Tuning the frequency of few-cycle femtosecond laser pulses by molecular phase modulation

We demonstrate theoretically how the time-dependent phase modulation induced by molecular alignment can be used to tune the frequency of few-cycle femtosecond laser pulses continuously. Using impulsively excited alignment in N(2), the central wavelength of an initial 800 nm, 5 fs Gaussian pulse can be tuned from 324.6 to 4237.3 nm. The aligned N(2) molecules are obtained by pretransmitting another 800 nm, 100 fs linearly polarized laser pulse of intensity 3.5x10(13) Wcm(-2). The number of optical cycles contained in the frequency-tuned pulse is almost unchanged after ideal chirp compensation.