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(Color online) Exploded view of the sample disk and environmental control features. The silicon substrate (A) is attached to the copper block (B) with restraints positioned above and below. The block is maintained at the correct temperature by a heating element (C) bonded to the back of the plate. The sample and block are thermally isolated by insulating ceramic posts (D) and can be quenched to lower temperature by flowing gas out of the quench tube (E). The base of the cell can be equipped with a variety of attachments, allowing for supply of quench gas (inlet F and effluent G), evacuation of the cell (vacuum flange H).
Typical temperature ramping behavior of the sealed sample cell. The feedback-loop resistive heater can stabilize at a set-point temperature within . The solid lines are sigmoidal fits to the data.
Temperature performance of the cell without active heating. Under vacuum (●), the cell holds temperature well, decaying with a time constant of . With a constant helium purge (◆), the cell’s temperature can be quenched to room temperature within (time constant ). The solid lines represent an exponential decay fit (for the helium purge data) and a biexponential decay (for the evacuated cell).
(Color online) Schematic of neutron reflectometry experiment with simultaneous laser irradiation. Two air-cooled lasers (on the right) are used as illumination source. One laser output is adjusted with a converging lens (L1) in order to account for slight differences in laser divergence. A beam from a second laser is reflected off of a mirror (M), making it nearly collinear with the first beam, and passing through a concerted optical train (the angular difference in the two beams has been exaggerated in the diagram, for clarity). The combined beam is converted from linearly polarized to circularly polarized using a wave plate . Diverging (L2) and converging (L3) lenses are then used to expand and collimate the beam, so that they overilluminate the sample with a uniform light intensity. The beam can be turned on and off via computer control of a shutter (S). On the far left, the sample can be seen attached to the copper heating plate, all enclosed in a sealed cell with a transparent window for laser illumination. The neutron beam passes through the sample housing, reflects off of the sample, and travels into a detector.
Comparison of fits used to analyze the neutron reflectivity curves. The data are the same in both cases (a thin azofilm before illumination), but have been offset vertically for clarity. The upper fit is a simple one-box model, which correctly describes the film thickness and density. The lower fit is a more elaborate model that includes a Gaussian distribution of film thickness values. This type of modeling correctly fits both the absolute intensity of the curve, as well as the depth of the minima.
Neutron reflectivity curves for an azofilm after various amounts of laser illumination. From top to bottom, the curves (offset vertically for clarity) correspond to before irradiation, after total irradiation, after irradiation, and after irradiation. The solid lines are the corresponding distribution-of-thickness fits. The shift of the fringes indicates that the film photoexpands with increased laser exposure.
Relative expansion of an azopolymer film as a function of total laser irradiation time, measured using neutron reflectometry. The data were fit with a biexponential rise-to-max.
Neutron reflectivity signals at (a) and (b). The black symbols refer to measurements during laser illumination, whereas the gray symbols refer to measurements in between successive laser irradiation steps. The monotonic data for smaller provide a more meaningful measure of photophysical change than do the data at larger .
Neutron reflectivity curves measured at different temperatures (offset vertically for clarity). The upper curve is a thin film, at room temperature, that has been laser irradiated. The lower curve is the same film, heated to, and maintained at . The distribution-of-thickness fits to the data (solid lines) show that the photoexpanded film thermally contracts.
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