Hyperbolic Phonon Polaritons Research Articles

Switchable strong coupling between dual hyperbolic phonon polaritons and photons in hybrid structure of metasurfaces and h-BN slab

We propose an all-dielectric hybrid structure combined with hexagonal boron nitride slab and strontium titanate (STO) metasurfaces to excite dual hyperbolic phonon polaritons (HPhPs) and an additional optical (TO) phonon, and achieve their strong coupling with photons. The metasurfaces, supporting tunable guided-mode resonance via adjusting the external temperature, consists of STO two-dimensional grating and STO layer. Thus, the strong coupling can be switched and tuned actively between the dual HPhPs and TO phonon via adjusting the external temperature of metasurfaces. This work has numerous potential applications on multi-channel biosensors, filters and tunable source and detectors.

Spontaneous emission modulation in biaxial hyperbolic van der Waals material

As a natural van der Waals crystal, α-MoO3 has excellent in-plane hyperbolic properties and essential nanophotonics applications. However, its tunable properties are generally neglected. Here, we achieve effective modulation of spontaneous emission (SE) from a single-layer flat plate by changing the crystal directions. Numerical results and theoretical analysis show that α-MoO3 exhibits good tunability when the crystal directions of α-MoO3 are different in y–z or x–y planes. A modulation factor of more than three orders of magnitude is obtained at 634 cm−1. This phenomenon is caused by the excitation of hyperbolic phonon polaritons in α-MoO3 at specific bands. However, when the crystal directions of α-MoO3 are different in the x–z plane, the SE of the material exhibits strong angle independence. Additionally, for the semi-infinite α-MoO3 flat structure, we determine the distribution of the modulation factor of SE using the wavenumber and rotation angle. Finally, we extend the calculation results from semi-infinite media to finite thickness films. We obtain the general evolution law of the peak angle of the modulation factor with thickness, increasing the modulation factor to approximately 2000, which exceeds the maximum modulation factor observed in previous works by 48 times. We believe this work could guide the SE modulation of anisotropic materials and benefit the field of micro-/nano-lasers and quantum computing.

Criterion for photonic topological transition in two-dimensional heterostructures.

The anisotropic van der Waals material α-MoO3 has recently attracted considerable attention because of the ability to support ellipse and hyperbolic phonon polaritons with extreme field confinement and long lifetimes, which can be used in topological transition and transformation polaritonics. However, the dispersion theory of some phonon polaritons in complex heterojunctions often requires tedious computation, which makes it difficult to simply judge and analyze the physical process of the photonic topological transition. Here we obtain the equivalent permittivity distribution of two-dimensional (2D) heterostructures by the effective medium theory and analyze the rotation-induced topological transitions and stack-dependent topological transitions of phonon polaritons. Unlike the previous discussion, we can predict the topological transition points by a parameter ɛx/y(i.e., the permittivity ratio along the in-plane crystal axis of the equivalent medium) and design precisely the phonon polaritons in the stacked materials by controlling the equivalent permittivity after simple calculation. The feasibility of the effective medium theory is verified based on the 2D approximation model and the non-2D approximation model under the limit of an ultrathin slab. Meanwhile, we compare the field distributions and dispersions of the 2D heterostructures and the corresponding equivalent structure. The simulation suggests that the elliptic/hyperbolic responses of the stacked materials depend on the sign of ɛx/y. The new, to the best of our knowledge, method not only provides an easier and clearer criterion for the study of photonic topological transition in anisotropic polaritons, but also shows great potential in designing some multilayer 2D heterostructures.

Reciprocal polariton-induced transparency in alpha-molybdenum trioxide-graphene heteronanostructures

An ultrathin heterogeneous nanostructure consisting of periodic orthorhombic-phase molybdenum trioxide (α-MoO3), graphene nanoribbons, and a dielectric spacer layer has been proposed. Coupling between a hyperbolic phonon polariton from α-MoO3 and a surface plasmon polariton from graphene leads to the presence of reciprocal polariton-induced transparency (RPoIT). The near-field coupling is further evaluated by combining the current distribution values of the multipole decomposition. The RPoIT can be controlled by changing the structural parameters and tuning the chemical potential of graphene. Effects of the incidence angle and polarization angle have been investigated thoroughly from the finite-difference time-domain calculations. The potential applications in the refractive index sensor and mid-infrared (MIR) absorbers have also been investigated. This research provides a clear understanding and practical guidance for the realization of tunable RPoIT, which is desirable from the development of nano-devices and multifunctional hybrid polarization devices in the MIR region.

Thermoradiative devices enabled by hyperbolic phonon polaritons at nanoscales

Theromoradiative (TR) devices, though proposed a decade ago, have seen little investigation due to their low-performance compared to other solid-state energy conversion technologies. Herein, we propose an InSb-hBN TR device with a nanoscale vacuum gap down to 10 nm that has the potential to achieve efficient waste heat recovery using solid-state technologies. By coupling the hyperbolic phonon polaritons of hBN with the interband transition of InSb, this TR device design can achieve an output power nearly 4 orders-of-magnitude higher than far-field scenarios and an upper bound efficiency up to 80% of the Carnot limit corresponding to only considering radiative loss. A detailed balance model combined with fluctuational electrodynamics for a multilayered anisotropic structure is developed to theoretically estimate the performance of TR devices from far- to near-field regime. Nonradiative losses are also investigated to elucidate their effects on performance. This work helps deepen the understanding of TR devices and provides a promising pathway to efficient low-grade (< 600 K) heat recovery.

Tuning infrared absorption in hyperbolic polaritons coated silk fibril composite

Advanced textiles for thermal management give rise to many functional applications and unveil a new frontier for the study of human thermal comfort. Manipulating the coated quasi-particles between the composite components offers a platform to study the advanced thermoregulatory textiles. Here, we propose that coating the hyperbolic polariton can be an effective tool to tune infrared absorption in hexagonal boron nitride-coated silk composite. Remarkably, we achieve significant tuning of the infrared absorption efficiency of silk fibrils through the designed hexagonal boron nitride film. The underlying mechanism is related to resonance coupling between hyperbolic phonon polaritons. We find a notably high infrared absorption efficiency, nearly 3 orders larger than that without hBN coating, which can be achieved in our composite system. Our results indicate the promising future of advanced polariton-coated textiles and open a pathway to guide the artificial-intelligence design of advanced functional textiles.

Near-field radiative heat transfer modulated by nontrivial topological surface states

Three-dimensional topological insulators (3D TIs), such as Bi2Se3 and Bi2Te3, possessing insulating bulk states and nontrivial topological surface states protected by time-reversal symmetry, have attracted considerable attention. In this paper, we investigate the near-field radiative heat transfer in 3D TIs, which can be strongly modulated by the non-trivial surface states. Full 3D TI properties including both the bulk and surface states are considered in our theoretical study. We show that the hybrid polaritons in single-material TI films, resulting from the coupling of Dirac plasmons on the top and bottom surfaces with the multiple phonon polariton modes supported by the bulk states including hyperbolic phonon polaritons (HPP), surface phonon polaritons (SPhP) and epsilon-near-pole (ENP) modes, can strongly mediate and enhance the near-field thermal radiation. The dispersions of hybrid polaritons intensely depend on the chemical potential of the surface and the film thickness, and by tuning both separately, we can control the coupling and competition between the surface and bulk modes, and thus modulate the radiative spectrum and heat flux. In addition, the nonlocal optical response of the surface states has a remarkable impact on the mode hybridization. This work demonstrates the potential of 3D TIs in manipulating near-field thermal radiation, which may provide a useful platform for energy conversion and thermal management.

Reconfigurable hyperbolic polaritonics with correlated oxide metasurfaces

Polaritons enable subwavelength confinement and highly anisotropic flows of light over a wide spectral range, holding the promise for applications in modern nanophotonic and optoelectronic devices. However, to fully realize their practical application potential, facile methods enabling nanoscale active control of polaritons are needed. Here, we introduce a hybrid polaritonic-oxide heterostructure platform consisting of van der Waals crystals, such as hexagonal boron nitride (hBN) or alpha-phase molybdenum trioxide (α-MoO3), transferred on nanoscale oxygen vacancy patterns on the surface of prototypical correlated perovskite oxide, samarium nickel oxide, SmNiO3 (SNO). Using a combination of scanning probe microscopy and infrared nanoimaging techniques, we demonstrate nanoscale reconfigurability of complex hyperbolic phonon polaritons patterned at the nanoscale with high resolution. Hydrogenation and temperature modulation allow spatially localized conductivity modulation of SNO nanoscale patterns, enabling robust real-time modulation and nanoscale reconfiguration of hyperbolic polaritons. Our work paves the way towards nanoscale programmable metasurface engineering for reconfigurable nanophotonic applications.

Spin-orbit-locked hyperbolic polariton vortices carrying reconfigurable topological charges

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 (HP2s) have been leveraged to drastically boost light-matter interactions. So far HP2 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 (HP2Vs) 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 HP2Vs. The complex interplay between excitation spin, spiral geometry and HP2 dispersion enables robust reconfigurability of the associated topological charges. Our results reveal unique opportunities to extend the application of HP2s into topological photonics, quantum information processing by integrating these phenomena with single-photon emitters, robust on-chip optical applications, sensing and nanoparticle manipulation.

Near-field probing of image phonon-polaritons in hexagonal boron nitride on gold crystals.

Near-field mapping has been widely used to study hyperbolic phonon-polaritons in van der Waals crystals. However, an accurate measurement of the polaritonic loss remains challenging because of the inherent complexity of the near-field signal and the substrate-mediated loss. Here we demonstrate that large-area monocrystalline gold flakes, an atomically flat low-loss substrate for image polaritons, provide a platform for precise near-field measurement of the complex propagation constant of polaritons in van der Waals crystals. As a topical example, we measure propagation loss of the image phonon-polaritons in hexagonal boron nitride, revealing that their normalized propagation length exhibits a parabolic spectral dependency. Furthermore, we show that image phonon-polaritons exhibit up to a twice longer normalized propagation length, while being 2.4 times more compressed compared to the case of the dielectric substrate. We conclude that the monocrystalline gold flakes provide a unique nanophotonic platform for probing and exploitation of the image modes in low-dimensional materials.

Nano‐Optical Engineering of Anisotropic Phonon Resonances in a Hyperbolic α‐MoO3 Metamaterial

AbstractLow‐dimensional α‐phase molybdenum trioxide (α‐MoO3), a layered van‐der‐Waals (vdW) semiconductor, has emerged as an attractive natural hyperbolic material supporting mid‐infrared hyperbolic phonon polaritons (PhPs), which exhibit strong spatial confinement and low loss. With the advantages of strong in‐plane optical anisotropy, many efforts have been devoted to investigating the properties of hyperbolic PhPs in α‐MoO3 using scanning near‐field optical microscopy. However, the studies of far‐field controlling of hyperbolic PhPs in α‐MoO3 have been quite limited so far. This work reports the first experimental demonstration of far‐field excitation and manipulation of hyperbolic phonon resonances in metamaterial structures consisting of α‐MoO3 nanodisks and slabs. The excited phonon resonances show a maximum spatial confinement factor (free space wavelength/period) of 32 and a quality factor of 76. It is shown that the in‐plane phonon resonances in α‐MoO3 can be selectively excited in the two Reststrahlen bands featuring hyperbolic dispersion relations along its two crystal directions, by simply controlling the incident polarization. In addition, the relative strength of resonances along different in‐plane crystal directions and the resultant field distributions can be continuously reconfigured by varying the incident polarization angle. These findings pave the way for future development of novel nanophotonic devices based on hyperbolic PhPs in vdW materials.

Modulation of Casimir Force between Graphene-Covered Hyperbolic Materials.

A flexible method for modulating the Casimir force is proposed by combining graphene and hyperbolic materials (HMs). The proposed structure employs two candidates other than graphene. One is hexagonal boron nitride (hBN), a natural HM. The other is porous silicon carbide (SiC), which can be treated as an artificial HM by the effective medium theory. The Casimir force between graphene-covered hBN (porous SiC) bulks is presented at zero temperature. The results show that covering HM with graphene increases the Casimir force monotonically. Furthermore, the force can be modulated by varying the Fermi level, especially at large separation distances. The reflection coefficients are thoroughly investigated, and the enhancement is attributed to the interaction of surface plasmons (SPs) supported by graphene and hyperbolic phonon polaritons (HPhPs) supported by HMs. Moreover, the Casimir force can be controlled by the filling factor of porous SiC. The Casimir force can thus be modulated flexibly by designing desired artificial HMs and tuning the Fermi level. The proposed models have promising applications in practical detection and technological fields.

Actively tunable hybrid plasmon-phonon polariton modes in ferroelectric/graphene heterostructure systems at low-THz frequencies

Attributed to hyperbolic phonon polaritons (HPPs) of ferroelectric materials such as lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) in low-THz frequencies (<20 THz), the ferroelectric/graphene heterostructure systems are put forward to obtain actively tunable hybrid plasmon-phonon polariton modes. When graphene layers are added in ferroelectric-based heterostructures, tunable hybrid plasmon-phonon polariton modes are obtained by the hybridization of ferroelectric phonon polaritons and graphene plasmons, and these characteristics also lead to dramatic regulation of spontaneous emission (SE) rates. Moreover, SE rates in ferroelectric/graphene heterostructure systems greatly rely on the ferroelectric thickness, graphene chemical potential and the emitter distance from the heterostructure. This work provides a meaningful instruction for actively tunable hybrid plasmon-phonon polariton modes and SE rate regulation in low-THz frequencies.

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 Planar Focusing Based on Hyperbolic Phonon Polaritons in α-MoO3.

Manipulation of the propagation and energy-transport characteristics of subwavelength infrared (IR) light fields is critical for the application of nanophotonic devices in photocatalysis, biosensing, and thermal management. In this context, metamaterials are useful composite materials, although traditional metal-based structures are constrained by their weak mid-IR response, while their associated capabilities for optical propagation and focusing are limited by the size of attainable artificial optical structures and the poor performance of the available active means of control. Herein, a tunable planar focusing device operating in the mid-IR region is reported by exploiting highly oriented in-plane hyperbolic phonon polaritons in α-MoO3 . Specifically, an unprecedented change of effective focal length of polariton waves from 0.7 to 7.4 μm is demonstrated by the following three different means of control: the dimension of the device, the employed light frequency, and engineering of phonon-plasmon hybridization. The high confinement characteristics of phonon polaritons in α-MoO3 permit the focal length and focal spot size to be reduced to 1/15 and 1/33 of the incident wavelength, respectively. In particular, the anisotropic phonon polaritons supported in α-MoO3 are combined with tunable surface-plasmon polaritons in graphene to realize in situ and dynamical control of the focusing performance, thus paving the way for phonon-polariton-based planar nanophotonic applications.

Gate tunable light–matter interaction in natural biaxial hyperbolic van der Waals heterostructures

Abstract The recent discovery of natural biaxial hyperbolicity in van der Waals crystals, such as α-MoO3, has opened up new avenues for mid-IR nanophotonics due to their deep subwavelength phonon polaritons. However, a significant challenge is the lack of active tunability of these hyperbolic phonon polaritons. In this work, we investigate heterostructures of graphene and α-MoO3 for actively tunable hybrid plasmon phonon polariton modes via electrostatic gating in the mid-infrared spectral region. We observe a unique propagation direction dependent hybridization of graphene plasmon polaritons with hyperbolic phonon polaritons for experimentally feasible values of graphene chemical potential. We further report an application to tunable valley quantum interference in this system with a broad operational bandwidth due to the formation of these hybrid modes. This work presents a lithography-free alternative for actively tunable, anisotropic spontaneous emission enhancement using a sub-wavelength thick naturally biaxial hyperbolic material.

Enhancement of Casimir Friction between Graphene-Covered Topological Insulator.

Casimir friction is theoretically studied between graphene-covered undoped bismuth selenide (Bi2Se3) in detail. In the graphene/Bi2Se3 composite structure, the coupling of the hyperbolic phonon polaritons supported by Bi2Se3 with the surface plasmons supported by graphene can lead to the hybrid surface plasmon–phonon polaritons (SPPPs). Compared with that between undoped Bi2Se3, Casimir friction can be enhanced by more than one order of magnitude due to the contribution of SPPPs. It is found that the chemical potential that can be used to modulate the optical characteristic of SPPPs plays an important role in Casimir friction. In addition, the Casimir friction between doped Bi2Se3 is also studied. The friction coefficient between doped Bi2Se3 can even be larger than that between graphene-covered undoped Bi2Se3 for suitable chemical potential due to the contribution of unusual electron surface states. The results obtained in this work are not only beneficial to the study of Casimir frictions but also extend the research ranges of topological insulators.

Near-field mapping of complex-valued wavevectors of in-plane hyperbolic phonon polaritons in α -MoO3

Hyperbolic phonon polaritons (HPhPs) in α-phase molybdenum trioxide (α-MoO3) have recently attracted significant attention. They propagate anisotropically along the flake of α-MoO3 and show anomalously concave wavefronts when being excited by a pointlike source. Such anisotropic propagation is governed by the anisotropic wavevectors of HPhPs, which have been studied in different works. However, extracting the complex-valued wavevectors of all HPhP modes directly from the observed anomalous wavefront remains elusive. Here, we theoretically and experimentally demonstrate that the complex-valued HPhP wavevectors can be accurately quantified by fitting the concave wavefront profiles with a modified damped sine-wave function in all allowed directions. To that end, HPhPs are launched by an infrared antenna on a thin flake of α-MoO3 and are imaged in real space by using scanning near-field optical microscope. From the recorded concave wavefronts of HPhPs, we have experimentally retraced both the real part and the imaginary part of polariton wavevectors along different propagation directions. Our results are of fundamental importance for analyzing the optical properties of HPhPs in α-MoO3, which can also be generic to other anisotropic optical/polaritonic systems.

Polarized Raman Scattering of In‐Plane Anisotropic Phonon Modes in α‑MoO3

AbstractThe recent discovery of in‐plane hyperbolic phonon polaritons in biaxial polar crystal—α‐MoO3, has aroused a huge research upsurge on anisotropic photonic quasiparticles in van der Waals materials. The physical origin of these highly anisotropic phonon polaritons is attributed to the strong anisotropy of crystal phonons. However, the orientation‐dependent phonon modes have not been fully understood from crystallographic point of view. This work systematically performs in situ angle‐resolved polarized Raman spectroscopy on α‐MoO3 to investigate its anisotropic photo‐phonon interactions. The sample orientation and Raman tensors are unambiguously determined under different polarization configurations. Specifically, the in‐plane anisotropic phonon modes of α‐MoO3 can be well characterized from the highly polarization‐dependent Raman‐active modes with periodic intensity variation. It can be concluded that these findings cannot only offer a better understanding of anisotropic phonon modes in α‐MoO3, but also provide a valuable reference for other anisotropic van der Waals crystals.

Hyperbolic phonon polaritons with positive and negative phase velocities in suspended α -MoO3

Sample suspension is a valuable method to improve the mechanical, thermal, electronic, and optical properties of low-dimensional materials. In terms of confined light-matter waves—the polaritons, sample suspension can elongate the wavelength of polaritons with a positive phase velocity. Previous work demonstrates a wavelength elongation of ∼10% for hyperbolic phonon polaritons (HPPs) in uniaxial crystals of hexagonal boron nitride (hBN). In this work, we report the alteration of HPPs in biaxial α-phase molybdenum trioxide (α-MoO3) by sample suspension. Our combined infrared nano-imaging experiments and electromagnetic theory reveal a wavelength elongation &amp;gt; 60% and a propagation length increase &amp;gt; 140%, due to the simultaneous wavelength elongation and dissipation elimination in the suspended specimen. We have also examined HPPs in α-MoO3 with a negative phase velocity. The sample suspension shortens the HPP wavelength and simultaneously reduces the dissipation due to the unique permittivity tensor. The HPPs with improved figures of merits in the suspended specimen may be developed for nano-polaritonic circuits, biochemical sensing, emission engineering, and energy transfer.