Johanna M. Dela Cruz

Quantitative investigation of the multiphoton intrapulse interference phase scan method for simultaneous phase measurement and compensation of femtosecond laser pulses

Femtosecond pulse characterization and compensation using multiphoton intrapulse interference phase scan (MIIPS) [Opt. Lett.29, 775 (2004)] was rigorously tested. MIIPS was found to have 3 mrad precision within the 90 nm bandwidth of the pulses. Group-velocity dispersion measurements of glass and quartz provided independent accuracy tests. Phase distortions from high-numerical-aperture objectives were measured and corrected using MIIPS, an important requirement for reproducible two-photon microscopy. Phase compensation greatly improved the pulse-shaping results through a more accurate delivery of continuous and binary phase functions to the sample. MIIPS measurements were possible through the scattering of biological tissue, a consideration for biomedical imaging.

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 8. Coherent control through scattering tissue.

We demonstrate experimentally that selective two-photon probe excitation using phase shaped pulses can be achieved even when the laser propagates through scattering tissue. The pre-optimized phase tailored femtosecond pulses were able to identify acidic and basic solutions of a pH sensitive chromophore hidden behind a slab of scattering tissue. This observation has important implications for future applications of coherent control for biomedical imaging and photodynamic therapy.

The MIIPS method for simultaneous phase measurement and compensation of femtosecond laser pulses and its role in two-photon microscopy and imaging

A number of nonlinear imaging modalities, such as two-photon excitation and second harmonic generation, have gained popularity during the last decade. These, and related methods, have in common the use of a femtosecond laser in the near infrared, with the short pulse duration making the nonlinear excitation highly efficient. Efforts toward the use of pulses with pulse duration at or below 10 fs, however, have been a great challenge, in part due to the fact that shorter pulses have been found to cause greater sample damage. Here we provide a brief review of the MIIPS method for correction of phase distortions introduced by high numerical aperture objectives and the introduction of simple phase functions capable of preventing three-photon induced damage, reducing autofluorescence, and providing selective probe excitation.

Converting Concepts and Dreams of Coherent Control into Applications

This chapter summarizes early concepts of laser control and turns them into robust applications. A method is applied to ensure that a specific phase is delivered to the sample. Most of the pulse characterization methods cannot be used in situ and require one to send the laser to a device with different dispersion characteristics than those surrounding the experimental sample. The development of muitiphoton intrapulse interference phase scan provides an advantage. The second breakthrough was the introduction of binary phase modulation for the control of nonlinear laser–matter excitation. This practice is formalized with a rigorous theoretical foundation where it is demonstrated that under certain conditions, binary phase functions provided ideal optimization. The experiments are initial examples of how the technology developed can be used to optimize laser-induced excitation and how this can be used for the purposes of identification.

Quantitative phase characterization and compensation by multiphoton intrapulse interference phase scan (MIIPS)

Accurate and reproducible research in the field of femtochemistry requires both the measurement and then subsequent correction of phase distortions inherent in femtosecond laser pulses. Spectral phase distortions, which are introduced by the transmission of an ultrashort laser pulse through a lens, optical fiber, or microscope objective, or by interaction with an optical surface such as a dielectric mirror, have a negative effect both on the ability to carry out and the ability to reproduce experiments in which pulse duration is of importance, or nonlinear optical processes are utilized. This chapter presents a quantitative study on the ability of multiphoton intrapulse interference phase scan (MIIPS) to both characterize phase distortions with better precision and reproducibility than other established methods, and to correct them to obtain transform-limited pulses. The MIIPS method provides accurate phase retrieval and phase correction at the sample. These are important functions for reproducible femtosecond laser pulse experiments, especially those involving nonlinear optics. The MIIPS setup can also be used to deliver accurate synthetic wavefunctions at the sample as would be required for laser control experiments.

Selective two-photon functional imaging through scattering media based on binary phase shaping

We demonstrate experimentally selective two-photon functional imaging through a scattering medium (a thin slice of chicken) based on concepts of coherent control. The selectivity, achieved using binary phase shaping, is maintained by the ballistic photons.