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.

Poking at phonon-polaritons at the nanoscale- Advances in Engineering
Reprinted with permission from American Chemical Society

About the author

Dr. Joshua Caldwell was awarded his Bachelors of Chemistry from Virginia Tech in 2000, then his PhD in Physical Chemistry in 2004 at the University of Florida. There he used magnetic resonance methods to investigate electron-nuclear spin coupling within low-dimensional quantum wells and heterostructures. He accepted a postdoctoral fellowship at the Naval Research Laboratory in 2005, using optical spectroscopy as a means of understanding defects within wide-band gap semiconductors. He was transitioned to permanent staff in 2007, where he began work in the field of nanophotonics, investigating coupling phenomena within plasmonic materials. More recently, Dr. Caldwell merged his prior work in wide band gap semiconductor materials with his efforts in nanophotonics, leading to his efforts to use undoped, polar dielectric crystals for low-loss, sub-diffraction optics.

Dr. Caldwell went on sabbatical at the University of Manchester with Prof. Kostya Novoselov in 2013-2014, investigating the use of van der Waals crystals such as hexagonal boron nitride for mid-IR to THz nanophotonics. He accepted a tenured Associate Professorship at Vanderbilt University within the Mechanical Engineering in June, 2017.

About the author

Andrea Centrone is a Project Leader in the Nanoscale Imaging and Spectroscopy Group. He received a Laurea degree and a Ph. D. in Materials Engineering from the Polytechnic University of Milan, Italy, working on nanoporous materials for hydrogen storage applications. Andrea performed postdoctoral work at the Massachusetts Institute of Technology, first as a Rocca Fellow in the Department of Material Science and Engineering, studying the phase separation of molecules self-assembled on metal nanoparticles. He continued his postdoctoral work in in the Department of Chemical Engineering, investigating the use of metal-organic frameworks for small molecules separation and gold nanorods for in vivo cancer detection and treatment.

Andrea joined the CNST in 2010, where he is developing new measurements methods (such as PTIR and STIRM) that combine wavelength tunable lasers with scanning probe techniques to provide correlated optical, chemical and thermal property maps of materials with nanoscale resolution. In collaboration with CNST users, Andrea leads multiple projects aimed at answering outstanding questions in nanotechnology and material science across several materials systems such as: organic inorganic perovskites solar cells, plasmonic and polaritonics nanostructures, drug delivering nanoparticles, polypeptide nanostructures, metal-organic frameworks, etc.



Brown, L., Davanco, M., Sun, Z., Kretinin, A., Chen, Y., & Matson, J., Vurgaftman, I., Sharac, N., Giles, A., Fogler, M., Taniguchi, T., Watanabe, K., Novoselov, K., Maier, S., Centrone, A., & Caldwell, J. (2018). Nanoscale Mapping and Spectroscopy of Nonradiative Hyperbolic Modes in Hexagonal Boron Nitride Nanostructures. Nano Letters, 18, 1628-1636.

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