A modified FDTD algorithm for processing ultra-wide-band response

Finite-difference time-domain (FDTD) is an effective algorithm for resolving Maxwell equations directly in time domain. Although FDTD has obtained sufficient development, there still exists some improvement space for it, such as ultra-wide-band response and frequency-dependent nonlinearity. In order to resolve these troubles, a modified version of FDTD called complex-field frequency-decomposition (CFFD) FDTD method is introduced, in which the complex-field is adopted to eliminate pseudo-frequency components when computing nonlinearity and the frequency-decomposition is adopted to transform an ultra-wide-band response into a series of narrow-band responses when computing the interaction of ultra-short pulse with matters. Its successful applications in several typical situations and comparison with other methods sufficiently verify the uniqueness and superiority in processing ultra-wide-band response and frequency-dependent nonlinearity.Comment: 8 figure

Tunable enhancement of harmonic radiation in coupled quantum wells

A few-cycle ultrashort laser field propagating through strongly coupled quantum wells (CQW) is numerically investigated. The results show that the harmonic signal can be tuned by the structure-control of CQW or enhanced due to propagation effects. If the structure of the CQW is spatial inversion symmetric, a disguised harmonic at the second-order harmonic position is disclosed within the normal odd-order harmonic sequence. However, if the structure of the CQW is adjusted to break the inversion symmetry, the odd-order and even-order harmonics both occur, whose intensities are also influenced by propagation effects

High‐Quality Femtosecond Laser Surface Micro/Nano‐Structuring Assisted by A Thin Frost Layer

Abstract Femtosecond laser ablation has been demonstrated to be a versatile tool to produce micro/nanoscale features with high precision and accuracy. However, the use of high laser fluence to increase the ablation efficiency usually results in unwanted effects, such as redeposition of debris, formation of recast layer, and heat‐affected zone in or around the ablation craters. Here this limitation is circumvented by exploiting a thin frost layer with a thickness of tens of microns, which can be directly formed by the condensation of water vapor from the air onto the exposed surface whose temperature is below the freezing point. When the femtosecond laser beam is focused onto the target surface covered with a thin frost layer, only the local frost layer around the laser‐irradiated spot melts into water, helping to boost ablation efficiency, suppress the recast layer, and reduce the heat‐affect zone, while the remaining frost layer can prevent ablation debris from adhering to the target surface. By this frost‐assisted strategy, high‐quality surface micro/nano‐structures are successfully achieved on both plane and curved surfaces at high laser fluences, and the mechanism behind the formation of high‐spatial‐frequency (HSF) laser‐induced periodic surface structures (LIPSSs) on silicon is discussed