Use of coherent control methods through scattering biological tissue to achieve functional imaging

Abstract

We test whether coherent control methods based on ultrashortpulse phase shaping can be applied when the laser light propagates through biological tissue. Our results demonstrate experimentally that the spectral-phase properties of shaped laser pulses optimized to achieve selective two-photon excitation survive as the laser pulses propagate through tissue. This observation is used to obtain functional images based on selective two-photon excitation of a pH-sensitive chromophore in a sample that is placed behind a slice of biological tissue. Our observation of coherent control through scattering tissue suggests possibilities in multiphoton-based imaging and photodynamic therapy.pulse shaping ͉ two-photon ͉ femtosecond laser ͉ intrapulse interference I nterest in coherent laser control has grown steadily, given its potential for influencing quantum-mechanical laser-molecule interactions (1-7). With the use of pulse shapers and computer learning algorithms, scientists have controlled the excitation of isolated atoms and molecules (8-10) and, more recently, large molecules in solution (11-13). Population transfer between different electronic states of large organic molecules can be maximized or minimized through manipulation of the spectral phase of ultrashort laser pulses (14, 15). The benefits of coherent laser control of large organic molecules can be extended to the biological field through selective two-photon chemical microenvironment probing (16) and microscopy (17). Control of lasermatter interactions could revolutionize biomedical applications. Coherent control schemes, for example, could enhance methods such as two-photon imaging (18, 19), which has provided higher resolution, lower background scattering, and better sample penetration than traditional techniques. Selective two-photon excitation would be desirable for distinguishing healthy from cancerous tissue based on differences in their chemical properties, such as pH, or for activating a photodynamic therapy (PDT) agent only when it is absorbed by cancer cells by using twophoton activated PDT (20). These improvements would be possible only if shaped laser pulses maintain their unique properties as they transmit through scattering biological tissue.Coherent control of laser-induced processes is based on the quantum interference among multiple excitation pathways (3). Progress in this field has been fueled by advances in pulseshaping technology, allowing control of the phase and amplitude across the bandwidth of ultrashort laser pulses (21). Control schemes depend on introducing intricate phase structures on the laser pulses that, by means of constructive and͞or destructive interference, optimize the desired outcome and minimize other pathways. This dependence on the accurate phase structure of the pulse suggests that these methods may not be applicable to situations involving transmission through scattering biological tissue.When a laser beam passes through a turbid scattering medium, its coherence (a property of waves defined as the degree with which w...

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