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Register a new userUnusual reflection of electromagnetic radiation from a stack of graphene layers at oblique incidence
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We study the interaction of electromagnetic (EM) radiation with single-layer graphene and a stack of parallel graphene sheets at arbitrary angles of incidence. It is found that the behavior is qualitatively different for transverse magnetic (or p-polarized) and transverse electric (or s-polarized) waves. In particular, the absorbance of single-layer graphene attains minimum (maximum) for p (s) polarization, at the angle of total internal reflection when the light comes from a medium with a higher dielectric constant. In the case of equal dielectric constants of the media above and beneath graphene, for grazing incidence graphene is almost 100% transparent to p-polarized waves and acts as a tunable mirror for the s-polarization. These effects are enhanced for the stack of graphene sheets, so the system can work as a broad band polarizer. It is shown further that a periodic stack of graphene layers has the properties of an one-dimensional photonic crystal, with gaps (or stop--bands) at certain frequencies. When an incident EM wave is reflected from this photonic crystal, the tunability of the graphene conductivity renders the possibility of controlling the gaps, and the structure can operate as a tunable spectral--selective mirror.
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Finite-difference time-domain method is employed to investigate the optical properties of semiconductor thin films patterned with circular holes. The presence of holes enhances the coupling of the incident plane wave with the thin film and greatly enhances the absorption performance. For a typical 100 nm thin film, the optimal hole pattern is achieved when the hole radius is 180 nm and volume fraction is about $30%$. Disorderness can alter the absorption spectra and has an impact on the broadband absorption performance. The non-uniform radius of holes can slightly broaden the absorption peaks and enhance the integrated absorption. Random hole position can completely change the shape of the absorption spectra and the averaged integrated absorption efficiency is slightly smaller than the optimized ordered nanohole pattern. Compared to random positioned nanoholes or ordered nanohole, amorphous arrangement of nanoholes will result in a much better absorption performance. However, it is also found that the absorption enhancement of amorphous pattern over an ordered pattern is weak when the incident angle departures from normal or when the intrinsic material absorption is strong.
We observed quantum reflection of ultracold atoms from the attractive potential of a solid surface. Extremely dilute Bose-Einstein condensates of ^{23}Na, with peak density 10^{11}-10^{12}atoms/cm^3, confined in a weak gravito-magnetic trap were normally incident on a silicon surface. Reflection probabilities of up to 20 % were observed for incident velocities of 1-8 mm/s. The velocity dependence agrees qualitatively with the prediction for quantum reflection from the attractive Casimir-Polder potential. Atoms confined in a harmonic trap divided in half by a solid surface exhibited extended lifetime due to quantum reflection from the surface, implying a reflection probability above 50 %.
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