Amplified spontaneous emission ASE

The chapter is organized as follows in Section 8.2 a brief overview of ultrafast optical dynamics in polymers is given in Section 8.3 we present m-LPPP and give a summary of optical properties in Section 8.4 the laser source and the measuring techniques are described in Section 8.5 we discuss the fundamental photoexcitations of m-LPPP Section 8.6 is dedicated to radiative recombination under several excitation conditions and describes in some detail amplified spontaneous emission (ASE) Section 8.7 discusses the charge generation process and the photoexcitation dynamics in the presence of an external electric field conclusions are reported in the last section. 

An alternative way to preform a channeled plasma consists in exploiting the nanosecond precursor that usually precedes a short femtosecond pulse in the output of a multi-terawatt laser system. In fact, the amplified spontaneous emission (ASE) pedestal has typically an intensity 106-1010 times lower than the main pulse, which, however, can be sufficient to ionize a gas-jet or a solid target. This drawback can be turned into a benefit assuming that this long precursor can prepare the plasma channel for the short pulse propagation. 

Figure 3.21. Effective index of TE and TM optical modes in a thin film of spiro-sexiphenyl on a glass substrate at the amplified spontaneous emission (ASE) wavelength. Note the different cutoff thicknesses that determine the number of possible modes for a certain film thickness. 

Fig. 15 Tunable amplified spontaneous emission (ASE) spectra of a 47-nm-thick terfluorene (compound 50) film measured as deposited and after annealing at 200 °C for 1, 2, and 4 min. Used with permission [128]...

In the literature there are extensive reports on the observation of amplified spontaneous emission (ASE) of polyfluorene [73-76]. ASE occurs at high excitation intensities when it is possible to create a transient excited state population which is greater than the population of the lower lying state to which it radiatively decays, i.e. forming a population inversion. If there is some feedback mechanism of emitted photons, stimulated emission can build... 

The last example of this section deals with the optical function of hole-transport layers. In OLEDs some light is waveguided in the combined anode/organic layers due to total internal reflection [44], By increasing the thickness of the hole conductor the emitter can be optically decoupled from the contact. In a series of devices with increasing thickness of the hole conductor, deposited subsequently from solution, it was shown that a HTL thickness larger than 300 nm was necessary to accomplish this. As a very sensitive measure the threshold of amplified spontaneous emission (ASE) was used (Fig. 9.18, [45]). This is an important result with respect to a potential electrically driven organic laser diode. 

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