Subfemtosecond-pulse generation by rotational molecular modulation and pulse-spacing increase

A technique is demonstrated to increase readily the pulse spacing of a subfemtosecond pulse train generated by rotational molecular modulation. Theoretical analysis and numerical calculations based on the fundamental rotational transition of molecular hydrogen showed that the pulse spacing of the generated periodic subfemtosecond pulse trains can readily be doubled, tripled, or even quadrupled when pairs of driving laser beams with properly selected carrier frequencies are used. This provides a practical technique for increasing the pulse spacings of periodic subfemtosecond pulse trains in a controllable way.