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.

We explore and demonstrate the use of phase-modulated ultrafast laser pulses for controlling nonlinear optical processes in large molecules, proteins, and solid materials. Our experiments illustrate that in condensed phases, when spectra are broad, the spectrum of the nth-order electric field, determined by multiphoton intrapulse interference, plays a major role in controlling multiphoton excitation. These findings determine key parameters (amplitude, period, and symmetry of the phase function) for coherent femtosecond laser control in condensed phases.

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 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.

Order of magnitude enhancement in the concerted elimination pathway leading to I 2 product formation in the photodissociation reaction of CH 2 I 2 by the use of positively chirped 312 nm femtosecond laser pulses is demonstrated. The maximum yield is found for chirps of 2400 fs 2 while the minimum is found near Ϫ500 fs 2. Multiphoton excitation with 624 nm pulses results in the opposite effect, where the maximum yield is found near Ϫ500 fs 2. 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.