[show abstract][hide abstract] ABSTRACT: a * Since its first experimental realization, tip-enhanced Raman spectroscopy (TERS) has emerged as a potentially powerful nanochemical analysis tool. However, questions about the comparability and reproducibility of TERS data have emerged. This interlaboratory comparison study addresses these issues by bringing together different TERS groups to perform TERS mea-surements on nominally identical samples. Based on the spectra obtained, the absolute and relative peak positions, number of bands, peak intensity ratios, and comparability to reference Raman and surface-enhanced Raman spectroscopy (SERS) data are discussed. Our general findings are that all research groups obtained similar spectral patterns, irrespective of the setup or tip that was used. The TERS (and SERS) spectra consistently showed fewer bands than the conventional Raman spectrum. When comparing these three methods, the spectral pattern match and substance identification is readily possible. Absolute and relative peak positions of the three major signals of thiophenol scattered by 19 and 9 cm À1 , respectively, which can prob-ably be attributed to different spectrometer calibrations. However, within the same group (but between different tips), the signals only scattered by 3 cm À1 on average. This study demonstrated the suitability of TERS as an analytical tool and brings TERS a big step forward to becoming a routine technique.
[show abstract][hide abstract] ABSTRACT: Optical spectroscopic imaging has taken a leap into the intramolecular
regime with an approach that achieves subnanometre spatial resolution.
The technique should find applications in photochemistry and
nanotechnology. See Letter p.82
[show abstract][hide abstract] ABSTRACT: We study the decrease in group velocity of broadband surface plasmon polariton propagation on a conical tip, using femtosecond time-domain interferometry. The group delay of (9±3) fs measured corresponds to a group velocity at the apex of less than 0.2c. The result agrees in general with the prediction from adiabatic plasmonic nanofocusing theory, yet is sensitive with respect to the exact taper geometry near the apex. This, together with the sub 25 fs<sup>2</sup> second-order dispersion observed, provides the fundamental basis for the use of plasmons for broadband slow-light applications.
[show abstract][hide abstract] ABSTRACT: We demonstrate a generalized route to generate nanometer spatially
confined ultrafast optical pulses with arbitrary deterministic
femtosecond waveform control using surface plasmon polarition
nanofocusing in 3D tapered noble metal tips.
[show abstract][hide abstract] ABSTRACT: The simultaneous control of optical fields on both nanometer spatial and
femtosecond time scales would enable direct spectroscopic access to the
elementary electronic and vibrational excitations in matter. Here, we
utilize adiabatic surface plasmon polariton (SPP) nanofocusing on
free-standing 3D tapered metal tips in order to generate nanometer
confined field localization at the tip apex. Using the second harmonic
generation (SHG) at the tip apex we perform MIIPS pulse optimization and
frequency-resolved optical gating (FROG) characterization of the
nanofocused pulses. With the combination of high bandwidth coupling
using a chirped grating, pulse-shaping, and low-dispersion nanofocusing,
we can achieve full optical control on the nanoscale, from < 16 fs
pulse duration to arbitrary optical waveforms. This technique enables
linear and non-linear plasmon-enhanced spectroscopy, with the
simultaneous temporal control over ultrashort pulses opening the
possibility for true time-resolved scanning-probe imaging. We
demonstrate this capability for background-free probing of individual
molecular and nanocrystalline systems.
[show abstract][hide abstract] ABSTRACT: Strongly correlated electron materials display diverse complex phenomena
such as metal-insulator transitions and ferroelectric and ferromagnetic
ordering, with characteristic lengths on the nanometer scale. In order
to directly access and study the associated nano-phase behavior and
domains for a wide range of materials, we have developed a low
temperature tip-enhanced scattering-type scanning near-field optical
microscope (s-SNOM). A microscopy flow cryostat reservoir is coupled to
a shear-force atomic force microscope, with illumination of
electrochemically etched Au tips provided by an on-axis high numerical
aperture parabolic mirror. We will discuss the use of this system for
the study and imaging of ferroic ordering in multiferroic and
ferroelectric materials through the symmetry selectivity provided by
tip-enhanced second harmonic generation (SHG) and nano-Raman
crystallography via the tensor based selection rules.
[show abstract][hide abstract] ABSTRACT: In addition to its metal-insulator transition (MIT), VO2 exhibits a rich phase behavior of insulating monoclinic (M1,M2) and triclinic (T) phases. By using micro-Raman spectroscopy and independent control of temperature and uniaxial strain in individual single-crystal microbeams, we map these insulating phases with their associated structural changes as represented by their respective phonon frequencies. The competition between these structural forms is dictated by the internal strain due to differing lattice constants, the experimentally applied external strain, and the temperature-dependent phase stability. We identify the nature of the triclinic phase as a continuously distorted variant of the M1 monoclinic phase, while a discontinuous transition into the M2 phase occurs from both the M1 and T phases. The results suggest that understanding the driving forces that determine the interplay between M1, M2, and T phases near the MIT could be critical for the identification of the underlying mechanism behind the MIT itself.
[show abstract][hide abstract] ABSTRACT: The simultaneous nanometer spatial confinement and femtosecond temporal control of an optical excitation has been a long-standing challenge in optics. Previous approaches using surface plasmon polariton (SPP) resonant nanostructures or SPP waveguides have suffered from, for example, mode mismatch, or possible dependence on the phase of the driving laser field to achieve spatial localization. Here we take advantage of the intrinsic phase- and amplitude-independent nanofocusing ability of a conical noble metal tip with weak wavelength dependence over a broad bandwidth to achieve a 10 nm spatially and few-femtosecond temporally confined excitation. In combination with spectral pulse shaping and feedback on the second-harmonic response of the tip apex, we demonstrate deterministic arbitrary optical waveform control. In addition, the high efficiency of the nanofocusing tip provided by the continuous micro- to nanoscale mode transformation opens the door for spectroscopy of elementary optical excitations in matter on their natural length and time scales and enables applications from ultrafast nano-opto-electronics to single molecule quantum coherent control.
[show abstract][hide abstract] ABSTRACT: The spatial confinement and temporal control of an optical excitation on
nanometer length scales and femtosecond time scales has been a long-standing
challenge in optics. It would provide spectroscopic access to the elementary
optical excitations in matter on their natural length and time scales and
enable applications from ultrafast nano-opto-electronics to single molecule
quantum coherent control. Previous approaches have largely focused on using
surface plasmon polariton (SPP) resonant nanostructures or SPP waveguides to
generate nanometer localized excitations. However, these implementations
generally suffer from mode mismatch between the far-field propagating light and
the near-field confinement. In addition, the spatial localization in itself may
depend on the spectral phase and amplitude of the driving laser pulse thus
limiting the degrees of freedom available to independently control the
nano-optical waveform. Here we utilize femtosecond broadband SPP coupling, by
laterally chirped fan gratings, onto the shaft of a monolithic noble metal tip,
leading to adiabatic SPP compression and localization at the tip apex. In
combination with spectral pulse shaping with feedback on the intrinsic
nonlinear response of the tip apex, we demonstrate the continuous micro- to
nano-scale self-similar mode matched transformation of the propagating
femtosecond SPP field into a 20 nm spatially and 16 fs temporally confined
light pulse at the tip apex. Furthermore, with the essentially wavelength and
phase independent 3D focusing mechanism we show the generation of arbitrary
optical waveforms nanofocused at the tip. This unique femtosecond nano-torch
with high nano-scale power delivery in free space and full spectral and
temporal control opens the door for the extension of the powerful nonlinear and
ultrafast vibrational and electronic spectroscopies to the nanoscale.
[show abstract][hide abstract] ABSTRACT: We demonstrate independent spatiotemporal control of optical fields on nanometer and femtosecond scales, enabled by the adiabatic and mode-matched surface plasmon polariton nanofocusing ability of 3D Au tips, combined with femtosecond pulse-shaping.
[show abstract][hide abstract] ABSTRACT: The metal-insulator transition (MIT) of VO2 exhibits a rich
phase behavior involving two monoclinic (M1, M2), triclinic, and
tetragonal phases that can form a complex domain structure and accompany
the electronic transition. The interplay between these structural
variants arises from strain due to differing lattice constants,
temperature-dependent phase stability, and possible external strain from
the substrate; the coupling between these effects renders a systematic
study of the phase behavior difficult. We report on phase mapping of the
structural changes through independent control of temperature and
uniaxial strain in individual single-crystal nanorods, using Raman
spectroscopy and near-field imaging. This allows us to investigate the
transformation between the various insulating phases, elucidating the
nature of the triclinic phase as a continuously distorted variant of the
M1 monoclinic phase, intermediate in the first-order transformation into
the monoclinic M2 phase.
[show abstract][hide abstract] ABSTRACT: True nanoscale optical spectroscopy requires the efficient delivery of light for a spatially nanoconfined excitation. We utilize adiabatic plasmon focusing to concentrate an optical field into the apex of a scanning probe tip of 10 nm in radius. The conical tips with the ability for two-stage optical mode matching of the surface plasmon polariton (SPP) grating-coupling and the adiabatic propagating SPP conversion into a localized SPP at the tip apex represent a special optical antenna concept for far-field transduction into nanoscale excitation. The resulting high nanofocusing efficiency and the spatial separation of the plasmonic grating-coupling element on the tip shaft from the near-field apex probe region allows for true background-free nanospectroscopy. As an application, we demonstrate tip-enhanced Raman spectroscopy (TERS) of surface molecules with enhanced contrast and its extension into the near-IR with 800 nm excitation.Keywords: tip-enhanced Raman; TERS; plasmon; adiabatic nanofocusing; scanning near-field optical microscopy; grating coupling
Journal of Physical Chemistry Letters - J PHYS CHEM LETT. 11/2010;