Enhancement of Intermediate-Field Two-Photon Absorption by Rationally-Shaped Femtosecond Pulses

We extend the powerful frequency-domain analysis of femtosecond two-photon absorption to the intermediate-field regime, which involves both two- and four-photon transitions. Consequently, we find a broad family of shaped pulses that enhance the absorption over the transform-limited pulse. It includes any spectral phase that is anti-symmetric around half the transition frequency. The spectrum is asymmetric around it. The theoretical framework and results for Na are verified experimentally. This work opens the door for rational femtosecond coherent control in a regime of considerable absorption yields

Pulse-Bandwidth Dependence of Coherent Phase Control of Resonance-Mediated (2+1) Three-Photon Absorption

We study in detail coherent phase control of femtosecond resonance-mediated (2+1) three-photon absorption and its dependence on the spectral bandwidth of the excitation pulse. The regime is the weak-field regime of third perturbative order. The corresponding interference mechanism involves a group of three-photon excitation pathways that are on resonance with the intermediate state and a group of three-photon excitation pathways that are near resonant with it. The model system of the study is atomic sodium (Na), for which experimental and numerical-theoretical results are obtained. Prominent among the results is our finding that with simple proper pulse shaping an increase in the excitation bandwidth leads to a corresponding increase in the enhancement of the three-photon absorption over the absorption induced by the (unshaped) transform-limited pulse. For example, here, a 40-nm bandwidth leads to an order-of-magnitude enhancement over the transform-limited absorption.Comment: 23 pages, 5 figure

Quantum control by von Neumann measurements

A general scheme is presented for controlling quantum systems using evolution driven by non-selective von Neumann measurements, with or without an additional tailored electromagnetic field. As an example, a 2-level quantum system controlled by non-selective quantum measurements is considered. The control goal is to find optimal system observables such that consecutive non-selective measurement of these observables transforms the system from a given initial state into a state which maximizes the expected value of a target operator (the objective). A complete analytical solution is found including explicit expressions for the optimal measured observables and for the maximal objective value given any target operator, any initial system density matrix, and any number of measurements. As an illustration, upper bounds on measurement-induced population transfer between the ground and the excited states for any number of measurements are found. The anti-Zeno effect is recovered in the limit of an infinite number of measurements. In this limit the system becomes completely controllable. The results establish the degree of control attainable by a finite number of measurements

Reprint of: Femtosecond transition-state spectroscopy of iodine: From strongly bound to repulsive surface dynamics

The application of femtosecond transition-state spectroscopy (FTS) to molecular iodine is reported. The real-time motion of wave packets prepared coherently in the bound B state is observed. In addition, the motion is probed near and above the dissociation limit for the reaction: I_2 → I (^2P_(3/2)) + I∗(^2P_(1/2)). FTS measurements of the dynamics on repulsive surfaces are also reported

Femtosecond real-time probing of reactions. V. The reaction of IHgI

The dissociation reaction of HgI2 is examined experimentally using femtosecond transition-state spectroscopy (FTS). The reaction involves symmetric and antisymmetric coordinates and the transition-state is well-defined: IHgI*-->[IHgI][double-dagger]@B|Q[sub S[script ']]Q[sub a[script ']]q-->HgI+I. FTS is developed for this class of ABA-type reactions and recurrences are observed for the vibrating fragments (symmetric coordinate) along the reaction coordinate (antisymmetric coordinate). The translational motion is also observed as a "delay time" of the free fragments. Analysis of our FTS results indicates that the reaction wave packet proceeds through two pathways, yielding either I(2P3/2) or I*(2P1/2) as one of the final products. Dissociation into these two pathways leads to HgI fragments with different vibrational energy, resulting in distinct trajectories. Hence, oscillatory behaviors of different periods in the FTS transients are observed depending on the channel probed (~300 fs to ~1 ps). These results are analyzed using the standard FTS description, and by classical trajectory calculations performed on model potentials which include the two degrees of freedom of the reaction. Quantum calculations of the expected fluorescence of the fragment are also performed and are in excellent agreement with experiments

Pulse generation without gain-bandwidth limitation in a laser with self-similar evolution

With existing techniques for mode-locking, the bandwidth of ultrashort pulses from a laser is determined primarily by the spectrum of the gain medium. Lasers with self-similar evolution of the pulse in the gain medium can tolerate strong spectral breathing, which is stabilized by nonlinear attraction to the parabolic self-similar pulse. Here we show that this property can be exploited in a fiber laser to eliminate the gain-bandwidth limitation to the pulse duration. Broad (̃200 nm) spectra are generated through passive nonlinear propagation in a normal-dispersion laser, and these can be dechirped to ̃20-fs duration

Intravital Imaging Study on Photodamage Produced by Femtosecond Near-infrared Laser Pulses in Vivo

© 2016 The American Society of Photobiology.Ultrashort femtosecond pulsed lasers may provide indispensable benefits for medical bioimaging and diagnosis, particularly for noninvasive biopsy. However, the ability of femtosecond laser irradiation to produce biodamage in the living body is still a concern. To solve this biosafety issue, results of theoretical estimations as well as the in vitro and in situ experiments on femtosecond biodamage should be verified by experimental studies conducted in vivo. Here, we analyzed photodamage produced by femtosecond (19, 42 and 100 fs) near-infrared (NIR; ∼800 nm) laser pulses with an average power of 5 and 15 mW in living undissected Drosophila larvae (in vivo). These experimental data on photodamage in vivo agree with the results of theoretical modeling of other groups. Femtosecond NIR laser pulses may affect the concentration of fluorescent biomolecules localized in mitochondria of the cells of living undissected Drosophila larva. Our findings confirm that the results of the mathematical models of femtosecond laser ionization process in living tissues may have a practical value for development of noninvasive biopsy based on the use of femtosecond pulses

Coherent control improves biomedical imaging with ultrashort shaped pulses

Abstract Although ultrashort pulses are advantageous for multiphoton excitation microscopy, they can be difficult to manipulate and may cause increased sample damage when applied to biological tissue. Here we present a method based on coherent control that corrects phase distortions introduced by high numerical aperture (NA) microscope objectives, thereby achieving the full potential of ultrashort pulses. A number of useful phase functions are recommended to gain selectivity that is similar to that which can be achieved by tuning a longer laser pulse; however this one involves no moving parts and maintains perfect optimization. This capability is used to demonstrate functional imaging by selective two-photon excitation of a pH-sensitive chromophore. Finally, we show that phase functions can also be introduced to minimize multiphoton excitation damage, while maintaining a high efficiency of two-photon excitation