Steering and cloaking of hyperbolic polaritons at deep-subwavelength scales

Abstract Polaritons are well-established carriers of light, electrical signals, and even heat at the nanoscale in the setting of on-chip devices. However, the goal of achieving practical polaritonic manipulation over small distances deeply below the light diffraction limit remains elusive. Here, we implement nanoscale polaritonic in-plane steering and cloaking in a low-loss atomically layered van der Waals (vdW) insulator, α-MoO3, comprising building blocks of customizable stacked and assembled structures. Each block contributes specific characteristics that allow us to steer polaritons along the desired trajectories. Our results introduce a natural materials-based approach for the comprehensive manipulation of nanoscale optical fields, advancing research in the vdW polaritonics domain and on-chip nanophotonic circuits

Gate-tunable negative refraction of mid-infrared polaritons

| openaire: EC/H2020/820423/EU//S2QUIP | openaire: EC/H2020/834742/EU//ATOP | openaire: EC/H2020/965124/EU//FEMTOCHIPNegative refraction provides a platform to manipulate mid-infrared and terahertz radiation for molecular sensing and thermal emission applications. However, its implementation based on metamaterials and plasmonic media presents challenges with optical losses, limited spatial confinement, and lack of active tunability in this spectral range. We demonstrate gate-tunable negative refraction at mid-infrared frequencies using hybrid topological polaritons in van der Waals heterostructures. Specifically, we visualize wide-angle negatively refracted polaritons in a-MoO 3 films partially decorated with graphene, undergoing reversible planar nanoscale focusing. Our atomically thick heterostructures weaken scattering losses at the interface while enabling an actively tunable transition of normal to negative refraction through electrical gating. We propose polaritonic negative refraction as a promising platform for infrared applications such as electrically tunable super-resolution imaging, nanoscale thermal manipulation, enhanced molecular sensing, and on-chip optical circuitry.Peer reviewe

Nanocone-shaped Carbon Nanotubes Field Emitter Array Fabricated by Laser Ablation

The nanocone-shaped carbon nanotubes field-emitter array (NCNA) is a near-ideal field-emitter array that combines the advantages of geometry and material. In contrast to previous methods of field-emitter array, laser ablation is a low-cost and clean method that does not require any photolithography or wet chemistry. However, nanocone shapes are hard to achieve through laser ablation due to the micrometer-scale focusing spot. Here, we develop an ultraviolet (UV) laser beam patterning technique that is capable of reliably realizing NCNA with a cone-tip radius of ≈300 nm, utilizing optimized beam focusing and unique carbon nanotube–light interaction properties. The patterned array provided smaller turn-on fields (reduced from 2.6 to 1.6 V/μm) in emitters and supported a higher (increased from 10 to 140 mA/cm(2)) and more stable emission than their unpatterned counterparts. The present technique may be widely applied in the fabrication of high-performance CNTs field-emitter arrays

Tunable Planar Focusing Based on Hyperbolic Phonon Polaritons in alpha-MoO3

| openaire: EC/H2020/820423/EU//S2QUIP | openaire: EC/H2020/965124/EU//FEMTOCHIP | openaire: EC/H2020/872049/EU//IPN-Bio | openaire: EC/H2020/834742/EU//834742 Funding Information: The authors acknowledge Dr. Pablo Alonso‐González, Dr. Javier Martín‐Sánchez, and Dr. Jiahua Duan (Departamento de Física, Universidad de Oviedo) for valuable discussions and constructive comments, and acknowledge Nanofab Lab @ NCNST for helping with sample fabrication. This work was supported by the National Key Research and Development Program of China (Grant No. 2020YFB2205701), the National Natural Science Foundation of China (Grant Nos. 51902065, 52172139, 51925203, U2032206, 52072083, and 51972072), Beijing Municipal Natural Science Foundation (Grant No. 2202062), and Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos. XDB36000000 and XDB30000000). F.J.G.A. acknowledges the ERC (Advanced Grant 789104‐eNANO), the Spanish MICINN (PID2020‐112625GB‐I00 and SEV2015‐0522), and the CAS President's International Fellowship Initiative (PIFI) for 2021. Z.P.S. acknowledges the Academy of Finland (Grant Nos. 314810, 333982, 336144 and 336818), The Business Finland (ALDEL), the Academy of Finland Flagship Programme (320167,PREIN), the European Union's Horizon 2020 research and innovation program (820423,S2QUIP; 965124, FEMTOCHIP), the EU H2020‐MSCA‐RISE‐872049 (IPN‐Bio), and the ERC (834742). Publisher Copyright: © 2022 Wiley-VCH GmbH.Manipulation of the propagation and energy-transport characteristics of sub-wavelength 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 alpha-MoO3. Specifically, an unprecedented change of effective focal length of polariton waves from 0.7 to 7.4 mu m 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 alpha-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 alpha-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.Peer reviewe

Doping-driven topological polaritons in graphene/α-MoO3 heterostructures

Funding Information: We acknowledge P. Alonso-González and J. Duan (Departamento de Física, Universidad de Oviedo) for valuable discussions and constructive comments. This work was supported by the National Key Research and Development Program of China (grant no. 2021YFA1201500, to Q.D.; 2020YFB2205701, to H.H.), the National Natural Science Foundation of China (grant nos. 51902065, 52172139 to H.H.; 51925203, U2032206, 52072083 and 51972072, to Q.D.), Beijing Municipal Natural Science Foundation (grant no. 2202062, to H.H.) and the Strategic Priority Research Program of Chinese Academy of Sciences (grant nos. XDB30020100 and XDB30000000, to Q.D.). F.J.G.d.A. acknowledges the ERC (Advanced grant no. 789104-eNANO), the Spanish MICINN (PID2020-112625GB-I00 and SEV2015-0522) and the CAS President’s International Fellowship Initiative for 2021. S.F. acknowledges the support of the US Department of Energy (grant no. DE-FG02-07ER46426). Z.S. acknowledges the Academy of Finland (grant nos. 314810, 333982, 336144 and 336818), The Business Finland (ALDEL), the Academy of Finland Flagship Programme (320167, PREIN), the European Union’s Horizon 2020 research and innovation program (820423, S2QUIP and 965124, FEMTOCHIP), the EU H2020-MSCA-RISE-872049 (IPN-Bio) and the ERC (834742). P.L. acknowledges the National Natural Science Foundation of China (grant no. 62075070). | openaire: EC/H2020/820423/EU//S2QUIP | openaire: EC/H2020/834742/EU//ATOP | openaire: EC/H2020/965124/EU//FEMTOCHIPControl over charge carrier density provides an efficient way to trigger phase transitions and modulate the optoelectronic properties of materials. This approach can also be used to induce topological transitions in the optical response of photonic systems. Here we report a topological transition in the isofrequency dispersion contours of hybrid polaritons supported by a two-dimensional heterostructure consisting of graphene and α-phase molybdenum trioxide. By chemically changing the doping level of graphene, we observed that the topology of polariton isofrequency surfaces transforms from open to closed shapes as a result of doping-dependent polariton hybridization. Moreover, when the substrate was changed, the dispersion contour became dominated by flat profiles at the topological transition, thus supporting tunable diffractionless polariton propagation and providing local control over the optical contour topology. We achieved subwavelength focusing of polaritons down to 4.8% of the free-space light wavelength by using a 1.5-μm-wide silica substrate as an in-plane lens. Our findings could lead to on-chip applications in nanoimaging, optical sensing and manipulation of energy transfer at the nanoscale.Peer reviewe

Active control of micrometer plasmon propagation in suspended graphene

| openaire: EC/H2020/820423/EU//S2QUIP | openaire: EC/H2020/834742/EU//ATOP | openaire: EC/H2020/965124/EU//FEMTOCHIPDue to the two-dimensional character of graphene, the plasmons sustained by this material have been invariably studied in supported samples so far. The substrate provides stability for graphene but often causes undesired interactions (such as dielectric losses, phonon hybridization, and impurity scattering) that compromise the quality and limit the intrinsic flexibility of graphene plasmons. Here, we demonstrate the visualization of plasmons in suspended graphene at room temperature, exhibiting high-quality factor Q~33 and long propagation length > 3 μm. We introduce the graphene suspension height as an effective plasmonic tuning knob that enables in situ change of the dielectric environment and substantially modulates the plasmon wavelength, propagation length, and group velocity. Such active control of micrometer plasmon propagation facilitates near-unity-order modulation of nanoscale energy flow that serves as a plasmonic switch with an on-off ratio above 14. The suspended graphene plasmons possess long propagation length, high tunability, and controllable energy transmission simultaneously, opening up broad horizons for application in nano-photonic devices.Peer reviewe