Classification of light sources and their interaction with active and passive environments

Emission from a molecular light source depends on its optical and chemical environment. This dependence is different for various sources. We present a general classification in terms of constant-amplitude and constant-power sources. Using this classification, we have described the response to both changes in the local density of states and stimulated emission. The unforeseen consequences of this classification are illustrated for photonic studies by random laser experiments and are in good agreement with our correspondingly developed theory. Our results require a revision of studies on sources in complex media.

Figure 1

Illustration of emission directionality below threshold (black arrows) and above threshold (thick red arrows) in (a) a conventional laser and (b) a random laser. In a random laser the emitted light by an ensemble of sources is always omnidirectional. (c) Experimental emission spectra below and narrowed spectra above random laser threshold for three different light sources: Rhodamine 640 P (red dashed lines), Cresyl Violet (purple solid lines), and Nile Blue (blue dotted lines). The $\beta$ factor is determined by the quotient of the area of the normalized spectra above and below threshold.

Figure 2

Input-output diagrams for random lasers consisting of light sources with low and high quantum efficiencies. Peak spectral radiance versus pump power for three random lasers with different molecular light sources on (a) a linear and (b) a log-log scale. The solid lines are fits to the experimental data. Integrated spectral radiance versus pump power for three random lasers on (c) a linear and (d) a log-log scale. The solid lines are theoretical calculations. The Rhodamine 640 P random laser does not show a clear threshold. All data points in (a)–(d) were normalized to the values at 2.1 $\mu$W, and the results for the Nile Blue and Cresyl Violet random lasers were shifted vertically for clarity.

Figure 3

Diagram of a Green function describing propagation from a constant-amplitude source at ${\mathbf{r}}_0$ to $\mathbf{r}$ with one possible scattering event at ${\mathbf{R}}_{\mathrm{s}}$.