Index matching of TE and TM modes in organic multilayer waveguides

We investigate transverse electric (TE) and magnetic (TM) mode propagation in organic multilayers consisting of aluminum quinoline (Alq$_{\mathrm{3}})$ and perylenetetracarboxylic dianhydride (PTCDA). In particular, we analyze two multilayer waveguides, Alq$_{\mathrm{3}}$-PTCDA-Alq$_{\mathrm{3}}$ and PTCDA-Alq$_{\mathrm{3}}$-PTCDA, engineered to give index matching according to modeling. The waveguides were grown on a glass substrate via organic molecular beam deposition. Fabry-Perot oscillations observed from reflection measurements were used to confirm the individual layer thicknesses. We were able to observe refractive index matching between TE$_{\mathrm{0}}$ and TE$_{\mathrm{1}}$, as well as TE$_{\mathrm{2}}$ and TE$_{\mathrm{3}}$ modes for the PTCDA-Alq$_{\mathrm{3}}$-PTCDA waveguide due to the light propagation through the top and bottom PTCDA layers, respectively. In addition, we were able to match TE$_{\mathrm{1}}$ and TM$_{\mathrm{1}}$, as well as TE$_{\mathrm{3}}$ and TM$_{\mathrm{3}}$ modes in the Alq$_{\mathrm{3}}$-PTCDA-Alq$_{\mathrm{3}}$ multilayer due to the birefringence of the PTCDA layer. Furthermore, we are able to create mode matching for a range of wavelengths due to the similar effective refractive index dispersion of different waveguide modes. The ability to phase match different waveguide modes opens a wide range of potential applications including polarization-insensitive propagation and mode switching by adding a thin magnetic metal film within the waveguide and applying an external magnetic field.