I. A. McIntyre

Studies of multiphoton production of vacuum-ultraviolet radiation in the rare gases

Measurements of the vacuum-ultraviolet (<80-nm) radiation produced by intense ultraviolet (248-nm) irradiation (1015–1016 W/cm2) of rare gases have revealed the copious presence of both harmonic radiation and fluorescence from excited levels. The highest harmonic observed was the seventeenth (14.6 nm) in Ne, the shortest wavelength ever produced by that means. Strong fluorescence was seen from ions of Ar, Kr, and Xe, with the shortest wavelengths observed being below 12 nm. Furthermore, radiation from inner-shell excited configurations in Xe, specifically the 4d95s5p → 4d105s manifold of Xe7+ at ~17.7 nm, was detected. These experimental findings, in alliance with other studies concerning multielectron processes, give evidence for a role of electron correlations in a direct nonlinear process of inner-shell excitation.

Subpicosecond KrF* excimer-laser source.

A subpicosecond KrF* laser system capable of producing 20 +/- 2-mJ pulses has been developed. The means of producing ultrashort seed pulses for the KrF* amplifier system and characteristics of the full system are described. It is shown that efficient subpicosecond energy extraction is possible.

High power ultrafast excimer lasers

Rare‐gas–halide excimer lasers are suitable for the production of extremely high peak brightness output in the ultraviolet. The basic properties of these systems are examined and the various techniques employed to produce high power (multiterawatt) operation are described. A specific system using short‐pulse injection is examined and its ability to focus to intensities above ∼1019 W cm−2 is discussed. Various applications of such high intensity subpicosecond light sources are also considered.

Shortening of KrF* laser pulses using stimulated Brillouin scattering

Pulses as short as 43 psec were generated by Brillouin backscattering at 248 nm in cyclohexane. The measurements show that the process is accompanied by considerable spectral broadening of the reflected radiation.

Dynamic absorption effects in KrF* amplifiers

The results of transient loss measurements performed in a self-sustained discharge KrF* amplifier are reported. Analysis of these results gives a minimum value of 20 for the effective gain to loss ratiog0/αeff, indicating that efficient extraction of energy in subpicosecond KrF* amplifiers in the ∼1 J range should be achievable.

Fifth-harmonic production in neon and argon with picosecond 248-nm radiation

The results of a study of fifth-harmonic production in neon and argon irradiated with 248-nm picosecond laser pulses are presented. Focused intensities range from 1013 to 1015 W/cm2. Data for fifth-harmonic intensity as a function of both target density and focused laser intensity are presented and compared with theory. For the laser intensities and medium densities studied, estimates for the linear and nonlinear components of Δk, the wave-vector mismatch between the fundamental and harmonic waves, indicate that the nonlinear component is much greater than the linear component.

Multiphoton ionization in intense ultraviolet laser fields

The mechanism of collision-free multiphoton ionization of rare gases irradiated with ultraviolet radiation at an intensity of up to approx.10/sup 16/ W/cm/sup 2/ at 248 nm and a pulse length of approx.0.5 ps and approx.10/sup 15/ W/cm/sup 2/ at 193 nm and approx.5ps, respectively, has been studied by observing the electron energy spectra and ion charge state threshold intensities. The formation of multiply charged ions by a sequential process of ionization has been directly detected in the electron energy spectra in the form of a characterisitic pattern of interwoven above threshold ionization (ATI) ladder line series. The threshold intensities for ion production have been compared with the Keldysh model and were found to be in good agreement for light ions (Ne) and consistently lower for heavier ions (Xe), scaling with the atomic number. These measurements, together with estimates of the ion population dynamics during the rise of the laser pulse can be reasonably understood on the basis of a single electron picture. 16 refs., 5 figs.

Tunable hybrid mode-locked dye laser operating in the range 735–760 nm

Abstract Hybrid mode-locking of two near infra-red dyes, Pyridine 2 and Rhodamine 700 is reported. Five different absorber dyes are used to produce the optimum gain/absorber dye combination for operation at 745 nm. The laser, tuned using a birefringent filter, produces pulses down to 325 fs long and the untuned laser, incorporating intracavity dispersion compensation, generates pulses

Multi-quantum processes at high field strengths

Subpicosecond ultraviolet laser technology is enabling the exploration of nonlinear atomic interactions with electric field strengths in excess of an atomic unit. As this regime is approached, experiments studying multiple ionization, photoelectron energy spectra, and harmonically produced radiation all exhibit strong nonlinear coupling. Recent findings in connection with the ion spectra produced by such interactions are described, and it is shown that energy transfer from the radiation field to the atom depends importantly on the risetime of the field. The nonlinear atomic interaction is also discussed with respect to high laser intensities (≳1018 W/cm2) in combination with high atomic density (≳1018 cm−3), for which inter‐atomic as well as intraatomic coupling may play a significant role.

Subpicosecond Studies Of Collision-Free Multiple Photon Processes In The Ultraviolet

In recent years a strong interest has developed in the study of the basic coupling mechanisms governing high order multiphoton processes in atomic systems.1 Such interest has been stimulated by the rapid advances in short pulse generation techniques which allow atomic interactions at an intense field strength of the order of atomic field to be studied. Furthermore, good understanding of the highly nonlinear photon interactions is necessary to evaluate the possibility of using multiphoton processes2 as a means to couple energy into excited states emitting in the x-ray region.