Figure 3.

Effect of phase-only shaping on single-molecule fluorescence intensity. (a) Phase oscillations (red) in the frequency domain produce an equally spaced pulse train in time domain. The interpulse delay is scanned for two different oscillation phases ϕ. (b) The pulse is spectrally split at 18 280 cm^–1 (547 nm), and the blue part is delayed by a linear phase wrapped between 0 and 2π. (c) In a closed loop approach, a genetic algorithm varied the phase mask to selectively maximize or minimize the fluorescence intensity. The algorithm has a genetic pool of 40 initially random phase masks ϕ₁, ϕ₂, ϕ₃..., for which in each generation the fitness is evaluated as the ratio of fluorescence intensity with the phase mask to be tested and with a spectrally flat phase mask. The genetic pool is modified by fitness-based breeding steps. The traces show the optimum and average fitnesses of all individuals in each generation. The optimum fitness intrinsically deviates from unity even with no control effect, because the algorithm selects the outer values of a normal distribution.