"Acceleration" of laser pulses due to a phase memory

A change in the shape of short relatively weak laser pulses, leading to an apparent acceleration, was observed. The effect occurred during propagation of ruby laser pulses in Rb₂ vapor in thermodynamic equilibrium at 200°C. A comparison of the theory with the experimental results yielded the modulus of the matrix element of the dipole moment of the resonance transition involved in this effect: The value of this modulus was 5 · 10^–18 cgs esu.