Theoretical and experimental progress of high-order harmonics and attosecond pulse generation based on solid overdense plasma

Abstract

<p indent="0mm">Since the laser was invented, harmonic generation and wavelength conversion have been important research topics in nonlinear optics regime. In 1987, McPherson A et al. used sub-picosecond KrF laser <sc>(248 nm</sc> wavelength) to interact with noble gas to obtain high-order harmonics for the first time. In 2001, the first single attosecond <sc>(10^–18 s)</sc> pulse with a pulse width of 650 as was measured experimentally by Krausz et al. by using the broadband high-order harmonic generation. Since then, high-order harmonics and attosecond pulses have been widely used in atomic and molecular physics, materials science and other fields. However, due to the low efficiency of attosecond pulses generated by the gas media, the energy of attosecond pulses is limited, which limits the detection methods of attosecond time dynamics research (currently mainly IR+XUV pumping/detection) and its applications in many fields. How to obtain high-brightness, high-energy attosecond pulses has always been the pursuit in this field since the generation of the first attosecond pulse. With the development of femtosecond laser technology, the output peak power of the femtosecond laser system has been rapidly increased in the past two decades. At present, the laser system can output the pulse with a peak power of up to 10 PW (1 PW<sc>=10¹⁵ W).</sc> Its intensity can be much higher than <sc>10¹⁸ W/cm²</sc> and its interaction with the overdense plasma can efficiently generate high-order harmonics at the level of hundreds of eV or even keV, which brings new opportunities for the development and application of high-order harmonics and attosecond pulses. Generally speaking, the interaction of high peak power femtosecond laser pulses with solid-density plasmas has unique advantages in high-luminance, high-energy high-order harmonics and attosecond pulse generation. In the ELI-ALPS plan, solid plasma high-order harmonics and attosecond pulse generation are very important parts, especially to promote their high brightness and their high photon energy. In terms of the attosecond pulse energy, the goal of their first phase is that the attosecond pulse energy reaches the μJ level, which will be advanced to the mJ level in the next phase, and the photon energy of the attosecond pulse will reach a few keV. This review gives a brief introduction to its physical mechanisms and their current experimental research progress. Three mechanisms (coherent wake emission (CWE), relativistic oscillating mirror (ROM), coherent synchrotron emission (CSE)) will be discussed. They are working under different laser intensity. CWE can emit high efficiency harmonics at the laser intensity below <sc>10¹⁸ W/cm².</sc> But when the peak laser intensity is very high, the research of the CSE mechanism shows the generation of high-luminance high-order harmonics under super-relativistic light intensity, which can be used to produce extremely high field-strength pulse of attoseconds, even tens of zeptoseconds. If the laser parameters and the plasma condition can be well controlled, the laser pulse may even generate higher harmonic radiation with field strength much higher than the pump laser field strength, which may be a possible way to study the 39th question (“What is the most powerful laser researchers can build? ”) among the 125 most challenging scientific questions published by <italic>Science</italic> on its 125th anniversary.