34 research outputs found
Narrowband spectroscopy by all-optical correlation of broadband pulses
High peak power ultrafast lasers are widely used in nonlinear spectroscopy
but often limit its spectral resolution because of the broad frequency
bandwidth of ultrashort laser pulses. Improving the resolution by achieving
spectrally narrow excitation of, or emission from, the resonant medium by means
of multi-photon interferences has been the focus of many recent developments in
ultrafast spectroscopy. We demonstrate an alternative approach, in which high
resolution is exercised by detecting narrow spectral correlations between
broadband excitation and emission optical fields. All-optical correlation
analysis, easily incorporated into the traditional spectroscopic setup, enables
direct, robust and simultaneous detection of multiple narrow resonances with a
single femtosecond pulse.Comment: 5 pages, 4 figures, submitted to PR
Noise auto-correlation spectroscopy with coherent Raman scattering
Ultrafast lasers have become one of the most powerful tools in coherent
nonlinear optical spectroscopy. Short pulses enable direct observation of fast
molecular dynamics, whereas broad spectral bandwidth offers ways of controlling
nonlinear optical processes by means of quantum interferences. Special care is
usually taken to preserve the coherence of laser pulses as it determines the
accuracy of a spectroscopic measurement. Here we present a new approach to
coherent Raman spectroscopy based on deliberately introduced noise, which
increases the spectral resolution, robustness and efficiency. We probe laser
induced molecular vibrations using a broadband laser pulse with intentionally
randomized amplitude and phase. The vibrational resonances result in and are
identified through the appearance of intensity correlations in the noisy
spectrum of coherently scattered photons. Spectral resolution is neither
limited by the pulse bandwidth, nor sensitive to the quality of the temporal
and spectral profile of the pulses. This is particularly attractive for the
applications in microscopy, biological imaging and remote sensing, where
dispersion and scattering properties of the medium often undermine the
applicability of ultrafast lasers. The proposed method combines the efficiency
and resolution of a coherent process with the robustness of incoherent light.
As we demonstrate here, it can be implemented by simply destroying the
coherence of a laser pulse, and without any elaborate temporal scanning or
spectral shaping commonly required by the frequency-resolved spectroscopic
methods with ultrashort pulses.Comment: To appear in Nature Physic
Ultrafast nano-focusing with full optical waveform control
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.Comment: Contains manuscript with 4 figures as well as supplementary material
with 2 figure
Allelopathic interactions of linoleic acid and nitric oxide increase the competitive ability of Microcystis aeruginosa
The frequency and intensity of cyanobacterial blooms are increasing worldwide with major societal and economic costs. Interactions between toxic cyanobacteria and eukaryotic algal competitors can affect toxic bloom formation, but the exact mechanisms of interspecies interactions remain unknown. Using metabolomic and proteomic profiling of co-cultures of the toxic cyanobacterium Microcystis aeruginosa with a green alga as well as of microorganisms collected in a Microcystis spp. bloom in Lake Taihu (China), we disentangle novel interspecies allelopathic interactions. We describe an interspecies molecular network in which M. aeruginosa inhibits growth of Chlorella vulgaris, a model green algal competitor, via the release of linoleic acid. In addition, we demonstrate how M. aeruginosa takes advantage of the cell signaling compound nitric oxide produced by C. vulgaris, which stimulates a positive feedback mechanism of linoleic acid release by M. aeruginosa and its toxicity. Our high-throughput system-biology approach highlights the importance of previously unrecognized allelopathic interactions between a broadly distributed toxic cyanobacterial bloom former and one of its algal competitors
Near-Field Infrared Vibrational Dynamics and Tip-Enhanced Decoherence
Ultrafast infrared spectroscopy can reveal the dynamics
of vibrational
excitations in matter. In its conventional far-field implementation,
however, it provides only limited insight into nanoscale sample volumes
due to insufficient spatial resolution and sensitivity. Here, we combine
scattering-scanning near-field optical microscopy (<i>s</i>-SNOM) with femtosecond infrared vibrational spectroscopy to characterize
the coherent vibrational dynamics of a nanoscopic ensemble of C–F
vibrational oscillators of polytetrafluoroethylene (PTFE). The near-field
mode transfer between the induced vibrational molecular coherence
and the metallic scanning probe tip gives rise to a tip-mediated <i>radiative</i> IR emission of the vibrational free-induction
decay (FID). By increasing the tip–sample coupling, we can <i>enhance</i> the vibrational dephasing of the induced coherent
vibrational polarization and associated IR emission, with dephasing
times up to <i>T</i><sub>2</sub><sup>NF</sup> ≃ 370 fs in competition against the
intrinsic far-field lifetime of <i>T</i><sub>2</sub><sup>FF</sup> ≃ 680 fs as dominated
by nonradiative damping. Near-field antenna-coupling thus provides
for a new way to modify vibrational decoherence. This approach of
ultrafast <i>s</i>-SNOM enables the investigation of spatiotemporal
dynamics and correlations with nanometer spatial and femtosecond temporal
resolution
Femtosecond optical control on the nanoscale
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
Femtosecond optical control on the nanoscale
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
Defects and Deformation of Boron Nitride Nanotubes Studied by Joint Nanoscale Mechanical and Infrared Near-Field Microscopy
Boron nitride nanotubes (BNNTs) are a one-dimensional material with high mechanical strength, good electric insulation, exceptional thermal conductivity, and high temperature stability. The presence of defects and deformations in BNNTs at the nanoscale affects their mechanical properties. In this article, we jointly utilize nanomechanical characterization and infrared scattering type near-field microscopy on the same individual BNNTs to study the defects and deformations. A characteristic structural defect of BNNT is revealed by a blue-shift of infrared absorption and discontinuity in adhesion. Polygonal facets have been observed in BNNTs, as supported by their mechanical responses and spectroscopic signatures. The presence of polygonal facets implies one should expect reactive chemical and hyperbolic optical properties of BNNTs