The rise time of intense radiation determines the maximum field strength atoms can be exposed to before their polarizability dramatically drops due to the detachment of an outer electron. Recent progress in ultrafast optics has allowed the generation of ultraintense light pulses comprising merely a few field oscillation cycles. The arising intensity gradient allows electrons to survive in their bound atomic state up to external field strengths many times higher than the binding Coulomb field and gives rise to ionization rates comparable to the light frequency, resulting in a significant extension of the frontiers of nonlinear optics and (nonrelativistic) high-field physics. Implications include the generation of coherent harmonic radiation up to kiloelectronvolt photon energies and control of the atomic dipole moment on a subfemtosecond $(1{\mathrm{f}\mathrm{s}=10}^{\mathrm{\ensuremath{-}}15}\mathrm{}\mathrm{s})$ time scale. This review presents the landmarks of the 30-odd-year evolution of ultrashort-pulse laser physics and technology culminating in the generation of intense few-cycle light pulses and discusses the impact of these pulses on high-field physics. Particular emphasis is placed on high-order harmonic emission and single subfemtosecond extreme ultraviolet/x-ray pulse generation. These as well as other strong-field processes are governed directly by the electric-field evolution, and hence their full control requires access to the (absolute) phase of the light carrier. We shall discuss routes to its determination and control, which will, for the first time, allow access to the electromagnetic fields in light waves and control of high-field interactions with never-before-achieved precision.

Intense few-cycle laser fields: Frontiers of nonlinear optics

Coherent extreme-ultraviolet radiation extending to wavelengths below the carbon K edge at 4.37 nanometers (nm) has been generated at a repetition rate of 1 kilohertz by focusing 5-femtosecond near-infrared (780 nm) laser pulses into a helium gas jet. The incident light field performs just a few oscillations, which results in the emission of an x-ray supercontinuum rather than discrete harmonics. Owing to the extremely short rise time of the driving pulses, neutral atoms can be exposed to high fields before they are depleted by ionization. As a result, the observed x-ray radiation extends well into the water window and is delivered in a well-collimated beam (divergence less than 1 milliradian). The high repetition rate and spatial coherence result in a brightness of about 5 × 108 photons per square millimeter per square milliradian per second in a 1-percent bandwidth at 4.37 nm, the carbon edge of the water window. The compact laboratory system holds promise as a source for biological holography and nonlinear optics in the x-ray regime.

https://science.sciencemag.org/content/sci/278/5338/661.full.pdf

Generation of Coherent X-rays in the Water Window Using 5-Femtosecond Laser Pulses

Publisher Summary This chapter illustrates that the development of lasers capable of delivering short pulses of very intense radiation, over a wide frequency range, has led to the discovery of new, nonperturbative multiphoton processes in laser interactions with atomic systems. In this article, we first give a survey of the main properties of multiphoton processes such as the multiphoton ionization of atoms, the emission by atoms of high-order harmonics of the exciting laser light, and laser-assisted electron-atom collisions. It then review the theory of these processes, giving particular attention to ab-initio nonperturbative methods such as the Sturmian-Floquet approach, the R-matrix-Floquet theory, and the numerical integration of the time-dependent Schrodinger equation. The chapter discusses relativistic effects that occur at ultra-high intensities. The chapter concludes by considering possible future developments of high-intensity laser-atom physics.

High-intensity laser-atom physics

High harmonic generation has been investigated in the atomic gases Ar and Xe as well as in the diatomic molecular gases H2, D2, N2 and O2 using a linearly polarized picosecond dye laser (594 nm) and a linearly polarized subpicosecond Ti:sapphire/Nd:glass laser (1053 nm). The harmonic conversion efficiencies are compared by choosing different gas pairs as a function of three parameters: (1) field-free ionization potential (2) mass and (3) static average polarizability. It is shown that the ionization potential and mass are not sensitive parameters for harmonic efficiency, but the polarizability is an important factor.

https://iopscience.iop.org/article/10.1088/0953-4075/27/20/024/pdf

High harmonic generation in atomic and diatomic molecular gases using intense picosecond laser pulses-a comparison

A novel laser-based apparatus is presented utilizing high harmonic radiation for visible pump–EUV probe experiments on ultrafast processes. True femtosecond temporal resolution is achieved by a monochromator making use of dedicated narrowband multilayer mirrors rather than gratings for selection of single harmonic orders in the photon energy range between 66 and 73 eV. First applications of this new light source for electron spectroscopy on gas phase helium and xenon demonstrate the selection of a single high harmonic order with the intensity ratio between the selected and its adjacent harmonic not exceeding 10:1. A pump–probe study of hot electron production on a solid Pt(110) surface yields a cross-correlation corresponding to a temporal system resolution of 100 fs.

Laser-based apparatus for extended ultraviolet femtosecond time-resolved photoemission spectroscopy

We present systematic experimental studies of high-order harmonic generation in He and Ne that have been performed with an intense, femtosecond Ti:sapphire laser at intensities in the tunneling regime, where special attention is concentrated on the strong-field effect. A broad plateau is formed in the harmonic distribution that includes harmonic peaks at low and high orders and the observed highest orders (shortest wavelengths) are the 103rd (7.6 nm) for He and 95th (8.2 nm) for Ne. The laser-ellipticity-dependent harmonics observed suggest different generation processes for the low orders below and around the atomic ionization limit and for the high orders in and above the plateau region. The medium ionization is found to induce strong self-defocusing of the incident laser beam, and its effects on the harmonic generation are studied. The results for the defocusing and the blueshifts of the harmonic spectra observed show that the generation of a higher-order harmonic requires a higher intensity in time and space. The theoretically predicted harmonic cutoff is demonstrated to agree well with experiment when we take into account the interaction intensity limited in the ionizing medium. The characteristic harmonic behaviors observed are discussed based on the interaction intensity and the propagation effect in the medium.

High-order harmonic generation in the tunneling regime.

High-order harmonic generation in rare gases with subpicosecond XeCl laser pulses.

PROBLEM TO BE SOLVED: To manufacture a high-quality compound thin film solar cell which prevents cell conversion efficiency from being reduced, the reduction in the cell conversion efficiency being caused by a heat damage layer when separating the compound thin film solar cell in accordance with laser scribing method.SOLUTION: A second laser scribing step is applied to a laminated portion of a compound semiconductor light absorption layer 13, a buffer layer 14 and a high-resistance buffer layer 15, and continuously, a third laser scribing step is applied to a structure formed with a transparent conductive film layer 16. In the second and third laser scribing steps, a division groove is formed by performing linear scanning under a predetermined irradiation condition of an ultra-short pulse laser. The irradiation condition is set in such a way that a pulse repetition in which a time interval after which the next pulse is irradiated is determined while reducing a laser energy density per pulse so as not to attain groove formation in one time of linear scanning, and a beam overlapping degree of the pulse laser lights on an irradiated surface is suppressed equal to or less than a fixed value.

Manufacturing method of compound thin film solar cell, and compound thin film solar cell

Femtosecond Ti:sapphire Laser Oscillators (Special Issue:Basic Technologies for X-Ray Lasers)

http://ieeexplore.ieee.org/iel5/8035/22193/01034343.pdf

Hideyuki Takada

Papers

High-order harmonic generation in the tunneling regime.

High-order harmonic generation in rare gases with subpicosecond XeCl laser pulses.

Manufacturing method of compound thin film solar cell, and compound thin film solar cell

Femtosecond Ti:sapphire Laser Oscillators (Special Issue:Basic Technologies for X-Ray Lasers)

Singleshot Measurements of Carrierenvelope Phase Using Timedependent Polarization Pulse