October 2, 2017

2D material photodetectors get even more sensitive

Substantial progress in photonic devices based on 2D materials, due to their exceptional electronic and optical properties, has been achieved, converting them into a potential material for optoelectronic applications that range from laser applications to the next generation of photodetectors or flexibles devices. However, to date, their sensitivity has not been able to surpass those of other standard CMOS current technologies like Silicon detectors.

In a recent study published in Nature Communications, ICFO researchers Nengjie Huo and ICREA Prof. at ICFO Gerasimos Konstantatos report on the development of an ultrasensitive two-dimensional photodetector employing an in-plane phototransistor with an out-of-plane vertical MoS2 p–n junction as a completely novel sensitiive scheme.

In their study, the team of scientists exfoliated a few layer MoS2 flakes on the SiO2/Si substrate using a micromechanical exfoliation method. To fabricate the MoS2 out-of-plane PN junction, they used AuCl3 P-type chemical surface doping and made the bottom N-MoS2 serve as the carrier transport channel while the top P-MoS2 was effectively isolated from the metal contacts. The PN junction served as a novel sensitizing scheme for all-2D based phototransistors by providing a charge separation mechanism that suppresses recombination in MoS2 and therefore has allowed to reach a quantum efficiency of nearly 10%, orders of magnitude higher than prior reports in 2D based phototransistors. Moreover, the device achieves a photoconductive gain of >105 electrons per photon, a responsivity of 7?×?104?A?W?1, and a time response on the order of tens of ms. Most importantly the device exhibits very low noise offering specific detectivity on the order of 1014 Jones in the visible, almost an order of magnitude better than the standard high performance silicon detectors.

The results of this study pave the way towards the use of these ultra-sensitive phototransistors in other 2D semiconductors or in combination of those to facilitate sensitization, particularly those possessing low band gap to extend the spectral coverage of the 2D materials realm.

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September 13, 2017

Falling Walls Lab Barcelona

On September 12th, the Universitat Pompeu Fabra and FECYT hosted the qualifying round of the Falling Walls Lab satellite event in Barcelona. The Falling Walls Lab is a challenging, inspiring and interdisciplinary format for outstanding talented professionals. It offers the opportunity to excellent academics and professionals to present their innovative ideas, research projects and social initiatives.

The first Falling Walls Lab took place in 2011 in Berlin one day prior to the Falling Walls Conference. Thanks to the huge success of the Falling Walls Lab 2011, the Falling Walls Foundation extended the format to a global scale in 2012. Since then, international Labs take place in different vibrant cities around the world throughout the year. So far, Falling Walls Labs have taken place in almost 50 countries.

This year, the Barcelona event selected thirteen candidates to present their work, in under 3 minutes, to a prestigious science and business jury. Presentations included ideas from different fields such as energy production, health monitoring, dyslexia, lead based devices, accessibility in playing music, freedom of education, shareholder value maximization, among others.

Dr. Emre Ozan Polat, a postdoctoral researcher in the Quantum Nano-optoelectronics research group at ICFO, was proclaimed winner of the event with his presentation “Breaking the Wall of personal health monitoring. Wearable wellness patches showing health status”

As winner of the Falling Walls Lab Competition in Barcelona, Emre will now continue on to the gran finale competition, held each year in Berlin on November 8th. Here winners of the international Labs from around the world, numbering over 100 participants, will compete for the final prize.

Congratulations and good luck Emre!!!

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September 12, 2017

Graphene @ MWC Americas

The Mobile World Congress Americas debuts this year in the Unites States and there is no better place to start than in San Francisco. From September 12-14 this renowned congress intends to showcase how mobile is creating the connected life, transforming how individuals, businesses and entire industries communicate, interact and innovate.

This event will highlight core mobile technologies, consumer and industrial applications in the Internet of Things, the intersection of mobile with entertainment, content and media, and the leading role of the Americas region in driving global innovation.

The Graphene Pavilion, already a consolidated exhibition stand of the MWC in Barcelona and Shangai, will also be present in the MWC Americas. Organized by ICFO – The Institute of Photonic Sciences, the National Graphene Association and GSMA, the pavilion will be showcasing a wide range of graphene-based technologies such as 3D printing, e-tattoo for sensing EMG, ECG skin temperature and hydration, graphene biosensors, NFC antennas printable, flexible wifi receivers and proximity sensors based on advanced photodetection.

The exhibitors for this Mobile World Congress edition include ICFO, Graphene Tech, Graphene 3D Lab, University of Texas, National Graphene Association, Consiglio Nazionale delle Ricerche CNR, Argonne National Laboratory, AMO, and Nanomedical Diagnostics.

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September 4, 2017

L4G Seminar GIULIO CERULLO ‘Two-dimensional Electronic Spectroscopy from the Visible to the Ultraviolet’

Two-dimensional (2D) spectroscopy is the “ultimate” ultrafast optical experiment, since it provides the maximum amount of information that can be extracted from a system within third-order nonlinear spectroscopy. The first applications were with IR pulses, resonant with vibrational transitions. Recently, 2D techniques have been extended to the visible and UV ranges, targeting electronic transitions. 2D electronic spectroscopy (2DES) allows fundamentally new insights into the structure and dynamics of multi-chromophore systems, measuring how the electronic states of molecules within a complex interact with one another and transfer electronic excitations [1]. This presentation will review the experimental techniques currently used to perform 2DES in the visible range and will introduce our approach to 2DES, based on a passive birefringent interferometer for the generation of phase-locked pump pulses [2]. We will present a few exemplary results on multi-chromophoric systems and nanostructures [3, 4] and finally discuss the prospects of extending 2D techniques to the UV range [5], of interest for biomolecules such as DNA and proteins.

Seminar, September 4, 2017, 15:00. Blue Lecture Room

Hosted by Frank Koppens

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August 29, 2017

Graphene Single Photon Detectors

Considerable interest in new single-photon detector technologies has been scaling in this past decade. Nowadays, quantum optics and quantum information applications are, among others, one of the main precursors for the accelerated development of single-photon detectors. Capable of sensing an increase in temperature of an individual absorbed photon, they can be used to help us study and understand, for example, galaxy formation through the cosmic infrared background, observe entanglement of superconducting qubits or improve quantum key distribution methods for ultra-secure communications.

Current detectors are efficient at detecting incoming photons that have relatively high energies, but their sensitivity drastically decreases for low frequency, low energy photons. In recent years, graphene has shown to be an exceptionally efficient photo-detector for a wide range of the electromagnetic spectrum, enabling new types of applications for this field.

Thus, in a recent paper published in the journal Physical Review Applied, and highlighted in APS Physics, ICFO researcher and group leader Prof. Dmitri Efetov, in collaboration with researchers from Harvard University, MIT, Raytheon BBN Technologies and Pohang University of Science and Technology, have proposed the use of graphene-based Josephson junctions (GJJs) to detect single photons in a wide electromagnetic spectrum, ranging from the visible down to the low end of radio frequencies, in the gigahertz range.

In their study, the scientists envisioned a sheet of graphene that is placed in between two superconducting layers. The so created Josephson junction allows a supercurrent to flow across the graphene when it is cooled down to 25 mK. Under these conditions, the heat capacity of the graphene is so low, that when a single photon hits the graphene layer, it is capable of heating up the electron bath so significantly, that the supercurrent becomes resistive – overall giving rise to an easily detectable voltage spike across the device. In addition, they also found that this effect would occur almost instantaneously, thus enabling the ultrafast conversion of absorbed light into electrical signals, allowing for a rapid reset and readout.

The results of the study confirm that we can expect a rapid progress in integrating graphene and other 2D materials with conventional electronics platforms, such as in CMOS-chips, and shows a promising path towards single-photon-resolving imaging arrays, quantum information processing applications of optical and microwave photons, and other applications that would benefit from the quantum-limited detection of low-energy photons.

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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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