June 26, 2017

Ultra-compact phase modulators based on graphene plasmons

Modulating the amplitude and phase of light is a key ingredient for many of applications such as wavefront shaping, transformation optics, phased arrays, modulators and sensors. Performing this task with high efficiency and small footprint is a major challenge for the development of optoelectronic devices.

In a recent paper published in Nature Photonics, ICFO researchers Dr. Achim Woessner and Dr. Mark Lundeberg, led by ICREA Prof. at ICFO Frank Koppens, in collaboration with Prof. Rainer Hillenbrand from CIC Nanogune, Iacopo Torre and Prof. Marco Polini from IIT and Dr. Yuanda Gao and Prof. James Hone from Columbia University, have developed a phase modulator based on graphene capable of tuning the light phase between 0 and 2? in situ.

To achieve this, they exploited the unique wavelength tunability of graphene plasmons, light coupled to electrons in graphene. In their experiment, they used ultra-high quality graphene and build a fully functional phase modulator with a device footprint of only 350 nm, which is 30 times than the wavelength of the infrared light used for this experiment. A near-field microscope was used to excite and image the plasmons, allowing an unprecedented insight into the plasmon properties such as their wavelength and phase.

This new type of phase modulator enables graphene plasmons to be used for ultra-compact light modulators and phase arrays with the possibility to control, steer and focus light in situ. This has potential applications for on-chip biosensing and two dimensional transformation optics.

This research has been partially supported by the European Research Council, the European Graphene Flagship, the Government of Catalonia, Fundació Cellex and the Severo Ochoa Excellence program of the Government of Spain.

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June 8, 2017

Quantum nanoscope

Researchers have studied how light can be used to “see” the quantum nature of an electronic material. They managed to do that by capturing light in a net of carbon atoms and slowing down light it down so that it moves almost as slow as the electrons in the graphene. Then something special happens: electrons and light start to move in concert, unveiling their quantum nature at such large scale that it could observed with a special type of microscope.

The experiments were performed with ultra-high quality graphene. To excite and image the ultra-slow ripples of light in the graphene (also called plasmons), the researchers used a special antenna for light that scans over the surface at a distance of a few nanometers. With this near field nanoscope they saw that the light ripples on the graphene moved more than 300 times slower than light, and dramatically different from what is expected from classical physics laws.

The work has been published in Science by ICFO researchers Dr. Mark Lundeberg, Dr. Achim Woessner, led by ICREA Prof. at ICFO Frank Koppens, in collaboration with Prof. Hillenbrand from Nanogune, Prof. Polini from IIT and Prof. Hone from Columbia University.

In reference to the accomplished experiments, Prof. Koppens comments: “Usually it is very difficult to probe the quantum world, and to do so it requires ultra-low temperatures; here we could just “see” it with light and even at room temperature”.

This technique paves now the way for exploring many new types quantum materials, including superconductors where electricity can flow without energy consumption, or topological materials that allow for quantum information processing with topological qubits. In addition, Prof. Hillenbrand states that “this could just be the beginning of a new era of near field nanoscopy”.

Prof. Polini adds that “This discovery may eventually lead to understanding in a truly microscopic fashion complex quantum phenomena that occur when matter is subject to ultra-low temperatures and very high magnetic fields, like the fractional quantum Hall effect”

This research has been partially supported by the European Research Council, the European Graphene Flagship, the Government of Catalonia, Fundació Cellex and the Severo Ochoa Excellence program of the Government of Spain.

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May 29, 2017

Graphene and Quantum Dots put in motion a CMOS-integrated camera that can see the invisible

Over the past 40 years, microelectronics has advanced by leaps and bounds thanks to silicon and CMOS (Complementary metal-oxide semiconductors) technology, making possible computing, smartphones, compact and low-cost digital cameras, as well as most of the electronic gadgets we rely on today. However, the diversification of this platform into applications other than microcircuits and visible light cameras has been impeded by the difficulty to combine semiconductors other than silicon with CMOS.

This obstacle has now been overcome. ICFO researchers have shown for the first time the monolithic integration of a CMOS integrated circuit with graphene, resulting in a high-resolution image sensor consisting of hundreds of thousands of photodetectors based on graphene and quantum dots (QD). They operated it as a digital camera that is highly sensitive to UV, visible and infrared light at the same time. This has never been achieved before with existing imaging sensors. In general, this demonstration of monolithic integration of graphene with CMOS enables a wide range of optoelectronic applications, such as low-power optical data communications and compact and ultra sensitive sensing systems.

The study was published in Nature Photonics, and highlighted on the front cover image. The work was carried out by ICFO researchers Stijn Goossens, Gabriele Navickaite, Carles Monasterio, Shuchi Gupta, Juan Jose Piqueras, Raul Perez, Gregory Burwell, Ivan Nitkitsky, Tania Lasanta, Teresa Galan, Eric Puma, and led by ICREA Professors Frank Koppens and Gerasimos Konstantatos, in collaboration with the company Graphenea. The graphene-QD image sensor was fabricated by taking PbS colloidal quantum dots, depositing them onto the CVD graphene and subsequently depositing this hybrid system onto a CMOS wafer with image sensor dies and a read-out circuit. As Stijn Goossens comments, “No complex material processing or growth processes were required to achieve this graphene-quantum dot CMOS image sensor. It proved easy and cheap to fabricate at room temperature and under ambient conditions, which signifies a considerable decrease in production costs. Even more, because of its properties, it can be easily integrated on flexible substrates as well as CMOS-type integrated circuits.”

As ICREA Prof. at ICFO Gerasimos Konstantatos, expert in quantum dot-graphene research comments, “we engineered the QDs to extend to the short infrared range of the spectrum (1100-1900nm), to a point where we were able to demonstrate and detect the night glow of the atmosphere on a dark and clear sky enabling passive night vision. This work shows that this class of phototransistors may be the way to go for high sensitivity, low-cost, infrared image sensors operating at room temperature addressing the huge infrared market that is currently thirsty for cheap technologies”.

“The development of this monolithic CMOS-based image sensor represents a milestone for low-cost, high-resolution broadband and hyperspectral imaging systems” ICREA Prof. at ICFO Frank Koppens highlights. He assures that “in general, graphene-CMOS technology will enable a vast amount of applications, that range from safety, security, low cost pocket and smartphone cameras, fire control systems, passive night vision and night surveillance cameras, automotive sensor systems, medical imaging applications, food and pharmaceutical inspection to environmental monitoring, to name a few”.

This project is currently incubating in ICFO’s Launchpad. The team is working with the institute’s tech transfer professionals to bring this breakthrough along with its full patent portfolio of imaging and sensing technologies to the market.

This research has been partially supported by the European Graphene Flagship, the European Research Council, the Government of Catalonia, Fundació Cellex and the Severo Ochoa Excellence program of the Government of Spain

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May 15, 2017

Energy decay in Graphene Resonators

Energy dissipation is a key ingredient in understanding many physical phenomena in thermodynamics, photonics, chemical reactions, nuclear fission, photon emissions, or even electronic circuits, among others.

In a vibrating system, the energy dissipation is quantified by the quality factor. If the quality factor of the resonator is high, the mechanical energy will dissipate at a very low rate, and therefore the resonator will be extremely accurate at measuring or sensing objects thus enabling these systems to become very sensitive mass and force sensors, as well as exciting quantum systems.

Take, for example, a guitar string and make it vibrate. The vibration created in the string resonates in the body of the guitar. Because the vibrations of the body are strongly coupled to the surrounding air, the energy of the string vibration will dissipate more efficiently into the environment bath, increasing the volume of the sound. The decay is well known to be linear, as it does not depend on the vibrational amplitude.

Now, take the guitar string and shrink it down to nano-meter dimensions to obtain a nano-mechanical resonator. In these nano systems, energy dissipation has been observed to depend on the amplitude of the vibration, described as a non-linear phenomenon, and so far no proposed theory has been proven to correctly describe this dissipation process.

In a recent study, published in Nature Nanotechnology, ICFO researchers Johannes Güttinger, Adrien Noury, Peter Weber, Camille Lagoin, Joel Moser, led by Prof. at ICFO Adrian Bachtold, in collaboration with researchers from Chalmers University of Technology and ETH Zurich, have found an explanation of the non-linear dissipation process using a nano-mechanical resonator based on multilayer graphene.

In their work, the team of researchers used a graphene based nano-mechanical resonator, well suited for observing nonlinear effects in energy decay processes, and measured it with a superconducting microwave cavity. Such a system is capable of detecting the mechanical vibrations in a very short period of time as well as being sensitive enough to detect minimum displacements and over a very broad range of vibrational amplitudes.

The team took the system, forced it out-of-equilibrium using a driving force, and subsequently switched the force off to measure the vibrational amplitude as the energy of the system decayed. They carried out over 1000 measurements for every energy decay trace and were able to observe that as the energy of a vibrational mode decays, the rate of decay reaches a point where it changes abruptly to a lower value. The larger energy decay at high amplitude vibrations can be explained by a model where the measured vibration mode “hybridizes” with another mode of the system and they decay in unison. This is equivalent to the coupling of the guitar string to the body although the coupling is nonlinear in the case of the graphene nano resonator. As the vibrational amplitude decreases, the rate suddenly changes and the modes become decoupled, resulting in comparatively low decay rates, thus in very giant quality factors exceeding 1 million. This abrupt change in the decay has never been predicted or measured until now.

Therefore, the results achieved in this study have shown that nonlinear effects in graphene nano-mechanical resonators reveal a hybridization effect at high energies that, if controlled, could open up new possibilities to manipulate vibrational states, engineer hybrid states with mechanical modes at completely different frequencies, and to study the collective motion of highly tunable systems.

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May 11, 2017

Congratulations to New ICFO PhD graduate

Dr. Achim Woessner received his Master degree in Photonics Engineering, Nanophotonics and Biophotonics from Europhotonics coming from the Karlsruhe Institute of Technology (KIT), in Germany, before joining the Quantum Nano Optoelectronics research group led by ICREA Prof. at ICFO Frank Koppens. At ICFO, he centered his doctoral work in studying and understating the fundamental properties and capabilities of graphene plasmonics for the future development of new applications. Dr. Achim Woessner´s thesis, entitled “Exploring Flatland Nano-Optics with Graphene Plasmons” has been supervised by Prof. Dr. Frank Koppens.


Plasmons are charge oscillations coupled to electromagnetic radiation. One of their most intriguing properties is their deep subwavelength confinement resulting in strongly enhanced light-matter interaction. Metal plasmons have received tremendous interest over the last decades and have sparked the development of a range of new fields such as plasmonic nanophotonic components, metamaterials, metasurfaces and more exotic research areas such as quantum plasmonics. One of the main drawbacks of conventional metal plasmonics is that the plasmon lifetime is extremely short when the light is confined to deep subwavelength scales and that their wavelength is not tunable in situ. This is where graphene, a one atom thick semimetal consisting of carbon atoms arranged in a two-dimensional honeycomb lattice, comes into play. In graphene plasmons can be confined to extreme subwavelength scales while still having a long lifetime and their wavelength is tunable in situ. Graphene plasmonics is a relatively new research area but has already attracted a lot of attention. This is undoubtedly due to the fact that graphene plasmons are extremely versatile. They are a unique platform for exploring the limits of light matter interaction, two dimensional transformation optics, biosensing, and mid-infrared integrated optics.

The goal of this thesis is to explore the frontiers of graphene plasmonics both to understand the fundamental properties and limitations as well as to use the gained understanding to develop new concepts towards applications. We will mainly be using graphene encapsulated in hexagonal boron nitride (h-BN). This material has already shown that it is an excellent substrate for graphene as graphene fully encapsulated by h-BN at room temperature shows a mobility limited by the lattice vibrations of the graphene itself. Furthermore, it has very intriguing optical properties as it is a natural hyperbolic material which we will explore in the second part of the thesis. As measurement apparatus for the studies presented here we mainly used scattering-type scanning near-field optical microscopy (s-SNOM) as well as a technique based on s-SNOM we developed called near-field photocurrent nanoscopy. These techniques provide great insight into the working mechanisms of graphene optoelectronics with a nanometer resolution over a broad frequency range from the mid-infrared to the terahertz.

This thesis is split into a general introduction chapter and two main parts with experimental results. In the beginning I will give an introduction to graphene and its opto-electronic properties, graphene devices and their fabrication. I will also introduce h-BN, a dielectric layered material commonly used as substrate for graphene (Chapter 1). Then in the first main part of the thesis I will introduce the background and fundamentals of graphene plasmons (Chapter 2). In the following I will then introduce an experiment where we explore the limitations of the graphene plasmon lifetime at room temperature (Chapter 3). I will then show am optical phase modulator which is capable of tuning the phase in situ from 0 to 2? with a footprint of only 350nm exploiting the unique capability of tuning the graphene plasmon wavelength (Chapter 4). In the second part of the thesis I will give a background on photodetection with graphene (Chapter 5). I will then introduce a new measurement technique called infrared photocurrent nanoscopy (Chapter 6) and show how it can be used to study the optoelectronic properties of a variety of graphene devices in the infrared at the nanoscale (Chapter 7). Then I will show how graphene plasmons can be detected electrically using this technique (Chapter 8). Finally I will introduce a way of detecting phonon polaritons in h-BN electrically using graphene and show how this can be used to greatly enhance the photoresponse of graphene photodetectors in the mid-infrared (Chapter 9).

Thesis Committee

Prof. Dr. Harald Giessen-University of Stuttgart

Prof. Dr. Javier Garcia de Abajo- ICFO

Prof. Alexey Kuzmenko- University of Ginebra

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May 3, 2017

L4G SeminarCLAUDIA DRAXL ‘Graphene Revisited by Many-Body Theory: From Orbital Mapping to its …

Despite the numberless amount of investigations on graphene performed over the last years, this material still offers a variety of fascinating aspects to explore, in particular in view of its excitations. Combining density-functional theory with many-body perturbation theory provides a powerful framework for this purpose. Based on this methodology, I will address the following questions: Can we “see” orbitals in an electron microscope, and what kinds of images are to be expected? Can we introduce novel spectral features by stacking 2D materials? How are first- and second-order Raman spectra are affected by strain, that may be induced by an underlying substrate? How does graphene as a substrate, in turn, impact the photo-switching behavior of molecules?

Seminar, May 3, 2017, 12:00. Seminar Room

Hosted by Prof. Jens Biegert

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April 24, 2017

L4G SeminarTOBIAS GRASS ‘About Synthetic Graphene in Real Bilayers and Synthetic Bilayers of …

The physics of graphene, outstanding through its relativistic electron dispersion, can also be studied in ’synthetic’ graphene, built from cold atoms in a honeycomb lattice. In the first part of my talk, I will present a flexible setting in which such synthetic graphene is coupled to a second atomic layer with non-relativistic band structure. Different effects of this proximity will be discussed, ranging from modifications in the free bandstructure to many-body effects, including the semimetal-to-superfluid phase transition and magnetized phases in the Mott insulating regime. In the second part of my talk, I will switch to a system of real graphene in the quantum Hall scenario. In this context, I will discuss the possibility of engineering a synthetic bilayer structure by coupling different Landau levels using light. This strategy not only allows to create bilayer quantum Hall phases, but may also be used for controlling quasiparticle excitations.

Seminar, April 24, 2017, 12:00. Seminar Room

Hosted by Prof. Maciej Lewenstein

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April 10, 2017

Growing 2D and 3D graphene nano-structures on glass

Since its discovery, different techniques on how to grow high quality graphene over large planar area substrates have been investigated, developed and implemented. Among these techniques, processes like chemical vapor deposition (CVD) and plasma-enhanced CVD (PECVD) use transition metal foils that act as catalysts to favor the dissociation of a hydrocarbon gas. These processes have a major drawback; they require the transfer of graphene from the metal foils onto the desired substrate, in the process leaving organic residues on the substrate, reducing the performance and quality of the device.

In order to avoid contamination due to residue, research has been focused on direct growth on dielectric surfaces, using metal films as catalysts that can retract during growth, leaving the graphene on the dielectric area. Any additional residue would be removed using etching processes.

In a recent study published in 2D Materials, ICFO researchers Miriam Marchena and Josep Canet Ferrer led by ICREA Prof. Valerio Pruneri at ICFO in collaboration with researchers from Corning and Cornell University of New York, report on the use of Copper (Cu) catalytic templates for the growth of graphene onto 2D- and 3D-G structures.

To demonstrate the versatility of their proposed technique, the team of researchers investigated the growth of three graphene structures with different optical, electrical and morphological properties, by properly defining the initial catalytic Cu templates. These graphene structures were: the arrangement of non-aggregated copper nanoparticles (Cu NPs) in different layers to produce the formation of a 3D-G sponge-like (3D-GS) structure; one layer of isolated Cu NPs to produce 3D-graphene nanoballs (3D-GB), and the aggregation of Cu NPs to form larger catalytic structures that produced 2D graphene (2D-G) networks.

The growth of the graphene onto the substrate was performed in three different steps. They first created a Cu pattering by dip-coating Copper-oxide particles on the substrate¬¬ or by thermally evaporating the Cu from a Cu foil; then they grew the graphene by CVD methods and finally they removed any remaining Cu by wet etching, sublimation or both.

In all, the synthesis of the graphene structures for all three scenarios was properly achieved. Even more, a very high optical transmission was maintained while also preserving electrical properties of the material, a very promising feature for applications such as transparent electrodes and interfacial layers.

The results of this study are a major step forward towards the development of new surfaces that could be used for a wide variety of applications, such as antiglare display screens, solar cells, light-emitting diodes, and gas and biological plasmonic sensors, among others.

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March 28, 2017

Graphene 2017 Conference in Barcelona

The Phantoms Foundation, in collaboration with ICFO and the Institut Catala de Nanociencia i Nanotecnologia (ICN2) are organizing the 7th edition of the Graphene Conference series, the largest European event in Graphene and 2D materials.


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March 27, 2017

L4G SeminarSEONGJUN PARK ‘Two Dimensional Materials for Electronic Devices’

Two dimensional (2D) materials are crystalline materials with layered structures, including Graphene, h-BN, and Transition Metal Di-chalcogenides (TMD’s). Each of their layers is consisting of one or a few atomic layers and it forms van der Waals interactions with neighbouring layers. Atomically thin 2D materials range from semi-metallic graphene, semiconducting TMD’s to insulating h-BN. They have been studied intensively due to their extraordinary material properties.

We have been investigated 2D materials in two directions. One is to enhance the performance and the processibility of Si technology for near term applications. Especially, we have focused 2D materials as interface materials due to their atomically thin nature. For example, they are good candidates for diffusion barrier and interface materials between metal and Si to reduce the Schottky barrier heights and contact resistance in source and drain, which is one of the most critical issues for scaling down. We demonstrated a graphene hybrid interconnect for conventional Si semiconducting devices and demonstrated converting the Schottky nature of the M-S junctions into the Ohmic contact with 2D materials.

The other direction is to replace Si with 2D materials for post-Si technology and for functional devices that Si technology cannot cover well. We explored the possibility of 2D materials for photo detector and sub-10 nm graphene nanoribbons (GNRs) for a transistor channel. Also we demonstrated atomic layer deposition (ALD) on Graphene, one of the most fundamental challenges for the successful incorporation of 2D materials in electronic devices using physisorbed-precursor-assisted ALD.

In this talk, we will cover most of the topics listed above.

Seminar, March 27, 2017, 12:00. Seminar Room

Hosted by Prof. Valerio Pruneri

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