Quantum reflection has provided a major breakthrough in solid surface atomic mirror. It is noteworthy that bare solid surface without any applied fields can be atomic mirror under quantum refleciton. Since quantum reflection the attractive part of the atom-surface interaction. Since it occurs away from the surface, local variation on the surface harldy affect the reflectivity. Namely, surface-preparation is not required. In order to investigate quantum reflection experimentally, the particle’s kinetic energy perpendicular to the surface must be sufficiently small. One way to achieve this prerequisite is by using a grazing incidence condition. In typical experimental conditions, a grazing incidence angle qin of a few mrads provides a sufficiently small z-component kinetic energy Ez. perpendicular to the surface.
Recently, the quantum reflection of a thermal atomic beam has been reported at a grazing incidence angle of a few milliradians [28-31]. Under grazing incidence condition which is another way to enhance the wave nature of the particle, the vertical component of the wave vector for a particle is decreased and corresponding de Broglie wavelength becomes larger. Hence, when an incident matter-wave propagates almost parallel to a surface, even fast atoms can be coherently reflected from the surface. In such circumstances, the reflectivity from a solid surface can reach 1 as grazing incidence angle goes to 0, which is an important requisite for the practical matter-wave mirror. In other words, those fast atoms without complex preparation steps like trapping or cooling processes can ///complement/// the ultra-cold atoms as the proper matter wave sources for the atomic mirror.////
Furthermore, under the above-mentioned condition, resolved diffraction peaks have been observed from micro-fabricated reflection-gratings [ref]. Figure 1a and 1b show the schematics of matter-wave scattering from a transmission grating with the period of 100 nm and a square-wave grating whose period is 400 um, respectively. For the sake of convenience, the incidence angle \(\theta_{in}\) and the diffraction angle \(\theta_n\) of the nano-transmission grating are defined with respect to the normal line while those are determined by the angle between the incident beam and the grating-surface plane at the square-wave grating. A sign of the diffraction order is defined in a way that diffraction angles of positive orders are larger than negative orders./// In order to see the difference between two gratings, \(n\)-th-order diffraction angles over certain ranges of incidence angles from each grating are calculated by grating equation [2008-19] in Fig 1c and 1d, respectively. Here, matter-wave de Broglie wavelength is fixed at 136 pm. Assuming the angular resolution of the atomic beam is around 100 urad, up to 5th order diffraction peaks can be observed at grazing incidence angles from the micro-fabricated reflection-grating as it can be from a nano-transmission grating at a normal incidence condition. Considering the fact that the reflection and diffraction efficiencies are increased as incidence angle goes smaller from the reflection-type grating, diffraction efficiencies from square-wave gratings are expected to be higher as much as those efficiencies from transmission grating. Therefore, micro-fabricated structures can act as a matter-wave-grating and it appears important for the generality of matter-wave elements due to its lower price and size-limit than nano-fabricated structures like the nano-transmission grating. Thus, micro-fabricated solid structures can be a good candidate for practical matter-wave optical instruments as exploiting grazing incidence condition.