Side Absorption Research Articles

Ultrafast dynamic response and temporal shaping modulation of tungsten disulfide irradiated by femtosecond laser

Femtosecond laser treatment has been widely used for modulating different kinds of materials as a convenient and efficient approach. In the process of laser modulation, the ionization caused by femtosecond laser irradiation may significantly affect the propagation and energy deposition of laser pulse inside the material, and thus finally influencing the surface morphology and optimizing the material properties. In this work, the ablation of WS<sub>2</sub> is conducted in a wide range of laser fluence by single pulse. With the increase of injected energy, the expansion of craters goes through a process from rapid growth to stabilization both in the direction of diameter and in the depth direction. And a plasma model is proposed to track the dynamic response of the excited material and the transfer and deposition of the laser energy in the irradiation of WS<sub>2</sub>. The calculated results reveal that a great number of free electrons will generate after the incidence of laser pulse and leads the dense plasma zone to form. In this zone, the reflection on the surface and the absorption inside of WS<sub>2</sub> are both enhanced due to the rapid increase of free electron density, which affects the injection and deposition of laser energy, thus resulting in the deposition of most energy in the shallow area below the surface. With the increasing of the laser fluence, the majority of laser energy is deposited on the surface of WS<sub>2</sub>, which leads the ablation crater to reach the saturation state. Meanwhile, a double-pulse train generated by temporal shaping is utilized to modulate the diameter of craters. By adjusting the pulse delay, the smallest diameter of the crater can be obtained at 0.7 ps. The results pave the way for potential applications of the effective method in controlling the material removal and improving the catalytic performance of pristine WS<sub>2</sub>.

Statistical wave scattering through classically chaotic cavities in the presence of surface absorption

We propose a model to describe the statistical properties of wave scattering through a classically chaotic cavity in the presence of surface absorption. Experimentally, surface absorption could be realized by attaching an "absorbing patch" to the inner wall of the cavity. In our model, the cavity is connected to the outside by a waveguide with N open modes (or channels), while an experimental patch is simulated by an "absorbing mirror" attached to the inside wall of the cavity; the mirror, consisting of a waveguide that supports N(a) channels, with absorption inside and a perfectly reflecting wall at its end, is described by a subunitary scattering matrix S(a). The number of channels N(a) , as a measure of the geometric cross section of the mirror, and the lack of unitarity P(a) = [symbol: see text]N(a) - S(a)(+)S(a) , as a measure of absorption, are under our control: these parameters have an important physical significance for real experiments. The absorption strength in the cavity is quantified by gamma(a) = tr P(a). The statistical distribution of the resulting S matrix for N = 1 open channel and only one absorbing channel, N(a) = 1, is solved analytically for the orthogonal and unitary universality classes, beta = 1 and beta = 2, respectively, and the results are compared with those arising from numerical simulations. The relation with other models existing in the literature, in some of which absorption has a volumetric character, is also studied.