Controlling the Infrared Dielectric Function through Atomic-Scale Heterostructures

Abstract

Surface phonon polaritons (SPhPs), the surface-bound electromagnetic modes of a polar material resulting from the coupling of light with optic phonons, offer immense technological opportunities for nanophotonics in the infrared (IR) spectral region. However, once a particular material is chosen, the SPhP characteristics are fixed by the spectral positions of the optic phonon frequencies. Here, we provide a demonstration of how the frequency of these optic phonons can be altered by employing atomic-scale superlattices (SLs) of polar semiconductors using AlN/GaN SLs as an example. Using second harmonic generation (SHG) spectroscopy, we show that the optic phonon frequencies of the SLs exhibit a strong dependence on the layer thicknesses of the constituent materials. Furthermore, new vibrational modes emerge that are confined to the layers, while others are centered at the AlN/GaN interfaces. As the IR dielectric function is governed by the optic phonon behavior in polar materials, controlling the optic phonons provides a means to induce and potentially design a dielectric function distinct from the constituent materials and from the effective-medium approximation of the SL. We show that atomic-scale AlN/GaN SLs instead have multiple Reststrahlen bands featuring spectral regions that exhibit either normal or extreme hyperbolic dispersion with both positive and negative permittivities dispersing rapidly with frequency. Apart from the ability to engineer the SPhP properties, SL structures may also lead to multifunctional devices that combine the mechanical, electrical, thermal, or optoelectronic functionality of the constituent layers. We propose that this effort is another step toward realizing user-defined, actively tunable IR optics and sources.

Optical Nonlocality in Polar Dielectrics

“…By comparison to experimental data, we demonstrate our approaches' validity for a variety of systems down to the nanometre scale. Our theory allows us to explain recently reported discrepancies between predictions of the dielectric theory and the experimental data [31], providing the first experimental evidence of nonlocal effects in polar dielectrics and demonstrating its relevance as a design tool for SPhP devices. For the sake of clarity, most of the technical details of the calculations have been placed in the Appendixes.…”

“…We then study epsilon-near-zero (ENZ) resonances in aluminum nitride thin films and build on these results to model crystal hybrids composed of aluminum nitride (AlN)/gallium nitride (GaN) atomic-scale superlattices. Our method provides an agile, numerically lightweight toolset to simulate and understand the nonlocal response of nanoscale polar structures, without the drawbacks of numerically taxing molecular dynamics [29] or density functional calculations [30,31]. By comparison to experimental data, we demonstrate our approaches' validity for a variety of systems down to the nanometre scale.…”

“…Recently, the fabrication and optical characterization of atomic-scale superlattices or crystal hybrids, composed of alternating nanoscopic layers of GaN and AlN grown on a semi-insulating SiC substrate, were reported [31]. Such a work demonstrated the failure of a standard local response theory to describe the observed optical properties of the superlattice, requiring instead density functional perturbation theory to correctly reproduce its features [30].…”

“…Using a nonlocal extension of the scattering matrix approach, we simulated the two heterostructures, labeled A and B, characterized in Ref. [31]. Heterostructure A consists of 50 alternating layers of GaNð2.2 nmÞ=AlNð2.2 nmÞ on a 50 nm AlN buffer layer.…”

Surface Phonon Polariton Resonance Imaging Using Long-Wave Infrared-Visible Sum-Frequency Generation Microscopy

“…For heterogeneous materials, such as semiconductor superlattices, this approach could also serve as a contrast mechanism between the individual constituents. 2,45 Finally, we expect LWIR-VIS SFG microscopy introduced here to also be implemented for studies for biological and chemical systems, for instance for probing the spatial heterogeneity of transient low-frequency modes of transient species at electro-chemical interfaces. [63][64][65] In summary, we have demonstrated scanning-probe LWIR-VIS SFG microscopy of surface phonon polariton resonances in subdiffractional nanostructures.…”