Experimental study on light transmission through Au wedge-shaped films

Metal film materials have been widely used in optical devices, green energy field, biosensor technologies, and so on. In these applications, the properties of metal surface play the significant role. However, propagation characteristics of light in metal-based materials are still confusing. Wavelength-dependent refraction going from negative to positive at air/metal interface was reported in previous researches [1, 2]. The experiment results show that classical Snell's law fails to describe light transmission at metal surface. This is because the optical constants of metals are presented as a complex form, whose imaginary part can not be neglected. It's necessary to get more experimental evidences for exploring more deeply mechanisms. In this work, we observed that refraction continuously changes from positive to negative at Au/air interface in the spectrum from 470nm to 700 nm. In the experiment, a series of Au wedge-shaped films with different wedge angles were radio frequency sputtered on K9 glass. Continuity and uniformity of the wedge-shaped samples were verified by AFM and step profiler. A collimated laser beam with continuously variable wavelength generated by SuperK EXTREME supercontinuum laser was normal incident on the glass substrate then emerged at air/Au interface, where refraction took place. The measurement interval was 10 nm. The shifts of spot Δx on screen at far-field (14.6 m) were detected by CCD. Then effective index of refraction n
e
was calculated. The results indicate that n
e
of samples with different wedge-shaped angles correspond well with each other, which means n
e
is an intrinsic characteristic of the material and irrelevant to the shape of samples. Different from the real part of complex refractive n, which is always positive in natural materials, n
e
reaches to negative in the spectrum starting from about 520 nm. The positive and negative in the experiment can be explained qualitatively by the real valued boundary conditions [3]. The results might also be conducive to the further quantitative study on negative refraction at metal/dielectric interface.