How bright can you get?
Medium energy backscattering spectrometry has been used to investigate ion impact‐induced incorporation of oxygen in silicon. The sample was first exposed to 20‐keV Xe bombardment at base pressure (oxygen‐to‐xenon flux density ratio ν̇(O2)/Φ̇(Xe)⩽0.5, beam incidence ∼10° off normal). Having arrived at the steady‐state level in xenon content, bombardment was continued at deliberately enhanced oxygen partial pressures in the target chamber (5⩽ν̇/Φ̇⩽500). This resulted in significant beam‐induced incorporation of oxygen which manifests itself not only in a corresponding signal in the backscattering spectra (aligned geometry) but also in an enhanced reemission of xenon and a reduction in the erosion rate of the substrate (up to a factor of 5). The impact‐induced oxygen incorporation efficiency α can become very large. At low (added) fluences and for ν̇/Φ̇≳100 we found α0 = (3±1) atoms/ion. This result is supported by sputtering yield measurements on SiO2 and oxygen‐exposed silicon, which indicate that the steady‐state partial sputtering yield of oxygen at high flux density ratios is Y p,O = (4.3±0.5) atoms/ion. It is suggested that oxygen incorporation and subsequent migration to larger depths is essentially a defect‐controlled process. The high incorporation efficiency is thus due to the large (average) number of defects arriving at the surface after impact of a single ion. Analysis of the experimental data suggests that ion‐induced desorption sets an upper limit to the incorporation efficiency. Oxygen reduces the xenon retention capacity of the substrate at all concentration levels. The most pronounced effects are observed near the surface where incorporation of oxygen causes drastic reemission of xenon. The steady‐state xenon content of an oxidized sample is controlled by the oxygen concentration and the erosion rate.
During the development of optical trapping using the near-field generated by plasmonic nanostructures, we have observed that as an object is trapped, it becomes brighter. This effect has been studied in detail in a system consisting of a gold disk (GD) and a latex sphere (LS). The reason to use a gold disk instead of a continuous film, is that such a disk can function as plasmonic trap integrated into the experiment.
This effect is illustrated in this figure, which shows three sequential measurements of the signal scattered by a system formed by a GD and a LS, for three different positions of the LS. The GD is deposited on a glass substrate and the LS moves from right to left in a microfluidic channel.16 The relative positions along the x-axis of the two scatterers are explicitly observed by plotting the normalized intensity along the dashed red lines (the background intensity is suppressed in these plots). Pannel (a) shows the moment when the LS approaches the GD from the right: the right edge of the GD and the left edge of the LS just begin to overlap. However, the LS is still far away from the GD in the z-direction since it is clearly out of focus in the image. Pannel (b) shows the moment when the LS almost completely overlaps the GD. The intensity profile of the GD does not change a lot: the two peaks corresponding to the left and right edges of the GD are still clearly visible, indicating that the LS is still far away above the GD because of the profile of the laminar flow in the microfluidic channel. In this overlapping position, the plasmonic trap begins to act on the LS, which is rapidly attracted toward the surface of the GD. At this moment, the LS shines up and appears as a very bright spot, pannel (c).
In addition to giving qualitative information to the trapping, this phenomenon can also be used to determine the exact position of the particle within the trap, as illustrated in the following figure, which shoes the normalized scattered intensity as a function of the vertical separation distance between the trapped particle and the plasmonic structure. The intensities measured on the three panels of the figure at the top of the page shown as a, b, and c and can be used to obtain the separation distance between the GD and the LS.
This work opens the way to quantitatively determine the position of a nanoparticle or a cell within a plasmonic trap.