Hierarchically Doped Plasmonic Nanocrystal Metamaterials

Abstract

Assembling plasmonic nanocrystals in regular superlattices can produce effective optical properties not found in homogeneous materials. However, the range of these metamaterial properties is limited when a single nanocrystal composition is selected for the constituent meta-atoms. Here, we show how continuously varying doping at two length scales, the atomic and nanocrystal scales, enables tuning of both the frequency and bandwidth of the collective plasmon resonance in nanocrystal-based metasurfaces, while these features are inextricably linked in single-component superlattices. Varying the mixing ratio of indium tin oxide nanocrystals with different dopant concentrations, we use large-scale simulations to predict the emergence of a broad infrared spectral region with near-zero permittivity. Experimentally, tunable reflectance and absorption bands are observed, owing to inand out-of-plane collective resonances. These spectral features and the predicted strong near-field enhancement establish this multiscale doping strategy as a powerful new approach to designing metamaterials for optical applications.

Wavelength Tunable Infrared Perfect Absorption in Plasmonic Nanocrystal Monolayers

“…To integrate the NCs with the photonic cavity, we placed a water droplet on top of the Ge spacer, then dispensed a droplet containing oleate-capped ITO NCs (dispersed in 2:1 hexane:toluene) on top. As we previously reported, , slow evaporation of the solvent produces a compact monolayer of ITO NCs (Figure a). Although Akselrod et al showed that isolated NCs on the top of a spacer-PEC geometry can exhibit perfect absorption behavior, close-packed layers of NCs increase their extinction , (and thus, the imaginary part of permittivity) due to the collective plasmon coupling behavior.…”

“…Close-packed monolayer films of ITO NCs exhibit a resonant peak in their optical extinction spectra and associated effective complex permittivity similar to other resonantly absorbing materials, as shown schematically in Figure a. As shown previously, ITO NC monolayer films have a distinct extinction peak [Figure a (ii)], which is associated with the CPR of the NC array (at λ CPR ). Due to the dense packing and uniformity of the monolayer film, its optical properties are well represented by an effective complex permittivity with a finite width, resonant absorption feature, centered at the peak wavelength of the CPR of the NC array, as shown in Figure a (iii).…”

“…Since we use ultrathin NC films, the critical conductivity value requires a large ε″, which is reached at or near λ CPR , as shown in Figure c where the critical conductivity value, Re (σ) = 1/ Z 0 , is shown as a dashed line. Equation can immediately predict whether a given ITO NC monolayer can exhibit PA in this cavity architecture; for example, for 3.5 μm light and 31 nm NC diameter, the PA condition requires ε″ ∼ 17, which is easily achievable with previously demonstrated ε eff at λ CPR in ITO NC monolayers …”

“…The transparency of metal oxide NCs to visible light also ensures that their presence does not compromise any visible light response, making them suitable for thermal absorbers in sunlight management . We recently showed that the tin-doped indium oxide (ITO) NCs can form a compact monolayer where NCs couple strongly to each other, giving rise to a collective plasmon response (CPR) . The NC monolayer was shown to effectively act as a mid-IR metasurface with frequency-tunable reflection, absorption, and epsilon near zero (ENZ) properties .…”

“…We recently showed that the tin-doped indium oxide (ITO) NCs can form a compact monolayer where NCs couple strongly to each other, giving rise to a collective plasmon response (CPR) . The NC monolayer was shown to effectively act as a mid-IR metasurface with frequency-tunable reflection, absorption, and epsilon near zero (ENZ) properties . In that study, the tunable resonant frequency of the ITO NCs made them versatile meta-atoms, where the effective properties of the optical layer could be varied by incorporating ITO NCs with two different Sn concentrations (and therefore different LSPR frequencies) within a single monolayer.…”