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Infrared Optics Using Low-Loss Materials with Refractive Index below Unity 
Date/Time:
April 18, 2017   8:00am - 8:45am
 
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Research in designing and utilizing nontrivial effective refractive index values of synthetic materials has opened up exciting opportunities in optics. For example, recently, materials and metamaterials with near-zero refractive index values (i.e. “epsilon-near-zero” or “mu-near-zero”) have attracted attention due to a variety of predicted properties including the decoupling of wavelength and frequency, a divergence of the density of states, and other remarkable effects. However, material systems with refractive indices near zero are often rather lossy, limiting their utility. In this work, we explored new optical phenomena enabled by refractive indices slightly below unity, rather than very close to zero. We found that in certain polar materials close to their intrinsic phonon resonances, refractive indices less than one can be achieved with relatively low losses. These materials include silicon dioxide (SiO2), aluminum oxide, aluminum nitride, and many others. For example, in SiO2, the real part of refractive index (n) is below one in the wavelength range of 7.37 micron to 7.67 micron, where the extinction coefficient (k) remains below 0.03. Here, we present experimental demonstrations of two new optical phenomena using SiO2 in this wavelength region: frustration of external reflection, and direct coupling to surface plasmon polaritons (SPPs) from free space. When light is incident on a low-loss medium with a refractive index less than one beyond a critical angle, it is reflected with high efficiency, similar to the case of total internal reflection. While this phenomenon of external reflection (ER) is widely used in x-ray optics, it is rarely observed at optical frequencies. We demonstrate that, using SiO2 at infrared frequencies, we can observe both ER and the related phenomenon of frustrated external reflection (FER). We utilized SiO2 films with thicknesses in the range of a few micron, on the order of the evanescent decay length in the SiO2 when light is incident on the film at an oblique angle of incidence beyond the critical angle. Decreasing the film thickness is shown to increase the amount of light transmitted through the film, providing evidence of the frustration mechanism. In parallel, we demonstrated direct excitation of SPPs from free space without the need of typical momentum matching structures such as prisms and gratings. Conventionally, direct coupling of free space light to SPPs is impossible because the wave vector of the SPP is always greater than the free-space wave vector. We showed that SPPs can be excited from free space at the interface between a metal and SiO2, enabled by a reduction of the SPP wave vector in the region where the refractive index of the SiO2 is less than one and the losses are not too large. 
 


 
 
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