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Experimental strong-field physics is a fast-growing area of nonlinear optics, triggered by the development of isolated attosecond (as; 1 as = 10−18 s) pulses (IAP) [1]/attosecond pulse trains (APT) [2-5]. These extreme ultraviolet (EUV or XUV) pulses generated through the process of high-order harmonic generation (HHG) [6] exhibits exceptional laser-like properties, capable of providing attosecond timescales and angstrom space-scale resolution. Hence, these pulses are promising candidates for the investigation of a broad class of atomic and/or molecular ultra-fast processes that require the aforementioned spatio-temporal resolution. For example, studies on charge migration and intermediate auger processes during ionization, femtosecond (fs) pump-probe spectroscopy with attosecond precision, high-resolution imaging, nonlinear processes in the XUV photon energies and attosecond chemistry, to name a few, are feasible via as pulses. Given these possibilities, research at LASE is aimed towards building such a facility capable of investigating atomic, molecular, and optical (AMO) physics using attosecond pulses within the Light and Matter Physics (LAMP) theme of the Raman Research Institute (RRI).
LASE would be completely operational in the upcoming years, allowing AMO physics experiments using attosecond pulses within the capacity of RRI. Following are some of the proposed research directions of LASE.
Laser produced plasmas (LPP) are excellent candidates for systems that have different constituents (atoms, molecules, ions, nanoparticles etc.) whose appearance depends on the time after which the plasma is formed. Probing such a transient and dynamic system completely has always been a challenge for many years and insight into processes occurring inside LPP would open doors to many unanswered questions. One of the research directions of LASE involves high harmonic generation (HHG) from LPPs, where plasmas would be used as the medium to generate harmonics.
HHG [7] is an intrinsic pump-probe experiment where the free electron wave function probes the bound part after some time delay such that any variation in the bound part can be recorded. One of the interesting effects that appears in molecules is that the emitted photon’s phase undergoes a large shift when the internuclear separation is approximately equal to the wavelength of the returning electron wavepacket. This is called two-centre interference (TCI). Aligning the molecular axis relative to the polarisation direction of the laser allows for the projected component of the internuclear separation along the polarisation axis to be altered relative to the returning electron wavepacket. On the other hand, Guoy-phase interferometry (GPI) utilizes an on-axis design such that two successive sources are positioned within the focus of the HH driving laser. The small Guoy-phase contribution is scaled by the harmonic number to give the corresponding phase shift observed in that harmonic. GPI measures the phase shift of HHG between two aligned molecular samples for investigating TCI directly and it is capable of measuring small phase shifts between two sources of HHG precisely with a timing resolution better than 200 zeptoseconds [8,9]. These innovative approaches will push the limits of the current technique beyond the state-of-the-art experiment.
References
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