Hyperbolic nanoparticles provide a versatile platform to widely tune light-matter interactions. Active nanophotonics can be realized by controlling the optical properties of materials with external magnetic fields. Here, we explore the influence of optical anisotropy on the magneto-optical response of hyperbolic nanoparticles across the visible and near infrared spectral range. By using a perturbative approach, we establish a model where the magneto-optical activity of the system is described in terms of the coupling of fundamental electric and magnetic dipole modes, which are induced by the hyperbolic dispersion, with a static magnetic field. Finally, an analytical model is established in the framework of Mie theory to describe the magneto-optical response and identify the contribution of electric and magnetic modes to the total spectrum.
In the framework of magneto-photonics, the optical properties of a material can controlled by an external magnetic field, providing active functionalities for applications, such as sensing and nonreciprocal optical isolation. For noble metals in particular, the inherently weak magnetooptical coupling of the bulk material can be greatly enhanced via excitation of localized surface plasmons (LSP) in nanostructured samples. Hyperbolic metamaterials therein provide the ideal platform to tune the plasmonic properties via careful design of the effective permittivity tensor. Here, we report on the magnetic circular dichroism of electric and magnetic dipole modes of a type II hyperbolic metasurface. Disk-shaped nanoparticles consist in stacks of alternating dielectric and metallic layers. Using an effective medium theory, we show that the optical properties of the system can be perfectly described by an anisotropic homogenized permittivity. Magnetic circular dichroism spectroscopy experiments are compared with plain gold disk samples and reveal a broadband magneto-optical response across the visible and near infrared spectral range. In particular, derivative-like spectral signatures at the resonances of the nanoparticles prove the induced dichroism for the two modes of the system. Results are interpreted in terms of magnetically induced spatial confinement/broadening of circular currents in the nanoparticles and are compared with a comprehensive numerical model based on the finite elements method using the real dimensions of the nanostructure. Spherical particles are employed as an analytical model system, allowing to generalize the contribution of electric and magnetic modes to the total magneto-optical response. More in detail, interaction cross sections are calculated as a weighted sum of the corresponding Mie coefficients. Utilizing a perturbative approach, we describe the magneto-optical effect in terms of linear changes in the cyclotron frequency of free charge carriers in the metal. By comparing our analytical model with full-wave numerical results, we can identify the contribution of electric and magnetic dipole modes to the spectrum and reproduce the spectral line shape we observe in the experiments for the hyperbolic nanoparticles.
Nanoplasmonic sensing is a very active and diverse field with a wide variety of applications in chemistry, biomolecular and materials science [1]. Optically resonant molecular systems often display what is called a strong coupling to the nanophotonic systems. This is primarily explored for the nanophotonics active control and in the studies of quantum optics [2]. At the same time, the strong coupling of the molecular resonances to the nanoplasmonic antennas has not been addressed to follow the light-induced molecular processes. Here we combine an exemplary molecular photo-switch, from the spiropyran photochromic family, with anisotropic nanoplasmonic antennas to earn the monitoring tool for the light-activated processes using molecular and nanoplasmonic resonances strong coupling regime. We follow the reversible photo-isomerization of the spiropyran photoswitch from the spiro form to the merocyanine form by tuning in the nanoplasmon antenna to the excitonic state of the merocyanine form (at 570 nm), prompting the formation of a hybrid excitonic-plasmonic state. Our anisotropic nanoantenna provides two polarization-dependent spectrally separated resonances in the visible region, allowing for separate monitoring of the plasmon-exciton strong coupling and the conventional enhanced optical near-field refractive index sensing. This system uncovers a new modality in polaritonic chemistry and optical label-free monitoring of the photo-activated processes and can find applications in photocatalysis, biosensing and in hybrid molecular-nanoantenna actively modulated systems.
[1] M. I. Stockman, Science 348, 287 (2015); A. Dmitriev (Ed.), Nanoplasmonic Sensors, Springer NY (2012).
[2] Yoshie, T. et al. Nature 432, 200 (2004) ; Kasprzak, J. et al. Nature Mater. 9, 304 (2010); Reinhard, A. et al. Nature Photon. 6, 93 (2012).
The enhancement of the electromagnetic field in the surroundings of nanoparticles via surface plasmon resonance offers
promising possibilities for biomedical applications. Here we report on the selective triggering of antibacterial activity
using a new type of silver nanoparticles coated with silica, Ag@silica, irradiated at their surface plasmon frequency. The
nanoparticles are able to bind readily to the surface of bacterial cells, although this does not affect bacterial growing
since the silica shell largely attenuates the intrinsic toxicity of silver. However, upon simultaneous exposure to light
corresponding to the absorption band of the nanoparticles, bacterial death is triggered selectively on the irradiated zone.
Because of the low power density used in the treatments, we discard thermal effects as the cause of cell killing. Instead,
we propose that the switched toxicity is due to the enhanced electromagnetic field in the proximity of the nanoparticles,
which either directly (through membrane perturbation) or indirectly (through induced photochemical reactions) is able to
cause cell death.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.