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Soviet Journal of Quantum Electronics

Parametric frequency conversion of ultrashort light pulses

Valery S. Butylkin, V.S. Grigoryan, M. E. Zhabotinskiǐ

Published Mar 31, 1980• 0 citations
Publisher

Abstract

A study is made of the evolution of ultrashort pulses of interrogation and generated fields, traveling across a medium after a pump pulse under two-photon resonance conditions. It is shown that, in spite of the absence of overlap of the pulses in space and time, the generated wave becomes phase-locked. The area theorem is obtained for the process in question. It is found that at large distances the second pulse should give rise to a subpulse of the interrogation field and a proportional subpulse whose area tends to zero or to 2π. This conclusion about the formation of subpulses and their limiting areas applies also in the two-photon interaction case. A report is given of an estimate of the threshold intensities of an interrogation pulse in cesium and sodium vapors. The maximum conversion efficiency is found as a function of the pump energy, parameters of the investigated medium, and frequencies of the interacting fields. It is shown that an overall conversion efficiency close to 100% can be obtained when the intensities of the pump and interrogation fields are within the capabilities of the currently available lasers.

DOI
10.1070/qe1980v010n03abeh010095

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A study is made of the evolution of ultrashort pulses of interrogation and generated fields, traveling across a medium after a pump pulse under two-photon resonance conditions. It is shown that, in spite of the absence of overlap of the pulses in space and time, the generated wave becomes phase-locked. The area theorem is obtained for the process in question. It is found that at large distances the second pulse should give rise to a subpulse of the interrogation field and a proportional subpulse whose area tends to zero or to 2π. This conclusion about the formation of subpulses and their limiting areas applies also in the two-photon interaction case. A report is given of an estimate of the threshold intensities of an interrogation pulse in cesium and sodium vapors. The maximum conversion efficiency is found as a function of the pump energy, parameters of the investigated medium, and frequencies of the interacting fields. It is shown that an overall conversion efficiency close to 100% can be obtained when the intensities of the pump and interrogation fields are within the capabilities of the currently available lasers.
An analysis is made of the evolution of high-power laser pulses in the case of coherent two-photon interaction with resonant media. It is shown that pulses of frequencies ω 0 and ω s satisfying the equalit ω 0 ±ω 0 =ω 21 (ω 21 is the resonance frequency of the medium) experience splitting into separate subpulses followed by a reduction in their duration and increase in their power. The efficiency of this process depends on the initial ratio of the amplitudes E 0 and E s . It is shown that such effects can be observed in paramagnetic acoustic resonance.
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.
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Four-photon parametric up-conversion of a weak infrared signal under conditions of two-photon resonance of a constant nonmonochromatic pump field is analyzed allowing for population migration, transient response of the medium, Stark shift of the levels, and width of the pump frequency spectrum. It is found that under steady-state conversion conditions in a nonmonochromatic pump field, it is possible to achieve an average conversion efficiency differing only negligibly from the maximum conversion efficiency in a monochromatic pump field. An analysis is made of the influence of optically induced relaxation on the average conversion efficiency in the transient regime. Numerical estimates are made of the transition parameters for an alkali metal vapor in a frequency-degenerate pump field exciting a two-photon transition.
A theoretical analysis is made of the interaction between fields whose frequencies satisfy the relationship ωp–ωs=ωg±ωa≈ω21 , where ωp, ωs, ωg and ωa are the frequencies of the pump, Stokes, generated, and auxiliary fields; ω21 tne frequency of a transition between two levels in the investigated susbstance. The conditions are established for the maximum efficiency of the conversion of the fields ωp and ωa into the field ωg. It is shown that the optimal conversion conditions are obtained for a certain ratio of the pump and auxiliary field powers when the phases of the interacting waves are locked. Numerical estimates are obtained of the conversion efficiency and interaction length when the active substance is potassium vapor or compressed hydrogen.