Bragg Diffraction of Large Organic Molecules

We demonstrate Bragg diffraction of the antibiotic ciprofloxacin and the dye molecule phthalocyanine at a thick optical grating. The observed patterns show a single dominant diffraction order with the expected dependence on the incidence angle as well as oscillating population transfer between the undiffracted and diffracted beams. We achieve an equal-amplitude splitting of $14\hslash k$ (photon momenta) and maximum momentum transfer of $18\hslash k$. This paves the way for efficient, large-momentum beam splitters and mirrors for hot and complex molecules.

Figure 1

(a) The experiments are performed with the antibiotic ciprofloxacin (left) and the organic dye phthalocyanine (right). (b) A thermal beam of molecules is produced by microevaporation and collimated vertically (${\mathrm{S}}_x$) and horizontally (${\mathrm{S}}_y$). After 1.5 m of free flight the molecules are diffracted at a thick laser grating created by retroreflecting a 532 nm laser at a highly reflective mirror. The angle of the mirror with respect to the molecular beam ${\theta }_{\text{grat}}$ can be controlled with $\mu \mathrm{rad}$ precision. The molecular diffraction pattern is recorded after further 0.57  m of free flight by laser-induced fluorescence microscopy.

Figure 2

False color image of the experimental (a) and simulated (b) Bragg diffraction pattern of the antibiotic ciprofloxacin. The laser grating waists are $w_z=7.04(5)\mathrm{mm}$, $w_y=55(5)\mu \mathrm{m}$, and the collimation slit is set to $14\mu \mathrm{m}$.

Figure 3

Bragg diffraction pattern of the organic dye molecule phthalocyanine at an incidence angle ${\theta }_{\text{grat}}=5(5)\mu \mathrm{rad}$. Panel (a) shows the false color diffraction image. Panel (b) shows the averages of $20\mu \mathrm{m}$ high stripes, smoothed with median and Savitzky–Golay [38] filters, and annotated with their corresponding velocities in $\mathrm{m}/\mathrm{s}$. The velocities are determined by comparison with a diffraction pattern produced by a material grating [21]. The laser grating waist for this measurement is $w_y=57(3)\mu \mathrm{m}$ and the collimation slit width is $11.5\mu \mathrm{m}$.

Figure 4

Angular dependence of Bragg diffraction of the dye molecule phthalocyanine. Panels (a)–(c) show diffraction images for the incidence angles 48 (a), $-5$ (b) and $-69\mu \mathrm{rad}$ (c). The images are 197 by $197\mu \mathrm{m}$ and the scale bars are $50\mu \mathrm{m}$ long. Panel (d) shows the integrated intensity profiles for the incidence angle varying from $-69$ to $48\mu \mathrm{rad}$ in steps of $10\mu \mathrm{rad}$. The profiles are averages of $16\mu \mathrm{m}$ high stripes of the diffraction images corresponding to a velocity range of 234 to $255\mathrm{m}/\mathrm{s}$. The curves are horizontally aligned to center the undiffracted beam (which is the right peak for negative incidence and the left peak for positive incidence). For $\pm 5\mu \mathrm{rad}$ we observe diffraction to both sides of the initial beam and hence align the traces with respect to their center of gravity. The laser grating waist for this measurement is $w_y=65(5)\mu \mathrm{m}$ and the collimation slit width is $14.8\mu \mathrm{m}$.