An intense laser pulse can transiently turn a dielectric into a conducting medium by exciting electron-hole pairs. If the pulse is as short as a few optical cycles, and the excitation process is highly nonlinear, then injected charge carriers form a highly non-equilibrium state. We investigate the optical conductivity of such transient states. Understanding the electrical properties of these states is important to utilize a fast change of optical conductivity for ultrafast metrology. We find that the average effective mass of laser-excited electrons and holes significantly increases with the peak intensity of the laser pulse. Also, we find that the optical conductivity induced by an intense pulse is sensitive to its carrier-envelope phase.
We performed simulations using our recently developed numerical model where the time-dependent Schrödinger equation (TDSE) or semiconductor Bloch equations (SBE) are solved in three spatial dimensions using the basis of Kohn-Sham orbitals . As the input, our code uses band energies and momentum matrix elements obtained from DFT codes (Abinit, Wien2k, GPAW). The equations of motion are solved in the velocity gauge within the independent-particle approximation. Evaluating the polarization response, we benefit from our method for correcting artifacts typical to velocity-gauge simulations .
As an example, the average effective mass of charge carriers excited by a 750-nm 4-fs laser pulse in diamond increases by a factor of 1.7 as the amplitude of the laser pulse increases from 0.5 V/Å to 1.4 V/Å. For SiO2, this is a factor of 3.9. This result is important for interpreting pump-probe measurements designed to study the temporal dynamics of strong-field charge-carrier injection, where a probe pulse accelerates electrons and holes generated by an intense few-cycle pump pulse. Also, the significant increase of the effective mass for intense pulses must be taken into account when the Drude model is applied to describe excitation-induced changes of optical properties of dielectrics and semiconductors.
The effects that we study two main origins: First, a stronger field populates a larger number of bands. Second, multiphoton and tunneling transitions driven by a strong field populate a large part of the Brillouin zone, especially if the amplitude of the vector potential is comparable to or exceeds the size of the Brillouin zone.
 Wismer, M.S., M.I. Stockman, V.S. Yakovlev. Ultrafast optical Faraday effect in transparent solids. arXiv:1612.08433 [cond-mat.other], 2016.
 V. S. Yakovlev, M. Wismer. Adiabatic corrections for velocity-gauge simulations of electron dynamics in periodic potentials. Comp. Phys. Comm. 217, 82 (2017).
We report on the experimental study of optical breakdown induced in multilayer thin-films by ultrashort pulses at kHz and MHz repetition rates, while keeping all other parameters similar. The investigated samples were coatings composed of TiO2, Ta2O5, HfO2, or Al2O3 as high-index material and SiO2 as low-index material. We compared the distinct band gap dependencies obtained in the two regimes.
Improved performance of Free Electron Laser (FEL) light sources in terms of timing stability, pulse shape and spectral
properties of the amplified FEL pulses is of interest in many fields of science. A promising scheme is direct seeding with
High-Harmonic Generation (HHG) in a noble gas target. A Free-Electron-Laser seeded by an external XUV-source is
planned for FLASH II at DESY in Hamburg. The requirements for the XUV/soft X-ray source can be summarized as
follows: A repetition rate of at least 100 kHz in a 10 Hz burst is needed at variable wavelengths from 10 to 40 nm and
pulse energies of several nJ within single harmonics.
This application requires a laser amplifier system with exceptional parameters, mJ-level pulse energy, sub-10 fs pulse
duration at 100 kHz (1 MHz) burst repetition rate. A new OPCPA system is under development in order to meet these
requirements, and very promising results has been achieved. In parallel to this development, a new High- Harmonic
Generation concept is necessary to sustain the high average power of the driving laser system and for the need of high
conversion efficiencies. Highest conversion efficiency in High Harmonic Generation has been shown using gas-filled
capillary targets, up to now. For our application, only a free-jet target is applicable for high harmonic generation at high
repetition rate, to overcome damage threshold limitations of HHG target optics. A new multi-jet target is under
development and first tests show a good performance of this nozzle configuration.
A compact, low cost prismless Titanium:sapphire laser with 154nm bandwidth and 20mW output power was developed and ultrahigh resolution OCT ex vivo imaging in an animal model with sub-2μm and in vivo imaging in patients with 3μm axial resolution is demonstrated. This light source not only significantly reduces costs for broadband OCT light sources, but has also great potential for clinical OCT applications due to its small footprint (500x200mm including pump laser), user-friendliness and power stability.
Active mode-locking itself provides pulses not shorter than picoseconds. Nevertheless, this regime is useful for many laser systems including powerful ones where strict synchronization with external periodic signal is needed. Another growing field where active mode-locking is widely used is fiber laser systems. 30 years passed since D. Kuizenga and A. Siegman (K-S) developed their (present well- known) theoretical model for active mode-locking. Hence many other models have been published, all of them (including K- S) are not completely adequate for the real laser systems. That conclusion was pointed out by one of the authors for the gas, dye and solid-state lasers and presented in many publications and conference papers. In this paper we present results of successful numerical modeling for the mode-locked Ar-ion laser, which are in qualitative agreement with the experiment. Disagreement of the known models (or not revealed stuff from them) with the experiment can be formulated in such common items: (1) an optimum pulse position inside the temporal mode-locker window: dependence on the gain, mode-locker parameters; (2) phase characteristics (pulse position inside the mode-locker window); and (3) pulse amplitude and duration versus the mode-locker parameters.
The problems of radio astronomy.radio intelligence,early radio-locating radiation detection.signal processing for satellite communication etc demand creation of devices with analysis band UD to some tens of gigahertz.In the caDacitv of one of suitable technoloay for aoplication of fiber-ootic is offered [1,2). The fiber-optic sDectrum analyser <FOSA) basis is the fiber-ootic structure CFOS),which is shown in fiaure 1.Comina sianals are introduced in the fiber-oDtic structure bv intensitv.oolarization or Dhase modulation of laser oDtic radiation.FOS ensures time delav and sianal combination bv means of fibers and connection elements between them. The Dhotodetector block Provides outout of sianals from analvser. In this wav freauencv filter or inout devices for soectrum analvsis are realized. The same FOS mav be used for Drocessina of manv sianals simultaneouslv with division bv liaht wave lenath ~. That is one of imoortant advantaae o~ this technoloav.
This paper presents the design for wide band radio frequencies panoramic search to detect and discriminate radio signal disclosure. We refer to this system as the panoramic, discriminator (PD). Its main distinguishing feature is use of an acousto-optic power spectrum analyzer for conversion to the panoramic power spectrum representation of input signals.