optics – Progress in Research

Metasurfaces for ultra-fast light switching and sensor technology

Posted on 10/01/202406/08/2024 by Erik Franco

One of the goals of optics is to realise ultra-fast devices capable of transmitting and manipulating information with switching times limited only by an optical cycle of light (thousands of attoseconds). This is possible by exploiting ‘non-linear’ optical processes, in which an optical signal is altered by the presence of a second light stimulus; these processes require mediation by specific materials.

A group of researchers from the Department of Physics of Politecnico di Milano used a metasurface, i.e., a two-dimensional matrix less than a micron thick – a hundred times thinner than a hair – composed of ‘meta-atoms’, elements smaller than the wavelength of light. The research, coordinated by Professors Michele Celebrano and Marco Finazzi and published in Nature Nanotechnology, demonstrates how a metasurface, by exploiting interference between nonlinear optical processes, is able to perform true optical switching of emitted light, converting incident infrared light into visible light and offering the possibility, in principle, of processing information at the rate of one trillion bits per second.

Over the past decade, metasurface research has been revolutionising the field of optics and is a cornerstone of the new EssilorLuxottica Smart Eyewear Lab, the research centre in collaboration with Politecnico di Milano for the design of the smart glasses of the future.

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Optical wireless may no longer have any obstacles

Posted on 19/12/202306/08/2024 by Erik Franco

Optical wireless may no longer have any obstacles. A study by Politecnico di Milano, conductedtogether with Scuola Superiore Sant’Anna in Pisa, the University of Glasgow and Stanford University, and published in the prestigious journal Nature Photonics, has made it possible to create photonic chips that mathematically calculate the optimal shape of light to best pass through any environment, even one that is unknown or changing over time.

The problem is well known: light is sensitive to any form of obstacle, even very small ones. Think, for example, of how we see objects when looking through a frosted window or simply when our glasses get foggy. The effect is quite similar on a beam of light carrying data streams in optical wireless systems: the information, while still present, is completely distorted and extremely difficult to retrieve.

The devices developed in this research are small silicon chips that serve as smart transceivers: working in pairs, they can automatically and autonomously ‘calculate’ what shape a beam of light needs to be in order to pass through a generic environment with maximum efficiency. Not only that: at the same time they can also generate many overlapping beams, each with its own shape, and direct them without them interfering with each other. This makes it possible to significantly increase transmission capacity, just as required by next-generation wireless systems.

Our chips are mathematical processors that make calculations on light very quickly and efficiently, almost with no energy consumption. The optical beams are generated through simple algebraic operations, essentially sums and multiplications, performed directly on the light signals and transmitted by micro-antennas directly integrated on the chips. This technology offers many advantages: extremely easy processing, high energy efficiency and an enormous bandwidth exceeding 5000 GHz
Francesco Morichetti, Head of the Photonic Devices Lab

‘Today, all information is digital, but in fact, images, sounds and all data are inherently analogue. Digitisation does allow for very complex processing, but as the volume of data increases, these operations become increasingly less sustainable in terms of energy and computation. Today, there is great interest in returning to analogue technologies, through dedicated circuits (analogue co-processors) that will serve as enablers for the 5G and 6G wireless interconnection systems of the future. Our chips work just like that’, stresses Andrea Melloni, Director of Polifab, Politecnico di Milano’s micro and nanotechnology centre.

The activity is co-funded under the NRRP by the RESTART research and development programme ‘RESearch and innovation on future Telecommunications systems and networks, to make Italy more smart’, in which Prof. Andrea Melloni of Politecnico di Milano and Prof. Piero Castoldi of the TeCIP Institute of the Scuola Superiore Sant’Anna in Pisa are coordinating the HePIC and Rigoletto projects, which aim to build prototypes in integrated photonics and future optical communications networks enabling the 6G infrastructure.

SeyedinNavadeh, S., Milanizadeh, M., Zanetto, F. et al.
Determining the optimal communication channels of arbitrary optical systems using integrated photonic processors.
Nat. Photon. (2023).

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Pioneering study sheds light on poorly understood aspect of cancer

Posted on 19/09/202306/08/2024 by Erik Franco

A new scientific study published in the journal Science Advances has investigated a still poorly understood aspect of cancer, therapy-induced senescence in tumor cells. The study, the result of collaboration between researchers from Politecnico di Milano, Johns Hopkins University in Baltimore, the National Cancer Institute in Milan, and the National Research Council, expands our understanding of cancer biology and paves the way for future therapeutic advancements.

The team worked to uncover the biological mechanisms behind the formation of “therapy-induced senescent” (TIS) cells, a small percentage of treated tumor cells that exhibits resistance to conventional therapies (chemotherapy and radiation therapy), leading to tumor quiescence and ultimately, recurrence.

This result is a clear example of how cutting-edge technologies, multidisciplinary expertise, and strong international collaborations are crucial in addressing the most pressing biological questions, such as the early reaction mechanisms of tumor cells to anticancer therapies.
Arianna Bresci, first author of the study and doctoral student at Department of Physics

Researchers utilized advanced optical microscopy techniques, combining three-dimensional holograms of tumor cells with ultra-short pulses of laser light. They explored both the chemical and morphological aspects of TIS cells in human tumors, without the use of invasive techniques, preserving the natural state of the cells.

The research group was able to distinguish key features of TIS cells in human tumor cells: the reorganization of the mitochondrial network, overproduction of lipids, cell flattening, and enlargement. By analyzing a considerable number of cells, researchers established a clear timeline for the development of these distinctive signs.

This discovery may lead to applications in the development of personalized treatments and the possibility of refining current screening protocols for oncology therapy.

Our findings provide important insights into the complex world of TIS in human tumor cells. In our laboratory at Politecnico di Milano, we have developed a new non-invasive laser microscope that has allowed us to understand the initial stages of this phenomenon.
Dario Polli, associate professor at Department of Physics and coordinator of the study

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).

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The official METAFAST project website

Quantum tunnelling of electrons in bidimensional materials

Posted on 26/07/202306/08/2024 by Erik Franco

Sustainable optical computers based on photonic logic gates with low power consumption, but also nano-scaled (one billionth of a metre) optical chips and novel sensors with high sensitivity: the research carried out by an international team opens those new intriguing perspectives for the near future. The team is coordinated by Politecnico di Milano – Department of Physics in collaboration with University of Sheffield (UK); researchers from University of Manchester and Exeter (UK) collaborated too.

The researchers observed that the effect of quantum tunnelling of electrons between two adjacent layers of atomically thin semiconductors drastically modifies their transparency, after being illuminated by laser light.

The work has been recently published on the prestigious journal Nature Communications.

More in details, the research team explored the effects of this bidirectional “transport” of electrons between a layer of an atomically thin material to another one, the so-called quantum tunnelling.

Because of this transfer, the electrons are delocalized among the layers and they compete with the electrons localized in only one layer to occupy the same energetic state. This phenomenon follows the so-called Pauli exclusion principle, which also hinders the light absorption if the states are already occupied by the electrons. This process can strongly modify the optical properties of the employed materials, increasing their transparency after the illumination with laser light.

In summary, the competition between electrons generates a drastic decrease of the light absorption in such materials, increasing their laser-induced transparency.

The observation of such properties paves the way for new research horizons in the field of photonics and materials science, for future applications in optical and quantum computing.

The work was partially funded by the European Union in the framework of the Graphene Flagship (project “GrapheneCore3” lead by Prof. Giulio Cerullo) and the Marie Curie Individual Fellowship project “Enosis” lead by Dr. Armando Genco.

The paper on Nature Communications

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

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A bridge between artificial intelligence and optics

Posted on 07/06/202111/10/2021 by admin

With a study published in the prestigious journal Optica, researchers in the Physics Department at the Politecnico di Milano have built a connection between two fields: artificial intelligence, which has been increasingly studied in recent years, and non-linear optics.

The research, conducted by Carlo Michele Valensise, Giulio Cerullo, and Dario Polli, together with Alessandro Giuseppi from Sapienza Università di Roma, began about a year ago during the first lockdown. It is based on the study of Deep Reinforcement Learning (DRL), that is, the branch of artificial intelligence related to programming agents that can learn to control automated systems. In other words, a DRL agent “learns” thanks to the independent interaction with the system in front of it.

Laboratory experiments then confirmed that the application of DRL to non-linear optics allows for simplification of some processes and, more generally, to speed up experimentation. One possible application, for example, is found in the generation of white light, one of the most common phenomena in this field of research.