Strong-field approximation and its extension for high-order harmonic generation with mid-infrared lasers

Abstract

In recent years intense mid-infrared lasers with wavelength of a few microns have become the standard tools for research in strong field physics laboratories worldwide. These lasers offer the opportunities to extend the traditional study of high-order harmonics generation and attosecond sciences from the extreme ultraviolet to soft x-rays. In this tutorial we revisit the familiar strong field approximation and its simplification-the quantum orbits theory. We draw special emphasis on the factorization of laser induced dipole moment as the product of a returning electron wave packet with the photo-recombination dipole transition matrix element. The former depends on the laser properties only (up to a normalization constant) while the latter is related to laser-free photoionization transition dipole. The factorization leads to the suggested modification beyond the strong field approximation-the quantitative rescattering theory. In applying these theories to mid-infrared lasers, we analyze the behavior of the returning electron wave packet and its scaling properties vs the wavelength of the laser. A few examples are given to demonstrate how the quantitative rescattering theory is capable of reproducing experimental harmonic spectra under various conditions. Future opportunities in employing harmonics generated by optimized mid-infrared lasers for probing molecular structure and for serving as useful table-top coherent light sources up to the x-ray region are also discussed.

Influence of permanent dipole and dynamic core-electron polarization on tunneling ionization of polar molecules

“…In a similar manner to the SC-MOADK theory, the MO-PPT theory [16] can also be extended to include the Stark shifts into the ionization potential, for the treatment of polar molecules. We will call such an extension Stark-corrected MO- PPT (or simply SC-MOPPT).…”

“…In fact, ionization from atomic targets has been quite well understood based on the Perelomov, Popov, and Terentev (PPT) [13] theory or its quasistatic tunneling limit, the socalled Ammosov-Delone-Krainov (ADK) theory [14]. The extensions to molecules, the so-called MO-ADK [15] and MO-PPT [16,17], have been found to work reasonably well for diatomic homonuclear molecules, such as H 2 , N 2 , and O 2 [18,19]. These results lead to a general belief that ionization yields reflect the shape of the highest occupied molecular orbital (HOMO), as one would expect based on the intuitive picture within the MO-ADK theory.…”

The plateau in above-threshold ionization: the keystone of rescattering physics

“…The topics to be considered have been covered by many more general review papers [34][35][36][37][38][39][40][41]. Some historical remarks can be found in [39].…”

Enhancing high-order-harmonic generation by time delays between two-color, few-cycle pulses

“…The rapidly developing field of attosecond physics, which is based upon HHG, has greatly advanced the investigation of ultrafast processes [8,10,11]. Moreover, the HHG spectra themselves are increasingly being utilized to probe or image molecular structures [4,10,[12][13][14].…”

“…Although the physical process of HHG is mostly well understood [9,10,[14][15][16][17], its low conversion efficiency remains a major limitation that prevents HHG from becoming an important new light source. Investigations of ways to address the low efficiency issue focus on two physical regimes that are important for HHG.…”