Exciton-exciton annihilation in organic polariton
microcavities
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Akselrod, G.M. et al. “Exciton-exciton annihilation in organic
polariton microcavities.” Lasers and Electro-Optics, 2009 and
2009 Conference on Quantum electronics and Laser Science
Conference. CLEO/QELS 2009. Conference on. 2009. 1-2. ©
2009 Institute of Electrical and Electronics Engineers.
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© 2009 OSA/CLEO/IQEC 2009
Exciton-Exciton Annihilation in Organic Polariton
Microcavities
Gleb M. Akselrod1, Jonathan R. Tischler1, Elizabeth R. Young2, M. Scott Bradley1, Daniel G. Nocera2,
Vladimir Bulović1
1
Department of Electrical Engineering and Computer Science
2
Department of Chemistry
Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139
email: akselrod@mit.edu
Abstract: Sublinear intensity dependence of photoluminescence from organic exciton-polariton
microcavities under non-resonant excitation in two power regimes is shown. The sublinearity is
attributed to exciton-exciton annihilation, which could compete with polariton-polariton scattering
in these devices.
©2008 Optical Society of America
OCIS codes: (230.3990) Micro-optical devices; (240.5420) Polaritons
Excitons in a solid can be coupled to the electromagnetic field by placing the material inside a resonantly tuned
microcavity. If the decay rates of the excitons and the cavity mode are slower than the rate of energy exchange, the
system takes on new eigenstates which are light-matter superpositions known as exciton-polaritons, and the limit of
strong coupling is achieved. Recent work has demonstrated the use of organic thin films [1, 2] as the excitonic layer
in polaritonic structures and the characteristic linear properties of these devices showed strong coupling. Here we
present the first in-depth study of high intensity optical excitation of such organic exciton-polariton devices.
The excitonic component of our devices was made of the J-aggregated cyanine dye TDBC (5,6-dichloro-2-[3[5,6-dichloro-1-ethyl-3-(3-sulfopropyl)-2(3H)-benzimidazolidene]-1-propenyl]-1-ethyl-3-(3-sulfopropyl)
benzimidazolium hydroxide, inner salt, sodium salt). To achieve the necessary dye density, a 5.1 ± 0.1 nm film was
assembled by sequential immersion in the polyelectrolyte PDAC (polydialylldimethylammonium chloride) and
TDBC, producing a layer with a very large peak absorption of 106 cm-1 [3]. The cavity was formed by sputterdepositing a 4.5 pair distributed Bragg reflector (DBR) on a quartz substrate, followed by a λ/4n SiO2 spacer layer,
where n is the index of refraction and λ = 595 nm, the peak of the J-aggregate emission (Fig. 1a). The J-aggregate
film was then deposited, followed by a 100±1 nm spin coated layer of PVA (poly vinyl alcohol, 99.8% hydrolized),
which enhances the photoluminscence quantum yield of the J-aggregate film and acts as a spacer layer. A variable
thickness thermally evaporated TAZ [3-(Biphenyl-4-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4- triazole] layer forms
the remainder of the cavity spacer, and the structure is capped with a thermally evaporated silver mirror, giving a
cavity Q of ~60.
Figure 1. (a) DBR-metal microcavity with a J-aggregate excitonic layer and a total optical path length of ~λ/2 where λ = 595 nm. (b) The
reflectivity of devices having different cavity-exciton detunings and the corresponding photoluminescence. (c) Energies of the upper and lower
polaritons as a function of detuning. The bare exciton and cavity dispersions are shown as dashed lines.
The total thickness of the cavity region was varied by changing the thickness of the TAZ layer, thereby
changing the detuning between the J-aggregate exciton (Eex = 2.08 eV) and the cavity mode. Figure 1a shows the
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normal mode splitting in the reflectivity for devices with differen cavity-exciton detunings. The energy of lower
polariton branch photoluminescene (PL) is observed to follow the lower polariton reflectivity. The polaritonic
dispersion relation for these devices is shown in Fig. 1b, demonstrating an anti-crossing at zero detuning with a Rabi
splitting of 160 meV.
To test for evidence of polariton lasing, the devices were pumped non-resonantly with TM polarization at λ =
535 nm at 60° relative to normal through the DBR, in order to populate the exciton reservoir. PL was collected at
normal incidence and imaged on a CCD spectrometer. To fully characterize the behavior of the devices in a wide
range of power regimes, three pump sources were utilized: a CW laser at 532 nm, a 10 ns pulsed laser at 535 nm,
and a 150 fs pulsed laser at 535 nm. With CW excitation, all of the devices showed linear PL intensity as a function
of input power. With 10 ns excitation, the PL began to show a sublinear power law dependence (p = 0.535) as a
function of the pump intensity, with the effect becoming more pronounced with 150 fs excitation (p = 0.348) (Fig.
2a and b). Devices with a range of tunings as well as cavities with higher Q (~115) were tested and all showed the
same qualitative sublinear behavior.
To ellucidate the role of the microcavity versus the excitonic layer in the sublinear PL dependence, a Jaggregate film was grown on a SiO2 substrate (i.e. the active layer without the cavity). A similar sublinear
dependence was observed for this thin film, indicating that the excitonic component of the device is responsible for
this behavior (Fig. 2c and d). The absorption saturation and photodamage of the sample were investigated, but the
effect of both is not enough to account for the sublinearity.
Figure 2. PL intensity dependence for: microcavity pumped with 535 nm (a) 10 ns laser and a (b) 150 fs laser ; and a J-aggregate thin film
pumped with (c) 10 ns laser and a (d) 150 fs laser. The data is fitted to a power law, where p is the power.
We discuss the process of exciton-exciton annhilation [4] as a possible mechanism to explain the reduction of
quantum yield with increasing intensity. Previous studies have shown the existence of exciton-exciton annihilation
in cyanine dye J-aggregates in both solution [5] and solid state [6], and it is a phenomenon observed in other
excitonic materials which are candidates for organic polariton lasing. Annihilation would be a process directly in
competition with polariton-polariton scattering—inherently an exciton-exciton interaction—which is a possible
mechanism for populating the k = 0 state of the polariton dispersion and achieving room temperature organic
polariton lasing.
References
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[2] J. R. Tischler, M. S. Bradley, Q. Zhang, T. Atay, A. Nurmikko, V. Bulovic, Org. Electron. 8, 94 (2007).
[3] M. S. Bradley, J. R. Tischler, V. Bulovic, Adv. Mater. 17, 1881 (2005).
[4] M. A. Baldo, C. Adachi, and S. R. Forrest, Phys. Rev. B 62, 10967 - 10977 (2000)
[5] L. Kelbauskas*, S. Bagdonas, W. Dietel, R. Rotomskis, J. Lumin. 101, 253-262 (2003).
[6] S. Ozcelik and D. L. Akins, J Phys. Chem. B 101, 3021-3024 (1997).