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Journal of the Optical Society of America B

Generation of infrared and extreme-ultraviolet radiation in krypton with picosecond irradiation at 193 nm

Mahnaz Shahidi, Ting S. Luk, C. K. Rhodes

Published Nov 1, 1988• 7 citations
Publisher

Abstract

The nonlinear interaction of neutral krypton atoms with picosecond 193-nm radiation with intensities ≤1013 W/cm2 has been studied. Because of the relatively high intensity of the applied radiation field and the two-photon resonance of the 4p–6p transition, several competing nonlinear processes were observed that result in the generation of coherent radiation. Amplified spontaneous emission (ASE) from the two-photon-resonant level was observed in the IR frequency at ~2 μm. Additionally, a tunable IR wave, in the same frequency range, was generated by stimulated hyper-Raman scattering of the incident laser. The observed self-generated IR waves induced other nonlinear interactions, which resulted in the generation of extreme-ultraviolet (XUV) radiation in the same medium. Tunable XUV frequency was shown to be generated by four-wave sum mixing of the laser and the hyper-Raman wave. Vacuum-ultraviolet frequency was shown to be generated by four-wave oscillation, and other nontunable XUV frequencies resulted from anti-Stokes Raman scattering of the ASE IR waves.

DOI
10.1364/josab.5.002386

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The nonlinear interaction of neutral krypton atoms with picosecond 193-nm radiation with intensities ≤1013 W/cm2 has been studied. Because of the relatively high intensity of the applied radiation field and the two-photon resonance of the 4p–6p transition, several competing nonlinear processes were observed that result in the generation of coherent radiation. Amplified spontaneous emission (ASE) from the two-photon-resonant level was observed in the IR frequency at ~2 μm. Additionally, a tunable IR wave, in the same frequency range, was generated by stimulated hyper-Raman scattering of the incident laser. The observed self-generated IR waves induced other nonlinear interactions, which resulted in the generation of extreme-ultraviolet (XUV) radiation in the same medium. Tunable XUV frequency was shown to be generated by four-wave sum mixing of the laser and the hyper-Raman wave. Vacuum-ultraviolet frequency was shown to be generated by four-wave oscillation, and other nontunable XUV frequencies resulted from anti-Stokes Raman scattering of the ASE IR waves.
Resonant and near-resonant two-photon excitations with ultrashort laser pulses of forbidden atomic transitions give access to a wide class of nonlinear-optical processes such as two-photon absorption, stimulated Raman scattering, four-wave sum- and difference-frequency mixing. The latter has proofed to be a powerful tool for efficient femtosecond pulse generation in the VUV-spectral range [1]. Nevertheless, up to now only few experimental results have been obtained which allow the direct observation of the coherent dynamics in two-photon induced radiation-matter interactions (see experimental papers [2-5] and monograph [6]).
We find that intense short pulses of XUV radiation can be produced by cooperative spontaneous emission from visible or IR laser pulses driving atoms or ions. The process depends on the generation and utilization of atomic coherence as is the case in lasing without inversion. However, the radiation process is not stimulated emission, but is rather cooperative spontaneous emission in the sense of Dicke. More precisely, the many atom mathematics of superradiance has much in common with coherent anti-Stokes Raman scattering. The generation of Raman coherence is considered from the perspectives of multi-photon excitation and breaking of adiabaticity. In particular, we find that the advent of an ultrashort (e.g. single cycle) pulse rewards rethinking of conventional (nanosecond) nonlinear optics with new insights.
Two-photon resonant and near-resonant four-wave difference-frequency mixing in gases in the interaction regime when laser pulse durations are comparable to or shorter than the medium polarization relaxation time ${T}_{2}^{\ensuremath{'}}$ is investigated. The results of experimental studies of the process in Ar and Kr using pump pulses from the ArF-excimer laser are presented demonstrating a generation of tunable short pulse radiation in the range 102--124 nm. The results are discussed in terms of a theoretical model based on a self-consistent solution of the Bloch equations for the atomic transitions and the Maxwell equations for the fields. This enables one to interpret specific nonstationary resonant and quasiresonant phenomena involved in the frequency conversion process. It is shown that the femtosecond-pulse four-wave frequency-mixing technique with probe pulses significantly shorter than the pump pulses makes it possible to study the coherent dynamics of an atomic transition exposed to an intense field. Using atomic Kr as the nonlinear medium, coherent Rabi oscillations and the subsequent phase relaxation of excitation were observed under the condition of two-photon interaction of Kr with femtosecond 193-nm laser pulses. The obtained information is important for controlling and optimizing processes of two-photon resonant frequency conversion and for time-resolved studies of Rydberg states in atoms and molecules.
Coherent radiation at several wavelengths in the vacuum ultraviolet (VUV) has been generated by four-wave parametric oscillation in mercury vapor. When a powerful ultraviolet pump laser of frequency omega(p) is tuned to a two-photon resonance, VUV signal photons at frequency omega(s), as well as idler photons at frequency omega(i), are generated such that omega(s) + omega(i) = 2omega(p). A frequency-doubled dye laser tuned to two-photon resonance with the 6s6d(1)D(2) level near 280.3 nm produced output at 184.9 and 143.5 nm. A tunable krypton fluoride excimer laser tuned to two-photon resonance with the 6s10s (1)S(0) level near 248.7 nm produced output at 140.1, 130.7, 130.1, 125.9, and 125.0 nm. Approximately 1-microJ, 200-W pulses were observed at 143.5 and 125.9 nm.
The generation of tunable, nanosecond-pulsed extreme-ultraviolet (EUV) light (84.92nm(84.92\mathrm{nm}(84.92nm to 84.97nm84.97\mathrm{nm}84.97nm $\ensuremath{\sim}14.6\mathrm{eV})$ using four-wave mixing in optically deep krypton gas was investigated experimentally and theoretically. The sum-frequency mixing scheme generated light from two overlapping autoionizing resonances, ${17s}^{\ensuremath{'}}$ and ${15d}^{\ensuremath{'}}$. We investigated whether the laser-induced atomic coherence effect of electromagnetically induced transparency (EIT) could enhance the intensity of the EUV light generated. The maximum EUV output was seen from the region where the ${15d}^{\ensuremath{'}}$ autoionizing resonance has a transparency window due to Fano-type quantum interference. An analytical model using appropriate atomic parameters explains how background absorption can prevent EIT enhancement of EUV generation; and shows how a background Raman-like coupling term in the nonlinear susceptibility can well describe the measured EUV generation spectrum.
Sum-frequency generation enhanced by electromagnetically induced transparency (EIT) has been extended to the production of extreme ultraviolet (XUV) radiation in the range 97.3--92.6 nm from $np\ensuremath{-}1s$ transitions (n=4−−8)(n=4--8)(n=4−−8) in atomic hydrogen. Pulsed radiation was generated by strong coupling of the np\mathrm{np}np and 2s2s2s levels with laser radiation at Balmer wavelengths, and simultaneously (weaker) coupling of the 2s2s2s and 1s1s1s levels by two-photon resonance with 243-nm laser radiation. Investigations were carried out over a range of laser intensities and products of interaction length and atomic density (NL).(\mathrm{NL}).(NL). As the product NL\mathrm{NL}NL increased, the XUV intensities were greatly enhanced by EIT, and at high NL\mathrm{NL}NL values, the spectral distributions of some of the generated signals were found to be distorted by the presence of molecular hydrogen. Calculated profiles, modified to incorporate the effects of nearby molecular resonances, were shown to be in good agreement with the observed spectra.