A graphene scanner permits pigments to be distinguished without the need for samples for the first time

A group of European researchers are developing a next generation graphene based scanner which will allow hitherto unknown aspects of works of art and other historic objects to be revealed. The equipment will enable the viewing of hidden images on canvases and unveil what is hidden inside three dimensional objects sealed centuries ago.

Testing the augmented reality based application in a real scenario. / Insidde.
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As well as showing sketches or previous paintings that have remained hidden beneath a particular art work, “the scanner will allow us to see preliminary drawings or identify overpaintings”. So explains Javier Gutiérrez, researcher at Treelogic and co-ordinator of Insidde, the Seventh Framework Programme – European Union funded project within which this work falls, and in which eight organisations from five European countries are participating.

The scanner allows the way in which the brushes have been applied to be observed, information which is opening up new fields of work for the scientists of Insidde: “The scanner may show details on how a painting was made that are of great art-historical value, because they tell us more about how the painter worked; these details are also extremely helpful for conservators when preparing for the restoration of paintings”, notes Laurens van der Maaten from Delft University of Technology.

An unexpected result

 Although the scanner is still in its trial and calibration phase, the researchers have already managed to use it to identify some of the materials found in specific works of art. This is a result that experts from the participating museums have been very positive about. In the words of Marta Flórez (Museum of Fine Arts of Asturias), “we didn’t expect to get this type of information, but with the prototype we have been able to clearly distinguish between some pigments, which in some cases will avoid having to puncture the painting in order to find out what materials the artist used”.

Besides inspecting paintings, which have a planar surface, the researchers are testing and adjusting the scanner in order to be able to generate images of sealed three-dimensional objects. For this, a structured-light scanner will be combined with the terahertz scanner.

(a) Still life. (b) Reproduction used for test and validation purposes. (c) Samples to be analysed with the terahertz scanner consisting of multiple pigments found in the original painting.

Geert Willems, Director R&D of 4DDynamics explains the benefit: “By reconstructing the 3D shape of the objects, we can help guide the terahertz scanner to the optimal positions around the curved shape of the objects, while making sure the scanner does not come in actual contact with the artwork”.

In the near future they will be scanning various Bulgarian pots from the 3^rd century which were sealed when they were found, and whose contents are unknown. Reneta Karamanova, restorer at the Regional Museum of History Stara Zagora, adds "Another valuable application of the scanner to archaeologists and restorers is its use to identify painted, carved or embossed decoration of pottery on which surface there are deposits of dust or limestone. The ascertainment of the condition of the ceramic surface by terahertz analysis would prevent the damage that can be caused to the vessel by manual cleaning of the deposits." 

More frequencies, more information

The graphene scanner is seen “as a new instrument which in no way damages the materials being studied and which will extend the investigated spectral region, and make more accessible the use of THz imaging analysis in the world of art” comment Raffaella Fontana and Marco Barucci from CNR-INO, who participate in the development of the focusing system. Mounted on what is referred to as the “XYZ table”, which measures 1.50 x 1.50 metres and is 1.20 metres high, the scanner comprises multiple heads which incorporate graphene emitters and receptors and can move three-dimensionally across the 2 square metre work area.

With regard to whether this scanner could replace the other methods that exist for obtaining hidden images in works of art such as scanners using x-rays, infra-red or ultraviolet radiation, the researchers at the University of Oviedo, in charge of the system,  are quite clear: “Each frequency range has a different capability in terms of penetrating the different layers of a piece of work, so the information that is recovered with each technique is complementary to the others”, explains Samuel Ver Hoeye, technical coordinator of the project.

(a) Ceramic from the III century. (b) Setup for 3D acquisition with a structured light scanner. (c) 3D model resulting from raw data before post-processing.

Why graphene?

Considered one of the materials of the future, graphene is formed by carbon atoms in a single layer only one atom thick. One of its many peculiarities is that, when submitted to electromagnetic waves, it behaves in a non-linear way. In other words, “It functions like a kind of frequency multiplier. If we make a wave of a particular frequency impinge on graphene, the graphene has the ability to emit another, higher, frequency”, agree David Gómez and Nuria Campos from ITMA Materials Technology.

This property of graphene is allowing scientists to emit, in the terahertz band, a band of frequencies which until now have mostly been achieved in experimental settings  and which are lower than infra-red but higher than the frequencies used by mobile phones and satellite communications. For this reason, “beginning to use it means filling a niche that exists between the frequency bands of other technologies that have already been developed”, acknowledges Javier Gutiérrez.

Sharing results

Beyond disseminating the results of their work in specialised international conferences, one of the priorities of the Insidde project is to ensure that the results also reach the general public, and as such the researchers will be making the images discovered by the graphene scanner publicly available. In this vein, the consortium is developing various means by which to popularise this new knowledge, for example through an augmented reality app for mobile phones which can be used in museums and art galleries. And, without having to leave home, the images can be seen on the internet through the open network Europeana. 

Members of the Insidde consortium

Led by Treelogic, the following seven organisations from five European countries are participating in the Insidde project (Integration of technological solutions for imaging detection and digitisation of hidden elements in artworks): from Spain, ITMA Materials Technology, the University of Oviedo and the Museum of Fine Arts of Asturias; from The Netherlands, Delft University of Technology; from Italy the Istituto Nazionale di Ottica; from Bulgaria, the Regional Museum of History of Stara Zagora, and from Belgium the company 4DDynamics.

Yield from graphene much higher than first thought

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Generating terahertz light via graphene may soon be an interesting alternative to the current methods used to generate THz light. This is the conclusion reached by researchers in the Optics Research Group and recently published in an article in ACS Nano.

‘We have discovered that after illuminating it with an ultrashort (femtosecond) laser pulse, a single layer of graphene emits a short flash of terahertz light,’ says Prof. Paul Planken. Light with a terahertz frequency (around 1012 Hz) can be used for a wide range of practical applications, making this an interesting development for advancements in imaging and spectroscopy, for example. The main advantage is that light of this frequency is not harmful, unlike X-rays. ‘The light can be used to look inside and through packaging, for example, and to analyse works of art.’

Relatively strong

Terahertz light is normally generated via completely different methods, such as by using antennas fabricated on semi-conductors. The idea of illuminating ultrathin graphene layers has come up before, but it was thought that the THz light yield would be too low. Planken: ‘In absolute terms, the terahertz light is weak, but considering the fact that graphene is just 1 atom thick, it is actually relatively strong. The mechanism responsible for the partial conversion of laser light into THz light occurs when ultrafast electric impulses are directly generated in the graphene as, in a manner of speaking, the light particles in the laser pulse collide with the electrons in the graphene.’

Gold

‘Even more important is the discovery that when we lay the graphene on a layer of gold and then induce so-called surface plasmons (optical surface waves) on the surface of the gold using femtosecond laser pulses, the terahertz yield of the graphene rises by up to a factor of 300!’, continues Planken. This makes graphene an interesting possible alternative. ‘We had expected that there would be an enhancement factor if we induced plasmons, but we hadn’t expected it to be this significant.’ Researchers have not yet got to the bottom of the mechanism responsible for this huge increase, but the high pump light intensity in the graphene layer of the surface plasmons plays an important role.

Well before 2030

‘Just two years ago, the Nobel Prize winner Novoselov wrote that he did not expect to see a practical THz generator using graphene before 2030, but I now think that it could be developed, at least an optically pumped one, well before 2030. In any case, we will definitely know whether it’s a possibility within the next few years.’

The article entitled Plasmon enhanced terahertz emission from single-layer graphene published inACS Nano is the result of collaboration between the Optics Research Group and the research group of Prof. Dai-Sik Kim and other researchers at Seoul National University and Ajou University in South Korea.

Super-terahertz heterodyne spectrometer using a quantum cascade laserq

http://repository.tudelft.nl/view/ir/uuid:1311361d-f737-40a5-98ef-aa19ad5e21f0/

Ren, Y.


High-resolution spectroscopy at super-terahertz frequencies (2-6 THz) can play a vital role in astronomical observation and atmospheric remote sensing. It provides unique and important information on the history of our universe and its evolution, by getting into the insight of the physical and chemical conditions. Moreover, it can help to address questions about our own atmosphere such as ozone layer depletion and climate change problems. However, up to now this frequency region has rarely been accessible for high-resolution spectroscopy due to the lack of suitable local oscillator technology. The recently developed terahertz quantum cascade lasers become the most promising candidate as a novel solid-state terahertz source.
Terahertz quantum cascade lasers, after one decade development from the first demonstration in 2002, are now capable of delivering milliwatts or more of continuous-wave coherent radiation over the terahertz frequency range. For the local oscillator application, a heterodyne sensitivity measurement has been performed, which proved a prominent power stability with no additional inher- ent noise for a terahertz quantum cascade laser. However, a finial and crucial step to demonstrate a heterodyne receiver system is a direct high-resolution heterodyne spectroscopic experiment, which is also an important approach to characterize the performance of the entire system.
In this thesis, we have realized a high-resolution heterodyne spectrometer by introducing a terahertz quantum cascade laser as a local oscillator, a super- conducting hot electron bolometer as a mixer and a Fast Fourier Transform Spectrometer as a back-end spectrometer. The first molecular spectrum by using a terahertz quantum cascade laser as local oscillator was obtained at a frequency of 2.9 THz. We push further this heterodyne spectrometer up to 3.5 THz with a more advanced terahertz quantum cascade laser, where the first 3.5 THz methanol (CH3OH) spectra were obtained with ∼1 GHz tuning range from the local oscillator. Excellent agreement between the measured spectra to the theoretical calculation was achieved with respect to both line intensity and frequency.
Furthermore, we have explored the frequency locking capability of such terahertz quantum cascade lasers by using a terahertz molecular absorption line as a reference frequency. Based on a compact gas cell and a power detector, the frequency stabilization is achieved with a minimal linewidth of 18 kHz and a Gaussian-like shape. Such kHz linewidth with a compact locking scheme is favorable for any space- or ballon-borne instrument.
For actual observation applications, the effective integration time is a cru- cial issue that determines the efficiency of the observation. A robust exper- imental scheme has been demonstrated to simultaneously stabilize the fre- quency and amplitude of a terahertz quantum cascade laser. The frequency stabilization has been realized using a methanol absorption line, a power de- tector and a proportional-integral-derivative loop. The amplitude stabilization of the incident power has been achieved using a swing-arm voice coil actuator as a fast optical attenuator, and using the direct detection output of a super- conducting mixer in combination with a 2nd feedback loop. As a result, a fully stabilized heterodyne spectrometer at super-terahertz freqeuencies was demon- strated, with improved Allan Variance times, and also supported by measured heterodyne molecular spectra.
Based on all this work, terahertz quantum cascade lasers become techno- logically much more mature and convincing to be used as local oscillator. A di- rect outcome is a new NASA mission: Galactic/Xgalactic Ultra long duration balloon Spectroscopic Stratospheric THz Observatory (GUSSTO), in which terahertz quantum cascade lasers have been proposed as local oscillators for the 4.7 THz receiver channel. Within the Phase-A-Concept study period, in collaboration with Q. Hu’s group at MIT and C. Walker’s group at University of Arizona, we demonstrated a heterodyne receiver using an advanced third- order distributed feedback quantum cascade laser as a local oscillator, whose emission frequency is only a few GHz away from the OI line at 4.7448 THz. Excellent receiver sensitivity together with a heterodyne spectrum have been demonstrated. All these efforts should lead soon to the first realization of a terahertz quantum cascade laser for astronomical application in a telescope. Also the local oscillator technology described in this thesis, offers the technique for other instruments such as Oxygen Heterodyne Camera (OCAM) proposed on SOFIA and also creates new mission opportunities in the future.

Enhanced terahertz emission from thin film semiconductor/metal interfaces

http://repository.tudelft.nl/view/ir/uuid:d69d7778-c5fc-4d2c-9b17-f3aaf2ee5f82/
Ramakrishnan, G.
Terahertz light is electromagnetic radiation, similar to visible light. The photons that the terahertz light is comprised of carry a much smaller amount of energy compared to the visible light photons. Unlike visible light, terahertz light can pass through materials like plastic, cardboards, wood etc.; a very useful property which enables it to replace harmful X-rays in many security applications. However, it is not possible to see the terahertz photons with our naked eyes, and it requires special detectors to observe them.
A lot of attention has been drawn to terahertz radiation recently because of its potential use in various applications in national security (as mentioned before), and in the biomedical and the semiconductor industries. Essential to any terahertz device is a suitable terahertz source. There are different methods to generate this type of radiation. After the advent of ultrafast lasers, an optical technique was developed which became very popular afterwards. In very simple terms, this technique can be considered as producing an extremely quick disturbance in a suitable material using an extremely quick flash of laser light. Here the phrase `extremely quick' refers to femtosecond time scales where one femtosecond is one millionth of one billionth of a second. The quick electromagnetic disturbance can lead to the emission of a pulse of electromagnetic radiation of a different frequency: terahertz light. Certainly, this process depends on the material in which the disturbance is created, which we will see in a bit more detail below. It is this method of terahertz generation we focus on in this thesis.
Let us now have a closer look at this. Only certain materials have this property of converting a flash of laser light efficiently into a flash of terahertz light, for example, some semiconductors. What type of a disturbance can a flash of laser light, (a laser pulse), create in such a material? In the case of semiconductors, the incident light pulse can lead to the excitation of mobile conduction electrons by providing them with the required energy. The semiconductor becomes momentarily a conductor. If it was initially kept under an external voltage bias, a momentary current is thus induced by the light pulse. A time-varying current can act as a source of electromagnetic waves. The emitted flash of light in this case is a terahertz pulse. Similar momentary disturbances can also be produced in certain nonlinear crystals without really exciting electrons from their bound states, but by causing an ultrafast displacement of the bound charges. In both these cases, the emitted light pulse carries information about the material's response to the femtosecond flash of light, which in fact is information about the material per se. For example, we see that the illumination of graphite with femtosecond laser pulses results in the emission of terahertz light pulses. The properties of the emitted terahertz pulse are suggestive of a transient photocurrent produced in the material. Graphite consists of stacks of atomic planes of carbon which are loosely attached to each other. Electric conductivity along a direction perpendicular to these planes is known to be very low as in this case electrons have to jump from one plane to the other. However, in our experiments the emitted terahertz pulses indicate a resultant photocurrent flowing in that direction.
Oxidized copper surfaces are known to act as a semiconductor-diode. A semiconductor diode is a device which restricts the electric current to flow through it in only one direction. In the case of oxidized copper surfaces, this is possible by a potential barrier formed at the interface between copper and cuprous oxide. When a femtosecond light pulse excites electrons at such an interface, and frees electrons in it, a quick pulse of current flows across the interface. This transient current emits a terahertz pulse.
The same idea can also be applied to different other semiconductor-metal interfaces. We have shown that terahertz pulses can be produced by exciting thin films of germanium and silicon deposited on a gold substrate. If the thin films of these semiconductors prepared on a glass substrate are illuminated with femtosecond light pulses, the emitted terahertz pulses are very feeble. When the thin films of the semiconductors are on a gold substrate, a surprising enhancement of the generated terahertz light from such thin films is observed. The later part of the thesis concentrates on the different possible ways in which the gold substrate can contribute to the enhancement of terahertz radiation from thin films.
When coherent laser light is incident on an extremely thin film of a semiconductor material deposited on a metal surface, light reflected from the top and the bottom of the film can result in a complete or partial reduction of the reflected light. It is equivalent to trapping the light inside the film, which leads to enhanced absorption in the thin film. This is sometimes called `coherent optical absorption'. Very strong absorption of the pump light can be achieved in thin films as compared to bulk materials, as a result of this. When light is strongly absorbed by the semiconductor, more electrons will be freed and a stronger transient current can be produced which can result in a stronger terahertz emission. This leads to the counter-intuitive result that less material emits more terahertz light.
The concentration of laser light inside the terahertz generation material can also be done by making use of surface plasmon excitation. Surface plasmons are light waves bound to the interface between a metal and a dielectric. In our case, since the terahertz generation takes place at the interface between a metal and a semiconductor, surface plasmons can play a role in the process. As surface plasmons are bound to the interface, they can enhance the local intensity of the pump light at the interface where the generation of terahertz radiation takes place. Using this method, we demonstrate the enhancement of terahertz emission from a layer as thin as a single molecular layer of a nonlinear optical material called hemicyanine. We also go on to show that enhanced terahertz emission can be achieved from semiconductors deposited on a nanostructured metal surface, where again surface plasmons are excited. Concentration of the laser light intensity using plasmonics also leads to terahertz emission from the metal surfaces itself, i.e., without any semiconductor on top.
In short, this thesis discusses the possibilities of terahertz generation from ultrathin semiconductor layers, metals and their interfaces, and on different optical techniques to enhance the terahertz emission. These techniques not only help in the study of the properties of ultrathin layers of materials, but can also help in miniaturizing terahertz sources for various applications.

TU Delft professor Andrea Neto wins ERC grant

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TU Delft EEMCS professor of Applied Electromagnetics Andrea Neto has been awarded a grant of € 1.5 million by the European Research Council.
To win an ERC grant, it is not enough to be the best in your field. “The Valorisation Centre made me rewrite the proposal several times”, Andrea Neto recalls. ”Coming from an industrial background, I am always careful not to promise anything I cannot deliver, so my statements were not ambitious enough. As I was also advised to quantify my goals, I finally claimed my antenna technology would make time-domain sensing systems 1000 times more efficient. This bold statement got me through the first evaluation phase, and later the reviewers said: ‘even if you accomplish less, it’s money well spent’. So the strategy worked!”
Dispersion
Andrea, who joined EEMCS a year and a half ago, is happy in his new environment. “My colleagues in the Microelectronics department are almost all IEEE-fellows”, he says admiringly. “I hope to follow in their footsteps.” He is an international expert on Terahertz (THz) antennas. On the electromagnetic spectrum THz is located between the upper end of the microwave range and the far infrared, a region that is almost entirely unexplored. “Broadband imaging, my specialty, is essentially not done”, says Andrea. “In my proposal I addressed one of the fundamental problems: dispersion. In order to radiate efficiently antennas have to be large. However, in large antennas the ‘phase centre’, the point from which the radiation spreads outward, moves according to frequency. I pioneered an antenna where it always remains in the same position. It’s a scientific breakthrough with applications ranging from deep space investigation and environmental monitoring, to security screening and biomedical imaging.”
Revealing the origins of the universe
The most striking element of Andrea’s CV is his experience in Spectroscopic Space Science. After his PhD research in this field, he designed the THz antennas for the European Space Agency’s Herschel Planck space observatory. “I expect us to also design the antennas for Planck’s successor, the Japanese-European SPICA space telescope. Chances are high because of our collaboration with SRON, the Netherlands Institute for Space Research, which is the prime investigator of SPICA’s spectrometer. One of the three PhDs I am hiring with the grant will be working on the design of focal plane arrays for the imaging of cosmic background radiation. It’s exiting that the THz spectrum may reveal the mechanisms at the origin of the universe.”
Radar agenda
Another PhD student is going to work on the improvement of time-domain sensing instruments, a joint project with the THz Research group of professor Paul Plancken (Applied Physics). “They use THz waves for non-destructive structural analysis”, Andrea explains, “for example to detect hidden layers of paint in paintings. With the right antennas, we may see things that were not possible before. In principle time domain sensing can also be used for the diagnosis of skin cancer and for the evaluation of the air quality.” A third PhD student will be involved in a joint project on 600 to 700 GHz radar systems. “The Microelectronics department is developing a road map for this technology; I am trying to build bridges between the department and other groups. The radar agenda can attract a lot of researchers, because it has many applications, such as the monitoring of the atmosphere or the detection of concealed weapons. NXP Semiconductors and the Ministry of Defence have also expressed an interest.” Finally, Andrea wants to hire an experienced postdoc to help him develop most of the theory.
Best of both worlds
With two more postdocs bringing their own financing, the move of Johan Lager to his group and a vacancy to fulfill, he will be leading a group of nine. “I have been looking forward to this moment”, he says cheerfully. “It has been hard work to obtain funding for my research, because it’s largely fundamental and there are no tangible results yet.” Andrea was educated as an electromagnetics theorist in Italy, but as it was difficult to find a job, he moved towards applied research. He regrets that in North European universities many aspects of electromagnetic theory are not taught anymore. “In southern countries there is less interaction between industry and universities, so universities feel less pressure to be market & technology-driven. Fortunately EEMCS has now been accredited by the prestigious European School of Antennas, so our students can take courses from universities that excel in theoretical education. This way they get the best of both worlds.”
Source: TU Delft.