∼90 km/h for the movement of the defect front along the laser cavity.

Figure 2 (click to enlarge)  
rogue-event Electron microscopy image of a device damaged by COD. The front facet is indicated by the dotted line on the left side. The material extrusion is clearly visible there. The degradation of the active laser layer starts at the front facet and is expanded up to the right edge of the image. A crystallographic interpretation of the changes of the material composition in the damaged volume indicates the presence of a temperature around 1600°C
Figure 3 (click to enlarge))  


Dr. Martin Hempel, +49 (0) 30-6392 1453

Generation of terahertz radiation by ionising two-color femtosecond pulses in gases

23rd January 2012

Scientists of the Max-Born- Institute have elucidated theoretically in cooperation with external partners the basic mechanism of the emission of Terahertz radiation in gases, provided an experimental evidence for this and opened new possibilities for the control of the Terahertz spectral parameters. Terahertz radiation (1 terahertz=1THz=1012Hz=1012cycles per second) is light with extremely large wavelength of about 0.3 mm. A frequency of 1 THz is about 50 times higher than the working frequency of cell phones. Nowadays THz radiation finds broad applications, such as for wireless data transfer or for the analysis of materials. Also the full-body scanners at airports use THz radiation for the observations of objects. In research THz pulses of extremely short duration are used for the investigation of basic properties of solids and liquids, e.g. charge transport and electric resistance. These investigations require the generation of short THz flashes which can be realized by the ionisation of gases with ultrashort light pulses. Among the various THz sources, employing two-color femtosecond pulses in gases provides striking performance in terms of high peak fields (up to the MV/cm range) as well as broad bandwidth (that can exceed 100 THz). Although already observed in the year 2000 and after that studied and applied in many papers, the physical mechanism behind the THz radiation is still controversy discussed in the literature. Initially, this process has been explained by rectification via third-order nonlinearity and later by the plasma current generated by the two-color laser field. Our theoretical study [1] and experiments at the MBI [2] related with this study show that THz generation in gases is intrinsically connected to the optically-induced stepwise increase of the plasma density near the maximum amplitudes of the pump fields due to tunneling ionisation leading to the associated emission of a discrete set of attosecond-scale, ultra-broadband bursts. The spectrum is therefore determined by the interference of contributions arising from different ionisation events, showing a remarkable analogy to the linear diffraction theory of gratings. Comprehensive (3+1)-dimensional numerical simulations confirmed this model which offers simple explanations for recent experimental observations and opens new avenues for the governing of THz parameters and THz pulse shapes based on temporal control of the ionization events (such as by the frequency offset of the pump pulses) [1]. The implementation of experiments at the MBI enabled to test this new understanding of THz emission directly by experimental observations [2]. The measured strong broadening of the THz spectra with increasing gas pressure has shown, in good agreement with (3+1)-dimensional numerical simulations, a sensitive dependence on the gas pressure enabling important insight into the basic mechanism of THz emission and the prominent role of propagation effects of the pump pulses. Plasma-induced blueshifts of the driving pulses play a key role in the broadening of the THz spectra with increasing pressure, and also deliver an experimental evidence of the above described mechanism in which THz emission is associated with the step-wise modulation of the tunnelling ionisation current.

Fig.: The mechanism of THz generation. The two-color field E(t) (red) generates free electrons with a step-wise modulation of the electron density (green) near the field amplitude maximum via tunnelling ionisation. This leads to a current (blue pointed) which acts as a source of THz emission. Inset: scheme of the experimental setup.





[1] I. Babushkin, S. Skupin, A. Husakou, C. Köhler, E. Cabrera-Grenado, L. Berge and J. Herrmann “Tailoring terahertz radiation by controlling tunnel ionisation events in gases “, New Journal of Physics 13, 123029 (2011)

[2] I. Babushkin, W. Kuehn, C. Köhler, S. Skupin, L. Berge, K. Reiman, M. Woerner, J. Herrmann and T. Elsaesser“, Ultrafast spatiotemporal dynamics of terahertz radiation by ionising two-color femtosecond pulses in gases “,Phys. Rev. Lett. 105, 053903 (2010)


Dr. J. Herrmann, Tel. 030 63921278



The onset of electrical resistance

16th December 2011

Researchers at the Max-Born-Institute, Berlin, Germany, observed the extremely fast onset of electrical resistance in a semiconductor by following electron motions in real-time.

When you first learned about electric currents, you may have asked how the electrons in a solid material move from the negative to the positive terminal. In principle, they could move ballistically or ‘fly’ through the solid, without being affected by the atoms or other charges of the material. But this actually never happens under normal conditions because the electrons interact with the vibrating atoms or with impurities. These collisions typically occur within an extremely short time, usually about 100 femtoseconds (10-13 seconds, or a tenth of a trillionth of a second). So the electron motion along the material, rather than being like running down an empty street, is more like trying to walk through a very dense crowd. Typically, electrons move only with a speed of 1m per hour, they are slower than snails.

Though the electrons collide with something very frequently in the material, these collisions do take a finite time to occur. Just like if you are walking through a crowd, sometimes there are small empty spaces where you can walk a little faster for a short distance. If it were possible to follow the electrons on an extremely fast (femtosecond) time scale, then you would expect to see that when the battery is first turned on, for a very short time, the electrons really do fly unperturbed through the material before they bump into anything. This is exactly what scientists at the Max-Born-Institute in Berlin recently did in a semiconductor material and report in the current issue of the journal Physical Review Letters [volume 107, 256602 (2011)]. Extremely short bursts of terahertz light (1 terahertz = 1012 Hz, 1 trillion oscillations per second) were used instead of the battery (light has an electric field, just like a battery) to accelerate optically generated free electrons in a piece of gallium arsenide. The accelerated electrons generate another electric field, which, if measured with femtosecond time resolution, indicates exactly what they are doing. The researchers saw that the electrons travelled unperturbed in the direction of the electric field when the battery was first turned on. About 300 femtoseconds later, their velocity slowed down due to collisions.

In the attached movie, we show a cartoon of what is happening in the gallium arsenide crystal. Electrons (blue balls) and holes (red balls) show random thermal motion before the terahertz pulse hits the sample. The electric field (green arrow) accelerates electrons and holes in opposite directions. After onset of scattering this motion is slowed down and results in a heated electron-hole gas, i.e., in faster thermal motion.

The present experiments allowed the researchers to determine which type of collision is mainly responsible for the velocity loss. Interestingly, they found that the main collision partners were not atomic vibrations but positively charged particles called holes. A hole is just a missing electron in the valence band of the semiconductor, which can itself be viewed as a positively charged particle with a mass 6 times higher than the electron. Optical excitation of the semiconductor generates both free electrons and holes which the terahertz bursts, our battery, move in opposite directions. Because the holes have such a large mass, they do not move very fast, but they do get in the way of the electrons, making them slower.
Such a direct understanding of electric friction will be useful in the future for designing more efficient and faster electronics, and perhaps for finding new tricks to reduce electrical resistance.

Two former PhD students of the Max-Born-Institute receive the Carl Ramsauer Prize 2011

14th November 2011

lDr. Christian Eickhoff and Dr. Wilhelm Kühn are among this year's winners of the Carl Ramsauer Prize of the Physikalische Gesellschaft zu Berlin (PGzB), an award for outstanding PhD work.
Christian Eickhoff (third from left in the picture) received his doctor degree at the Freie Universität Berlin and Wilhelm Kühn (second from left in the picture) is awarded for his doctoral thesis at Humboldt University. They did their research work at the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (MBI). The prizes will be awarded on November 17th, 2011, at the Physics Department of FU Berlin.

Dr. Christian Eickhoff's dissertation was devoted to “Time-resolved two-photon photoemission at the Si(001)-surface: Hot electron dynamics and two-dimensional Fano resonance.” He investigated the (100) surface of silicon, the basic material of the present semiconductor technology. The ongoing miniaturization and increasing efficiency of devices require a basic understanding of the electronic and and dynamic properties of carriers at the Si surface. Eickhoff designed a complex experiment consisting of a laser system which provides widely tunable photon energies, and a ultrahigh vacuum setup including a two-dimensional detector for photoelectrons. He observed the dynamics of optically excited electrons in surface states and in the conduction band of silicon with a time resolution of several tens of femtoseconds (1 fs = 1015 sec). He also investigated the influence of elastic and inelastic scattering processes on energy relaxation and surface recombination. He showed how optical excitation of carriers leads to complex two-dimensional interference phenomena which are accounted for by Fano resonances in the initial and intermediate state. His work explained why the elevated temperature of hot electrons in the conduction band is maintained over an unexpectedly long period of many picoseconds (1 ps = 1012 sec). The extraction of hot electrons may find application for increasing the efficiency of third-generation solar cells.

Dr. Wilhelm Kühn has developed a novel method of nonlinear spectroscopy in the THz frequency domain (1 THz = 1012 Hz) and applied it in basic solid state research. The nonlinear interaction of light and matter is measured in two independent dimensions in time to derive two-dimensional (2D) spectra in the frequency domain. Such spectra give insight into the couplings between different excitations of the system under study and their time evolution. THz oscillations are extremely slow compared to visible light, an oscillation period lasts approximately 250 fs. Focusing the THz beam allows for generating high electric fields (ca. 300 kV/cm) and applying them for the acceleration of charge carriers in solids. With this novel method, Kühn studied the transport properties of electrons in the semiconductor gallium arsenide (GaAs). He observed that strongly accelerated electrons move without appreciable friction ('ballistic motion'). At very high kinetic energies, however, they slow down and are accelerated into the opposite direction, due to their negative effective mass. In this way, the resulting circular motion of electrons, the so-called Bloch oscillations, was observed for the first time in a bulk crystal.
In a second experiment, Kühn investigated a semiconductor model system displaying an extremely strong coupling of electrons and the crystal lattice. He was able to show the formation of a new particle, the polaron which consists of an electron and a cloud of phonons. His results which were confirmed by theoretical calculations, also demonstrate how and into which channels electron energy is transferred to the crystal lattice.

Electrons and Lattice Vibrations—A Strong Team in the Nano World

4th August 2011

Semiconductor electronics generates, controls, and amplifies electrical current in devices like the transistor. The carriers of the electric current are mobile electrons, which move with high velocities through the crystal lattice of the semiconductor. Doing this, they lose part of their kinetic energy by causing atoms in the lattice to vibrate. In semiconductors like gallium arsenide the positively and negatively charged ions of the crystal lattice vibrate with an extremely short period of 100 fs (1 fs = 10-15 s = 1 billionth part of one millionth of a second). In the microcosm of electrons and ions such vibrations are quantized. This means that the vibrational energy can only be an integer multiple of a vibrational quantum, also known as a phonon. When an electron interacts with the crystal lattice (the so called electron-phonon interaction), energy is transferred from the electron to the lattice in the form of such vibrational quanta.

Berlin researchers report in the latest issue of the scientific journal Physical Review Letters that the strength of the electron-phonon interaction depends sensitively on the electron size, i.e., on the spatial extent of its charge cloud. Experiments in the time range of the lattice vibration show that reducing the electron size leads to an increase of the interaction by up to a factor of 50. This results in a strong coupling of the movements of electrons and ions. Electron and phonon together form a new quasi particle, a polaron.

To visualize this phenomenon, the researchers used a nanostructure made from gallium arsenide and gallium aluminum arsenide, in which the energies of the movements of electrons and ions were tuned to each other. The coupling of both movements was shown by a new optical technique. Several ultrashort light pulses in the infrared excite the system under study. The subsequent emission of light by the moving charge carriers is measured in real time. In this way two-dimensional nonlinear spectra (see Fig.) are generated, which allow the detailed investigation of coupled transitions and the determination of the electron-phonon coupling strength. From the coupling strength one finds the size of the electron cloud, which is just 3-4 nanometers (1 nanometer = 10-9 m = 1 billionth of one meter). Furthermore, this new method shows for the first time the importance of electron-phonon coupling for optical spectra of semiconductors. This is of interest for the development of optoelectronic devices with custom-tailored optical and electric properties.


Negative Mass and High Speed: How Electrons Go Their Own Ways

12th April 2010

Wilhelm Kuehn beim Einjustieren des ExperimentsIsaac Newton found in the 17th century that a force causes a body to accelerate. The inertial mass of the body is the ratio between force and acceleration, thus, given the same force, a light body is accelerated more strongly than a heavy body. A body's mass is positive, meaning that the acceleration is in the same direction as the force. Charged elementary particles as the free electron, which has a mass of only 10-30 = 0. ...(29 zeros !)...1 kilograms, can be accelerated in electric fields to extremely high speeds. If the electric field is small, the motion of electrons in crystals is governed by the same laws. In this regime, the mass of a crystal electron is only a small part of the mass of a free electron.
Wilhelm Kuehn, Michael Woerner, Klaus Reimann and Thomas Elsaesser from the MBI have now demonstrated that crystal electrons in extremely high electric fields exhibit a completely different behavior. Their mass even becomes negative. They report in the latest issue of Physical Review Letters 104, 146602 (2010) that the electron is accelerated within the extremely short time of 100 femtoseconds = 0.000 000 000 000 1 seconds to a speed of 4 million kilometers per hour. Afterwards the electron comes to a stop and even moves backward. This means that the acceleration is in opposite direction to the force, which can only be explained by a negative inertial mass of the electron.

In the experiments, electrons in the semiconductor crystal gallium arsenide are accelerated by an extremely short electrical pulse with a field strength of 30 million Volts per meter. At the same time the speed of the electrons is measured with high precision as a function of time. The duration of the electric pulse is only 300 femtoseconds. This extremely short duration is essential as otherwise the crystal could be damaged.

DThe new results agree with calculations of the Nobel Prize winner Felix Bloch undertook more than 80 years ago. They open up a new regime of charge transport with new possibilities for future microelectronics devices. The observed frequencies are in the terahertz range (1 THz = 1000 GHz = 1012 Hz), about 1000 times higher than the clock rate of the newest PCs.