Abstract
We present an analytic quantum theory of high harmonic generation by low frequency laser fields for a two-state hydrogenlike atom model. In the case of an initial coherent superposition of two states, the theory clearly explains why high conversion efficiency can be obtained. Furthermore, we apply this model to generate an intense isolated attosecond pulse by the proposed combined field. By mixing an infrared chirped pulse with a fundamental chirped pulse, a supercontinuum spectrum can be formed. The long quantum trajectory is suppressed, and the short one is enhanced and an intense isolated 95-as pulse is obtained by He+ ion. Our simulation shows that the produced pulses with good signal-to-noise ratio are obtained without any phase compensation. In particular, these results are analyzed by using both classical and time-frequency analyses.
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