light – 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

fastMOT: revolutionising medical imaging

Posted on 27/06/202306/08/2024 by Erik Franco

With its innovative fast gated, ultra-high quantum efficiency single-photon sensor, the fastMOT (fast gated superconducting nanowire camera for Multi-functional Optical Tomograph) project will enable deep body imaging with diffuse optics. Implemented in the new Multifunctional Optical Tomograph, the light sensor will achieve a 100x improvement of signal-to-noise ratio compared to using existing light sensors.

The fastMOT project will receive a total of 3 million euro in funding: 2.49 million euro from the European Innovation Council programme and 525,000 Euro from the UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee.

Traditionally, organ monitoring and deep-body functional imaging are performed using ultrasound, Xray (including CT), PET or MRI. However, these techniques allow only extremely limited measurements of functionality and are usually combined with exogenous and radioactive agents. To overcome this limitation, six partners, coordinated by the Dutch SME Single Quantum, have joined forces to develop an ultra-high performance light sensor in different imaging techniques to radically improve the performance of microscopy and imaging.

The novel sensor is based on superconducting nanowire single-photon detectors (SNSPDs), which have been shown to be ultra-fast and highly efficient. However, the active area and number of pixels have so far been limited to micrometre diameters and tens of pixels.

The fastMOT consortium now aims at developing new techniques to overcome this limit and scale to 10,000 pixels and millimetre diameter. In addition, new strategies for performing time domain near infrared spectroscopy (TDNIRS) and time domain speckle contrast optical spectroscopy (TD-SCOS) will be developed to optimally use this new light sensor with Monte-Carlo simulations. The new light sensor will be implemented in an optical tomograph and will achieve a 100x improvement of signal-to-noise ratio compared to using existing light sensors.

The new sensing technology will have a major impact on a wide range of sectors: not only will it improve microscopy and imaging performance, but it will also enable groundbreaking applications that will lead to new insights and a major economic boost. The proposed Multifunctional Optical Tomograph will make it possible to image deep organ and optical structures and monitor body functions such as oxygenation, haemodynamics, perfusion and metabolism. It also has the potential to significantly improve the accuracy of non-invasive breast cancer diagnosis, reducing the risk of false positive biopsies, with benefits for patients’ quality of life and improved sustainability for the healthcare systems.

In addition to Single Quantum, the participating institutions are the Center for Ultrafast Science and Biomedical Optics CUSBO at the Department of Physics of the Politecnico di Milano in Italy (unit responsible Prof. Alberto Dalla Mora), the Institute of Photonic Sciences ICFO in Spain, the Technische Universiteit Delft in the Netherlands, the network of European laser research infrastructures Laserlab-Europe AISBL in Belgium, the Forschungsverbund Berlin e.V. in Germany, and the University College London in the United Kingdom.

The fastMOT website

fastMOT is funded by the EU’s HORIZON EUROPE programme (grant agreement 101099291) and by the UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee (grant number 10063660).

New, crucial information on the validity of the Floquet Theory applied to very short light pulses

Posted on 21/12/202206/08/2024 by Erik Franco

The Floquet Theory, particularly important for the development of new concepts in electro-optics, is used to create time crystals and induce new properties in materials. A study published in Nature Communications presents new, crucial information on the validity of this theory when applied to very short light pulses.

Researchers in the Department of Physics at the Politecnico di Milano, in partnership with the Institute of Photonics and Nanotechnology (IFN-CNR), the University of Tsukaba (Japan) and the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg (Germany), have discovered that a time crystal with unique properties can be induced even with very short pulses, lasting a few millionths of a billionth of a second, or femtoseconds.

The researchers managed to observe the creation of Floquet state of a free electron on this ultrashort time scale, thanks to experiments conducted at the Attosecond Research Center of the Department of Physics at the Politecnico di Milano as part of the ERC project AuDACE (Attosecond Dynamics in AdvanCed matErials). Using simulations based on advanced theoretical models, they demonstrated that the Floquet Theory can be extended to these regimes.

These are significant findings because the possibility to induce new properties in matter with ultrashort light paves the way to the realization of new devices, impossible to obtain with standard techniques.
Matteo Lucchini, professor from the Department of Physics and lead author of the study

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