Ultra-low-loss Polaritons in Isotopically Pure Materials: A New Approach (1705.05971v1)

Abstract: Conventional optical components are limited to size-scales much larger than the wavelength of light, as changes in the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultra-thin, co-called "flat" optical components that beget abrupt changes in these properties over distances significantly shorter than the free space wavelength. While high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a three-fold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach towards realizing the loss control necessary for the development of PhP-based nanophotonic devices.

• The paper achieves a three-fold improvement in polariton lifetime by isotopically enriching hBN.
• It shows reduced Raman linewidth and extended propagation lengths, lowering optical losses significantly.
• The study outlines a practical framework for optimizing material purity, paving the way for advanced nanophotonic applications.

Ultra-Low-Loss Polaritons in Isotopically Pure Materials: A New Approach

The utilization of phonon polaritons (PhPs) in nanophotonics introduces unique opportunities for the realization of sub-diffraction-limit optical components, especially in the infrared (IR) to terahertz (THz) spectral domains. This paper presents a significant advancement in reducing optical losses for polaritonic devices by leveraging isotopic enrichment of hexagonal boron nitride (hBN), a polar dielectric material. The specific isotopic modification strategy elucidated in this paper has been practically demonstrated to dramatically enhance polariton propagation length, largely due to extended phonon lifetimes.

The investigation begins with an exploration of the fundamental differences between plasmon polaritons (PPs) and PhPs, emphasizing that PhPs offer lower optical losses due to their longer phonon lifetimes. This paper demonstrates a three-fold improvement in the polariton lifetime through isotopic enrichment of hBN. By replacing the naturally occurring boron isotope composition in hBN (approximately 80% ^11B and 20% ^10B) with either 99.2% ^11B or 98.7% ^10B, the research shows significant phononic and polaritonic performance improvements.

From the analysis, it emerges that this isotopic enrichment leads notably to a reduction in Raman linewidth and an increase in phonon lifetime with a corresponding shift in optic phonon energy. Experimentally, ultra-low optical losses are further evidenced by a marked increase in propagation lengths of the hyperbolic phonon polaritons (HPhPs) these materials support. Although only a portion of the predicted lifetime gains were realized due to increased carbon impurities in enriched samples, the paper outlines a strategic path for optimizing material purity to enhance optoelectronic performance.

The implications of these findings are substantial for the development of advanced nanophotonic devices. Enhanced PhP lifetimes facilitate improvements in integrated photonics, hyperlenses, waveguides, and other applications requiring efficient light-matter interactions at the nanoscale. Furthermore, the approach can be extrapolated to other polar crystals, suggesting broader applicability.

In terms of future research, the paper highlights possible improvements via a more immaculate control of carbon impurities and suggests the hybridization of electromagnetic mechanisms as a means to achieving even longer propagation lengths. Such efforts could pave the way for the development of multifunctional, high-efficiency polaritonic devices, including but not limited to metamaterials and complex metasurfaces, with improved spatial resolution and propagation capabilities in the IR and THz regimes.

In conclusion, this paper not only advances the theoretical understanding but also provides a practical framework for enhancing phonon-polaritonic interactions via isotopic purity, indicating promising advances for future nanophotonic technologies.