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Hyperbolic phonon polaritons in bc plane. a) HAADF image of a MoO3 flake fabricated by FIB. Inset shows the corresponding electron diffraction pattern, confirming its crystal orientation. b,c) Line profiles with the beam scanning along the b and c directions. Horizontal dashed lines mark the edge of the flake. d) Energy‐filtered EELS maps at selected energy marked by triangular markers of corresponding color. Insets illustrate isofrequency surfaces calculated by Fresnel equation. e) Simulated EELS maps of several polariton eigenmodes corresponding to (d). The white dashed boxes denote the boundary of the flake.
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Hyperbolic phonon polaritons (HPhPs) in orthorhombic-phase molybdenum trioxide (α-MoO3 ) show in-plane hyperbolicity, great wavelength compression, and ultralong lifetime, therefore holding great potential in nanophotonic applications. However, its polaritonic response in the far-infrared (FIR) range remains unexplored due to challenges in experime...
Citations
... As a biaxial vdW material with a low crystalline structural symmetry, it has multiple RBs ranging from the middle-infrared to far-infrared spectral region 20,34,35 , as shown in Fig. 1a and b. Notable, there are additional spectral regimes where theses RBs overlap, resulting in fascinating polaritonic properties. ...
The canalization effect of phonon polaritons (PhPs) shows highly directional, and diffraction-less propagation characteristics in van der Waals (vdW) materials, offering new opportunities to mold the light flow at nanoscale for near-field energy, information and thermal managements. Previously, canalized PhPs have only been experimentally realized in the hexagonal boron nitride metasurface, heterostructures of twisted α -phase molybdenum trioxide ( α -MoO 3 ) crystal flakes or the hybridized system. However, these systems typically have complex structures, and require strict operational conditions, such as fine structural parameters, a specific photonic magic angle or a doping level of graphene, for realizing polariton canalization with a modest performance. Here, we demonstrate the high-quality PhPs canalization in a single-layer natural α -MoO 3 crystal flake. The canalized PhPs exhibit the highly directional, and diffraction-free propagation features, associated with lateral confinement ratio up to λ 0 /80 (where λ 0 is the free-space wavelength of the incident laser). We believe this work is important to effectively manipulate PhPs in natural vdW materials, with potential applications in nanoimaging, directional energy transfer and enhanced nonlinearity at the deep subwavelength scale.
... This low symmetric crystalline structure of the α-MoO 3 crystal gives rise to rich phonon modes, which are infrared-active along three principle crystalline directions and thus yields various RBs [11]. In each RB that spans from the longitudinal optical (LO) over the transverse optical (TO) phonon frequency of the α-MoO 3 crystal, the Re(ε) is expected to be negative and hyperbolic PhPs modes are supported by the α-MoO 3 crystal [45,46]. In Fig. 1(d), there are three RBs in the mid-infrared range of 700 to 1050 cm −1 . ...
... The enhanced efficiency of edge-launched hyperbolic PhPs in the stacked α-MoO 3 flakes could be attributed to a combined effect that includes the nanoscale air gap between stacked sample and the wavenumber direction of the incident laser perpendicular to the sample edge. Notably, it is still a challenge to perform monochromatic scattering-type scanning near-field optical microscopy (s-SNOM) to probe hyperbolic PhPh at the RB 1 owing to the external interferometric detection method and the limitation of the available light sources [9,45]. Besides s-SNOM, PiFM could be an alternative optical near-field experimental instrument, especially for ω below 880 cm −1 [42,55]. ...
In this work, we reported a systemic study on the enhanced efficiency of launching hyperbolic phonon polaritons (PhPs) in stacked α-phase molybdenum trioxide (α-MoO3) flakes. By using the infrared photo-induced force microscopy (PiFM), real-space near-field images (PiFM images) of mechanically exfoliated α-MoO3 thin flakes were recorded within three different Reststrahlen bands (RBs). As referred with PiFM fringes of the single flake, PiFM fringes of the stacked α-MoO3 sample within the RB 2 and RB 3 are greatly improved with the enhancement factor (EF) up to 170%. By performing numerical simulations, it reveals that the general improvement in near-field PiFM fringes arises from the existence of a nanoscale thin dielectric spacer in the middle part between two stacked α-MoO3 flakes. The nanogap acts as a nanoresonator for prompting the near-field coupling of hyperbolic PhPs supported by each flake in the stacked sample, contributing to the increase of polaritonic fields, and verifying the experimental observations Our findings could offer fundamental physical investigations into the effective excitation of PhPs and will be helpful for developing functional nanophotonic devices and circuits.
... according to the hyperbolic IFCs, with more exotic forms of inplane hyperbolicity reported in lower symmetry crystals such as hyperbolic shear polaritons within monoclinic crystals. [30,31] This extremely anisotropic response offers new opportunities for confining and configuring electromagnetic waves at deepsubwavelength scales, especially in localized energy confinement, [32][33][34][35][36][37][38][39] topological transitions, [40][41][42][43][44][45][46] and planar polariton optics. [47][48][49][50][51][52][53][54] In addition, through stacking the in-plane hyperbolic medium with another polaritonic medium to form heterostructures and twisting the heterostructures to a prescribed misorientation angle, recent studies have opened the door for the emerging field of twist-optics. ...
Hyperbolic phonon polaritons (HPhPs) are stimulated by coupling infrared (IR) photons with the polar lattice vibrations. Such HPhPs offer low‐loss, highly confined light propagation at subwavelength scales with out‐of‐plane or in‐plane hyperbolic wavefronts. For HPhPs, while a hyperbolic dispersion implies multiple propagating modes with a distribution of wavevectors at a given frequency, so far it has been challenging to experimentally launch and probe the higher‐order modes that offer stronger wavelength compression, especially for in‐plane HPhPs. In this work, we report the experimental observation of higher‐order in‐plane HPhP modes stimulated on a 3C‐SiC nanowire (NW)/α‐MoO3 heterostructure where leveraging both the low‐dimensionality and low‐loss nature of the polar NWs, higher‐order HPhPs modes within two‐dimensional α‐MoO3 crystal are launched by the one‐dimensional 3C‐SiC NW. We further study the launching mechanism and determine the requirements for efficiently launching of such higher‐order modes. In addition, by altering the geometric orientation between the 3C‐SiC NW and α‐MoO3 crystal, we demonstrate the manipulation of higher‐order HPhP dispersions as a method of tuning. This work illustrates an extremely anisotropic low dimensional heterostructure platform to confine and configure electromagnetic waves at the deep‐subwavelength scales for a range of infrared applications including sensing, nano‐imaging, and on‐chip photonics. This article is protected by copyright. All rights reserved
... α-MoO 3 as an advanced material has attracted wide attention in recent years owing to its phenomenal phonon polariton (PhP, infrared [IR] light interacting with phonons in polar materials) properties and wide range of applications. [17][18][19][20][21][22][23][24][25][26][27][28][29] Therefore, a systematic lattice dynamics study of α-MoO 3 may be useful for understanding the anisotropy of the propagation behavior of PhP to guide and optimize the rational design of related devices. Moreover, the ultra-wide band gap up to 3.8 eV [30,31] and the sufficiently large birefringence of transparent α-MoO 3 makes it an excellent platform for studying individually the influence of birefringence on the ARPRS response. ...
Optical anisotropy, which is quantified by birefringence (Δn) and linear dichroism (Δk), can significantly modulate the angle‐resolved polarized Raman spectroscopy (ARPRS) response of anisotropic layered materials (ALMs) by external interference. This work studies the separate modulation of birefringence on the ARPRS response and the intrinsic response by selecting transparent birefringent crystal α‐MoO3 as an excellent platform. It is found that there are several anomalous ARPRS responses in α‐MoO3 that cannot be reproduced by the real Raman tensor widely used in non‐absorbing materials; however, they can be well explained by considering the birefringence‐induced Raman selection rules. Moreover, the systematic thickness‐dependent study indicates that birefringence modulates the ARPRS response to render an interference pattern; however, the amplitude of modulation is considerably lower than that by linear dichroism as occurred in black phosphorous. This weak modulation brings convenience to the crystal orientation determination of transparent ALMs. Combining the atomic vibrational pattern and bond polarizability model, the intrinsic ARPRS response of α‐MoO3 is analyzed, giving the physical origins of the Raman anisotropy. This study employs α‐MoO3 as an example, although it is generally applicable to all transparent birefringent ALMs.
... (a) IFCs in the momentum space and the z component of the electric field in real space of surface plasmons supported by BP in purely anisotropic, quasi-circular, and hyperbolic regimes [231] . (b), (c), (d) Hyperbolic regimes induced by anisotropic phonons, plasmons, and excitons for phonon [232] , plasmon [39] , and exciton polaritons [110] , respectively. the hyperbolic polariton in metasurfaces cannot be large. ...
... Yellow: fitted background. Red: extracted five major vibrational signals [232] . octahedron, and a unit layer contains two layers of such octahedrons. ...
... The inset shows an electron diffraction pattern obtained near the ribbon edge. (f) EELS maps taken in the yellow dashed box in (e) [232] . ...
... tunable energy range. [1][2][3][4] Among the emerging natural hyperbolic materials, a particular interest has been focused on orthorhombic-phase molybdenum trioxide (α-MoO 3 ) for sustaining extremely anisotropic HPhPs in the mid-IR range, which also provides multiple photonic operation routes such as the layer-twisting, [5][6][7][8] heterojunction, [9] artificial structure, [10][11][12][13][14][15] doping, [16,17] and ambient dielectric modulation. [11,18,19] These highly anisotropic HPhPs of α-MoO 3 stem from three socalled Reststrahlen bands (RBs) with opposite-signed refractive indices along the three principal crystal orientations in the mid-IR range, which results from the strong anisotropic structure. ...
... [1,3,12] Alternatively, Gao et al. employed an electron beam instead of a light source through electron energy loss spectroscopy (EELS) to investigate HPhPs in a large energy range that covers all three full RBs in mid-IR. [4] However, this nonoptical technique has a short penetration depth and a low spectral energy resolution limit. [4] The in-plane anisotropic HPhPs properties, especially in the spectral range that covers both RB1 Anisotropic hyperbolic phonon-polaritons (HPhPs) in a van der Waals material, orthorhombic-phase molybdenum trioxide (α-MoO 3 ), provide strategies in multiple dimensions to manipulate photons at the nanoscale. ...
... [4] However, this nonoptical technique has a short penetration depth and a low spectral energy resolution limit. [4] The in-plane anisotropic HPhPs properties, especially in the spectral range that covers both RB1 Anisotropic hyperbolic phonon-polaritons (HPhPs) in a van der Waals material, orthorhombic-phase molybdenum trioxide (α-MoO 3 ), provide strategies in multiple dimensions to manipulate photons at the nanoscale. However, the nano-imaging studies of the in-plane HPhPs along orthogonal crystal orientations are still rare. ...
Anisotropic hyperbolic phonon‐polaritons (HPhPs) in a van der Waals material, orthorhombic‐phase molybdenum trioxide (α‐MoO3), provide strategies in multiple dimensions to manipulate photons at the nanoscale. However, the nano‐imaging studies of the in‐plane HPhPs along orthogonal crystal orientations are still rare. In this work, the launching and propagating properties of HPhPs are studied in α‐MoO3 upon a holey silicon nitride microcavity through a photon‐induced force microscope (PiFM) over a broad tunable mid‐infrared region of 780–1010 cm⁻¹. Near‐field patterns stemming from the orthogonal polariton momenta are observed in this spectral band. A simple model based on the geometrical optics of elliptic resonance cones is used to explain the near‐field phonon‐polariton features. These near‐field optical properties indicate abundantly possible operations and applications for nanophotonics.
... The orthorhombic lattice symmetry endows α-MoO 3 with three distinct but orthogonal major polarizability axes, which results in strong birefringence, or even hyperbolicity within several distinct spectral ranges. [4,27] This extreme optical anisotropy is highly associated with the anisotropic crystal phonons. [23,25,28] The inplane anisotropic phonon modes of α-MoO 3 and their basic physical parameters, such as the symmetry of vibration and Raman tensor, have been revealed in several previous works by using polarization-resolved Raman spectroscopy. ...
The effect of birefringence in anisotropic materials has been a long‐term issue for polarized Raman scattering. In this work, the polarization‐dependent Raman scattering in anisotropically birefringent materials is modeled with birefringence considered in the fundamental polarization selection rule. The birefringence‐induced polarization transformation is treated as a tensor in the calculation of Raman scattering intensity. The validity of this theory is further demonstrated in experiments by taking angle‐resolved polarized Raman measurements on the basal and cross planes of α‐MoO3—a typical biaxial van der Waals crystal with strong in‐plane and out‐of‐plane anisotropy. The anomalous angular dependency of polarized Raman scattering intensity can be well reproduced by the modified theoretical model with fitted real‐valued Raman tensors and birefringence‐related parameters. It can be concluded that this work can provide a valuable reference and guidance for quantitative analyses of anisotropic materials by using polarization‐resolved Raman scattering spectroscopy.
... For natural HMs, on the other hand, such as hexagonal boron nitride (hBN) [23][24][25][26] and α-phase molybdenum trioxide (α-MoO 3 ) [27][28][29], this limitation can be ignored because the lattice constant is on the order of subnanometers, thereby natural HMs have unique advantages. The excitation of volume-confined hyperbolic polaritons (VHPs) and surface-confined hyperbolic polaritons (SHPs) in natural HMs could further enhance NFRHT. ...
Hyperbolic materials (HMs), whose components of the permittivity tensor have opposite signs, can excite hyperbolic phonon polaritons in a wide frequency range, paving a way to control light. However, most HMs studied previously are artificial structures constructed with periodically stacked subwavelength metallic and dielectric layers, whose hyperbolic properties are limited by the tangential wavevector component. In comparison, the lattice constants of natural HMs are sub-nanometer in size, there is no need to consider this limitation. In this chapter, we investigated the near-field radiative heat transfer (NFRHT) between 2D natural HMs, including hBN and α-MoO3, whose excellent 2D properties can be obtained by mechanical exfoliation. The near-field radiative heat flux is calculated using the fluctuation-dissipation theorem and the modified 4 × 4 transfer matrix method. Numerical results show that the NFRHT between 2D natural HMs can be significantly enhanced in the hyperbolic region. Moreover, we pointed out the regions in the wavevector space where volume-confined hyperbolic polaritons (VHPs) and surface-confined hyperbolic polaritons (SHPs) can exist and proved that VHPs and SHPs excited in natural HMs is the main reason for the large radiative heat flux. In particular, we discussed the essential role of natural HMs in enhancing NFRHT, considering the effects of optical axis orientation, film thickness, and material types. We believe this chapter will open a novel path for the research on NFRHT and are expected to be applied to next-generation high-efficiency energy conversion devices.
... The HAADF images were acquired using Nion U-HERMES200 operated at 60 kV. [14,25] EELS Data Processing: All the acquired vibrational spectra were processed by using a custom-written MATLAB code and Gatan Microscopy Suite. More specifically, the EEL spectra were first aligned by their normalized cross-correlation. ...
... The spectra were summed along the direction parallel to the interface for obtaining line-scan data with a good signal-to-noise ratio. [14,25] Theoretical Calculations: We employ a finite element method implemented in COMSOL Multiphysics to simulate the EELS spectra. The Radio Frequency Toolbox is used for performing retarded simulations (solving Maxwell's equations) to evaluate the electric field in the presence of a MXene. ...
Two‐dimensional metal carbides and nitrides (MXene) are promising candidates for electromagnetic (EM) shielding, saturable absorption, thermal therapy, and photocatalysis due to their excellent performance in EM absorption. Although the plasmon resonance in MXene can play a role in the EM absorption, such an effect has never been quantitatively analyzed yet due to the lack of knowledge of the plasmon dispersion of MXene, which is challengeable to measure for optical methods. Here, we measure the plasmon dispersions (frequency‐momentum relation) for individual MXene nanoflakes with varied thicknesses by using the high‐spatial‐resolution electron energy‐loss spectroscopy, which is corroborated by EM theoretical calculation. The plasmon dispersions show that plasmon frequency can be continuously modulated in the ultrabroad band (frequency domain below near‐infrared) by changing the momentum (e.g., via control of the flake size). The increasing plasmon compression effect in flakes with decreasing layer number indicates that increasing the interlayer distance can effectively improve the plasmon compression and thus increase the EM absorption. For example, a 3 nm interlayer distance can nearly double the plasmon‐enhanced EM absorption in MXene nanostructures. Our work provides guidance for the design of advanced ultra‐thin EM absorption materials for wide applications such as EM absorbers, EM shielding devices, photothermal and photocatalysis devices. This article is protected by copyright. All rights reserved
... In this scheme, the spatial resolution can reach 0.2 nm with an ultra-compensated momentum up to 10 8 cm −1 (refs. [62][63][64] ). ...
Polaritons, originating from the interactions between photons and material excitations, have attracted attention because of their strong field compression and deeply subdiffractional scales. For practical applications, it is crucial to manipulate polaritons efficiently, but doing so has remained challenging because of the relatively poor tunability of traditional polaritonic media. Fortunately, in the past decade, polaritons hosted by van der Waals (vdW) materials have allowed new opportunities to tackle this difficulty. We review the state of the art in the manipulation of polaritons at the extreme scale in vdW materials. Benefiting from the large and expanding catalogue of vdW materials and associated architectures and techniques, more accessible manipulation strategies are expected, not only offering control of light at the nanoscale with new degrees of freedom, but also offering insight into nanophotonics, meta-optics, topological physics and quantum materials.
