Multiphoton Intrapulse Interference 3: Probing Microscopic Chemical Environments Johanna M. Dela Cruz, Igor Pastirk, Vadim V. Lozovoy, Katherine A. Walowicz, and Marcos Dantus* Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824 ReceiVed: July 23, 2003; In Final Form: September 3, 2003 Phase modulation, through its influence on the probability of two-photon excitation at specific frequencies, is used to probe a molecule’s microscopic chemical environment. The spectral phase required is designed according to the principles of multiphoton intrapulse interference. We show experimental results where phase modulation of 20 fs pulses from a titanium sapphire laser oscillator controls the intensity of two-photon induced fluorescence from the pH-sensitive dye 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) in solution. We show that the dependence of fluorescence yield on the phase can be utilized as a means of distinguishing molecules in different environments. Consequently, this method can be used to achieve selective multiphoton microscopy. To illustrate this capability, we show images where phase modulation was used to selectively excite HPTS in acidic or basic microscopic regions. 1. Introduction This work represents a continuing effort from our group focused on the design of tailored femtosecond pulses to achieve control of nonlinear optical excitation in large molecules based on the concept of multiphoton intrapulse interference (MII). 1-3 Our goal is to elucidate well-defined and reproducible pulse shapes that can be used to enhance or suppress particular nonlinear optical transitions in large molecules such as laser dyes and proteins in solution. In this paper, we demonstrate the use of MII to probe the local and microscopic environment of molecules by selective two-photon laser induced fluorescence (LIF). There has been much interest in the use of tailored ultrafast laser pulses to control the excitation of large molecules in condensed phase. 4-7 Coherent control of large molecules in the condensed phase has been considered very challenging because of the large numbers of intra- and intermolecular degrees of freedom involved. 8 Further complications arise from the ex- tremely fast homogeneous (<100 fs) and inhomogeneous (<10 fs) electronic dephasing in these systems. 9 Many of these studies have been based on the concept of optimal pulse shaping, whereby a learning algorithm optimizes the spectral phase of the laser field. 10-13 These methods have the advantage of convergence to an “optimal” pulse shape. However, because of the sensitivity of high-intensity multiphoton excitation to a large number of experimental parameters (for example, pulse intensity, saturation, transverse-mode quality, pulse bandwidth, and spectral phase which is affected by transmission through lenses and windows and reflection from dielectric mirrors), it is difficult to learn principles from these experiments that can be applied consistently to different samples in different labo- ratories. The influence of phase on two-photon excitation was explored by Broers et al., who proposed that certain phase functions could focus the second harmonic spectrum in the frequency domain like a Fresnel lens focuses light in the spatial domain. They demonstrated this principle by controlling the two-photon excitation of Rb atoms. 14 Meshulach et al. used the same principle to demonstrate coherent control on the two-photon excitation of Cs atoms. 15 Their work focused on the narrow two-photon absorption resonance; they showed that the coherent two-photon enhancement was absent in the broadband excitation of large molecules in solution. 15 Work from our group has demonstrated that the changes introduced by pulse shaping on two- and three-photon transitions arise from destructive interfer- ence and not on focusing in the frequency domain. 2 The theoretical framework developed by our group went beyond the narrow resonance condition and allowed us to develop MII for controlling multiphoton excitation in large molecules in solu- tion. 1,2 The fundamental advantage of our work was the early realization that very short pulses, with concomitant large bandwidths, would be needed to control the nonlinear excitation of complex systems. We realized that in the condensed phase inhomogeneous broadening would prevent the coherent ma- nipulation of vibronic levels, as can be done in the gas phase. Therefore, we developed a method to control the delivery of energy to the molecule that was less dependent on intramolecular dynamics. MII has now proven to be a robust method that is amenable to a number of applications. 3 The first application of phase tailored pulses for sensing pH involved intense linearly chirped laser pulses that saturated the one-photon resonant transition of a pH-sensitive dye. 16 The ratio between the LIF signal obtained for positively and negatively chirped pulses was found to vary from 1.9 to 1.2 for different pH values. However, this ratio did not vary monotonically, being the same for pH 7 and pH 9. The method presented here is completely different from the early experiments of Buist et al. 16 Our method involves very weak pulses inducing two-photon transitions. The samples are thus transparent at the carrier wavelength of the pulse, the transitions are not saturated, and the results are independent of probe molecule concentration. The mechanism we used for pH probing is understood and theoretical simulations are in close agreement with the data. The probability for two-photon excitation, S (2) , using phase- modulated pulses at frequency ω ) Δ + ω 0 , where ω 0 is the * To whom correspondence should be addressed. E-mail: dantus@msu.edu. 53 J. Phys. Chem. A 2004, 108, 53-58 10.1021/jp036150o CCC: $27.50 © 2004 American Chemical Society Published on Web 11/25/2003