Igor Pastirk

Multiphoton intrapulse interference. IV. Ultrashort laser pulse spectral phase characterization and compensation

We introduce a noninterferometric single beam method to characterize and compensate the spectral phase of ultrashort femtosecond pulses accurately. The method uses a pulse shaper that scans calibrated phase functions to determine the unknown spectral phase of a pulse. The pulse shaper can then be used to synthesize arbitrary phase femtosecond pulses or it can introduce a compensating spectral phase to obtain transform-limited pulses. This method is ideally suited for the generation of tailored spectral phase functions required for coherent control experiments.

Multiphoton intrapulse interference. II. Control of two- and three-photon laser induced fluorescence with shaped pulses

Nonlinear optical processes are controlled by modulating the phase of ultrafast laser pulses taking advantage of multiphoton intrapulse interference. Experimental results show orders of magnitude control over two- and three-photon excitation of large organic molecules in solution using specific phase functions. We show simulations on the effect of phase modulation on the second- and third-order amplitude of the electric field spectrum, and demonstrate that the observed control is not caused by simple changes in peak intensity.

Selective two-photon microscopy with shaped femtosecond pulses

Selective two-photon excitation of fluorescent probe molecules using phase-only modulated ultrashort 15-fs laser pulses is demonstrated. The spectral phase required to achieve the maximum contrast in the excitation of different probe molecules or identical probe molecules in different micro-chemical environments is designed according to the principles of multiphoton intrapulse interference (MII). The MII method modulates the probabilities with which specific spectral components in the excitation pulse contribute to the two-photon absorption process due to the dependence of the absorption on the power spectrum of E2(t) [1-3]. Images obtained from a number of samples using the multiphoton microscope are presented.

Multiphoton intrapulse interference 6; binary phase shaping

We demonstrate a new approach to laser control using binary phase shaping. We apply this method to the problem of spectrally narrowing multiphoton excitation using shaped laser pulses as required for selectivity in two-photon microscopy. The symmetry of the problem is analyzed from first principles and a rational solution is proposed. Successful experimental implementation and simulations are presented using 10 fs ultrashort pulses. The proposed solution is a factor of 6 better than the sinusoidal phase used previously by our group. An evolutionary learning algorithm was used to efficiently improve the solution by a further factor of 2.5 because of the greatly reduced search space afforded by binary phase shaping.

Quantum control of the yield of a chemical reaction

Order of magnitude enhancement in the concerted elimination pathway leading to I2 product formation in the photodissociation reaction of CH2I2 by the use of positively chirped 312 nm femtosecond laser pulses is demonstrated. The maximum yield is found for chirps of 2400 fs2 while the minimum is found near −500 fs2. Multiphoton excitation with 624 nm pulses results in the opposite effect, where the maximum yield is found near −500 fs2. The enhancement as a function of chirp is found to depend on the wavelength and intensity of the laser pulses. These results offer new experimental evidence for quantum control of chemical reactions.

Population and coherence control by three-pulse four-wave mixing

Control of coherence and population transfer between the ground and excited states is reported using three-pulse four-wave mixing. The inherent vibrational dynamics of the system are utilized in timing the pulse sequence that controls the excitation process. A slight alteration in the pulse sequence timing causes a change in the observed signal from coherent vibration in the ground state to coherent vibration in the excited state. This control is demonstrated experimentally for molecular iodine. The theoretical basis for these experiments is discussed in terms of the density matrix for a multilevel system.

No loss spectral phase correction and arbitrary phase shaping of regeneratively amplified femtosecond pulses using MIIPS

Spectral phase correction (TBP<1.005) of femtosecond regeneratively amplified pulses using a MIIPS-enabled pulse shaper positioned between the oscillator and amplifier is performed. Rigorous characterization of the shaped output pulses will be presented.

Cascaded free-induction decay four-wave mixing

Abstract We report the observation of cascaded optical free-induction decay four-wave mixing (FID-FWM) signal. This process can take place when nonlinear optical measurements are carried out with pulses that are orders of magnitude shorter than the dephasing time of the sample. Experimental observations and theoretical calculations show that the coherent emission from the first laser pulse participates as a time-delayed local electric field to yield the cascaded signal. We arrive at this conclusion based on pulse sequences of degenerate noncollinear femtosecond pulses for which three-pulse FWM is forbidden. Further confirmation was obtained from experiments where the time delay between two pulses were used to form ground or excited state populations, the signal reflected the corresponding ground or excited state dynamics. Although FID is long lived, the femtosecond resolution was found to be maintained in our measurements on gas phase molecular iodine. This is because the FID is modulated in the femtosecond time scale by the molecular dynamics of the system; its intensity and modulation were confirmed using femtosecond time-gated up-conversion measurements.

The role of pulse sequences in controlling ultrafast intramolecular dynamics with four-wave mixing

This article seeks to provide a fundamental understanding of time-resolved four-wave mixing (FWM) processes based on a large body of experimental measurements on a model system consisting of isolated iodine molecules. The theoretical understanding is based primarily on a diagrammatic approach. Doublesided Feynman diagrams are used to classify and describe the coherent FWM processes involved in the signal obtained with each pulse sequence. Different pulse sequences of degenerate femtosecond pulses are shown to control the optical phenomena observed, that is transient grating, reverse-transient grating, photon echo and virtual photon echo. The experimental data reveal clear differences between the nonlinear optical phenomena. We find that the virtual photon echo sequence k1 - k2 + k3 is the most efficient for controlling the observation of ground - or excited-state dynamics. The strategy followed to make this assessment was to compare transients when the time delay between two of the three pulses set in or out of phase with the excited-state vibrational dynamics. We have obtained a signal from pulse sequences k1 + k2 - k3 for which FWM signal generation for this two-electronic-level system is forbidden. This signal can be explained by the cascading of a first-order polarization and a second-order process to generate the FWM signal. The implications of our findings are discussed in the context of multiple-pulse methods for the control of intramolecular dynamics.

Femtosecond photon echo and virtual echo measurements of the vibronic and vibrational coherence relaxation times of iodine vapor

Abstract Femtosecond three-pulse four-wave mixing (FWM) techniques are used to sort and measure the processes that contribute to coherence relaxation. We compare the observed relaxation times for vibronic coherence using photon echo (PE) and reverse transient grating (RTG) measurements at different temperatures to isolate inhomogeneous and homogeneous components. Different pulse sequences are used to select ground or excited state vibrational coherences. Measurements of ground and excited state wave packet spreading times due to anharmonicity, a process that does not involve energy dissipation or phase relaxation, are also presented.