Excitation fluences

At high excitation fluence (2.4 mJ/cm2) the emission spectrum collapses into a narrow line located at 2.53 eV (see Fig. 8-2 a, dashed line), slightly blue-shifted with respect to the low intensity emission. The process strongly depends on the shape of the excitation area. For a rectangular spot, line narrowing occurs at 55 pj/cm2, i.e. 45 times a lower fluence than for a small circular spot, while the... 

Figure 8-7. SE decay al 2.53 cV in m-LPPP films il T=77 K versus pump-probe delay at lhrce different excitation fluences (from Ref. 125] with permission).

Fig. 3.9. Left photoelectron intensity from TbTe3 surface as a function of energy and momentum for different time delays, showing the ultrafast closing of the CFW gap marked with a dot. Right Time-dependent binding energy of the Te band (lower trace) and the CB (upper trace), exhibiting a periodic modulation at 2.3 and 3.6 THz, respectively, under strong excitation fluence (2mJ/ cm2). From [22]...

Dephasing of the Vibrational Coherence Excitation Fluence Dependence [33, 35]... 

Different types of set-up have been reported in the literature. A typical transient absorption set-up with a continuum probe and subpicosecond time resolution, which can be used with laser pulses of a few hundred femtoseconds of duration, is shown in Fig. 7.15. The sample is excited by the pump pulses P. The pump flu-ence (number of photons per cm ) is set to be large enough to obtain an appreciable population of the excited state, which however can be small when an excitation source of high repetition rate is used (Ti-sapphire femtosecond lasers run at 1 kHz) because it allows fast accumulation of weak amplitude signals. With low repetition lasers (10 Hz) the excitation fluence should be dose to the saturation fluence ( l/ca, where (7 is the absorption cross section of the solute at the pump wave-... 

Figure 8-6. SE spectra of m-LPPP films at T=ll K (a) at 0.6 ps pump-probe delay for increasing pump fluences (b) for 7.2 mJ/cm excitation fluence at two pump-probe delays (from Ref. [25] with permission).

Figure 8-11. Solid line AF at 2.53 eV for different excitation fluences. Dashed lines fitting curves obtained from the theoretical model described in the text (from Ref. [33] with permission).

Figure 11.17 Spectral linewidths (FWHM) as a function of the excitation fluence for various widths of the excitation beam. Reprinted with permission from K. Shimizu, Y. Mori and S. Hotta, Laser oscillation from hexagonal crystals of a thiophene/phenylene co-oligomer, J. Appl. Phys., 99(6), 063505 (2006). Copyright 2006, American Institute of Physics...

To understand the dynamics of radiative recombination of the M-plane MQWs, time-resolved PL measurements at 7 K and at various excitation fluences were performed. We used a frequency-doubled Ti sapphire laser with a photon energy of 3.4 eV, a pulse width of about 200 fs, and a repetition rate of 4.75 MHz. For detection, a Hamamatsu C5680 streak camera was used. Figure 6.20a shows TRPL transients for a 20-period M-plane (In,Ga)N/GaN MQW after direct excitation of the wells for different excitation fluences. The initial decay is rapid, faster than that observed for C-plane MQWs with similar... 

Figure 6.20b shows the transient obtained with the highest excitation fluence plotted as ln(ln[Io/I(t)j) versus ln(t), in which a stretched exponential would be a straight line. Obviously, the data cannot be represented by a stretched exponential. The key for understanding the recombination dynamics for this sample is displayed in Figure 6.21. The PL band exhibits rapid spectral diffusion during the first 500 ps after excitation. For longer times, the PL band continues to redshift, but is significantly slower.

Figure 11.10 Complex, frequency dependent conductivities for PbS measured 10 ps after photoexcitation for two different 266 nm excitation fluences (black and grey dots). The data is described well by the Drude expression (solid lines) yielding the plasma frequency (directly related to density) and the carrier scattering time.

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