Nonlinear photoionization of transparent solids: a nonperturbative theory obeying selection rules

We provide a nonperturbative theory for photoionization of transparent solids. By applying a particular steepest-descent method, we derive analytical expressions for the photoionization rate within the two-band structure model, which consistently account for the $selection$ $rules$ related to the parity of the number of absorbed photons ($odd$ or $even$). We demonstrate the crucial role of the interference of the transition amplitudes (saddle-points), which in the semi-classical limit, can be interpreted in terms of interfering quantum trajectories. Keldysh's foundational work of laser physics [Sov. Phys. JETP 20, 1307 (1965)] disregarded this interference, resulting in the violation of $selection$ $rules$. We provide an improved Keldysh photoionization theory and show its excellent agreement with measurements for the frequency dependence of the two-photon absorption and nonlinear refractive index coefficients in dielectrics

Developing the drilling tool for trenchless pipeline construction

The most promising methods of underground pipeline construction are discussed. To increase the efficiency of the drilling tool for underground pipeline construction the following research methods have been used in the article: optimization method of Evolutionary strategy, finite element modeling. The influence of geometric parameters of the drilling tool on force characteristics of enlarging the drill hole has been investigated

Dynamic Material Parameters in Molecular Dynamics and Hydrodynamic Simulations on Ultrashort-Pulse Laser Ablation of Aluminum

Recent developments on the open-source Molecular Dynamics Code IMD from the Institute of Functional Materials and Quantum Technologies at Stuttgart University are reported. Whereas IMD supports laser ablation comprising the Two-Temperature Model by default, reliable simulation data mainly can be found for the femtosecond regime, since laser-matter interaction is implemented by the Lambert-Beer law solely. For laser-matter interaction in the picosecond regime, IMD algorithms were modified in order to enable the dynamic recalculation of the dielectric permittivity of each FD cell for every timestep following the corresponding implementation in the open-source hydrodynamic code Polly-2T from the Joint Institute of High Temperatures at the Russian Academy of Sciences, Moscow. Thus, optical changes in material and jet due the temporal evolution of temperature, density and mean charge are taken into account. Moreover, as implemented in Polly-2T as well, a dynamic model for the thermal conductivity of the electron gas is introduced in IMD to consider effects for a wide range of temperatures. Thus, hydrodynamic (HD) and Molecular Dynamics (MD) simulations are compared extracting the residual effects of the different model approaches which persist in a particle-based method (IMD) using an Embedded-Atom potential (EAM) and a finite cell based approach (Polly-2T) employing the semi-empirical equations of state (EOS) including ionization. Simulation results are compared with experimental results and literature data

Suppression of ablation in femtosecond double pulse experiments

We report the physical reasons of a curious decrease in the crater depth observed for long delays in experiments with femtosecond double pulses. Detailed hydrodynamic modeling demonstrates that the ablation mechanism is dumped when the delay between the pulses exceeds the electron-ion relaxation time. In this case, the interaction of the second laser pulse with the expanding target material leads to the formation of the second shock wave suppressing the rarefaction wave created by the first pulse. The evidence of this effect follows from the pressure and density profiles obtained at different delays after the first laser pulse.Comment: Submitted to one of the APS Journal

Vibrational and structural properties of P2O5P_2O_5 glass: Advances from a combined modeling approach

We present experimental measurements and ab initio simulations of the crystalline and amorphous phases of $P_2O_5$. The calculated Raman, infrared, and vibrational density of states (VDOS) spectra are in excellent agreement with experimental measurements and contain the signatures of all the peculiar local structures of the amorphous phase, namely, bridging and nonbridging (double-bonded or terminal) oxygens and tetrahedral $PO_4$ units associated with $Q^2$, $Q^3$, and $Q^4$ species ($Q^n$ denotes the various types of $PO_4$ tetrahedra, with $n$ being the number of bridging oxygen atoms that connect the tetrahedra to the rest of the network). In order to reveal the internal structure of the vibrational spectrum, the characteristics of vibrational modes in different frequency ranges are investigated using a mode-projection approach at different symmetries based on the $T_d$ symmetry group. In particular, the VDOS spectrum in the range from $∼ 600$ to $870$ $cm^-$$^1$ is dominated by bending ($F_2$$_b$) motions related to bridging oxygen and phosphorus ($∼ 800$ $cm^-$$^1$ band) atoms, while the high-frequency doublet zone ($∼ 870 – 1250$ $cm^-$$^1$ is associated mostly with the asymmetric (($F_2$$_s$) and symmetric ($A_1$) stretching modes, and most prominent peak around $1400$ $cm^-$$^1$ (exp. $1380$ $cm^-$$^1$) is mainly due to asymmetric stretching vibrations supported by double-bonded oxygen atoms. The lower-frequency range below $600$ $cm^-$$^1$ is shown to arise from a mixture of bending ($E$ and ($F_2$$_b$) and rotation ($F_1$) modes. The scissors bending ($E$) and rotation ($F_1$) modes are well localized below $600$ $cm^-$$^1$, whereas the ($F_2$$_b$ bending modes spread further into the range $∼ 600 – 870$ $cm^-$$^1$. The projections of the eigenmodes onto $Q^2$, $Q^3$, and $Q^4$ species yield well-defined contributions at frequencies in striking correspondence with the positions of the Raman and infrared bands