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Dispersion of the refractive indices of thiophene/phenylene co-oligomer single crystals
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Image of FIG. 1.
FIG. 1.

Structural formulae of BP1T, AC5, and AC’7.

Image of FIG. 2.
FIG. 2.

(Color online) Micrographs of the slab crystals of (a) BP1T, (b) AC5, and (c) AC’7. For (c) we used the AC’7 crystal #1 (see Table I). Their crystal axes (a, b, and c) were determined by referring the characteristic directions and angles of the crystals to the crystallographic structures. A pair of parallel crystal edges (the bc-planes for BP1T and AC’7; the ca-planes for AC5) constitutes an optical resonator (i.e., reflecting mirrors). The white arrows indicate the crystal width that defines the resonator length.

Image of FIG. 3.
FIG. 3.

(Color online) (a) Schematic diagram of the experimental setup including the crystal. (b) Cross section of the slab crystal cut along the dotted line in (a). Both the vertical edge facets and horizontal slab planes act as a resonator. The emissions occurring inside the crystal are propagated back and forth in the crystal and multiply-reflected at the vertical facets. Instead of the excitation light source (UV or laser), a white light is used for the measurements of reflection spectra (not shown in the diagram).

Image of FIG. 4.
FIG. 4.

(a) Emission spectrum of the AC’7 crystal #1. The full spectrum is obtained by joining the segmented data containing 1024 data points for every ∼10 nm. (b) Detailed profile of the spectrum around 530 nm. Emission spectra polarized (c) perpendicular and (d) parallel to the crystal ab-plane. For the measurements, the polarizer was placed between the crystal and the optical fiber; see Fig. 3(a).

Image of FIG. 5.
FIG. 5.

Laser oscillation spectrum of the AC’7 crystal #1. The excitation pulse energy was 6.7 mJ/cm2.

Image of FIG. 6.
FIG. 6.

Comparison of the broadband interference modulation (top) and the laser oscillation spectrum (bottom) for the AC’7 crystal #1. Notice that the intensity counts of the ordinates on either side resulted from the different measuring methods and different excitation light sources. Grids are put on the diagram for the sake of emphasizing the phase shifts of modulated peaks. To avoid complexity only six grids are drawn. The phase shift was 24° toward the shorter wavelength region for the longest-wavelength peak of the broadband emission relative to the corresponding peak of the laser oscillation. The shift was 107° toward the shorter wavelength region for the shortest-wavelength peak of the broadband emission.

Image of FIG. 7.
FIG. 7.

(Color online) (a) Wavelength dependence of the group refractive indices of the BP1T, AC5, and AC’7 crystals. The optimized solid curves are plotted using Eq. (5). The data plotted by filled circles (BP1T), filled triangles (AC5), and filled diamonds (AC’7) were obtained by the broadband interference modulation of emissions. An open circle, an open triangle, and open diamonds show the group indices estimated from the mode intervals of the multimode laser oscillations. (b) Dispersion curves of the phase refractive indices. These are plotted using the Sellmeier equation [Eq. (4)] with the optimized parameters A, B, and C (listed in Table II).


Generic image for table
Table I.

Major parameters including group indices for thiophene/phenylene co-oligomer (TPCO) crystals. The crystals were used for both the broadband interference measurements of the emission spectra and the laser beam excitation measurements. These indices were pertinent to the direction perpendicular to the ab-plane. V and L stand for the crystal growth methods in the vapor phase and the liquid phase, respectively.

Generic image for table
Table II.

Estimated parameters of the Sellmeier equation for the TPCO crystals. The parameters A, B, and C are included in Eq. (4).

Generic image for table
Table III.

Group indices along the a- and b-axes of AC’7 crystals together with their thicknesses. These were calculated from the interference modulation of the reflection spectra. The crystals were grown in the liquid phase.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Dispersion of the refractive indices of thiophene/phenylene co-oligomer single crystals