Quantum reflection of helium atom beams from a microstructured grating

We observe high-resolution diffraction patterns of a thermal-energy helium atom beam reflected from a microstructured surface grating at grazing incidence. The grating consists of $10\text{−}\mu \mathrm{m}$-wide Cr strips patterned on a quartz substrate and has a periodicity of $20\mu \mathrm{m}$. Fully resolved diffraction peaks up to the seventh order are observed at grazing angles up to $20\mathrm{mrad}$. With changes in de Broglie wavelength or grazing angle the relative diffraction intensities show significant variations which shed light on the nature of the atom-surface interaction potential. The observations are explained in terms of quantum reflection at the long-range attractive Casimir–van der Waals potential.

Figure 1

(Color online) Scheme of the experimental setup. Diffraction patterns are recorded by scanning the detection angle $\theta$, which is defined with respect to the reflection-grating surface plane. The inset in the upper right shows an enlargement of the grating indicating the directions of the zeroth- and first-order diffraction beams.

Figure 2

(Color online) (a) Semilogarithmic plot of diffraction patterns observed for a He atom beam at $T_0=20\mathrm{K}$ at various incident grazing angles as indicated. Numbers indicate the diffraction order assigned to the peaks. (b) Comparison between the observed diffraction angles and those calculated by the grating equation. The gray-shaded region indicates the regime beneath the surface given by ${\vartheta }_n<-{\theta }_{\mathrm{in}}$.

Figure 3

(Color online) Diffraction patterns observed for He atom beams at various source temperatures for ${\theta }_{\mathrm{in}}=4.9$ (a) and $7.2\mathrm{mrad}$ (b). Numbers above the arrows indicate the diffraction order assigned to the peaks. Relative diffraction intensities, which are normalized to the zeroth-order intensity, are plotted for both series in (c) and (d), respectively, as a function of $k_{\mathrm{perp}}$, the normal component of the incident wave vector. The relative error of these data points is around 10%.

Figure 4

(Color online) Observed coherent reflection probabilities for beams of He atoms [dots connected by solid lines, different lines correspond to different $T_0$ $\left(6.7\text{to}20\mathrm{K}\right)$], He trimers (open diamonds), and Ne atoms (open squares). The dash-dotted line presents a prediction for classical reflection from a hard-wall surface, tentatively assuming a roughness $\sigma =4\mathrm{nm}$. The dashed lines present predictions for quantum reflection at the long-range attractive branch of the surface potential. The kink is marked by an arrow.