Figure 4
(a) Laser spectrum (shaded grey curve), initial spectral phase measured with MIIPS (red curve) and the inverse phase mask obtained with the closed-loop optimization on QDs (blue curve). The light blue shaded region represents the uncertainty on the determination of the phase mask, corresponding to the standard deviation calculated repeating the maximization process several times. (b) Laser spectrum (shaded grey curve), absorption (blue solid curve) and TPPL (red curve) spectrum of the QDs. The red-colored region is the portion of the TPPL we detected in the experiment using a 680 nm short pass filter to block the laser. The blue-dashed curve is the magnified (100 × ) absorption spectrum in the 700 nm range. The blue arrow at 400 nm indicates the spectral region where the TPA takes place. (c) Sketch of two possible sum frequency processes, for electric fields E(ω0−Δ)+E(ω0+Δ) or E(ω0)+E(ω0), leading to the same final state. Here ω0 is the central frequency of the laser and E(ω0±Δ) represents a frequency component within the laser bandwidth. The total TPA is maximal when all the possible pathways constructively interfere.
