Understanding quantum emitters in plasmonic nanocavities with conformal transformation: Purcell enhancement and forces.

Nanogaps supporting cavity plasmonic modes with unprecedented small mode volume are attractive platforms for tailoring the properties of light-matter interactions at the nanoscale and revealing new physics. Hitherto, there is a concerning lack of analytical solutions to divide the complex interactions into their different underlying mechanisms to gain a better understanding that can foster enhanced designs. Bowtie apertures are viewed as an effective and appealing nanocavity and are studied here within the analytical frame of conformal transformation. We show how the non-radiative Purcell enhancement of a quantum emitter within the bowtie nanocavity depends strongly not only on the geometry of the nanocavity, but also on the position and orientation of the emitter. For a 20 nm diameter (∅ 20 nm) bowtie nanocavity, we report a change of up to two orders of magnitude in the maximum non-radiative Purcell enhancement and a shift in its peak wavelength from green to infra-red. The changes are tracked down to the overlap between the emitter field and the gap plasmon mode field distribution. This analysis also enables us to understand the self-induced trapping potential of a colloidal quantum dot inside the nanocavity. Since transformations can be cascaded, the technique introduced in this work can also be applied to a wide range of nanocavities found in the literature.

Localized photonic jets from flat, three-dimensional dielectric cuboids in the reflection mode.

A photonic jet (a terajet at terahertz frequencies) commonly denotes a specific, spatially localized region in the near field on the front side of a dielectric particle with a diameter comparable with the wavelength illuminated by a plane wave on its back side (i.e., the jet emerges from the shadow surface of a dielectric particle). In this Letter, the formation of a photonic jet is demonstrated using the recently proposed three-dimensional (3D) dielectric cuboids working in the "reflection" mode when the specific, spatially localized region is localized in the direction of the incident wavefront. The results of the simulations based on the Finite Integration Technique are discussed. All dimensions are given in wavelength units so that all results can be scaled to any frequency of interest, including optical frequencies, thus simplifying the fabrication process compared with spherical dielectrics. The results presented here may be of interest for novel applications, including microscopy techniques and sensors.

Exploiting the dispersion of the double-negative-index fishnet metamaterial to create a broadband low-profile metallic lens.

Metamaterial lenses with close values of permittivity and permeability usually display low reflection losses at the expense of narrow single frequency operation. Here, a broadband low-profile lens is designed by exploiting the dispersion of a fishnet metamaterial together with the zoning technique. The lens operates in a broadband regime from 54 GHz to 58 GHz, representing a fractional bandwidth ~7%, and outperforms Silicon lenses between 54 and 55.5 GHz. This broadband operation is demonstrated by a systematic analysis comprising Huygens-Fresnel analytical method, full-wave numerical simulations and experimental measurements at millimeter waves. For demonstrative purposes, a detailed study of the lens operation at two frequencies is done for the most important lens parameters (focal length, depth of focus, resolution, radiation diagram). Experimental results demonstrate diffraction-limited ~0.5λ transverse resolution, in agreement with analytical and numerical calculations. In a lens antenna configuration, a directivity as high as 16.6 dBi is achieved. The different focal lengths implemented into a single lens could be potentially used for realizing the front end of a non-mechanical zoom millimeter-wave imaging system.

Multifrequency focusing and wide angular scanning of terajets.

In the past, it has been demonstrated that it is possible to produce terajets with high resolution at its focus using 3D dielectric cuboids under plane-wave illumination. Here, a systematic study of the harmonic and angular response of terajets using cuboids is performed. Mutifrequency focusing is demonstrated at the fundamental frequency and two higher frequency harmonics showing an intensity enhancement of ∼10, ∼18, and ∼14 for each case. This capability to use 3D dielectric cuboids to produce terajets at the fundamental frequency and first harmonic is experimentally evaluated at sub-THz frequencies, with good agreement with numerical results. Moreover, a robust angular response is demonstrated numerically and experimentally showing that the intensity at the focal position is maintained in a wide angular range (from 0° to 45°), demonstrating the capability to work as a wide scanning terajet-focusing lens. The results here presented may be scaled at different frequency bands such as optical frequencies and may be used in microscopy techniques and sensors.