### Recent Submissions

• #### NIST-certified secure key generation via deep learning of physical unclonable functions in silica aerogels

(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.
• #### Photonics based perfect secrecy cryptography: Toward fully classical implementations

(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.

(Scientific Reports, Springer Science and Business Media LLC, 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

(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.
• #### Broadband vectorial ultra-flat optics with experimental efficiency up to 99% in the visible via universal approximators

(arXiv, 2020-05-05) [Preprint]
Integrating conventional optics into compact nanostructured surfaces is the goal of flat optics. Despite the enormous progress of this technology, there are still critical challenges for real world applications due to a limited efficiency in the visible, on average lower than $60\%$, which originates by absorption losses in wavelength thick ($\approx 500$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 $99\%$) ultra-flat (down to $50$nm thick) optics for vectorial light control and broadband input-output responses on a desired wavefront shape. The approach leverages on a hidden network of universal approximators, which exist in the physical layer of suitably engineered semiconductor nanostructures. Near unity performance results from the ultra-flat nature of these surfaces, which reduces absorption losses to almost negligible values. Experimentally, we discuss polarizing beam splitters, comparing their performances 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 colors by using only two sub-pixels, overcoming the limitations of conventional LCD/OLED technologies that require three sub-pixels for each composite color. Our devices are manufactured with a complementary metal-oxide-semiconductor (CMOS) compatible process, making them scalable for mass production at inexpensive costs.
• #### On-Chip Hyperuniform Lasers for Controllable Transitions in Disordered Systems

(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

(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.
• #### Perfect secrecy cryptography via mixing of chaotic waves in irreversible time-varying silicon chips.

(Nature communications, Springer Science and Business Media LLC, 2019-12-20) [Article]
Protecting confidential data is a major worldwide challenge. Classical cryptography is fast and scalable, but is broken by quantum algorithms. Quantum cryptography is unclonable, but requires quantum installations that are more expensive, slower, and less scalable than classical optical networks. Here we show a perfect secrecy cryptography in classical optical channels. The system exploits correlated chaotic wavepackets, which are mixed in inexpensive and CMOS compatible silicon chips. The chips can generate 0.1 Tbit of different keys for every mm of length of the input channel, and require the transmission of an amount of data that can be as small as 1/1000 of the message's length. We discuss the security of this protocol for an attacker with unlimited technological power, and who can access the system copying any of its part, including the chips. The second law of thermodynamics and the exponential sensitivity of chaos unconditionally protect this scheme against any possible attack.
• #### Terahertz Time-Dependent Random Metamaterials

(OSA, 2019-10-21) [Conference Paper]
Plasmonic metamaterials enable access to extremely nonlinear regimes with remarkable full-field control. We theoretically and experimentally demonstrate a novel form of photo-induced semiconducting Time-Dependent metamaterial at THz frequencies.
• #### Metal Contact and Carrier Transport in Single Crystalline CH3NH3PbBr3 Perovskite

(Nano Energy, Elsevier BV, 2018-09-21) [Article]
Organic-inorganic perovskites have arrived at the forefront of solar technology due to their impressive carrier lifetimes and superior optoelectronic properties. By having the cm-sized perovskite single crystal and employing device patterning techniques, and the transfer length method (TLM), we are able to get the insight into the metal contact and carrier transport behaviors, which is necessary for maximizing device performance and efficiency. In addition to the metal work function, we found that the image force and interface charge pinning effects also affect the metal contact, and the studied single crystal CH3NH3PbBr3 features Schottky barriers of 0.17 eV, 0.38 eV, and 0.47 eV for Au, Pt, and Ti electrodes, respectively. Furthermore, the surface charges lead to the thermally activated transport from 207 K to 300 K near the perovskite surface. In contrast, from 120 K to 207 K, the material exhibited three-dimensional (3D) variable range hopping (VRH) carrier transport behavior. Understanding these fundamental contact and transport properties of perovskite will enable future electronic and optoelectronic applications.
• #### Photo-induced THz Plasmonics in Black Silicon

(Advanced Photonics 2018 (BGPP, IPR, NP, NOMA, Sensors, Networks, SPPCom, SOF), The Optical Society, 2018-06-28) [Conference Paper]
We experimentally investigated a novel form of photo-induced plasmonic response, in nanostructured silicon, at THz frequencies which can be employed to precisely control the full-wave properties, i.e. amplitude and phase, of the generated THz pulse.
• #### X-ray created metamaterials: applications to metal-free structural colors with full chromaticity spectrum and 80 nm spatial resolution

(Conference on Lasers and Electro-Optics, The Optical Society, 2018-05-07) [Conference Paper]
We created new types of metamaterials by hard X-rays possessing high fluency. We discuss applications in structural colors that show full spectrum of Cyan, Yellow, Magenta, Black (CYMK), realized in transparent dielectrics with 80 nm resolution.
• #### Complex epsilon-near-zero metamaterials for broadband light harvesting systems

(Physics, Simulation, and Photonic Engineering of Photovoltaic Devices VII, SPIE-Intl Soc Optical Eng, 2018-02-17) [Conference Paper]
We engineered an epsilon-near-zero (ENZ) material from suitably disordered metallic nanostructures. We create a new class of dispersionless composite materials that efficiently harnesses white light. By means of Atomic Force Microscopy (AFM) and Photoluminescence (PLE) measurements we experimentally demonstrate that this nanomaterial increases up to a record value the absorption of ultra-thin light harvesting films at visible and infrared wavelengths. Moreover, we obtained a 170% broadband increase of the external quantum efficiency (EQE) when these ENZ materials are inserted in an energy-harvesting module. We developed an inexpensive electrochemical deposition process that enables large-scale production of this material for energy-harvesting applications.
• #### High performance nanostructured Silicon heterojunction for water splitting on large scales

(2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Institute of Electrical and Electronics Engineers (IEEE), 2017-11-02) [Presentation]
In past years the global demand for energy has been increasing steeply, as well as the awareness that new sources of clean energy are essential. Photo-electrochemical devices (PEC) for water splitting applications have stirred great interest, and different approach has been explored to improve the efficiency of these devices and to avoid optical losses at the interfaces with water. These include engineering materials and nanostructuring the device's surfaces [1]-[2]. Despite the promising initial results, there are still many drawbacks that needs to be overcome to reach large scale production with optimized performances [3]. We present a new device that relies on the optimization of the nanostructuring process that exploits suitably disordered surfaces. Additionally, this device could harvest light on both sides to efficiently gain and store the energy to keep the photocatalytic reaction active.
• #### Structural colours via metal free disordered nanostructures with nm resolution and full CYMK colour spectrum

(2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Institute of Electrical and Electronics Engineers (IEEE), 2017-11-02) [Presentation]
Engineering colors through optical properties of nanostructures represents a research area of great interest, due to the many applications that can be enabled by this technology, from adaptive camouflage to micro-images for security and biomimetic materials [1-4].
• #### Ultrafast pulse generation in integrated arrays of anapole nanolasers

(2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Institute of Electrical and Electronics Engineers (IEEE), 2017-11-02) [Presentation]
One of the main challenges in photonics is the integration of ultrafast coherent sources in silicon compatible platforms at the nanoscale [1]. Generally, the emission of ultra-short pulses is achieved by synchronizing the cavity modes of the system via external active components, such as, e.g., Q-switch or saturable absorbers. Consequently, the required optical setups are complex and difficult to integrate on-chip. To address these difficulties, we propose a novel type of integrated source based on the spontaneous synchronization of several near-field nanolasers. We design our near-field lasers by considering the nonlinear amplification of non-radiating Anapole modes [2]. Anapoles represent an intriguing non-conventional state of radiation, whose excitation is responsible for the formation of scattering suppression states in dielectric nanostructures [3]. Due to their inherent near-field emission properties, an ensemble of anapole-based nanolasers represent an ideal candidate to investigate and tailor spontaneous synchronization phenomena in a silicon-compatible framework. Additionally, their mutual non-linear interaction can be precisely controlled within standard nanofabrication tolerances.
• #### Route to Photo-Enabled Random Terahertz Metasurfaces

(Advanced Photonics 2017 (IPR, NOMA, Sensors, Networks, SPPCom, PS), The Optical Society, 2017-07-17) [Conference Paper]
We experimentally investigate the enhancement and phase change of the THz emission from photo-excited randomly structured black silicon substrates mediated by an induced metallic state of the nanostructured surface.
• #### Anapole nanolasers for mode-locking and ultrafast pulse generation

(Nature Communications, Springer Nature, 2017-05-31) [Article]
Nanophotonics is a rapidly developing field of research with many suggestions for a design of nanoantennas, sensors and miniature metadevices. Despite many proposals for passive nanophotonic devices, the efficient coupling of light to nanoscale optical structures remains a major challenge. In this article, we propose a nanoscale laser based on a tightly confined anapole mode. By harnessing the non-radiating nature of the anapole state, we show how to engineer nanolasers based on InGaAs nanodisks as on-chip sources with unique optical properties. Leveraging on the near-field character of anapole modes, we demonstrate a spontaneously polarized nanolaser able to couple light into waveguide channels with four orders of magnitude intensity than classical nanolasers, as well as the generation of ultrafast (of 100 fs) pulses via spontaneous mode locking of several anapoles. Anapole nanolasers offer an attractive platform for monolithically integrated, silicon photonics sources for advanced and efficient nanoscale circuitry.
• #### Near-Field Coupling and Mode Competition in Multiple Anapole Systems

(Applied Sciences, MDPI AG, 2017-05-24) [Article]
All-dielectric metamaterials are a promising platform for the development of integrated photonics applications. In this work, we investigate the mutual coupling and interaction of an ensemble of anapole states in silicon nanoparticles. Anapoles are intriguing non-radiating states originated by the superposition of internal multipole components which cancel each other in the far-field. While the properties of anapole states in single nanoparticles have been extensively studied, the mutual interaction and coupling of several anapole states have not been characterized. By combining first-principles simulations and analytical results, we demonstrate the transferring of anapole states across an ensemble of nanoparticles, opening to the development of advanced integrated devices and robust waveguides relying on non-radiating modes.