Laser absorption and third-harmonic generation in free-electron nanofilms

Abstract

The collective collisionless dynamics of the electron gas in free-electron nanofilms irradiated by an obliquely incident p-polarized laser wave are considered in the classical hydrodynamic and jellium-model approximations. The two cases of cold metallic nanofilms and hot free-electron nanofilms laser ionized and laser heated by a pump-laser prepulse are investigated with proper electron statistics. Both linear and nonlinear properties of the plasma resonance excitation in the nanofilms are studied in detail for different film parameters ͑film thickness, thickness of the diffuse film boundary, outer-ionization degree for hot laser-ionized/heated films, etc.͒. The significant role of the diffuse film boundaries for both linear absorption of the laser field and third-harmonic generation is demonstrated. For this goal, we do not use the standard dielectric-permittivity approach with boundary conditions between two different media but solve continuously over all space the full set of hydrodynamic and electrodynamic equations in nonrelativistic one-dimensional approximation. It is shown that collisionless edge absorption may be dominant in thin nanofilms, while in cold metal nanofilms it results in the appearance of several linear-absorption resonances below the bulk-plasma resonance frequency. For hot nanofilms, drastic broadening of the linear-plasma-resonance profile is obtained in calculations when the film thickness is reduced. In our model, the third-harmonic generation is determined by the density gradient in the diffuse film edges. Additional resonances in third-harmonic generation as a function of laser frequency are obtained for cold metal nanofilms. They differ from the standard third-order nonlinear resonance, which is located at one third of the plasma resonance frequency. The important role of the outer-ionization degree in forming the third-order nonlinear response of the hot laser-ionized film is also analyzed and discussed.

Plasmon‐Enhanced Second‐ and Third‐Order Harmonic Generation in Spherical Free‐Electron Nanoclusters with Diffuse Surface

“…(A.1), the equations for the third-order scalar functions are obtained as ( ρ) given by Eqs. (28), but in terms of the tilded variables. Next, the third-order tensor function T 3αβ can be presented in the general tensor decomposition form as…”

“…which result from the second-order continuity equation 2in 2 = divq 2 , were used in reducing Eqs. (28). Finally, from Eq.…”

“…One of them corresponds to sufficiently high laser fields, when the amplitude of the forced electron oscillations inside the cluster is larger than the electric charge separation length while still smaller than the thickness of the diffuse surface. This cluster model was developed on the basis of the similar onedimensional collisionless hydrodynamic model [28] for thin nanofilms interacting with p-polarized laser light. [52], this case was referred to as the charge compensation approximation (CCA).…”

“…This case we call "beyond the CCA". This cluster model was developed on the basis of the similar onedimensional collisionless hydrodynamic model [28] for thin nanofilms interacting with p-polarized laser light. In the first nanocluster paper [52], having developed the complete nonlinear model for the laser-cluster interaction, we restricted ourselves to the detailed study of the linear electromagnetic response of spherical nanoclusters,…”

Harmonics generation in ultra-thin nanofilms irradiated by intense nonrelativistic laser pulses

“…In case of nanofilms exhaustive investigation of the nonlinear responce of thin films to p-polarized laser pulses in the case ξ 1 was presented recently [34]. Hydrodynamic approximation was used to describe a nanofilm behavior in laser fields of moderate intensities (10 14 .. 10 16 W/cm 2 ).…”