Title:
Thermochromic phase-transitions of GafChromic films studied by z-scan and temperature-dependent absorbance measurements.

Abstract:
Upon irradiation the active diacetylene layer of radiochromic films undergoes dose-dependent polymerization in the blue polydiacetylene form. Dose assessment is currently based on linear absorbance measurements. The scope of the present study was designed for the further understanding of the linear and, in particular, the nonlinear optical characteristics of such films and the utilization of these characteristics for devising alternative "reading" procedures.
The nonlinear optical interactions of laser light with previously irradiated GafChromic(®) HD-810 and MD55-2 films were studied by z-scan techniques. The focused laser beam strikes on the film, which is located at various distances from the focal spot. The beam induces linear and nonlinear absorption and refraction. The measurement of the transmitted power (open-aperture z-scan) allows the study of the nonlinear absorption. On the other hand, the measurement of the axial region of the transmitted beam (closed-aperture z-scan) is related to both nonlinear absorption and refraction effects. A 10 mW, 633 nm, linearly polarized He/Ne laser was employed as the light source. The transmittance measurements were coupled with absorbance measurements carried out over the whole visible spectrum and at various reading temperatures, in an attempt to clarify the underlying mechanisms dictating the observed effects. These effects were incorporated to an open-aperture z-scan model, developed for the purpose of comparison to the corresponding experimental curves.
The transmittance data obtained by both open- and closed-aperture z-scan were found to be dose-dependent, thus allowing such transmittance techniques to be employed for dose assessment. Low power open-aperture z-scan measurements (sensitive to absorptive nonlinearities) revealed a stepwise two-photon excitation of the active layer, through an intermediate state. At higher laser intensities (and/or absorbed dose), the shapes of the z-scan curves were found to be more complicated. These shapes were attributed to a temperature-dependent (thermochromic) transition from the blue form to the conformer red form of the active material, a transition which appears to be irreversible at high intensities. This transition was incorporated into the open-aperture z-scan model which reproduced satisfactorily the corresponding experimental curves. Finally, the use of a chopped laser beam in the closed-aperture z-scan technique allowed artifact-free, high spatial resolution dose measurements using a laser beam of relatively high power.
The model developed in the present study elucidates some of the effects related to film reading and the causes of some of the artifacts reported in the literature. Two-dimensional dose distributions can be assessed by z-scan; however, the merits of such a technique relative to those currently in use have to be further investigated.