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We show that attosecond pulse trains have a natural application in the control of strong field processes. In combination with an intense infrared laser field, the pulse train can be used to microscopically select a single quantum path contribution to a process that would otherwise consist of several interfering components. We present calculations that demonstrate this by manipulating the time-frequency properties of high order harmonics at the single atom level. This quantum path selection can also be used to define a high resolution attosecond clock.

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The 2.94 μm Er:YAG laser Q-switched mechanically by a rotating mirror was developed. The laser generated pulses of 30 mJ energy and duration below 290 ns, which corresponds to over 100 kW peak power. It operated at the repetition rate of up to 25 Hz. To our knowledge it is the most powerful erbium laser operating at so high repetition rate. The developed laser can be successfully applied in medicine -e.g. in microsurgery of soft biological tissues.Laser beam interaction with a gelatin sample

Attosecond pulse trains ͑APT͒ are natural tools for controlling strong field processes, due to their periodicity and short duration. Here we present nonadiabatic calculations of the macroscopic harmonic signal created by a gas of helium atoms exposed to a strong infrared ͑IR͒ pulse in combination with an APT. We find that the harmonic yield can be enhanced by two to four orders of magnitude for the optimal delays between the IR and the APT pulses. The large enhancement is due to the change in the IR-intensity dependence of both the harmonic strength and phase caused by the presence and timing of the APT. This leads to enhancement of the harmonic yield and improved phase matching conditions over a large volume.