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The prospect of massive parallelism of optics enabling fast and low energy cost operations is attracting interest for novel photonic circuits where 3-dimensional (3D) implementations have a high potential for scalability. Since the technology for data input–output channels is 2-dimensional (2D), there is an unavoidable need to take 2D-nD transformations into account. Similarly, the 3D-2D and its reverse transformations are also tackled in a variety of fields such as optical tomography, additive manufacturing, and 3D optical memories. Here, we review how these 3D-2D transformations are tackled using iterative techniques and neural networks. This high-level comparison across different, yet related fields could yield a useful perspective for 3D optical design.

Metasurfaces have recently gained extensive interest because of their extraordinary optical behavior as artificial material interfaces with ultrahigh compactness. In this framework, dielectric platforms have newly become very promising for nonlinear nanophotonics, providing opportunities, especially for ultrafast optical switching, and high harmonic generation, opening the research field of nonlinear metaoptics. Up to now, nonlinear metaoptics have been mostly explored using single metasurfaces. However, in a long-term vision, the stacking of optical metasurfaces, very challenging in terms of fabrication, is one key goal of this research field. Here, we demonstrate a three-layer metasurface in the AlGaAs-on-insulator platform, which improves the second harmonic generation efficiency by more than one order of magnitude with respect to its one-layer counterpart. Our achievement paves the way toward phase-shaping multilayer and multifunctional all-dielectric metasurfaces.

International audience; A novel theoretical non-Hermitian formalism for analyzing nonlinear nano-optics is proposed. Its main strength lies in the unique way it analytically incorporates the quasinormal modes of the nanostructures, which have to be resonantly excited to achieve significant nonlinear efficiency. Owing to the analyticity, the formalism is computationally more effective than the usual multipolar Mie-scattering expansions for nonlinear nano-optics. It is also more general and applies to nanostructures laying on substrates or embedded in thin films. It additionally provides guidelines for the multiparameter design and optimization of nonlinear photonic nanostructures. In particular, it reveals an important phase-matching condition at the subwavelength scale between the linear and nonlinear harmonics, which was not clarified in earlier theoretical works based on Hermitian theory for waveguides and high-Q cavities. Its closed form is a major asset that enables us to propose a systematic approach to design phased-matched nanostructures offering drastic second harmonic generation enhancements with engineered χ(2) and pump beams.

We propose a physics-informed neural network for the scattering problem from biological samples. We use this network as a forward model in an optimization task for optical diffraction tomography to reconstruct the refractive index distribution.

We propose a physics-informed neural network as the forward model for tomographic reconstructions of biological samples. We demonstrate that by training this network with the Helmholtz equation as a physical loss, we can predict the scattered field accurately. It will be shown that a pretrained network can be fine-tuned for different samples and used for solving the scattering problem much faster than other numerical solutions. We evaluate our methodology with numerical and experimental results. Our physics-informed neural networks can be generalized for any forward and inverse scattering problem.

We demonstrate optically tunable control of second-harmonic generation in all dielectric nano-antennas: by using a control beam which is absorbed by the nano-resonator, we thermo-optically change the refractive index of the radiating element to modulate amplitude and phase of the second-harmonic signal. For a moderate temperature increase, sizeable modulation is demonstrated; the large tunability of the single meta-atom response paves the way to exciting avenues for reconfigurable homogeneous and heterogeneous metasurfaces.

Metasurfaces have recently enabled a further miniaturization of many optical components which are typically bulky and made of different materials. Notably, nonlinear optics in dielectric metasurfaces has been investigated achieving significant Second Harmonic (SH) generation efficiency in χ (2) materials [1] , [2] . In this work, we use a symmetry broken meta-atom shaped as an AlGaAs "nano-chair", obtained by removing one quarter of the volume from a nanocylinder with elliptic base, see Fig. 1(a) . The proposed geometry is promising for technological applications, because capable of emitting the SH light along the vertical direction ( z axis) [3] . We thus propose to use such structures in a metasurface that is concurrently excited by a Fundamental Frequency (FF) pump in the infrared region (1.55 μm) and by a Continuous Wave (CW) beam in the visible (550 nm). By varying the CW intensity, it is possible to control the temperature inside the meta-atom. The temperature increasing is responsible for a modification in the AlGaAs refractive index. Thus, by varying the CW light intensity, I 0 , we can optimize the temperature profile so as to change the refractive index of the meta-atom, achieving in such manner the tuning of the linear resonant response and consequently obtaining a detectable variation in the SH light. In Fig. 1(b) we elucidate that is possible to create a look-up table for the emitted SH magnitude and phase coming from the meta-atom as a function of I 0 . We verify that for the studied CW intensity range, the SH emission pattern preserves a main lobe ( y -polarized) towards the vertical direction with small amplitude alterations.

Photon-pair sources based on spontaneous parametric down-conversion (SPDC) are the backbone of a vast number of experimental implementations in quantum optics. Sub-wavelength-sized dielectric nanoresonators have recently emerged as a new platform for the implementation of SPDC sources [1] , that promise much potential in generating photons with engineered properties, as these nanoresonators offer a strong control over many parameters of the scattered light (directionality, polarization, …) However, the many degrees of freedom in these highly multi-modal and non-Hermitian systems makes design and formal description of pair-generation challenging.

Dielectric optical metasurfaces provide a powerful and versatile way to explore exotic physics on engineered systems as well as conceive novel miniaturized optical devices for wavefront shaping, harmonic generation and quantum photonics [1] . Since their sub-wavelength constitutive elements are non Hermitian due to low quality factors, such assemblies of optical nanoantennas can implement a wide range of functionalities. Non-Hermitian components were formerly studied in plasmonics [2] , and more recently their dielectric counterparts have also drawn a growing interest [3] , [4] . In this work, we theoretically study one-dimensional chains of N dissipative dielectric nanoresonators by building an analytical non-Hermitian Hamiltonian model. The latter has been fed with complex coupling parameters obtained from a numerical simulation in the case of N=2 , and then validated with longer chains ( N>2 ) by comparison with brute-force calculations. It thereby becomes a predictive tool, essential to overcome numerical limitations to study and design large systems.

We experimentally demonstrate phase encoding in SHG with transparent all-dielectric metasurfaces. While a similar task was previously achieved with plasmonic metasurfaces for THG beam shaping, here we obtain three-order-of-magnitude higher generation efficiency without thermal dissipation.

High refractive index dielectric nanostructures can be exploited to enhance nonlinear optical processes via the strong light confinement by their resonant modes. The sensitive dependence of these m...

Nanoscale optical integration is nowadays a strategic technological challenge and the ability of generating and manipulating nonlinear optical processes in sub-wavelength volumes is pivotal to realize efficient sensing probes and photonic sources for the next-generation communication technologies. Yet, confining nonlinear processes below the diffraction limit remains a challenging task because phase-matching is not a viable approach at the nanoscale. The localized fields associated to the resonant modes of plasmonic and dielectric nanoantennas offer a route to enhance and control nonlinear processes in highly confined volumes. In my talk I will discuss two nonlinear platforms based on plasmonic and dielectric nanostructures. The first relies on a broken symmetry antenna design, which brings about an efficient second harmonic generation (SHG). We recently applied this concept to an extended array of non-centrosymmetric nanoantennas for sensing applications. I will also show the evidence of a cascaded second-order process in Third Harmonic Generation (THG) in these nanoantennas. Recently, dielectric nanoantennas emerged as an alternative to plasmonic nanostructures for nanophotonics applications, thanks to their sharp magnetic and electric Mie resonances along with the low ohmic losses in the visible/near-infrared region of the spectrum. I will present our most recent studies on the nonlinear properties of AlGaAs dielectric nanopillars. The strong localized modes along with the broken symmetry in the crystal structure of AlGaAs allow obtaining more than two orders of magnitude higher SHG efficiency with respect to plasmonic nanoantennas with similar spatial footprint and using the same pump power. I will also discuss a few key strategies we recently adopted to optically switch the SHG in these antennas even on the ultrafast time scale. Finally, I will show how to effectively engineer the sum frequency generation via the Mie resonances in these nanoantennas. These results draw a viable blueprint towards room-temperature all optical logic operation at the nanoscale.

We demonstrate photon-pair generation via spontaneous parametric down-conversion (SPDC) from two types of metasurfaces composed by AlGaAs nanocylinders: 1) monolithically fabricated on a selectively oxidized layer of AlAs epitaxially grown on a GaAs wafer; 2) fabricated by reporting the AlGaAs nanostructures on a transparent wafer via wafer bonding. In these samples, we observed SPDC both in back- and forward-scattering configurations, under excitation with a CW pump around 775 nm and single-photon detection on the signal and idler channels. The Bragg modulation of Mie-resonances enables paraxial SPDC, which demonstrates the potential of all-dielectric metasurfaces for quantum applications like on-axis quantum imaging.

All-dielectric optical metasurfaces consist of 2D arrangements of nanoresonators and are of great importance for shaping polarization, phase and amplitude of both linear and harmonic fields. Here, we demonstrate the generation of second harmonic (SH) with zero-order diffraction from nonlinear AlGaAs metasurfaces with spatial period comparable with a pump wavelength in the near-IR. Upon normal incidence of the pump, we demonstrate paraxial SH light into the zero order. SH polarization is effectively controlled via either the meta-atom shape or the pump polarization, with potential applications for on-axis imaging and free-space communication systems.

There are two physical effects that are exploited nowadays to implement flat metalenses requiring 2𝜋-phase excursion, either subwavelength guidance implementing varying propagation delays, or resonant confinement combining two resonances. We compare both approaches and identify possible limitations with the second approach.

High-permittivity III-V semiconductor nanocavities have shown huge potential for enhanced nonlinear light-matter interactions at the nanoscale. In particular, Second Harmonic (SH) generation in AlGaAs nanoantennas can be extremely efficient; however, vertical emission is difficult to achieve, due to the zincblende χ(2) tensor and epitaxially growth on (100) substrates. Here, we demonstrate that we can shape the second harmonic radiation pattern from a single AlGaAs nanostructure by exploiting a geometrical symmetry breaking optimization approach. The optimized design allows to redirect the SH signal toward the normal direction and to increase the SH power collection efficiency by 2 orders of magnitude in a small numerical aperture of 0.1 with respect to the symmetrical counterpart structure.

We demonstrate an AlGaAs metasurface that implements second harmonic generation with engineered phase-front. Nonlinear holography is obtained, with three-order-of-magnitude higher conversion efficiency than in χ(3) plasmonic metasurfaces.

We propose a robust design for both second harmonic generation and its wavefront control in an AlGaAs-on-sapphire metasurface. In analogy with optical phased arrays, we achieve 0-2π phase modulation on the second harmonic.

The omnipresence of salts in biofluids creates a pervasive challenge in designing sensors suitable for in vivo applications. Fluctuations in ion concentrations have been shown to affect the sensitivity and selectivity of optical sensors based on single-walled carbon nanotubes wrapped with single-stranded DNA (ssDNA-SWCNTs). We herein observe fluorescence wavelength shifting for ssDNA-SWCNT-based optical sensors in the presence of divalent cations at concentrations above 3.5 mM. In contrast, no shifting was observed for concentrations up to 350 mM for sensors bioengineered with increased rigidity using xeno nucleic acids (XNAs). Transient fluorescence measurements reveal distinct optical transitions for ssDNA- and XNA-based wrappings during ion-induced conformation changes, with XNA-based sensors showing increased permanence in conformational and signal stability. This demonstration introduces synthetic biology as a complementary means for enhancing nanotube optoelectronic behavior, unlocking previously unexplored possibilities for developing nanobioengineered sensors with augmented capabilities.

Optical dielectric metasurfaces composed of arrayed nanostructures are expected to enable arbitrary spatial control of incident wavefronts with subwavelength spatial resolution. For phase modulation, one often resorts to two physical effects to implement a 2\pi-phase excursion. The first effect relies on guidance by tall nanoscale pillars and the second one exploits resonant confinement by nanoresonators with two degenerate Mie-resonances. The first approach requires high aspect ratios, while the second one, known as Huygens metasurfaces, is much flatter, and thus easier to manufacture. We compare the two approaches, more focusing on conceptual rather than technological issues, and identify fundamental limitations with the latter. We explain the origin of the limitations based on general arguments, such as reciprocity, multimode/monomode operation and symmetry breaking., Comment: 6 figures

All-dielectric nanophotonics has recently raised an increasing interest because the optical response of high-permittivity dielectric nanoparticles exhibits negligible dissipative losses and strong magnetic multipole resonances [1–4] in the visible and near-IR. We recently reported a second harmonic generation (SHG) conversion efficiency higher than 10−5 at 2 = 1554 nm (pump intensity ≈ 1.6 GW/cm2; pulse width ≈ 150 fs; repetition rate = 80 MHz) in all-dielectric Al 0.18 Ga 0.82 As-on-AlOx nanodisks [5–7]. Such a remarkable efficiency is achieved thanks to the exploitation of magnetic and electric resonances emerging at coincidence with both the pump and SHG wavelength. In addition, the choice of both the pump and, consequently, the SHG (≈ 777 nm) wavelength, set below the energy of the material bandgap, allow minimizing the two-photon absorption of the pump and the re-absorption of the SHG.

All-dielectric optical metasurfaces consist of two-dimensional arrangements of nanoresonators and are of great importance for shaping polarization, phase and amplitude of both linear and harmonic light waves. Up to date, their reported nonlinear optical properties have been dominated by the local features of the individual nanoresonators [1–3]. However, collective responses typical of either Mie-resonant metamaterials or photonic crystals can potentially boost the control over such optical properties. In this work, we demonstrate the generation of second harmonic with zero-order diffraction from quadratically nonlinear metasurfaces, made out of AlGaAs-on-AlOx nanocylinders with spatial period comparable with a pump wavelength in the optical telecom range. Upon normal incidence of the pump beam, we demonstrate that the modulation of Mie-resonances via Bragg scattering at both fundamental and second harmonic frequencies enables paraxial second harmonic light diffraction back into the zero order, based on effective optical properties. This zero-order second harmonic (SH) diffraction results in a 50-fold increase of the detected harmonic power within a solid angle of 5° (see Figure 1). Moreover, 90% of the incident polarization is converted in the SH cross-polarized component, in contrast to around 40% measured previously for isolated nano-antennas [4–7]. This is achieved by a stronger coupling of the incident light with the metasurface resonances at both fundamental and harmonic frequencies with respect to the original Mie-resonances of the isolated constituents. In addition, upon pumping a polarization-gradient χ(2) metasurface with varying incident polarizations, highly horizontal and diagonal linear polarized zero-order SH states can be achieved. Our results demonstrate that higher control of polarization, phase, and amplitude of the harmonic wavefront can be achieved in diffractive all-dielectric nonlinear metasurfaces, with applications for on-axis optical imaging.

International audience; All-dielectric metasurfaces consist of two-dimensional arrangements of nanoresonators and are of paramount importance for shaping polarization, phase, and amplitude of both fundamental and harmonic optical waves. To date, their reported nonlinear optical properties have been dominated by local features of the individual nanoresonators. However, collective responses typical of either Mie-resonant metamaterials or photonic crystals can potentially boost the control over such optical properties. In this work we demonstrate the generation of a second harmonic optical wave with zero-order diffraction, from a metasurface made out of AlGaAs-on-AlOx nanocylinders arranged with spatial period comparable to the pump telecom wavelength. Upon normal incidence of the pump beam, the modulation of Mie resonances via Bragg scattering at both fundamental and second harmonic frequencies enables paraxial second harmonic light generation by diffraction into the zero order, with a 50-fold increase in detected power within a solid angle of 5°. Exquisite control of a higher harmonic wavefront can be thus achieved in all-dielectric nonlinear metasurfaces, with potential applications for on-axis optical systems.

We experimentally demonstrate the capability of semiconductor monolithic metasurfaces to control the polarization state of optical second-harmonic generation. Based on the properties of single Mie-resonator scattering, we design and fabricate AlGaAs-on-insulator metasurfaces whose spatial periodicity allows us to decouple polarization and diffraction features. This enables collection of a polarization-engineered harmonic field in a small solid angle around the zero-diffraction order. Our metasurfaces rely on a AlGaAs/AlAs/GaAs metal-organic chemical vapor deposition structure, which makes them promising for optoelectronic integration.

Waste is classified (art.184, sub. 1, L.D. n.152/2006) as urban or special depending on its origin and whether it is dangerous or not, as well as the degree of danger. The reuse and recycling of materials are two of the main features that characterize the concept of the circular economy. Every firm operating in the field of the circular economy should adopt an industrial approach based on resource efficiency and the use and supply of sustainable raw materials, which can be achieved through innovative technologies, innovative methodologies and new business models. GE.S.P.I, has become a leading firm in the sector of hazardous industrial waste disposal, adding value to waste through a groundbreaking technology. The result of the process is the production of energy, as well as the creation of ash; this ash is then treated in order to separate dangerous heavy metals from the ash through the technique of eddy currents. Metals and purified ash are then put on the market. The aim of this paper is the price definition of this special waste thanks to the analysis of the specific second raw materials market. In this firm, incineration is a process where emissions are strongly controlled by innovative instruments in order to excel in the social and environmental respect. Through SWOT analysis we moreover describe how a company can turn the weaknesses of a high risk area into opportunities and value becomes a useful problem solving instrument to analyze logistic, marketing, and social responsibility, in the perspective of the optimization of eco-management material flows.

The omnipresence of salts in biofluids creates a pervasive challenge in designing sensors suitable for in vivo applications. Fluctuations in ion concentrations have been shown to affect the sensitivity and selectivity of optical sensors based on single-walled carbon nanotubes wrapped with single-stranded DNA (ssDNA-SWCNTs). We herein observe fluorescence wavelength shifting for ssDNA-SWCNT-based optical sensors in the presence of divalent cations at concentrations above 3.5 mM. In contrast, no shifting was observed for concentrations up to 350 mM for sensors bioengineered with increased rigidity using xeno nucleic acids (XNAs). Transient fluorescence measurements reveal distinct optical transitions for ssDNA- and XNA-based wrappings during ion-induced conformation changes, with XNA-based sensors showing increased permanence in conformational and signal stability. This demonstration introduces synthetic biology as a complementary means for enhancing nanotube optoelectronic behaviour, unlocking previously unexplored possibilities for developing nano-bioengineered sensors with augmented capabilities.

The unique optoelectronic properties of single-walled carbon nanotubes (SWCNTs) motivate their use as next-generation optical sensors. The intrinsic SWCNT near-infrared fluorescence offers many advantages in terms of bio-transparency, sensitivity, and photo-stability. Characteristics required for in vivo continuous sensing, such as selectivity, biocompatibility, and solubility, can be tuned by suspending SWCNTs with biomolecules such as single-stranded DNA (ssDNA). Despite the ubiquitous use of DNA-wrapped SWCNTs for a variety of in vivo sensing applications, their behaviour in the presence of varying concentrations of the different ions present in biological media has yet to be studied. In this work, we present a systematic exploration of the effects of various cations and anions on the fluorescence of ssDNA-wrapped SWCNTs. The presence of multivalent cations was found to induce fluorescence shifting of near-infrared fluorescence through the transient formation of a metastable state. The incorporation of artificial oligonucleotides, or xeno nucleic acids, was shown to mitigate the effects of ion-induced conformal drift through improved conformational resiliency. The synthetic biology approach presented herein offers a powerful demonstration for enhancing sensor selectivity and stability while enabling a previously unexplored space for engineering oligonucleotide-based SWCNT sensors.

The purpose of this study was to determine the diagnostic performance of magnetic resonance (MR) obtained with intra-articular contrast medium in the evaluation of recurrent meniscal tears using low-field extremity-only and high-field whole-body magnets. Postoperative standard MR examinations and MR arthrographies of 95 knees were reviewed. Patients experiencing pain and disability after meniscal repair underwent standard MR and MR arthrography (Gadoterate meglumine 0.0025 mmol/ml) on both a 0.2-T and 1.5-T magnet. In 52 of 95 patients, second-look arthroscopy was performed; in the remaining 43 of 95 patients, clinical follow-up was used as the standard of reference. Sensitivity, specificity, positive and negative predictive values as well as accuracy of MRI/MR arthrographic signs as meniscal morphologic changes and the presence of contrast medium tracking into the tear at T1- and T2-weighted sequences in the detection of recurrent meniscal tears were determined. All MR and MR arthrograpic signs were sensitive in the detection of recurrent tears (range 80–91%). Abnormal meniscal morphology had low specificity [26% (13/50)] for both the 0.2-T and 1.5-T scanner, whereas accuracy was 55% (52/95) and 57% (54/95), respectively. The presence of contrast medium within the meniscus substance on T2-weighted images had higher value of specificity [84% (42/50)] and accuracy [84% (80/95)] by using low field strength magnet than by using high field strength magnet [74% (37/50) and 81% (77/95), respectively]. Whereas, the increased intrameniscal signal intensity extending to the meniscal surface at T1-weighted sequences after intra-articular contrast medium administration had lower specificity and accuracy on 0.2-T images [84% (42/50) and 82% (78/95), respectively] than on 1.5-T images [90% (45/50) and 88% (84/95), respectively]. A diagnosis of recurrent meniscal tear in a previously arthroscopically repaired meniscus can be made both on 0.2-T and 1.5-T magnets on the basis of increased signal on T2-weighted and T1-weighted images in the presence of intra-articular contrast material.

We demonstrate optically tunable control of second-harmonic generation in all-dielectric nanoantennas: by using a control beam that is absorbed by the nanoresonator, we thermo-optically change the refractive index of the radiating element to modulate the amplitude of the second-harmonic signal. For a moderate temperature increase of roughly 40 K, modulation of the efficiency up to 60% is demonstrated; this large tunability of the single meta-atom response paves the way to exciting avenues for reconfigurable homogeneous and heterogeneous metasurfaces.