Recent Submissions

  • The science of harnessing light’s darkness

    Bogdanov, Andrey A.; Fratalocchi, Andrea; Kivshar, Yuri (Nanophotonics, Walter de Gruyter GmbH, 2021-11-12) [Article]
    Nonradiative sources of light such as anapoles and bound states in the continuum (BICs) were initially proposed in quantum mechanics and astrophysics, and they did not attract much attention in photonics for a long time. However, primarily due to the rapid development of metamaterials and metaphotonics, it was recognized that such states are very prospective for efficient trapping of light, amplification of local fields, control of scattering, and also nonlinear manipulation of light at the nanoscale. Metaphotonics provides a broad variety of resonant dielectric structures, including optical gratings, metasurfaces, photonic crystals, and single resonators for a precise engineering of high values of quality factor (Q-factor) of the resonant states and their optical response. In the last ten years, nonradiating states matured from pure conceptual fundamental works to experimental demonstrations and original applications in photonics and radiophysics. They promised functional tools for controlling electromagnetic radiation of different spectral ranges from visible light to microwaves.
  • Terahertz emission mediated by ultrafast time-varying metasurfaces

    Tunesi, J.; Peters, L.; Gongora, J. S. Totero; Olivieri, L.; Fratalocchi, Andrea; Pasquazi, A.; Peccianti, M. (Physical Review Research, American Physical Society (APS), 2021-10-18) [Article]
    Systems with ultrafast time-varying dielectric properties represent an emerging physical framework. We demonstrate here the observation of subcycle dynamics interacting directly with an electromagnetic source comprised of morphologically constrained photoexcited carriers in a surface nanostructure. A transition to a metallic metasurface state occurs on time scales faster than the terahertz-field period, inducing large nonlinear ultrafast phase shifts in the terahertz emission and exposing an interesting physical setting.
  • Metasurface design platform for highly efficient wavefront engineering

    Makarenko, Maksim; Burguete-Lopez, Arturo; Getman, Fedor; Fratalocchi, Andrea (IEEE, 2021-09-30) [Conference Paper]
    In this work, we propose a universal design platform for the development of wavefront engineering structures. We demonstrate this approach's efficiency by producing a series of highly efficient common optical devices.
  • Terahertz Sources Based on Time-Dependent Metasurfaces

    Tunesi, J.; Peters, L.; Gongora, J. S. Totero; Olivieri, L.; Fratalocchi, Andrea; Pasquazi, A.; Peccianti, M. (IEEE, 2021-09-30) [Conference Paper]
    Novel metamaterial platforms exhibiting time-dependent electromagnetic properties enable the investigation of previously unexplored light-matter interactions [1] - [2] . A variation in the dielectric function on a timescale shorter than the electric field period is perceived as an ultrafast temporal boundary [3] , thus resulting in a time-dependent "Snell’s law" which connects the polarisation field frequency before and after the transition [4] , [5] . The onset of a frequency shift therefore enables the engineering of exotic nonlinear phenomena such as time refraction and photon acceleration [5] - [7] , with key fundamental and practical implications [8] . In one scheme, a temporal boundary is induced via photoexcitation of semiconducting metamaterials excited by ultrashort optical pulses. Above-bandgap photons drive an ultrafast transition from a dielectric to a metallic state. Temporal-boundaries-mediated nonlinearities become relevant for transition times shorter than the wave-period timescale, a challenging regime in optics. This condition is achieved in a hybrid approach, exploiting the interaction of terahertz (THz) fields and ultrafast photo-excited transients. The peculiar advantage of THz Time-Domain-Spectroscopy techniques is that they allow the agile reconstruction of full-field dynamics with sub-wave-period resolution. In addition, in all the considered schemes, terahertz waves impinge from vacuum onto a positionally static transient, in other words a large velocity mismatch always exists. An unexplored physical scenario is then when the transient is applied directly to a source where the transient exists in the same positional reference as the THz wave.
  • Large-scale, high-resolution, wide-gamut structural coloration of flexible substrate

    Li, Ning; Fratalocchi, Andrea (IEEE, 2021-09-30) [Conference Paper]
    Manipulating the interaction between light and matter for generating colors is becoming a hot spot of research in photonics, revolutionizing many areas, such as holographic data storage [1] , light filtering [2] , high resolution displays [3] , security [4] , and integrated opto-electronics components [5] .
  • Clean Carbon Cycle via High-Performing and Low-Cost Solar-Driven Production of Freshwater

    Mazzone, Valerio; Bonifazi, Marcella; Aegerter, Christof M.; Cruz, Aluizio M.; Fratalocchi, Andrea (Advanced Sustainable Systems, Wiley, 2021-09-12) [Article]
    While renewable power available worldwide costs increasingly less than the least expensive option based on fossil fuels, countries continue to increase their coal-fired capacity, which should conversely fall by 80% within a decade to limit global warming effects. To address the challenges to the implementation of such an aim, here, a path is explored that leverages on a previously unrecognized aspect of coal, opening to a new solar-driven carbon cycle that is environmentally friendly. By engineering the porosity matrix of coal into a suitably designed compressed volumetric structure, and by coupling it with a network of cotton fibers, it is possible to create a record performing device for freshwater production, with a desalination rate per raw material cost evaluated at 1.39 kg h −1 $ −1 at one sun intensity. This value is between two and three times higher than any other solar desalination device proposed to date. These results could envision a clean and socially sustainable cycle for carbon materials that, while enabling an enhanced water economy with global access to freshwater and sanitation, poses zero risks of reinjecting 𝐶𝑂2 into the environment through competing economies in the fossil's market.
  • Robust and Scalable Flat-Optics on Flexible Substrates via Evolutionary Neural Networks

    Makarenko, Maksim; Wang, Qizhou; Burguete-Lopez, A.; Getman, Fedor; Fratalocchi, Andrea (Advanced Intelligent Systems, Wiley, 2021-08-31) [Article]
    In the past 20 years, flat-optics has emerged as a promising light manipulation technology, surpassing bulk optics in performance, versatility, and miniaturization capabilities. As of today, however, this technology is yet to find widespread commercial applications. One of the challenges is obtaining scalable and highly efficient designs that can withstand the fabrication errors associated with nanoscale manufacturing techniques. This problem becomes more severe in flexible structures, in which deformations appear naturally when flat-optics structures are conformally applied to, for example, biocompatible substrates. Herein, an inverse design platform that enables the fast design of flexible flat-optics that maintain high performance under deformations of their original geometry is presented. The platform leverages on suitably designed evolutionary large-scale optimizers, equipped with fast-trained neural network predictors based on encoder decoder architectures. This approach supports the implementation of flexible flat-optics robust to both fabrication errors or user-defined perturbation stress. This method is validated by a series of experiments in which broadband flexible light polarizers, which maintain an average polarization efficiency of 80% over 200 nm bandwidths when measured under large mechanical deformations, are realized. These results could be helpful for the realization of a robust class of flexible flat-optics for biosensing, imaging, and biomedical devices.
  • Metasurface enabled single measurement recovery of thickness and refractive index maps of cells

    Burguete-Lopez, Arturo; Makarenko, Maksim; Getman, Fedor; Fratalocchi, Andrea (The Optical Society, 2021-06-27) [Conference Paper]
    We present a bio-imaging technique that exploits thin film interference on cells placed on top of a nanostructured broadband light coupling metasurface. Initial results show a 10-2 refractive index sensitivity and 50 nm spatial resolution.
  • Broadband vectorial ultrathin optics with experimental efficiency up to 99% in the visible region via universal approximators

    Getman, Fedor; Makarenko, M.; Burguete-Lopez, A.; Fratalocchi, Andrea (Light: Science & Applications, Springer Nature, 2021-03-04) [Article]
    AbstractIntegrating conventional optics into compact nanostructured surfaces is the goal of flat optics. Despite the enormous progress in this technology, there are still critical challenges for real-world applications due to the limited operational efficiency in the visible region, on average lower than 60%, which originates from absorption losses in wavelength-thick (≈ 500 nm) structures. Another issue is the realization of on-demand optical components for controlling vectorial light at visible frequencies simultaneously in both reflection and transmission and with a predetermined wavefront shape. In this work, we developed an inverse design approach that allows the realization of highly efficient (up to 99%) ultrathin (down to 50 nm thick) optics for vectorial light control with broadband input–output responses in the visible and near-IR regions with a desired wavefront shape. The approach leverages suitably engineered semiconductor nanostructures, which behave as a neural network that can approximate a user-defined input–output function. Near-unity performance results from the ultrathin nature of these surfaces, which reduces absorption losses to near-negligible values. Experimentally, we discuss polarizing beam splitters, comparing their performance with the best results obtained from both direct and inverse design techniques, and new flat-optics components represented by dichroic mirrors and the basic unit of a flat-optics display that creates full colours by using only two subpixels, overcoming the limitations of conventional LCD/OLED technologies that require three subpixels for each composite colour. Our devices can be manufactured with a complementary metal-oxide-semiconductor (CMOS)-compatible process, making them scalable for mass production at low cost.
  • Silicon-Based Photocatalysis for Green Chemical Fuels and Carbon Negative Technologies

    Li, Ning; Xiang, Fei; Fratalocchi, Andrea (Advanced Sustainable Systems, Wiley, 2021-01-18) [Article]
    Silicon, an earth-abundant material with mature technology, low-cost manufacturing, and high stability, holds promise to enable the sustainable exploitation of solar energy resources currently under utilized at the world-scale. Apart from traditional interest in the photovoltaic industry, recent years have seen increasingly large activity in the study of Si-based photo-electro-chemical (PEC) cells for water splitting and CO2 reduction. This research established an exciting area with the potential to address the present environmental crisis originating from unregulated CO2 emission levels. In this review, the recent work on Si-based PEC devices for large scale production of hydrogen via water splitting, and carbon-negative technologies for the solar-driven reduction of CO2 into chemical fuels of industrial interest are summarized. Bias-assisted and bias-free PEC architectures are discussed, highlighting the motivations, challenges, functional mechanisms, and commenting on the perspectives related to this field of research both as a science and engineering.
  • Paired Ru‒O‒Mo ensemble for efficient and stable alkaline hydrogen evolution reaction

    Li, Huang Jing Wei; Liu, Kang; Fu, Junwei; Chen, Kejun; Yang, Kexin; Lin, Yiyang; Yang, Baopeng; Wang, Qiyou; Pan, Hao; Cai, Zhoujun; Li, Hongmei; Cao, Maoqi; Hu, Junhua; Lu, Ying Rui; Chan, Ting Shan; Cortés, Emiliano; Fratalocchi, Andrea; Liu, Min (Nano Energy, Elsevier BV, 2021-01-18) [Article]
    Electrocatalytic hydrogen evolution reaction (HER) in alkaline media is a promising electrochemical energy conversion strategy. Ruthenium (Ru) is an efficient catalyst with a desirable cost for HER, however, the sluggish H2O dissociation process, due to the low H2O adsorption on its surface, currently hampers the performances of this catalyst in alkaline HER. Herein, we demonstrate that the H2O adsorption improves significantly by the construction of Ru–O–Mo sites. We prepared Ru/MoO2 catalysts with Ru–O–Mo sites through a facile thermal treatment process and assessed the creation of Ru–O–Mo interfaces by transmission electron microscope (TEM) and extended X-ray absorption fine structure (EXAFS). By using Fourier-transform infrared spectroscopy (FTIR) and H2O adsorption tests, we proved Ru–O–Mo sites have tenfold stronger H2O adsorption ability than that of Ru catalyst. The catalysts with Ru–O–Mo sites exhibited a state-of-the-art overpotential of 16 mV at 10 mA cm–2 in 1 M KOH electrolyte, demonstrating a threefold reduction than the previous bests of Ru (59 mV) and commercial Pt (31 mV) catalysts. We proved the stability of these performances over 40 h without decline. These results could open a new path for designing efficient and stable catalysts.
  • Terahertz Emission from Ultrafast Time-Varying Metamaterials

    Tunesi, J.; Peters, L.; Gongora, J. S. Totero; Olivieri, L.; Fratalocchi, Andrea; Pasquazi, A.; Peccianti, M. (IEEE, 2021) [Conference Paper]
    We demonstrate a time-dependent dielectric metasurface with sub-cycle dynamics coupled with a photoexcited electromagnetic source. The ultrafast photoexcitation of nanostructured Silicon acts as a temporal discontinuity affecting the nonlinear response responsible for terahertz emission.
  • NIST-certified secure key generation via deep learning of physical unclonable functions in silica aerogels

    Fratalocchi, Andrea; Fleming, Adam; Conti, Claudio; Di Falco, Andrea (Nanophotonics, Walter de Gruyter GmbH, 2020-11-03) [Article]
    AbstractPhysical unclonable functions (PUFs) are complex physical objects that aim at overcoming the vulnerabilities of traditional cryptographic keys, promising a robust class of security primitives for different applications. Optical PUFs present advantages over traditional electronic realizations, namely, a stronger unclonability, but suffer from problems of reliability and weak unpredictability of the key. We here develop a two-step PUF generation strategy based on deep learning, which associates reliable keys verified against the National Institute of Standards and Technology (NIST) certification standards of true random generators for cryptography. The idea explored in this work is to decouple the design of the PUFs from the key generation and train a neural architecture to learn the mapping algorithm between the key and the PUF. We report experimental results with all-optical PUFs realized in silica aerogels and analyzed a population of 100 generated keys, each of 10,000 bit length. The key generated passed all tests required by the NIST standard, with proportion outcomes well beyond the NIST’s recommended threshold. The two-step key generation strategy studied in this work can be generalized to any PUF based on either optical or electronic implementations. It can help the design of robust PUFs for both secure authentications and encrypted communications.
  • makamoa/alfred:

    Getman, Fedor; Fratalocchi, Andrea; Makarenko, M.; Burguete-Lopez, A. (Github, 2020-07-13) [Software]
  • Photonics based perfect secrecy cryptography: Toward fully classical implementations

    Mazzone, Valerio; Falco, Andrea Di; Cruz, Al; Fratalocchi, Andrea (Applied Physics Letters, AIP Publishing, 2020-06-29) [Article]
    Developing an unbreakable cryptography is a long-standing question and a global challenge in the internet era. Photonics technologies are at the frontline of research, aiming at providing the ultimate system with capability to end the cybercrime industry by changing the way information is treated and protected now and in the long run. Such a perspective discusses some of the current challenges as well as opportunities that classical and quantum systems open in the field of cryptography as both a field of science and engineering.
  • Generalized Maxwell projections for multi-mode network Photonics.

    Makarenko, M; Burguete-Lopez, A; Getman, Fedor; Fratalocchi, Andrea (Scientific Reports, Springer Nature, 2020-06-03) [Article]
    The design of optical resonant systems for controlling light at the nanoscale is an exciting field of research in nanophotonics. While describing the dynamics of few resonances is a relatively well understood problem, controlling the behavior of systems with many overlapping states is considerably more difficult. In this work, we use the theory of generalized operators to formulate an exact form of spatio-temporal coupled mode theory, which retains the simplicity of traditional coupled mode theory developed for optical waveguides. We developed a fast computational method that extracts all the characteristics of optical resonators, including the full density of states, the modes quality factors, and the mode resonances and linewidths, by employing a single first principle simulation. This approach can facilitate the analytical and numerical study of complex dynamics arising from the interactions of many overlapping resonances, defined in ensembles of resonators of any geometrical shape and in materials with arbitrary responses.
  • Harnessing complex photonic systems for renewable energy

    Tian, Y.; Li, Ning; Bonifazi, Marcella; Fratalocchi, Andrea (Advances in Physics: X, Informa UK Limited, 2020-05-25) [Article]
    The study of efficient mechanisms of photon conversion processes into electronic, thermal and chemical energy is an interdisciplinary research field spanning physics, chemistry and material science. In recent years, different physical mechanisms sustained by the engineering of diverse complex photonic structures have emerged to offer significant advances in the area of thermal energy generation, photocatalytic and photoelectrochemical energy transformation. The efficient behavior of these systems results from the integration, with different levels of complexity, of dielectric and metallic optical nanostructures into hierarchical disordered architectures, which have shown to significantly improve broadband light-harvesting, electronic charges extraction and light energy confinement. The review aims to concisely highlight the most recent progress in this field, with emphasis on discussing the physics and applications of complex lightwave systems for the realization of efficient processes of photon energy harvesting.
  • On-Chip Hyperuniform Lasers for Controllable Transitions in Disordered Systems

    Lin, Ronghui; Mazzone, Valerio; Alfaraj, Nasir; Liu, Jianping; Li, Xiaohang; Fratalocchi, Andrea (Laser & Photonics Reviews, Wiley, 2020-01-15) [Article]
    Designing light sources with controllable properties at the nanoscale is amain goal in research in photonics. Harnessing disorder opens manyopportunities for reducing the footprints of laser devices, enabling physicalphenomena and functionalities that are not observed in traditional structures.Controlling coherent light–matter interactions in systems based onrandomness, however, is challenging especially if compared to traditionallasers. Here, how to overcome these issues by using semiconductor laserscreated from stealthy hyperuniform structures is shown. An on-chip InGaNhyperuniform laser is designed and experimentally demonstrated, a new typeof disordered laser with controllable transitions—ranging from lasing curveslopes, thresholds, and linewidths— from the nonlinear interplay betweenrandomness and hidden order created via hyperuniformity. Theory andexperiments show that the addition of degrees of order stabilizes the lasingdynamics via mode competition effects, arising between weak lightlocalizations of the hyperuniform structure. The properties of the laser areindependent from the cavity size or the gain material, and show very littlestatistical fluctuations between different random samples possessing thesame randomness. These results open to on-chip lasers that combine theadvantages of classical and random lasers into a single platform.
  • Improved performance and stability of photoelectrochemical water-splitting Si system using a bifacial design to decouple light harvesting and electrocatalysis

    Fu, Hui-Chun; Varadhan, Purushothaman; Tsai, Meng Lin; Li, Wenjie; Ding, Qi; Lin, Chun-Ho; Bonifazi, Marcella; Fratalocchi, Andrea; Jin, Song; He, Jr-Hau (Nano Energy, Elsevier BV, 2020-01-13) [Article]
    Photoelectrochemical (PEC) splitting of water into hydrogen and oxygen is a promising way for the production of clean, and storable form of fuel but the PEC efficiency has remained low. Herein, we demonstrate enhanced light harvesting, charge carrier separation/transfer, and catalyst management with bifacial design for the Si-based photocathodes to achieve best-in-class hydrogen generation with excellent electrochemical stability. Decoupling the light harvesting side from the electrocatalytic surface nullifies parasitic light absorption and enables Si photocathodes that exhibit a photocurrent density of 39.01 mA/cm2 and stability over 370 h in 1 M H2SO4(aq) electrolyte due to fully covered a 15 nm Pt without any intentional protective layer. Furthermore, the bifacial Si photocathode system with semi-transparent Pt layer of 5 nm developed herein are capable of collecting sunlight not only on the light harvesting side but also on the back side of the device, resulting in a photocurrent density of 61.20 mA/cm2 under bifacial two-sun illumination, which yields 56.88% of excess hydrogen when compared to the monofacial PEC system. Combining the bifacial design with surface texturing and antireflection coating enables excellent omnidirectional light harvesting capability with a record hydrogen (photocurrent) generation, which provides a promising way to realize practical PEC water splitting applications.
  • Broadband Ultra-flat Optics with Experimental Efficiencies Exceeding 99% at Visible Wavelengths

    Burguete-Lopez, Arturo; Getman, Fedor; Makarenko, Maksim; Fratalocchi, Andrea (The Optical Society, 2020) [Conference Paper]
    We present a platform by which high experimental efficiency (up to 99.2%), ultraflat (down to 50nm) optics such as polarizer beam splitters, dichroic mirrors and polarization dependent colour filters can be produced in the visible.

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