24 research outputs found
Enabling infinite factors in absorbing optical systems
Resonant optical systems have widespread applications in science and
technology. However, their quality () factors can be significantly
deteriorated, if some of their parts exhibit optical absorption. Here, we show
that by coupling a lossy mode of such a structure to two independent lossless
modes, one can create a nonradiating and absorption-free bound state in the
continuum (BIC). The factor of such a BIC is theoretically unlimited
despite interaction with an absorbing structure. We use this mechanism to
design a plasmonic metasurface with factors that are close to in the
visible spectral range. The proposed mechanism is general and can be used to
engineer ultrahigh- resonances in various absorbing structures.Comment: 9 pages, 5 figure
Nonlinear nonlocal metasurfaces
Optical metasurfaces have recently emerged as the game changer in light manipulation and opened up new perspectives in many subfields of optics and photonics. Recent developments in nonlocal metasurfaces, in which the nanoscale building blocks respond to the incoming light collectively rather than as individual objects, are especially promising for enhancing and controlling the nonlinear optical phenomena. In this article, we provide a brief overview of the basic principles of nonlocal metasurfaces in the context of their nonlinear optical functionalities. We discuss the origin and the regimes of the nonlocal response, covering the aspects of multiple scattering, radiation damping, quality factor, local-field enhancement, and temporal dynamics. Some important aspects are illustrated by computational examples. We also give our personal viewpoint on the selected ideas and research directions in nonlocal and nonlinear metasurfaces, including the role of spatial symmetry in nonlocal interactions, the effects of phase and momentum matching in frequency conversion, as well as the possibilities offered by new material platforms and novel concepts, such as bound states in the continuum, parity-time symmetry, and time-variant metasurfaces.publishedVersionPeer reviewe
Badanie nieliniowych wĆaĆciwoĆci optycznych nanostruktur plazmonicznych
The aim of this thesis and the underlying research work is to demonstrate the benefits emerging from combination of the peculiar properties of plasmonic nanostructures with the most interesting aspects of nonlinear optics. For this purpose, analytical and numerical modeling was combined with experimental work, which included nanofabrication and measurements performed by means of polarization-resolved nonlinear confocal microscopy and by modified Z-scan technique (called "f-scan").It has been shown that the effective anisotropy of the second-harmonic generation in plasmonic crystals (formed by rectangular arrays of tetrahedral recesses in silver surface) can be controlled by proper choice of lattice constants. It also has been shown that this anisotropy arises mainly from the anisotropic photonic band structure, exhibiting plasmonic band gap with plasmonic band edge states, enabling enhancement of the local electric field.Two-dimensional chiral arrangements of triangular gold nanoparticles, forming plasmonic enantiomeric "meta-molecules", have been studied by nonlinear microscopy operating with circularly polarized light and by numerical modeling, revealing strong chiroptical effect in backscattered second-harmonic radiation. Small size of individual enantiomers allows to create "watermarks", encoded by the chirality of meta-molecules, which can be readout by imaging of second-harmonic generation excited by circularly polarized laser beam.Quantitative characterization of the third-order optical nonlinearity and saturable absorption efficiency of aqueous solutions of graphene and gold-nanoparticle decorated graphene has been performed by novel "f-scan" technique, which has been created and developed by incorporation of a focus-tunable lens into traditional Z-scan. These studies have shown that the graphene exhibits very efficient ultrafast saturable absorption, which is occasionally suppressed by reverse saturable absorption. Moreover, it turns out that decoration of graphene by gold nanoparticles may cause a slight improvement of the saturable absorption efficiency parameter within spectral range of their plasmon resonances.In summary, the following thesis presents various nonlinear optical properties of plasmonic nanostructures. Different possibilities of controlling these properties by means of nano-engineering, supported by analytical and numerical modeling, is also analyzed and demonstrated. This work opens up new perspectives for fabrication and rational design of novel photonic nano-materials and nano-devices based on nonlinear nanoplasmonic phenomena.Le but de cette thĂšse et de la recherche associĂ©e est une dĂ©monstration des avantages dâune combinaison de propriĂ©tĂ©s inhabituelles de nanostructures plasmoniques avec des aspects parmi les plus intĂ©ressants de lâoptique non-linĂ©aire. Pour cet effet, la modĂ©lisation analytique et numĂ©rique a Ă©tĂ© combinĂ© avec le travail expĂ©rimental, qui comprenait la production de nanostructures et les mesures effectuĂ©es au moyen de la microscopie confocale non-linĂ©aire rĂ©solue en polarisations et de la technique Z-scan modifiĂ©e (nommĂ©e âf-scanâ).Il a Ă©tĂ© montrĂ© que lâanisotropie efficace de gĂ©nĂ©ration de seconde-harmonique dans les cristaux plasmoniques (formĂ©s par des rĂ©seaux rectangulaires de cavitĂ©s tĂ©traĂ©driques sur une surface dâargent) peut ĂȘtre contrĂŽlĂ©e par un choix appropriĂ© des paramĂštres de maille. Il a aussi Ă©tĂ© montrĂ© que cette anisotropie provient principalement dâune structure de bande photonique elle-mĂȘme anisotrope, prĂ©sentant une bande interdite plasmonique avec des Ă©tats plasmoniques en bord de bande, permettant de renforcer le champ Ă©lectrique local. Les arrangements chiraux bidimensionnels de nanoparticules triangulaires dâor, forment des âmeta-molĂ©culesâ plasmoniques Ă©nantiomĂ©riques, ont Ă©tĂ© analysĂ©s par microscopie non-linĂ©aire Ă la lumiĂšre polarisĂ©e circulairement et par modĂ©lisation numĂ©rique, rĂ©vĂ©lant un fort effet chiroptique par gĂ©nĂ©ration de seconde harmonique en rĂ©tro-rĂ©flexion. La petite taille des Ă©nantiomĂšres uniques permet de crĂ©er âdes filigranesâ (âwatermarksâ) codĂ©s par la chiralitĂ© des meta-molĂ©cules, qui peuvent ĂȘtre lu par imagerie de la gĂ©nĂ©ration de seconde harmonique excitĂ©e par un rayon polarisĂ© circulairement. Les caractĂ©ristiques quantitatives de la non-linĂ©aritĂ© optique du troisiĂšme ordre et de lâefficacitĂ© dâabsorption saturable des solutions aqueuses de fragments de graphĂšne et de graphĂšne dopĂ© par des nanoparticules dâor a Ă©tĂ© effectuĂ©e par une nouvelle technique âf-scanâ, qui a Ă©tĂ© crĂ©Ă©e et dĂ©veloppĂ©e par incorporation dâune lentille Ă distance focale accordable dans une technique de Z-scan traditionnelle. Ces Ă©tudes ont montrĂ©es que le graphĂšne prĂ©sente une absorption saturable ultra-rapide trĂšs efficace, qui est parfois convertie en absorption saturable inverse. Il apparaĂźt alors quâune dĂ©coration du graphĂšne par des nanoparticules dâor peut causer une lĂ©gĂšre amĂ©lioration du paramĂštre dâefficacitĂ© dâabsorption saturable dans la plage spectrale de leurs rĂ©sonances plasmoniques. En rĂ©sumĂ©, cette thĂšse prĂ©sente une variĂ©tĂ© de propriĂ©tĂ©s optiques non-linĂ©aires apparaissant dans les nanostructures plasmoniques. DiffĂ©rentes possibilitĂ©s de contrĂŽle de ces propriĂ©tĂ©s au moyen dâune dĂ©marche de nano-ingĂ©nierie, soutenue par des modĂ©lisations Ă la fois analytique et numĂ©rique ont Ă©tĂ© dĂ©montrĂ©es et analysĂ©es. Ces travaux ouvrent la voie Ă la fabrication et Ă lâoptimisation sur mesure de nouveaux nano-matĂ©riaux et nano-dispositifs photoniques reposant sur des effets de nano-plasmonique non-linĂ©aire
Enabling infinite Q factors in absorbing optical systems
Resonant optical structures have widespread applications in science and technology. However, their quality (Q) factors can be significantly deteriorated, if some of their parts exhibit optical absorption. Here, we show that by coupling a lossy mode of such a structure to two independent lossless modes, one can create a nonradiating and absorption-free bound state in the continuum (BIC). The Q factor of such a BIC is theoretically unlimited despite interaction with an absorbing structure. We use this mechanism to design a plasmonic metasurface with Q factors that are close to 107 in the visible spectral range. The proposed mechanism is general and can be used to engineer ultrahigh-Q resonances in various systems containing absorbing structures
Gain-induced scattering anomalies of diffractive metasurfaces
Photonic nanostructures with gain and loss have long been of interest in the context of diverse scattering anomalies and light-shaping phenomena. Here, we investigate the scattering coefficients of simple gain-doped diffractive metasurfaces, revealing pairs of scattering anomalies surrounded by phase vortices in frequencyâmomentum space. These result from an interplay between resonant gain, radiative loss, and interference effects in the vicinity of Rayleigh anomalies. We find similar vortices and singular points of giant amplification in angle-resolved reflectivity spectra of prism-coupled gain slabs. Our findings could be of interest for gain-induced wavefront shaping by all-dielectric metasurfaces, possibly employing gain coefficients as low as âŒ50 cmâ1
Hybridization of electromagnetic multipoles in a nanoscatterer in the presence of another nanoscatterer
Coupling between multipolar modes of different orders has not been investigated in depth, despite its fundamental and practical relevance in the context of optical metamaterials and metasurfaces. Here, we use an electromagnetic multipole expansion of both the scattered fields and the oscillating electric currents to reveal the multipolar excitations in a nanoparticle positioned close to another nanoparticle. The considered single-particle multipoles radically differ from multipoles excited in a pair of nanoparticles. Using the expansion, we reveal the multipole character of the electric currents and the contributions of the multipole moments to the scattering cross section of each particle, including the effect of their interaction. We find that light scattered by the particles plays the role of an inhomogeneous incident field for each of the particles, leading to hybridization of the originally independent orthogonal multipole resonances. For an incident plane wave polarized along the nanoparticle pair, the hybridization of the dipole and quadrupole resonances gives rise to a significant narrowband resonance in the spectrum of the dipole scattering, which can be of interest for various applications, e.g. in surface-enhanced fluorescence and Raman spectroscopy. In general, this work shows that the multipole-multipole interaction between nanoparticles must be treated by taking into account also such hybridized multipole resonances
Crosstalk Reduction between Closely Spaced Optical Waveguides by Using Higher-Order Modes
Funding Information: This work is funded by the Academy of Finland (308394). A significant part of the numerical calculations was performed using computational resources within the Aalto University School of Science âScience-ITâ project. We also acknowledge the Academy of Finland Flagship Programme, Photonics Research and Innovation (Grant No. 320167, PREIN). Publisher Copyright: © 2022 authors. Published by the American Physical Society.Optical waveguide structures can make the state-of-the-art micro- and nanofabricated devices faster and less energy consuming. However, on-chip optical components must be placed at relatively large distances from each other, on the order of the wavelength λ, to eliminate the crosstalk between them. This makes on-chip optical structures too large to compete with their electronic counterparts. In this work, we explore the possibility of suppressing the crosstalk between closely spaced dielectric waveguides by making use of higher-order modes. We show that the crosstalk between two waveguides can be essentially eliminated, even if the waveguides are separated by a distance of λ/10 only. We also study arrays of more than two waveguides and find the conditions for the efficient crosstalk reduction in them. Our results can lead to further miniaturization of photonic integrated circuits.Peer reviewe