NanoPlasMeta 2022 Abstracts

Area 1 - Nanophotonics, Plasmonics and Metamaterials

Area 1 - Nanophotonics, Plasmonics and Metamaterials

Nr: 3
Title:

Machine Learning Enhanced Optical Force Calculation in the Geometrical Optics Approximation
Authors:

David Bronte-Ciriza, Alessandro Magazzù, Agnese Callegari, Maria Antonia Iati, Onofrio M. Maragò and Giovanni Volpe
Abstract: Light can exert forces by exchanging momentum with particles. Since the pioneering work by Ashkin in the 1970’s, optical forces have played a fundamental role in fields like nanotechnology, atomic physics, or biology. Optical tweezers, which are instruments that, by tightly focusing a laser beam, are capable of confining particles in three dimensions, have become a common tool for manipulation of micro- and nano-particles, as well as a force and torque transducer with sensing capabilities at the femtonewton level [6,7]. Optical tweezers have also been successfully employed to explore novel phenomena, including protein folding and molecular motors, or the optical forces and Brownian motion of 1D and 2D materials. Numerical simulations play a fundamental role in the planning of experiments and in the interpretation of the results. Under some conditions, the optical trap can be approximated by a harmonic potential. However, there are many situations where this approximation is insufficient, for example in the case of a particle escaping an optical trap or for particles that are moving on an optical landscape but are not trapped. In these cases, a more complex treatment of the light-matter interaction is required for a more accurate calculation of the forces. This calculation is computationally expensive and prohibitively slow for numerical simulations when the forces need to be calculated many times in a sequential way. Recently, machine learning has been demonstrated to be a promising approach to improve the speed of these calculations and therefore, to expand the applicability of numerical simulations for experimental design and analysis. In this work, we explore the geometrical optics regime, valid when the particles are significantly bigger than the wavelength of the incident light. This is typically the case in experiments with micrometer-size particles. The optical field is described by a collection of N light rays and the momentum exchange between the rays and the particle is calculated employing the tools of geometrical optics. The limitation of considering a discrete N number of light rays introduces artifacts in the force calculation. We show that machine learning can be used to improve not only the speed but also the accuracy of the force calculation. This is first demonstrated by training a neural network for the case of a spherical particle with 3 degrees of freedom accounting for the position of the particle. Starting from these results for 3 degrees of freedom, the work has been expanded to 9 degrees of freedom by including in one NN all the relevant parameters for the optical forces calculation considering also different refractive indexes, shapes, sizes, positions and orientations of the particle besides different numerical apertures of the objective that focuses the light. This work proves machine learning as a compact, accurate, and fast approach for optical forces calculation and presents a tool that can be used to study systems that, due to computation limitations, were out of the scope of the traditional ray optics approach.
Nr: 15
Title:

Plasmon-assisted Antibunching in Plasmonic Nanocavities
Authors:

Antonio I. Fernandez Dominguez
Abstract: In this talk, I will explore the quantum-optical properties of the light emitted by a nanoparticle-on-mirror cavity filled with a single quantum emitter [1]. Inspired by recent experiments [2], I will consider a dark-field set-up and explore the photon statistics of the scattered light under grazing laser illumination. Exploiting analytical solutions to Maxwell’s equations [3], I will discuss the quantization of the nanophotonic cavity fields and describe the formation of plasmon-exciton polaritons (or plexcitons) in this system. Finally, I will reveal how the rich plasmonic spectrum of the nanocavity offers unexplored mechanisms for nonclassical light generation that are more efficient than the usual, resonant interaction between the emitter natural transition and the brightest optical mode [4]. [1] R. Sáez-Blázquez et al., Optica 4, 1363-1367 (2017), Phys. Rev. A. 98, 013839 (2018). [2] R. Chikkaraddy et al., Nature 535, 127 (2016). [3] A. Cuartero-González et al. Phys. Rev. B 101, 035403 (2020). [4] R.Sáez-Blázquez et al., submitted (2021).
Nr: 16
Title:

Harnessing Modulated Electrons to Probe Light-matter Strong Coupling
Authors:

Jaime Abad Arredondo, Antonio I. Fernandez Dominguez and Francisco José García Vidal
Abstract: Due to recent advances in the control of the quantum properties of collimated free-electron beams, these appear to be one of the most promising probes for quantum matter at the nanoscale. In this work, we provide a model Hamiltonian describing the quantum interaction between a modulated electron wave-packet and a hybrid photonic-excitonic system comprising a quantum emitter and an optical nanocavity. This Hamiltonian is constructed using macroscopic QED ideas and fully parameterized in terms of the electromagnetic Dyadic Green’s function. We explore the Jaynes-Cummings polariton ladder of the cavity-emitter system through both the free-electron and photon spectra and demonstrate the power of modulated electrons as near-field probes of light-matter interactions in the strong-coupling regime.
Nr: 18
Title:

Designing Colloidal Interactions with Random Optical Fields
Authors:

Luis S. Froufe-Pérez, Augustin Muster, Diego Romero-Abujetas and Frank Scheffold
Abstract: Interaction forces between objects induced by electromagnetic fields are far from being negligible in many situations. From intermolecular dispersion forces to colloidal interactions, optical fields with some degree of randomness induce interaction forces between bodies of all sizes and shapes. Since the pioneering work of Ashkin [1], optical tweezers have been employed in trapping and manipulation of particles ranging from the sub-micron to few-micron sizes and made of different materials. As a consequence of the advent of new technologies and the better understanding of radiation-matter interactions, many techniques allowing for the collective manipulation of increasingly large numbers of particles have emerged, as clearly shown in time-shared optical tweezers set-ups [2], for instance. The vast majority of current experimental approaches to optical manipulation rely on the careful design of the input beams, typically monochromatic, and adapted to the optical properties of the particles to be manipulated. On the opposite extreme, and in analogy to black-body radiation induced forces, a random optical field can be designed to have a particular energy density spectrum such that the optically induced interaction between pairs of particles corresponds to a predefined potential [3]. Here we present some previous results showing that, with single frequency random fields, the optical response of the particles has to be carefully chosen such that the induced interaction obeys a predefined one. In particular we discuss the “mock-gravity” induced forces (1/r2 interactions). In the case of small dipole metal particles, the frequency of the otherwise random optical field has to be tuned to a Fröhlich resonance of the nanoparticle [4]. Nevertheless, at least for high refractive index submicron-sized particles, it is possible to design random optical fields involving several frequencies with different energy densities, such that the interaction potential obeys a certain predefined pair-interaction. We show that, tailoring the spectral characteristics of the random field, opens the way to design colloidal interactions in a remarkably flexible way. We shall discuss the particular case of an interaction that, in principle, leads to stealthy hyperuniform structures [5] in equilibrium. [1] A. Ashkin. Acceleration and trapping of particles by radiation pressure. Phys. Rev. Lett., 24, 156 (1970); D. G. Grier. A revolution in optical manipulation. Nature, 424, 810 (2003). [2] P. H. Jones, O. M. Maragó, and G. Volpe. Optical Tweezers: Principles and Appilactions. Cambridge University Press, Cambridge, 2015 [3] G. Brügger, L. S. Froufe Pérez, F. Scheffold, and J. J. Sáenz. Controlling dispersion forces between small particles with artificially created random light fields. Nat. Comm., 6, 7460 (2015). [4] J. Luis-Hita, M. I. Marqués, R. Delgado-Buscalioni, N. de Sousa, L. S. Froufe-Pérez, F. Scheffold, and J. J. Sáenz. Light induced inverse-square law interactions between nanoparticles: “mock gravity” at the nanoscale. Phys. Rev. Lett., 123, 143201 (2019). [5] S. Torquato. Hyperuniform states of matter. Phys. Rep. 745, 1 (2018).
Nr: 19
Title:

Chiral Sensing with Semiconductor Nanophotonics
Authors:

Alberto G. Curto
Abstract: Chirality plays a pivotal role in the functionality of biomolecules such as proteins, amino acids, and carbohydrates. Circular dichroism can distinguish enantiomers thanks to a small difference in the absorption of circularly polarized light. However, chiral sensing faces significant limitations due to inherently weak chiroptical signals. It is thus severely limited by low sensitivity and low spatial resolution. As a result, it is challenging to resolve the chirality of individual nanoscale objects using light for critical applications such as detecting protein aggregates linked to various diseases. In this presentation, I will discuss our progress to push the limits of optically resolvable chirality through new concepts in semiconductor nanophotonics. First, I will show several strategies to optimize chiral molecular sensors based on silicon metasurfaces to detect low molecular concentrations. Specifically, I will present our recent results on tailoring silicon nanostructures to enhance polarized fluorescence and Raman spectroscopies, increase optical chirality, and maximize chirality transfer [1,2]. Second, I will introduce an approach to molecular sensing based on excitons in atomically thin semiconductors. I will show how monolayer semiconductors can exhibit strong fluorescence fluctuations [3] that report on charge transfer events to nearby nano-objects. References [1] E Mohammadi, A Tittl, KL Tsakmakidis, TV Raziman, AG Curto, ACS Photonics, 8, 6, (2021)1754. [2] TV Raziman, RH Godiksen, MA Müller, AG Curto, ACS Photonics 6, 10, (2019) 2583. [3] RH Godiksen, S Wang, TV Raziman, MHD Guimaraes, JG Rivas, AG Curto Nano Lett. 20, 7, (2020) 4829.
Nr: 20
Title:

Ultrasensitive Chiral Detection through Enhanced Polarized Fluorescence with Dielectric Nanoresonators
Authors:

Ershad Mohammadi, Raziman Thottungal Valapu and Alberto G. Curto
Abstract: Detecting the chirality of molecular enantiomers is crucial for numerous applications in medicine and biochemistry. Conventional chiral sensing techniques such as circular dichroism (CD) exploit the dissymmetric absorption of large ensembles of chiral molecules for right- and left-circularly polarized light. Nanophotonics offers a viable route to increase chiral sensitivity through enhanced superchiral fields. However, it remains elusive to reach the detection limit of a single chiral molecule using absorption-based methods. In contrast, emission-based chiroptical methods such as circularly polarized luminescence or fluorescence-detected circular dichroism are promising for probing molecular chirality at the single-molecule level. Such analysis could uncover the conformational dynamics of individual biological molecules, usually masked by ensemble averaging. Circularly polarized luminescence (CPL) exploits the dissymmetry in circularly polarized emission for a chiral molecule. It is quantified by the degree of circular polarization (DOCP), taking values between -1 and 1 for perfectly right- and left-handed circularly polarized emission. For a small chiral molecule, DOCP values are typically very small (below 0.001). Tailored chiral fields can enhance this weak polarization contrast [1, 2]. However, most studies thus far rely on suppressing the electric field at the location of the chiral molecule, which essentially degrades the fluorescence signal. This suppressed emission has been a critical limitation towards achieving single-molecule sensitivity because of the poor signal-to-noise ratio. Here, we introduce nanophotonic resonators to boost chiral sensing based on circularly polarized luminescence. Our approach enhances both the degree of circular polarization and the total fluorescence intensity. To increase these two quantities together, we propose a high-index dielectric system composed of a silicon holey disk surrounded by a ring. Our approach enables simultaneous 1000- and 10-fold enhancements in fluorescence signal and polarization, respectively. We model the interactions between a chiral emitter and a general nanoantenna through a rigorous analytical framework based on the reciprocity theorem. Our analysis reveals that the extrinsic polarization due to the nanostructure should be taken into account to be able to distinguish opposite enantiomers. Our proposed method promises outstanding performance for single-molecule chiroptical detection in diverse applications. [1] Ershad Mohammadi, Andreas Tittl, Kosmas L. Tsakmakidis, T. V. Raziman, and Alberto G. Curto. ACS Photonics, 8, 6, 1754 (2021) [2] T. V. Raziman, Rasmus H. Godiksen, Moos A. Müller, and Alberto G. Curto. ACS Photonics, 6, 10, 2583 (2019)
Nr: 21
Title:

Radical Polymerization Mediated by Plasmon
Authors:

Aldine Ali, Frédéric Amiard, Sandie Piogé and Marc Lamy de La Chapelle
Abstract: Thanks to their remarkable optical and plasmonic properties, metallic nanoparticles (NPs) can induce different physical phenomena such as local heat, local near-field enhancement or transfer of hot electrons to molecules[1]. Such phenomena can be exploited to modify molecules, activate chemical reactions or improve their kinetics or yields[2]. Our work proposes to explore a new process of radical activation (formation of primary radicals) of polymerization via the exploitation of the plasmonic properties of gold NPs in order to elaborate well defined polymers. The plasmon is used to induce the formation of primary radicals necessary for the polymerization of vinyl monomers under different light irradiations (458, 515, 611 or 630 nm). A systematic study of the experimental conditions (NPs concentration, NPs size, plasmon resonance position, monomer concentration) of such a system allowed us to demonstrate that the plasmon improves the synthesis performances (kinetics and polymerization yield) at 458 nm and that these performances are optimal at 515 nm, wavelength close to the plasmon resonance of gold NPs. On the contrary, no plasmon effect on the polymerization reaction was detected at 611 and 630 nm (non-resonance wavelengths). Moreover, even if the concentration of NPs influences the polymerization kinetics, it should be noted that a very low concentration of NPs (a few pmol/L) is sufficient to improve kinetic performances. This study allows us to better understand the phenomena involved during the radical activation in photoinduced polymerization and to evaluate the real influence of the plasmon. Thanks to this study, the plasmonic parameters such as the position of the resonance are identified as those involved in the process of radical activation in polymerization. [1] Baffou, G. et al., Chemical Society Reviews 43, 3898 (2014) [2] Xiao, M. et al., Journal of Materials Chemistry A 1, 5790 (2013)
Nr: 22
Title:

Designing Plasmonic Nanoheaters at the Nanoscale: A Comprehensive Analysis
Authors:

Javier González-Colsa, Guillermo Serrera, José María Saiz, Dolores Ortiz, Francisco González, Fernando Bresme, Fernando Moreno and Pablo Albella
Abstract: Nowadays, efficient local heat generation using metallic nanoparticles is a highly active research field. These optically excited structures often show large resistive losses induced by the generation of Localized Surface Plasmons Resonances (LSPR), so that they are capable of efficiently convert optical energy into thermal energy [1]. The features of the obtained temperature distribution depend on the geometry and composition of the nanoparticles; therefore, different nanostructures can be designed to serve as nanoheaters for different purposes, opening a wide range of fascinating applications in areas from materials science to biomedicine. Some examples are micropollutant degradation [2] or photothermal therapy (PTT) [3]. Particularly, the success of the PTT relies strongly on the heating source i.e., the optically absorbing agent employed. A wide set of materials, including bovine serum albumin heterojunctions [4], graphene nanoparticles and carbon nanotubes [5], are being investigated for PTT applications, but the most widespread and promising PTT agents are built on gold due to its biocompatibility, weak cytotoxicity, and low reactivity. Furthermore, gold LSPR can be taken to the NIR, close to the so-called “biological windows” (700-900 nm, 1000-1400 nm) [6], in which human tissues are dispersive -causing partial light depolarization- but low absorbing. Therefore, the design of particles with main absorption in this wavelength range is a key objective in PTTs, as they provide optimum heating conditions for the nanoparticles minimizing the non-selective heating of healthy tissues. In this talk, we will present our most recent results on this topic, showing a comprehensive comparison of different efficient and tunable nanoheater prototypes. We will analyse their relative ability to transform light into heat. In addition, we will show how the different orientations that nanoparticles immersed in a fluid can present affect their thermal performance. REFERENCES [1] G. Baffou. Cambridge: Cambridge University Press, 2017. [2] H. Wei, Stephanie K. Loeb, Naomi J. Halas, J. Kim. PNAS 117 (27) 15473-15481, 2020. [3] André M. Gobin, Min Ho Lee, Naomi J. Halas, William D. James, Rebekah A. Drezek, and Jennifer L. West. Nano Letters 7 (7), 1929-1934, 2007. [4] Z. Guo, S. Zhu, Y. Yong, X. Zhang, X. Dong, J. Du, J. Xie, Q. Wang, Z. Gu, and Y. Zhao. Adv. Mater. 29(44), 1–12 (2017). [5] Z. M. Markovic, L. M. Harhaji-Trajkovic, B. M. Todorovic-Markovic, D. P. Kepić, K. M. Arsikin, S. P. Jovanović, A. C. Pantovic, M. D. Dramićanin, and V. S. Trajkovic. Biomaterials 32(4), 1121–1129 (2011). [6] E. Hemmer, A. Benayas, F. Légaré, and F. Vetrone. Nanoscale Horizons 1(3), 168–184 (2016). ACKNOWLEDGMENTS Authors would like to thank A. Franco and C. R. Crick for the interesting discussions. We gratefully acknowledge financial support from Spanish national project (No. PGC2018-096649-B-I), the UK Leverhulme Turst (Grant No. RPG-2018-384), UK-EPSRC (EP/J003859/1) and Imperial College Europeans Partner Fund grant. J. G-C. thanks the Ministry of science of Spain for his FPI grant. G. S. thanks the Ministry of education for his collaboration grant and P.A. acknowledges funding for a Ramon y Cajal Fellowship (Grant No. RYC-2016-20831).
Nr: 27
Title:

Comparing the Near-field Pattern of a Dielectric Metasurface Measured by Dual-tip Scanning Near-field Optical Microscopy to Full-wave Simulations
Authors:

Angela I. Barreda, Najmeh Abbasirad, Dennis Arslan, Michael Steinert, Stefan Fasold, Carsten Rockstuhl, Frank Setzpfandt, Thomas Pertsch and Isabelle Staude
Abstract: The emission of light of a quantum emitter depends on the photonic environment into which the emitter is placed. The localization of quantum emitters in the surroundings or inside metallic or dielectric nanostructures has increased the power that can be extracted. For that reason, the knowledge of the local density of optical states (LDOS) of the photonic systems is crucial to understand the possible modes to which a quantum emitter can couple. The LDOS is related to the imaginary part of the Green's function. Measurements of the near-field Green's function require a point-source excitation and simultaneous near-field detection below the diffraction limit. This experimental challenge can be addressed using the dual-tip near-field optical microscope (SNOM). This work analyses the position-dependent near-field intensity distribution and directional mode propagation in an all-dielectric metasurface upon dipole emission. In particular, we concentrate on the aspects regarding the numerical simulations of the LDOS in realistic experimental systems and their comparison with experimental data obtained from dual-tip SNOM measurements. We demonstrate that the integrated mapped near-field intensity by dual-tip SNOM is proportional to the partial LDOS. This research contributes to the development of novel single-photon technologies for quantum information and quantum sensing.
Nr: 34
Title:

Exploring the Interaction of Mie-resonant Nanoparticles with Plane Waves, Dipoles, and Electron Beams
Authors:

Christos Tserkezis
Abstract: Dielectric nanoparticles that support Mie resonances have proven efficient as nanoantennas and building blocks for optical metamaterials [1], thus consituting promising candidates for enhanced light—matter interactions at the nanoscale. Here we discuss our recent efforts to understand and tailor light—matter interactions in Mie-resonant nanoparticles coupled to light, to dipolar or extended excitonic emitters, and to electron beams. We first discuss the weak-coupling regime, and focus on the fluorescence of molecules placed in the vicinity of silicon nanospheres [2]. Through exact analytic solutions, we show that the richness of optical modes supported by such particles enables appreciable fluorescence enhancement for almost any molecule position and orientation [3], a response that is not easy to achieve with plasmonic environments, where an optimal emitter orientation with respect to the incident field is required [4]. We then turn to the strong-coupling regime [5], and we show that complex core—shell nanoparticles combining a silicon and an excitonic component have nothing to envy their plasmonic counterparts, leading to resonance splittings of the order of 200 meV [6]. Depending on the Mie mode to which the excitons couple, the resulting hybrid polaritons can have either magnetic or electric character, enabling selective manipulation of electric or magnetic-dipolar transitions. The relatively lower losses in silicon also allow to observe Rabi-like oscillations in the electric and magnetic field beat patterns, facilitating real-time monitoring of the strong coupling. This strong-coupling behaviour is verified experimentally [7], and we discuss how the broken symmetry provided by cylindrical nanoparticlse allows to better separate the electric and magnetic modes. The response of the resulting hybrid Mie-exciton polaritons can be further manipulated with static magnetic fields [8]. Finally, we discuss how the nature of the different Mie modes supported by silicon nanoparticles can be monitored and visualised in an electron microscope through cathodoluminescence spectroscopy, and discuss how care must be taken when interpreting measurements, as interference with transition radiation can strongly affect the recorded spectra [9]. [1] I. Staude & J. Schilling, Nat. Phot. 11, 274 (2017) [2] P. Albella et al., J. Phys. Chem. C 117, 13573 (2013) [3] P. E. Stamatopoulou & C. Tserkezis, OSA Continuum 4, 918 (2021) [4] T. Härtling et al., Opt. Express 15, 12806 (2007) [5] C. Tserkezis et al., Rep. Prog. Phys. 83, 082401 (2020) [6] C. Tserkezis et al., Phys. Rev. B 98, 155439 (2018) [7] F. Todisco et al., Nanophotonics 9, 803 (2020) [8] P. E. Stamatopoulou et al., Phys. Rev. B 102, 195415 (2020) [9] S. Fiedler et al., Disentangling cathodoluminescence spectra in nanophotonics: particle eigenmodes vs transition radiation, submitted
Nr: 36
Title:

Directional Plasmon Emission in a Mixed Dielectric-plasmonic Structure: Application to Chiral Sensing
Authors:

Guillermo Serrera, Javier González-Colsa, Vincenzo Giannini, José M. Saiz and Pablo Albella
Abstract: The concept of chirality, related to objects and structures which present non-superimposable mirror images in any plane of symmetry, has attracted a lot of attention recently in research fields like life sciences, where for example, it has been suggested as the underlying cause of Parkinson’s and Alzheimer’s diseases [1]. Enantiomers or optical isomers are chiral molecules which can have different effects as drugs, and therefore their distinction is relevant in the pharmacological industry, where single-enantiomer drugs are being developed to improve their effectiveness and compatibility [2]. In nanophotonics, intensive research is being performed to develop novel structures and devices capable of sensing enantiomeric proportions by enhancing the intrinsically weak Circular Dichroism (CD) optical effect. Our work focuses on the use of chiral structures capable of enhancing this effect. Special attention is paid to the distribution of near-field optical chirality, seeking for homogeneous high-dissymmetry factor areas available to analytes [3]. Here I will present the results of a recent study where using computational FDTD methods we demonstrate the capabilities of a High Refractive Index Dielectric (HRID) metasurface to offer both, a high far field enhancement of the CD effect and a strong near-field chiral enhancement. This HRID metasurface is then set on a gold layer to support directional Surface Plasmon Polariton (SPP) generation [4] [5], thus obtaining a directional plasmonic chiral sensor device [6]. REFERENCES [1] C.M. Dobson, Protein folding and misfolding, Nature (London) 426, 884, 2003. [2] J. McConathy and J. Owens, Stereochemistry in drug action, Prim. Care Companion J. Clin. Psychiatry 5, 70, 2003. [3] F. Reyes Gómez, O. N. Oliveira, Jr., P. Albella and J. R. Mejía-Salazar, “Enhanced chiroptical activity with slotted high refractive index dielectric nanodisks”, Physical Review B 101, 155403, 2020. [4] J. González-Colsa, G. Serrera, J. M. Saiz, F. González, F. Moreno and P. Albella, "On the performance of a tunable grating-based high sensitivity unidirectional plasmonic sensor", Optics Express 29, 9, 13733-13745, 2021. [5] J. Lin et al., “Polarization-controlled tunable directional coupling of surface plasmon polaritons,” Science 340, 6130, 331–334, 2013. [6] G. Serrera, J. González-Colsa, V. Giannini, J. M. Saiz and P. Albella, "Enhanced chiral sensing via directional emission of Surface Plasmon Polaritons" (under review).
Nr: 37
Title:

Extraordinarily Transparent Compact Metallic Metamaterials
Authors:

Vincenzo Giannini
Abstract: Metals are highly opaque, yet we show numerically and experimentally that densely packed arrays of metallic nanoparticles can be more transparent to infrared radiation than dielectrics such as germanium, even for arrays that are over 75% metal by volume. Despite strong interactions between the metallic particles, these arrays form effective dielectrics that are virtually dispersion-free, making possible the design of optical components that are achromatic over ultra-broadband ranges of wavelengths from a few microns up to millimetres or more. Furthermore, the local refractive indices may be tuned by altering the size, shape, and spacing of the nanoparticles, allowing the design of gradient-index lenses that guide and focus light on the microscale (see figure a). The electric field is also strongly concentrated in the gaps between the metallic nanoparticles, and the simultaneous focusing and squeezing of the electric field produces strong ‘doubly-enhanced’ hotspots (see figure b) which could boost measurements made using infrared spectroscopy and other non-linear processes over a broad range of frequencies, with minimal heat production.
Nr: 38
Title:

Distinguishing Thermal from Non-thermal (``hot'') Carriers in Illuminated Molecular Junctions
Authors:

Yonatan Sivan
Abstract: We provide a theory for using plasmonic molecular junctions that shows how non-thermal electrons can be measured directly and separately from the unavoidable thermal response, and discuss the relevance of our theory to recent experiments.
Nr: 39
Title:

The Transition Matrix Technique in Nanoplasmonics
Authors:

Maria Antonia Iati and Rosalba Saija
Abstract: The Transition matrix technique is a powerful approach to study the optical properties of metal nanoparticles and nano-aggregates [1,2]. Such technique, based on the multipole expansions of the electromagnetic fields, can be applied to model the optical behavior of many scatterer morphologies, from nano-shell particles to inhomogeneous spherical particles and clusters of particles. One of the most powerful aspects of this technique is that it offers both an analytical and numerical solution to the problems under study. It allows a rigorous description of the scattering processes, surpassing other commonly used techniques in terms of computational efficiency. A unique feature of the Transition matrix is that it allows to perform an analytical evaluation of the scattering characteristics averaged over particle orientation. After a general introduction to the method, I will focus on the description of plasmon coupling in metal nanoparticles, going from nano-dimers to larger aggregates and nano-shell particles. The distance dependence of the near-field plasmon interaction will be discussed through the presentation of several numerical results. We acknowledge financial contribution from the agreement ASI-INAF n. 2018-16-HH.0, project “SPACE Tweezers” and the MSCA ITN (ETN) project “Active Matter”. [1] F. Borghese, P. Denti, R. Saija. "Scattering from model nonspherical particles". 2nd ed., Springer, Berlin, 2007 [2] V. Amendola et al., “Surface plasmon resonance in gold nanoparticles: a review", Journal of Physics: Condensed Matter 29 (2017), 203002
Nr: 41
Title:

Accessing Anapole-exciton Polaritons using Electron Energy Loss Spectroscopy
Authors:

Carlos Alberto M. Escudero
Abstract: Strong light-matter interaction occurs when the coherent energy exchange between an electromagnetic mode confined in an optical cavity (light) and a dipolar excitation (matter) exceeds both the photonics mode and dipole decay rates. As a consequence, new hybrid modes (polaritons) are formed exhibiting inseparable light and matter properties which can lead, for instance, to modification of the system chemical reactivity [1], increase of charge transport [2], among others. Recently, strong coupling between excitons and optical anapole states has emerged as an intriguing research topic [3,4], including reports on the properties of optical anapoles as non-radiating current configurations with vanishing scattering (dark scattering states). These states can be understood as the superposition of an electric and toroidal multipole. The radiation pattern of both multipoles leads to destructive interference in the far-field and thus to its characteristic radiation – less behaviour [5]. In this work, we theoretically identify and characterize anapole states in high refractive index nanodisks with large diameter-to-height aspect ratio by using low-loss electron energy loss spectroscopy (EELS). Our theoretical results show that the optical anapole state supported in the nanodisk emerges as a dip in the electron energy loss spectra. To experimentally validate anapole excitation and detection in EELS, we fabricate nanodisks made of Tungsten Disulfide (WS2) and compare the experimental EELS signal with numerical calculations. Remarkably, the anapole wavelength evolves linearly with the disk size and thus, by varying the WS2 nanodisk dimensions, the non-radiating state can be tuned to coincide with an WS2 exciton transition leading to anapole-exciton hybridization. Our studies constitute an avenue for engineering light-matter interactions using dark scattering states which can now be accessed by electron microscopy. [1] J. Galego, F. J. García-Vidal, and J. Feist, “Suppressing photochemical reaction reactions with quantized light fields”, Nat. Commun. 7, 13841 (2016). [2] E. Orgiu, J. George, J. A. Hutchison, E. Devaux, J. F. Dayen, B. doudin, F. Stellacci, C. Genet, J. Schachenmayer, C. Genes, G. Pupillo, P. Samorí, and T. W. Ebbesen, “Conductivity in organic semiconductors hybridized with the vacuum field”, Nat. Mater. 14, 1123-1129 (2015). [3] R. Verre, D. G. Baranov, B. Munkhbat, J. Cuadra, M. Käll, and T. Shegai, “Transition metal dichalcogenide nanodisk as high index dielectric Mie nanoresonators”, Nat. Nanotechnol. 14, 679-683 (2019). [4] K. As’ham, I. Al-Ani, L. Huang, A. E. Miroschnichenko, and H. T. Hattori, “Boosting Strong Coupling in a Hybrid WSe2 Monolayer-Anapole-Plasmon System”, ACS Photonics 8, 48496 (2021). [5] A. E. Miroshnichenko, A. B. Evlyukhin, Y. F. Yu, R. M. Bakker, A. Chipouline, A. I. Kuznetsov, B. Luk’yanchuk, B. N. Chichkov and Y. S. Kivshar, “Nonradiating anapole modes in dielectric nanoparticles”, Nat. Commun. 6, 8069 (2015).
Nr: 43
Title:

Focusing of In-plane Hyperbolic Polaritons in Van der Waals Crystals with Tailored Infrared Nanoantennas
Authors:

Javier Martín-Sánchez
Abstract: Polaritons -hybrid light-matter excitations- play a crucial role in fundamental and applied sciences, as they enable control of light on the nanoscale [1]. The recent emergence of low-loss van der Waals (vdW) materials opens the door to achieving anisotropic optical phenomena owing to their layered crystal structure, which leads to an intrinsic and strong out-of-plane (perpendicular to the layers) optical anisotropy. A prominent example is given by hyperbolic phonon polaritons (PhPs) -infrared light coupled to lattice vibrations in layered polar materials- in hexagonal boron nitride (h-BN) [2], which exhibit long lifetimes, ultra-slow propagation and hyper-lensing effects. Only recently, PhPs with in-plane hyperbolic dispersion, a key requirement for on-chip planar optical circuitry, have been demonstrated in natural slabs of α-phase molybdenum trioxide (α-MoO3) [3-5] and vanadium pentaoxide (α-V2O5) [6]. In this work, we demonstrate focusing of infrared ray-like hyperbolic PhPs into deep subwavelength focal spots along the surface of α-MoO3 crystals by using metal antennas with an optimized design. Specifically, field confinement is achieved in focal spots with a size of λp/4.5=λ0/50 (λp is the polariton wavelength and λ0 is the photon wavelength in free space). Moreover, the achievable focal distance in in-plane hyperbolic α-MoO3 can be tuned to values well below the diffraction limit in in-plane isotropic materials, along with a better performance in terms of near field confinement and optical absorption. Our findings set the grounds for planar polaritonic technologies at the nanoscale [7]. [1] T. Low et al. Nat. Mater. 16, 182 (2017) [2] S. Dai et al. Science 343, 1125 (2014) [3] W. Ma et al. Nature 562, 557 (2018) [4] J. Duan et al. Nano Lett. 20, 5323 (2020) [5] J. Duan et al. Sci. Adv. 7, eabf2690 (2021) [6] J. Taboada-Gutiérrez et al. Nat. Mater. 19, 964 (2020) [7] J. Martín-Sánchez et al. Sci. Adv. 7, eabj012 (2021)
Nr: 44
Title:

Quantum Many-body Study of the Optical Response of Plasmonic Nanoparticles Coupled to Single Emitters
Authors:

Ruben Esteban, Antton Babaze, Andrei G. Borisov and Javier Aizpurua
Abstract: The interaction of a molecule, a quantum dot or another single emitter with a nearby metallic nanostructure can lead to a rich variety of physical effects, due to the excitation of optical resonances called localized surface plasmon polaritons. These effects are exploited in different surface-enhanced spectroscopy techniques and other applications such as single photon emission. A classical treatment based on the solution of Maxwell’s equations is enough to model the plasmonic structure-emitter interaction in many practical situations. However, extreme situations, such as current configurations where a molecule is placed between two metallic particles separated by a ~1nm gap, can require a quantum treatment [1,2]. In this contribution, we use the quantum many-body methodology Time-Dependent Density Functional Theory (TD-DFT) to be able to capture the influence of electronic correlations or charge transfer processes in the optical plasmonic response. We focus on two different effects. First, we show that a quantum emitter can strongly affect the non-linear response of a plasmonic system, enabling the emission of light at twice the energy of the illumination laser for configurations where this process would be symmetry forbidden [3]. Further, we demonstrate that, for very narrow gaps, the electronic coupling between a quantum emitter and a metallic nanostructure can completely modify the strength and frequency of the resonant modes that dominate the optical response [4]. The electronic coupling can not only lead to the emergence of charge transfer plasmons at low energies, but also induces an electronic quenching of the excited states of the emitter. This electronic quenching reduces the optical coupling between the emitter and the nanostructure, inducing a blueshift and a gradual disappearance of optical resonances and even hindering the capability of plasmonic systems to reach the regime of strong coupling. Thus, including the electronic coupling can be critical for systems exhibiting (sub)nanometer gaps. [1] M. S. Tame et al., Nature Physics 9, 329–340 (2013) [2] T. Neuman et al., Nano Lett. 18, 2358−2364 (2018) [3] A. Babaze et al., ACS Photonics 7, 701−713 (2020) [4] A. Babaze et al., Nano Lett. 21, 8466−8473 (2021).
Nr: 45
Title:

Large Area Metasurfaces for Thermoplasmonics and Thermoelectric Applications
Authors:

Josep Canet-Ferrer
Abstract: Resistive loss in metals is one of the main limitations in Nanophotonics. However, thermoplasmonic devices can convert this limitation into opportunities for further applications in fields as different as Optoelectronics, Photocatalysis or Biosensing. Depending on the application, we can find chemical methods (based on the synthesis and functionalization of gold NPs) and nanofabrication methods (based on nanolithography). In both cases, most successful approaches have been based on gold nanostructures which have demonstrated unrivalled performance for patterning temperature gradients on a surface, in terms of spatial resolution and thermal amplitude. Making a step forward, in this work we will focus on the generation of thermal gradients for improving thermoelectric performance. Based on different strategies, our approaches combine chemical and nanofabrication methods to obtain large area thermoplasmonic surfaces and perfect absorbers operating in a broad range of the spectra, as required for the fabrication of thermoelectrics panels. The numerical design of our devices is carried out using simple analytical methods while the experimental realization is developed by means of scalable processes using inexpensive materials. As a result, we have achieved an increase up to the 15% in those devices exploiting thermoplasmonics with respect to their non-thermoplasmonic counterparts.
Nr: 48
Title:

Optical Trapping of Individual Dust Particles in the T-matrix Formalism
Authors:

Abir Saidi, Rosalba Saija, M. A. Iatì, Alessandro Magazzù, D. Bronte Ciriza, Anna Musolino, Luigi Folco, A. Rotundi and Onofrio M. Maragò
Abstract: Cosmic dust is the key ingredient in the formation of stars, planets, comets, and asteroids all across the universe. Moreover, the surface of cosmic dust grains serves as a platform for the formation of molecular hydrogen and simple organic compounds. For all these reasons the study of the composition, shape, and optical properties of dust grains is a relevant question in space science. To this end, some space missions have been planned over the years with the aim of recovering and sending dust samples to Earth for chemical and physical analysis in the laboratory. More challenging is the possibility to analyze dust grains in situ so as to avoid contamination with the terrestrial environment. One of the possible solutions to this demand, intriguing from a scientific point of view, is the design and realization of a scientific instrument that can be sent into space, to the heart of which there is an optical tweezers system capable of contactless trapping and manipulation of extra-terrestrial particle matter. In this context, it is also necessary to have a reliable theoretical approach through which, thanks to the study of the interaction with the radiation field, we can not only verify whether the extra-terrestrial conditions would allow the trapping and manipulation of particles but also infer the main information about the nature of the trapped grains. To meet some of these needs, we utilize the full light scattering theory in the T-matrix formalism to analyse the optical behaviour and to compute optical trapping properties of a class of complex nano and micro particles whose nature is similar to that found in the extra-terrestrial environment. Using the real mineral composition of trapped dust particles as a main basis, we report the main results considering different strategies: firstly, to speed up the simulations, we use spherical grains with an effective refractive index using Bruggeman mixing rule, after, taking into account the non-homogeneity in the matter distribution, we consider a spherical grain with a series of different homogeneous inclusions representing the real composition of the material. Finally, choosing an even more complex and more computationally demanding model, we took into account both the anisotropy in shape and the non-homogeneity in composition using a cluster model. Theoretical results will be compared with experimental data. We acknowledge support by the agreement ASI-INAF n.2018-16-HH.0, project “SPACE Tweezers” and the MSCA ITN (ETN) project “ActiveMatter”, project number 812780.
Nr: 51
Title:

Unidirectional Emission of Single-molecule based on DNA Origami Assembled Ultracompact Antenna
Authors:

Guillermo P. Acuna, Maria Sanz-Paz, Fangjia Zhu and Mauricio Pilo-Pais
Abstract: Optical antennas have been widely used for manipulating light-matter interactions at the nanoscale in order control the emission intensity, lifetime and directivity of single molecules [1,2]. However, to date, precisely controlling the interaction between molecules and antennas at the single level is still challenging. In this contribution, we exploit the DNA origami technique [3,4] to self-assemble ultra-compact antennas [5] based on two gold nanorods using a T-shaped host structure. Here, we show that these antennas are capable of directing the emission of a single fluorophore that is placed with nanometric precision in the vicinity of one nanorod tip. We use the image in the back focal plane of our microscope to visualize and quantify this directional emission. [1] P. Biagioni et al., Nanoantennas for Visible and Infrared Radiation, Reports Prog. Phys. 75(2), 024402 (2012) [2] A. G. Curto et al., Unidirectional Emission of a Quantum Dot Coupled to a Nanoantenna, Science, 329(5994), 930–933 (2010) [3] P. W. K. Rothemund, Folding DNA to Create Nanoscale Shapes and Patterns, Nature, 440, 297–302 (2006) [4] M. Pilo-Pais et al., Sculpting Lightby Arranging OpticalComponents with DNA Nanostructures, MRS Bull, 42(12), 936–942 (2017) [5] T. Pakizeh et al., Unidirectional Ultracompact Optical Nanoantennas, Nano Lett., 9(6), 2343–2349 (2009)
Nr: 52
Title:

Controlling Light Generation in Disordered Fractal Networks of Nanostructures
Authors:

Giorgio Volpe
Abstract: The use of light as a way to communicate and process information to and from the nanoscale is one of the technological milestones that is advancing innovation in modern times across many areas, from computer sciences and renewable energies to personalized healthcare and sensing technologies. While, to date, control of light flow at the nanoscale has mainly been achieved with periodically structured materials, disordered photonic nanostructures are slowly emerging as suitable easy-to-fabricate designs that can lead to performances superior to those offered by conventional photonic structures in, e.g., imaging and photovoltaics. Here, I will present recent experimental results where disordered fractal nanostructures allow us to generate and control light at the nanoscale. These novel disordered arrays of nanostructures can play a key role in controlling the light transport properties of complex photonic systems and, thus, in controlling their final optical properties, which are ultimately of interest to develop next generation optical devices.
Nr: 54
Title:

Planar Nano-optics in Anisotropic Media: Refraction and Lensing of In-plane Hyperbolic Polaritons
Authors:

Pablo Alonso-González
Abstract: As one of the fundamental optical phenomena, refraction between isotropic media is characterized by light bending towards the normal to the boundary when passing from a low- to a high-refractive-index medium. However, in anisotropic media, refraction is a much more exotic phenomenon. In this talk, we will show the first visualizations of refraction of electromagnetic waves between two strongly anisotropic (hyperbolic) media. To do this we image polaritons confined to the nanoscale in a low-loss natural medium: α-MoO3. As they traverse planar nanoprisms tailored in α-MoO3, refracted polaritons exhibit non-intuitive directions of propagation, enabling to unveil an exotic optical effect: bending-free refraction. Furthermore, we will also show results on the first in-plane refractive hyperlens, which yields foci as small as λp/6, being λp the polariton wavelength (λ0/50 with respect to the wavelength of light in free space).