arXiv subjects

R. Hillenbrand

Publications and source records attributed to R. Hillenbrand.

12 recordsLinked to original sources

arXiv2023

Dual-band coupling between nanoscale polaritons and vibrational and electronic excitations in molecules

Strong coupling (SC) between light and matter excitations such as excitons and molecular vibrations bear intriguing potential for controlling chemical reactivity, conductivity or photoluminescence. So far, SC has been typically achieved either between mid-infrared (mid-IR) light and molecular vibrations or between visible light and excitons. Achieving SC simultaneously in both frequency bands may open unexplored pathways for manipulating material properties. Here, we introduce a polaritonic nanoresonator (formed by h-BN layers placed on Al ribbons) hosting surface plasmon polaritons (SPPs) at visible frequencies and phonon polaritons (PhPs) at mid-IR frequencies, which simultaneously couple to excitons and atomic vibration in an adjacent molecular layer (CoPc). Employing near-field optical nanoscopy, we first demonstrate the co-localization of strongly confined near-fields at both visible and mid-IR frequencies. After covering the nanoresonator structure with a layer of CoPc molecules, we observe clear mode splittings in both frequency ranges by far-field transmission spectroscopy, unambiguously revealing simultaneous SPP-exciton and PhP-vibron coupling. Dual-band SC may be exploited for manipulating the coupling between excitons and molecular vibrations in future optoelectronics, nanophotonics, and quantum information applications.

physics.optics↗

arXiv2021

Substrate matters: Surface-polariton enhanced infrared nanospectroscopy of molecular vibrations

Infrared nanospectroscopy based on Fourier transform infrared near-field spectroscopy (nano-FTIR) is an emerging nanoanalytical tool with large application potential for label-free mapping and identification of organic and inorganic materials with nanoscale spatial resolution. However, the detection of thin molecular layers and nanostructures on standard substrates is still challenged by weak signals. Here, we demonstrate a significant enhancement of nano-FTIR signals of a thin organic layer by exploiting polariton-resonant tip-substrate coupling and surface polariton illumination of the probing tip. When the molecular vibration matches the tip-substrate resonance, we achieve up to nearly one order of magnitude signal enhancement on a phonon-polaritonic quartz (c-SiO2) substrate, as compared to nano-FTIR spectra obtained on metal (Au) substrates, and up to two orders of magnitude when compared to the standard infrared spectroscopy substrate CaF2. Our results will be of critical importance for boosting nano-FTIR spectroscopy towards the routine detection of monolayers and single molecules.

physics.optics↗

arXiv2021

Enhanced Light-Matter Interaction in B-10 Monoisotopic Boron Nitride Infrared Nanoresonators

Phonon-polaritons, mixed excitations of light coupled to lattice vibrations (phonons), are emerging as a powerful platform for nanophotonic applications. This is because of their ability to concentrate light into extreme sub-wavelength scales and because of their longer phonon lifetimes than their plasmonic counterparts. In this work, the infrared properties of phonon-polaritonic nanoresonators made of monoisotopic B-10 hexagonal-boron nitride (h-BN) are explored, a material with increased phonon-polariton lifetimes compared to naturally abundant h-BN due to reduced photon scattering from randomly distributed isotopes. An average relative improvement of 50% in the nanoresonators Q factor is obtained with respect of nanoresonators made of naturally abundant h-BN. Moreover, the monoisotopic h-BN nano-ribbon arrays are used to sense nanometric-thick films of molecules, both through surface-enhanced absorption spectroscopy and refractive index sensing. In addition, strong coupling is achieved between a molecular vibration and the phonon-polariton resonance in monoisotopic h-BN ribbons.

physics.optics↗

arXiv2021

Planar nano-optics in anisotropic media: anomalous refraction and diffraction-free lensing of highly confined polaritons

As one of the most fundamental optical phenomena, 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, in anisotropic media, refraction is a much more exotic phenomenon. The most general case of refraction between two anisotropic media remains unexplored, particularly in natural media and at the nanoscale. Here, we visualize and comprehensively study refraction of electromagnetic waves between two strongly anisotropic (hyperbolic) media, and, importantly, we do it with the use of polaritons confined to the nanoscale in a low-loss natural medium, alpha-MoO3. Our images show refraction of polaritons under the general case in which both the direction of propagation and the wavevector are not collinear. As they traverse planar nanoprisms tailored in alpha-MoO3, refracted polaritons exhibit non-intuitive directions of propagation, enabling us to unveil an exotic optical effect: bending-free refraction. Furthermore, we succeed in developing the first in-plane refractive hyperlens, which yields foci as small as lamdap/6, being lamdap the polariton wavelength (lamda0/50 with respect to the wavelength of light in free space). Our results set the grounds for planar nano-optics in strongly anisotropic media, with potential for unprecedented control of the flow of energy at the nanoscale.

physics.optics↗

arXiv2018

Efficient coupling of light to graphene plasmons by compressing surface polaritons with tapered bulk materials

Graphene plasmons promise exciting nanophotonic and optoelectronic applications. Owing to their extremely short wavelengths, however, the efficient coupling of photons to graphene plasmons - critical for the development of future devices - can be challenging. Here, we propose and numerically demonstrate coupling between infrared photons and graphene plasmons by the compression of surface polaritons on tapered bulk slabs of both polar and doped- semiconductor materials. Propagation of the surface phonon polaritons (in SiC) and surface plasmon polaritons (in n-GaAs) along the tapered slabs compresses the polariton wavelengths from several micrometers to around 200 nm, which perfectly matches the wavelengths of graphene plasmons. The proposed coupling device allows for a 25% conversion of the incident photon energy into graphene plasmons and, therefore, could become an efficient route towards graphene plasmon circuitry.

physics.optics↗

arXiv2018

THz nanofocusing with cantilevered THz-resonant antenna tips

We developed THz-resonant scanning probe tips, yielding strongly enhanced and nanoscale confined THz near fields at their tip apex. The tips with length in the order of the THz wavelength (λ = 96.5 μm) were fabricated by focused ion beam (FIB) machining and attached to standard atomic force microscopy (AFM) cantilevers. Measurements of the near-field intensity at the very tip apex (25 nm radius) as a function of tip length, via graphene-based (thermoelectric) near-field detection, indicate their first and second order geometrical antenna resonances for tip length of 33 and 78 μm, respectively. On resonance, we find that the near-field intensity is enhanced by one order of magnitude compared to tips of 17 μm length (standard AFM tip length), which is corroborated by numerical simulations that further predict remarkable intensity enhancements of about 107 relative to the incident field. Because of the strong field enhancement and standard AFM operation of our tips, we envision manifold and straightforward future application in scattering-type THz near-field nanoscopy and THz photocurrent nanoimaging, nanoscale nonlinear THz imaging, or nanoscale control and manipulation of matter employing ultrastrong and ultrashort THz pulses.

physics.optics↗

arXiv2018

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

yperbolic polaritons in van der Waals 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 polarities, 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 stronger field confinement, 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.

physics.optics↗

arXiv2018

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 sub-diffraction limited length scales. Here, we show that hyperbolic phonon polarity,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 scatteringtype scanning near field optical microscopy ,s SNOM, 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 non-linear optics to optical circuitry.

physics.optics↗

arXiv2018

Acoustic Graphene Plasmon Nanoresonators for Field Enhanced Infrared Molecular Spectroscopy

Field-enhanced infrared molecular spectroscopy has been widely applied in chemical analysis, environment monitoring, and food and drug safety. The sensitivity of molecular spectroscopy critically depends on the electromagnetic field confinement and enhancement in the sensing elements. Here we propose a concept for sensing, consisting of a graphene plasmonic nanoresonator separated from a metallic film by a nanometric spacer. Such a resonator can support acoustic graphene plasmons, AGPs; that provide ultra-confined electromagnetic fields and strong field enhancement. Compared to conventional plasmons in graphene, AGPs exhibit a much higher spontaneous emission rate, higher sensitivity to the dielectric permittivity inside the AGP nano resonator, and remarkable capability to enhance molecular vibrational fingerprints, of nanoscale analyte samples. Our work opens novel avenues for sensing of ultra-small volume of molecules, as well as for studying enhanced light-matter interaction, e.g. strong coupling applications.

physics.optics↗

arXiv2013

Plasmons in graphene on uniaxial substrates

Placing graphene on uniaxial substrates may have interesting application potential for graphene-based photonic and optoelectronic devices. Here we analytically derive the dispersion relation for graphene plasmons on uniaxial substrates and discuss their momentum, propagation length and polarization as a function of frequency, propagation direction and both ordinary and extraordinary dielectric permittivities of the substrate. We find that the plasmons exhibit an anisotropic propagation, yielding radially asymmetric field patterns when a point emitter launches plasmons in the graphene layer.

cond-mat.mes-hall↗

arXiv2013

Electronic and plasmonic phenomena at graphene grain boundaries

Graphene, a two-dimensional honeycomb lattice of carbon atoms, is of great interest in (opto)electronics and plasmonics and can be obtained by means of diverse fabrication techniques, among which chemical vapor deposition (CVD) is one of the most promising for technological applications. The electronic and mechanical properties of CVD-grown graphene depend in large part on the characteristics of the grain boundaries. However, the physical properties of these grain boundaries remain challenging to characterize directly and conveniently. Here, we show that it is possible to visualize and investigate the grain boundaries in CVD-grown graphene using an infrared nano-imaging technique. We harness surface plasmons that are reflected and scattered by the graphene grain boundaries, thus causing plasmon interference. By recording and analyzing the interference patterns, we can map grain boundaries for a large area CVD-grown graphene film and probe the electronic properties of individual grain boundaries. Quantitative analysis reveals that grain boundaries form electronic barriers that obstruct both electrical transport and plasmon propagation. The effective width of these barriers (~10-20 nm) depends on the electronic screening and it is on the order of the Fermi wavelength of graphene. These results uncover a microscopic mechanism that is responsible for the low electron mobility observed in CVD-grown graphene, and suggest the possibility of using electronic barriers to realize tunable plasmon reflectors and phase retarders in future graphene-based plasmonic circuits.

cond-mat.mes-hall↗

arXiv2013

Experimental Verification of the Spectral Shift between Near- and Far-Field Peak Intensities of Plasmonic Nanoantennas

Theory predicts a distinct spectral shift between the near- and far-field optical responses of plasmonic antennas. Here we combine near-field optical microscopy and far-field spectroscopy of individual infrared-resonant nanoantennas to verify experimentally this spectral shift. Numerical calculations corroborate our experimental results. We furthermore discuss the implications of this effect in surface-enhanced infrared spectroscopy (SEIRS).

physics.optics↗