Thickness dependence of infrared reflectance of ultrathin metallic films: Influence of quantum confinement

In this letter we discuss experimental results on optical reflectance of ultrathin metallic films. The laser light source was tuned at the infrared wavelength of λ=9.2μmλ=9.2μm. Three metals were tested: aluminum, niobium, and nickel. The thin films were coated on various wafers, namely, fused glass, α-quartz and amorphous silicon, with the technique of magnetic field assisted sputtering modulated at radio frequencies. The infrared reflectance was recorded while the thickness of the film varied from 5 to little more than 100 Å. A phenomenon is observed, i.e. a periodic oscillation appeared modulated on the otherwise classic thickness dependence of metallic films. The effects are attributed to the influence of quantum confinement induced intraband redistribution of conduction electrons. A one-dimensional quantum well model was employed to simulate the system, and the numerical results confirmed the existence of the quantum size effect.