Sub-diffractional cavity modes of terahertz hyperbolic phonon polaritons in tin oxide

Abstract

Hyperbolic phonon polaritons have recently attracted considerable attention in nanophotonics mostly due to their intrinsic strong electromagnetic field confinement, ultraslow polariton group velocities, and long lifetimes. Here we introduce tin oxide (SnO2) nanobelts as a photonic platform for the transport of surface and volume phonon polaritons in the mid- to far-infrared frequency range. This report brings a comprehensive description of the polaritonic properties of SnO2 as a nanometer-sized dielectric and also as an engineered material in the form of a waveguide. By combining accelerator-based IR-THz sources (synchrotron and free-electron laser) with s-SNOM, we employed nanoscale far-infrared hyper-spectral-imaging to uncover a Fabry–Perot cavity mechanism in SnO2 nanobelts via direct detection of phonon-polariton standing waves. Our experimental findings are accurately supported by notable convergence between theory and numerical simulations. Thus, the SnO2 is confirmed as a natural hyperbolic material with unique photonic properties essential for future applications involving subdiffractional light traffic and detection in the far-infrared range.

Far-Infrared Near-Field Optical Imaging and Kelvin Probe Force Microscopy of Laser-Crystallized and -Amorphized Phase Change Material Ge3Sb2Te6

“…In particular, GST326 was shown to switch reversibly between amorphous and crystalline states when investigated with mid-infrared SNOM 21 and the related compound GST124 was shown to possess far-infrared SNOM contrast between the crystalline and amorphous states. 58 In this work, SNOM is used with illumination from a CO 2 laser and the infrared-to-THz free-electron laser (FEL) FELBE in Dresden, Germany, 57,59,63,71,80 to investigate the nanoscale optical properties and free charge carriers of GST326 in the mid-and far-infrared range, i.e., at 885 cm −1 (11.3 μm, 26.5 THz) and 168 cm −1 (59.5 μm, 5.04 THz), respectively, complemented by simultaneous KPFM measurements (cf. the Supporting Information for experimental details).…”

“…Scattering-type scanning near-field optical microscopy (SNOM, Figure c) fulfills these criteria by using strongly confined optical near-fields at the apex of a sharp metallic tip to probe nanoscale areas of the sample. , These near-fields extend beyond the immediate surface, typically penetrating up to 100 nm into the sample, − and thus provide additional subsurface information. Optical frequencies in the infrared range can be used to investigate characteristic phonon resonances and free charge carriers, giving insight into local electronic and structural properties with nanoscale resolution. − For plasmons of low charge carrier density or phonons of heavy atoms, near-field contrast is typically found at low energies, requiring SNOM at far-infrared frequencies. ,,− The spectral region of 33–334 cm –1 (1–10 THz, 30–300 μm) is difficult to access experimentally and is therefore often referred to as the “THz gap” . Despite these challenges, SNOM at THz frequencies has recently become a vibrant field of research, providing fundamental insight into low-energy and high-momentum excitations, such as nonlocal effects in single-layer graphene, subwavelength metal structures, , and phonon polaritons. , Similar to SNOM, Kelvin probe force microscopy (KPFM) possesses the high resolution of several nanometers typical for atomic force microscopy (AFM) techniques. − However, KPFM measures the local surface potential instead of the optical properties.…”

Ultrabroadband Nanocavity of Hyperbolic Phonon–Polaritons in 1D-Like α-MoO3

“…In this work, we exploit the polaritonic response of α-MoO 3 nanobelts, with reduced dimensionality approaching “1D-like” (Figure c), using synchrotron infrared nanospectroscopy (SINS) − and scattering-scanning near-field optical microscopy (s-SNOM). As reported, such nanoscopy techniques (see details in Supporting Information) possess a high spatial resolution and momentum nanoprobe allowing for full characterization of HP 2 waves.…”

Broadband Metamaterial Solar Absorbers Based on the Earth First-Rate Temperature-Insensitivity Metals for High Temperature Energy Harvesting

“…It is found that the absorption strength of AP 2 in all the proposed MSA achieved the maximum value when the thickness reached S H = 50 nm. It can be explained by the fact that AP 2 is excited by the cavity modes ( [26,27])b e t w e e nt h e layers of upper grating and the tungsten lower plate. The insets in Figure 2 show the excitation of the cavity modes at λ = 1455 nm for tungsten, at λ = 1200 nm for rhenium, at λ = 1260 nm for tantalum, and at λ = 1245 nm for molybdenum.…”