Burst-Driven Collective Ultrafast Interactions for Confined Optical Absorption

Abstract

Ultrafast bursts, groups of closely spaced femtosecond pulses, can drive light-matter interactions that are not possible with single-pulse excitations. This occurs when earlier pulses create a transient effect that nonlinearly alters the interaction of later pulses, provided that the inter-pulse spacing is shorter than the relaxation time of the transient. A long-standing trade-off in optical excitation is between multiphoton absorption, which provides strong spatial confinement but low energy-transfer efficiency, and linear absorption, which is efficient but offers limited spatial localization. Here, we show that a burst of ultrafast pulses can collectively build up a population of free carriers in semiconductor nanoparticles via multiphoton absorption. The free carriers absorb linearly and strongly, and dissipate rapidly once the burst ends. The result is a more efficient absorption process that is strongly gated spatially and temporally by multiphoton absorption and burst duration, respectively. We develop an analytical model that couples carrier generation with Auger and surface recombination, yielding a piecewise-exact description of the carrier dynamics and an intuitive scaling relation for the collective energy-deposition enhancement. A signature consequence of collective absorption is its dependence on one extra power of the light intensity, effectively raising the order of an m-photon process to m+1 without requiring higher intensities for the individual pulses. While we focus on nonlinear photoacoustic imaging as a potential application, we anticipate diverse applications, including nonlinear fluorescence microscopy.