and opened up the possibility of developing new techniques using matter-wave. For example, development of electron wave optics including electromagnetic lens have motivated an electron microscopy, which still remains indispensable device to reveal the ultrastructure such as microorganism [ref]. Thus, matter-wave-based instruments have been achieved based on the matter-wave optical elements such as mirror, beam splitter and lens are required.
In the past few decades, a new type of matter-wave microscopy have been developed by using neutral He atoms as a probe source. Taking advantages of He atoms’ low energy (20-100 ̃ meV) and chemical inertness, neutral He atom microscopy have drawn attention as a surface sensitive microscopy, imaging delicate structure without damage the target materials. However, neutral He atom microscopy has been limited by a lack of proper neutral atom optics such as an atomic mirror.
Recently, Amongst them, mirrors are key components for realization of matter-wave optics (e.g.; keys to a focusing material for microscopy). However, unlike electrons and neutrons, atoms Despite of a decade of efforts towards developing a focusing material, technical limitation. Development of the atom optics and atomic mirrors is strongly dependent on the technical development.
In this introduction, I outline briefly about atom optics and atomic mirrors along with the technical development. There are several underlying physical principles for atomic reflection (i.e., classical and quantum reflections). Here, several types of atomic mirrors are described depending on such reflection mechanisms.
1.1 Atom optics
The early stage of atom optics was in need of further steps while electron and neutron optics were on a fast track [3-7, 1994-review]. In truth, since 1930, when Stern and Estermann showed diffraction of He atoms from the crystal surface of lithium fluoride [2], following studies on atom optics were halted for next 40 years. One of the obstacles for realization of atom optics was a lack of intense and coherent atomic source. Without a proper atomic source, neither static electric or electromagnetic field nor solid materials are suitable to change their motion while preserving their coherence while both of them easily affect motions of neutrons and electrons. There are two reasons for this: 1) electric, magnetic and optical forces upon thermal neutral atoms are too weak to change their motion [8]; and 2) atoms collide with rather than penetrate through the solid material, causing diffusive scattering.
Development of a supersonic beam, in the 1970s, appeared to be a starting point to address the aforementioned problems [ref]. Under supersonic expansion, atoms in the high-pressure environment, pass through an orifice into low pressure or vacuum region. Their random thermal motions are, thereby, collimated, i.e.; atomic wave sources with well-defined kinetic energy become available [9]. Such supersonic beams serving as the coherent atomic source enabled various studies on the physisorption or chemisorption as well as the atomic diffraction from the crystal surface [10-18?/scattering//]. Thus, an advent of the proper atomic source played an important role in the revival of atom optics.
The further progress in atom optics has been accelerated by the advanced technical developments in laser and nanotechnology such as improved quality and tunability of laser and nano-fabrication technique; Enhanced coherence and intensity of the laser enabled us to observe Bragg scattering and the Kapitza-Dirac effect of atomic waves by generating a standing wave [19-22]. Besides that, intense tunable laser motivated the first approach to an evanescent wave mirror [23,24], by adjusting the detuning frequency with respect to the transition frequency of atoms. In addition, the establishment of nano-fabricated devices has served as atomic optical elements such as transmission grating [25-27] and Fresnel zone plate [28-31]. Until now, those instruments have been the basis of matter-wave interferometry [32-35] as well as atomic de Broglie microscopy [28-31].
Moreover, attempts to utilize advanced laser technology to explore the atomic motion has led to the laser-cooling and -trapping techniques in the late 20th century. By exploiting such techniques to atoms, ultracold atoms (below \(\mu K\)) are produced, providing a high-flux and much slower atomic source. It made a significant advance in atomic mirrors: 1) normal incidence atomic reflection from the evanescent wave mirror has been achieved [ref], 2) the atomic reflection from an external magnetic field, known as magnetic mirror, has been performed for the first time [in evanescent thesis]. Furthermore, since ultracold atoms have low kinetic energy, their de Broglie wavelength is much larger than thermal atoms'. Such a large atomic de Broglie wavelength emphasize the wave nature of atoms, giving rise to the quantum mechanical phenomena, e.g.; quantum reflection. As described below, the quantum reflection have provided a major breakthrough in discovery of solid atomic mirror (see 1.2.2).
1.2 Atomic mirror