de Broglie's hypothesis postulating wave nature of matter broke new ground for matter-wave optics. Subsequent observations about various optical phenomena with electrons have confirmed his hypothesis [1] and opened up the possibility of developing new techniques and devices for matter-waves such as interferometer and microscopy. However, since 1930, when Stern and Estermann showed diffraction of He atoms from the crystal surface of lithium fluoride [2], further studies on atom optics were halted until the 1970s, whereas matter-wave optics with neutrons and electrons were actively developed [3-7]. Hence, the early stage of matter-wave optics was in need of further steps for atoms while those studies with neutrons and electrons were on a fast track.
Early-stage technology was unable to satisfy the prerequisite for atom optics. This was mainly due to the difficulty of manipulating atoms while preserving their coherence. While static electric or magnetic field and collision against solid materials easily affect motions of neutrons and electrons, neither of them are suitable for thermal neutral atoms. There are two reasons for this: 1) electric, magnetic and optical forces upon them are too weak to change their motion [atom_optics ref 7-8]; and 2) they collide with the surface of the solid material rather than penetrate through matter, causing decoherence on atomic waves.
Development of a supersonic beam appeared to be a starting point to address the aforementioned problems. In the supersonic expansion, atoms which are originally located in the high-pressure region, pass through an orifice into low pressure or vacuum region. In this process, their random thermal motion turns to the directed center-of-mass motion, i.e., atomic wave sources with well-defined kinetic energy are available [ref]. Various studies on the chemisorption or physisorption as well as atomic diffraction from the crystal surface have been successfully performed by employing the supersonic beam as the atomic source [ref]. Thus, such a breakthrough played an important role in the revival of atom optics.
A combination of the supersonic nozzle with advanced technological developments in laser and nanotechnology accelerated the progress of atom optics. Ability to tune the laser frequency enabled us to observe Bragg scattering and the Kapitza-Dirac effect of atomic waves by adjusting the detuning frequency with respect to the transition frequency of atoms [11-14]. It motivated the first approach to the practical atomic mirror, known as the evanescent wave mirror [15-17]. In addition, the establishment of nano-fabricated devices has served as atomic optical elements such as transmission grating[18-20] and Fresnel zone plate [21]. Until now, those instruments have been the basis of matter-wave interferometry [22-26] as well as atomic de Broglie microscopy [21 - 보딜].
Nonetheless, previous studies of atomic reflection by surface carried out with thermal beams have shown the efficiency limit due to its high sensitivity to the surface quality. Since atomic de Broglie wavelengths of such beams (around sub-angstroms) are much smaller than the characteristic surface roughness (usually 100 angstroms), this reflection occurs at the repulsive potential. Such a mechanism is called classical reflection. When atoms are classically reflected from the surface, they are close to the surface atoms, feeling the local variation over the surface. In order to avoid an incoherent reflection which may be caused by adsorbates on the surface, complicated surface-preparation processes like annealing and sputtering are crucial to render surfaces smooth at the atomic-level [8-10]. Besides, even a perfectly clean surface would nonetheless experience unexpected surface defects, decreasing reflectivity [ref].
Therefore, the initial stage of atomic mirrors has been proposed in a way that prevents atom-surface collisions because atom-surface interaction has been known to deteriorate coherent reflection from the surface. For example, the evanescent wave mirror[ref] and the magnetic mirror [ref] use the evanescent wave and the magnetic field applied along the surface, respectively, to prevent atom-surface collisions. The artificial repulsive potentials induced by those field push atoms away from the surface before they collide against it. However, the delicate fabrication process and lithography technique are necessary to construct the magnetic mirror and evanescent wave mirror is only applicable to specific atoms. In addition, such mirrors are difficult to make big in size without compromising accuracy.
In the late 20th century, the advent of atomic-cooling and -trapping techniques gives rise to the creation of laser-cooled and trapped atoms. Such ultracold atoms whose much larger de Broglie wavelength emphasize the wave nature of atoms and it made a significant advance in atomic mirrors. Magneto-optically trapped(MOT) metastable neon atoms, for instance, has led to the first observation of a quantum reflection of the atomic wave from the solid surface [27]. Driven by such a pioneering research, numerous studies both in theory and experiment have been carried out with various solid structures such as silicon or glass surface and even with structured surfaces [ref]. The solid material is found to be the proper atomic mirror whose reflectivity can reach 1 at certain conditions. Here, it is noteworthy that bare solid surface without any applied field was finally appeared to be a decent atomic mirror.
Quantum reflection is a quantum mechanical phenomenon in which a particle is reflected by the attractive part of the atom-surface interaction. Since it occurs away from the surface, an atomic scale irregularity on the surface hardly affects the reflectivity, i.e., surface-preparation is not required. In order to investigate the quantum reflection experimentally, the particle’s kinetic energy perpendicular to the surface must be sufficiently small. There are two ways to achieve this prerequisite: 1) using ultracold atoms to exploit their larger wavelength; 2) employing a grazing incidence condition.
Instead of generating ultracold atoms, we have studied the quantum reflection of thermal atoms at a grazing incidence angle through high angular resolution matter-wave optics apparatus [28-31]. In typical experimental conditions, the grazing incidence angle of a few mrads provides a sufficiently small vertical kinetic energy, perpendicular to the surface. Hence, when an incident matter-wave propagates almost parallel to a surface, even fast atoms can be coherently reflected from the surface. Furthermore, in such circumstances, distinct resolved diffraction peaks have been observed from micro-fabricated reflection-gratings [ref].