| 1. |
- Baranov, Denis, 1990, et al.
(författare)
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Circular dichroism mode splitting and bounds to its enhancement with cavity-plasmon-polaritons
- 2020
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Ingår i: Nanophotonics. - : Walter de Gruyter GmbH. - 2192-8614. ; 9:2, s. 283-293
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Tidskriftsartikel (refereegranskat)abstract
- Geometrical chirality is a widespread phenomenon that has fundamental implications for discriminating enantiomers of biomolecules. In order to enhance the chiral response of the medium, it has been suggested to couple chiral molecules to resonant optical cavities in order to enhance the circular dichroism (CD) signal at the resonant frequency of the cavity. Here, we studied a distinctly different regime of chiral light-matter interaction, wherein the CD signal of a chiral medium splits into polaritonic modes by reaching the strong coupling regime with an optical microcavity. Specifically, we show that by strongly coupling chiral plasmonic nanoparticles to a non-chiral Fabry-Pérot microcavity one can imprint the mode splitting on the CD spectrum of the coupled system and thereby effectively shift the initial chiral resonance to a different energy. We first examined the effect with the use of analytical transfer-matrix method as well as numerical finite-difference time-domain (FDTD) simulations. Furthermore, we confirmed the validity of theoretical predictions in a proof-of-principle experiment involving chiral plasmonic nanoparticles coupled to a Fabry-Pérot microcavity.
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| 2. |
- Verre, Ruggero, 1985, et al.
(författare)
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Polarization conversion-based molecular sensing using anisotropic plasmonic metasurfaces
- 2016
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Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 8:20, s. 10576-10581
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Tidskriftsartikel (refereegranskat)abstract
- Anisotropic media induce changes in the polarization state of transmitted and reflected light. Here we combine this effect with the refractive index sensitivity typical of plasmonic nanoparticles to experimentally demonstrate self-referenced single wavelength refractometric sensing based on polarization conversion. We fabricated anisotropic plasmonic metasurfaces composed of gold dimers and, as a proof of principle, measured the changes in the rotation of light polarization induced by biomolecular adsorption with a surface sensitivity of 0.2 ng cm(-2). We demonstrate the possibility of miniaturized sensing and we show that experimental results can be reproduced by analytical theory. Various ways to increase the sensitivity and applicability of the sensing scheme are discussed.
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| 3. |
- Andrén, Daniel, 1991, et al.
(författare)
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Probing Photothermal Effects on Optically Trapped Gold Nanorods by Simultaneous Plasmon Spectroscopy and Brownian Dynamics Analysis
- 2017
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 11:10, s. 10053-10061
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Tidskriftsartikel (refereegranskat)abstract
- Plasmonic gold nanorods are prime candidates for a variety of biomedical, spectroscopy, data storage, and sensing applications. It was recently shown that gold nanorods optically trapped by a focused circularly polarized laser beam can function as extremely efficient nanoscopic rotary motors. The system holds promise for-applications ranging from nanofluidic flow control and nanorobotics to biomolecular actuation and analysis. However, to fully exploit this potential, one needs to be able to control and understand heating effects associated with laser trapping. We investigated photothermal heating of individual rotating gold nanorods by simultaneously probing their localized surface plasmon resonance spectrum and rotational Brownian dynamics over extended periods of time. The data reveal an extremely slow nanoparticle reshaping process, involving migration of the order of a few hundred atoms per minute, for moderate laser powers and a trapping wavelength close to plasmon resonance. The plasmon spectroscopy and Brownian analysis allows for separate temperature estimates based on the refractive index and the viscosity of the water surrounding a trapped nanorod. We show that both measurements yield similar effective temperatures, which correspond to the actual temperature at a distance of the order 10-15 nm from the particle surface. Our results shed light on photothermal processes on the nanoscale and will be useful in evaluating the applicability and performance of nanorod motors and optically heated nanoparticles for a variety of applications.
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| 4. |
- Andrén, Daniel, 1991, et al.
(författare)
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Surface Interactions of Gold Nanoparticles Optically Trapped against an Interface
- 2019
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:26, s. 16406-16414
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Tidskriftsartikel (refereegranskat)abstract
- Particles that diffuse in close proximity to a surface are expected to behave differently than in free solution because the surface interaction will influence a number of physical properties, including the hydrodynamic, optical, and thermal characteristics of the particle. Understanding the influence of such effects is particularly important in view of the increasing interest in laser tweezing of colloidal resonant nanoparticles for applications such as nanomotors and optical printing and for investigations of unconventional optical forces. Therefore, we used total internal reflection microscopy to probe the interaction between a glass surface and individual ∼100 nm gold nanoparticles trapped by laser tweezers. The results show that particles can be optically confined at controllable distances ranging between ∼30 and ∼90 nm from the surface, depending on the radiation pressure of the trapping laser and the ionic screening of the surrounding liquid. Moreover, the full particle-surface distance probability distribution can be obtained for single nanoparticles by analyzing temporal signal fluctuations. The experimental results are in excellent agreement with Brownian dynamics simulations that take the full force field and photothermal heating into account. At the observed particle-surface distances, translational friction coefficients increase by up to 60% compared to freely diffusing particles, whereas the rotational friction and thermal dissipation are much less affected. The methodology used here is general and can be adapted to a range of single nanoparticle-surface interaction investigations.
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| 5. |
- Block, Stephan, 1978, et al.
(författare)
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Antenna-Enhanced Fluorescence Correlation Spectroscopy Resolves Calcium-Mediated Lipid-Lipid Interactions
- 2018
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 12:4, s. 3272-3279
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Tidskriftsartikel (refereegranskat)abstract
- Fluorescence correlation spectroscopy (FCS) has provided a wealth of information on the composition, structure, and dynamics of cell membranes. However, it has proved challenging to reach the spatial resolution required to resolve biophysical interactions at the nanometer scale relevant to many crucial membrane processes. In this work, we form artificial cell membranes on dimeric, nanoplasmonic antennas, which shrink the FCS probe volume down to the ∼20 nm length scale. By analyzing the autocorrelation functions associated with the fluorescence bursts from individual fluorescently tagged lipids moving through the antenna "hotspots", we show that the confinement of the optical readout volume below the diffraction limit allows the temporal resolution of FCS to be increased by up to 3 orders of magnitude. Employing this high spatial and temporal resolution to probe diffusion dynamics of individual dye-conjugated lipids, we further show that lipid molecules diffuse either as single entities or as pairs in the presence of calcium ions. Removal of calcium ions by addition of the chelator EDTA almost completely removes the complex contribution, in agreement with previous theoretical predications on the role of calcium ions in mediating transient interactions between zwitterionic lipids. We envision that antenna-enhanced FCS with single-molecule burst analysis will enable resolving a broad range of challenging membrane biophysics questions, such as stimuli-induced lipid clustering and membrane protein dynamics.
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| 6. |
- Bosio, Noemi, 1993, et al.
(författare)
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Plasmonic versus All-Dielectric Nanoantennas for Refractometric Sensing: A Direct Comparison
- 2019
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Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 6:6, s. 1556-1564
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Tidskriftsartikel (refereegranskat)abstract
- In comparison to nanoplasmonic structures, resonant high-index dielectric nanoantennas hold several advantages that may benefit nanophotonic applications, including CMOS compatibility and low ohmic losses. One such application area might be label-free refractometric sensing, where changes in individual antenna resonance properties are used to quantify changes in the surrounding refractive index, for example, due to biomolecular binding. Here, we analyze and compare the sensing performance of silicon and gold nanodisks using a common and unbiased testing framework. We find that the all-dielectric system is fully capable of effectively monitoring small changes in bulk refractive index and biomolecular coverage, but the sensitivity is five to ten times lower than the plasmonic counterpart. However, this drawback is partly compensated for by a more linear response to adsorbate layer thickness changes and an approximately four times smaller susceptibility to photothermal heating. Finally, dielectric sensors may show promise if certain strategies are employed to improve their performance, which could thus bridge the gap between the two systems.
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| 7. |
- Odebo Länk, Nils, 1988, et al.
(författare)
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Directional scattering and multipolar contributions to optical forces on silicon nanoparticles in focused laser beams
- 2018
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Ingår i: Optics Express. - : Optical Society of America. - 1094-4087 .- 1094-4087. ; 26:22, s. 29074-29085
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Tidskriftsartikel (refereegranskat)abstract
- Nanoparticles made of high index dielectric materials have seen a surge of interest and have been proposed for various applications, such as metalenses, light harvesting and directional scattering. With the advent of fabrication techniques enabling colloidal suspensions, the prospects of optical manipulation of such nanoparticles becomes paramount. High index nanoparticles support electric and magnetic multipolar responses in the visible regime and interference between such modes can give rise to highly directional scattering, in particular a cancellation of back-scattered radiation at the first Kerker condition. Here we present a study of the optical forces on silicon nanoparticles in the visible and near infrared calculated using the transfer matrix method. The zero-backscattering Kerker condition is investigated as an avenue to reduce radiation pressure in an optical trap. We find that while asymmetric scattering does reduce the radiation pressure, the main determining factor of trap stability is the increased particle response near the geometric resonances. The trap stability for non-spherical silicon nanoparticles is also investigated and we find that ellipsoidal deformation of spheres enables trapping of slightly larger particles.
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| 8. |
- Odebo Länk, Nils, 1988, et al.
(författare)
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Electromagnetic Energy Distribution in Resonant Quasi Porous Silicon Nanostructures
- 2019
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Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 6:7, s. 1706-1714
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Tidskriftsartikel (refereegranskat)abstract
- Geometric resonances in high refractive index dielectric nanoantennas enhance the local density of optical states, increasing the decay rate of electric and magnetic dipolar emitters. Due to low losses, dielectric antennas exhibit less quenching than their plasmonic counterparts. However, enhanced fields associated with these resonances, in contrast to plasmonic ones, are confined to the particle interior, complicating efficient coupling strategies. Previous research has focused on emitters either placed next to dielectric antennas or incorporated into them during fabrication. Making the nanoantenna porous enables access to the internal fields while being flexible during fabrication. Herein, a model porous silicon antenna is analyzed, and the available electromagnetic energy within it is investigated. A porosity of 30% achieves an optimal balance between antenna quality and energy available in the pores. To minimize disturbance to the optical modes, the pores should be no larger than 5% of the size of the antenna. Moreover, the magnetic dipole resonance of these structures is remarkably robust to perturbations and is thus a promising target for applications due to its tolerance to fabrication error. Calculations show that nonradiative decay is important for electric emitters despite relatively low material losses, while magnetic dipole decay rate enhancement is completely dominated by radiative yield.
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| 9. |
- Odebo Länk, Nils, 1988, et al.
(författare)
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Large-Scale Silicon Nanophotonic Metasurfaces with Polarization Independent Near-Perfect Absorption
- 2017
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 17:5, s. 3054-3060
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Tidskriftsartikel (refereegranskat)abstract
- Optically thin perfect light absorbers could find many uses in science and technology. However, most physical realizations of perfect absorption for the optical range rely on plasmonic excitations in nanostructured metallic metasurfaces, for which the absorbed light energy is quickly lost as heat due to rapid plasmon decay. Here we show that a silicon metasurface excited in a total internal reflection configuration can absorb at least 97% of incident near-infrared light due to interferences between coherent electric and magnetic dipole scattering from the silicon nanopillars that build up the metasurface and the reflected wave from the supporting glass substrate. This "near-perfect" absorption phenomenon loads more than 50 times more light energy into the semiconductor than what would be the case for a uniform silicon sheet of equal surface density, irrespective of incident polarization. We envisage that the concept could be used for the development of novel light harvesting and optical sensor devices.
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| 10. |
- Odebo Länk, Nils, 1988
(författare)
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Nanostructured electromagnetic metasurfaces at optical frequencies
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electromagnetic metasurfaces are broadly defined as optically thin layersthat are structured on the subwavelength scale. In general, metasurfacesthus consist of nanoparticles, or other kinds of ”meta-atom”, arranged insome pattern where both the individual particle sizes and the inter-particledistances are much smaller than the wavelength. With advances in nanofabrication,it has become feasible to precisely engineer metasurface constituentelements to fulfil certain functions.This thesis, focuses on the properties of metasurfaces assembled by colloidallithography. In contrast to most metasurfaces studied in the literature,samples produced by colloidal lithography lack long-range periodicity.Two different approaches to metasurface design are investigated. In appendedpapers I and II, the individual elements are plasmonic gold nanoparticleswhile appended paper III deals with the geometric resonances supportedin silicon nanoparticles. The investigated systems are able to convertpropagating electromagnetic fields into localized ones and vice-versa.Hence, we can measure information about local properties in the far-field.In paper I, the individual particles were progressively tilted with respectto the substrate normal, resulting in an overall directional response.This directionality was manifested in enhanced fluorescence emission inparticular directions.In paper II, the ability of anisotropic individual particles to alter thepolarization of the incoming light beam was utilized to develop a sensingscheme based on the detection of rotation of polarization. The change inrotation and ellipticity of the light was shown to be sensitive to the localrefractive index around the particles. Refractometric biosensing was performedby tracking these changes in real time.The interaction between the incident light and geometric electric andmagnetic resonances was studied in paper III. At certain illumination conditions,it was shown that the interference between interface reflection andthe coherent scattering from the electric and magnetic dipole resonancesgave rise to almost complete light absorption independent of polarization.The ability to design metasurfaces with specific properties is of importancefor future applications. The results presented in this thesis contributeto the understanding of the properties of the individual particles that composea metasurface and how structuring of these particles affects its overallproperties.
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