Transient optical non-linearity in p-Si induced by a few cycle extreme THz field.

We study the impact of a few cycle extreme terahertz (THz) radiation (the field strength ETHz ∼1-15 MV/cm is well above the DC-field breakdown threshold) on a p-doped Si wafer. Pump-probe measurements of the second harmonic of a weak infrared probe were done at different THz field strengths. The second harmonic yield has an unusual temporal behavior and does not follow the common instantaneous response, ∝ETHz2. These findings were attributed to: (i) the lattice strain by the ponderomotive force of the extreme THz pulse at the maximal THz field strength below 6 MV/cm and (ii) the modulation of the THz field-induced impact ionization rate at the optical probe frequency (due to the modulation of the free carriers' drift kinetic energy from the probe field) at the THz field strength above 6-8 MV/cm.

Circular ripple patterns on silicon induced by bubble-diffracted femtosecond laser pulses in liquid.

We report on a new technique of silicon surface nanostructuring in liquid with a pair of Gaussian-shaped femtosecond laser pulses. The bubble, generated in liquid near the molten silicon surface by the first pulse, serves as a dynamic microscale obstacle for spatial modulation of the intensity profile of the second pulse following at a certain delay via scattering processes. As a result, the circular ripple patterns with anomalously high surface-relief modulation, undersurface annular nanocavities, and interfacial smoothness are produced at the surface. The possibility of the control over the specific pattern through the laser intensity variation is shown.

Transient Second Harmonic Generation Induced by Single Cycle THz pulses in Ba0.8Sr0.2TiO3/MgO.

The ability to switch ferroics (magnets, ferroelectrics, multiferroics) between two stable bit states is the main principle of modern data storage technology. Due to many new ideas, originating from fundamental research during the last 50 years, this technology has developed in a breath-taking fashion. Ever increasing demands for faster and more energy efficient data storage strongly motivate fundamental studies of dynamics in ferroics triggered by ultrashort stimuli. It has been recently realized that nearly single cycle intense THz pulses and the phenomenon of the second harmonic generation are appealing tools for excitation and detection of poorly understood ultrafast dynamics of electric polarization in ferroelectrics at the picosecond timescale. Here we investigate picosecond dynamics of second harmonic from near-infrared pulse in ferroelectric heterostructure Ba0.8Sr0.2TiO3/MgO triggered by the electric field of a nearly single cycle intense THz pulse. The dynamics of the nonlinear optical signal is characterized by a step and oscillations at the frequency of about 1.67 THz. Although the observations can be mistakenly interpreted as oscillations of the electric polarization at the frequency of the soft mode and switching of the order parameter to another metastable state, here we show that the THz modulation of second harmonic generation in Ba0.8Sr0.2TiO3/MgO has a purely optical origin. The observation can be explained assuming that the THz pulse is a relativistically propagating inhomogeneity which induces center of symmetry breaking and linear birefringence. Our work reveals the role of propagation effects in interpretation of time-resolved non-linear optical experiments and thus it has important implications for experimental studies of ultrafast dynamics in ferroics.

THz Electric Field-Induced Second Harmonic Generation in Inorganic Ferroelectric.

Second Harmonic Generation induced by the electric field of a strong nearly single-cycle terahertz pulse with the peak amplitude of 300 kV/cm is studied in a classical inorganic ferroelectric thin film of (Ba0.8Sr0.2)TiO3. The dependences of the SHG intensity on the polarization of the incoming light is revealed and interpreted in terms of electric polarization induced in the plane of the film. As the THz pulse pumps the medium in the range of phononic excitations, the induced polarization is explained as a dynamical change of the ferrolectric order parameter. It is estimated that under action of the THz pulse the ferroelectric order parameter acquires an in-plane component up to 6% of the net polarization.

Generation of hard x rays by femtosecond laser pulse interaction with solid targets in atmosphere.

X ray radiation as high as 50 keV, including K(α) of Ba and Mo, have been observed from a solid target during the interaction of low energy ~0.65 mJ, 1 kHz 40 femtosecond laser pulses focused in air at atmospheric pressure. Energetic electrons generating such x rays are possibly produced when the field strength in laser pulse wake exceeds the runaway threshold in air. Two dimensional particle-in-cell simulations that include optical field ionization of air and elastic collisions support this mechanism.