The impact of nonlocal response on metallo-dielectric multilayers and optical patch antennas

We analyze the impact of nonlocality on the waveguide modes of metallo-dielectric multilayers and optical patch antennas, the latter formed from metal strips closely spaced above a metallic plane. We model both the nonlocal effects associated with the conduction electrons of the metal, as well as the previously overlooked response of bound electrons. We show that the fundamental mode of a metal-dielectric-metal waveguide, sometimes called the gap-plasmon, is very sensitive to nonlocality when the insulating, dielectric layers are thinner than 5 nm. We suggest that optical patch antennas, which can easily be fabricated with controlled dielectric spacer layers and can be interrogated using far-field scattering, can enable the measurement of nonlocality in metals with good accuracy

Origin of second-harmonic generation enhancement in optical split-ring resonators

We present a study of the second-order nonlinear optical properties of metal-based metamaterials. A hydrodynamic model for electronic response is used, in which nonlinear surface contributions are expressed in terms of the bulk polarization. The model is in good agreement with published experimental results, and clarifies the mechanisms contributing to the nonlinear response. In particular, we show that the reported enhancement of second-harmonic in split-ring resonator based media is driven by the electric rather than the magnetic properties of the structure

Impact of surface charge depletion on the free electron nonlinear response of heavily doped semiconductors

We propose surface modulation of the equilibrium charge density as a technique to control and enhance, via an external static potential, the free electron nonlinear response of heavily doped semiconductors. Within a hydrodynamic perturbative approach, we predict a two order of magnitude boost of free electron third-harmonic generation

Third-harmonic generation in the presence of classical nonlocal effects in gap-plasmon nanostructures

Classical nonlocality in conducting nanostructures has been shown to dramatically alter the linear optical response by placing a fundamental limit on the maximum field enhancement that can be achieved. This limit directly extends to all nonlinear processes, which depend on field amplitudes. A numerical study of third-harmonic generation in metal film-coupled nanowires reveals that for subnanometer vacuum gaps, the nonlocality may boost the effective nonlinearity by 5 orders of magnitude as the field penetrates deeper inside the metal than that predicted assuming a purely local electronic response. We also study the impact of a nonlinear dielectric placed in the gap region. In this case the effect of nonlocality could be masked by the third-harmonic signal generated by the spacer. By etching the dielectric underneath the nanowire, however, it is possible to muffle such contributions. Calculations are performed for both silver and gold nanowire

Free electron nonlinearities in heavily doped semiconductors plasmonics

Heavily doped semiconductors have emerged as tunable low-loss plasmonic materials at mid-infrared frequencies. In this article we investigate nonlinear optical phenomena associated with high concentration of free electrons. We use a hydrodynamic description to study free electron dynamics in heavily doped semiconductors up to third-order terms, which are usually negligible for noble metals. We find that cascaded third-harmonic generation due to second-harmonic signals can be as strong as direct third-harmonic generation contributions even when the second-harmonic generation efficiency is zero. Moreover, we show that when coupled with plasmonic enhancement free electron nonlinearities could be up to two orders of magnitude larger than conventional semiconductor nonlinearities. Our study might open a new route for nonlinear optical integrated devices at mid-infrared frequencies

Optical properties of plasmonic core-shell nanomatryoshkas: a quantum hydrodynamic analysis.

Plasmonic response of the metallic structure characterized by sub-nanometer dielectric gaps can be strongly affected by nonlocal or quantum effects. In this paper, we investigate these effects in spherical Na and Au nanomatryoshka structures with sub-nanometer core-shell separation. We use the state-of-the-art quantum hydrodynamic theory (QHT) to study both near-field and far-field optical properties of these systems: results are compared with the classical local response approximation (LRA), Thomas-Fermi hydrodynamic theory (TF-HT), and the reference time-dependent density functional theory (TD-DFT). We find that the results obtained using the QHT method are in a very good agreement with TD-DFT calculations, whereas other LRA and TF-HT significantly overestimate the field-enhancements. Thus, the QHT approach efficiently and accurately describes microscopic details of multiscale plasmonic systems whose sizes are computationally out-of-reach for a TD-DFT approach; here, we report results for Na and Au nanomatryoshka with a diameter of 60 nm