Thermally activated population of microcavity polariton states under optical and electrical excitation

We examine the luminescence from optical microcavities containing an organic semiconductor in the regime of strong exciton-photon coupling. The ratio of luminescence from the upper polariton branch to the lower polariton branch is studied as a function of microcavity detuning and temperature under both optical and electrical excitation. The population of the upper polariton branch is modeled by means of thermal activation from an uncoupled exciton reservoir. Here, the activation energy for the population of the upper polariton branch is equal to the energetic separation between the exciton reservoir and the upper branch at the angle of detection. Agreement is obtained with this model under both optical and electrical excitation using measurements of angle-resolved microcavity reflectivity and luminescence.

Figure 1

(a) Absorption coefficient, photoluminescence (PL, broken line), and electroluminescence (EL, solid line) for a 70-nm-thick film of TPP collected at 300 K. The PL spectrum was collected under excitation at a wavelength of 405 nm. The EL spectrum was collected using the bottom-emitting microcavity structure described in Sec. 2 with the Ag anode omitted. The driving current density was 100 mA/cm${}^2$. Also shown are microcavity architectures for luminescence measurements at (b) room temperature and (c) variable temperature.

Figure 2

Exciton relaxation and microcavity polariton formation according to the proposed model. A full explanation of the model is provided in the text.

Figure 3

(a) Reflectivity spectra collected at angles of incidence ranging from 15° to 75° in steps of 15° for the microcavity of Fig. 1b. The broken lines highlight the dispersion of strongly coupled features with angle. The uncoupled exciton energy is denoted by a solid line. (b) Dispersion relation obtained from reflectivity. Solid lines are fits to the data based on a coupled-oscillator model. The broken line indicates the position of the exciton reservoir.

Figure 4

(a) Logarithmic plot of angle-resolved (steps of 10${}^{ˆ}$) photoluminescence (PL) and (b) the corresponding dispersion relation. (c) Logarithmic plot of angle-resolved (steps of 10${}^{ˆ}$) electroluminescence (EL) and (d) the corresponding dispersion relation. Under both optical and electrical excitation, luminescence is collected for the microcavity of Fig. 1b. For parts (a) and (c), broken lines indicate the position of the strongly coupled features while the solid lines indicate the position of the uncoupled exciton energy of TPP. For the dispersion relations of parts (b) and (d), solid lines are fits to the data based on a coupled-oscillator model while the uncoupled exciton energy is indicated by a broken line.

Figure 5

Photoluminescence (a) and (b) and electroluminescence (c) and (d) spectra collected at normal incidence as a function of temperature for the microcavity of Fig. 1c. Data for microcavities containing 55 nm of TPP are shown in parts (a) and (c) while data for microcavities containing 65 nm of TPP are shown in parts (b) and (d). The vertical solid line in each figure denotes the position of the uncoupled exciton energy of TPP, while the arrows denote the direction of increasing temperature. The labels Unc., LB, and UB indicate emission from uncoupled reservoir excitons, lower branch polaritons, and upper branch polaritons, respectively.

Figure 6

Multipeak fitting for a photoluminescence spectrum from Fig. 5 collected at 210 K on a device containing 65 nm of TPP (open circles). The spectrum is deconvoluted by multipeak fitting with four Gaussian peaks representing uncoupled emission (Unc., two peaks), lower branch emission (LB), and upper branch emission (UB).

Figure 7

(a) Temperature-dependent ratio of upper branch to lower branch photoluminescence intensity for the microcavity of Fig. 1c. Dispersion relations obtained from angle-resolved reflectivity for microcavities containing either (b) 55 nm or (c) 65 nm of TPP. In (b) and (c), the solid lines are fits based on a coupled-oscillator model while broken lines denote the position of the uncoupled exciton reservoir.

Figure 8

(a) Temperature-dependent ratios of upper branch to lower branch electroluminescence intensity for the microcavity of Fig. 1c. Dispersion relations obtained from angle-resolved reflectivity for microcavities containing either (b) 55 nm or (c) 65 nm of TPP. In (b) and (c), the solid lines are fits based on a coupled-oscillator model while broken lines denote the position of the uncoupled exciton reservoir.