METANANO 2021 Conference Program

Excitons are bound pairs of excited electrons and holes and play a crucial role in many photophysics processes occurring in Nature, such as photosynthesis and light absorption in organic and inorganic semiconductor materials. On the other hand, one of the most important phenomena in Quantum Electro-Dynamics (QED) is the so-called ``Strong Coupling’’ regime, which appears when the interaction between light and excitons in matter is so strong that the photon and matter components mix to create hybrid light/matter states, called polaritons. Traditionally, this hybrid character has been used to achieve new functionalities in which polaritons are thought of as dressed photons, e.g. by exploiting exciton-exciton coupling to construct interacting photons. However, over the last years, it has become clear that the strong coupling regime can be used with an alternative purpose: to significantly modify internal material properties and dynamics by dressing the excitons. In this keynote talk I plan to illustrate how the merging of these two fields of research (Excitons and QED) allows managing excitons to enhance energy transport in organic materials by taking advantage of the delocalized character of the polaritons and to alter the energy landscape of the organic molecules in such a way that photochemical reactions and even ground-state chemical reactions can be altered.

Microcavities are structures composed of two mirrors that are placed either side of a semiconductor thin film. Within the strong-coupling regime, a hybridization occurs between confined photon modes and the semiconductor excitons, creating new types of quasi-particles called cavity polaritons. As cavity polaritons are bosons, they can undergo a condensation process at high occupation density, creating a non-equilibrium polariton condensate that is trapped at the bottom of the polariton dispersion curve. The emission of light from such condensates carries the same high degree of spatial and temporal coherence as the condensate itself, and represents a class of light-source called a polariton laser. Firstly, I present recent work on the fabrication of strongly-coupled microcavities based on the organic dye BODIPY. I present evidence for polariton condensation in BODIPY-based microcavities, and demonstrate that condensates have a high degree of spatial coherence (up to 30 m), temporal coherence (up to 1.2 ns), with their emission energy being tunable through 125 meV by controlling cavity length. I then show that polariton condensation and lasing can be observed in cavities based on relatively simple cavity mirrors. I then present microcavities in which a periodic lattice of defects has been written into the active cavity layer using a serial optical patterning technique. Using a combination of Fourier-space imaging together with optical tomography, we characterize the fluorescence emission from the cavity, and show that a full 2D band-structure of the optical lattice within the cavity can be clearly evidenced.

Recent experiments [1-8] have demonstrated strong light–matter coupling between electromagnetic nanoresonators and pristine sheets of two-dimensional semiconductors, and it has been speculated whether these systems can enter the quantum regime operating at the few-polariton level. To address this question, we present a first-principles microscopic quantum theory [9] for the interaction between excitons in an infinite sheet of two-dimensional material and a localised electromagnetic resonator. We find that the light-matter interaction breaks the symmetry of the otherwise translation-invariant system and thereby effectively generates a localised exciton mode, which is coupled to an environment of residual exciton modes. This dissipative coupling increases with tighter lateral confinement, and our analysis reveals this to be a potential challenge in realising nonlinear exciton-exciton interaction. Nonetheless, we predict that polariton blockade due to nonlinear exciton-exciton interactions is well within reach for nanoresonators coupled to transition-metal dichalcogenides, provided that the lateral resonator mode confinement can be sufficiently small that the nonlinearity overcomes the polariton dephasing caused by phonon interactions [10]. [1] J. Wen, et al., Nano Letters 17, 4689 (2017). [2] D. Zheng et al., Nano Letters 17, 3809 (2017). [3] M.-E. Kleemann et al., Nature Communications 8, 1 (2017). [4] J. Cuadra et al., Nano Letters 18, 1777 (2018). [5] M. Stührenberg et al., Nano Letters 18, 5938 (2018). [6] X. Han et al., ACS Photonics 5, 3970 (2018). [7] M. Geisler et al., ACS Photonics 6, 994 (2019). [8] J. Qin, et al., Physical Review Letters 124, 063902 (2020). [9] E. V. Denning et al., arXiv:2103.14488 (2021). [10] E. V. Denning et al., arXiv:2103.14484 (2021).

Polydopamine (PDA) is one of the promising materials of modern nanotechnology, possessing record high adhesion, chemical stability, high oxidative capacity, and biocompatibility. However, PDA was found to be a fluorescence quencher by Foerster resonance energy transfer and/or photoinduced electron transfer mechanisms, which may affect the fluorescence intensity of the attached dye molecules. In this work, we show that the quenching can be significantly suppressed due to the formation of a strong coupling of dye molecules in PDA shell grown around plasmonic nanoparticle. In this case, the energy exchange between the dye molecules and the plasmon nanoparticle takes place at times much faster than the characteristic times of dye molecules quenching inside the of PDA material alone (without plasmonic nanoparticle). This makes it possible to weaken the quenching process of the dye molecules in the PDA shell by about 1000 times when the strong coupling limits are reached. Besides we show that the implementation of a strong optical coupling in such a system makes it possible to reach 30-fold reduction of dye molecules photodegradation process, which is a fundamental limitation of the use of dye molecules in nanophotonics and optical sensorics.

Nonlinear optical phenomena are central to a myriad of applications in light sources and microscopy. Nonlinear optical effects are fundamentally enhanced in materials with a high refractive index, following the so-called Miller’s rule, as well as by the presence of a resonant photonic environment. This has triggered a quest for nonlinearity enhancement in nanoscale resonators, where resonant dielectric metasurfaces play a key role. However, the applicability of high-index single-crystal metasurfaces for an enhanced nonlinear light generation has remained limited. This is hindered by difficulties in the fabrication of nanostructures from high-index crystalline materials on a transparent substrate, as well as by the strongly diffractive nature of the nonlinear harmonic generation, e.g. emission at multiple diffractive orders. In this talk, we present the use of different dielectric materials for nonlinear applications of metasurfaces, including III-V semiconductors, transition-metal-dichalcogenides and lithium niobate. We further demonstrate how the process of sum-frequency mixing of an infrared image with a homogeneous pump beam in GaAs resonant metasurfaces can result in coherent infrared to visible conversion of images for infrared imaging applications. Our results open new opportunities for the development of compact night-vision devices operating at room temperature and have multiple applications in defence and life sciences.

Recently it was shown that nonlinear metasurfaces can be used to generate broadband THz radiation. Here I will discuss different generation mechanisms, their selection rules, and show that complete spatiotemporal control over the polarization and phase of the THz waves is obtained, allowing a new class of functional and spin controlled THz emitters.

Optical angular momentum-based photonic technologies demonstrate the key role of the optical spin–orbit interaction that usually refers to linear optical processes in spatially engineered optical materials. Re-examining the basics of nonlinear optics of homogeneous crystals under circularly polarized light, we report experiments on the enrichment of the spin–orbit angular momentum spectrum of paraxial light. The demonstration is made within the framework of second-harmonic generation using a crystal with three-fold rotational symmetry. Four spin–orbit optical states for the second harmonic field are predicted from a single fundamental state owing to the interplay between linear spin–orbit coupling and nonlinear wave mixing; three of these states are experimentally verified. Besides representing a spin-controlled nonlinear route to orbital angular multiplexing, modal vortex light sources, high-dimensional parametric processes and multi-state optical magnetization, our findings suggest that the fundamentals of nonlinear optics are worth revisiting through the prism of the spin–orbit interaction of light.

We report on the development of a new approach for studying the internal structure of polymer integrated nanophotonics devices using phase-sensitive optical coherence microscopy. Visualization and flaw detection of devices and their internal structure was carried out using the example of coupling gratings and prisms for a miniature Otto configuration with a characteristic gap height of 50-300 nm.

Flat optical devices based on semiconductor nanostructures are considered a potentially powerful tool for processing optical information. However, these elements are mostly passive and cannot provide tunable operation. In this work, we develop and numerically investigate reconfigurable metasurfaces based on GaAs material, which can be switched between different amplitude-phase profiles under asymmetric optical pumping. We demonstrate the possibility to use them as a tunable Fourier filter for image processing tasks. Achieved results can open the way to create optical computing devices that are lightweight, compact, and remarkably fast compared to conventional analogues.

In this contribution, we present the analysis and numerical verification of the scattering phenomenon from a temporal interface in a parallel-plate waveguide realized by suddenly modifying the dimensions of the waveguide while the wave is propagating. As it is well known in guided wave theory, at the interface between two different waveguides there exists a change of the effective refractive index and wave impedance perceived by the propagating wave within the device, which inevitably scatters at the interface into a reflected and refracted wave. In analogous way, by suddenly changing the effective material properties within the whole waveguide, it is possible to realize the so-called temporal interface, as well. Here, we theoretically and numerically investigate on the scattering from a waveguide temporal interface induced by the abrupt change of the waveguide dimension, which in turn realize a change of the effective material properties perceived by the wave.

Many areas of major interest in Science and Engineering require one to solve infinite-dimensional optimization problems, also called functional optimization problems. In such a context, one has to minimize (or maximize) a functional with respect to admissible solutions belonging to infinite-dimensional spaces of functions, often dependent on a large number of variables. This is the case, for example, with analysis and design of large-scale networks, stochastic optimal control of nonlinear dynamic systems with a large number of state variables, optimal management of complex team organizations, freeway traffic congestion control, reconstruction of unknown environments, Web exploration, etc. Functional optimization problems can be solved analytically only if special assumptions are verified. When optimal solutions cannot be found analytically and/or numerical solutions are not easily implementable, classical approaches to find approximate solutions often incur the so-called “curse of dimensionality” (e.g., an extremely fast growth - with respect to the number d of variables of the admissible solutions - of the computational load required to obtain suboptimal solutions within a desired accuracy). The approximate method that we discuss is based on two steps. First, the decision functions are constrained to take on the structure of linear combinations of basis functions containing free parameters to be optimized. This approximation scheme, which is an extension to the Ritz method (for which fixed basis approximating functions) are used, is called “variable-basis approximation” and models a large variety of connectionistic models used in machine learning (e.g., neural networks, radial basis functions, etc.). Then, the functional optimization problem can be approximated by nonlinear programming problems. For certain classes of functional optimization problems, using variable.-basis connectionistic models may require a number of parameters increasing moderately with the number d of variables, whereas classical fixed-basis approximation may be ruled out by the curse of dimensionality.

During fabrication of photonic integrated circuits (ICs), the performance of each chip is being improved. The main method for this is the topology optimization (TO) of photonic devices. Previous studies have mainly relied on the TO application on a rectangular grid and did not take into account the limitations of a nanofab. Thus, structures could not be produced whose target optimization functions would not be subject to the stage of modifications (due to the adjustment of the device mask discretization to the nanofab constraints) following the TO. In addition, well-known software packages (such as Synopsys FullWave) require a lot of computing resources. We propose a new TO approach for passive components of photonic ICs (PICs) using machine learning and GFIEM (Green's Function Integral Equation Method), which uses GPGPU-accelerated implementation of GMRES (Generalized Minimal Residuals Method). The proposed software package showed this TO approach computation time is significantly less compared to Synopsys FullWave and takes into account the requirements of nanofab sampling. It can be applied in addition to existing programs as a utility. Analysis of world experience shows reducing the cost and increasing the competitiveness of devices and equipment are primarily associated with the transition from electronic ICs to PICs and with a TO usage of devices. The proposed TO approach promotes a faster transition to photonic PICs and enables to accelerate a PIC fabrication, reduce its size as well as the cost.

In this work, we corrected a parabolic X-Ray lens model, taking into account the voxel size. In the simplest case for constant laser power, the problem can be solved analytically. To prove the corrected model's efficiency, we compared the vertical sections of lenses printed with and without correction. To conclude, we have corrected the 3D model of the parabolic lens for direct laser writing (DLW). The research can be helpful for further advance of DLW-fabrication of X-Ray optical elements.

A promising technique for the spectral design of acoustic metamaterials is based on the formulation of suitable constrained nonlinear optimization problems. Unfortunately, the straightforward application of classical gradient-based iterative optimization algorithms to the numerical solution of such problems is typically highly demanding, due to the complexity of the underlying physical models. Nevertheless, supervised machine learning techniques can reduce such a computational effort, e.g., by replacing the original objective functions of such optimization problems with more-easily computable approximations. In this framework, the present article describes the application of a related unsupervised machine learning technique, namely, principal component analysis, to approximate the gradient of the objective function of a band gap optimization problem for an acoustic metamaterial, with the aim of making the successive application of a gradient-based iterative optimization algorithm faster. Numerical results show the effectiveness of the proposed method (joint work with Andrea Bacigalupo, Francesca Fantoni, Daniela Selvi).

Focus is on the design of an innovative class of tunable periodic metamaterials, conceived for the realization of high performance acoustic metafilters with settable real-time capabilities. In this framework the tunability is due to the presence of a piezoelectric phase shunted by a suitable electrical circuit with adjustable impedance/admittance. It follows that the acoustic properties of the metamaterial can be properly modified in an adaptive way, opening up new possibilities for the control of pass- and stop-bands.

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Fluorescence of the modified GFP chromophore diethyl-ABDI-BF2 dispersed in PMMA matrix is studied on top of glass, continuous and perforated optically thin silver films. In polymer, the fluorescence decay kinetics becomes non-exponential and can be described by the distribution of rate constants. The results demonstrate shortening of the excited state lifetime in the presence of silver and broadening of the lifetime distribution caused by the nanoholes.

An environment-friendly method of pulsed laser ablation in liquids is successfully employed for structural modification of silicon nanoparticles leading to considerable narrowing of their size distribution accompanied with reduction of the mean size. Contamination-free conditions of synthesis ensure the chemical purity of formed nanostructures that may reduce toxicity issues. Such a laser-induced modification leads to the appearance of plasmonic properties in semiconductor-based nanomaterials. Their spectral position can easily be varied in the whole visible range. Combined in one nanoparticle properties of semiconductors and noble metals can strongly promote applications of composite laser-synthesized nanoparticles for biosensing (using their plasmonic-based surface-enhanced ability) and bioimaging (using their both optical and magnetic abilities) purposes.

Using direct femtosecond laser patterning of metal-insulator-metal sandwich designed to support Fabry-Perot mode in the visible spectral range we demonstrate new practically relevant strategy for high-resolution color printing. Up-scalable ablation-free laser fabrication method paves the way towards various applications ranging from large-scale structural color printing at a lateral resolution of 25,000 dots per inch.

In spite of the presence of exact analytic and numerical solutions, the popularity of alternative intuitive description of light scattering, and other plasmonic properties, in more simple terms appears to be greater than ever before. Our works reports on new and important development of the so-called modified long wavelength approximation (MLWA), which has been known to very accurately approximate the T-matrix by a simple ratio $iR/(F+D-iR)$, where $F$ is a size-independent Fr\"ohlich term, $R$ is a radiative reaction term, and $D$ is a dynamic depolarization term. With $F$ and $R$ fixed, our main finding is that there is a one parameter freedom in selecting an optimized $D$. By exploiting this unnoticed design freedom it became suddenly possible to accurately approximate the T-matrix for much larger particle sizes than it has been till recently deemed possible. As demonstrated on a broad choice of plasmonic materials, involving the traditional (Ag,Au), increasingly popular (Al), and a very recent alternative (Mg) plasmonic metals, the optimized MLWA is shown to yield surprisingly accurate results for plasmonic spheres for a broad range of their radii up to $160$~nm, including higher order multipoles ($\ell>1$). The precision of our optimized MLWA is truly remarkable, while essentially doubling its expected range of validity.

Quantum hydrodynamic theory (QHT) is a powerful method to calculate the optical response of metallic nanoparticles (NPs) since it takes into account nonlocality and spill-out effects. Nevertheless, the absorption spectra of metallic nanoparticles obtained with conventional QHT, i.e., incorporating Thomas-Fermi (TF) and von Weizsäcker (vW) kinetic energy (KE) contributions, can be affected by several spurious resonances at energies higher than the main localized surface plasmon (LSP). These peaks are not present in reference time-dependent density-functional-theory TD-DFT spectra, where, instead, only a broad shoulder exists. Moreover, we show here that these peaks incorrectly reduce the LSP peak intensity and have a strong dependence on the simulation domain size so that a proper calculation of QHT absorption spectra can be problematic. To overcome this issue, we propose to complement QHT with functionals that depend on the Laplacian of the electronic density, thus, beyond the gradient-only dependence of the TFvW functional. By doing this, we obtain the absorption spectrum that is free of spurious peaks, with LSP resonance of correct intensity and numerically stable Bennett state. Finally, we present a novel Laplacian-level KE energy functional that is very accurate for the description of the optical properties of NPs with different sizes as well as for dimers. Thus, the Laplacian-level QHT represents a novel, efficient, and accurate platform to study plasmonic systems.

We provide the first complete electronic and photonic theory of luminescence from Drude metals. We resolve a series of arguments about the basic nature of the emission, its spectral shape and electric field dependence.

On two new optical forces acting on magnetoelectric particles Manuel Nieto-Vesperinas1, Xiaohao Xu2, Cheng-Wei Qiu3 1.Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas (CSIC), Madrid 28049, Spain. 2.Institute of Nanophotonics, Jinan University, Guangzhou 511443, China. 3.Department of Electrical and Computer Engineering, National University of Singapore Singapore 117583, Singapore. We show two new optical forces that we have recently discovered. One derives from a vortex density of azimuthal imaginary Poynting momentum (IPM) which we show to be built by a superposition of radially and azimuthally polarized beams. Further this azimuthal IPM density may exist with a donut beam intensity distribution, and with zero azimuthal component of all other momenta. This discovery is important because while the spin momentum of a light beam loops in circularly polarized beams and the orbital momentum spirals in helically phased beams, such a behavior of the IPM had not yet been found. The resulting novel effect for optical manipulation is the rotation of spherical particles in absence of incident spin and orbital angular momenta. The second force that we address derives from the interference between the electric and magnetic dipoles induced by light in a magnetodielectric particle, (which gives rise to those well-known Kerker interference effects in the angular distribution of scattered intensity), producing an intensity gradient force which, in contrast with the well-known gradient force employed in standard optical tweezers, is either attractive or repulsive, as well as non-conservative on illumination of the object with either linearly or elliptically polarized Gaussian beam. This new force is directionally anisotropic, tending to repel particles away from the beam axis. Such repulsive effects enhance the sensitivity of the optical manipulation configuration to the particle size. On the basis of this behavior, all-optical sorting of Si nanoparticles is theoretically demonstrated, with tunable size-selection criterion and accuracy.

Nanomechanical resonators with ultra-low dissipation constitute the ideal systems for applications ranging from high-precision sensing to quantum transduction between disparate quantum systems. Traditionally, the improvement of the resonator’s performance through nanomechanical engineering such as dissipation dilution and strain engineering has been driven by human intuition and insight. Such an approach is inefficient and leaves aside a plethora of unexplored mechanical designs that potentially achieve better performance. Here, we present a computer-aided approach known as topology optimization to structurally design mechanical resonators with optimal performance of the fundamental mechanical mode. Using this approach we fabricate and characterize new nanomechanical resonators with record-breaking performance of its fundamental mode [1]. In addition, we present another approach based on a novel type of phononic crystal engineering that creates full bandgaps with high quality modes. With this approach, we make nanomechanical resonators with extremely high quality factors. These approaches open up a new paradigm for designing ultra-coherent nanomechanical resonators for cutting-edge technology and applications. [1] Dennis Høj, Fengwen Wang, Wenjun Gao, Ulrich Busk Hoff, Ole Sigmund, Ulrik Lund Andersen, arXiv: 2103.15601

Liquid-core fibers are a versatile platform for nonlinear light-matter interactions, combining highly nonlinear materials like carbon disulfide or nitrobenzene with the confinement and long interaction length of optical fibers. While there is no strain inside the fiber core due to the liquid phase, pressure effects have a more dominant role depending on the thermodynamic regime and filling of the fiber core. We experimentally demonstrate localized and distributed optoacoustic interactions through Brillouin Optical Correlation Domain Analysis (BOCDA). Due to the associated refractive index change, this response is strain, pressure and temperature dependent. The strong and tunable Brillouin response in sealed carbon disulfide-filled liquid-core fibers offers a unique opportunity to study fundamental optoacoustics in liquids and to realize SBS-based applications, such as signal processing and sensing.

Targeted drug delivery is one the main research directions in biophysical investigations with far-going practical applications. Here we will demonstrate our approach to capsules delivery, based on optomechnical tools. The emphasis will be made on optical and optomechanical properties of nanostructured mesoporous vaterite particles and vaterite-based metamaterials.

In this talk I will introduce a novel technique for airborne particle metrology based on hollow-core photonic crystal fiber. It relies on optical forces that automatically capture airborne particles in front of the hollow core and propel them into the fibre. The resulting transmission drop, together with the time-of-flight of the particles passing through the fibre, provide unambiguous mapping of particle size and refractive index with high accuracy. The technique can be directly applied to monitor air pollution in the open atmosphere and precise particle characterization in a local environment.

Phonon polaritons - light coupled to optical lattice vibrations - in 2D materials can exhibit ultra-short wavelengths, long lifetimes and strong field confinement, which allows for manipulating infrared light at the nanometer scale. Here, we demonstrate how they can be utilised to achieve vibrational strong coupling with nanoscale amounts of organic molecules. To that end, we employ far-field spectroscopy and real-space nanoimaging experiments of ultra-confined infrared phonon polariton in hexagonal boron nitride (h-BN) nanostructures and layers that are adjacent to molecule layers.

When the coupling between a confined electromagnetic mode and the electronic degrees of freedom of a solid-state system becomes large enough, the interaction can modify the electronic wavefunctions and the related material properties. These effects become more dramatic in systems with continuum electronic degrees of freedom, in which the gapless spectra enhance the electronic malleability. Solid-state cavity quantum electrodynamics can thus become a tool for quantum material engineering, allowing us to drastically enrich the catalogue of materials available for scientific and technological applications. In this talk I will present the first experimental demonstration of cavity-induced single-photon wavefunction modification in which we measured a change of 30% for the Bohr radius in microcavity embedded quantum wells [1]. Using doped quantum wells characterised by a continuum spectrum we then predicted [2] and demonstrated [3] the formation of novel excitons bound by photon exchange. We also observed related polaritonic nonlocal effects [4] reducing the achievable field enhancement and thus the achievable strength of the light-matter coupling. [1] Experimental verification of the very strong coupling regime in a GaAs quantum well microcavity. S. Brodbeck et al.,Phys. Rev. Lett. 119, 027401 (2017) [2] Strong coupling of ionising transitions.
E. Cortese et al., Optica 6, 354 (2019) [3] Excitons bound by photon exchange.
E. Cortese et al.,
Nature Physics 17, 31 (2021) [4] Polaritonic non