Figure 3. a) Images of azimuthal ss- and sp-polarized IR-DCP
transmission and absolute phase spectra. Tsp exhibits a
baseline value of 1 in the absence of anisotropy (e.g., for azimuthal
sample rotations of 0° and 90°) due to the spectral normalization to a
measurement without a sample. b) Azimuthal variation of the C–O–C band
height in the absolute ss- and pp-polarized phases as well as their
difference.
The parallel-polarized Tss and Δssimages capture all vibrational contributions along the ss polarization
axis and therefore also include isotropic contributions. In contrast,
the cross-polarized Tsp and Δsp images
are only sensitive to signals from anisotropic contributions, qualifying
them for the direct inspection of anisotropies. The anisotropic
absorption behavior of the nanofibers is directly identified in the
cross-polarized images, which show band maxima at 45° azimuthal angle,
in agreement with maximum in-plane p–s anisotropy for aligned
nanofibers. The parallel-polarized Δss phase image shows
azimuthally homogeneous areas beside the vibrational band contributions,
which could potentially be used as a measure of optical thickness.
Figure 3 (b) shows the azimuthal variation of the C–O–C band heights
in the ss- and pp-polarized phase spectra as well as their difference.
Classical FTIR polarimetry cannot measure absolute phases, but only
relative ones. In contrast, IR-DCP can separate the information encoded
in the polarization-dependent absolute phases.
In summary, the spectral polarimetric amplitudes and phases
simultaneously provide complementary information on sample structure and
thickness. Moreover, IR-DCP acquires absolute phase data for arbitrary
polarization configurations, allowing, for instance, the direct
measurement of Δsp without the need for rotating optical
elements such as polarizers. Fast sample imaging applications of
parallel- and cross-polarized band amplitude and phase properties thus
become available in the mid‑IR spectral range.