We show that meta-antennas made of a composite material displaying type II hyperbolic dispersion enable precise and controlled spectral separation of absorption and scattering processes in the visible/near-infrared frequency range. The experimental evidence is supported by a comprehensive theoretical study. We demonstrate that the physical mechanism responsible for the aforementioned effect lies in the different natures of the plasmonic modes excited within the hyperbolic meta-antennas. We prove that it is possible to have a pure scattering channel if an electric dipolar mode is induced, while a pure absorption process can be obtained if a magnetic dipole is excited. Also, by varying the geometry of the system, the relative weight of scattering and absorption can be tuned, thus enabling an arbitrary control of the decay channels. Importantly, both modes can be efficiently excited by direct coupling with the far-field radiation, even when the radiative channel (scattering) is almost totally suppressed, hence making the proposed architecture suitable for practical applications. E 0 k n = 1.5 n = 1.5 J J J J n= 1.5 = 860 nm = 1120 nm = 1100 nm = 1300 nm = 860 nm = 1120 nm = 1100 nm = 1300 nm APPENDIX A: NUMERICAL SIMULATIONS AND THEORETICAL ANALYSIS 1. Optimization of the hyperbolic meta-antennas dimensions and composition
A colorimetric
immunosensor based on
local surface plasmon resonance by gold nanoparticles is presented,
and its application for the detection of human immunoglobulin G (IgG)
is demonstrated. The color change of the colloidal solution is produced
by nanoparticle aggregation, a process that can be tuned by the presence
of the analyte once the nanoparticles are functionalized. In comparison
to common functionalization techniques, the procedure described here
is simpler, low-cost, and effective in binding antibodies upright
on the gold surface. The dose–response curve is similar to
that resulting in typical immunoassay platforms and is satisfactorily
described by the proposed theoretical model. Human IgG at concentration
levels of few hundreds of nanograms per milliliter can be detected
by eyes within a few minutes, thereby making the colorimetric immunosensor
proposed here a powerful tool in several areas, with urine test in
medical diagnostics being the most immediate.
Harmonic
generation mechanisms are of great interest in nanoscience
and nanotechnology, since they allow generating visible light by using
near-infrared radiation, which is particularly suitable for its countless
applications in bionanophotonics and optoelectronics. In this context,
multilayer metal–dielectric nanocavities are widely used for
light confinement and waveguiding at the nanoscale. They exhibit intense
and localized resonances that can be conveniently tuned in the near-infrared
and are therefore ideal for enhancing nonlinear effects in this spectral
range. In this work, we experimentally investigate the nonlinear emission
properties of multilayer metal–dielectric nanocavities. By
engineering their absorption efficiency and exploiting their intrinsic
interface-induced symmetry breaking, we achieve an almost 2 orders
of magnitude higher second-harmonic generation efficiency compared
to gold nanostructures featuring the same geometry and optical resonant
behavior. In particular, while both the third-order nonlinear susceptibility
and conversion efficiency are comparable with those of the Au nanoresonators,
we estimate a second-order nonlinear susceptibility of the order of
1 pm/V, which is comparable with that of typical nonlinear crystals.
We envision that our system, which combines the advantages of both
plasmonic and dielectric materials, might enable the realization of
composite and multifunctional nanosystems for the efficient manipulation
of nonlinear optical processes at the nanoscale.
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