Modes interaction and light transport in bidimensional organic random lasers in the weak scattering limit

We report on the modes interaction and light transport in weakly scattering neat films of small molecular weight organic molecules, showing coherent random lasing. The lasing modes interaction exhibits peculiar properties, with spatially overlapped modes at low excitation density and modes competition at high excitation density. This results in a progressive decrease and in a saturation of the number of lasing modes with increasing the excitation density. The weak scattering results in diffusive light transport with a transport mean free path in the range $14.6–125\mu \mathrm{m}$, which is much higher that the lasing wavelength $\lambda \approx 620\mathrm{nm}$. The average transport properties are correlated with the single scattering event cross section. We demonstrate that the light propagation is mainly affected by single scattering properties rather than collective effects.

Figure 1

Emission spectra of the S1 sample as a function of the excitation density for a stripe length of $6.5\mathrm{mm}$. The gray lines are fit of the spectra without the contribution of random lasing peaks. The dotted vertical lines and the numbers indicate three modes with different intensity dependence on excitation density. Inset: Chemical structure of the T5oCx molecule ($\mathrm{Hex}=\text{Hexyl}$ chain, $\mathrm{Cx}=\text{Ciclohexyl}$ group).

Figure 2

Emission spectra of the S2 sample as a function of the excitation density.

Figure 3

Intensity of the modes 1, 2, and 3 of Fig. 1 as a function of the excitation density. The lines are guide for the eyes. The intensity decrease at high excitation density for the modes 1 and 2 is evident.

Figure 4

Number of random lasing mode in S1 spectra as a function of the excitation density for three different stripe lengths.

Figure 5

Number of random lasing mode as a function of the excitation density for S2 and S3 samples. The S3 point are vertically traslated for clarity. The lines are guides for the eyes.

Figure 6

Emission spectra of sample S1 as a function of the stripe length from $3.5\mathrm{mm}\text{to}6.5\mathrm{mm}$ at fixed excitation density of $1.55\mathrm{mJ}{\mathrm{cm}}^{-2}$. The dotted lines indicate some modes that appear at some stripe length and persist for longer stripes.

Figure 7

Number of lasing modes as a function of the excited stripe length showing the presence of a threshold length followed by a linearly increasing number of lasing modes.

Figure 8

$2\times 2\mu \mathrm{m}$ AFM images of the three investigated samples. The roughness is in the range $5–6\mathrm{nm}$.

Figure 9

Typical power Fourier transform of the S1 emission spectra over the random lasing threshold. The dotted lines represent the best fit curves of a multi-Gaussian fitting of the spectrum.

Figure 10

Height correlation function of the three samples. The curves are normalized to their saturation value and vertically translated for clarity.

Figure 11

Transport mean free path obtained from the calculated cross section (full dots) compared with the values obtained from the diffusion equation (empty dots). The good agreement is evident.