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.

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.

Controlling how fast graphene cools down

Collaborative paper published in ACS Nano.

An international study, published in ACS Nano, has demonstrated an unprecedented level of control of the optical properties of graphene. The work has promising applications in different technological fields ranging from photonics to telecommunications.

Graphene is the thinnest material ever produced, with the thickness of a single atomic layer, thinner than a billionth of a meter, it is able to efficiently absorb light from the visible to the infrared through the photoexcitation of its charge carriers. After light absorption, its photoexcited charge carriers cool down to the initial equilibrium state in a few picoseconds, corresponding to a millionth of a millionth of a second. The remarkable speed of this relaxation process makes graphene particularly promising for a number of technological applications, including light detectors, sources and modulators.

A recent study published in ACS Nano has shown that the relaxation time of graphene charge carriers can be significantly modified by applying an external electrical field. The research was conceived within an international collaboration between the CNR-IFN, Politecnico di Milano, the University of Pisa, the Graphene Center of Cambridge (UK) and the Catalan Institute of Nanoscience and Nanotechnology (ICN2, Barcelona).

The change in the relaxation time of charge carriers in graphene that we have observed, demonstrates an unprecedented level of control on the optical response of a crystal and allows to obtain a large variety of behaviors using a single material,” says Dr Eva Pogna, researched from CNR-IFN, first author of the work. This study paves the way to the development of devices that exploit the control of the relaxation time of charge carriers to support novel functionalities. For example, if graphene is used as saturable absorber in a laser cavity to generate ultrashort light pulses, by changing the relaxation time of the charge carriers, we can control the duration of the output pulses.

The specific device that we have used to study graphene, proved to be crucial to observe the strong tunability of its optical properties with the external electric field, allowing to change the number of charge carriers over a broad range by exploiting ionic liquid gating, which is a state-of-the-art technology introduced to study superconductors,” explains Prof. Andrea Ferrari, director of the Graphene Center in Cambridge.

The graphene-based device has been studied by ultrafast spectroscopy, which allowed to monitor the variation of the relaxation time of the charge carriers. “This work represents the latest step of a long-standing research collaboration devoted to the study of the ultrafast carrier dynamics in graphene, aimed at exploring the great potential of this fascinating material,” added Dr Klaas-Jan Tielrooij, leader of the Ultrafast Dynamics in Nanoscale Systems Group at ICN2.

This discovery is of large interest for a number of technological applications, ranging from photonics, for pulsed laser sources or optical limiters that prevent optical components damaging, to telecommunication, for ultrafast detectors and modulators,” concludes Prof. Giulio Cerullo, professor at the Physics Department of Politecnico di Milano.

Image: Graphene charge carriers lying on different energetic levels represented by the Dirac cones, which, depending on the number of charge carriers, are occupied up to the neutrality point (blue level on the left cone) or well into the conduction band (blue level on the right cone). In the two cases, the photoexcited charge carriers relax with faster (left side) or slower (right side) dynamics.

Thin is cool: thermal dissipation of layered semiconductors down to the monolayer

New work published in Adv. Mater.

A study published in “Advanced Materials” reveals the thermal transport properties of ultrathin crystals of molybdenum diselenide, a two-dimensional material of the transition metal dichalcogenide (TMD) family. Outperforming silicon, TMD materials prove to be outstanding candidates for electronic and optoelectronic applications, such as flexible and wearable devices. This research, which involved researchers belonging to four ICN2 groups and from ICFO (Barcelona), Utrecht University (the Netherlands), the University of Liège (Belgium) and the Weizmann Institute of Science (Israel), was coordinated by ICN2 group leader Dr Klaas-Jan Tielrooij.

The increasing demand for extremely small components and devices has led scientists to search for new materials that could best meet these needs. Two-dimensional layered materials (2D materials) – which can be as thin as one or a few atomic layers and are strongly bonded only in the in-plane direction – have attracted the attention of both academia and industry, and do not cease to amaze with their peculiar and remarkable properties. Among them, transition metal dichalcogenides (TMDs) are promising for a variety of electronic, optoelectronic and photonic applications.

When it comes to the integration and miniaturization of devices, a key aspect to take into account is the thermal transport properties of materials: in most applications overheating is a crucial factor limiting performance and lifetime. Therefore, in order to take advantage of the electronic and optical properties of TMDs, a deep understanding and control of heat flow in these materials is required. In particular, comprehending the effects of crystal thickness – down to just one layer – and the environment on thermal transport are key to applications.

Influence of crystal thickness on thermal dissipation properties

A combined experimental and theoretical study recently published in Advanced Materials investigates the thermal conductivity of molybdenum diselenide (MoSe2), which is an archetypal TMD material. David Saleta Reig, PhD student and first author of the work explains: “We performed a systematic study of the effects of crystal thickness and surrounding environment on heat flow. This fills an important gap in the scientific literature about 2D materials.” Indeed, performing either reliable experimental studies or computer simulations of thermal transport over a broad range of thicknesses from bulk down to a single molecular monolayer is not an easy task. The authors of this research were able to overcome these challenges and produce protocols and results that are valid not only for the case study, MoSe2, but also for a broader range of 2D materials.

Ultrathin MoSe2 transports heat faster than ultrathin silicon

The experimental measurements, in combination with numerical simulations, led to a remarkable result: “We found that the in-plane thermal conductivity of the samples decreases only marginally when reducing the thickness of the crystal all the way to a monolayer with sub-nanometer thickness,” explains Sebin Varghese, PhD student and second author of the study. This behaviour originates from the layered nature of MoSe2 and sets TMD materials apart from non-layered semiconductors, such as the industry standard, silicon. In the latter, the thermal conductivity decreases dramatically when the thickness approaches the nanometer, due to increased scattering at the surface. This effect is much less significant in layered materials, such as MoSe2.

First principles thermal transport simulations reproduced the experimental results in an excellent way, and led to another surprising result: “For the thinnest films, the heat is carried by different phonon modes than for thicker ones,” says Dr Roberta Farris, postdoctoral researcher who developed and carried out the ab initio simulations. Finally, this study also clarifies the influence of the material’s environment on heat dissipation, demonstrating that ultrathin MoSe2 is able to dissipate heat very efficiently to surrounding air molecules.

Dr Klaas-Jan Tielrooij, who coordinated the work, comments: “This work shows that TMD crystals with (sub)nanometer thickness have the potential to outperform silicon films both in terms of electrical and thermal conductivity in this ultrathin limit”. These results thus demonstrate the excellent prospects of TMDs for applications that require thicknesses on the order of a few nanometers or less, for example in the case of flexible and wearable devices and nanoscale electronic components. “Of course it remains to be seen if TMDs will live up to their promises”, concludes Dr Tielrooij, “as there are many hurdles to overcome before these materials will be applied on an industrial scale. At least we now know that their thermal properties are – in principle – not a show-stopper.”

About the study

The authors of this study used the Raman thermometry technique to measure the thermal conductivity of a large set of suspended, crystalline, and clean MoSe2 crystals with systematically varied thickness, taking care to identify and suppress possible thickness-dependent artifacts. They compared the experimental results with ab initio simulations –based on density functional theory and Boltzmann transport theory— performed with the SIESTA method and software, which is particularly suitable for atomistic simulations with a large number of atoms.

This research, coordinated by Dr Klaas-Jan Tielrooij, leader of the ICN2 Ultrafast Dynamics in Nanoscale Systems Group, involved several ICN2 researchers and group leaders belonging to: the Phononic and Photonic Nanostructures Group, led by ICREA Prof. Clivia Sotomayor Torres, the Theory and Simulation Group, led by Prof. Pablo Ordejón, and the Physics and Engineering of Nanodevices Group, led by ICREA Prof. Sergio Valenzuela. Researchers ICFO (Barcelona), Utrecht University (the Netherlands) –with Prof. Zeila Zanolli, former member of Prof. Ordejón’s group—, the University of Liége (Belgium) –with Prof. Matthieu Verstraete, former visiting scientist at ICN2— and the Weizmann Institute of Science (Israel) were also involved.