THz commensurate echoes: Periodic rephasing of molecular transitions in free-induction decay

Abstract

We report the first study of coherent transients excited by ultrafast pulses of THz radiation. Using a newly developed optoelectronic source of well-collimated beams of subpicosecond pulses of THz radiation to excite ${\mathrm{N}}_{2}$O vapor, we have observed the subsequent emission from the vapor of coherent THz pulse trains extending to as long as 1 nsec. The origin of these subpicosecond THz pulses is a periodic rephasing, during the free-induction decay, of the more than fifty coherently excited rotational lines with commensurate transition frequencies. We term this new coherent phenomenon ``THz commensurate echoes.'' The echo repetition rate is equal to the frequency spacing between adjacent lines. From the decay and reshaping of the echoes the coherent relaxation time ${\mathit{T}}_{2}$ and the anharmonicity factor for the ${\mathrm{N}}_{2}$O molecule are evaluated.