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Soviet Journal of Quantum Electronics

Compression of parametrically amplified pulses using ultrashort high-intensity adiabatic pumping

Б. В. Крыжановский

Published Feb 28, 1989• 1 citation
Publisher

Abstract

The problem of transient four-wave parametric interaction in a two-level medium is solved taking into account the group delay without assuming a weak exciting field. It is found that a high-power narrow peak whose duration τs is considerably shorter than the pump duration τp is observed against the background of a slowly varying envelope of the weak signal during its amplification process. In the case of a high pump intensity (ΩmτP−1) the lowest possible value of τS that can be achieved for a suitable angle of incidence and input signal frequency is governed by τp/Ωm1/2, where Ωm is the peak Stark shift of the resonance frequency.

DOI
10.1070/qe1989v019n02abeh007796

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The problem of transient four-wave parametric interaction in a two-level medium is solved taking into account the group delay without assuming a weak exciting field. It is found that a high-power narrow peak whose duration τs is considerably shorter than the pump duration τp is observed against the background of a slowly varying envelope of the weak signal during its amplification process. In the case of a high pump intensity (ΩmτP−1) the lowest possible value of τS that can be achieved for a suitable angle of incidence and input signal frequency is governed by τp/Ωm1/2, where Ωm is the peak Stark shift of the resonance frequency.
The results are given of a theoretical investigation of the interaction between two ultrashort pulses J1 and J2 of frequencies ω31 and ω21 with a three-level amplifying medium. It is assumed that the signal J1 of frequency ω31 is a steady-state π pulse and the signal J2 is weak so that its influence on the level populations can be ignored. The case when the signal J2 has a steady-state amplitude profile and an exponential gain is considered. The analysis is made ignoring the group delay effects and also allowing for the mismatch of the group velocities of the signals. The results obtained can be used in studies of the amplification of ultrashort pulses in molecular media such as organic dyes or CO2 molecules.
A theoretical investigation is made of an essentially nonstationary double resonance excited in a system with a common upper level in the field of a 2π pump pulse and of constant low-intensity signal radiation. The conditions are found for the formation of an exponentially amplified signal pulse of asymptotically constant profile moving at the velocity of the pump pulse. A study is made of the relationship between the signal pulse parameters and the parameters of the medium and of the pump pulse. It is shown that competition between the local and wave transient processes is responsible for the existence of an optimal pump pulse energy which depends on the values of the detuning from a resonance in the pump and signal channels. When this optimal energy is exceeded, the output signal pulse energy decreases. It is demonstrated that when the initial values of the detuning in the two channels are unequal the formation of a signal pulse is accompanied by pulling of the signal frequency to the value which satisfies the conditions of equality of the detuning values. During the early stages of formation of the signal pulse this results in splitting into a train of several pulses which are shorter than the pump pulse.
Four-wave parametric amplification is discussed when the incident pump frequency is close to a two-photon resonant transition in the material. Expressions for the amplification factor are obtained both under steady-state and pulsed excitation and also for a nonmonochromatic pump. For ultrashort-pulse excitation an ’’energy theorem’’ is derived and discussed. In steady state, expressions for optimum frequency and intensity are obtained. The amplification factor under broad-band excitation decreases with increasing pump linewidth.
The case is considered of a weak Gaussian harmonic pulse of frquency $(2s+1)\ensuremath{\omega}$ passed through an atomic medium collinearly with an intense Gaussian laser pulse of frequency $\ensuremath{\omega}.$ The harmonic beam may be amplified or deamplified according to the balance between stimulated emission and absorption. A simple analytic expression for the gain is derived in the tunneling limit of ionization. It predicts a positive (negative) gain for positive (negative) delay of the harmonic pulse with respect to the laser pump pulse. The gain may be significant and proportional to the delay for delays of a few laser periods, much shorter than the pulse envelopes. Possibilities for experimental observations are briefly discussed.
An experimental investigation was made of a four-wave parametric interaction (FWPI) in a field of ultrashort optical pulses (λp ≈0.55 μ) acting on a 1S–1P0 two-level system of barium atomic vapor. It was observed that a pair of new frequencies ν3 and ν4 appeared as a result of FWPI, ν3 being close to ν'tps = 2νp – ν'sp and ν4 being close to ν'sp. This process was observed on approaching the resonance from the low-frequency side. When the resonance was approached from the high-frequency side, conical emission was recorded. It was found that a FWPI in a two-level system subjected to a strong pump field is governed by the third- and fifth-order nonlinear susceptibilities χ(3) and χ(5).