Photodynamic therapy (PDT) is a treatment based on the interaction of light, photosensitizing agents and tissue oxygen.
The light delivery in PDT is usually optimized by controlling the intensity, the spectrum, and/or the dosage of excitation
light. In this paper, we introduce a novel method that aims to improve the efficiency of PDT by controlling the <i>phase</i> of
the excitation light, an important and so far neglected parameter. This coherent control approach utilizes the coherence
properties of light-matter interaction and aims to manipulate the quantum interferences between various available
reaction pathways. In general, an outcome of a photochemical reaction can be optimized by enhancing the desired
reaction pathways and suppressing other unwanted pathways. Such optimizations can be done by appropriate tailoring of
the electric field profile of a broadband coherent excitation light, i.e. ultrafast laser pulse. Here, we used a femtosecond
laser source with adaptive pulse shaping together with a molecular feedback in a learning loop to search for and
synthesize such 'smart' laser pulses. Our control objective is to enhance the triplet yield of a model photosensitizer zinc
phthalocyanine (ZnPc), which then leads to enhancement of the overall PDT process. We use two coherent control
schemes where we optimize the ratio between the excited singlet state (S) and triplet state (T) ZnPc molecules both ways
(S/T and T/S). We demonstrate a control of 15% over the triplet yield between the found best and the worst pulse shapes.
Our preliminary results show that phase shaping can indeed be used in manipulating photosensitizer photophysics and
correspondingly the yield of singlet oxygen.