Reduced lasing threshold from organic dye microcavities

We demonstrate an unexpected tenfold reduction in the lasing threshold of an organic vertical microcavity under subpicosecond optical excitation. In contrast to conventional theory of lasing, we find that the lasing threshold depends on the rate at which excitons are created rather than the total energy delivered within the exciton lifetime. The threshold reduction is discussed in the context of microcavity-enhanced super-radiant coupling between the excitons. The interpretation of super-radiance is supported by the temporal relaxation dynamics of the microcavity emission, which follows the super-radiance time rather than the cavity lifetime. This demonstration suggests that room-temperature super-radiant effects could generally lower the threshold in four-level lasing systems of similar relaxation dynamics.

Figure 1

(a) Architecture and cross-sectional scanning electron microscope image of an organic microcavity with DCM gain material situated between a DBR and an Ag mirror (low-$Q$ cavity) or a second DBR mirror (high-$Q$ cavity). (b) and (c) Normalized emission intensity as a function of excitation density for the (b) $Q=300$ and (c) $Q=3000$ microcavities, showing different thresholds under pulsed excitation with durations of 80 fs and 8 ns.

Figure 2

Cavity emission under fast (80 fs) excitation for the $Q=300$ cavity. (a) Emission spectra collected over angles −20° to 20° at increasing excitation densities, showing a spectral line narrowing to 1 nm above threshold where $E_{\mathrm{th}}$ is the threshold energy density. (b) A wide emission cone below threshold narrows to a width of 8° above threshold. (c) The degree of polarization of the emission $\left(T_M-T_E\right)/\left(T_M-T_E\right)$ as a function of excitation density shows highly polarized (0.94) emission above threshold.

Figure 3

Threshold reduction under fast excitation for three cavity lengths with $Q\sim 300$. (a) λ/2 cavity, (b) λ cavity, and (c) 3λ/2 cavity. Open circles are data for slow (8-ns) excitation, and filled circles are for fast (80-fs) excitation.

Figure 4

Energy-level diagram of DCM. The pump state is $m$, and the fluorescent state is $n$.

Figure 5

Comparison of cavity dynamics with (a) 100-fs excitation pulse width, and (b) 1-ns excitation pulse width. The emission pulse is offset from $t=0$ because the excitation pulse has a positive offset to avoid negative time values in the simulation. The oscillations above threshold are due to the repumping of state $n$ from state $m$ due to vibrational relaxation.

Figure 6

Simulated cavity emission as a function of pump energy density for a range of pump pulse widths. The dashed line indicates the level at which the threshold is determined.

Figure 7

Threshold energy density as a function of excitation pulse width for the low-$Q$ cavity. Experiment shows a dramatic reduction in threshold energy density for shorter pulses, whereas simulation of the conventional lasing threshold does not show the same dependence, indicating that phenomena beyond conventional lasing are responsible for the reduced threshold.

Figure 8

(a) For regular lasing $({\tau }_{\mathrm{pump}}=8\mathrm{ns})$, photons in the microcavity are in phase, whereas the radiating excitons are not, and under pulsed excitation, the emission pulse is determined by the cavity lifetime ${\tau }_c$. (b) In super-radiant lasing $({\tau }_{\mathrm{pump}}=80\mathrm{fs})$, the excitons are also coherently coupled to one another and hence radiate with an effective cross section enhanced by ${\tau }_R/{\tau }_c$, and the emission pulse is determined by the SR time ${\tau }_R$, resulting in a reduced lasing threshold excitation density. (c)–(f) Simulated and measured emissions from the low-$Q$ cavity following 80-fs pulsed excitation. The pulse decay times predicted for the conventional laser model are (c) 200 fs for excitation density $2E_{\mathrm{th}}$ and (e) 110 fs for excitation density $3E_{\mathrm{th}}$, approaching ${\tau }_c=95$ fs for high excitation densities. In contrast, the measured pulse decay times above threshold are as follows: (d) 1100 fs for excitation density $2E_{\mathrm{th}}$ and (f) 690 fs for excitation density $3E_{\mathrm{th}}$. The observed pulse decay times are $\sim 2{\tau }_R$, in agreement with the SR regime of operation.

Figure 9

Time-resolved emission from the high-$Q$ cavity above threshold under 80-fs excitation showing a decay time constant of 5900 fs, in good agreement with the SR time predicted by Eq. (3).