An official website of the United States government
Phonons are important lattice vibrations that affect the thermal, electronic, and optical properties of materials. In this work, we studied infrared phonon resonance in a prototype van der Waals (vdW) material—hexagonal boron nitride (hBN)—with the thickness ranging from monolayers to bulk, especially on ultra-thin crystals with atomic layers smaller than 20. Our combined experimental and modeling results show a systematic increase in the intensity of in-plane phonon resonance at the increasing number of layers in hBN, with a sensitivity down to one atomic layer. While the thickness-dependence of the phonon resonance reveals the antenna nature of our nanoscope, the linear thickness-scaling of the phonon polariton wavelength indicates the preservation of electromagnetic hyperbolicity in ultra-thin hBN layers. Our conclusions should be generic for fundamental resonances in vdW materials and heterostructures where the number of constituent layers can be conveniently controlled. The thickness-dependent phonon resonance and phonon polaritons revealed in our work also suggest vdW engineering opportunities for desired thermal and nanophotonic functionalities.
more »
« less
Low Dielectric Medium for Hyperbolic Phonon Polariton Waveguide in van der Waals Heterostructures
Polar van der Waals (vdW) crystals, composed of atomic layers held together by vdW forces, can host phonon polaritons—quasiparticles arising from the interaction between photons in free-space light and lattice vibrations in polar materials. These crystals offer advantages such as easy fabrication, low Ohmic loss, and optical confinement. Recently, hexagonal boron nitride (hBN), known for having hyperbolicity in the mid-infrared range, has been used to explore multiple modes with high optical confinement. This opens possibilities for practical polaritonic nanodevices with subdiffractional resolution. However, polariton waves still face exposure to the surrounding environment, leading to significant energy losses. In this work, we propose a simple approach to inducing a hyperbolic phonon polariton (HPhP) waveguide in hBN by incorporating a low dielectric medium, ZrS2. The low dielectric medium serves a dual purpose—it acts as a pathway for polariton propagation, while inducing high optical confinement. We establish the criteria for the HPhP waveguide in vdW heterostructures with various thicknesses of ZrS2 through scattering-type scanning near-field optical microscopy (s-SNOM) and by conducting numerical electromagnetic simulations. Our work presents a feasible and straightforward method for developing practical nanophotonic devices with low optical loss and high confinement, with potential applications such as energy transfer, nano-optical integrated circuits, light trapping, etc.
more »
« less
- PAR ID:
- 10576860
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Nanomaterials
- Volume:
- 14
- Issue:
- 16
- ISSN:
- 2079-4991
- Page Range / eLocation ID:
- 1344
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Polar van der Waals (vdW) crystals that support phonon polaritons have recently attracted much attention because they can confine infrared and terahertz (THz) light to deeply subwavelength dimensions, allowing for the guiding and manipulation of light at the nanoscale. The practical applications of these crystals in devices rely strongly on deterministic engineering of their spatially localized electromagnetic field distributions, which has remained challenging. The polariton interference can be enhanced and tailored by patterning the vdW crystalα‐MoO3into microstructures that support highly in‐plane anisotropic phonon polaritons. The orientation of the polaritonic in‐plane isofrequency curve relative to the microstructure edges is a critical parameter governing the polariton interference, rendering the configuration of infrared electromagnetic field localizations by enabling the tuning of the microstructure size and shape and the excitation frequency. Thus, the study presents an effective rationale for engineering infrared light flow in planar photonic devices.more » « less
-
Abstract Electromagnetic hyperbolicity has driven key functionalities in nanophotonics, including super-resolution imaging, efficient energy control, and extreme light manipulation. Central to these advances are hyperbolic polaritons—nanometer-scale light-matter waves—spanning multiple energy-momentum dispersion orders with distinct mode profiles and incrementally high optical momenta. In this work, we report the mode conversion of hyperbolic polaritons across different dispersion orders by breaking the structure symmetry in engineered step-shape van der Waals (vdW) terraces. The mode conversion from the fundamental to high-order hyperbolic polaritons is imaged using scattering-type scanning near-field optical microscopy (s-SNOM) on both hexagonal boron nitride (hBN) and alpha-phase molybdenum trioxide (α-MoO3) vdW terraces. Our s-SNOM data, augmented with electromagnetics simulations, further demonstrate the alteration of polariton mode conversion by varying the step size of vdW terraces. The mode conversion reported here offers a practical approach toward integrating previously independent different-order hyperbolic polaritons with ultra-high momenta, paving the way for promising applications in nano-optical circuits, sensing, computation, information processing, and super-resolution imaging.more » « less
-
Abstract Silicon waveguides have enabled large‐scale manipulation and processing of near‐infrared optical signals on chip. Yet, expanding the bandwidth of guided waves to other frequencies will further increase the functionality of silicon as a photonics platform. Frequency multiplexing by integrating additional architectures is one approach to the problem, but this is challenging to design and integrate within the existing form factor due to scaling with the free‐space wavelength. This paper demonstrates that a hexagonal boron nitride (hBN)/silicon hybrid waveguide can simultaneously enable dual‐band operation at both mid‐infrared (6.5–7.0 µm) and telecom (1.55 µm) frequencies, respectively. The device is realized via the lithography‐free transfer of hBN onto a silicon waveguide, maintaining near‐infrared operation. In addition, mid‐infrared waveguiding of the hyperbolic phonon polaritons (HPhPs) supported in hBN is induced by the index contrast between the silicon waveguide and the surrounding air underneath the hBN, thereby eliminating the need for deleterious etching of the hyperbolic medium. The behavior of HPhP waveguiding in both straight and curved trajectories is validated within an analytical waveguide theoretical framework. This exemplifies a generalizable approach based on integrating hyperbolic media with silicon photonics for realizing frequency multiplexing in on‐chip photonic systems.more » « less
-
Hyperbolic phonon polaritons (HPhPs) enable strong confinements, low losses, and intrinsic beam steering capabilities determined by the refractive index anisotropy—providing opportunities from hyperlensing to flat optics and other applications. Here, two scanning-probe techniques, photothermal induced resonance (PTIR) and scattering-type scanning near-field optical microscopy (s-SNOM), are used to map infrared (6.4–7.4 µm) HPhPs in large (up to 120 × 250 µm2) near-monoisotopic (>99% 10B) hexagonal boron nitride (hBN) flakes. Wide (≈40 µm) PTIR and s-SNOM scans on such large flakes avoid interference from polaritons launched from different asperities (edges, folds, surface defects, etc.) and together with Fourier analyses (0.05 µm−1 resolution) enable precise measurements of HPhP lifetimes (up to ≈4.2 ps) and propagation lengths (up to ≈25 and ≈17 µm for the first- and second-order branches, respectively). With respect to naturally abundant hBN, we report an eightfold improved, record-high (for hBN) propagating figure of merit (i.e., with both high confinement and long lifetime) in ≈99% 10B hBN, achieving, finally, theoretically predicted values. We show that wide near-field scans critically enable accurate estimates of the polaritons’ lifetimes and propagation lengths and that the incidence angle of light, with respect to both the sample plane and the flake edge, needs to be considered to extract correctly the dispersion relation from the near-field polaritons maps. Overall, the measurements and data analyses employed here elucidate details pertaining to polaritons’ propagation in isotopically enriched hBN and pave the way for developing high-performance HPhP-based devices.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/nano14161344
