Graphene Nano-Optics in the Terahertz Gap

Graphene nano-optics at terahertz (THz) frequencies (ν) is theoretically anticipated to feature extraordinary effects. However, interrogating such phenomena is nontrivial, since the atomically thin graphene dimensionally mismatches the THz radiation wavelength reaching hundreds of micrometers. Greater challenges happen in the THz gap (0.1-10 THz) wherein light sources are scarce. To surpass these barriers, we use a nanoscope illuminated by a highly brilliant and tunable free-electron laser to image the graphene nano-optical response from 1.5 to 6.0 THz. For ν < 2 THz, we observe a metal-like behavior of graphene, which screens optical fields akin to noble metals, since this excitation range approaches its charge relaxation frequency. At 3.8 THz, plasmonic resonances cause a field-enhancement effect (FEE) that improves the graphene imaging power. Moreover, we show that the metallic behavior and the FEE are tunable upon electrical doping, thus providing further control of these graphene nano-optical properties in the THz gap.

Van der Waals materials as dielectric layers for tailoring the near-field photonic response of surfaces

Epsilon near-zero photonics and surface polariton nanophotonics have become major fields within optics, leading to unusual and enhanced light-matter interaction. Specific dielectric responses are required in both cases, which can be achieved, e.g., via operation near a material's electronic or phononic resonance. However, this condition restricts operation to a specific, narrow frequency range. It has been shown that using a thin dielectric layer can adjust the dielectric response of a surface and, therefore, the operating frequency for achieving specific photonic excitations. Here, we show that a surface's optical properties can be tuned via the deposition/transference of ultra-thin layered van der Waals (vdW) crystals, the thicknesses of which can easily be adjusted to provide the desired response. In particular, we experimentally and theoretically show that the surface phonon resonance of a silica surface can be tuned by ∼50 cm^-1 through the simple deposition of nanometer-thick exfoliated flakes of black phosphorus. The surface properties were probed by infrared nanospectroscopy, and results show a close agreement with the theory. The black phosphorus-silica layered structure effectively acts as a surface with a tunable effective dielectric constant that presents an infrared response dependent on the black phosphorus thickness. In contrast, with a lower dielectric constant, hexagonal boron nitride does not significantly tune the silica surface phonon polariton. Our approach also applies to epsilon near-zero surfaces, as theoretically shown, and to polaritonic surfaces operating at other optical ranges.

Sub-diffractional cavity modes of terahertz hyperbolic phonon polaritons in tin oxide

Hyperbolic phonon polaritons have recently attracted considerable attention in nanophotonics mostly due to their intrinsic strong electromagnetic field confinement, ultraslow polariton group velocities, and long lifetimes. Here we introduce tin oxide (SnO2) nanobelts as a photonic platform for the transport of surface and volume phonon polaritons in the mid- to far-infrared frequency range. This report brings a comprehensive description of the polaritonic properties of SnO2 as a nanometer-sized dielectric and also as an engineered material in the form of a waveguide. By combining accelerator-based IR-THz sources (synchrotron and free-electron laser) with s-SNOM, we employed nanoscale far-infrared hyper-spectral-imaging to uncover a Fabry-Perot cavity mechanism in SnO2 nanobelts via direct detection of phonon-polariton standing waves. Our experimental findings are accurately supported by notable convergence between theory and numerical simulations. Thus, the SnO2 is confirmed as a natural hyperbolic material with unique photonic properties essential for future applications involving subdiffractional light traffic and detection in the far-infrared range.

Dipole modelling for a robust description of subdiffractional polariton waves

The nanophotonics of van der Waals (vdW) materials relies critically on the electromagnetic properties of polaritons defined on sub-diffraction length scales. Here, we use a full electromagnetic Hertzian dipole antenna (HDA) model to describe the hyperbolic phonon polaritons (HP²s) in vdW crystals of hexagonal boron nitride (hBN) on a gold surface. The HP² waves are investigated by broadband synchrotron infrared nanospectroscopy (SINS) which covers the type I and type II hyperbolic bands simultaneously. Basically, polariton waves, observed by SINS, are assigned to the resultant electric field from the summation over the irradiated electric fields of dipoles distributed along the crystal edge and at the tip location and a non-propagating field. The values of polariton momenta and damping extracted from the HDA model present excellent agreement with theoretical predictions. Our analysis shows that the confinement factor of type I HP²s exceeds that of the type II ones by up to a factor of 3. We extract anti-parallel group velocities (v_g) for type I (v_g,typeI = -0.005c, c is the light velocity in a vacuum) in relation to type II (v_g,typeII = 0.05c) polaritonic pulses, with lifetimes of ∼0.6 ps and ∼0.3 ps, respectively. Furthermore, by incorporating consolidated optical-near field theory into the HDA model, we simulate real-space images of polaritonic standing waves for hBN crystals of different shapes. This approach reproduces the experiments with a minimal computational cost. Thus, it is demonstrated that the HDA modelling self-consistently explains the measured complex-valued polariton near-field, while being a general approach applicable to other polariton types, like plasmon- and exciton-polaritons, active in the wide range of vdW materials.

Anisotropic Flow Control and Gate Modulation of Hybrid Phonon-Polaritons

Light-matter interaction in two-dimensional photonic or phononic materials allows for the confinement and manipulation of free-space radiation at sub-wavelength scales. Most notably, the van der Waals heterostructure composed of graphene (G) and hexagonal boron nitride (hBN) provides for gate-tunable hybrid hyperbolic plasmon phonon-polaritons (HP³). Here, we present the anisotropic flow control and gate-voltage modulation of HP³ modes in G-hBN on an air-Au microstructured substrate. Using broadband infrared synchrotron radiation coupled to a scattering-type near-field optical microscope, we launch HP³ waves in both hBN Reststrahlen bands and observe directional propagation across in-plane heterointerfaces created at the air-Au junction. The HP³ hybridization is modulated by varying the gate voltage between graphene and Au. This modifies the coupling of continuum graphene plasmons with the discrete hBN hyperbolic phonon polaritons, which is described by an extended Fano model. This work represents the first demonstration of the control of polariton propagation, introducing a theoretical approach to describe the breaking of the reflection and transmission symmetry for HP³ modes. Our findings augment the degree of control of polaritons in G-hBN and related hyperbolic metamaterial nanostructures, bringing new opportunities for on-chip nano-optics communication and computing.

Low-aberration beamline optics for synchrotron infrared nanospectroscopy

Synchrotron infrared nanospectroscopy is a recently developed technique that enables new possibilities in the broadband chemical analysis of materials in the nanoscale, far beyond the diffraction limit in this frequency domain. Synchrotron infrared ports have exploited mainly the high brightness advantage provided by electron storage rings across the whole infrared range. However, optical aberrations in the beam produced by the source depth of bending magnet emission at large angles prevent infrared nanospectroscopy to reach its maximum capability. In this work we present a low-aberration optical layout specially designed and constructed for a dedicated synchrotron infrared nanospectroscopy beamline. We report excellent agreement between simulated beam profiles (from standard wave propagation and raytracing optics simulations) with experimental measurements. We report an important improvement in the infrared nanospectroscopy experiment related to the improved beamline optics. Finally, we demonstrate the performance of the nanospectroscopy endstation by measuring a hyperspectral image of a polar material and we evaluate the setup sensitivity by measuring ultra-thin polymer films down to 6 nm thick.

Infrared Vibrational Nanospectroscopy by Self-Referenced Interferometry

Infrared vibrational scattering scanning near-field optical microscopy (s-SNOM) has emerged as a new frontier in imaging science due to its potential to provide nanoscale spatially resolved chemical spectroscopy for the investigation of molecular, soft-matter, and biological materials. As a phase-sensitive technique able to yield the full complex dielectric function of materials, different interferometric schemes have been developed involving asymmetric interferometry between sample and reference arms. In this work, we take advantage of a greatly simplified symmetric geometry that uses the spatially coherent background scattered light from within the confocal sample volume as a reference field for signal amplification in both self-homodyne and self-heterodyne interferometry. On the basis of a simple model for tip-sample scattering and interferometric detection, we demonstrate the measurement of the vibrational response of molecular materials in good agreement with established values. In addition to a compact design, enhanced signal levels, and a reduced sensitivity to fluctuations and drift, including those from the light source, self-referenced interferometry brings benefits for routine s-SNOM chemical spectroscopy, remaining robust even under a wide range of challenging experimental environments.

Graphene/h-BN plasmon-phonon coupling and plasmon delocalization observed by infrared nano-spectroscopy

We observed the coupling of graphene Dirac plasmons with different surfaces using scattering-type scanning near-field optical microscopy integrated into a mid-infrared synchrotron-based beamline. A systematic investigation of a graphene/hexagonal boron nitride (h-BN) heterostructure is carried out and compared with the well-known graphene/SiO2 heterostructure. Broadband infrared scanning near-field optical microscopy imaging is able to distinguish between the graphene/h-BN and the graphene/SiO2 heterostructure as well as differentiate between graphene stacks with different numbers of layers. Based on synchrotron infrared nanospectroscopy experiments, we observe a coupling of surface plasmons of graphene and phonon polaritons of h-BN (SPPP). An enhancement of the optical band at 817 cm(-1) is observed at graphene/h-BN heterostructures as a result of hybridization between graphene plasmons and longitudinal optical phonons of h-BN. Furthermore, longitudinal optical h-BN modes are preserved on suspended graphene regions (bubbles) where the graphene sheet is tens of nanometers away from the surface while the amplitude of transverse optical h-BN modes decrease.