Bound states in the continuum can be defined as non-radiating resonant modes within open environments. These modes share a defining characteristic of being dark, displaying an exceptional degree of field localization. However, their practical accessibility lies in their quasi-bound form, which needs the introduction of perturbations in the system's geometry or material properties. Despite a finite, albeit high, quality factor, the quasi-bound modes manage to retain their characteristic strong field localization. In this presentation, our focus will be directed towards the exploration of symmetry-protected bound states in the continuum, delving into a comprehensive analysis of the impact that the introduction of various types of asymmetries can have on the formation and behavior of their quasi-bound counterparts. In particular, we will focus our attention on metasurfaces made of BaTiO3, whose constituent elements are periodically arranged nanowires. By investigating the topological features that contribute to certain mode selection rules, our analysis aims to provide a deeper understanding of the underlying mechanisms governing the formation and behavior of these modes. Our findings provide a strategic roadmap for optimizing the implementation of quasi-bound modes and provide a clear path to exploit them in specific applications such as sensing and nonlinear optical processes.
We propose a dual-beam pumping scheme whereby a pulse of telecom frequency ω (1550 nm wavelength) is mixed with its frequency-doubled replica at 2ω. This brings about sum-frequency generation at ω+2ω = 3ω on top of third harmonic at ω+ω+ω = 3ω. We exploited the diffraction by an AlGaAs metasurface for realizing the symmetry breaking which is key to avoid averaging out the interference between the two coherent frequency-tripling pathways. The upconverted light can thereby be routed into different diffraction orders with high selectivity, based on the pumps’ relative phase, providing a platform for GHz-rate amplitude modulation and information encoding.
Infrared imaging is a crucial technique in a multitude of applications, including night vision, autonomous vehicle navigation, optical tomography, and food quality control. Conventional infrared imaging technologies, however, require the use of materials such as narrow bandgap semiconductors, which are sensitive to thermal noise and often require cryogenic cooling. We demonstrate a compact all-optical alternative to perform infrared imaging in a metasurface composed of GaAs semiconductor nanoantennas, using a nonlinear wave-mixing process. We experimentally show the upconversion of short-wave infrared wavelengths via the coherent parametric process of sum-frequency generation. In this process, an infrared image of a target is mixed inside the metasurface with a strong pump beam, translating the image from the infrared to the visible in a nanoscale ultrathin imaging device. Our results open up new opportunities for the development of compact infrared imaging devices with applications in infrared vision and life sciences.
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.
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.
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.
Dielectric metasurfaces have recently shown to be an excellent candidate for efficient frequency mixing at the nanoscale due to the excitation of Mie resonances. Among various dielectric materials, GaAs-based nanostructures have been reported to have high-efficiency of second-order nonlinear processes due to their high quadratic nonlinear susceptibility. Efficient frequency up-conversion can thereby be realised in GaAs-based metasurfaces through the process of sum-frequency generation (SFG), thereby opening new opportunities for nonlinear imaging and infrared vision not possible before. Here we demonstrate for the first time, infrared imaging based on nonlinear mixing of an infrared image with a pump beam in a GaAs resonant metasurface. The nonlinear mixing process generates visible images (Fig. 1a), which can be time resolved with femtosecond resolution and can be observed on a conventional CMOS sensor. Our results open new opportunities for the development of compact night-vision devices operating at room temperature and have multiple applications in defense and life sciences.
Along with increasing demands and rapid development of nanotechnology in various fields, the combination of photonics, nano-optics, biology and chemistry opens new opportunities of developing optical sensors with subwavelength elements or micrometric structures. An innovative sensing mechanism based on the variations in the sensor radiation diagram in correspondence of changes in the refractive index of the surrounding material is illustrated. Particularly, we present a theoretical investigation of two different prototypes of dielectric structures for sensitive and compact chemical optical sensing through the radiation pattern.
We investigate the impact of different substrates on the radiation efficiency of nonlinear processes from dielectric nanoantennas. Several approaches have been considered to optimize the nonlinear radiation efficiency from these structures, but all the strategies have generally failed at limiting the amount of radiation at the harmonic frequencies that is lost in the substrate. It is well known that epsilon-near-zero (ENZ) materials, namely material showing a zero-crossing for the real part of the dielectric permittivity, have peculiar linear properties, such as their ability to realize perfect electromagnetic tunneling and re-direct antennas electromagnetic radiation. Here we first compare the second harmonic signal radiated from a cylindrical AlGaAs nanoantenna placed over different materials, including an ENZ substrate and found that the ability to re-direct the electromagnetic radiation is particularly favored when the ENZ condition occurs at the second harmonic frequency. These results let us foresee a novel approach to improve nonlinear processes at the nanoscale and the possibility to realize novel functionalities, such as beam steering and tailored antenna directivities thanks to the tunability of ENZ materials.
Boosting nonlinear frequency conversion in extremely confined volumes remains a crucial task in nano-optics, all-optical signal processing and nanomedicine. Photon-photon interactions at the nanoscale are intrinsically weak and can only be excited using very high light intensities. Optical materials featuring large nonlinearities and low absorption losses offer a valuable solution to circumvent this limitation and effectively generate nonlinear effects in nanoscale volumes. Metal-less 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 in the visible and near-IR. We have recently proposed all-dielectric AlGaAs-on-AlOx nanodisks and coupled dimers featuring an intense second harmonic generation (SHG) with conversion efficiency up to 10-^5 upon pumping with an ultrafast Erbium-doped fiber laser centered at 1554 nm (150 fs pulses, 80 MHz repetition rate) at 1.6 GW/cm^2 fluency, which is in excellent agreement with our numerical simulations.
To further enhance the SHG efficiency, we propose a metal-dielectric hybrid nanostructure based on a nanodisk surrounded by a gold plasmonic nanoring. The role of the plasmonic nanoantenna is to efficiently collect the illumination light and convert it to match the anapole mode supported by the AlGaAs nanodisk, to significantly amplify the electric field inside the disk. We theoretically predict and experimentally demonstrate that the SHG conversion efficiency in this nanostructure can be boosted by one order of magnitude with respect to the isolated nanodisk.
We demonstrate monolithic aluminum gallium arsenide (AlGaAs) optical nanoantennas for enhanced second harmonic generation (SHG) at telecom wavelengths. From measurements on nanocylinders of 400 nm height and varying radius pumped with femtosecond pulses delivered at 1554-nm wavelength, we estimated a peak conversion efficiency exceeding 10−5. Our measurements are in excellent agreement with frequency-domain numerical simulations, revealing the microscopic nature of the SHG process in our nanoresonators.
We model the linear and nonlinear optical response of disk-shaped AlGaAs nanoantennas. We design nanoantennas with
a magnetic dipole resonant mode in the near-infrared wavelength range, and we analyze volume second-harmonic
generation driven by a magnetic dipole resonance by predicting a conversion efficiency exceeding 10-3 with 1 GW/cm2
of pump intensity.
We designed cylindrical AlGaAs-on-aluminium-oxide all-dielectric nanoantennas with magnetic dipole resonance at near-infrared wavelengths. Our choice of material system offers a few crucial advantages with respect to the silicon-oninsulator platform for operation around 1.55μm wavelength: absence of two-photon absorption, high χ(2) nonlinearity, and the perspective of a monolithic integration with a laser. We analyzed volume second-harmonic generation associated to a magnetic dipole resonance in these nanoantennas, and we predict a conversion efficiency exceeding 10-3 with 1GW/cm2 of pump intensity.
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