Beyond Noble Metals: High Q-Factor Aluminum Nanoplasmonics

Abstract

Aluminum, with its distinctively favorable dielectric characteristics down to deep ultraviolet (UV) regime, has recently emerged as a broad-band and low-cost alternative to noble metals. However, low Q-factor resonances (Q ∼ 2−4), offered by Al nanostructures, pose a fundamental bottleneck for many practical applications. Here, we show that it is possible to realize Al-nanoantenna with remarkably large extinction cross sections and strong resonance characteristics surpassing those of their noble metal counterparts. By quenching radiation damping through far-field coherent dipolar interactions, we experimentally demonstrate exceptionally narrow line width (∼15 nm) and high Q-factor (∼27) dipolar plasmonic resonances in the blue-violet region of the optical spectrum (∼3 eV) beyond the practical operational limits of traditional plasmonic metals. To realize high Q-factor Al resonators, we introduce a novel space mapping algorithm enabling inverse design of Al nanoantenna arrays at arbitrary sub/superstrate material interfaces with diminished radiative losses. We show that radiatively coupled Al nanoantenna arrays offer remarkably high-Q factor (27 ≤ Q ≤ 53) resonances over the entire visible spectrum and readily outperform similarly optimized silver (Ag) nanoantenna arrays in green-blue-violet wavelengths (≤550 nm) and near UV regime. This report shows that it is possible to realize high Q-factor aluminum resonators by suppressing radiative losses and that Al-based plasmonics holds enormous potential as a viable and low-cost alternative to noble metals. Our inverse-design technique, on the other hand, provides a general and efficient approach in engineering of high Q-factor resonator arrays, independently from the metals and sub/superstrates used.

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“…Traditional trial-and-error-based design approaches often require computationally expensive and timeconsuming three-dimensional rigorous electromagnetic simulations of tens to hundreds of candidate structures [121,122]. In contrast, we showed that our space mapping-based inverse design technique enables ultrafast and accurate retrieval of the fittest sets of structural PNA parameters, yielding the desired optical characteristics with a minimal number of high-fidelity and computationally expensive simulations [92]. Our inverse design approach establishes a one-to-one mapping scheme [123,124] enabling effective approximation of time-consuming finite difference time domain (FDTD) simulations using an analytical coupled dipole approximation (CDA) model that is not resource hungry.…”

“…The latest advances in nanoscale fabrication and nanophotonic characterization techniques, as well as powerful electromagnetic simulation methods, have enabled scientists and engineers to understand and harness plasmonic excitations in metallic PNAs with high efficiencies [14,31,75,76]. By employing these well-established toolsets, sophisticated devices can be readily designed with desired functionality through rigorous parametric optimization techniques [92,93]. For example, a well-engineered array of PNAs can support long-lived and high quality (Q)-factor optical resonances and high field enhancements over a broad spectral range down to deep UV regime.…”

“…To date, gold (Au), silver (Ag), and aluminum (Al) remain the most commonly used metals for plasmonics because of their high free-electron density and low intrinsic losses [76]. However, to fully exploit strong light-matter interactions for practical purposes, considerable efforts have yet to be made to control plasmon damping processes in realization of high-quality factor (Q-factor) plasmon resonances [22,92,93,101,102]. Metallic resonators, such as PNAs, yield broad linewidth resonances due to rapid dephasing of coherent oscillations within 2-10 fs [101,103].…”

“…Hence, quenching of radiative damping is particularly effective in realization of pronounced line width narrowing within the green-blue-violet visible and near-UV regime. To minimize radiative losses, we introduced an elegant and universal approach enabling rapid and efficient inverse engineering of remarkably narrow linewidth and high Q-factor PNAs [92]. Our powerful and versatile inverse-design technique enables optimization of radiatively coupled PNA arrays fabricated on refractive index mismatched super-/substrates ( Figure 3A).…”

“…Real-world adaptation of nanoscale technologies critically depends on the development of high-throughput and high-yield nanofabrication techniques, offering structural uniformity at nanoscale over large areas. During the last few decades, we have witnessed remarkable advancements in high-throughput nanofabrication techniques based on topdown electron-beam [126], ion-beam [127], nanostencil [26,128], nanosphere [129,130] Recently, we have demonstrated an effective strategy in tackling these challenges by employing a high-throughput wafer-scale lift-off free fabrication approach based on UV interference lithography and reactive ion etching [92] ( Figure 4A). We demonstrated low-cost large-area nanofabrication of Al nanodisk arrays fabricated over 8-inch fused silica wafers with excellent structural uniformity with sub-30 nm feature sizes ( Figure 4B-C).…”