Photofragmentation Beam Splitters for Matter-Wave Interferometry

Extending the range of quantum interferometry to a wider class of composite nanoparticles requires new tools to diffract matter waves. Recently, pulsed photoionization light gratings have demonstrated their suitability for high mass matter-wave physics. Here, we extend quantum interference experiments to a new class of particles by introducing photofragmentation beam splitters into time-domain matter-wave interferometry. We present data that demonstrate this coherent beam splitting mechanism with clusters of hexafluorobenzene and we show single-photon depletion gratings based both on fragmentation and ionization for clusters of vanillin. We propose that photofragmentation gratings can act on a large set of van der Waals clusters and biomolecules which are thermally unstable and often resilient to single-photon ionization.

Figure 1

Time-domain interference using single-photon fragmentation gratings. A 500 K pulsed nozzle source emits organic molecules, here, hexafluorobenzene (HFB) or vanillin. Supersonic expansion in an intense neon pulse leads to the formation of clusters. Three standing light-wave gratings form the matter-wave interferometer. At the antinodes of the light gratings, the clusters may fragment or ionize after absorption of a single 7.9 eV (VUV) photon. This leads to a pulsed and spatially periodic labeling of clusters and their effective removal from the beam. Only clusters transmitted through the absorptive light comb contribute to the interference pattern.

Figure 2

Quantum interference of clusters of hexafluorobenzene (a) and vanillin (b). We monitor the normalized signal contrast ($S_N$) which compares the cluster transmission for the on resonant and off resonant setting of the grating pulse separation times. Resonances can be seen in the mass spectrum when the pulse separation time is close to an integer multiple of the Talbot time $T_T$. (c,d) The resonant character of quantum interference can be seen by varying the difference of the two pulse separation times $\mathrm{\Delta }T=\mathrm{\Delta }{T}_{12}-\mathrm{\Delta }{T}_{23}$ between subsequent diffraction gratings in the reference mode. Interference occurs only when both times are equal. A temporal detuning of several dozen nanoseconds suffices to destroy the effect. The dips in (c) and (d) were measured for the detected cluster number $n_{\text{det}}=1$ and $n_{\text{det}}=5$ of HFB. The error bars represent one standard deviation of statistical error. The solid lines are Gaussian fits.

Figure 3

(a) Verifying the multiphoton character of the cluster ionization process: clusters propagate freely between the source $S$ and the detector $D$ where they are ionized upon photoabsorption. (b) Photoionization of HFB clusters: the detected ion signal depends nonlinearly on the laser energy $E_D$. This is a clear sign of a multiphoton process. The curves represent $n_{\text{det}}=2$ (crosses, orange), 4 (open circle, dark orange), $5$ (plus, light orange) HFB molecules per detected cluster. (c) Photoionization of vanillin clusters: $n_{\text{det}}=3$ (crosses, blue), 7 (open circle, dark blue), $11$ (plus, light blue). We observe a high-order nonlinearity for $n_{\text{det}}=3$ and an indication of single-photon events at $n_{\text{det}}=11$. This is consistent with the expectation that the ionization efficiency increases with increasing cluster number. (d) Verifying the single-photon character of the beam depletion process in the laser grating $G$: The mirror has been retracted beyond the coherence length of the $F_2$ laser to limit the cluster-light interaction to absorption in a running wave. (e) Beam depletion, case of HFB: the curves are well reproduced by single-photon events in a Poissonian process (exponential fits) for the detected cluster numbers $n_{\text{det}}=2$ (crosses, orange), 4 (open circle, dark orange), $5$ (plus, light orange). (f) Beam depletion, case of vanillin: $n_{\text{det}}=3$ (crosses, blue), 7 (open circle, dark blue), $11$ (plus, light blue). Similarly to HFB, vanillin also shows single-photon fragmentation events. The constants $c_1,\dots ,c_4$ define the scale of the measured laser energy and are different for all four panels.