16 research outputs found
Seeing the vibrational breathing of a single molecule through time-resolved coherent anti-Stokes Raman scattering
The motion of chemical bonds within molecules can be observed in real time,
in the form of vibrational wavepackets prepared and interrogated through
ultrafast nonlinear spectroscopy. Such nonlinear optical measurements are
commonly performed on large ensembles of molecules, and as such, are limited to
the extent that ensemble coherence can be maintained. Here, we describe
vibrational wavepacket motion on single molecules, recorded through
time-resolved, surface-enhanced, coherent anti-Stokes Raman scattering. The
required sensitivity to detect the motion of a single molecule, under ambient
conditions, is achieved by equipping the molecule with a dipolar nano-antenna
(a gold dumbbell). In contrast with measurements in ensembles, the vibrational
coherence on a single molecule does not dephase. It develops phase fluctuations
with characteristic statistics. We present the time evolution of discretely
sampled statistical states, and highlight the unique information content in the
characteristic, early-time probability distribution function of the signal.Comment: 17 pages, 5 figure
Ultrafast Microscopy of Spin-Momentum-Locked Surface Plasmon Polaritons
Using
two-photon photoemission electron microscopy (2P-PEEM) we
image the polarization dependence of coupling and propagation of surface
plasmon polaritons (SPPs) launched from edges of a triangular, micrometer
size, single-crystalline Ag crystal by linearly or circularly polarized
light. 2P-PEEM records interferences between the optical excitation
field and SPPs it creates with nanofemto space-time resolution. Both
the linearly and circularly polarized femtosecond light pulses excite
spatially asymmetric 2PP yield distributions, which are imaged. We
attribute the asymmetry for linearly polarized light to the relative
alignments of the laser polarization and triangle edges, which affect
the efficiency of excitation of the longitudinal component of the
SPP field. For circular polarization, the asymmetry is caused by matching
of the spin angular momenta (SAM) of light and the transverse SAM
of SPPs. Moreover, we show that the interference patterns recorded
in the 2P-PEEM images are cast by phase shifts and amplitudes for
coupling of light into the longitudinal and transverse components
of SPP fields. While the interference patterns depend on the excitation
polarization, nanofemto movies show that the phase and group velocities
of SPPs are independent of SAM of light in time-reversal invariant
media. Simulations of the wave interference reproduce the polarization
and spin-dependent coupling of optical pulses into SPPs
Ultrafast Microscopy of Spin-Momentum-Locked Surface Plasmon Polaritons
Using
two-photon photoemission electron microscopy (2P-PEEM) we
image the polarization dependence of coupling and propagation of surface
plasmon polaritons (SPPs) launched from edges of a triangular, micrometer
size, single-crystalline Ag crystal by linearly or circularly polarized
light. 2P-PEEM records interferences between the optical excitation
field and SPPs it creates with nanofemto space-time resolution. Both
the linearly and circularly polarized femtosecond light pulses excite
spatially asymmetric 2PP yield distributions, which are imaged. We
attribute the asymmetry for linearly polarized light to the relative
alignments of the laser polarization and triangle edges, which affect
the efficiency of excitation of the longitudinal component of the
SPP field. For circular polarization, the asymmetry is caused by matching
of the spin angular momenta (SAM) of light and the transverse SAM
of SPPs. Moreover, we show that the interference patterns recorded
in the 2P-PEEM images are cast by phase shifts and amplitudes for
coupling of light into the longitudinal and transverse components
of SPP fields. While the interference patterns depend on the excitation
polarization, nanofemto movies show that the phase and group velocities
of SPPs are independent of SAM of light in time-reversal invariant
media. Simulations of the wave interference reproduce the polarization
and spin-dependent coupling of optical pulses into SPPs
Ultrafast Microscopy of Spin-Momentum-Locked Surface Plasmon Polaritons
Using
two-photon photoemission electron microscopy (2P-PEEM) we
image the polarization dependence of coupling and propagation of surface
plasmon polaritons (SPPs) launched from edges of a triangular, micrometer
size, single-crystalline Ag crystal by linearly or circularly polarized
light. 2P-PEEM records interferences between the optical excitation
field and SPPs it creates with nanofemto space-time resolution. Both
the linearly and circularly polarized femtosecond light pulses excite
spatially asymmetric 2PP yield distributions, which are imaged. We
attribute the asymmetry for linearly polarized light to the relative
alignments of the laser polarization and triangle edges, which affect
the efficiency of excitation of the longitudinal component of the
SPP field. For circular polarization, the asymmetry is caused by matching
of the spin angular momenta (SAM) of light and the transverse SAM
of SPPs. Moreover, we show that the interference patterns recorded
in the 2P-PEEM images are cast by phase shifts and amplitudes for
coupling of light into the longitudinal and transverse components
of SPP fields. While the interference patterns depend on the excitation
polarization, nanofemto movies show that the phase and group velocities
of SPPs are independent of SAM of light in time-reversal invariant
media. Simulations of the wave interference reproduce the polarization
and spin-dependent coupling of optical pulses into SPPs
Efficient Plasmon-Mediated Energy Funneling to the Surface of Au@Pt Core-Shell Nanocrystals
Ultrafast Microscopy of Spin-Momentum-Locked Surface Plasmon Polaritons
Using
two-photon photoemission electron microscopy (2P-PEEM) we
image the polarization dependence of coupling and propagation of surface
plasmon polaritons (SPPs) launched from edges of a triangular, micrometer
size, single-crystalline Ag crystal by linearly or circularly polarized
light. 2P-PEEM records interferences between the optical excitation
field and SPPs it creates with nanofemto space-time resolution. Both
the linearly and circularly polarized femtosecond light pulses excite
spatially asymmetric 2PP yield distributions, which are imaged. We
attribute the asymmetry for linearly polarized light to the relative
alignments of the laser polarization and triangle edges, which affect
the efficiency of excitation of the longitudinal component of the
SPP field. For circular polarization, the asymmetry is caused by matching
of the spin angular momenta (SAM) of light and the transverse SAM
of SPPs. Moreover, we show that the interference patterns recorded
in the 2P-PEEM images are cast by phase shifts and amplitudes for
coupling of light into the longitudinal and transverse components
of SPP fields. While the interference patterns depend on the excitation
polarization, nanofemto movies show that the phase and group velocities
of SPPs are independent of SAM of light in time-reversal invariant
media. Simulations of the wave interference reproduce the polarization
and spin-dependent coupling of optical pulses into SPPs
Isomerization of One Molecule Observed through Tip-Enhanced Raman Spectroscopy
While exploring photoisomerization
of azobenzyl thiols (ABT) adsorbed on Au(111), through joint scanning
tunneling microscopy (STM) and tip-enhanced Raman scattering (TERS)
studies, the reversible photoisomerization of one molecule is captured
in TERS trajectories. The unique signature of single molecule isomerization
is observed in the form of anticorrelated flip-flops between two distinct
spectra with two discrete, on- and off-levels. The apparently heterogeneously
photocatalyzed reaction is assigned to cis–trans isomerization
of an outlier, which is chemisorbed on the silver tip of the STM.
Otherwise, the ensemble of ABT molecules that lie flat on Au(111)
remain strongly coupled to the surface, excluding the possibility
of photoisomerization or detection through TERS
Chemically Selective Imaging of Individual Bonds through Scanning Electron Energy-Loss Spectroscopy : Disulfide Bridges Linking Gold Nanoclusters
As proof-of-principle of chemically selective, spatially resolved imaging of individual bonds, we carry out electron energy-loss spectroscopy in a scanning transmission electron microscopy instrument on atomically precise, thiolate-coated gold nanoclusters linked with 5,5′-bis(mercaptomethyl)-2,2′-bipyridine dithiol ligands. The images allow the identification of bridging disulfide bonds (R–S–S–R) between clusters, and X-ray photoelectron spectra support the finding.peerReviewe
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Seeing the vibrational breathing of a single molecule through time-resolved coherent anti-Stokes Raman scattering
The motion of chemical bonds within molecules can be observed in real time,
in the form of vibrational wavepackets prepared and interrogated through
ultrafast nonlinear spectroscopy. Such nonlinear optical measurements are
commonly performed on large ensembles of molecules, and as such, are limited to
the extent that ensemble coherence can be maintained. Here, we describe
vibrational wavepacket motion on single molecules, recorded through
time-resolved, surface-enhanced, coherent anti-Stokes Raman scattering. The
required sensitivity to detect the motion of a single molecule, under ambient
conditions, is achieved by equipping the molecule with a dipolar nano-antenna
(a gold dumbbell). In contrast with measurements in ensembles, the vibrational
coherence on a single molecule does not dephase. It develops phase fluctuations
with characteristic statistics. We present the time evolution of discretely
sampled statistical states, and highlight the unique information content in the
characteristic, early-time probability distribution function of the signal
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Photon momentum enabled light absorption in silicon
Photons do not carry sufficient momentum to induce indirect optical
transitions in semiconducting materials such as silicon, necessitating the
assistance of lattice phonons to conserve momentum. Compared to direct bandgap
semiconductors, this renders silicon a less attractive material for a wide
variety of optoelectronic applications. In this work, we introduce an
alternative strategy to fulfill the momentum-matching requirement in indirect
optical transitions. We demonstrate that when confined to scales below ~3 nm,
photons acquire sufficient momentum to allow electronic transitions at the band
edge of Si without the assistance of a phonon. Confined photons allow
simultaneous energy and momentum conservation in two-body photon-electron
scattering; in effect, converting silicon into a direct bandgap semiconductor.
We show that this less explored concept of light-matter interaction leads to a
marked increase of the absorptivity of Si from the UV to the near-IR. The
strategy provides opportunities for more efficient use of indirect
semiconductors in photovoltaics, energy conversion, light detection and
emission