Saül Vélez

Controlling graphene plasmons with resonant metal antennas and spatial conductivity patterns.

A controlled launch for plasmons To create nanophotonic devices, engineers must combine large-scale optics with tiny nanoelectronics. Plasmons, the collective light-induced excitations of electrons at a metals surface, can bridge that difference in size scales. Alonso-Gonzalez et al. placed structured gold “antennas” on top of a graphene layer to launch and propagate plasmonic excitations into the graphene. By carefully designing the antennas, the researchers could engineer the wavefronts of the plasmons and control the direction of propagation. This approach illustrates a versatile approach for the development of nanophotonics. Science, this issue p. 1369 Structured gold antennas are used to launch plasmons into graphene, engineer their wavefronts, and control their propagation. 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.

Spin Hall magnetoresistance at Pt/CoFe2O4 interfaces and texture effects

We report magnetoresistance measurements on thin Pt bars grown on epitaxial (001) and (111) CoFe2O4 (CFO) ferrimagnetic insulating films. The results can be described in terms of the recently discovered spin Hall magnetoresistance (SMR). The magnitude of the SMR depends on the interface preparation conditions, being optimal when the Pt/CFO samples are prepared in situ, in a single process. The spin-mixing interface conductance, the key parameter governing SMR and other relevant spin-dependent phenomena, such as spin pumping or spin Seebeck effect, is found to be different depending on the crystallographic orientation of CFO, highlighting the role of the composition and density of magnetic ions at the interface on spin mixing.

Hanle Magnetoresistance in Thin Metal Films with Strong Spin-Orbit Coupling.

We report measurements of a new type of magnetoresistance in Pt and Ta thin films. The spin accumulation created at the surfaces of the film by the spin Hall effect decreases in a magnetic field because of the Hanle effect, resulting in an increase of the electrical resistance as predicted by Dyakonov [Phys. Rev. Lett. 99, 126601 (2007)]. The angular dependence of this magnetoresistance resembles the recently discovered spin Hall magnetoresistance in Pt/Y(3)Fe(5)O(12) bilayers, although the presence of a ferromagnetic insulator is not required. We show that this Hanle magnetoresistance is an alternative simple way to quantitatively study the coupling between charge and spin currents in metals with strong spin-orbit coupling.

Acoustic terahertz graphene plasmons revealed by photocurrent nanoscopy

Terahertz (THz) fields are widely used for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, λp, compared with the photon wavelength, λ0, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far-only GPs with nearly free-space wavelengths have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (λp ≈ λ0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.

Nanofocusing of Hyperbolic Phonon Polaritons in a Tapered Boron Nitride Slab

Nanofocusing of light offers new technological opportunities for the delivery and manipulation of electromagnetic fields at subdiffraction-limited length scales. Here, we show that hyperbolic phonon polariton (HPP) modes in the mid-infrared as supported by a hexagonal boron nitride (h-BN) slab can be nanofocused (i.e., both field enhanced and wavelength compressed) by propagation along a vertical taper. Via numerical simulations, we demonstrate that field enhancement factors of 90—for steep tapers—and wavelength compression of more than an order of magnitude—for adiabatic tapers—can be expected. Employing scattering-type scanning near-field optical microscopy, we provide, for the first time, proof-of-principle experimental evidence of a significant HPP wavelength compression. We expect these functionalities to provide diverse applications, from biosensing and nonlinear optics to optical circuitry.

Optical Nanoimaging of Hyperbolic Surface Polaritons at the Edges of van der Waals Materials

Hyperbolic polaritons in van der Waals (vdW) materials recently attract a lot of attention, owing to their strong electromagnetic field confinement, ultraslow group velocities, and long lifetimes. Typically, volume-confined hyperbolic polaritons (HPs) are studied. Here we show the first near-field optical images of hyperbolic surface polaritons (HSPs), which are confined and guided at the edges of thin flakes of a vdW material. To that end, we applied scattering-type scanning near-field optical microscopy (s-SNOM) for launching and real-space nanoimaging of hyperbolic surface phonon polariton modes on a hexagonal boron nitride (h-BN) flake. Our imaging data reveal that the fundamental HSP mode exhibits a stronger field confinement (shorter wavelength), smaller group velocities, and nearly identical lifetimes, as compared to the fundamental HP mode of the same h-BN flake. Our experimental data, corroborated by theory, establish a solid basis for future studies and applications of HPs and HSPs in vdW materials.

Nanoimaging of resonating hyperbolic polaritons in linear boron nitride antennas

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.

Probing low-energy hyperbolic polaritons in van der Waals crystals with an electron microscope

Van der Waals materials exhibit intriguing structural, electronic, and photonic properties. Electron energy loss spectroscopy within scanning transmission electron microscopy allows for nanoscale mapping of such properties. However, its detection is typically limited to energy losses in the eV range—too large for probing low-energy excitations such as phonons or mid-infrared plasmons. Here, we adapt a conventional instrument to probe energy loss down to 100 meV, and map phononic states in hexagonal boron nitride, a representative van der Waals material. The boron nitride spectra depend on the flake thickness and on the distance of the electron beam to the flake edges. To explain these observations, we developed a classical response theory that describes the interaction of fast electrons with (anisotropic) van der Waals slabs, revealing that the electron energy loss is dominated by excitation of hyperbolic phonon polaritons, and not of bulk phonons as often reported. Thus, our work is of fundamental importance for interpreting future low-energy loss spectra of van der Waals materials.Here the authors adapt a STEM-EELS system to probe energy loss down to 100 meV, and apply it to map phononic states in hexagonal boron nitride, revealing that the electron loss is dominated by hyperbolic phonon polaritons.

Spin Hall Magnetoresistance as a Probe for Surface Magnetization in Pt=CoFe2O4 Bilayers

This work is supported by the European Union under the NMP project (263104-HINTS) and the European Research Council (257654-SPINTROS), by the Spanish MINECO (MAT2012-37638, MAT2015-65159-R, MAT2014-56063- C2-1R, and SEV-2015-0496), by the Basque Government (PC2015-1-01), and by the Catalan Government (2014 SGR 734). M. I. and E. S. acknowledge the Basque Government and the Spanish MECD, respectively, for a Ph.D. fellowship (BFI-2011-106 and FPU14/03102). J. F. acknowledges stimulating discussions with Xavier Marti.

Infrared hyperbolic metasurface based on nanostructured van der Waals materials

Patterning a hyperbolic metasurface Structured metasurfaces potentially enable the control of the propagation direction of excitations on the materials surface. However, the high losses associated with the materials used to date has led to relatively short lifetimes for the excitations. Li et al. patterned a subwavelength grating into a layer of hexagonal boron nitride (hBN) and found that the lifetime and propagation length of the excitations could be much longer. Direct imaging of the polariton excitations illustrates that hBN can be a viable platform for nanophotonic circuits. Science, this issue p. 892 Directional propagation of polaritons is observed in a patterned layer of hexagonal boron nitride. Metasurfaces with strongly anisotropic optical properties can support deep subwavelength-scale confined electromagnetic waves (polaritons), which promise opportunities for controlling light in photonic and optoelectronic applications. We developed a mid-infrared hyperbolic metasurface by nanostructuring a thin layer of hexagonal boron nitride that supports deep subwavelength-scale phonon polaritons that propagate with in-plane hyperbolic dispersion. By applying an infrared nanoimaging technique, we visualize the concave (anomalous) wavefronts of a diverging polariton beam, which represent a landmark feature of hyperbolic polaritons. The results illustrate how near-field microscopy can be applied to reveal the exotic wavefronts of polaritons in anisotropic materials and demonstrate that nanostructured van der Waals materials can form a highly variable and compact platform for hyperbolic infrared metasurface devices and circuits.