Coherent control experiments with fully coherent multicolor pulses
K. C. Prince
Elettra Synchrotron Trieste
17:00 - 18:00 Tuesday 06 December 2016 KFU The hallmarks of pulsed optical lasers are high intensity, ultrashort duration, variable polarization, transverse coherence and longitudinal coherence. The majority of short wavelength Free Electron Lasers (FELs) possess the first four of these five characteristics, but lack the fifth. These properties have enabled a range of exciting experiments in completely new wavelength ranges, some of which are analogous to those with optical lasers, and some of which are new and unique to FELs. Most FELs are based on Self Amplified Spontaneous Emission, which is an intrinsically stochastic process, and produces pulses which are spiky in both the temporal and frequency domains. Until recently no short wavelength FEL experiment was based on longitudinal coherence.
The seeded FEL FERMI is the first designed to produce fully coherent pulses. As well, commensurate wavelengths (different harmonics of the same seed wavelength) were predicted to be mutually phase coherent. We have demonstrated experimentally the longitudinal phase correlation between two colours (first and second harmonics), and applied it to coherently control a photoionization experiment [1]. Neon was ionized at wavelengths of 63.0 and 31.5 nm, and the asymmetry of the 2p photoelectron angular distribution (PAD) was manipulated by adjusting the phase, in a Brumer-Shapiro type experiment [2]. The outgoing 2p electrons, ionized by one (second-harmonic) photon or two (first-harmonic) photons interfere to give an asymmetric PAD whose asymmetry depends on the relative phase of the two photon fields.
The relative phase of the two wavelengths was locked and tuned with temporal resolution of about 3 attoseconds. In the optical region various methods exist for tuning phase, with temporal resolution down to about 30-40 attoseconds for normal incidence or 200 attoseconds for grazing incidence optics (the preferred geometry for short wavelengths.) The key to the extremely precise manipulation of the phase in the present experiment lies in the use of phase shifters located between the radiators of FERMI. This innovative approach provides an extremely high degree of control.
The present results open the door to new coherent control experiments on atoms and molecules in the XUV and soft X-ray region, with ultrahigh time/phase resolution. The design of FERMI is flexible and has permitted new operating modes of the machine, such as two-colour, double-pulse operation; production of two coherent, incommensurate colours; and single-colour, phase locked double pulses [3]. Very recent results will be shown of applications which are unique to Free Electron Lasers with phase control, and prospects for further development will be discussed.
References:
[1] K. C. Prince et al, Nature Photonics 10, 176 (2016).
[2] P. Brumer, and M. Shapiro. “Principles of the Quantum Control of Molecular Processes”, Wiley-VCH, Berlin (2003).
[3] E. Ferrari et al, Nat. Comm. 7, 10343 (2015). E. Roussel et al, Phys. Rev. Lett. 115, 214801 (2015). D. Gauthier et al, Phys. Rev. Lett. 116,
024801 (2016).
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