Laser-induced atomic adsorption: A mechanism for nanofilm formation

Abstract

We demonstrate and interpret a technique of laser-induced formation of thin metallic films using alkali atoms on the window of a dense-vapour cell. We show that this intriguing photo-stimulated process originates from the adsorption of Cs atoms via the neutralisation of Cs + ions by substrate electrons. The Cs + ions are produced via two-photon absorption by excited Cs atoms very close to the surface, which enables the transfer of the laser spatial intensity profile to the film thickness. An initial decrease of the surface work function is required to guarantee Cs + neutralisation and results in a threshold in the vapour density. This understanding of the film growth mechanism may facilitate the development of new techniques of laser-controlled lithography, starting from thermal vapours.

Coverage threshold for laser-induced lithography

“…When the substrate work function W is lower than the atom energy of ionization, an electron can be transferred from the substrate to an ion, thus neutralizing it [27,4]. W switches from larger to smaller than the Cs atom energy of ionization I around a coverage of the order of 10 −2 , which is much lower than the threshold coverage θ th ≈ 1 measured in this work.…”

“…We have systematically measured the thickness growth rate as a function of the key parameters of the process (laser intensity 1 and frequency, window temperature and vapor pressure). From the dependence of the growth rate, cubic with the pump laser intensity and linear with the vapor pressure, we have inferred that the role of the pump beam is to ionize the cesium atoms in the vapor [4]. At the high atomic densities where the formation of the film can actually be observed (see Appendix), the laser absorption by the vapor is very strong, resulting in a very small penetration of the beam.…”

“…An important avenue has been opened in the control of the deposition of cold atoms on surfaces by the application of optical forces with lasers, that guide particles through optical patterns [2]. More recently, a new technique of control of the atomic binding to a surface was explored [3,4], whereby the manipulation of the internal state of atoms very close to the surface is a key element of the process and that present the important feature of depositing a metallic film, with arbitrary shape and nanometric thickness resolution, directly from a thermal atomic vapor. This technique is the subject of the present study.…”

“…This laser-induced adsorption process allows for the growth, on a dielectric surface, of a metallic film of nanometric thickness whose shape is determined by the intensity profile of the light. Systematic measurements [4] have shown that the film is grown from ions created by the interaction of the laser with the vapor and that, for a given surface temperature, a minimum vapor pressure is needed for the laser-induced film to start growing. In this letter, we investigate the origin of this vapor-pressure threshold, assuming it is related to the requisite that the ions be neutralized at the surface in order to suppress repulsion between them and allow for their accumulation as a metallic film.…”

Photo-induced modifications of the substrate-adsorbate interaction in K-loaded porous glass

“…A deep understanding of adsorption/desorption and nucleation processes is of great importance for novel engineered materials [1,2,3,4]. Indeed, the possibility to recognise and modify the underlying mechanisms would allow to manipulate the overall atomic transport dynamics [5,6,7]. In this context, because of the increasing demand of nano-sized devices, the scientific research has focused on the study of surface interactions at the micro-and nanoscale.…”