Tuning high-harmonic generation by controlled deposition of ultrathin ionic layers on metal surfaces

Abstract

High harmonic generation (HHG) from semiconductors and insulators has become a very active area of research due to its great potential for developing compact HHG devices. Here we show that by growing monolayers (ML) of insulators on single-crystal metal surfaces, one can tune the harmonic spectrum by just varying the thickness of the ultrathin layer, not the laser properties. This is shown from numerical solutions of the time-dependent Schrödinger equation for nML NaCl/Cu(111) systems (n = 1 − 50) based on realistic potentials available in the literature. Remarkably, the harmonic cutoff increases linearly with n and as much as an order of magnitude when going from n = 1 to 30, while keeping the laser intensity low and the wavelength in the near-infrared range. Furthermore, the degree of control that can be achieved in this way is much higher than by varying the laser intensity. The origin of this behavior is the reduction of electronic "friction" when moving from the essentially discrete energy spectrum associated with a few-ML system to the continuous energy spectrum (bands) inherent to an extended periodic system. PACS numbers: 42.65. Ky,78.66.Nk, Discovered in the 1980's [1-3], high harmonic generation (HHG) has become the fundamental tool of modern attoscience [4][5][6]. In HHG from atomic or molecular gases, a strong laser field ionizes an electron, which then gains energy from the field and returns to the parent ion, where it finally recombines converting the gained energy into high-frequency radiation [7][8][9][10]. The process repeats every half-cycle of the ionizing field, thus leading to a sequence of attosecond light bursts that contain multiples of the fundamental laser frequency, ω 0 . Since electronic motion in atomic and molecular systems occurs in the attosecond time scale, light pulses arising from HHG are currently used to probe electron dynamics in those systems [11][12][13][14][15][16][17][18]. Furthermore the HHG process itself contains the signature of the parent-ion dynamics occurring during the round trip of the traveling electron. Thus the analysis of the HHG spectral features can also reveal important aspects of such dynamics [19][20][21][22][23][24][25].HHG from condensed matter systems was first observed in the mid 90s [26,27]. In these early experiments, bulk metals and dielectrics irradiated with very intense near-infrared laser fields (of the order of 10 18 W/cm 2 ) were shown to emit harmonic radiation as a result of plasma oscillations induced in the system (see also [28]) Due to the high intensity of the field, HHG was always accompanied by sample damage. More recent experiments have made use of nano tips and nano spheres [29][30][31][32][33], from which HHG has been produced by using relatively moderate fields (of the order of 10 12 W/cm 2 ). HHG has also been observed in bulk semiconductors and insulators [34][35][36][37][38][39][40] by using similar low intensities (even lower than those usually needed to generate high harmonics in the gas phase) and longer wavelengths, down ...

Tuning the electronic band structure of metal surfaces for enhancing high-order harmonic generation

“…The pulse is characterized by a chirp parameter β and has 1.27 µm central wavelength, 43 fs pulse duration and 2×10 13 W/cm 2 as the maximum of the peak intensity. The choice of the pulse intensity is limited by the material damage, as discussed in 41 . Figure 2(a) shows a strong sensitivity of the spectrum to the change of the chirp parameter β .…”

“…The electron dynamics induced inside metal surfaces is modelled using a 1D-model with the use of a one-electron pseudo potential as in previous works 41,52,53 . In a 3D-model, the metal surface electron feels the potential in the direction normal to the surface, while it is considered to move freely with the electron momentum k in the parallel direction.…”

“…Note that the pseudo potential in Eq. ( 2) is multiplied by a damping function of the form [1 + tanh(z)]/2, similar to that used in 41 , to ensure the continuity of the potential at the boundaries. The time-dependent interaction H I (t) in Eq.…”

“…where the function S(z) accounts for screening of the electric field inside the Cu(111) surface (see, e.g., Refs. 41,52 ) and takes the form…”

“…In these works, the generated har-monics were limited to the lower-order region and up to the 25th order, and the used laser intensities, although operate in the non-perturbative regime, cannot be too high to prevent the material from damage. Alternative studies address the HHG from metal surfaces Cu(111) and precisely from the surface state in which the electron wavepacket is initially localized and the control scheme explores the effect of growing ultrathin layers of the insulator NaCl on single-crystal metal surfaces 41 .…”