Arkaprabha Konar

Solvent Environment Revealed by Positively Chirped Pulses

The spectroscopy of large organic molecules and biomolecules in solution has been investigated using various time-resolved and frequency-resolved techniques. Of particular interest is the early response of the molecule and the solvent, which is difficult to study due to the ambiguity in assigning and differentiating inter- and intramolecular contributions to the electronic and vibrational populations and coherence. Our measurements compare the yield of fluorescence and stimulated emission for two laser dyes IR144 and IR125 as a function of chirp. While negatively chirped pulses are insensitive to solvent viscosity, positively chirped pulses are found to be uniquely sensitive probes of solvent viscosity. The fluorescence maximum for IR125 is observed near transform-limited pulses; however, for IR144, it is observed for positively chirped pulses once the pulses have been stretched to hundreds of femtoseconds. We conclude that chirped pulse spectroscopy is a simple one-beam method that is sensitive to early solvation dynamics.

Optical Response of Fluorescent Molecules Studied by Synthetic Femtosecond Laser Pulses

The optical response of the fluorescent molecule IR144 in solution is probed by pairs of collinear pulses with intensity just above the linear dependence using two different pulse shaping methods. The first approach mimics a Michelson interferometer, while the second approach, known as multiple independent comb shaping (MICS), eliminates spectral interference. The comparison of interfering and non-interfering pulses reveals that linear interference between the pulses leads to the loss of experimental information at early delay times. In both cases, the delay between the pulses is controlled with attosecond resolution and the sample fluorescence and stimulated emission are monitored simultaneously. An out-of-phase behavior is observed for fluorescence and stimulated emission, with the fluorescence signal having a minimum at zero time delay. Experimental findings are modeled using a two-level system with relaxation that closely matches the phase difference between fluorescence and stimulated emission and the relative intensities of the measured effects.

Controlling S2 Population in Cyanine Dyes Using Shaped Femtosecond Pulses.

Fast population transfer from higher to lower excited states occurs via internal conversion (IC) and is the basis of Kashas rule, which states that spontaneous emission takes place from the lowest excited state of the same multiplicity. Photonic control over IC is of interest because it would allow direct influence over intramolecular nonradiative decay processes occurring in condensed phase. Here we tracked the S2 and S1 fluorescence yield for different cyanine dyes in solution as a function of linear chirp. For the cyanine dyes with polar solvation response IR144 and meso-piperidine substituted IR806, increased S2 emission was observed when using transform limited pulses, whereas chirped pulses led to increased S1 emission. The nonpolar solvated cyanine IR806, on the other hand, did not show S2 emission. A theoretical model, based on a nonperturbative solution of the equation of motion for the density matrix, is offered to explain and simulate the anomalous chirp dependence. Our findings, which depend on pulse properties beyond peak intensity, offer a photonic method to control S2 population thereby opening the door for the exploration of photochemical processes initiated from higher excited states.

Stimulated Emission Enhancement Using Shaped Pulses

Controlling stimulated emission is of importance because it competes with absorption and fluorescence under intense laser excitation. We performed resonant nonlinear optical spectroscopy measurements using femtosecond pulses shaped by π- or π/2-step phase functions and carried out calculations based on density matrix representation to elucidate the experimental results. In addition, we compared enhancements obtained when using other pulse shaping functions (chirp, third-order dispersion, and a time-delayed probe). The light transmitted through the high optical density solution was dominated by an intense stimulated emission feature that was 14 times greater for shaped pulses than for transform limited pulses. Coherent enhancement depending on the frequency, temporal, and phase characteristics of the shaped pulse is responsible for the experimental observations.

Ultrafast X-ray Absorption Near Edge Structure Reveals Ballistic Excited State Structural Dynamics

Polarized ultrafast time-resolved X-ray absorption near edge structure (XANES) allows characterization of excited state dynamics following excitation. Excitation of vitamin B12, cyanocobalamin (CNCbl), in the αβ-band at 550 nm and the γ-band at 365 nm was used to uniquely resolve axial and equatorial contributions to the excited state dynamics. The structural evolution of the excited molecule is best described by a coherent ballistic trajectory on the excited state potential energy surface. Prompt expansion of the Co cavity by ca. 0.03 Å is followed by significant elongation of the axial bonds (>0.25 Å) over the first 190 fs. Subsequent contraction of the Co cavity in both axial and equatorial directions results in the relaxed S1 excited state structure within 500 fs of excitation.

Long-lived electronic polarization and nonlinear optical effects of fluorescent molecules in solution

Summary form only given. Preparation and probing of coherent superposition of multiple quantum states leading to the generation of quantum coherences in condensed phase systems has been the topic of intense research for over a decade and is one of the most interesting and least understood of the quantum phenomena. Evolution of ultrashort broadband excitation sources has over the years provided us with new tools for revisiting problems in every field pertaining to light matter interaction. One such problem is the formation of electronic coherence through the reversible non-radiative electronic coupling and superposition of electronic states. Understanding the role of electronic coherence in condensed phase systems is central to elucidating the mechanism of energy transfer in photosynthetic pigment protein complexes [1] and several tubular nanostructures [2]. The recent observation of long-lived electronic coherence in chemical and biological system at room temperatures [3] forces one to reconsider the existing theories on energy transfer and devise an efficient and smart way in designing of synthetic light harvesting systems.Here we explore the linear and nonlinear optical response of solvated fluorescent molecules under saturation conditions by studying the changes in fluorescence and stimulated emission upon interacting with a pulse pair generated by pulse shaper assisted phase and amplitude modulation. In particular we focus on the observation of long lived oscillations that are detected in the stimulated emission signal during an interferometric measurement. Interferometric time-delay scans are measured while simultaneously monitoring the fundamental, fluorescence and stimulated emission, as shown in Fig. 1a. The pump pulse is fixed at time zero while the probe pulse is scanned. All signals are normalized to unity at time zero when the system is being interrogated by a single transform-limited pulse. The early portion of the scan (for IJ <; 60 fs) has strong interferometric modulation caused by the linear optical interference between the laser pulses. The different asymptotic levels of fluorescence and stimulated emission are due to the nonlinear effect of strong field excitation. The most interesting feature is the observed long lived oscillations in the stimulated emission signal which persists beyond ~150 fs. The observed π phase shift of the delayed modulation indicates that the physical source of the observed long lived oscillations is most likely caused by the induced linear polarization of the medium induced by the electronic coherence between ground and excited state. A simulation (using phenomenological formulas) of the stimulated emission showing the saturation, appearance and phase shift of the signal is shown in Fig 1b.

Electronic Coherence Mediated Quantum Control of Chemical Reactions in Polyatomic Molecules

We report order-of-magnitude quantum coherent control of the yield of photofragment ions following the creation of electronic coherence in the Rydberg states of dicyclopentadiene (C10H12) molecules using a pair of phase-locked femtosecond near-infrared laser pulses.