Experimental Measurement And Study Of The Visible Electromagnetic Wave Propagated At The Metal/Media Interface

The Snell law reveals the propagation and refraction law of light when it passes through the interface composed by the mediums with different optical constants. It is the foundation of modern physical and geometrical optics. And it is also the physical origin which limits the fine properties of the optics device and instrument to be achieved in application. The way in great effort to overcome those optical barriers is trying to find the optical material with a negative refractive index matching to the material with a positive refractive index. By this way, the perfect optical lens in application can be realized in expectation. This interesting prediction has been theoretically studies for many years, but the natural material with the negative optical refraction index has not yet been found in a very long period of time.With the progress and development of the technologies that can be applied to fabricate the artificial material composed by the micro or nano-structures, it is found that the artificially metal-based micro-structure materials can produce a negative refraction effect in the microwave frequency range. This leads to great interest to study the optical properties of the structures and materials in a wide optical frequency range. Both in theory and experiment, however, there are serious controversies over its physical origin of the negative refraction happened in the metal-based artificial structure, since the phenomena observed for those materials are against the conventional optical theory. In this work, we have fabricated a series of noble-metal-based samples (Au and Ag) with precisely controlled incident angles. We quantitatively measured the refraction of the light passing through the metal/air interface, and observed that the refraction can change from negative to positive in the visible range at the simplest interface of the natural noble metal. In terms of the refraction angle measured at the metal/air interface, we quantitatively obtained the relationship between the value of pseudo refractive index and incidence angle. Through careful discussion and data analysis, the mechanism to result in the negative refraction has been studied and proposed. The results shown in this work, therefore, will stimulate people to understand more clearly the origin of the negative refraction in the metal-based artificial materials and will hopefully help its application in design and fabrication of the photonics material and device in the future.In this work, firstly, we give a historic survey of the optical principle based on Snell's law which was discovered more than three hundred years ago. It is the foundation of modern optics physics and engineer. Although there are some theoretical studies trying to understand the propagation of light at the metal interface, it is still very difficult to carry out the experiment to verify the theoretical predication due to very short optical penetration depth in the metal. Therefore, we described the experiment procedure, showing in detail how to correctly measure the light refraction at the metal/air interface by overcoming those experiment difficulties in every experiment step.We choose the noble metals like Au and Ag that have the refractive index less than 1 (close to 0) in visible light region as well as a reference sample of Si. All prism-like samples were fabricated to have a series of wedged angles by the RF sputtering method. The spectra of the optical constant s of the metals were measured with Spectroscopic Ellipometry in the visible range. Three lasers with the wavelengths ofλ=473nm, 532nm, 632.8nm, respectively, in the visible range were incident on the metal/air interface with different incident angles. The slight positive or negative shift of the laser beam position on the screen located far away from the sample was accurately measured by a CCD camera.In terms of the result to show the refractive angles changing from negative to positive at different wavelengths and at the metal/air interface, we studied and discussed those possible mechanisms used to explain the physical origin of the negative refraction effect in past years, such as the surface and bulk Plasmon resonance effect, negative permeability effect, Goos-H(a|¨)nchen effect, slow and fast photon effect, predication of the light path at the metal-based interface, and so on. Through detailed calculation and analysis of the dispersion characteristic of the noble metals like Au and Ag, we found that the group velocity combined with the group refractive index can change from negative to positive with increase of the photons energy in the visible Drude region and has the spectral property which is in agreement with the experimental results obtained in this work.As all above, results shown in this work will help to understand the fundamental principle of optics both in theory and experiment based on Snell's law. More studies to explore the true physical origins regarding to those interesting phenomena happened at the metal-based interface will be expected to be followed in the future to help the design and fabrication of the new type of photonics materials and devices in wide applications.