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 and experimentally demonstrate a large-scale, high-performance photonic computing platform that simultaneously combines light scattering and optical nonlinearity. The core processing unit consists in a disordered polycrystalline lithium niobate slab bottom-up assembled from nanocrystals. Assisted by random quasiphase-matching, nonlinear speckles are generated as the complex interplay between the simultaneous linear random scattering and the second-harmonic generation based on the quadratic optical nonlinearity of the material. Compared to linear random projection, such nonlinear feature extraction demonstrates universal performance improvement across various machine learning tasks in image classification, univariate and multivariate regression, and graph classification.
Nonlinear and electro-optic devices are present in our daily life with many applications: light sources for microsurgery, green laser pointers, or modulators for telecommunication. They mainly use bulk materials such as glass fibers or high-quality crystals, hardly integrable or scalable due to low signal and difficult fabrication. Here I will show several strategies to enhance optical signals by engineering metal-oxides.
First, I will explain some fundamental aspects of quadratic materials. Then, I will show several photonic systems relying on bottom-up assemblies of barium titanate nanoparticles either to obtain electro-optic metasurfaces or broadband Mie driven microspheres.
Nonlinear optics is present in our daily life with many applications, e.g. light sources for microsurgery or green laser pointer. All of them use bulk materials such as glass fibres or crystals. Generating nonlinear effects from materials at the nanoscale can expand the applications to biology as imaging markers or sensors, and to optoelectronic integrated devices. However, the nonlinear emission efficiency of nanostructures is low due to their small volumes. In our work, we show strategies to enhance the second harmonic generation (SHG) at the nanoscale with the goal of developing nonlinear photonics devices for a broad spectral range. So far, the SHG from metallic and semiconductor nanostructures as gold or gallium arsenide has been successfully shown. However, the application range of these materials is generally limited to the visible-near-infrared range by their high absorption. We use metal oxides such as barium titanate (BTO) and lithium niobate (LNO) as an alternative platform for nanoscale nonlinear photonics in a broad spectral range. Both BTO and LNO are noncentrosymmetric materials with high refractive index and high energy band gaps, transparent down to the near-ultraviolet range. We demonstrate linear Mie resonances in BTO and LNO nanostructures, such as nanospheres or nanocubes. Further, we show that these resonances enhance the SHG emission from the nanostructures. We also perform simulations to understand the underlying mechanism of this enhancement. Finally, we explore fabrication methods for BTO and LNO nanostructures that will allow the controlled integration of BTO and LNO nanostructures for nonlinear metasurfaces.
Switching the scattering direction of high-index dielectric nanoantennas between forward and backward, via Mie resonances in the linear regime, has been widely studied, recently. However, switching the harmonic emission of nanoantennas without applying any physical change to the antennas, such as geometry, or environment, is a chal- lenging task that has not been demonstrated yet. Here, we investigate multipolar second-harmonic switch from GaAs nanoantennas. Based on the peculiar nonlinearities of zinc-blende semiconductors, we demonstrate both theoretically and experimentally unidirectional nonlinear emission routing and switching via pump polarization control. Our results offer exciting opportunities for nonlinear nanophotonics technologies, such as nanoscale light routing elements, nonlinear light sources, nonlinear imaging, multifunctional flat optical elements.
In our work, we employ the resonant electromagnetic properties of III-V semiconductor nanowires to design building blocks for nonlinear all-dielectric metamaterials and devices. Contrary to widely used Si and Ge nanostructures, III-V materials, such as GaAs or AlGaAs, have a direct band gap and non-centrosymmetric crystal structure, which makes them promising for the development of nonlinear metamaterials. We developed an innovative approach to fabricate disk and rod nanoantennas by slicing bottom-up grown nanowires using a focused ion beam milling (FIB). The proposed method allows to significantly decrease the influence of the substrate on the electromagnetic field distribution inside the nanoantenna and it opens the possibility to use any substrate regardless of the nanostructure fabrication process. With this technique, we study the influence of geometry, design and crystal structure on the characteristics of all-dielectric nanoantennas. It offers unique opportunities to fabricate high-quality structures with variable radii, longitudinal heterostructures with lattice-mismatched materials, and structures with different refractive indexes and crystal phases that are not available in bulk materials.
Nonlinear optical processes are known to be weak in bulk materials and extremely small at the nanoscale since they mainly scale with the volume. Here I will show how we enhance second-harmonic generation in two typical χ2 non centrosymmetric nanomaterials.
First, in barium titanate nanoparticles, we take advantage of various resonances occurring in the visible wavelength range. Contrary to plasmonics occurring in metallic nanostructures, those resonances take place in dieletric nanoparticles and they are well described by the Mie scattering theory.
Second, in lithium niobate nanowires, we demonstrate phase-matching and use it to increase the guided second-harmonic power by a factor of more than 80. We also increase non-phase-matched guided second-harmonic by engineering the nanowire length. Those bright nanostructures can serve for developing compact efficient nonlinear optical sources or waveguides.
We use the nonlinear optical property of GaAs to directly visualize the path of the near infrared incident laser light
coupled into individual nanowires. We fully illuminate with near infrared pulse laser untapered and tapered GaAs
nanowires grown via the Au-assisted vapor-liquid-solid mechanism. We record second-harmonic generation (SHG)
signals in the visible spectrum. In some nanowires, an interference pattern is observed and investigated in terms of
distances between the maxima of the SHG signal taking into account the effective refractive index in such sub
wavelength structures with radius below 90 nm. We propose a model to explain the periodicity of the maxima in the
SHG interference pattern. The theoretical model includes the waveguiding and the Mie scattering theories for obtaining
the 2π periodicity fitting well the experiments. Moreover, we also measure interferences in tapererd nanowires with a
radius down to 76 nm. The possible effect of the gold in non radiative recombination and the presence of the gold
particle at the tip of some nanowires are also discussed.
Nanocrystals with second harmonic response is a new class of nonlinear optical nanoprobes with dramatically different
properties from fluorescent agents. Compared with two-photon fluorescence, second harmonic generation is an ultrafast,
lossless, and coherent process. In particular, the absence of photobleaching and emission intermittency in the optical
response of the second harmonic nanoparticles is likely to complement the fluorescent agents widely used today in many
imaging applications. Furthermore, the coherent emission from the second harmonic generation process provides unique
opportunities for the application of coherence domain techniques that are not available with fluorescent agents. We
review the application of the second harmonic nanocrystals in imaging applications, especially those pertaining to
biomedicine.
We report the hydrothermal synthesis of free-standing lithium niobate nanowires. We show that the versatile properties
of bulk lithium niobate such as nonlinear optical effects can be exploited at the nanoscale. We describe the fabrication of
polydimethylsiloxane (PDMS) microfluidics as well as indium tin oxide (ITO) electrodes with different design for
dedicated applications. The control of microfluidic channel dimensions and the corresponding particle concentration is
explored. Finally, the selection of fluidic conductivity for optimal dielectrophoretic trapping conditions is discussed.
We develop second-harmonic nanoparticles as the contrast agents for cell imaging. Second-harmonic nanoparticles show
promise as cell imaging probes due to their non-bleaching, non-blinking, and coherent signal. Nanoparticles of noncentrosymmetric
crystal structures have high second-harmonic generation (SHG) efficiency and provide high contrast in
a generally non-structured cell environment. Here, we use barium titanate (BaTiO3) nanoparticles with tetragonal crystal
structure as imaging probes. Cytotoxicity tests performed on BaTiO3 nanoparticles with mammalian cells did not result
in toxic effects. Specifically, we observed no change in the cell metabolism after 24 hours incubation of the cells with
high concentration of BaTiO3 nanoparticles. We demonstrate two methods of cell labeling with BaTiO3 nanoparticles for imaging. One is non-specific labeling via endocytosis of the cells, which results in a great number of the nanoparticles
randomly distributed inside the cells. The other is specific labeling via surface functionalization of the nanoparticles with
antibodies, which enables us to label specific cell membrane proteins with the nanoparticles. SHG imaging is compatible
to two-photon microscopy and the SHG signal from nanoparticles can be easily detected with a standard two-photon
confocal microscope. Our work provides the opportunity for long-term, three-dimensional cell tracking with secondharmonic
nanoparticles.
We have developed a luminescent marker using the second harmonic generation (SHG) from noncentrosymmetric
BaTiO3 nanocrystals. These nanoparticles are attractive due to their stable, non-saturating and coherent signal with a
femtosecond-scale response time and broad flexibility in the choice of excitation wavelength. In this paper, we report the
SHG response of BaTiO3 nanocrystals under a circularly polarized excitation. We observed a more uniform SHG signal
intensity from nanocrystals of different crystal orientations under a circularly polarized excitation. The threedimensional
(3D) SHG fields generated from the SHRIMPs were recorded by a harmonic holographic microscope. Submicron
resolution in both lateral and axial directions has been achieved. We show that the circularly polarized excitation
is useful for harmonic holographic microscopy.
Luminescent markers play a key role in imaging techniques for life science since they provide a contrast mechanism
between signal and background. We describe a new type of marker using second harmonic generation (SHG) from
noncentrosymmetric BaTiO3 nanocrystals. These nanoparticles are attractive due to their stable, non-saturating and
coherent signal with a femtosecond-scale response time and broad flexibility in the choice of excitation wavelength. In
this paper, we report the use of nanoparticles for cell imaging. We first stabilized the BaTiO3 nanoparticles in suspension
and characterized the optical properties. We also demonstrated the functionalization of BaTiO3 nanoparticles by
conjugating IgG antibody on the surface of the nanoparticles. These functionalized nanocrystals are capable of specific
labeling the antigens of interest.
Luminescent markers play a key role in imaging techniques for life sciences since they provide a contrast mechanism
between signal and background. We describe a new type of marker using second harmonic generation (SHG) from
noncentrosymmetric BaTiO3 nanocrystals. These nanoparticles are attractive due to their stable, non-saturating and
coherent signal with a femtosecond-scale response time and a broad flexibility in the choice of excitation wavelength.
We use the coherent SHG signal from BaTiO3 nanoparticles for three-dimensional (3D) imaging without scanning. We
built a harmonic holographic (H2) microscope which records digital holograms at the second harmonic frequency. Highresolution
3D distributions of these SHG markers in mammalian cells are successfully captured and interpreted by the H2
microscope.
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