Femtosecond X-ray generation in the water window using 25-fs laser pulses

Summary form only given. Recent advances in laser technology have led to high-harmonic generation using ultrashort pulses with pulse durations in the range of 5-25 fs. We have shown that the cutoff harmonic photon energy, is inversely proportional to the square of the log of the laser pulse. Our experimental results for argon gas, are in excellent agreement with our predictions. The above formula also explicitly shows how the cutoff photon energy changes with the laser wavelength, the atomic species, and the electron quantum state. From the formula, under our experimental conditions (25 fs laser and helium gas), we expect to observe harmonics up to order 333 (corresponding to an energy of 518 eV). Experimentally, using a 25-fs Ti:sapphire laser centered at 800 nm, we observed discrete harmonic peaks in He up to the 221st order.