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Optical characteristics of α-MoO3 and the structure under study a Schematic representation of atomic orientation in the bulk structure of α-MoO3 in xz and yz planes, (b) real and (c) imaginary parts of the dielectric function for α-MoO3. d Schematic illustration of the investigated multilayer structure.
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Exploiting polaritons in natural vdW materials has been successful in achieving extreme light confinement and low-loss optical devices and enabling simplified device integration. Recently, α-MoO3 has been reported as a semiconducting biaxial vdW material capable of sustaining naturally orthogonal in-plane phonon polariton modes in IR. In this study...
Citations
... Van der Waals materials have drawn considerable interest in the last decade as low-loss platforms for polaritons, hybrid light-matter modes that confine electric fields down to the nanoscale (19)(20). Surface plasmon polaritons in graphene (21) and hyperbolic phonon polaritons in hexagonal boron nitride (22) and molybdenum trioxide (23)(24), in particular, may enable numerous diverse applications including superresolution focusing (25), biosensing (26), negative refraction (27)(28) and reflection (29), polarization conversion (30), et cetera. Hyperbolic plasmon polaritons (HPPs), which occur in materials with highly anisotropic plasma frequencies, on the other hand, have yet to be widely applied because their host materials have been too lossy (13)(14) or even hygroscopic (12,15,31) and non-exfoliatable (32). ...
Correlated materials may exhibit unusually high resistivity increasing linearly in temperature, breaking through the Mott-Ioffe-Regel bound, above which coherent quasiparticles are destroyed. The fate of collective charge excitations, or plasmons, in these systems is a subject of debate. Several studies suggest plasmons are overdamped while others detect unrenormalized plasmons. Here, we present direct optical images of low-loss hyperbolic plasmon polaritons (HPPs) in the correlated van der Waals metal MoOCl2. HPPs are plasmon-photon modes that waveguide through extremely anisotropic media and are remarkably long-lived in MoOCl2. Many-body theory supported by photoemission results reveals that MoOCl2 is in an orbital-selective and highly incoherent Peierls phase. Different orbitals acquire markedly different bonding-antibonding character, producing a highly-anisotropic, isolated Fermi surface. The Fermi surface is further reconstructed and made partly incoherent by electronic interactions, renormalizing the plasma frequency. HPPs remain long-lived in spite of this, allowing us to uncover previously unseen imprints of electronic correlations on plasmonic collective modes.
... When ε x ̸ = ε y ̸ = ε z , the material is biaxial hyperbolic material, where the characteristic feature of this material is the presence of alpha-molybdenum (α-MoO 3 ). This crystal exhibits van der Waals bonding and also possesses an asymmetric crystalline structure, with three dielectric tensor principal values corresponding to three Reststrahlen frequency bands [9,10]. In contrast to hBN, the three Reststrahlen frequency bands of alpha-molybdenum trioxide exhibit overlapping regions that are theoretically more anisotropic. ...
... Likewise, the expression of the GH and IF shifts upon the isotropic media can be obtained with the same method. Consequently, Equations (10) and (15) can be simplified to a well-known result, which was cited in the relevant reference [24]. ...
This investigation focuses on the Goos–Hänchen (GH) and Imbert–Fedorov (IF) shifts on the surface of the uniaxial hyperbolic material hexagonal boron nitride (hBN) based on the biaxial hyperbolic material alpha-molybdenum (α-MoO3) trioxide structure, where the anisotropic axis of hBN is rotated by an angle with respect to the incident plane. The surface with the highest degree of anisotropy among the two crystals is selected in order to analyze and calculate the GH- and IF-shifts of the system, and obtain the complex beam-shift spectra. The addition of α-MoO3 substrate significantly amplified the GH shift on the system’s surface, as compared to silica substrate. With the p-polarization light incident, the GH shift can reach 381.76λ0 at about 759.82 cm⁻¹, with the s-polarization light incident, the GH shift can reach 288.84λ0 at about 906.88 cm⁻¹, and with the c-polarization light incident, the IF shift can reach 3.76λ0 at about 751.94 cm⁻¹. The adjustment of the IF shift, both positive and negative, as well as its asymmetric nature, can be achieved by manipulating the left and right circular polarization light and torsion angle. The aforementioned intriguing phenomena offer novel insights for the advancement of sensor technology and optical encoder design.
... The x and y axes represent the crystalline directions of [100] and [001] of the bottom α-MoO 3 layer, respectively, and the twist angle between the top and bottom α-MoO 3 layers is ϕ. In application, the bottom α-MoO 3 layer can be grown on a substrate using physical vapor deposition techniques, 17 followed by the deposition of the VO 2 layer using magnetron sputtering, 18 and then, the top α-MoO 3 layer can be twisted at the desired angle using the dry-transfer technique. 19,20 As a typical biaxial van der Waals (vdW) material, the α-MoO 3 crystal is an orthorhombic crystal featuring three different symmetries for oxygen atoms that give rise to the biaxial optical properties. ...
Flexible control of intrinsic chiroptical responses within compact nanostructures is crucial for flat optics, topological photonics, and chiroptics. However, previous approaches require complicated patterns with both in-plane and out-of-plane mirror symmetry breaking to achieve intrinsic chirality, and their chiroptical responses cannot be dynamically controlled as well. Herein, we demonstrated that near-perfect intrinsic circular dichroism (CD) can be achieved within a lithography-free structure consisting of the twisted bilayer α-MoO3 separated by a vanadium dioxide (VO2) film. By twisting the bilayer α-MoO3, dual-band intrinsic chiroptical responses can be realized due to the excitations of the hyperbolic phonon polaritons modes in the mid-infrared. It is the spin-selected average electric-field enhancement instead of the chiral absorption that is responsible for the intrinsic CD of the device. In addition, the chiroptical responses are insensitive to the variation of the thickness of the structure as well as the incident angle, and high contrast CD can be dynamically tuned by varying the volume fraction of VO2.
... Insulating antiferromagnets are magnetically hyperbolic materials in the case of no external magnetic field, whose hyperbolicity originates from their magnetic permeability with opposite-sign principal values in the far-infrared or THz range. [1][2][3] It is wellknown that antiferromagnets support surface magnon polaritons (SMPs) with TE polarization in the Voigt geometry, [4][5][6] where the antiferromagnetic easy axis and external magnetic field are parallel to each other, and the SMPs propagate along the direction perpendicular to the external field. In the case of no external magnetic field, these antiferromagnets become some magnetically hyperbolic crystals whose permeability is a diagonal tensor with opposite-sign principal values in their reststrahlen frequency bands, but the dielectric permittivity is a constant. ...
Magnetically ordering media support spin waves or magnons, which can couple with electromagnetic waves to form magnon polaritons. Based on insulating antiferromagnets, magnon polaritons are situated in the far-infrared or THz frequency range. We investigated Dyakonov surface magnon polaritons (DSMPs) at the antiferromagnetic surface in an external magnetic field, where the external field and antiferromagnetic easy axis lie in the surface plane and are normal to each other. Our numerical results are based on the MnF2 crystal, but the conclusions and qualitative results are also available to other insulating antiferromagnets. We predicted two field-tunable DSMPs and one tunable Dyakonov surface magnon. We discerned the main effects of the external magnetic field on the DSMPs; either DSMPs are sensitively modulated by the external field or there is a cutoff magnetic field. Their individual Poynting vector seriously deviates the propagation direction and is sensitively controlled by the external field. The spin angular momentum contains two components normal to each other, unlike conventional surface magnon polaritons. One of the DSMPs can carry a huge Poynting vector and spin angular momentum in the external magnetic field. These results are interesting for micromechanics and spintronics and relevant technologies in the far-infrared or THz domain.
... Besides their strong anisotropy related to optical phonons (ideal for polarization rotation and control), they allow strong field localization by the excitation of surface waves called surface phonon polaritons (SPhPs), achieved through the coupling of the electromagnetic field with lattice vibrations. Several works reported on the great potential of polar materials for mid-IR sensing applications up to the terahertz (THz) regime [14,15] and for the realization of compact IR photonic devices [16,17]. ...
In recent years, the excitation of surface phonon polaritons (SPhPs) in van der Waals materials received wide attention from the nanophotonics community. Alpha-phase Molybdenum trioxide (-MoO3), a naturally occurring biaxial hyperbolic crystal, emerged as a promising polaritonic material due to its ability to support SPhPs for three orthogonal directions at different wavelength bands (range 10–20 m). Here, we report on the fabrication, structural, morphological, and optical IR characterization of large-area (over 1 cm2 size) -MoO3 polycrystalline film deposited on fused silica substrates by pulsed laser deposition. Due to the random grain distribution, the thin film does not display any optical anisotropy at normal incidence. However, the proposed fabrication method allows us to achieve a single -phase, preserving the typical strong dispersion related to the phononic response of -MoO3 flakes. Remarkable spectral properties of interest for IR photonics applications are reported. For instance, a polarization-tunable reflection peak at 1006 cm-1 with a dynamic range of R=0.3 and a resonance Q-factor as high as 53 is observed at 45° angle of incidence. Additionally, we report the fulfillment of an impedance matching condition with the SiO2 substrate leading to a polarization-independent almost perfect absorption condition (R<0.01) at 972 cm-1 which is maintained for a broad angle of incidence. In this framework our findings appear extremely promising for the further development of mid-IR lithography-free, scalable films, for efficient and large-scale sensors, filters, thermal emitters, and label-free biochemical sensing devices operating in the free space, using far-field detection setups.
... Planar hyperbolic media are most relevant in the development of ultrathin polarization controllers as shown in Fig. 6f [122][123][124] . Size is a bottleneck in traditional polarization controllers, for instance, a state-ofthe-art quarter-wave plate uses bulky linear birefringent crystals, since it requires a significant propagation distance to establish the phase difference between orthogonal polarizations. ...
Anisotropic planar polaritons - hybrid electromagnetic modes mediated by phonons, plasmons, or excitons - in biaxial two-dimensional (2D) van der Waals crystals have attracted significant attention due to their fundamental physics and potential nanophotonic applications. In this Perspective, we review the properties of planar hyperbolic polaritons and the variety of methods that can be used to experimentally tune them. We argue that such natural, planar hyperbolic media should be fairly common in biaxial and uniaxial 2D and 1D van der Waals crystals, and identify the untapped opportunities they could enable for functional (i.e. ferromagnetic, ferroelectric, and piezoelectric) polaritons. Lastly, we provide our perspectives on the technological applications of such planar hyperbolic polaritons.
... The hBN is one hyperbolic material in the RFBs and is one ellipsoidal material outside the RFBs. The α-MoO 3 is a typical biaxial hyperbolic crystal that has three RFBs corresponding to the three principal values of its permittivity tensor, respectively [29][30][31][32]. In contract to the hBN, the three RFBs of α-MoO 3 are not separated so that there are partly overlaps. ...
... We use ε 0 and µ 0 to show the vacuum permittivity and permeability. For a biaxial hyperbolic crystal, the three primary elements can be uniformly expressed with a function by [15,[30][31][32] ε n = ε ∞,n ...
... The physical parameters included in Eq. (2) are given in are given in The chirality in twisted bilayer α-MoO3 was discussed [49] and the near-field radiative modulator driven by anisotropic hyperbolic polaritons in this biaxial hyperbolic material recently [50,51]. Table 1 [ [30][31][32], where the damping constants of optical phonons are ignored. The three diagonal elements of the primary permittivity change with frequency, as illustrated in Fig. 1(b). ...
We predicted peculiar ghost surface phonon polaritons in biaxially hyperbolic materials, where the two hyperbolic principal axes lie in the plane of propagation. We took the biaxially-hyperbolic α-MoO3 as one example of the materials to numerically simulate the ghost surface phonon polaritons. We found three unique ghost surface polaritons to appear in three enclosed wavenumber-frequency regions, respectively. These ghost surface phonon polaritons have different features from the surface phonon polaritons found previously, i.e., they are some hybrid-polarization surface waves composed of two coherent evanescent branch-waves in the α-MoO3 crystal. The interference of branch-waves leads to that their Poynting vector and electromagnetic fields both exhibit the oscillation-attenuation behavior along the surface normal, or a series of rapidly attenuated fringes. We found that the in-plane hyperbolic anisotropy and low-symmetric geometry of surface are the two necessary conditions for the existence of these ghost surface polaritons.
... However, the optic axis of these layered materials is typically out of the plane, and thus their utility in conventional optical systems is limited by the difficulty to funnel light into the small nanoscale area that parallels to its out-of-plane optical axis. Instead, in-plane birefringence is more conducive to the practical applications 2,30 , but the realization of large in-plane birefringence in these layered materials remains challenging. ...
Birefringence is at the heart of photonic applications. Layered van der Waals materials inherently support considerable out-of-plane birefringence. However, funnelling light into their small nanoscale area parallel to its out-of-plane optical axis remains challenging. Thus far, the lack of large in-plane birefringence has been a major roadblock hindering their applications. Here, we introduce the presence of broadband, low-loss, giant birefringence in a biaxial van der Waals materials Ta2NiS5, spanning an ultrawide-band from visible to mid-infrared wavelengths of 0.3–16 μm. The in-plane birefringence Δn ≈ 2 and 0.5 in the visible and mid-infrared ranges is one of the highest among van der Waals materials known to date. Meanwhile, the real-space propagating waveguide modes in Ta2NiS5 show strong in-plane anisotropy with a long propagation length (>20 μm) in the mid-infrared range. Our work may promote next-generation broadband and ultracompact integrated photonics based on van der Waals materials.
... Dereshgi and colleagues introduced a new approach to constructing IR polarization converters by using α-MoO 3 flakes, which does not involve lithography and purely relies on the use of orthogonal in-plane phonons. [37] In this paper, we take the advantages of α-MoO 3 biaxial hyperbolicity and strong in-plane anisotropy to achieve an effective improvement of spatial shift. Meanwhile, we utilize the hBN as a substrate. ...
Many optical systems that deal with polarization rely on the adaptability of controlling light reflection in the lithography-free nanostructure. In this study, we explored the Goos-Hänchen (GH) and Imbert-Fedorov (IF) shifts at a biaxial hyperbolic film on a uniaxial hyperbolic substrate. This research statistically analyzed and calculated the GH and IF shifts for the natural biaxial hyperbolic material (NBHM). We selected the surface with the strongest anisotropy within the NBHM and obtained the complex beam-shift spectra. By incorporating the NBHM film, the GH shift caused by a transversely magnetic incident-beam on the surface is significantly enhanced compared to that on the uniaxial hyperbolic material. The maximum of GH shift can reach 86 λ 0 at about 841 cm ⁻¹ when the thickness of NBHM is 90 nm, and the IF shift can approach 2.7 λ 0 for a circularly-polarized beam incident on a NBHM of 1700 nm. An increase in spatial-shift was found when a highly anisotropic hyperbolic polariton is excited in hyperbolic materials, where the shift spectra exhibits an oscillating behaviour along with sharp shift peaks (steep slopes). This large spatial shift may provide an alternative strategy to develop novel sub-micrometric optical devices and biosensors.
... In each direction, the RS band is situated between the longitudinal optical (LO) and transverse optical (TO) phonons. The dielectric tensor of α-MoO 3 is described using a Lorentzian model [34][35][36]: ...
... Given that the thickness of the Au substrate exceeds the skin depth of the incident light, the transmission can be neglected [18,38]. Therefore, we can calculate the absorption according to the expression A = 1 − R. The refractive index of Ge is n Ge = 4, and the dielectric constant of Au was obtained from Ref. [35]. The structure is designed so that the [100] direction is aligned along the x-axis ( Fig. 2(a)), enabling the realization of broadband absorption in the RS band 2, where the electric field is along the x-axis. ...
