HIGH ORDER HARMONIC GENERATION FROM SOLID TARGETS: TOWARDS INTENSE ATTOSECOND PULSES
Dr. A. Tarasevitch
The generation of attosecond light pulses offers a spectacular extension to the field of ultrafast science [1- 3]. Such pulses can provide enough temporal resolution for the direct observation of electron motion in nanostructures, molecules, and atom inner shells. At the present time, the established method of producing attosecond pulses is the generation of high order harmonics (HOHG) in rare gases. However, recent theoretical work has suggested that HOHG from solid surfaces can be orders of magnitude more efficient [4-8].
Our research group has a long experience in HOHG from surfaces (Publications) starting from [9]. We investigate the HOHG both experimentally and using particle-in-cell (PIC Further information...) simulations. The harmonics can in principle be generated due to relativistic and/or non-relativistic mechanisms (Further information...). With our laser set-up we can make use of both mechanisms, and we were the first to experimentally demonstrate the transition from one to the other [10].
For the surface HOHG high intensity ultrashort laser pulses have to be used. Our laser set-up is capable of producing high-quality pulses with the intensities up to few times 1019 W/cm2 and 40 fs duration (Further information...). At such intensity the target surface is nearly fully ionized already by the leading edge of the pulse. At the pulse maximum the velocity of the collective oscillations of the surface electrons approaches the speed of light.
Attosecond light pulses can generally be obtained by superimposing several harmonics of the carrier frequency of an intense laser pulse. Increasing the number of harmonics results in shorter pulses, provided that all harmonics are in phase. Generally, however, there are two difficulties in the generation of attosecond pulses using surface HOHG: i) the expected slope of the harmonic spectrum is very steep; ii) there is a certain phase mismatch between the individual harmonic components. In this case in order to produce attosecond pulses the harmonic radiation has to be spectrally filtered, which results in a huge energy loss. We suggest a different approach (zero-cycle attosecond pulses, two-beam HOHG) which allows high efficiency attosecond pulse generation without spectral filtering [11]. Further information...[1] P. Corkum and F. Krausz, Nature Phys., 3, 381, (2007).
[2] M. Nisoli and G. Sansone, Prog. Quantum Electron., 33, 17 (2009).
[3] F. Krausz and M. Ivanov, Rev. Mid. Phys, 81 , 163 (2009).
[4] G. Mourou et al., Rev. Mod. Phys., 78, 309 (2006).
[5] G. Tsakiris et al., New J. Phys., 8, 19 (2006).
[6] N. Naumova et al., Phys. Rev. Lett., 92, 063902 (2004).
[7] J. Nees et al., J. Mod. Opt., 52, 305 (2005).
[8] S. Gordienko et al., Phys. Rev. Lett., 93, 115002 (2004).
[9] D. von der Linde et al, Phys. Rev. A, 52, R25 (1995).
[10] A. Tarasevitch et al., Phys. Rev. Lett. 98, 103902 (2007).
[11] A. Tarasevitch et al., J. Phys. B, 42, 134006, (2009).