Solid state organic gain medium using optically-pumped dye molecules doped in a polymer host is considered as the top cladding of a long-range surface plasmon polariton (LRSPP) structure to enable active plasmonic devices with interesting applications operating in the near-infrared.
The gain media is a thin film of PMMA (poly (methyl methacrylate)) doped with ~ 0.9 wt% organic dye molecules of IR-140 and is pump optically using 8 nsec laser pulses at 810 nm to enable stimulated emission by excited dye molecules to the LRSPP mode at ~ 880 nm.
The gain media was modeled through rate equations for a four-level energy system, relating the small signal gain coefficient to the dye photo-physical parameters, dye concentration and pump irradiance. Distributed Bragg reflector (DBR) and distributed feedback (DFB) lasers were proposed using Bragg reflectors based on modulation of the metal stripe width, forming a stepped-in-width Bragg grating in the LRSPP waveguide. Single mode surface plasmon DFB and DBR lasers were designed at 882 nm, by applying coupled-mode theory and transfer matrix method (TMM).
The IR-140 doped PMMA gain medium was experimentally characterized. The maximum available material gain was identified for various pump intensities and two possible pump polarization in the gain media using the variable stripe length (VSL) method. The maximum available material gain agreed well with the theoretical gain modeling performed previously.
The DFB lasers and passive Bragg gratings were fabricated in the microfabrication laboratories at Center for Research in Photonics in University of Ottawa. Main fabrication processes included electron beam lithography to create stepped-in-width Bragg grating patterns with sharp corners and edges, with features as small as 150 nm.
Passive Bragg gratings were successfully characterized by my colleague showing a clear dip in the transmittance spectra (~ 40%) at the designed Bragg wavelength 882 nm.
DFB lasers were characterized and successfully demonstrated a highly narrowed (FWHM ~ 0.2 nm) single mode lasing peak at 882 nm. The mode profile from the DFBs’ output facet was captured by an infrared camera showing a tiny bright spot surrounded with dim spontaneous emission.