Recently, broken symmetry within crystals has been igniting tremendous research interest since it can be utilized to effectively manipulate the propagation of photons. In particular, low-symmetry Bravais crystals can support hyperbolic shear polaritons (HShPs), holding great promise for technological upgrading on the emerging research area of spinoptics. Herein, an Otto-type multilayer structure consisting of KRS5 prism, sensing medium, and monoclinic β-Ga2O3 crystal is designed to ameliorate the photonic spin Hall effect (PSHE). The surface of β-Ga2O3 is the monoclinic (010) plane (x-y plane). We show that giant spin Hall shifts with three (or two) orders of magnitude of the incident wavelength are obtained in the in-plane (or transverse) directions. The azimuthal dispersions of photonic spin Hall shifts present non‐mirror‐symmetric patterns at tuning the rotation angle of β-Ga2O3 around the z axis in plane. All of these exotic optical properties are closely correlated with the broken crystal lattice symmetry and the incurred excitation of HShPs in monoclinic β-Ga2O3 crystal. By virtue of the remarkably enhanced PSHE, our proposed Otto-type multilayer structure shows a superior biosensing performance in which the maximum sensitivity is two orders of magnitude larger than previously reported PSHE biosensors based on two-dimensional materials. In addition, the optimized physical and structural parameters including the incident angle, excitation wavelength, azimuth angle and doping concentration of β-Ga2O3, thickness and refractive index of sensing medium are also investigated and given. This work unequivocally confirms the strong influence of crystal symmetry on the PSHE, shedding important insights into understanding the rich modulation of spin-orbit interaction of light via shear polaritons and therefore facilitating potential applications in photoelectronic devices.
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Hyperbolic shear polaritons in low-symmetry crystals
Abstract The lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase 1–4 . Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light–matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales 5 . Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid-infrared to far-infrared due to shear phenomena in the dielectric response. This feature emerges in materials in which the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals in which several oscillators with non-orthogonal relative orientations contribute to the optical response 6,7 . Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic 1,3,4 and hexagonal 8,9 crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts and asymmetric responses. The interplay between diagonal loss and off-diagonal shear phenomena in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our results will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals 10 , many common oxides 11 and organic crystals 12 , greatly expanding the material base and extending design opportunities for compact photonic devices.
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- NSF-PAR ID:
- 10331461
- Date Published:
- Journal Name:
- Nature
- Volume:
- 602
- Issue:
- 7898
- ISSN:
- 0028-0836
- Page Range / eLocation ID:
- 595 to 600
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Structural anisotropy in crystals is crucial for controlling light propagation, particularly in the infrared spectral regime where optical frequencies overlap with crystalline lattice resonances, enabling light-matter coupled quasiparticles called phonon polaritons (PhPs). Exploring PhPs in anisotropic materials like hBN and MoO3has led to advancements in light confinement and manipulation. In a recent study, PhPs in the monoclinic crystal β-Ga2O3(bGO) were shown to exhibit strongly asymmetric propagation with a frequency dispersive optical axis. Here, using scanning near-field optical microscopy (s-SNOM), we directly image the symmetry-broken propagation of hyperbolic shear polaritons in bGO. Further, we demonstrate the control and enhancement of shear-induced propagation asymmetry by varying the incident laser orientation and polariton momentum using different sizes of nano-antennas. Finally, we observe significant rotation of the hyperbola axis by changing the frequency of incident light. Our findings lay the groundwork for the widespread utilization and implementation of polaritons in low-symmetry crystals.
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Abstract The topological features of optical vortices have been opening opportunities for free-space and on-chip photonic technologies, e.g., for multiplexed optical communications and robust information transport. In a parallel but disjoint effort, polar anisotropic van der Waals nanomaterials supporting hyperbolic phonon polaritons (HP 2 s) have been leveraged to drastically boost light-matter interactions. So far HP 2 studies have been mainly focusing on the control of their amplitude and scale features. Here we report the generation and observation of mid-infrared hyperbolic polariton vortices (HP 2 Vs) associated with reconfigurable topological charges. Spiral-shaped gold disks coated with a flake of hexagonal boron nitride are exploited to tailor spin–orbit interactions and realise deeply subwavelength HP 2 Vs. The complex interplay between excitation spin, spiral geometry and HP 2 dispersion enables robust reconfigurability of the associated topological charges. Our results reveal unique opportunities to extend the application of HP 2 s into topological photonics, quantum information processing by integrating these phenomena with single-photon emitters, robust on-chip optical applications, sensing and nanoparticle manipulation.more » « less
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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 crystal
α ‐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. -
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Abstract Optical nanoantennas are of great importance for photonic devices and spectroscopy due to their capability of squeezing light at the nanoscale and enhancing light–matter interactions. Among them, nanoantennas made of polar crystals supporting phonon polaritons (phononic nanoantennas) exhibit the highest quality factors. This is due to the low optical losses inherent in these materials, which, however, hinder the spectral tuning of the nanoantennas due to their dielectric nature. Here, active and passive tuning of ultranarrow resonances in phononic nanoantennas is realized over a wide spectral range (≈35 cm−1, being the resonance linewidth ≈9 cm−1), monitored by near‐field nanoscopy. To do that, the local environment of a single nanoantenna made of hexagonal boron nitride is modified by placing it on different polar substrates, such as quartz and 4H‐silicon carbide, or covering it with layers of a high‐refractive‐index van der Waals crystal (WSe2). Importantly, active tuning of the nanoantenna polaritonic resonances is demonstrated by placing it on top of a gated graphene monolayer in which the Fermi energy is varied. This work presents the realization of tunable polaritonic nanoantennas with ultranarrow resonances, which can find applications in active nanooptics and (bio)sensing.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41586-021-04328-y
