Enhanced dye lasing emission by guided-mode resonance grating with mesoporous silica as spacing layer

Abstract

<sec>The enhancement of lasing emission intensity of dye laser is particularly important and urgently required due to a broad range of optical and electrical applications. The guided-mode resonance (GMR) effect occurs in a periodic waveguide structure where an incident wave is coupled to a leaky waveguide mode, and yields a resonance peak. The resonance wavelength can be easily controlled by adjusting the period of the grating, thickness of the waveguide layer, and refractive index of the covering materials. By using band edge states, one may be able to excite optical resonances extended over the entire structure surface, thereby achieving field enhancement over a large area. In this study, mesoporous silica with low refractive index is introduced between the grating layer and the substrate layer of the GMR structure to significantly enhance the contact between local electric field and gain medium. For comparison, another structure using SU-8 with high refractive index as the spacing layer is also proposed. It is clearly observed that the maximum of the electric field intensity is highly localized inside the SU-8 waveguide grating layer. However, it is shifted upward to the gain medium layer in the mesoporous silica structure due to the reverse symmetry waveguide structure design. Therefore, the interaction between laser dye and electric field is increased to further enhance the lasing emission.</sec><sec>Besides the refractive index, the waveguide layer, other structural parameters such as thickness of each layer and grating period also affect the electric field distribution in the GMR structure. Based on the finite-difference time-domain method, the structural parameters are analyzed and optimized. According to the simulation results, the structure parameters T_WG = 3.5 μm, <i>Λ</i> = 700 nm, and <inline-formula><tex-math id="Z-20201229080229-1">\begin{document}$ T_{\rm TiO_2} = 20 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20201017_Z-20201229080229-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20201017_Z-20201229080229-1.png"/></alternatives></inline-formula> nm are chosen as the guideline for designing the dye laser, which generates the resonance wavelength of 820 nm the same as the absorption wavelength of dye molecules. Additionally, the laser characteristics excited by pump light with the wavelength of 820 nm are studied. The continuous laser output is obtained. The energy threshold is about 2.5 mJ/cm², and the linewidth is about 0.3 nm. The proposed structure can effectively regulate the surface local electric field and enhance the interaction between pump light and gain medium. It can not only be used in lasers, but also provide reference for designing other light-emitting devices.</sec>