Spectral effects of gain saturation in the β-phase of poly(9,9-dioctylfluorene)

Abstract

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Interplay between Amplified Spontaneous Emission, Förster Resonant Energy Transfer, and Self-Absorption in Hybrid Poly(9,9-dioctylfluorene)-CdSe/ZnS Nanocrystal Thin Films

“…Good fitting was obtained for α = 0.40 ± 0.01 (see inset of Figure ). The ASE intensity ( I ase PF8 ) shows instead an almost exponential increase with the excitation density up to about 500 μJ cm −2 , followed by a subexponential increase and a saturation at about 4.0 mJ cm −2 that, together with the ASE band rebroadening, suggests the presence of gain saturation . Good fitting of I ase PF8 (see inset of Figure ) was obtained by using the Linford formula, including gain saturation (see ref 24).…”

Dynamics of photoexcitation and stimulated optical emission in conjugated polymers: A multiscale quantum-chemistry and Maxwell-Bloch-equations approach

“…Light amplification at these 0-1 vibronic transitions has been observed in isolated polyfluorene chains using a pump-probe experiment 4,66 and in polyfluorene films with a small fraction of beta phase chromophores by detecting amplified spontaneous emission. 37,38,67,68 Figure 5 shows the temporal dynamics of the average total ͑averaged over the molecular layers and summed over all vibrational states͒ occupations of the S 0 and S 1 states together with incoming and transmitted pulses for two different relaxation times of vibrational quanta, ͑a͒ t rel = ប / ␥ = 1 ps and ͑b͒ t rel = 20 ps. The figure shows excitation ͑pumping͒ by the first pulse and amplification of the subsequent ͑probe͒ pulses as these propagate through the system ͑the occupation inversion is reduced accordingly͒.…”

“…Similarly, it has been discussed that gain saturation at high pump intensities and the wavelength shift of the amplified spontaneous emission can only be explained by including vibrational relaxation dynamics in the ground electronic state. 68 Next, we illustrate the role that light propagation plays in the pulse amplification shown in Fig. 5.…”

“…In this section, we focus on the role light propagation can play in the excitation dynamics in that it gives rise to ASE. 68 ASE being primarily a stimulated emission effect ͑only initially spontaneously triggered͒, we model it by introducing fluctuations in the classical optical field. In the numerical evaluation, enabled by the self-consistent solution of Maxwell and material ͑density-matrix͒ equations, the spontaneous radiative decay can be simulated by introducing random fluctuations in the classical light field at each chromophore site.…”

“…In an experiment, the pumping of the ␤-phase chromophores typically occurs due to efficient and fast ͑picosecond time scale͒ excitation transfer from the surrounding glassy phase molecules with larger excitation energies. 68 If the pumping is on a much longer time scale than is needed for the migration to occur, the details of the migration processes are rendered unimportant. Being mostly interested in the dynamics of the stimulated optical emission from the radiatively coupled ␤-phase chromophores, we imitate the real pumping situation by tuning the center frequency of the pump pulse to 3.4316 meV such that it excites the molecules from the vibronic ground state into a higher excited vibrational state of S 1 .…”