Poking at phonon-polaritons at the nanoscale

Recently, studies involving hyperbolic phonon polaritons (HPhPs) have attracted significant interest among researchers. The inherent crystal anisotropy of hexagonal boron nitride (hBN), for example, enables this material to support HPhPs within distinct spectral bands delimited by the high and low frequencies of its optic phonons. The interest in hBN can be attributed to these exciting optical properties, combined with very low optical losses and with its compatibility and ease of coupling with graphene and other van der Waals materials.

In general, polaritons are quasiparticles comprised of a photon of light coupled with a coherently oscillating charge, due to electrons in a metal or due to ions on a polar lattice (as in hBN). For a given wavelength of light, polaritonic media typically offer the ability to compress the wavelength down to nanoscale dimensions, resulting in enhanced optical fields confined to their surface. In addition, hyperbolic phonon polaritons have several interesting characteristics. For example, hyperbolic materials can support an arbitrary number of modes that propagate inside the material volume at defined angles. For nanostructures, this means that instead of a continuo of modes, only a large but discrete set of mode can exist, which are defined by three geometry-specific quantum numbers. Prior theoretical work predicted a broad array of these modes, for example, the spheroid nanoparticles they discussed are defined by azimuthal angular momentum (m), orbital angular momentum (l) and radial index (n).

Previously, one of the primary researchers on this highlighted article showed that frustum (truncated nanocones) shaped nanostructures can support 3D confined HPhP modes in the mid-infrared. However, only a few low-order HPhPs resonances (m=1, n=0, l=1,2,3…) were detected experimentally using far-field reflection and s-SNOM techniques. It is believed that the optical selection rules dictate that the previously unobserved higher order modes are only very weakly excited. However, although these higher-order resonances have been elusive, they provide a means to control HPhPs’ orbital angular momentum, i.e. yielding the ability of effectively tailoring their response to various applications. Therefore, there is an urgent need to develop methods for detecting these higher-order modes, including nonradiative HPhPs.

In a recent research article published in the journal Nano Letters, an international research collaboration led by Professor Joshua Caldwell at Vanderbilt University and Dr. Andrea Centrone at National Institute of Standards and Technology (NIST) implemented the near-field photothermal induced resonance (PTIR) technique for detecting the never before observed nonradiative HPhPs modes in hBN nanostructures. Their measurements identified up to four different series (‘limbs’) of HPhP modes with varying quantum numbers, demonstrating that these previously predicted modes do indeed exist. This development offers the opportunity to explore how the near-field profiles and resonance energies could be tailored for various applications, such as enhanced emitters or quantum information.

The PTIR technique leverage an atomic force microscopy (AFM) probe to transduce the sample thermal expansion that follow the absorption of a light pulse in the sample. The gold coated AFM probe, in contact with the sample, provides the ability couple light from the far-field to the deeply sub-diffractional polaritonic modes. Essentially, in PTIR the AFM probes functions as a near-filed detector and therefore overcoming the challenges posed by the far-field detection and enabling the detection of weakly absorbing modes. The authors successfully observed higher order nonradiative modes that were initially predicted only theoretically. For instance, five branches of four different limbs of the hyperbolic dispersion were detected.

This study is the first to use PTIR to observe nonradiative HPhPs modes in hBN. According to the authors, the rich PTIR spectra were directly compared to the calculations of the spheroidal nanoparticles as a function of aspect ratio (ratio of the nanostructure height over diameter), an approach that was successfully implemented in prior works with such nanostructures using far-field reflection and s-SNOM techniques. This comparison provides the basis for defining and assigning the quantum numbers of each limb and branch. Additionally, PTIR maps were also measured providing the ability to visualize the frequency-dependent evolution of the resonant absorption and suggesting that the HPhP mode properties can be modified with the nanostructure geometrical design. This capability could be leveraged engineer the resonance frequencies and their strengths independently.