Shining light on transport phenomena

Review paper on spatiotemporal microscopy published in Adv. Electron. Mater.

Transport phenomena like diffusion, convection, and drift play key roles in the sciences and engineering disciplines. They belong to the most omnipresent and important phenomena in nature that describe the motion of entities such as mass, charge or heat. Understanding and controlling these transport phenomena is crucial for a host of industrial technologies and applications, from cooling nuclear reactors to nanoscale heat-management in the semiconductor industry. For decades, macroscopic transport techniques have been used to access important parameters such as charge mobilities or thermal conductivities. While being powerful, they often require physical contacts, which can lead to unwanted effects. Over the past years, an exciting solution has emerged: a technique called spatiotemporal microscopy (SPTM) that accesses crucial transport phenomena in a contactless, all-optical, fashion. This technique offers powerful advantages in terms of accessible timescales, down to femtoseconds, and length scales, down to nanometres, and, further, selectively observes different species of interest. This tutorial review discusses common experimental configurations of SPTM and explains how they can be implemented by those entering the field. This review highlights the broad applicability of SPTM by presenting several exciting examples of transport phenomena that were unravelled thanks to this technique.

ERC consolidator grant awarded

New funding to develop THz components based on quantum materials

From TU/e website (link):

ERC Consolidator Grant of 2 million euros for three TU/e researchers

NOVEMBER 23, 2023

The researchers are all involved in building a digital future that is both fast and energy-efficient

Aida Todri-Sanial, Klaas-Jan Tielrooij en Yoeri van de Burgt. Photos: Vincent van den Hoogen en Angeline Swinkels

TU/e researchers Yoeri van de Burgt, Klaas-Jan Tielrooij, and Aida Todri-Sanial have each been awarded an ERC Consolidator Grant worth two million euros from the European Research Council (ERC). The researchers will use this money for research into organic computers, ultrafast components for wireless networks based on terahertz radiation, and developing a novel computing paradigm based on thermodynamics, respectively. We spoke to the three researchers about their plans for the coming five years.

Associate Professor Yoeri van de Burgt, working at the Department of Mechanical Engineering and institutes EAISI and EHCI, works on electronic systems that are nothing like the electronics in your laptop or cell phone. The heart of the devices in his lab are not made of the semiconductor silicon, but of organic materials that are also found in our bodies.

Moreover, this organic processor is more similar in design to the neural networks in our brains than to a classical computer chip. Van de Burgt now wants to connect such a processor to a biological environment consisting of nerve cells.

That connection to biological material is challenging, according to Van de Burgt. “We want to make a connection that transmits information to both sides, and that’s tricky,” he says. “How do you measure signals from biological cells and pass them on to the processor? Moreover, we want this connection to be ‘self-learning,’ like the flexible connections between brain cells.”

The latter means that the strength of the coupling depends on previous signals that passed through it, but also on the presence of certain neurotransmitters, or signaling substances, in the biological environment.

Yoeri van de Burgt. Photo: Angeline Swinkels

Spikes and neurotransmitters

The platform the researchers are building for this project is several centimeters in size and consists of fluid channels as well as sensors made of so-called mixed conducting polymers that can conduct both electrons and charged ions.

“This processor responds to two types of signals,” Van de Burgt says. “The first type of signals are normal electrical signals – or spikes – from nerve cells. The second signal is neurotransmitters such as dopamine or serotonin, signaling substances that you also find in the brain. The concentration of these substances determines the strength of the learning part.”

Finally, the processor is given a task: the researchers connect a “soft” gripper to it. Not only do they want to control the gripper with electrical signals from the cells. The idea is that the organic processor will eventually “learn” to deal with the gripper and, for example, be able to apply just the right amount of force to grip a particular object.

The promise, according to Van de Burgt, is that in the future the technology could serve for personal prosthetic devices that are directly coupled to the body. This kind of organic computer could eventually do the same work locally, inside the body, much more energy-efficiently than an external classical computer.

The project is called “neuro-labs” and will consist of three PhD students and two postdocs.

Klaas-Jan van Tielrooij. Photo: Vincent van den Hoogen

The power of tetrahertz radiation

Not so long ago people used to only send short text messages to each other; now they talk via video call and send large files effortlessly. This development requires ever faster connections and processing of data. Associate Professor Klaas-Jan Tielrooij of the Department of Applied Physics and Science Education is laying the foundation for wireless electronic components that can keep up with this trend in the future.

New materials and new physics should enable compact transmitters and receivers that process trillions of signals per second. These optoelectronic components use terahertz radiation to send and receive information. In theory, they are many times faster than the components now found in wireless communication networks.

Currently, terahertz components for wireless communications are often inefficient and quite costly. This is in contrast to infrared and optical light, which are widely used in fiber optics. Tielrooij will use extremely thin materials, such as graphene and other so-called quantum materials, which are only one or few atoms thick.

The dynamics of electrons

Essential for a good terahertz transmitter or receiver is an efficient conversion of light of another wavelength to terahertz radiation, and vice versa, and to electrical signals. To this end, Tielrooij is one of the first to apply a design strategy based on the so-called ultrafast thermodynamic properties of materials.

“Existing techniques mostly rely on an interaction between light and the charge of electrons in the material. If you can also use the dynamics of electron temperature then this conversion can be much more efficient,” he says. You can think of this electron temperature as the amount of energy of the electrons.

According to Tielrooij, the components he is developing will soon end up not only in components for wireless networks. Terahertz radiation is also suitable for safely scanning people and suitcases at airports, for example. And it could be used to perform medical scans to detect tumors, for example.

“The terahertz regime is not as developed compared to other wavelengths. It is important that we catch up, because there are many possible applications,” he says.

The project is called “Engineering QUAntum materials for TErahertz applications” (Equate) and consists of three PhD students and two postdocs.

Aida Todri-Sanal. Photo: Vincent van den Hoogen

Self-organizing neural networks

Aida Todri-Sanial, professor of Integrated Circuits in the Department of Electrical Engineering and EAISI, will develop a novel computing paradigm inspired by thermodynamic principles. By applying this to artificial neural networks in the lab this could lead to computers that can solve computationally hard problems in an energy-efficient way.

Todri-Sanial has been modeling and building artificial neural networks in the lab for many years. These are based on electrical oscillators on a chip that uses very little power. Now she wants to give these so-called oscillatory neural networks the ability to learn, similar to how the brain works. This requires rethinking the building blocks of such a system: to enable self-organization, reconfigurability, adaptability, and evolution, something that has not been done before in artificial neural networks.

Efficient and stable

Traits like self-organization, adaptation, and evolution are based on thermodynamics principles, which drive a system to evolve and adapt in an energy-efficient way towards stable configurations. These configurations represent the solution of the task that is given to the system. Self-organization is frequently observed in nature but not yet in our computing systems, Todri-Sanial says.

“In this project, I want to design a computing architecture that self-organizes to solve problems via collective dynamics of coupled oscillators. To do so, we will first develop computing models based on thermodynamics and then design analog integrated circuits for implementing a thermodynamic-inspired computing architecture.”

In the future self-organizing neural networks could solve problems that are typically hard for classical computers, like predicting the weather, find the shortest route between two points on a map or calculating exactly how two molecules will bind. The latter could be very useful in developing new medicines. The research could also lead to more efficient learning strategies for artificial intelligence.

The project “Thermodynamic-inspired computing with oscillatory neural networks” (THERMODON) will consist of three PhD students and three postdocs.

ERC Consolidator Grants

The ERC’s mission is to encourage the highest quality research in Europe through competitive funding and to support frontier research across all fields, based on scientific excellence. ERC grants allow Europe’s brightest minds to identify new opportunities and directions in any field of research and catalyze new and unpredictable scientific and technological discoveries.

The ERC Consolidator Grants are awarded to outstanding researchers of any nationality and age, with at least seven and up to twelve years of experience after a PhD, and a scientific track record showing great promise.

Carbon nanotube films for extreme ultraviolet lithography and effective heat management in electronic devices

Collaborative paper with imec

A study led by researchers at the ICN2 and published in ‘ACS Applied Materials & Interfaces’ demonstrates that carbon nanotube networks can be used to provide efficient heat dissipation in applications such as extreme ultraviolet lithography, as well as flexible electronics and interconnects in integrated circuits.

Heat dissipation and low power consumption have become key challenges for modern electronics, which must be addressed hand in hand with the development of new integration and miniaturisation technologies for electronic circuits and systems. In particular, as devices become smaller and more powerful, excess heat issues arise, which can compromise operation and durability. In this context, new materials and nanotechnologies can provide solutions for designing high-performance electronic devices that consume little power and are able to manage heat efficiently.

Carbon nanotube (CNT) networks –an emerging class of nanostructured materials that exhibit outstanding electrical and mechanical properties— are promising candidates for applications requiring efficient heat dissipation, such as extreme ultraviolet (EUV) lithography. EUV lithography is a key technology for the miniaturisation of integrated circuits that requires the use of protective films to avoid damaging photomasks during the process. Suitable materials for these shielding pellicles should be transparent to EUV light, mechanically robust, and thermally stable with high thermal conductivity.

As demonstrated in a research paper published today in ACS Applied Materials & Interfacesfree-standing CNT films are strong candidates for this application, due to their high EUV transparency and ability to withstand heat. The authors of this study –a team of researchers from the ICN2 Ultrafast Dynamics in Nanoscale Systems Group (led by Prof. Klaas-Jan Tielrooij), the former ICN2 Phononic and Photonic Nanostructures Group (led by Prof. Clivia Sotomayor-Torres), and from imec (Leuven, Belgium)— employed all-optical Raman thermometry to gain insight into the thermal properties of free-standing CNT networks.

The authors investigated two types of carbon nanotube films: single-walled CNTs (SWCNTs) and double-walled CNTs (DWCNTs). The first structure consists of a network of nanotubes, whose walls are one atom thick; the second is composed of a network of nanotubes each made of two single-walled nanotubes nested within each other. By Raman thermometry –which is a non-invasive technique— the researchers were able to measure the thermal conductivity of these two types of free-standing CNT films at temperatures ranging from 300 K to 700 K.

The study revealed thermal conductivities up to 50 W m−1 K−1 for films made of double-walled CNTs, a remarkable seven times higher than those of single-walled CNT films. “We believe that this enhanced thermal conduction is due to the additional wall in double-walled carbon nanotube networks, which introduces an extra heat-dissipation channel and reduces the impact of defects,” explains Prof. Klaas-Jan Tielrooij, the coordinator of this research and corresponding author of the paper. “Furthermore, the nanotubes in the studied CNT network films were randomly oriented; we think that aligning the double-walled CNTs within the network would lead to even higher thermal conductivity.”

These outcomes show that free-standing carbon nanotube network films, in particular the double-walled one, provide efficient heat dissipation, demonstrating their suitability for application as EUV pellicle materials. In addition, they hold promise for use in flexible electronics and interconnects in integrated circuits. In fact, the relatively high thermal conductivity of these films makes them ideal for efficient thermal management in devices where traditional materials fall short when dimensions are extremely reduced.

Experiments reveal that water can ‘talk’ to electrons in graphene, confirming theory of quantum friction

Collaborative paper published in Nature Nanotechnology

Water and carbon make a quantum couple: the flow of water on a carbon surface is governed by an unusual phenomenon dubbed quantum friction. A work published in ‘Nature Nanotechnology’ experimentally demonstrates this phenomenon –which was predicted in a previous theoretical study— at the interface between liquid water and graphene, a single layer of carbon atoms. These results could lead to applications in water purification and desalination processes and maybe even to liquid-based computers.

For the last 20 years, scientists have been puzzled by how water behaves near carbon surfaces. It may flow much faster than expected from conventional flow theories or form strange arrangements such as square ice. Now, an international team of researchers from the Max Planck Institute for Polymer Research of Mainz (Germany), the Catalan Institute of Nanoscience and Nanotechnology (ICN2, Spain), and the University of Manchester (England), reports in a study published today in Nature Nanotechnology that water can interact directly with the carbon’s electrons: a quantum phenomenon that is very unusual in fluid dynamics.

A liquid, such as water, is made up of small molecules that randomly move and constantly collide with each other. A solid, in contrast, is made of neatly arranged atoms that bathe in a cloud of electrons. The solid and the liquid worlds are assumed to interact only through collisions of the liquid molecules with the solid’s atoms: the liquid molecules do not “see” the solid’s electrons. Nevertheless, just over a year ago, a paradigm-shifting theoretical study proposed that at the water-carbon interface, the liquid’s molecules and the solid’s electrons push and pull on each other, slowing down the liquid flow: this new effect was called quantum friction. However, the theoretical proposal lacked experimental verification.

We have now used lasers to see quantum friction at work,” explains study lead author Dr Nikita Kavokine, a researcher at the Max Planck Institute in Mainz and the Flatiron Institute in New York. The team studied a sample of graphene – a single monolayer of carbon atoms arranged in a honeycomb pattern. They used ultrashort red laser pulses (with a duration of only a millionth of a billionth of a second) to instantaneously heat up the graphene’s electron cloud. They then monitored its cooling with terahertz laser pulses, which are sensitive to the temperature of the graphene electrons. This technique is called optical pump – terahertz probe (OPTP) spectroscopy.

To their surprise, the electron cloud cooled faster when the graphene was immersed in water, while immersing the graphene in ethanol made no difference to the cooling rate. “This was yet another indication that the water-carbon couple is somehow special, but we still had to understand what exactly was going on,” Kavokine says. A possible explanation was that the hot electrons push and pull on the water molecules to release some of their heat: in other words, they cool through quantum friction. The researchers delved into the theory, and indeed: water-graphene quantum friction could explain the experimental data.

It’s fascinating to see that the carrier dynamics of graphene keep surprising us with unexpected mechanisms, this time involving solid-liquid interactions with molecules none other than the omnipresent water,” comments Prof Klaas-Jan Tielrooij from ICN2 (Spain) and TU Eindhoven (The Netherlands). What makes water special here is that its vibrations, called hydrons, are in sync with the vibrations of the graphene electrons, called plasmons, so that the graphene-water heat transfer is enhanced through an effect known as resonance.

The experiments thus confirm the basic mechanism of solid-liquid quantum friction. This will have implications for filtration and desalination processes, in which quantum friction could be used to tune the permeation properties of the nanoporous membranes. “Our findings are not only interesting for physicists, but they also hold potential implications for electrocatalysis and photocatalysis at the solid-liquid interface,” says Xiaoqing Yu, PhD student at the Max Planck Institute in Mainz and first author of the work.

The discovery was down to bringing together an experimental system, a measurement tool and a theoretical framework that seldom go hand in hand. The key challenge is now to gain control over the water-electron interaction. “Our dream is to switch quantum friction on and off on demand,” Kavokine says. “This way, we could design smarter water filtration processes, or perhaps even fluid-based computers.”   


Water-graphene quantum friction [Credits: Lucy Reading-Ikkanda / Simons Foundation]

Ultrafast terahertz-induced visible light emission

Collaborative paper published in Nano Letters

A study carried out by researchers from the HZDR Institute of Radiation Physics, ICN2 Ultrafast Dynamics in Nanoscale Systems group, University of Exeter Centre for Graphene Science, and TU Eindhoven demonstrates that graphene-based materials can be used to efficiently convert high-frequency signals (in the terahertz regime) into visible light, and that this mechanism is tunable. These outcomes open the path to exciting applications in next-future information and communication technologies.

The ability to convert signals from one frequency regime to another is key to various technologies, in particular in telecommunications, where, for example, data processed by electronic devices are often transmitted as optical signals through fibres. To enable significantly higher data transmission rates, future 6G wireless communication systems will need to extend the carrier frequency above 100GHz up to the terahertz (THz) range. Therefore, a fast and controllable mechanism to convert THz waves into visible or telecom light will be required. Imaging and sensing technologies could also benefit from it.

What is missing is a material that is capable of upconverting photon energies by a factor of 1000 or so: from the milli-electronvolt (meV) range to around 1 eV. Researchers have recently identified the strong nonlinear response of so-called Dirac quantum materials, e.g. graphene and topological insulators, to THz light pulses (see this and this previous news item). This manifests in the highly efficient generation of high harmonics upon excitation with THz pulses. These harmonics are still within the THz range, however, there were also first observations of visible light emission from graphene upon infrared and terahertz excitation. Up to now, this effect was extremely inefficient and the underlying physical mechanism was not understood.

work published in Nano Letters demonstrates ultrafast and tunable conversion of THz light into visible light in graphene systems. The study was led by Dr Igor Ilyakov and Dr Sergey Kovalev–from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR, Germany)— and by Prof Klaas-Jan Tielrooij –group leader at Catalan Institute of Nanoscience and Nanotechnology (ICN2, Spain) and Eindhoven University of Technology (The Netherlands). They provide a physical explanation for this mechanism and show how the light emission can be strongly enhanced by using highly doped graphene (so-called GraphExeter) and by using a grating-graphene metamaterial. They also observed that the conversion occurs very rapidly –on the sub-nanosecond time scale— and that it can be controlled by electrostatic gating.

The authors ascribe the light frequency conversion in graphene to a THz-induced thermal radiation mechanism: the charge carriers absorb electromagnetic energy from the incident THz field; the absorbed energy rapidly distributes in the material, resulting in carrier heating; finally, this leads to emission of photons in the visible spectrum as a result of black-body radiation.

The tunability and speed of the THz-to-visible light conversion achieved in graphene-based materials has great potential for application in information and communication technologies. The underlying ultrafast thermodynamic mechanism could certainly produce an impact on THz-to-telecom interconnects, as well as in any technology that requires ultrafast frequency conversion of signals.

Negative Diffusion in Gold Films

Collaborative paper published in ACS Photonics

Ultrafast light-induced spatiotemporal dynamics in metals in the form of electron and/or phonon heating is a fundamental physical process that has tremendous practical relevance. In particular, understanding the resulting lateral heat transport is of key importance for various (opto)electronic applications and thermal management but has attracted little attention. Here, by using scanning ultrafast thermo-modulation microscopy to track the spatiotemporal electron diffusion in thin gold films, we show that a few picoseconds after the optical pump there is unexpected heat flow from phonons to electrons, accompanied by negative effective thermal diffusion, characterized by shrinking of the spatial region with increased temperature. Peculiarly, this occurs on the intermediate time scale, between the few picosecond long thermalization stage and the many picosecond stage dominated by thermoacoustic vibrations. We accurately reproduced these experimental results by calculating the spatiotemporal photothermal response based on the two-temperature model and an improvement of the standard permittivity model for gold. Our findings facilitate the design of nanoscale thermal management strategies in photonic, optoelectronic, and high-frequency electronic devices.

Introducing a novel technique for measuring the thermal diffusivity of thin films

Published in Rev. Sci. Instrum. as Editor’s Pick.

A paper recently published in the journal ‘Review of Scientific Instruments’ introduces a new all-optical technique for measuring thermal diffusion in nanoscale systems. The study was conducted by a team of researchers led by Prof. Klaas-Jan Tielrooij, senior group leader at ICN2. This “pre-time-zero spatiotemporal pump-probe microscopy” has several important benefits over existing techniques and provides an excellent tool for studying heat transport at the nanoscale.

Thermal transport in materials plays a critical role in many technological applications in electronics, optoelectronics, energy conversion, and more. Overheating is actually a major concern for many devices, since it can hinder their proper operation. Therefore, understanding the heat flow at the nanoscale is crucial to optimizing device performance. There are several optical techniques available for measuring heat transport, but they often require knowledge of multiple parameters of the material under study, and strong heating of the sample. 

Researchers from the ICN2 Ultrafast Dynamics in Nanoscale Systems Group, led by Prof. Klaas-Jan Tielrooij, have developed a novel all-optical technique for measuring thermal transport properties of thin films with high sensitivity (down to one degree) and without the need for any material input parameters. This work has been described in a paper recently published in Review of Scientific Instruments and selected as an Editor’s Pick.As explained in the article, the authors demonstrate the advantages of this technique using various semiconducting 2D materials –MoSe2, WSe2, MoS2 and WS2– as test cases.

The technique is based on spatiotemporal pump-probe microscopy, in which a fixed laser beam (the pump pulse) is used to excite the material and a second one (the probe) is used to scan the sample spatially and in time –i.e., at variable time delays after the excitation. Phonon heat diffusion is a slow process (as compared to electronic diffusion), but the diffusing heat produced by the pump laser is long-lived, so its effects can be observed at larger delays, as long as the temporal spacing between subsequent pump pulses. Thus, the researchers used a small negative pump-probe time delay to observe heat flow in their samples just before the arrival of a new pulse.

The authors termed this new measurement technique “pre-time-zero spatiotemporal pump-probe microscopy“, because, as said, they obtain the thermal diffusivity by examining the spatial profile that results from scanning the probe beam over the pump beam at a negative pump-probe delay. This effectively corresponds to a large pump-probe delay – specifically, 13 ns –, matching the inverse of the repetition rate of the ultrafast laser they used. 

Sebin Varghese, PhD student at ICN2 and first author of the paper, explains: “Our novel technique  combines advantageous elements from several well-established methods: it exploits the long-lived heat signal in pump-probe measurements similar to time-domain thermoreflectance; it is sensitive to phonon heat through a temperature-sensitive mode similar to Raman thermometry; and it uses two laser beams that are spatially scanned with respect to each other, which has similarities with two-laser optothermal microscopy.”

By bringing together the advantages of each of these techniques into a single method, the researchers were able to directly measure the thermal diffusivity of a few samples of semiconducting materials, finding excellent agreement with earlier studies and with theoretical calculations performed in collaboration with the research groups led by Prof. Pablo Ordejón at ICN2 (Spain) –Theory and Simulation Group—, by Prof. Zeila Zanolli at Utrecht University (the Netherlands), and by Prof. Matthieu Verstraete at Liege University (Belgium), respectively.

Thanks to this spatiotemporal scanning, the newly developed method is in fact a super-resolution technique that can resolve spatial spreading with an accuracy around 10 nm, which is much smaller than the laser spot size (of around 1 micron). Being highly sensitive and non-invasive, and not requiring knowledge of material parameters, this method will enable the study of diffusion processes in a large variety of systems.  “This new technique has great potential for advancing our understanding of nanoscale thermal transport phenomena, including in cases where Fourier’s law might no longer hold,” concludes Prof. Klaas-Jan Tielrooij, corresponding author of the paper.

Engineering defects to control the optoelectronic properties of new hetero-materials

Collaborative paper with Max Planck Institute of Polymer Research published in Nano Letters.

A study on van der Waals heterostructures unveils that the photoinduced processes at the interface between the different materials can be modified by electrically controlling the occupancy of the interfacial defects. The ICN2 Ultrafast Dynamics in Nanoscale Systems Group was involved in this research, just published in Nano Letters.

Atomically thin materials can be stacked one on top of the other to form novel heterostructures, which exhibit exotic optical and electronic properties that are relevant to various optoelectronic applications, including photodetection. At the interface, the different materials are coupled by van der Waals forces and an interfacial flow of charge carriers can occur, in particular upon triggering by a light pulse. Understanding and controlling the interfacial charge flow is key to the design of new devices, their operation, and their optimization to specific purposes.

Particularly interesting for their light-matter interaction properties and their high charge carrier mobility are the graphene-transition metal dichalcogenide heterostructures. Photoinduced processes at the interface between the two materials can be investigated by means of an experimental technique called ultrafast pump-probe spectroscopy, which uses ultrashort laser pulses to study ultrafast electronic dynamics.

In a work recently published in the ACS journal Nano Letters, a team of researchers –from the Max Planck Institute for Polymer Research, the Catalan Institute of Nanoscience and Nanotechnology (Ultrafast Dynamics in Nanoscale Systems Group) and the University of Mons— used operando optical-pump terahertz-probe spectroscopy to track and control the charge flow at the van der Waals interface between graphene and the transition metal dichalcogenide (TMD). This study revealed not only the fundamental role played by the material defects at the interface (which introduce extra electronic states) in the photoinduced behaviour of the heterostructure, but also –and more importantly– that the latter can be tuned by electrically controlling the occupancy of the interfacial defects.

This work provides important insights into the complex dynamics of interfacial charge carriers in van der Waals heterostructures and into how to reversibly modify it. Such relevant knowhow opens the way to the development of novel high-performance optoelectronic devices by defect and doping engineering of multi-layer heteromaterials.

Harmonic generation from topological insulators

Collaborative paper published in Light: Science & Applications

A paper recently published in ‘Light: Science & Applications’ demonstrates that quantum materials of the topological insulator family can efficiently upconvert electromagnetic radiation in the terahertz (THz) regime. These results open new avenues for THz photonics technology and its application in sensing, homeland security and sixth-generation mobile communications. The study was conducted by a team of researchers coordinated by ICN2 group leader Dr Klaas-Jan Tielrooij.

Terahertz light, radiation in the far-infrared part of the emission spectrum, is currently not fully exploited in technology, although it shows great potential for many applications in sensing, homeland security screening, and future (sixth generation) mobile networks. Indeed, this radiation is harmless due to its small photon energy, but it can penetrate many materials (such as skin, packaging, etc.). In the last decade, a number of research groups have focused their attention on identifying techniques and materials to efficiently generate THz electromagnetic waves: among them is the wonder material graphene, which, however, does not provide the desired results. In particular, the generated terahertz output power is limited.

Better performance has now been achieved by topological insulators (TIs) – quantum materials that behave as insulators in the bulk while exhibiting conductive properties on the surface—, according to a paper recently published in ‘Light: Science & Applications’. This study was carried out by members of the ICN2 Ultrafast Dynamics in Nanoscale Systems Group, led by Dr Klaas-Jan Tielrooij, and of the High-field THz Driven Phenomena Group at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR, Germany) –led by Dr Sergey Kovalev–, in collaboration with researchers from the ICN2 Physics and Engineering of Nanodevices Group, headed by ICREA Prof. Sergio O. Valenzuela, from the School of Physics and Astronomy of the University of Manchester (UK), and the Physics Institute of the University of Würzburg (Germany). The experiments were performed at the TELBE THz facility in Dresden.

Earlier studies had shown that materials which host electrons with zero effective mass enable efficient generation of terahertz harmonics, including the aforementioned graphene and topological insulators. The phenomenon of harmonic generation occurs when photons of the same frequency and energy interact non-linearly with matter, leading to the emission of photons whose energy is a multiple of that of the incident ones. This can be exploited, for example, to upconvert electronically generated signals in the high GHz regime into signals in the THz regime.

Dr Tielrooij and colleagues investigated the behaviour of two topological insulators –the prototypical Bi2Se3 and Bi2Te3– in direct comparison with a reference graphene sample. They observed that, while the maximum power of the harmonics generated in graphene is limited by saturation effects (which arise at high incident powers), in these quantum materials it continued to increase with the incident fundamental power. The performed experiments revealed an improvement in generated output power by orders of magnitude over graphene, approaching the milliwatt regime.

This significant divergence in behaviour is due to the fact that topological insulators can rely on a highly efficient cooling mechanism, in which the massless charges on the surface dissipate their electronic heat to those in the rest of the thin film. In other words, bulk electrons lend a helping hand to the surface-state electrons by sinking electronic heat. The highest output power for the terahertz third-harmonic –i.e. radiation with three times the same energy– was achieved in a metamaterial that contained a topological insulator film together with a metallic grating –consisting of metal strips separated by gaps on the surface of the material.

In this work we demonstrate that the saturation effect occurring in graphene is much less detrimental in topological insulators. This occurs thanks to a novel cooling mechanism between surface and bulk electrons of topological insulators,” explains Dr Klaas-Jan Tielrooij, first author of the paper. “These quantum metamaterials thus bring nonlinear terahertz photonics technology a big step closer.” Sergey Kovalev, last author of the paper, adds: “The obtained results furthermore offer interesting possibilities towards studying the quantum properties of these materials with prospects towards quantum technologies.”

Ultrafast heat dissipation through surface-bulk coupling in quantum materials

Collaborative paper with Alessandro Principi published in Phys. Rev. B.


The timescale of electronic cooling is an important parameter controlling the performance of devices based on quantum materials for optoelectronic, thermoelectric, and thermal management applications. In most conventional materials, cooling proceeds via the emission of phonons, a process that can bottleneck the carrier relaxation dynamics, thus degrading the device performance. Here we present the theory of near-field radiative heat transfer that occurs when a two-dimensional electron system is coupled via the nonretarded Coulomb interaction to a three-dimensional bulk that can behave as a very efficient electronic heat sink. We apply our theory to study the cooling dynamics of surface states of three-dimensional topological insulators and of graphene in proximity to small-gap bulk materials. The “Coulomb cooling” we introduce is alternative to the conventional phonon-mediated cooling, can be very efficient, and can dominate the cooling dynamics under certain circumstances. We show that this cooling mechanism can lead to a sub-picosecond timescale, significantly faster than the cooling dynamics normally observed in Dirac materials.