Active modulation of Fano resonance by light is demonstrated for an all-dielectric multilayer system containing an azo-dye-doped layer. The sample studied consists of a polystyrene layer doped with disperse red 1 (azo dye) molecules, a polyvinyl alcohol layer, and a pure polystyrene layer. In a Kretschmann attenuated-total-reflection configuration, angle-scan reflection spectra of the sample were measured with blue probe light under blue light pumping. The Fano line shape was found to change systematically depending on the intensity of the pump light. Analyses based on electromagnetic calculations of the spectra and field distributions in the layers indicate that the Fano resonance observed is generated by coupling between a broad half-leaky guided mode supported by the azo-dye-doped layer and a sharp planar waveguide mode supported by the pure polystyrene layer. The systematic changes in the Fano line shape under pump light irradiation can be well understood by a systematic decrease in light absorption in the azo-dye-doped polystyrene layer; the decrease in light absorption is due to a decrease in the extinction coefficient of the layer arising from the photoisomerization of azo dye molecules.

A method that allows direct monitoring of Fano resonant behaviours of local electric fields inside a waveguide layer in multilayer structures was developed. All-dielectric multilayer structures consisting of two polystyrene waveguide layers separated by a polyvinyl alcohol spacer layer were prepared. One of the waveguide layers was doped with fluorescent dye molecules. The fluorescence spectra of the sample were measured in a Kretschmann attenuated-total-reflection geometry as a function of the angle of incidence of the excitation light. The angle-scan fluorescence excitation spectra exhibited a sharp Fano line shape superposed on a broad band. Results of electromagnetic calculations of the electric field distribution inside the multilayer structure revealed that the local electric fields inside the dye-doped waveguide layer exhibit Fano resonant behaviours due to the near-field coupling to the waveguide mode supported by another waveguide layer. Using the calculated local electric fields, theoretical fluorescence spectra were calculated based on a point dipole model. The theoretical fluorescence spectra were found to reproduce the experimental ones very well, confirming that the observed Fano line shapes in the fluorescence spectra are the manifestation of the Fano resonant behaviours of the local electric fields inside the dye-doped layer.

Using a reverse attenuated-total-reflection geometry, we measured angle-scan fluorescence emission spectra of all-dielectric multilayer samples containing a waveguide layer doped with fluorescent dye molecules (fluorescent waveguide layer). A sample containing only one fluorescent waveguide layer showed a highly directional emission spectrum with a Lorentzian line shape caused by the radiative decay of an excited planar waveguide mode into a traveling wave in a decoupling prism. Addition of another waveguide layer containing absorptive dye molecules was found to greatly modify the spectrum and generate a Fano line shape in the emission spectrum. The observed Lorentzian and Fano emission spectra could be well reproduced by electromagnetic calculations based on the Lorentz reciprocity theorem. Calculated results of electric field distributions indicate that the Fano line shape is generated by the suppression of local electric fields inside the fluorescent waveguide layer resulting from coupling between two waveguide modes.

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We have succeeded in controlling the line shape of Fano-like resonances in all-dielectric multilayer structures in a wide range by UV light irradiation. Multilayer structures consisting of a waveguide layer supporting a half-leaky guided mode, a spacer layer, and another waveguide layer supporting a planar waveguide mode are known to exhibit Fano-like line shapes in attenuated total reflection spectra due to coupling between the half-leaky guided mode and the planar waveguide mode. Using a photochromic layer, i.e., a layer doped with spiropyran molecules, as one of the waveguide layers, we controlled the amount of light absorption in the waveguide layer by varying the UV irradiation dose. We demonstrated that the line shape changes dramatically depending on the UV dose, from the electromagnetically induced transparency-like to electromagnetically induced absorption-like line shape (or vice versa) passing through the Fano-like line shape. We also demonstrated that the photochromic response induced by UV irradiation in the Fano-resonant multilayer structure is enhanced by a factor of ∼100 relative to that in a single photochromic layer. Our analyses based on electromagnetic calculations suggest that the dramatic line shape change and the enhanced photochromic response are the consequences of enhanced local electric fields inside the photochromic waveguide layer combined with the photoinduced increase in the imaginary part of the dielectric constant.