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Journal of Physics B Atomic Molecular and Optical Physics

Maximal coherence in a three-level autoionizing atom for the generation of subpicosecond pulses in the extreme ultraviolet

Marco V. Tognetti, Roberto Buffa

Published Dec 8, 2003• 23 citations
Publisher

Abstract

We discuss the possibility of using a three-level ladder atomic system, with a final autoionizing state, for the efficient generation of short radiation pulses in the extreme ultraviolet through a process of sum-frequency mixing in the presence of maximal coherence. The results of numerical calculations are presented, investigating the effects of photoionization, dynamic Stark shift and electron impact dephasing on the conversion efficiency in a single atom model. The results of this investigation can be useful in identifying a real candidate for a possible experiment.

DOI
10.1088/0953-4075/37/1/001

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Optics Letters•Jan 15, 2002•2 citations
We discuss the possibility of using a three-level ladder atomic system, with a final autoionizing state, for the efficient generation of short radiation pulses in the extreme ultraviolet through a process of sum-frequency mixing in the presence of maximal coherence. The results of numerical calculations are presented, investigating the effects of photoionization, dynamic Stark shift and electron impact dephasing on the conversion efficiency in a single atom model. The results of this investigation can be useful in identifying a real candidate for a possible experiment.
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We present an experimental demonstration of sum-frequency generation toward tunable picosecond, broadband radiation pulses in the extreme-ultraviolet regime, supported by adiabatically prepared atomic coherences. We drive a two-photon transition in a dense medium of xenon atoms with a long (nanosecond) pump laser pulse at the Fourier-transform-limited bandwidth and with a wavelength of 225 nm. The frequency of the pump pulse is slightly detuned from exact two-photon resonance. In this configuration, the medium is adiabatically driven by a process of coherent population return. The adiabatic passage process generates a maximal coherent superposition of two quantum states with large energy spacing. An additional, short (picosecond) probe laser pulse at a wavelength of 540 nm beats with the maximal atomic coherence and generates a short (picosecond) signal radiation pulse at 93 nm. As compared to conventional (diabatic) frequency conversion, the adiabatically driven maximal atomic coherence yields enhanced conversion efficiency and significantly enhanced stability.
A scheme is analyzed for effcient generation of vacuum ultraviolet radiation through four-wave mixing processes assisted by the technique of Stark-chirped rapid adiabatic passage. These opportunities are associated with pulse excitation of laddertype short-wavelength two-photon atomic or molecular transitions so that relaxation processes can be neglected. In this three-laser technique, a delayed-pulse of strong oR-resonant infrared radiation sweeps the laser-induced Stark-shift of a two-photon transition in a such way that facilitates robust maximum two-photon coherence induced by the first ultraviolet laser. A judiciously delayed third pulse scatters at this coherence and generates short-wavelength radiation. A theoretical analysis of these problems based on the density matrix is performed. A numerical model is developed to carry out simulations of a typical experiment. The results illustrate a behavior of populations, coherence and generated radiation along the medium as well as opportunities of effcient generation of deep (vacuum) ultraviolet radiation.
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.
We present experimental data on adiabatically driven frequency conversion with ultrashort (picosecond) laser pulses towards the vacuum-ultraviolet regime, enhanced by coherent population return and preparation of maximal atomic coherences. We generate the sum frequency of an intense pump pulse and a probe pulse via a two-photon resonance in xenon. When we slightly detune the pump laser from two-photon resonance, the atomic populations are adiabatically driven forth and back from the atomic ground state to an excited state. This coherent population return (CPR) prepares the medium in a transient state of maximal atomic coherence, which enhances frequency mixing with a probe laser. We thoroughly study the variation of CPR-enhanced frequency conversion with experimental parameters and compare CPR with conventional, resonantly enhanced frequency conversion. In particular, we investigate the temporal evolution of the atomic coherence and the effect of inevitable ac Stark shifts. We find that ac Stark shifts induce an asymmetry in the spectral characteristics of CPR, which nevertheless permits enhanced frequency conversion. For the case of resonantly enhanced frequency conversion, we show that the atomic coherences are maintained for several tens of picoseconds after the pump pulse, which permits time-delayed frequency conversion. Moreover, we analyze the pressure dependence of the atomic coherence' lifetime and observe a free induction decay at the two-photon coherence with the generation of a second harmonic field. These findings shall push applications of adiabatic light-matter interactions also to the regime of nonlinear optics, driven by intense laser pulses.