Nanoscale‐Confined Terahertz Polaritons in a van der Waals Crystal

Abstract

Electromagnetic field confinement is crucial for nanophotonic technologies, since it allows for enhancing light–matter interactions, thus enabling light manipulation in deep sub‐wavelength scales. In the terahertz (THz) spectral range, radiation confinement is conventionally achieved with specially designed metallic structures—such as antennas or nanoslits—with large footprints due to the rather long wavelengths of THz radiation. In this context, phonon polaritons—light coupled to lattice vibrations—in van der Waals (vdW) crystals have emerged as a promising solution for controlling light beyond the diffraction limit, as they feature extreme field confinements and low optical losses. However, experimental demonstration of nanoscale‐confined phonon polaritons at THz frequencies has so far remained elusive. Here, it is provided by employing scattering‐type scanning near‐field optical microscopy combined with a free‐electron laser to reveal a range of low‐loss polaritonic excitations at frequencies from 8 to 12 THz in the vdW semiconductor α‐MoO3. In this study, THz polaritons are visualized with: i) in‐plane hyperbolic dispersion, ii) extreme nanoscale field confinement (below λo ⁄75), and iii) long polariton lifetimes, with a lower limit of >2 ps.

Van der Waals Phonon Polariton Microstructures for Configurable Infrared Electromagnetic Field Localizations

“…α ‐MoO 3 supports anisotropic PhPs throughout the mid‐infrared (545 to 1010 cm −1 ) and THz (267 to 400 cm −1 ) spectral range, [ 8,9,15,16 ] over much of which these modes are hyperbolic in nature. These hyperbolic spectral bands originate from the opposite signed permittivities along different crystalline directions: Re( ε i )·Re( ε j ) < 0, with i and j representing the [100] ( x ‐axis), [001] ( y ‐axis), and [010] ( z ‐axis) crystalline axes (Figure S1a,b, the Supporting Information).…”

Terahertz Nano-Imaging with s-SNOM

“…Another example of THz s-SNOM measurements of surface waves was recently reported in Ref. [76]. Here, the topic was the observation of phonon polaritons in the van der Waals semiconductor α-MoO 3 , and the proof of their hyperbolic (selfconfined) propagation in spectral regions where the dielectric permittivity is positive in one spatial direction and negative in another.…”

Microwaves Are Everywhere: “SMM: Nano-Microwaves”

“…The great advantage of the interferometric detection schemes, besides the ability to record magnitude and phase, was the possibility to perform measurements over an extremely wide frequency range, and hence to accomplish broad band spectroscopy on samples, while detecting extremely localized material characteristics (ε s ) that had been inaccessible beforehand [71]. Succeeding work focused on interpretation and extraction of specific material properties ( [72], for example), as well as many improvements to the measurement system, including the addition of new techniques for signal illumination and detection [73]- [83]. It should be noted that Keilmann, Hillenbrand and other colleagues, all have prodigious publication portfolios involving scanning nearfield techniques, but the majority of the implementations and applications fall into the infrared and optical regimes, and are not referenced here.…”

“…(right). There are many other recent source and detector implementations, both narrow band and broadband, for the Neaspec IR and THz near-field systems, as described in [74]- [83].…”