We present measurements of the local dichroism and birefringence of thin film specimens us ing techniques that combine near-field scanning optical microscopy (NSOM) and a novel polarization modulation (PM) polarimetry utilizing Fourier analysis of the detected intensity signal. Generally, quantitative near-field polarimetry is hampered by the optical anisotropy of NSOM probes. For exampl'dely used Al-coated pulled-fiber aperture probes typically exhibit a dichroism near 10%. Our analysis of aperture dichroism demonstrates that the usual techniques for nulling a PM polanimeter result in a non-zero redisual probe birefringence in the presence of a dichroic tip. However, we show that both dichroism and birefrin-ence of the sample can be determined if both the tip dichroism and birefringenceare explicitly measured and accounted for in the data. In addition, in thin films (<100 nm thick), where the sample birefringence and dichroism are often small, we show how to determine these polarimetric quantities without requiring alignment of the birefringent and dichroic axes, which is a more general casethan has been previously discussed. We demonstrate our techniques using two types of polymer film specimens: ultra-high molecular weight block copolymers (recently noted for their photonic activity) an isotactic polystyrene spherulites. Finally, we discuss how changes in the tip dichroism during datacollection can limit the accuracy of near-field polarimetry and what future steps can be taken to improve these techniques.
Citation: Applied Optics
Pub Type: Journals
birefringence, diblock copolymers dichroism, near-field scanning optical microscopy, NSOM, polarimetry, retardance, SNOM, sphenrulities, thin films