The initial study takes into consideration an array of silver nanopillars inside a thin film overlay with thickness
300nm and refractive index
2 on a bulk glass substrate. The pillars are arranged in a square lattice with spatial period
a and they are characterized by radius
r and height
d. The presence of pillars results in periodic modulation of the overlay thickness, which leads to the formation of guided Bloch modes with characteristic frequency
and in-plane wave vector
. The modulation strength depends on the ratio of pillar height and overlay thickness
which we set to 0.3. In addition to the Bloch modes traveling in the overlay (we refer to these modes as photonic), the nanopillars support surface lattice resonances (SLRs), resulting from far-field coupling of the individual LSPRs [
17] that are closely confined at the pillar surface. In case of significant overlap between the photonic and plasmonic modes, provided symmetry and polarization match, the modes will experience interference, which manifests itself by avoided crossing of the two bands in band dispersion diagram
[
18]. Assuming infinite dimensions of the structure, we model the band dispersion and mode profiles with FDTD simulation and the results are presented in the
Figure 1. At this stage, for simplicity and clear observation of the mode behavior, the overlay is considered lossless and the pillars are isolated from each other (
Figure 1a). The geometric parameters
400 nm,
90 nm,
90 nm and
300 nm were chosen to observe the crossing near 300 THz, with a large value of
close to the corner of the Brillouin zone
M. As shown in
Figure 1b, away from the crossing the modes have a predominant photonic or plasmonic characteristics, however near the intersection they become coupled, giving rise to hybrid modes, with significant contribution of both plasmonic and photonic parts. Due to coupling, the energy stored in such a mode is not bound to the photonic or plasmonic part, but can be transferred between them in time, as has been previously shown on the basis of coupled mode theory for plasmonic resonators integrated in waveguide structures [
19,
20]. This fact is particularly interesting in the context of designing plasmonic Schottky junctions for the purpose of light detection, where the dominant part of charge carriers is generated through absorption in a thin depletion region. SLR features will be responsible for enhancing the field intensity at this region, while it can be sustained for a longer time, because of the interaction with coupled photonic mode.
For practical implementation of the above concept in designing the photodetector junction, the pillars should be interconnected with a thin silver patch, acting as electrical contact and the overlay must be made of semiconducting material. The
2 refractive index may correspond to various kinds of perovskite materials, which can have different optoelectronic properties, including absorption, emission and carrier mobility. This study focuses mostly on optical behavior of the structures, and so the overlay material is simply characterized by
n and
(see
Section 4 for a more comprehensive discussion on specific materials and applications). The top electrode of the junction can be made of thin metal, transparent conductive oxides, polymers, or composite materials, such as transparent electrodes based on silver nanowires. Also, it can uniformly cover the overlay upper surface or can be positioned on the sides. In the presented FDTD simulations the top electrode is not included, as it does not play a significant role to the demonstrated effects.
Figure 2 shows the schematic design and the mode dispersion diagram of the proposed plasmonic-photonic heterostructure. In the guided region (below the light line), at the frequencies 280-300 THz, one can identify two modes which exhibit hybrid plasmonic-photonic character and they dominate the band structure in the specified frequency range. The marked zone on the band diagram is analogical to the crossing sketched in
Figure 1, however the presence of silver patch makes the band configuration more complex due to the formation of additional SPP modes. The demonstrated characteristics of the heterostructure implicate that 280-300 THz radiation with a high enough
will most favorably couple to the hybrid modes and this can be exploited to manage the optoelectronic properties of the junction.