Optical amplification of two different propagation modes in organic small molecular waveguide structure.

We investigate the emission spectra from the edge of optically pumped waveguide. The waveguide is based on vacuum-deposited thin films of small molecular, 2,5,2',5'-tetrakis(2,2-diphenylvinyl)biphenyl (TDPVBi). Narrowed emissions are observed both at high (> 6 KW/cm2) and low (< 1 W/cm2) pump power sources, which are attributed to two different propagation modes in the asymmetric slab waveguides, guided mode and cutoff mode, respectively. The peak wavelengths of the guided mode appear at the maximum of the photoluminescence (PL) spectrum of the TDPVBi film. In contrast, both the peak wavelength and polarization of the cutoff mode are thickness dependent. The optical gains of the two modes are measured by the variable stripe length (VSL) method. The amplification with an exceptional low threshold for the cutoff mode has been demonstrated. Our results suggest that the cutoff mode is a promising route for the reduction of lasing threshold.