Anomalous High-Energy Second Plateau in High Harmonic Generation from Fullerenes

Abstract

We explore with ab-initio theory high harmonic generation (HHG) from a series of gas-phase fullerenes (from C$_{20}$ to C$_{60}$, including isomers) under varying laser conditions (different ellipticities, angular orientations, intensities and wavelengths). We find that HHG emission from fullerenes exhibits a prominent high-energy second plateau, extending well above the expected semi-classical cutoff, e.g. for an 800 nm driving laser with a peak intensity of $10^{14}$ W/cm$^2$, the anomalous $2^{nd}$ plateau cutoff is 115 eV. We theoretically analyze the underlying $2^{nd}$ plateau physical mechanism and determine that: (i) It differs from the standard SFA-like mechanism; (ii) It is recombination- based; (iii) It has an unusual inverted cutoff scaling with wavelength (whereby the cutoff decreases with increasing wavelength, until reaching a saturation); and (iv) It exhibits a linear cutoff dependence with electric field amplitude E$_0$ . We further rule out real-space trajectory based pictures, indicating that the most likely culprits are sharp quantum mechanical resonances in the fullerene family. Our work establishes a new route for high energy coherent broadband emission without requiring mid-IR driving and uncovers a novel HHG mechanism in complex structure quantum systems.

Figures (5)

• Figure 1: (left) Illustration of set-up: C20 cage irradiated by an ultrashort intense linearly polarized laser pulse, causing HHG emission. (right top panel) Exemplary HHG spectrum showing a prominent wide-energy second plateau reaching up to 115 eV. (right bottom panel) Gabor time-frequency analysis corresponding to top panel HHG spectra. The simulation is performed at 800nm driving with a peak power of 10$^{14}$ W/cm$^2$.
• Figure 2: HHG and 2$^{nd}$ plateaus from the fullerene family in similar laser conditions to Fig. \ref{['fig:1']}. The geometry of each fullerene is shown in inset along with its spectrum. Plateaus are marked by background coloring, with dashed purple lines indicating typical semi-classical cutoffs from the HOMO and deepest orbital in the system.
• Figure 3: Ellipticity dependence of harmonic intensity for selected harmonics from each plateau are illustrated, calculated in similar conditions to Fig. \ref{['fig:1']} but with elliptical driving. Two lines are provided as a guide to eye, passing through the full width at half maximum (FWHM) of the ellipticity-decay distribution, and its corresponding ellipticity value. 2nd plateau harmonics indicate a much sharper decay with driving ellipticity.
• Figure 4: Physical investigations of 2nd plateau. (Top left panel) Illustration of the negligible roles of electronic correlation and multi-orbitals contribution to the harmonic generation process in C20 cage. (Top right panel) Dependence of HHG on the laser pulse duration. The second plateau regions are marked by two vertical lines. The bottom panels present cutoff scaling for the 2nd plateau with laser field strength E0 (left) and wavelength (right). All plots calculated in similar conditions to Fig. \ref{['fig:1']} apart from the direct variations in parameters in each case.
• Figure 5: Harmonic yield dependence on driving laser angle. HHG yields for select harmonics (both in the 1st and 2nd plateaus) are plotted with respect to different angular orientations of the laser electric field across all the three planes. Simulations performed in similar conditions to Fig. \ref{['fig:1']}.