A simple physical picture for quantum control of wave packet localization

In a short-pulse CO2 laser based on discharge excitation, there is a pulse tail that depends on the device configuration and operating conditions. The pulse tail is longer than the spike pulse and causes thermal effects such as a crack, heat-affected zones (HAZ), and so on. There are various types of glass having different physical constants related to heat, such as the thermal expansion coefficient and the softening point. Even if the same CO2 laser pulse is radiated onto glass, the processing results may differ depending on the glass material. Four types of glass, namely, crown glass, soda-lime glass, borosilicate glass, and synthetic quartz glass were irradiated with two types of short CO2 laser pulses, one with a large pulse tail and one with a small pulse tail, at a repetition rate of 200 Hz and a fluence per pulse of 22 J/cm2. As the processing characteristics, the ratios of the surface hole diameter and the HAZ diameter to the irradiation diameter, as well as the drilling depth, were investigated. The pulse-tail energy of the short CO2 laser pulses did not affect the surface hole diameter. In the glasses with small softening points of 740 °C or less, the pulse-tail energy of short CO2 laser pulses affected the HAZ with a large number of pulse irradiations with a total irradiation fluence of 2000 J/cm2 or more. The short CO2 laser pulses with a small tail produced a smaller HAZ than the short CO2 laser pulses with a large tail. In drilling with a large number of pulse irradiations, the short CO2 laser pulses with a small tail produced deeper drilling than the short CO2 laser pulses with a large tail. The glass material did not affect the surface hole diameter and the drilling depth. The glass material affected the HAZ.

The physical mechanism of strong field quantum control using chirped femtosecond laser pulses is investigated. Dressed state control is exerted by making explicit use of the temporal phase changes of the pulse. In our experiment, the dressed state population is mapped by photoelectron spectra from simultaneous excitation and ionization of potassium atoms as a function of the chirp parameter. We show that chirped pulses can be used to selectively steer ground state atoms temporarily into a single dressed state realizing transient Selective Population of Dressed States (SPODS).

We report generation of 5 TW peak power in a 2 ps pulse at 9.2 µm via chirped-pulse amplification (CPA) in mixed-isotope high-pressure CO2 laser amplifiers. This represents the highest peak power in a single picosecond pulse format and the shortest pulse at terawatt peak power ever achieved in the long-wave infrared (LWIR) spectral range. Maximum demonstrated energy of the compressed pulse is 14 J and minimum period between shots is 20 s. The successful implementation of the chirped pulse amplification technique in a short-pulse gas laser opens a route to development of LWIR laser facilities with tens of terawatts of peak power, thus approaching the number of photons per optical cycle of current near-infrared laser systems based on solid-state active media.

In the last years, laser beam drilling became increasingly important for many technical applications as it allows the contactless production of high quality drill holes. So far, mainly short laser pulses are of industrial relevance, as they offer a good compromise between precision and efficiency and combine high ablation efficiency with low thermal damage of the workpiece. Laser beam drilling in this pulse length range is still a highly thermal process. There are two ablation mechanisms: evaporation and melt expulsion. In order to achieve high quality processing results, a basic process understanding is absolutely necessary. Yet, process observations in laser beam drilling suffer from both the short time scales and the restricted accessibility of the interaction zone. Numerical simulations offer the possibility to acquire additional knowledge of the process as they allow a direct look into the drill hole during the ablation process. In this contribution, a numerical finite volume multi-phase simulation model for laser beam drilling with short laser pulses shall be presented. The model is applied for a basic study of the ablation process with μs and ns laser pulses. The obtained results show good qualitative correspondence with experimental data.

An analytic description for the yield, P(p), of high-energy electrons ionized from an atom by a short (few-cycle) laser pulse is obtained quantum mechanically. Factorization of P(p) in terms of an electron wave packet and the cross section for elastic electron scattering (EES) is shown to occur only for an ultrashort pulse, while in general P(p) involves interference of EES amplitudes with laser-field-dependent momenta. The analytic predictions agree well with accurate numerical results.

We discuss a method of solving the time dependent Schrodinger equation for atoms with two active electrons in a strong laser field, which we used in a previous paper [A. Scrinzi and B. Piraux, Phys. Rev. A 56, R13 (1997)] to calculate ionization, double excitation and harmonic generation in Helium by short laser pulses. The method employs complex scaling and an expansion in an explicitly correlated basis. Convergence of the calculations is documented and error estimates are provided. The results for Helium at peak intensities up to 10^15 W/cm^2 and wave length 248 nm are accurate to at least 10 %. Similarly accurate calculations are presented for electron detachment and double excitation of the negative hydrogen ion.

Introduction of chirped‐pulse amplification (CPA) enabled the latest revolution in production of high peak powers from lasers through amplification of very short (femtosecond) laser pulses to pulse energies previously available only from long‐pulse lasers. CPA has rapidly bridged the gap from its initial modest demonstrations to multi‐terawatt and petawattscale systems in research facilities and universities, as well as numerous lowerpower scientific and industrial applications.

This study presents a survey on quantum control theory and applications from a control systems perspective. Some of the basic concepts and main developments (including open-loop control and closed-loop control) in quantum control theory are reviewed. In the area of open-loop quantum control, the paper surveys the notion of controllability for quantum systems and presents several control design strategies including optimal control, Lyapunov-based methodologies, variable structure control and quantum incoherent control. In the area of closed-loop quantum control, this study reviews closed-loop learning control and several important issues related to quantum feedback control including quantum filtering, feedback stabilisation, linear-quadratic-Gaussian control and robust quantum control.

Self-focusing and the initial stages of plasma generation by short laser pulses

The breakdown of gases by short (typically sub-nanosecond) laser pulses when the two dominant mechanisms governing the plasma growth are multiphoton and cascade ionization is treated theoretically. It is shown that two important pressure régimes result: a low-pressure régime where the breakdown is governed solely by the multiphoton ionization process; and a high-pressure régime where the expression for threshold breakdown intensity involves both ionization processes as well as the effect of impurities in the gas.The predictions of the theory are compared with published experimental data for ruby and Nd3+ laser pulses. Good agreement is obtained in the case of ruby radiation, and a possible explanation of deviation from the experimental data obtained using a Nd3+ laser is offered.

The acceleration of proton beams during the interaction of an ultra short and ultra intense laser pulse with matter is possibly the most important application of compact laser systems with multi-terawatt and petawatt power. High quality beams with a small energy spread are required for applications where spatially accurate energy deposition is important. For some specific applications the target may be located far from the acceleration region, in which case space charge effects and the interaction with an ambient plasma may deteriorate the beam quality during its propagation.Here, first we review a recently proposed method for producing high quality proton beams using short laser pulses and a target made of a layer of heavy ions followed by a thin proton layer with a transverse size smaller than the pulse waist. Subsequently, we investigate the effects of the beam propagation through a plasma slab in a simplified one-dimensional model.

In this paper we review contamination and damage that has occurred since the HELEN laser was converted for use as a chirped pulse amplification (CPA) system. We concentrate on the largest components in the facility that operated in a vacuum environment in the short pulse (500fs) parts of the system. This experience will be valuable for planning commissioning and operations on the successor facility, ORION that is currently being constructed. The optical components located in the vacuum sections will be described. We have an interest in laser-induced damage and contamination from the long pulse only (~1ns), short pulse only (500fs) and combined regimes. For most of the operations the CPA system has operated at a wavelength of 1053nm (1ω). Some experiments have also been conducted with 527nm (2ω) CPA light derived by the use of a KDP doubling crystal. Damage to the infra red pulse compression gratings has occurred infrequently with a minority of high-energy shots. Contamination of multilayer dielectric plane turning mirrors has arisen from target disassembly. Focussing systems using off axis parabolic mirrors have sustained contamination from debris and a dielectric protected silver reflector used for green light suffered laser induced damage. Debris shields placed between the target and the parabolas have been used on selected experiments. The shields were anti-reflection coated with single layer, sol gel silica. These shields became contaminated on the target facing sides and in the case of 2ω operation also from blow off of the damaged mirror coating that caused a two-pass transmission loss in the system. A number of characterisation methods were used to evaluate and quantify the damage and contamination. These include macroscopic photography, microscopy, reflectometry and transmission spectroscopy.

Significant progress has been made in recent years using lasers for electron acceleration to high energies in plasmas. The main idea of such laser-plasma schemes is based on an opportunity to use the lasers for generation a large amplitude regular plasma wave with strong longitudinal electric field and relativistic phase velocity, which is capable of acceleration injected electrons. The great interest to these accelerators is due to their ability to sustain extremely large acceleration gradients (<100 GeV/m) essentially exceeding the acceleration gradients of conventional radio-frequency linear accelerators (>100 MeV/m). At the first stage, the main attention was attracted to plasma beat wave accelerator scheme (PBWA) where two-frequency laser radiation was used to create a plasma wave. With development of chirped pulse amplification (CPA) and powerful laser systems, the new methods for excitation of large amplitude relativistic plasma waves appear as the laser wake-field accelerator (LWFA) and self-modulated laser wake-field accelerator (SM-LWFA). This report is intended to give a brief overview of up to date results obtained for different schemes of laser-plasma based accelerators. The problems and plans of future investigations are also discussed.

A multiframe, high-time resolution pump-probe diagnostic consisting of a consecutive train of ultrashort laser pulses (approximately ps) has been developed for use with a chirped pulse amplification (CPA) system. A system of high quality windows is used to create a series of 1054 nm picosecond-laser pulses which are injected into the CPA system before the pulse stretcher and amplifiers. By adding or removing windows in the pulse train forming optics, the number of pulses can be varied. By varying the distance and thickness of the respective optical elements, the time in between the pulses, i.e., the time in between frames, can be set. In our example application, the CPA pulse train is converted to 527 nm using a KDP crystal and focused into a preformed plasma and the reflected laser light due to stimulated Raman scattering is measured. Each pulse samples different plasma conditions as the plasma evolves in time, producing more data on each laser shot than with a single short pulse probe. This novel technique could potentially be implemented to obtain multiple high-time resolution measurements of the dynamics of physical processes over hundreds of picoseconds or even nanoseconds with picosecond resolution on a single shot.

Micro structures in silicon are applied in different fields of industry, medicine and research. Examples are micro mechanical sensors for car security systems, nozzle plates for printer, and optical elements for X-ray beam splitting. Wherever the accuracy of etched silicon structures is not required, laser processes with short pulses and small wave length can be an option with the advantage of shorter process time High quality cutting and drilling of silicon can be achieved by application of diode pumped q-switched Nd:YAG-lasers and harmonics generation. The short pulses (15 ns) and the UV wave length (355 nm) of the Gator UV from Lambda Physik AG, for example, allow a reduction of thermal effects like deposition of molten material and heat effected zones at the edges. Especially in the case of deep structures the ablation plasma causes powerful heating of the walls. An reduction of the plasma temperatures and so the heat influence on the walls can be realized by a small laser wave length with low plasma absorption. Short laser pulse durations are necessary to reduce the heat effected depth or melting due to heat flow from the ablation area into the bulk material. Also the duration and intensity of plasma heating is reduced by short laser pulses. In this contribution the possibilities and limits of laser machining of Si by diode pumped Nd:YAG-lasers with harmonics generation will be presented by means of structures processed by application of a scanner with f-theta-optic. The results will be discussed concerning the experimental setup and the laser parameters.