Controlling semiconductor nanoparticle size distributions with tailored ultrashort pulses

Adjustments of dielectrics craters and their surfaces by ultrafast laser pulse train based on localized electron dynamics control

A quantum model with the consideration of laser wave-particle duality based on the plasma model is employed for the femtosecond laser pulse train processing of fused silica. Effects of the key pulse train parameters, such as the pulse separation time and the number of pulses per train on the distributions of free electron are discussed. The calculations show that the spatial/temporal distributions of free electron can be adjusted by transient localized electron dynamics control using femtosecond laser pulse train design; the results are ablation shapes of craters and subwavelength ripples. It is also found that the first pulse separation time (Δt1) can be used for rough adjustments of ablated structures, while the second pulse separation time (Δt2) can be used for the fine tuning of ablated structures, especially the shapes of craters.

Closed loop coherent control of electronic transitions in gallium arsenide

A genetic algorithm was used to control the photoluminesce-nce (PL) from GaAs(100). A spatial light modulator (SLM) used feedback from the emission to optimize the spectral phase profile of an ultrashort laser pulse. Most of the experiments were performed using a sine phase function to optimize the integrated PL spectrum over a specified wavelength range, with the amplitude and period of the phase function treated as genetic parameters. An order of magnitude increase in signal was achieved after only one generation, and an optimized waveform, consisting of three equally spaced pulses approximately 0.8 ps apart, was obtained after 15 generations. The effects of fluence, polarization, relative phase of the subpulses, and spectral range of the optimized PL were investigated. In addition, preliminary experiments were performed using the phases of individual pixels of the SLM as genetic variables. The PL spectrum is identified with recombination of electron-hole pairs in the L-valley of the Brillouin zone. Control is achieved by coherent manipulation of plasma electrons. It is proposed that hot electrons excite lattice phonons, which in turn scatter carriers into the L-valley.