Reflection of chirped laser from plasma and its application to attosecond pulse generation

Abstract

We have studied the pulse shaping of the chirped laser pulse by the reflection from the linear plasma density ramp. The temporal profile of the frequency in the chirped laser pulse manifests in distinct apparent reflection points inside the plasma having a linear density profile. The higher frequency component reflects from the deeper portion of the plasma as compared to the lower frequency component. These frequency-dependent variations in apparent reflection points introduce a natural phase lag, and the reflected pulse shortens, without compromising on the actual bandwidth and the energy of the pulse. Under optimum conditions, we find the peak field amplitude of the reflected pulse to be almost 2 times that of the incident pulse. We have deduced the condition for the efficient generation of the reflected pulse in terms of the incident pulse and target parameters. The higher harmonic generation by Helium atom through the above mentioned reflected pulse is studied by numerically solving the Time-Dependent Schrödinger equation. The enhanced field amplitude of the reflected pulse causes the extension of the harmonic cutoff as compare to the incident pulse. The shorter duration of the reflected pulse and extended harmonic cutoff results in the generation of the single attosecond pulse of duration ∼18 as under optimum condition.