The performance of organic donor-acceptor heterojunctions in solar cell devices is fundamentally determined by factors such as charge separation efficiency, charge carrier mobility, exciton diffusion lengths, and energy losses due to exciton recombination. Despite advancements in organic photovoltaic materials, exciton transport is frequently hindered by structural disorder, which limits overall device efficiency. Exciton-polaritons, formed through the strong coupling of cavity-coupled organic materials, exhibit delocalized states that enhance exciton transport and reduce the effects of disorder. Thus, it is of interest to elucidate the influence of strong coupling on charge transfer dynamics in organic photovoltaic materials. Using transient absorption spectroscopy, we explore the effect of cavity-strong-coupling in DBPC70 donor-acceptor blends integrated with a distributed Bragg reflector cavity. We find that the presence of the cavity slows the charge transfer process in such heterojunction systems.
In this work, we present a rapid scanning approach to fluorescence-detected two-dimensional electronic spectroscopy (FD-2DES). Our approach combines phase modulation with digital lock-in detection. Using phasemodulation each of the four pulses used to excite the sample is tagged with a specific radio-frequency. The resulting fluorescence signal is thus modulated at different linear combinations of these frequencies. Digital lock-in detection is used to retrieve complex linear and non-linear spectroscopic signals in a single measurement. The approach allows simultaneous tracking of interferometric time delays, correction of spectral phase distortions, and also enables accurate phasing of the data. We report the simultaneously acquired linear fluorescence excitation spectrum, rephasing, non-rephasing, and absorptive FD-2DES spectra of the laser dye IR-140 in DMSO.
Ultrafast pump-probe spectroscopy and pulse-shaping techniques are providing new modes of contrast for the field of
multiphoton microscopy. Endogenous species such as heme proteins show rich nonlinear spectroscopic signatures of
excited state absorption, stimulated emission and ground-state bleaching. Commercially available octave-spanning
Ti:sapphire oscillators offer new opportunities for imaging based on pump-probe contrast. Spatial light modulators take
advantage of this large bandwidth, shaping pulses of light to selectively excite molecular structures with similar spectral
properties. We present two-color pump-probe imaging of heme proteins solutions and red blood cells.
Fluorescence Resonance Energy Transfer (FRET) microscopy is a commonly-used technique to study problems
in biophysics that range from uncovering cellular signaling pathways to detecting conformational changes in single
biomolecules. Unfortunately, excitation and emission spectral overlap between the fluorophores create challenges in
quantitative FRET studies. It has been shown previously that quantitative FRET stoichiometry can be performed by
selective excitation of donor and acceptor fluorophores. Extending this approach to two-photon FRET applications is
difficult when conventional femtosecond laser sources are used due to their limited bandwidth and slow tuning response
time. Extremely broadband titanium:sapphire lasers enable the simultaneous excitation of both donor and acceptor for
two-photon FRET, but do so without selectivity. Here we present a novel two-photon FRET microscopy technique that
employs pulse-shaping to perform selective excitation of fluorophores in live cells and detect FRET between them.
Pulse-shaping via multiphoton intrapulse interference can tailor the excitation pulses to achieve selective excitation. This
technique overcomes the limitation of conventional femtosecond lasers to allow rapid switching between selective
excitation of the donor and acceptor fluorophores. We apply the method to live cells expressing the fluorescent proteins
mCerulean and mCherry, demonstrating selective excitation of fluorophores via pulse-shaping and the detection of twophoton
FRET. This work paves the way for two-photon FRET stoichiometry.
We present a systematic comparison between coherent and spontaneous Raman scattering under conditions
relevant for biological imaging. Using spectral domain imaging, we find that the signal levels for each method
are comparable at the low excitation power and low concentrations appropriate for biological samples. For
samples of polystyrene beads with a molecular concentration of 10 M, we determine the critical power at which
the two methods give equal signal levels to be ~1.3 mW. The advantages offered by coherent Raman methods
are mitigated by the low excitation power, low sample concentrations, and short interaction lengths involved
with biological imaging. We present calculations to support our measurements.
We compare imaging using coherent and spontaneous Raman scattering under biological imaging conditions. We
perform spectral domain imaging of polystyrene beads and find comparable signal levels for both methods at excitation
powers and concentrations most relevant for biological samples. The critical power at which the two methods provide
equivalent signal levels is found to be ~1.3 mW in 10 M polystyrene beads and ~7 mW in 13 M 2-propanol. The low
sample concentrations and low excitation power necessary for most biological imaging applications reduce the relative
advantages offered by coherent Raman methods.
We present a time domain Fourier transform coherent Raman microscopy. We show that with an added local electric
field, the signal can be amplified by an order of magnitude through homodyne detection. Our approach requires a single
broadband pulse to modulate, probe and amplify Raman coherence with passive phase stability to obtain high-resolution
coherent anti-Stokes Raman (CARS) spectra for microscopy and microspectroscopy applications.
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