An increase has been observed in the conversion efficiency (ηk) from laser energy to Kα X-ray energy produced by 60 fs frequency-doubled high-contrast laser pulses focused on a Cu foil, compared to the case with fundamental laser pulses. The value of ηkshows a strong dependence on the nonlinearly modified rising edge of the laser pulse. It reaches a maximum for a 100 fs negatively modified pulse. The efficient heating of the hot electrons leads to the enhancement of ηk. This demonstrates that high-contrast lasers are an effective tool for optimizing ηk, through increasing the quantity of hot electrons by vacuum heating. These results have been published in Phys. Rev. Lett.,2008,100:045004 .