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Researchers demonstrate graphene plasmon edge modes at infrared wavelengths. Such modes may offer additional electromagnetic field confinement compared with conventional sheet modes. Plasmons in graphene nanoresonators have many potential applications in photonics and optoelectronics, including room-temperature infrared and terahertz photodetectors, sensors, reflect arrays or modulators 1 , 2 , 3 , 4 , 5 , 6 , 7 . The development of efficient devices will critically depend on precise knowledge and control of the plasmonic modes. Here, we use near-field microscopy 8 , 9 , 10 , 11 between λ 0  = 10–12 μm to excite and image plasmons in tailored disk and rectangular graphene nanoresonators, and observe a rich variety of coexisting Fabry–Perot modes. Disentangling them by a theoretical analysis allows the identification of sheet and edge plasmons, the latter exhibiting mode volumes as small as 10 −8 λ 0 3 . By measuring the dispersion of the edge plasmons we corroborate their superior confinement compared with sheet plasmons, which among others could be applied for efficient 1D coupling of quantum emitters 12 . Our understanding of graphene plasmon images is a key to unprecedented in-depth analysis and verification of plasmonic functionalities in future flatland technologies.

Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy has been recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS 3 and h-BN crystals. This issue inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Waals interaction. To do this, we made correlative far- and near-field characterizations validated by first-principle calculations that reveal a huge birefringence of 1.5 in the infrared and 3 in the visible light for MoS 2 . Our findings demonstrate that this remarkable anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics. Optical anisotropy in a broad spectral range is pivotal to efficient light manipulation. Here, the authors measure a birefringence of 1.5 in the infrared range and 3 in the visible light for MoS 2 .

Graphene plasmons promise unique possibilities for controlling light in nanoscale devices and for merging optics with electronics. We developed a versatile platform technology based on resonant optical antennas and conductivity patterns for launching and control of propagating graphene plasmons, an essential step for the development of graphene plasmonic circuits. We launched and focused infrared graphene plasmons with geometrically tailored antennas and observed how they refracted when passing through a two-dimensional conductivity pattern, here a prism-shaped bilayer. To that end, we directly mapped the graphene plasmon wavefronts by means of an imaging method that will be useful in testing future design concepts for nanoscale graphene plasmonic circuits and devices.

Polaritons in layered materials—including van der Waals materials—exhibit hyperbolic dispersion and strong field confinement, which makes them highly attractive for applications including optical nanofocusing, sensing and control of spontaneous emission. Here we report a near-field study of polaritonic Fabry–Perot resonances in linear antennas made of a hyperbolic material. Specifically, we study hyperbolic phonon–polaritons in rectangular waveguide antennas made of hexagonal boron nitride (h-BN, a prototypical van der Waals crystal). Infrared nanospectroscopy and nanoimaging experiments reveal sharp resonances with large quality factors around 100, exhibiting atypical modal near-field patterns that have no analogue in conventional linear antennas. By performing a detailed mode analysis, we can assign the antenna resonances to a single waveguide mode originating from the hybridization of hyperbolic surface phonon–polaritons (Dyakonov polaritons) that propagate along the edges of the h-BN waveguide. Our work establishes the basis for the understanding and design of linear waveguides, resonators, sensors and metasurface elements based on hyperbolic materials and metamaterials. Here, the authors report a near-field study of hyperbolic phonon polaritons in linear antennas made of hexagonal boron nitride. Infrared nanospectroscopy and nanoimaging experiments reveal sharp Fabry-Perot resonances with large quality factors, exhibiting atypical modal behaviour.

Launching and manipulation of polaritons in van der Waals materials offers novel opportunities for field-enhanced molecular spectroscopy and photodetection, among other applications. Particularly, the highly confined hyperbolic phonon polaritons (HPhPs) in h-BN slabs attract growing interest for their capability of guiding light at the nanoscale. An efficient coupling between free space photons and HPhPs is, however, hampered by their large momentum mismatch. Here, we show —by far-field infrared spectroscopy, infrared nanoimaging and numerical simulations— that resonant metallic antennas can efficiently launch HPhPs in thin h-BN slabs. Despite the strong hybridization of HPhPs in the h-BN slab and Fabry-Pérot plasmonic resonances in the metal antenna, the efficiency of launching propagating HPhPs in h-BN by resonant antennas exceeds significantly that of the non-resonant ones. Our results provide fundamental insights into the launching of HPhPs in thin polar slabs by resonant plasmonic antennas, which will be crucial for phonon-polariton based nanophotonic devices. Momentum mismatch prevents efficient coupling between free space photons and hyperbolic phonon polaritons. The authors show, using far-field infrared spectroscopy, infrared nanoimaging and numerical simulations, that resonant metallic antennas can efficiently launch hyperbolic phonon polaritons in thin h-BN slabs.

Refraction between isotropic media is characterized by light bending towards the normal to the boundary when passing from a low- to a high-refractive-index medium. However, refraction between anisotropic media is a more exotic phenomenon which remains barely investigated, particularly at the nanoscale. Here, we visualize and comprehensively study the general case of refraction of electromagnetic waves between two strongly anisotropic (hyperbolic) media, and we do it with the use of nanoscale-confined polaritons in a natural medium: α-MoO 3 . The refracted polaritons exhibit non-intuitive directions of propagation as they traverse planar nanoprisms, enabling to unveil an exotic optical effect: bending-free refraction. Furthermore, we develop an in-plane refractive hyperlens, yielding foci as small as λ p /6, being λ p the polariton wavelength (λ 0 /50 compared to the wavelength of free-space light). Our results set the grounds for planar nano-optics in strongly anisotropic media, with potential for effective control of the flow of energy at the nanoscale. Refraction between anisotropic media is still an unexplored phenomenon. Here, the authors investigate the propagation of hyperbolic phonon polaritons traversing α-MoO3 nanoprisms, showing a bending-free refraction effect and sub-diffractional focusing with foci size as small as 1/50 of the light wavelength in free space.

The emergence of a topological transition of the polaritonic dispersion in twisted bilayers of anisotropic van der Waals materials at a given twist angle—the photonic magic angle—results in the diffractionless propagation of polaritons with deep-subwavelength resolution. This type of propagation, generally referred to as canalization, holds promise for the control of light at the nanoscale. However, the existence of a single photonic magic angle hinders such control since the canalization direction in twisted bilayers is unique and fixed for each incident frequency. Here we overcome this limitation by demonstrating multiple spectrally robust photonic magic angles in reconfigurable twisted α-phase molybdenum trioxide (α-MoO3) trilayers. We show that canalization of polaritons can be programmed at will along any desired in-plane direction in a single device with broad spectral ranges. These findings open the door for nanophotonics applications where on-demand control is crucial, such as thermal management, nanoimaging or entanglement of quantum emitters.The direction of polariton canalization—its diffractionless propagation—in twisted bilayers at the magic angle is hindered by the lack of multiple magic angles. By controlling the twist angles between three α-MoO3 layers, reconfigurable and spectrally robust polariton canalization along any in-plane direction is demonstrated.

Photonic crystals (PCs) are periodically patterned dielectrics providing opportunities to shape and slow down the light for processing of optical signals, lasing and spontaneous emission control. Unit cells of conventional PCs are comparable to the wavelength of light and are not suitable for subwavelength scale applications. We engineer a nanoscale hole array in a van der Waals material (h-BN) supporting ultra-confined phonon polaritons (PhPs)—atomic lattice vibrations coupled to electromagnetic fields. Such a hole array represents a polaritonic crystal for mid-infrared frequencies having a unit cell volume of 10 -5 λ 0 3  (with λ 0 being the free-space wavelength), where PhPs form ultra-confined Bloch modes with a remarkably flat dispersion band. The latter leads to both angle- and polarization-independent sharp Bragg resonances, as verified by far-field spectroscopy and near-field optical microscopy. Our findings could lead to novel miniaturized angle- and polarization-independent infrared narrow-band couplers, absorbers and thermal emitters based on van der Waals materials and other thin polar materials. Photonic crystals can steer, shape, and sculpture the flow of photons. Here, the author fabricate a deep-subwavelength photonic crystal slab that supports ultra-confined phonon polaritons, by patterning a nanoscale hole array in h-BN.

Mid-infrared radiation allows the analysis of a wide range of different material properties, including chemical composition and the structure of matter (Gutberlet in Science 324:1545, 2009, Li in Nature Phys. 4:532, 2008). Infrared spectroscopy is therefore an essential analytical tool in many sciences and technologies. The diffraction limit, however, challenges the study of individual molecules and nanostructures, as well as the development of highly integrated mid-infrared optical devices (Soref in Nature Photon. 4:495, 2010). Here, we experimentally demonstrate mid-infrared nanofocusing by propagating a mid-infrared surface wave along a tapered two-wire transmission line. The tapering results in a compression of the electromagnetic energy carried by the surface wave. By using infrared vector near-field microscopy (Schnell et al. in Nano Lett. 10:3524, 2010, Olmon in Phys. Rev. Lett. 105:167403, 2010), we directly visualize the evolution of the energy compression into a nanoscale confined infrared spot with a diameter of 60 nm ( λ /150) at the taper apex. Our work opens the way to the development of chemical and biological sensing tools based on infrared surface waves, including miniaturized spectrometers and lab-on-a-chip integrated (bio)sensors. Using a tapered two-wire transmission line, researchers experimentally focus mid-infrared energy to a nanoscale confined spot with a diameter of 60 nm at the taper apex.

Light scattering at nanoparticles and molecules can be dramatically enhanced in the 'hot spots' of optical antennas, where the incident light is highly concentrated. Although this effect is widely applied in surface-enhanced optical sensing, spectroscopy and microscopy, the underlying electromagnetic mechanism of the signal enhancement is challenging to trace experimentally. Here we study elastically scattered light from an individual object located in the well-defined hot spot of single antennas, as a new approach to resolve the role of the antenna in the scattering process. We provide experimental evidence that the intensity elastically scattered off the object scales with the fourth power of the local field enhancement provided by the antenna, and that the underlying electromagnetic mechanism is identical to the one commonly accepted in surface-enhanced Raman scattering. We also measure the phase shift of the scattered light, which provides a novel and unambiguous fingerprint of surface-enhanced light scattering. Light scattering from nanoscale objects can be dramatically enhanced in the proximity of optical antennas. Here, by studying the amplitude and phase of the light scattered from a tip located at the hot spot of an antenna, the underlying electromagnetic mechanism of this enhancement is resolved.