Abstract

Quasi one-dimensional (ID) conjugated polymers, such as polydiacetylenes, have thermally unpopulated C=C and C≡C stretching mode vibrational levels in the ¹A_g ground state (corresponding to v₁ and v₂, respectively) which are coupled to the exciton transition in the small signal limit. The participtation of these vibrational levels can lead to a variety of nonlinear optical effects not seen in inorganic semiconductors. In this work, the stimulated Raman gain (RG) spectrum of a ID singlecrystal polydiacetylene with the backbone structure (RC-C≡C-CR’)_n, where R=R’= 2,5- bis trifluoromethyl-phenyl, PDA-DFMP, is obtained with a ≈ 100 fs pump at 1.99 eV, which is detuned from the ¹B_u exciton transitions energy at 2.27 eV. The RG spectra for v₁ and v₂ are both found to shift to higher probe photon energies by ≈ 100 cm^–1 when the pump fluence is increased to ≈ 5.8 × 10¹⁰ W/cm². Previous measurements of the RG spectra in other 1D conjugated polymers did not detect any optical Stark shifts since the pump laser was resonant with exciton transition.^[1,2]

© 1992 IQEC