Multiphoton intrapulse interference 3: Probing microscopic chemical environments

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.