Hilmar Oberst

Ridged atomic mirrors and atomic nanoscope

Recent results on the reflection of waves from ridged mirrors are discussed. Numerical calculations, analytical estimates and the direct measurements of coefficient of specular reflection of atomic waves from solid-state mirrors are combined. The reflectivity is approximated as an elementary function of period L of ridges, their width l, wavenumber K, grazing angle θ and effective depth w of the van der Waals potential. In a special case L = l, the fit reproduces the reflectivity of flat surfaces. Our approximation allows us to optimize the L at given l and estimate the maximum performance of a ridged mirror. Such a mirror is suggested as a focusing element for the nano-scale imaging system.

Atomic-matter-wave scanner

We report on the experimental realization of an atom optical device that allows scanning of an atomic beam with a controllable diffraction grating for atomic waves. We used a time-modulated evanescent wave field above a glass surface to diffract a continuous beam of metastable neon atoms at grazing incidence. The diffraction angles and efficiencies were controlled by the frequency and form of modulation, respectively. With an optimized shape, obtained from a numerical simulation, we were able to transfer more than 50% of the atoms into the first order beam, which we were able to move over a range of 8 mrad.

Quantum reflection of cold atoms

The coherent reflection of ultra-cold neon and helium atoms from silicon surfaces and surface structures is measured and analyzed. When the silicon surface is flat, the reflection is explained by the quantum reflection due to van der Waals attractive potential near the silicon surface. When the silicon surface is modified to form a grating structure, the reflectivity increases dramatically. For certain ranges of surface structure parameters, the reflection may be interpreted as a coherent summation of Fresnel diffraction of the atomic wave at the ridges of the grating structure.

The spatial Zeno effect applied to atomic optics

The scattering of waves at a ridged surface is interpreted as the Zeno effect. This interpretation leads to a simple estimate for the coefficient of reflection. Our estimate shows good agreement with experimental data.

Atom optics using solid surfaces and surface structures

A cold beam of metastable helium atoms is used to study the quantum reflection on solid surfaces and surface structures. The results are applied to construct reflection-type atom optical devices, such as diffraction gratings. Simple solid surfaces and surface structures offer several advantages as precision reflectors for atomic waves: They are inherently stable, accurate and nearly dispersionless, and in addition they can be made very large. The reflection is coherent as long as the de Broglie wavelength is large compared to the surface roughness, which is easily realized with polished surfaces and laser-cooled atoms. We report here on new experiments with laser-cooled He* atoms to investigate atom-surface interactions and to construct atom optical components based on micro-scale surface structures. A cross-sectional view of the experimental setup is shown in Fig. 1. Metastable helium atoms in the 2S1 state are trapped and cooled in a magneto-optical trap using the transition at 1083 nm. The atoms are release from the trap by illuminating the cloud of trapped atoms from the top with short pulses of a focused, resonant laser beam. The sample surfaces are placed in the atomic beam line below the trap at grazing incident angle. The pattern of scattered atoms is detected on a micro-channel plate detector, which provides a spatial resolution below 100μm. The velocity of the atoms is adjusted with the length and intensity of the releasing pulses, and the detector is gated to detect only atoms that arrive within a short, chosen interval. A measurement of the reflectivity of He* on a flat, polished silicon surface as a function of the normal incident velocity component between 3 and 30 cm/s is shown in Fig. 2. On a flat solid surface the reflection can be explained as the quantum reflection at the steep slope of the van der Waals attraction near the surface. We compare our result to a calculation of the van der Waals surface potential using the frequencydependent dipole polarizability of He* and the dielectric properties of silicon, and analyze the influence of doping and of an oxide surface layer on the potential shape. The dashed line represents the reflectivity expected on a perfect conductor, the solid line on undoped silicon and the dashed line on undoped silicon covered with a 100nm-thick oxide layer. Although the silicon sample is doped and has an electrical conductivity of 0.02 (Ωm), the experimental data clearly deviate from the reflectivity expected on a conductor. For a conductivity below about 100 (Ωm) and a distance of several hundred nm, the presence of free charges has almost no influence on the potential shape. An oxide layer has an appreciable effect on the reflectivity only when the layer thickness exceeds about 20 nm. QTuG2-2