Photonic-band-edge-induced lasing in self-assembled dye-activated photonic crystals

We report experimental demonstration of photonic band-edge lasing in three-dimensionally ordered self-assembled photonic crystals consisting of rhodamine-B dye doped nanospheres of diameter 295 nm. Our all-solid photonic crystal shows a well-resolved photonic stop gap in the visible region. Laser-induced emission experiments reveal more than 51$\%$ inhibition in spontaneous emission intensity of dye molecules within the stop gap and an enhancement near the blue side of the stop gap. With increase in incident pump energy, we achieve photonic-band-edge-induced lasing at room temperature with a lasing threshold of 0.7 mJ. We explain the origin of lasing as due to the enhancement of local density of states near the band edges and the light field distribution within the photonic crystal structure. Our results indicate that such all-solid, dye-activated self-assembled nanostructures are a promising candidate for realizing photonic devices.

Figure 1

(a) FESEM image of the photonic crystals made of dye doped PS spheres with a diameter of 295 nm indicates proper ordering of spheres on the surface and in the bulk of the crystal. (b) The top surface shown is representative of the (111) plane of the fcc lattice. Very good ordering of spheres is observed in (a) and (b).

Figure 2

Reflectivity (black line), transmittance (red dash-dotted line), and diffuse scattered intensity (blue dotted line) measured from the (111) plane of the photonic crystal composed of dye doped PS sphere of diameter 295 nm. The peak in reflectivity spectra and trough in transmittance spectra are in good agreement, indicating the signature of a photonic stop gap. The absorption due to rhodamine-B dye is indicated using downward arrows in the transmittance spectra. The diffuse scattered intensity drops inside the stop gap but remains nonzero due to residual disorder present in the samples. The diffuse scattered intensity increases near the blue side of the stop gap. The inset shows the higher-order photonic stop gaps and their observation is a confirmation of superior optical quality of our samples.

Figure 3

(a) Emission spectra measured from the photonic crystals composed of dyed-PS spheres of diameter 295 nm (open symbols) and 617 nm (solid line). Emission spectra for photonic crystals of sphere diameter 295 nm show an abrupt decrease in emission intensity around 595 nm due to the photonic stop gap. The emission spectra for photonic crystals of sphere diameter 617 nm do not show any changes in intensity as there is no photonic stop gap in the spectral range of dye emission. (b) The intensity ratio between the emission spectra of photonic crystals with sphere diameter of 295 and 617 nm (solid line) together with corresponding stop gap (open circles) shows the suppression of spontaneous emission in the stop-gap wavelength ranges. The trough in intensity ratio is in good agreement with peak in reflectivity spectra. The intensity ratio is above unity on the blue side of the photonic stop gap.

Figure 4

Emission intensity as a function of wavelength with increase in incident pump energy for photonic crystals of sphere diameter (a) 295 nm and (b) 617 nm together with corresponding reflectivity spectra (open symbols). With increasing pump energy, the intensity of emission increases, and a new sharp peak (dashed vertical line) appears near the long-wavelength band edges of the photonic stop gap in (a) which is absent in (b). This represents the band-edge lasing peak. When the lasing peak becomes well resolved, spontaneous emission shows a clear trough (diamond symbol) on the blue of the lasing peak, indicating the removal of energy from spontaneous emission to the stimulated emission. (c) The zoomed-in spectra around the lasing peak at ∼606 nm for different incident pump energies of 0.53 mJ (dashed line), 2.66 mJ (dotted line), 4.09 mJ (dash-dotted line), 7.49 mJ (short dashed line), and 10.98 mJ (solid line). With increasing pump energy, the peak intensifies and slightly shifts to the red side.

Figure 5

(a) Amplified spontaneous emission at 586 nm as a function of incident pump energy. A gradual increase in spontaneous emission intensity is evident. (b) The emission intensity of the lasing peak at 606 nm as a function of incident pump energy shows a clear laser threshold at 0.7 mJ (dotted vertical line). Once the threshold is reached, the lasing intensity shoots up.