13 research outputs found
Moving beyond the electromagnetic enhancement theory
The electromagnetic enhancement theory describes surface-enhanced Raman scattering (SERS) as a Raman effect that takes place in the near-field of a plasmonic nanostructure. The theory has been very successful in explaining the fundamental properties of SERS, modelling the performance of different metals as enhancing materials and optimizing SERS hotspots for strongest possible enhancement. Over the last decade, a number of carefully designed experimental studies have examined predictions of the electromagnetic theory like the size and shape of SERS hotspots, the absolute magnitude of the enhancement and the width of the SERS resonance. Although the overall picture was quite satisfactory, the theory failed to predict key aspects of SERS, for example, the absolute magnitude of the plasmonic enhancement. We scrutinize these experiments and review them focusing on the results that require going beyond the electromagnetic enhancement theory. We argue that the results of these experiments create the need to develop the theory of SERS further, especially the exact role of plasmonic enhancement in inelastic light scattering
Quantum Nature of Plasmon-Enhanced Raman Scattering
We report plasmon-enhanced Raman scattering in graphene coupled to a single
plasmonic hotspot measured as a function of laser energy. The enhancement
profiles of the G peak show strong enhancement (up to ) and narrow
resonances (30 meV) that are induced by the localized surface plasmon of a gold
nanodimer. We observe the evolution of defect-mode scattering in a defect-free
graphene lattice in resonance with the plasmon. We propose a quantum theory of
plasmon-enhanced Raman scattering, where the plasmon forms an integral part of
the excitation process. Quantum interferences between scattering channels
explain the experimentally observed resonance profiles, in particular, the
marked difference in enhancement factors for incoming and outgoing resonance
and the appearance of the defect-type modes.Comment: Keywords: plasmon-enhanced Raman scattering, SERS, graphene, quantum
interferences, microscopic theory of Raman scattering. Content: 22 pages
including 5 figures + 11 pages supporting informatio
Dielectric Screening inside Carbon Nanotubes
Dielectric screening plays a vital role in determining physical properties at the nanoscale and affects our ability to detect and characterize nanomaterials using optical techniques. We study how dielectric screening changes electromagnetic fields and many-body effects in nanostructures encapsulated inside carbon nanotubes. First, we show that metallic outer walls reduce the scattering intensity of the inner tube by 2 orders of magnitude compared to that of air-suspended inner tubes, in line with our local field calculations. Second, we find that the dielectric shift of the optical transition energies in the inner walls is greater when the outer tube is metallic than when it is semiconducting. The magnitude of the shift suggests that the excitons in small-diameter inner metallic tubes are thermally dissociated at room temperature if the outer tube is also metallic, and in essence, we observe band-to-band transitions in thin metallic double-walled nanotubes
Spectroscopic and Interferometric Sum-Frequency Imaging of Strongly Coupled Phonon Polaritons in SiC Metasurfaces
Phonon polaritons enable waveguiding and localization of infrared light with
extreme confinement and low losses. The spatial propagation and spectral
resonances of such polaritons are usually probed with complementary techniques
such as near-field optical microscopy and far-field reflection spectroscopy.
Here, we introduce infrared-visible sum-frequency spectro-microscopy as a tool
for spectroscopic imaging of phonon polaritons. The technique simultaneously
provides sub-wavelength spatial resolution and highly-resolved spectral
resonance information. This is implemented by resonantly exciting polaritons
using a tunable infrared laser and wide-field microscopic detection of the
upconverted light. We employ this technique to image hybridization and strong
coupling of localized and propagating surface phonon polaritons in metasurfaces
of SiC micropillars. Spectro-microscopy allows us to measure the polariton
dispersion simultaneously in momentum space by angle-dependent resonance
imaging, and in real space by polariton interferometry. Notably, we directly
visualize how strong coupling affects the spatial localization of polaritons,
inaccessible with conventional spectroscopic techniques. We further observe the
formation of edge states at excitation frequencies where strong coupling
prevents polariton propagation into the metasurface. Our approach is applicable
to the wide range of polaritonic materials with broken inversion symmetry and
can be used as a fast and non-perturbative tool to image polariton
hybridization and propagation
Extreme light confinement and control in low-symmetry phonon-polaritonic crystals
Polaritons are a hybrid class of quasiparticles originating from the strong
and resonant coupling between light and matter excitations. Recent years have
witnessed a surge of interest in novel polariton types, arising from
directional, long-lived material resonances, and leading to extreme optical
anisotropy that enables novel regimes of nanoscale, highly confined light
propagation. While such exotic propagation features may also be in principle
achieved using carefully designed metamaterials, it has been recently realized
that they can naturally emerge when coupling infrared light to directional
lattice vibrations, i.e., phonons, in polar crystals. Interestingly, a
reduction in crystal symmetry increases the directionality of optical phonons
and the resulting anisotropy of the response, which in turn enables new
polaritonic phenomena, such as hyperbolic polaritons with highly directional
propagation, ghost polaritons with complex-valued wave vectors, and shear
polaritons with strongly asymmetric propagation features. In this Review, we
develop a critical overview of recent advances in the discovery of phonon
polaritons in low-symmetry crystals, highlighting the role of broken symmetries
in dictating the polariton response and associated nanoscale-light propagation
features. We also discuss emerging opportunities for polaritons in
lower-symmetry materials and metamaterials, with connections to topological
physics and the possibility of leveraging anisotropic nonlinearities and
optical pumping to further control their nanoscale response
Evaluating arbitrary strain configurations and doping in graphene with Raman spectroscopy
Raman spectroscopy is a powerful tool for characterizing the local properties
of graphene. Here, we introduce a method for evaluating unknown strain
configurations and simultaneous doping. It relies on separating the effects of
hydrostatic strain (peak shift) and shear strain (peak splitting) on the Raman
spectrum of graphene. The peak shifts from hydrostatic strain and doping are
separated with a correlation analysis of the 2D and G frequencies. This enables
us to obtain the local hydrostatic strain, shear strain and doping without any
assumption on the strain configuration prior to the analysis. We demonstrate
our approach for two model cases: Graphene under uniaxial stress on a PMMA
substrate and graphene suspended on nanostructures that induce an unknown
strain configuration. We measured for pure hydrostatic strain. Raman scattering with circular
corotating polarization is ideal for analyzing strain and doping, especially
for weak strain when the peak splitting by shear strain cannot be resolved
Exploiting plasmonic enhancement with light-emitting diode excitation in surface-enhanced Raman scattering
Surface-enhanced Raman scattering (SERS) is a well-established technique that enables the detection of very low molecular concentrations down to single molecules. Typical applications of SERS are the consistent identification of various samples used in chemistry, biology, and physics among others. In contrast to common SERS setups, where lasers are used as excitation source, we exploit SERS to perform Raman spectroscopy with a light-emitting diode (LED). We demonstrate the applicability of our approach on four different Raman reporters. We unambiguously distinguish two similar designer molecules 4-nitrothiophenol (p-NTP) and 5,5-dithio-bis-(2-nitrobenzoic acid) (DTNB) that are often used in SERS experiments. Additionally, we probe Rhodamine 6G that is used in many different applications and carbon nanotubes as a one-dimensional solid state nanosystem. The LED excited surface-enhanced Raman spectra reproduce the characteristic Raman modes of the different samples. We compare the LED spectra to Raman spectra excited with a laser at the same wavelength. We envision the combination of LED sources with SERS substrates in the next generation of handheld devices and low-cost Raman setups
Moir\'e-induced Vibrational Coupling in Double Walled Carbon Nanotubes
Moir\'e patterns are additional, long-range periodicities in twisted
crystalline bilayers. They are known to fundamentally change the electronic
states of the layers, but similar effects on their mechanical and vibrational
properties have not been discussed so far. Here we show that the Moir\'e
potential shifts the radial breathing mode in double walled carbon nanotubes
(DWCNTs). The change of frequency is expected to be proportional to the shift
in optical transition energies, which are induced by the Moir\'e patterns. To
verify our model we performed resonant Raman scattering on purified and sorted
semiconducting DWCNTs. We find that the radial breathing mode shifts up to 14
cm higher in energy followed by optical transitions energies
displacement up to 200 meV to lower energies, compared to the single-walled
tubes. We show how to identify the strong coupling condition in DWCNTs from
their phonon frequencies and construct a Kataura plot to aid their future
experimental assignment.Comment: main script 16 pages, 5 figures; supplementary script 7 pages, 3
figure
Moiré-Induced Vibrational Coupling in Double-Walled Carbon Nanotubes
Moiré patterns are additional, long-range periodicities in twisted crystalline bilayers. They are known to fundamentally change the electronic states of the layers, but similar effects on their mechanical and vibrational properties have not been discussed so far. Here we show that the moiré potential shifts the radial breathing mode in double-walled carbon nanotubes (DWCNTs). The change in frequency is expected to be proportional to the shift in optical transition energies, which are induced by the moiré patterns. To verify our model, we performed resonance Raman scattering on purified and sorted semiconducting DWCNTs. We find that the radial breathing mode shifts up to 14 cm–1 higher in energy followed by displacement of optical transition energies of up to 200 meV to lower energies, in comparison to the single-walled tubes. We show how to identify the strong coupling condition in DWCNTs from their phonon frequencies and construct a Kataura plot to aid their future experimental assignment
Exploiting plasmonic enhancement with light-emitting diode excitation in surface-enhanced Raman scattering
Surface-enhanced Raman scattering (SERS) is a well-established technique that enables the detection of very low molecular concentrations down to single molecules. Typical applications of SERS are the consistent identification of various samples used in chemistry, biology, and physics among others. In contrast to common SERS setups, where lasers are used as excitation source, we exploit SERS to perform Raman spectroscopy with a light-emitting diode (LED). We demonstrate the applicability of our approach on four different Raman reporters. We unambiguously distinguish two similar designer molecules 4-nitrothiophenol (p-NTP) and 5,5-dithio-bis-(2-nitrobenzoic acid) (DTNB) that are often used in SERS experiments. Additionally, we probe Rhodamine 6G that is used in many different applications and carbon nanotubes as a one-dimensional solid state nanosystem. The LED excited surface-enhanced Raman spectra reproduce the characteristic Raman modes of the different samples. We compare the LED spectra to Raman spectra excited with a laser at the same wavelength. We envision the combination of LED sources with SERS substrates in the next generation of handheld devices and low-cost Raman setups