photonics – Progress in Research

Crucial discovery on the ultrafast charge injection in semiconductors

Posted on 13/09/202306/08/2024 by Erik Franco

The capability to follow and control ultrafast electron dynamics in matter with light pulses is a long-sought goal, with important implications in many fields of technology and research. In a semiconductor, for example, charge injection by few-femtosecond infrared pulses could be used to turn the material into a conductive state, realizing ultrafast switches in opto-electronics, a milestone that promises to increase the limiting speed of data processing and information encoding. This technological breakthrough can only stem from a comprehensive knowledge of light-induced charge injection, a key challenge of modern solid-state physics and photonics.

A study published in Nature Photonics tackles this problem by investigating field-driven carrier injection in a prototype semiconductor (monocrystalline germanium) with attosecond transient reflection spectroscopy: the researchers from Politecnico di Milano, in collaboration with the Istituto di Fotonica e Nanotecnologie (IFN-CNR), the Istituto per la microelettronica e microsistemi (CNR-IMM), the Istituto Nanoscienze (CNR-NANO) and a group from the Università degli Studi di Salerno, have discovered a new light-matter interaction regime where charges are excited by diverse coexisting mechanism. These mechanisms compete and develop on different time scales, of the order of few millionths of billionth of a second.

The researchers succeeded in disentangling the complex charge injection regime on these extreme temporal scales thanks to the experiments performed by the Attosecond Research Centerwithin the ERC project AuDACE (Attosecond Dynamics in AdvanCed matErials) and the PRIN project aSTAR. By means of simulations based on advanced theoretical models, they have shown the complex interaction between diverse mechanisms in the quantum-mechanical response of the material, never observed before, with important implications in many fields as optics, photonics, and information technology.

Those are significant results because the knowledge of the excitation processes induced by light in semiconductors allows us to design new opto-electronic devices with optimized ratio between charge injection speed and dissipated power.
Matteo Lucchini, professor of the Department of Physics and last author of the study

Read the study online

New ultrafast titanium nitride-based photonics

Posted on 23/08/202306/08/2024 by Erik Franco

A study resulting from the collaboration between the Department of Energy and the Department of Physics of Politecnico di Milano and supported by an interdisciplinary PhD research by Silvia Rotta Loria has been published in the journal Advanced Optical Materials as a cover article.

The study explains the origin of the ultrafast optical response of titanium nitride (TiN). This material, already knownfor its refractory properties, is also attracting increasing interest because of its fast response to photo-excitation and the possibility of controlling its optical and electronic properties during synthesis.
TiN films have already been used for thermo-photovoltaic devices, for artificial photosynthesis or for micro super-capacitors on chips. Moreover, TiN is compatible with technologies used in digital electronics. Overall, it is therefore a material with a great potential for developing ultrafast photonic devices.

This collaboration has made it possible to thoroughly study this material of great technological interest and to clarify the origin of its peculiar response to light excitation, which can be engineered through the manufacturing procedure,
Prof. Margherita Zavelani Rossi, Department of Energy, co-author of the paper

The study was carried out thanks to the synergy between two Departments of Politecnico: TiN films were made in the NanoLab (Micro and Nanostructured Materials Lab) of the Department of Energy, were then characterised in the ultrafast spectroscopy laboratories of the Department of Physics, and the experimental data were interpreted using a model developed in the Department of Physics.

Thanks to the accurate numerical model developed, it is now possible to determine how the response of a titan nitride thin film can be controlled through light; this knowledge is crucial for developing new miniaturised opto-electronic and photonic devices,
Prof. Giuseppe Della Valle, Department of Physics, co-author of the paper

The experiment mentioned in the paper is one of the outcomes of the METAFAST project funded by the European Union’s H2020-FET-OPEN programme, coordinated by Prof. Giuseppe Della Valle. The project aims to develop a new class of ultrafast optical devices based on special nanostructured surfaces (nonlinear metasurfaces).

Read the study

The official METAFAST project website

The microscope that reveals the chemical composition of samples

Posted on 11/07/202306/08/2024 by Erik Franco

An international research team co-ordinated by the Institute for Photonics and Nanotechnologies of the National Research Council in Milan (CNR-IFN) and involving researchers from Politecnico di Milano’s Department of Physics, Columbia University and Stanford University, has developed an innovative optical microscope capable of producing detailed images of the chemical composition of a sample more effectively compared to the systems currently in use. The result has been published in the journal Optica.

This instrument represents a major breakthrough in the field of microscopy and spectroscopy, opening up new perspectives for research in the materials and life sciences. Indeed, it will be able to contribute to the study of innovative two-dimensional materials and to the characterisation of microplastics found in the environment and within animal tissues.

The benefits of the microscope stem from the unprecedented combination of two techniques, Raman spectroscopy and Fourier transform spectroscopy. The developed method allows Raman and fluorescence maps to be acquired in up to 100 times less time than with traditional instruments and to measure all sample points at the same time and with high efficiency, acquiring more data simultaneously.

The study online

Photonic chips for low-power neural networks

Posted on 15/05/202306/08/2024 by Erik Franco

A study by the Politecnico di Milano and Stanford University, published in the journal Science, shows that it is possible to create extremely efficient neural networks using photonic chips.

Neural networks are distributed computing structures inspired by the structure of a biological brain and aim to achieve cognitive performance comparable to that of humans. They are used in many areas, such as speech and image recognition and synthesis, autonomous driving and augmented reality systems, bioinformatics, genetic and molecular sequencing, and high-performance computing technologies.

Neural networks are trained with a large amount of known information, on the basis of which they become able to adapt their behaviour, working autonomously. However, their training is an extremely energy-intensive process.

Researchers from the Politecnico’s Photonic Devices Lab and Polifab, the university’s micro- and nano-technology centre, in collaboration with researchers from Stanford University, have sought a solution and developed a silicon microchip just a few square millimetres in size with an integrated photonic accelerator that allows calculations to be performed very quickly – in less than a billionth of a second – and efficiently. Thanks to this photonic chip, neural network operations take place with considerable energy savings.

In addition to neural networks, it will be possible to use this device as a computing unit for multiple applications where high computational efficiency is required, e.g., for graphics accelerators, mathematical coprocessors, data mining, cryptography and quantum computers.

The study online

The origin of the stains on leonardo da vinci’s codex atlanticus revealed

Posted on 02/05/202306/08/2024 by Erik Franco

An in-depth study by the Politecnico di Milano, published in Scientific Reports, has uncovered the cause of a number of black spots that appeared several years ago on the Codex Atlanticus, one of the most extensive and fascinating collections of Leonardo da Vinci’s sketches and writings.

The stains particularly affect the modern panel – known technically as the passepartout – that binds and frames Da Vinci’s original papers in the codex preserved at the Veneranda Biblioteca Ambrosiana library in Milan. The interdisciplinary research team coordinated by Lucia Toniolo, Professor of Material Science and Technology at the Politecnico di Milano, used a series of non-invasive and micro-invasive analysis techniques to examine and understand the nature and causes of the phenomenon of blackening that has been observed on some 210 pages of the Codex since 2006 and has caused great concern among museum curators and conservators, as well as scholars.

The research by the Politecnico di Milano focused on folio 843 of the Codex and, combining hyperspectral photoluminescence analysis, UV fluorescence imaging, with micro-ATR-IR imaging, revealed the presence of starch glue and vinyl glue in the areas where the staining is most concentrated, right at the edge of the folio.

In addition, the presence of rounded inorganic nanoparticles composed of mercury and sulphur was detected within the cavities formed between the cellulose fibres of the passepartout paper. These particles were identified as metacinnabar, a mercury sulphide in an unusual black crystalline phase.

In-depth studies on paper preservation methods have allowed us to formulate some hypotheses on the formation of metacinnabar. The presence of mercury could be linked to the addition of an anti-vegetative salt to the glue mixture used during the restoration of the Codex in the 1960s and 70s, which could have been applied in only certain areas of the passepartout paper, precisely where it holds Da Vinci’s folio, to ensure adhesion and prevent microbiological infestations on the Codex. The presence of sulphur, on the other hand, has been linked to air pollution (sulphur dioxide (SO2) levels were very high in Milan in the 1970s) or to the additives used in the glue, which over time would have led to a reaction with mercury salts and the formation of metacinnabar particles, responsible for the black stains.

A new chapter for nonlinear optics

Posted on 19/09/202206/08/2024 by Erik Franco

A new bidimensional semiconductor shows the highest nonlinear optical efficiency over nanometer thicknesses. This is the result of a new study recently published in Nature Photonics by Xinyi Xu, PhD student of Columbia University, and Chiara Trovatello, postdoctoral research scientist at the Department of Physics of Politecnico di Milano, together with Prof. Giulio Cerullo from the Department of Physics of Politecnico di Milano, Dmitri N. Basov and P. James Schuck from the Columbia University.

Optical fibers, bar code readers, light scalpels for precision surgery… the innumerable applications which have revolutionized our daily life rely exclusively on one tool: the laser. Each laser, however, emits light only at one specific wavelength and in order to generate new colors one can make use of specific crystals exploiting nonlinear optical processes. The miniaturization trend, which has dominated the world of electronics, enabling the realization of powerful consumer devices, such as smartphones and tablets, is now moving the world of lasers and their applications, which constitute the so-called field of photonics. For this reason, it is necessary to realize nonlinear processes inside thinner and thinner crystals.
Chiara Trovatello, author of the study

The typical nonlinear crystal thickness is on the order of a millimiter. In this study researchers have proven that a new nonlinear material – the 3R crystal phase of molybdenum disulfide – over a thickness of few hundreds of nanometers (1 nm = 10^-9 m) can achieve an unprecedent nonlinear optical gain. This study sets the ground for a new revolution in the field on nonlinear optics.

This new crystal opens innumerable future applications, which could be directly integrated on a micrometric optical chip, reducing the typical size of nonlinear optical devices. Among the most relevant applications: optical amplifiers, tunable lasers and quantum light generators over nanometer length scales.

On-chip nonlinear application will reinvent photonic devices through thinner and more compact designs.
Prof. Cerullo

Read the study

Optical wireless: the new frontier for communication

Posted on 08/07/202206/08/2024 by Erik Franco

In the field of cable transmission, the advent of optical fibres represented an epochal technological leap, allowing light to be used to transfer enormous amounts of data, and they now form the basic infrastructure of the Internet and global telecommunications systems.

For wireless communications too, it is expected that optical connections will soon represent the new frontier. Similarly to what happens in optical fibres, even in free space, light can travel in the form of beams having different shapes, called “modes”, and each of these modes can carry a flow of information. Generating, manipulating and receiving more modes therefore means transmitting more information. The problem is that free space is a much more hostile, variable and unpredictable environment for light than an optical fibre. Obstacles, atmospheric agents or more simply the wind encountered along the way, can alter the shape of the light beams, mix them and make them at first sight unrecognisable and unusable.

A study by the Politecnico di Milano, conducted together with Stanford University, the Scuola Superiore Sant’Anna in Pisa and the University of Glasgow and published in the prestigious journal Light: Science & Applications, has found a way to separate and distinguish optical beams even if they are superimposed and the form in which they arrive at their destination is drastically changed and unknown.

This operation is made possible by a programmable photonic processor built on a silicon chip of just 5 mm². The processor created is able to receive all the optical beams through a multitude of microscopic optical antennas integrated on the chip, to manipulate them through a network of integrated interferometers and to separate them on distinct optical fibres, eliminating mutual interference. This device allows information quantities of over 5,000 Ghz to be managed, at least 100 times greater than current high-capacity wireless systems.

The activity is funded by the European Horizon 2020 Superpixels project, which aims to create next-generation sensor and imaging systems by exploiting the on-chip manipulation of light signals.

The studio is authored among the others by Francesco Morichetti, head of the Photonic Devices Lab and Andrea Melloni, director of Polifab, the Politecnico di Milano centre for micro and nanotechnologies.

Read the study online

Controlling how fast graphene cools down

Posted on 09/03/202206/08/2024 by Erik Franco

Graphene is the thinnest material ever produced, with the thickness of a single atomic layer, thinner than a billionth of a meter.

A property of its is 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 ICN2 of Barcelona (Spain), and it is supported by the European project Graphene Flagship.

This work 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 theoretical modeling of the relaxation of the charge carriers of graphene as a function of the external electric field has allowed the identification of the physical mechanism underlying the observed phenomenon. 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 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
Giulio Cerullo, professor of the Department of Physics of Politecnico di Milano

The research on ACS Nano

Discovery of a new phase transition in quasi-crystals

Posted on 07/03/202206/08/2024 by Erik Franco

A team of researchers from the Politecnico di Milano and the University of Rostock (Germany) has discovered and observed in the laboratory a new type of phase transition in a quasi-crystal made of laser light.

The discovery of this new phase transition in quasi-crystals represents a breakthrough in the understanding of some fundamental phenomena of quantum matter.

Quasi-crystals are structures that are not perfectly ordered, like crystals, but not completely disordered and are among the rarest structures in nature. In order to study their characteristics, the team of experimental physicists made in the laboratory a quasi-crystal with laser light that propagates in an intertwined manner in kilometre-long optical fibres: the complex dynamics of light in these fibres closely mirrors the quantum motion of electrons in the quasi-crystal. During the experiment, the researchers observed a triple phase transition, in which the topological properties, conductivity, and energy exchange between the quasi-crystal and its surroundings change abruptly but at the exact same time.

The discovery was published in the journal Nature and could pave the way for a holistic understanding of the inner workings of complex or engineered materials and their use in advanced phase-controlled materials-based applications.

The discovery of this new phase transition in quasi-crystals represents a breakthrough in the understanding of some fundamental phenomena of quantum matter. It may also pave the way for the development of a new technology and type of material unlike anything we have seen before, the properties of which we will be able to simultaneously control and modify at will. It would be a new form of matter much more flexible and controllable than the one we currently know about.
Stefano Longhi, professor at the Department of Physics of the Politecnico and co-author of the study

The article published in Nature