Enabling enhanced emission and low-threshold lasing of organic molecules using special Fano resonances of macroscopic photonic crystals

Abstract

The nature of light interaction with matter can be dramatically altered in optical cavities, often inducing nonclassical behavior. In solid-state systems, excitons need to be spatially incorporated within nanostructured cavities to achieve such behavior. Although fascinating phenomena have been observed with inorganic nanostructures, the incorporation of organic molecules into the typically inorganic cavity is more challenging. Here, we present a unique optofluidic platform comprising organic molecules in solution suspended on a photonic crystal surface, which supports macroscopic Fano resonances and allows strong and tunable interactions with the molecules anywhere along the surface. We develop a theoretical framework of this system and present a rigorous comparison with experimental measurements, showing dramatic spectral and angular enhancement of emission. We then demonstrate that these enhancement mechanisms enable lasing of only a 100-nm thin layer of diluted solution of organic molecules with substantially reduced threshold intensity, which has important implications for organic light-emitting devices and molecular sensing.

Keywords: enhanced light–matter interaction; fluorescence enhancement.

Fig. 1.

Optofluidic platform of organic molecules coupled to Fano resonances of the macroscopic photonic crystal. (A) Schematic drawing of the two lowest singlet energy levels of a dye molecule and the transitions it undergoes during fluorescence emission. (B) Schematic drawing of the experimental setup of the angle-resolved fluorescence measurements of R6G dissolved in methanol at the concentration of 1 mM placed on top of the PhC. The gray substrate is the macroscopic PhC slab. The orange spheres are schematic drawings of the R6G molecules in solution. The blue surface represents the equal energy density surface of the Fano resonance. Fluorescence spectra of the organic solution were recorded using a high-resolution spectrometer placed close to the normal of the PhC. By tuning the position of the spectrometer, fluorescence spectra of the molecules along to X and to M were measured.

Fig. 2.

Significantly enhanced fluorescence emission from R6G molecules. Comparison of fluorescence spectra of R6G molecules measured in the normal direction on the PhC (solid lines) pumped on-resonance (blue) and off-resonance (red), as well as on a uniform unpatterned slab (dashed green line). *, blue line has been multiplied by a factor of 0.1 for the simplicity of comparison with others. By comparing the spectra, we obtain the excitation (), extraction , and total enhancement factors, which are compared with the theoretical predictions, as described in the main text. (Inset) FDTD calculation results of the band structure from which the incident angle (ϕ) for on-resonance coupling is determined , showing good agreement with experiments . a.u., arbitrary unit.

Fig. 3.

Comparison between theoretical model and experimental results of the enhancement mechanisms. (A) Band structure of the PhC along -to-M and -to-X directions. (B) Angle-resolved fluorescence measurements of R6G solution suspended on top of the PhC. The correspondence between the color and number of photons (arbitrary units) is given in the color bar on the side. (C) Total enhancement factors, , for mode 1 (blue line) and mode 4 (green line) calculated through the product of excitation enhancement, , and extraction enhancement, , using the theoretical model. (D) Theoretical prediction of the averaged total enhancement factor, , between 0° and , to be compared with experiments. (E) Total enhancement factor, , extracted from experimental results in B. A comparison between D and E for the same angle range (0–) shows good agreement not only in trend but also in values.

Fig. 4.

Low-threshold lasing of a 100-nm thin layer of R6G molecules in solution. Input–output energy characteristics of lasing through mode 4 (580 nm) under pulsed excitation are shown. The solid lines are analytical predictions from our lasing model, whereas the green circles are energies measured (Meas.) with a power meter. Red circles are measurement results using the spectrometer multiplied by an arbitrary constant for the simplicity of comparison. The jump in output power clearly indicates the onset of lasing. (Lower Inset) Same results in linear scale, where the output grows linearly with the pump energy beyond threshold. (Upper Inset) Measured spectrum of emission from the PhC slab at normal direction when pumped below (blue) and above (red) the lasing threshold. Single-mode lasing is attained at approximately (corresponding to an intensity of ).