Hyperbolic Plasmons Research Articles

Tunable anisotropic van der Waals films of 2M-WS2 for plasmon canalization

In-plane anisotropic van der Waals materials have emerged as a natural platform for anisotropic polaritons. Extreme anisotropic polaritons with in-situ broadband tunability are of great significance for on-chip photonics, yet their application remains challenging. In this work, we experimentally characterize through Fourier transform infrared spectroscopy measurements a van der Waals plasmonic material, 2M-WS2, capable of supporting intrinsic room-temperature in-plane anisotropic plasmons in the far and mid-infrared regimes. In contrast to the recently revealed natural hyperbolic plasmons in other anisotropic materials, 2M-WS2 supports canalized plasmons with flat isofrequency contours in the frequency range of ~ 3000-5000 cm−1. Furthermore, the anisotropic plasmons and the corresponding isofrequency contours can be reversibly tuned via in-situ ion-intercalation. The tunable anisotropic and canalization plasmons may open up further application perspectives in the field of uniaxial plasmonics, such as serving as active components in directional sensing, radiation manipulation, and polarization-dependent optical modulators.

Actively tuning anisotropic light–matter interaction in biaxial hyperbolic material α-MoO3 using phase change material VO2 and graphene

Anisotropic hyperbolic phonon polaritons (PhPs) in natural biaxial hyperbolic material α-MoO3 has opened up new avenues for mid-infrared nanophotonics, while active tunability of α-MoO3 PhPs is still an urgent problem necessarily to be solved. In this study, we present a theoretical demonstration of actively tuning α-MoO3 PhPs using phase change material VO2 and graphene. It is observed that α-MoO3 PhPs are greatly dependent on the propagation plane angle of PhPs. The insulator-to-metal phase transition of VO2 has a significant effect on the hybridization PhPs of the α-MoO3/VO2 structure and allows to obtain actively tunable α-MoO3 PhPs, which is especially obvious when the propagation plane angle of PhPs is 90°. Moreover, when graphene surface plasmon sources are placed at the top or bottom of α-MoO3 in α-MoO3/VO2 structure, tunable coupled hyperbolic plasmon–phonon polaritons inside its Reststrahlen bands (RBs) and surface plasmon–phonon polaritons outside its RBs can be achieved. In addition, the above-mentioned α-MoO3-based structures also lead to actively tunable anisotropic spontaneous emission (SE) enhancement. This study may be beneficial for realization of active tunability of both PhPs and SE of α-MoO3, and facilitate a deeper understanding of the mechanisms of anisotropic light–matter interaction in α-MoO3 using functional materials.

Highly directional and carrier density-independent plasmons in quasi-one-dimensional electron gas systems

Recent advancements in developing metahyperbolic surfaces through substrate patterning have enabled the realization of highly-directional hyperbolic surface plasmons, but the feasibility of reproducing the same properties in natural hyperbolic two-dimensional (2D) materials is still unexplored. In this study, we expand the possibility of natural 2D materials in achieving electromagnetic scenarios akin to those observed in metahyperbolic surfaces. Natural hyperbolic 2D materials provide inherent advantages for simplicity, predictability, and lower losses compared to meta-surfaces. By employing first-principles calculations, we find that realistic 2D material, specifically the RuOCl2 monolayer, are suitable alternatives to metahyperbolic surfaces. Indeed, RuOCl2 monolayer sustains carrier-density-independent and broadband low-loss hyperbolic responses across the terahertz to ultraviolet spectral range, owning to the highly-anisotropic electronic band structures characterized by quasi-one-dimensional electron gas. These findings shed light on the integration of hyperbolicity in natural 2D materials, opening new avenues for the design and development of optoelectronic devices and nanoscale imaging systems.

Plasmonic responses in Janus bAsP with elliptic-to-hyperbolic transition: an ab-initio study.

Plasmonic responses in materials with actively tunable elliptic-to-hyperbolic transition are rare in nature. Based on ab-initio calculations, we have theoretically predicted that Janus black arsenic phosphorus (bAsP) supports both elliptic and hyperbolic in-plane surface plasmon polaritons in the infrared after being doped with electrons. In the elliptic regime, anisotropic plasmonic responses have been observed, which can be explained by the anisotropic dispersions at the bottom of the conduction bands. In the hyperbolic regime, the total permittivity along the armchair/zigzag edge is negative/positive, which is the result of positive interband permittivities and largely different Drude plasma frequencies along two directions making the total permittivities change signs at different photon energies. In this material, changing the topology (elliptic or hyperbolic) of the plasmonic responses via doping is possible. Then, strains along the zigzag and armchair directions have been applied to modify the band structures as well as the plasmonic responses. Since plasmonic responses are mostly related to the bands near the Fermi energy, a relatively small strain along the zigzag direction can make bAsP become an indirect-bandgap material and change the Drude plasma frequencies under proper doping. With both strain and doping present in this material, we have even found a special case of hyperbolicity where the total permittivity in the zigzag/armchair direction is negative/positive, which is opposite to the normal case. In the end, we have extended our investigations to bAsP-graphene heterostructures. Since bAsP is a Janus material, such direct contact can change the Fermi energy through charge transfer making this heterostructure support strong plasmons without extra doping. Our investigations propose bAsP as a promising Janus material platform for plasmonic applications.

Data-driven design of multilayer hyperbolic metamaterials for near-field thermal radiative modulator with high modulation contrast

The thermal modulator based on the near-field radiative heat transfer has wide applications in thermoelectric diodes, thermoelectric transistors, and thermal storage. However, the design of optimal near-field thermal radiation structure is a complex and challenging problem due to the tremendous number of degrees of freedom. In this work, we have proposed a data-driven machine learning workflow to efficiently design multilayer hyperbolic metamaterials composed of α-MoO3 for near-field thermal radiative modulator with high modulation contrast. By combining the multilayer perceptron and Bayesian optimization, the rotation angle, layer thickness and gap distance of the multilayer metamaterials are optimized to achieve a maximum thermal modulation contrast ratio of 6.29. This represents a 97 % improvement compared to previous single layer structure. The large thermal modulation contrast is mainly attributed to the alignment and misalignment of hyperbolic plasmon polaritons and hyperbolic surface phonon polaritons of each layer controlled by the rotation. The results provide a promising way for accelerating the designing and manipulating of near-field radiative heat transfer by anisotropic hyperbolic materials through the data-driven style.

Interplay of hyperbolic plasmons and superconductivity

Interplay of hyperbolic plasmons and superconductivity

Tunable optical topological transitions of plasmon polaritons in WTe2 van der Waals films

Naturally existing in-plane hyperbolic polaritons and the associated optical topological transitions, which avoid the nano-structuring to achieve hyperbolicity, can outperform their counterparts in artificial metasurfaces. Such plasmon polaritons are rare, but experimentally revealed recently in WTe2 van der Waals thin films. Different from phonon polaritons, hyperbolic plasmon polaritons originate from the interplay of free carrier Drude response and interband transitions, which promise good intrinsic tunability. However, tunable in-plane hyperbolic plasmon polariton and its optical topological transition of the isofrequency contours to the elliptic topology in a natural material have not been realized. Here we demonstrate the tuning of the optical topological transition through Mo doping and temperature. The optical topological transition energy is tuned over a wide range, with frequencies ranging from 429 cm−1 (23.3 microns) for pure WTe2 to 270 cm−1 (37.0 microns) at the 50% Mo-doping level at 10 K. Moreover, the temperature-induced blueshift of the optical topological transition energy is also revealed, enabling active and reversible tuning. Surprisingly, the localized surface plasmon resonance in skew ribbons shows unusual polarization dependence, accurately manifesting its topology, which renders a reliable means to track the topology with far-field techniques. Our results open an avenue for reconfigurable photonic devices capable of plasmon polariton steering, such as canaling, focusing, and routing, and pave the way for low-symmetry plasmonic nanophotonics based on anisotropic natural materials.

Twist-Angle and Thickness-Ratio Tuning of Plasmon Polaritons in Twisted Bilayer van der Waals Films.

Stacking bilayer structures is an efficient way to tune the topology of polaritons in in-plane anisotropic films, e.g., by leveraging the twist angle (TA). However, the effect of another geometric parameter, the film thickness ratio (TR), on manipulating the plasmon topology in bilayers is elusive. Here, we fabricate bilayer structures of WTe2 films, which naturally host in-plane hyperbolic plasmons in the terahertz range. Plasmon topology is successfully modified by changing the TR and TA synergistically, manifested by the extinction spectra of unpatterned films and the polarization dependence of the plasmon intensity measured in skew ribbon arrays. Such TR- and TA-tunable topological transitions can be well explained based on the effective sheet optical conductivity by adding up those of the two films. Our study demonstrates TR as another degree of freedom for the manipulation of plasmonic topology in nanophotonics, exhibiting promising applications in biosensing, heat transfer, and the enhancement of spontaneous emission.

Hyperbolic plasmons on natural biphenylene surface

Meta-surfaces constructed of patterned graphene nanoribbons on dielectric substrates have enabled in-plane propagation of surface plasmons with hyperbolic dispersion. However, the current size limitations of these meta-surface result in a restricted ceiling for the plasmon wave vector, highlighting the importance of exploring natural hyperbolic surfaces that do not require structuring. Here, we unveil a promising natural hyperbolic surface: the newly-synthesized graphene allotrope – biphenylene (BPN) [Fan et al., Science 372, 852–856 (2021)]. We demonstrate the anisotropic framework of π orbitals of BPN endows broadband hyperbolic plasmons on the BPN surface. Undamped plasmons emerge along the x-direction with a broadband spectrum from THz to the ultraviolet regime, whereas those along the y-direction are restricted to a low frequency region (<0.74 eV). Notably, hyperbolic regimes are observed within specific energy ranges, 1.21–2.44 eV and 3.35–3.51 eV, which facilitate the directional propagation of surface plasmons with hyperbolic dispersion relations. These results open an avenue for the use of graphene technique in advanced optics and offer a promising strategy for designing hyperbolic natural surfaces.

Broadband hyperbolic plasmons in aluminum disulfide monolayer and its analogues: The role of orbital anisotropy

Two-dimensional (2D) materials with high in-plane anisotropy offer promising candidates for hosting highly directional surface plasmons, but few of them have been demonstrated to reach this goal thus far. In this paper, we propose a design principle of 2D hyperbolic materials (2D-HMs) based on orbital anisotropy and predict from first-principles calculations a family of 2D-HMs: aluminum disulfide monolayer and its analogues $X{Y}_{2}$ $(X=\mathrm{Al},\mathrm{Ga},\mathrm{In};Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$. These natural 2D-HMs exhibit broadband hyperbolic regimes across the near-infrared to ultraviolet spectrum, enabling the propagation of highly directional hyperbolic surface plasmons. Undamped plasmons emerge along the $x$ direction with the maximum wave vector of $\ensuremath{\sim}0.16\phantom{\rule{0.16em}{0ex}}{\AA{}}^{\ensuremath{-}1}$ and frequency of 4.6 eV, whereas the plasmons along the $y$ direction have low frequency $(&lt;0.6\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$ and decay rapidly to electron-hole pairs for the $\mathrm{Al}{\mathrm{S}}_{2}$ monolayer. By solving Maxwell's equation, we simulate the directional propagation of the surface waves with hyperbolic dispersion relations. We correlate the fascinating plasmonic properties with the unique electronic structures of these highly anisotropic 2D materials, which offers a promising strategy for the design of 2D-HMs.

Topological Insulator Metamaterials

Confinement of electromagnetic fields at the subwavelength scale via metamaterial paradigms is an established method to engineer light-matter interaction in most common material systems, from insulators to semiconductors and from metals to superconductors. In recent years, this approach has been extended to the realm of topological materials, providing a new avenue to access nontrivial features of their electronic band structure. In this review, we survey various topological material classes from a photonics standpoint, including crystal growth and lithographic structuring methods. We discuss how exotic electronic features such as spin-selective Dirac plasmon polaritons in topological insulators or hyperbolic plasmon polaritons in Weyl semimetals may give rise to unconventional magneto-optic, nonlinear, and circular photogalvanic effects in metamaterials across the visible to infrared spectrum. Finally, we dwell on how these effects may be dynamically controlled by applying external perturbations in the form of electric and magnetic fields or ultrafast optical pulses. Through these examples and future perspectives, we argue that topological insulator, semimetal and superconductor metamaterials are unique systems to bridge the missing links between nanophotonic, electronic, and spintronic technologies.

Strain engineering of hyperbolic plasmons in monolayer carbon phosphide: a first-principles study.

Natural and tunable in-plane hyperbolic plasmons have so far been elusive, and hence few two-dimensional hyperbolic materials have been theoretically and experimentally discovered. Here, comprehensive first-principles calculations were conducted to study the electronic and plasmonic properties of biaxially strained monolayer carbon phosphide (β-CP). We found that (i) a compressed β-CP hosts strong anisotropic Dirac-shaped fermions with robust modulated Fermi velocity, (ii) for biaxial strain of -3% an unprecedented ultra-wide hyperbolic window is extended continuously from terahertz (9 THz) to mid-visible (blue light, 693 THz), (iii) the tunable optical Van Hove singularity as the origin of hyperbolic plasmons in deformed β-CP is disclosed, (iv) an elliptic to hyperbolic transition in the σ-near-zero regime is demonstrated in terahertz frequencies (9 THz), (v) the propagation angle of the concave wavefront can be actively tuned using biaxial strains, and (vi) hyperbolic dispersion reorientation from one principal axis to another orthogonal one under compressive strains larger than 8% is observed. This study sheds new light on the unique properties of hyperbolic two-dimensional (2D) materials having exotic optoelectronic characteristics which are promising candidates for anisotropic light control with ultimate dexterity in the flat optics.

Tunable, Homoepitaxial Hyperbolic Metamaterials Enabled by High Mobility CdO

AbstractPropagating light exhibits hyperbolicity in strongly anisotropic materials where the principal components of the dielectric tensor are opposite in sign. While hyperbolicity occurs naturally in anisotropic polar dielectrics, wherein optical phonons along orthogonal crystal axes are nondegenerate, such optical anisotropy can also be engineered in hyperbolic metamaterials (HMMs): thin film superlattices of alternating dielectric and metallic layers. Contrasted with the severely limited tunability of natural hyperbolic materials, the hyperbolic behavior of HMMs can be tailored significantly both through superlattice design and material selection. However, so far HMMs have suffered from high optical losses, hindering their performance. In this report, broadly tunable (λ = 2–5 µm) Type I and II hyperbolic modes with low losses (quality (Q)‐factors up to 19.7) are observed through attenuated total reflectance measurements of monolithic, homoepitaxial superlattices of high‐ and low‐doped cadmium oxide (CdO). Further, the low losses offered by CdO enable the first demonstration of real‐space imaging of hyperbolic plasmon polaritons in nanoresonators by scattering‐type scanning near‐field optical microscopy—previously only possible for hyperbolic phonon polariton materials. Tunable, low‐loss CdO HMMs promise designability for applications such as on‐chip photonics, super‐resolution imaging (hyperlensing), enhanced emission, novel emitter designs, and possibly quantum nanophotonic and time variant metasurfaces.

Moiré-driven reconstitution on electromagnetic energy transfer

The past decade has witnessed an explosion in the field of twistronics concepts, opening the door to numerous extraordinary physical phenomena such as weak ferroelectric charge transfer, superconductivity, topologically protected states, and so on. Here, inspired by the twistronics concepts, we take a moiré three-body system consisting of three metasurfaces supporting highly confined hyperbolic surface plasmons as an example, introducing an analogous concept into the study of near-field electromagnetic energy transfer within three-body system. We demonstrate the possible existence of the moiré transistor effect induced by the twisted hyperbolic repeater in the polariton coupling system (source and drain parallel to each other), where the near-field electromagnetic energy transfer exchanged between the source and the drain can be reversibly switched, amplified, and modulated by action on moiré patterns. Moreover, we point out that for the symmetry-breaking situations (source and drain not parallel to each other), there is also a magic angle in this system and demonstrate that the twisted hyperbolic repeater can act as a moiré compensation effect at this magic angle, mitigating to a certain extent the electromagnetic energy attenuation caused by the malposition between source and drain. Our findings may open new opportunities for the development of near-field thermal energy transport, enabling new perspectives concerning thermal management at the nanoscale, near-field energy conversion, and infrared spectroscopy.

Enhancement and Manipulation of Near-Field Thermal Radiation Using Hybrid Hyperbolic Polaritons

Owing to a high electromagnetic confinement and a strong photonic density of states, hyperbolic surface plasmon polaritons (HSPPs) provide a fascinating promise for applications in thermal photonics. In this work, we theoretically predict a possibility for the improvement of the near-field radiative heat transfer on the basis of tailoring the electromagnetic state of hyperbolic metasurfaces by the uniaxial hyperbolic substrate. By using the photonic tunneling coefficient and the polaritons dispersion, we present a comprehensive study of the hybrid effect of the hyperbolic substrate on HSPPs. We find that due to the hybrid effect of the hyperbolic substrate, the anisotropy surface state of hyperbolic metasurfaces would undergo significant deformations and even topological transition. Moreover, we systematically exhibit the evolution of such hybrid hyperbolic mode with different thicknesses of the hyperbolic substrate and analyze the thickness effect on radiative properties of the hybrid system. It is shown that the resulting heat transfer with the assistance of the hybrid hyperbolic mode by optimizing the substrate parameters is many times stronger than that of monolayer hyperbolic metasurface at the same vacuum gap. Taken together, our results provide a platform to tailor 2D hyperbolic plasmons as a potential strategy toward passive or active control of the near-field heat transfer, and the hybrid hyperbolic mode presented here may facilitate the system design for near-field energy harvesting, thermal imaging, and radiative cooling applications.

Hyperbolic plasmons in massive tilted two-dimensional Dirac materials

We explore topological transitions in the type of propagation of surface electromagnetic modes in massive anisotropic tilted Dirac systems. The presence of tilting and mass gives rise to an indirect band gap that strongly modifies the joint density of states compared to the gapless system. New van Hove singularities appear, and the interplay between intra and interband transitions leads to an anisotropic optical conductivity with imaginary parts acquiring opposite signs in orthogonal directions, opening the possibility of having hyperbolic propagation of plasmons. Isofrequency contours and low plasmon losses, as obtained from the dispersion relation, show that transitions between purely anisotropic quasi-elliptical and well-defined, highly directional, hyperbolic modes are attainable only when tilt and mass coexist. Fine-tuning in the THz regime suggests that this might be useful as a dynamical probe of concurring tilt and mass in Dirac materials.

Enhanced Near‐Field Radiative Heat Transfer between Graphene/hBN Systems (Small 19/2022)

Radiative Heat Transfer In article number 2108032, Jinlin Song, Zixue Luo, Qiang Cheng, and co-workers measure the near-field radiative heat transfer between graphene/hexagonal boron nitride systems, where the coupling of hyperbolic phonon polaritons and surface plasmon polaritons is examined. The largest ever radiative heat flux is achieved for a gap space of ≈400 nm, which is six times higher than the blackbody limit.

Tunable Near-Field Radiative Effect in a Td – WTe2 Single Layer

In contrast to artificial metamaterials, natural two-dimensional (2D) hyperbolic materials are able to support higher electromagnetic confinement and stronger photonic density of states. This means natural 2D hyperbolic materials have fascinating potential for applications in thermal photonics. In this work, we investigate mechanisms of near-field thermal radiation (NFTR) in a ${\mathrm{T}}_{d}$--${\mathrm{WTe}}_{2}$ single layer at the nanoscale using the fluctuation-dissipation theorem and density-functional theory. The results show that NFTR of ${\mathrm{T}}_{d}$--${\mathrm{WTe}}_{2}$ can be 3 orders of magnitude larger than the blackbody limit. By using the photonic tunneling coefficient and the plasmon dispersion, we present a comprehensive study of plasmonic properties of ${\mathrm{T}}_{d}$--${\mathrm{WTe}}_{2}$. Moreover, our first-principles calculations predict that application of a certain amount of mechanical stress can trigger topological transition between the elliptic and the hyperbolic surface states by regulating the Fermi surface and interband excitation threshold. Lastly, we systematically exhibit the evolution of surface plasmon polaritons in ${\mathrm{T}}_{d}$--${\mathrm{WTe}}_{2}$ under mechanical stress and analyze the performance of using mechanical stress to modulate the corresponding NFTR. Our work explores the great potential of tunable radiative heat flux in ${\mathrm{T}}_{d}$--${\mathrm{WTe}}_{2}$, which is explained here through the peculiar nature of hyperbolic surface plasmons.

Monolayer NaW2O2Br6: a gate tunable near-infrared hyperbolic plasmonic surface.

Electrically tunable hyperbolic polaritons in two dimensional (2D) materials can offer unexplored opportunities in integrating photonics and nano-optoelectronics into a single chip. Here, we suggest that monolayer NaW2O2Br6 can host electrically tunable hyperbolic plasmon polaritons for infrared light via first-principles calculations. 2D monolayer NaW2O2Br6 exhibits an extremely anisotropic metallic property: conducting for one direction but almost insulating for the other direction, which could be considered as a 2D analogue of metal/dielectric multilayers, a typical structure for hyperbolic metamaterials. More interestingly, we also demonstrate that the hyperbolic properties in the near-infrared range, including the hyperbolic windows, figure of merit, and propagation directions of plasmon beams, can be effectively modulated by carrier doping at the order of 1013 cm−2, which even can be accessed by solid-gated field effect transistors. Thus, it is anticipated that monolayer NaW2O2Br6 has a great potential in constructing field programmable polariton nanodevices for emerging and diverse photonic applications.

Near-field thermal rectification driven by nonreciprocal hyperbolic surface plasmons

In this work, we theoretically demonstrate a pathway for highly efficient near-field thermal rectification. We propose the use of nonreciprocal hyperbolic surface plasmon polaritons (NHSPPs) supported by a drift-biased graphene grating to rectify the radiative heat flux between the two dielectric particles. At higher chemical potentials, we achieve theoretical rectification ratios over 88.67, which is a 460-fold improvement over the case of a drift-biased graphene sheet. By adjusting the value of the drift current or the orientation of the graphene ribbons, the NHSPPs can be actively manipulated, hence inducing the modulation of the rectification effect. We discuss the influence of the filling ratio of the grating and the height of the particles from its surface on the near-field radiative heat transfer. We find that the rectification ratio and the heat flux between the particles can be adjusted over a broad range by controlling the height of the particles. Compared with a drift-biased graphene sheet, the directional stability of the diode was also enhanced. This work provides an idea for the thermal management of micro- and nanoscale devices.