Sarah R. Nichols

Molecular fragmentation driven by ultrafast dynamic ionic resonances.

The authors time resolve molecular motion in bound state, ionic potentials that leads to bond cleavage during the interaction with intense, ultrafast laser fields. Resonances in molecular ions play an important role in dissociative ionization with ultrafast laser fields, and the authors demonstrate how these resonances evolve in time to produce dissociation after initial strong-field ionization. Exploiting such dynamic resonances offers the possibility of controlled bond breaking and characterizing time-dependent molecular structure.

Creation of multihole molecular wave packets via strong-field ionization

We demonstrate the creation of vibrational wave packets on multiple electronic states of a molecule via strong-field ionization. Furthermore, we show that the relative contribution of the different electronic states depends on the shape of the laser pulse which launches the wave packets.

Pulse shaping multiphoton FRET microscopy

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.

Pulse-shaping multiphoton FRET microscopy

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.

Comparing coherent and spontaneous Raman scattering signals for biological imaging applications

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.

A comparison between coherent and spontaneous Raman scattering for biological imaging

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.

Dissociative ionization of an aligned molecular sample

We investigate dissociative ionization of aligned N2 molecules using intense ultrafast laser pulses. We find surprising differences in the yields of N2++, N+(1, 0) and N+(1, 1) as a function of molecular axis-laser polarization angle.