Optical gain

Fig. 1. The energy levels in a semiconductor. Shown are the valence and conduction bands and the forbidden gap in between where represents an occupied level, ie, electrons are present O, an unoccupied level and -3- an energy level arising from a chemical defect D and occurring within the forbidden gap. The electrons in each band are somewhat independent, (a) A cold semiconductor in pitch darkness where the valence band levels are filled and conduction band levels are empty, (b) The same semiconductor exposed to intense light or some other form of excitation showing the quasi-Fermi level for each band. The energy levels are occupied up to the available voltage for that band. There is a population inversion between conduction and valence bands which can lead to optical gain and possible lasing. Conversely, the chemical potential difference between the quasi-Fermi levels can be connected as the output voltage of a solar cell. Fquilihrium is reestabUshed by stepwise recombination at the defect levels D within the forbidden gap.

Figure 7-22. Optical gain in P3BT films normalized phoioresponse ul 1.8 eV in a Ihin film (solid line) and a llitck lilm (broken line).

Vlasov YuA, Luterova K, Pelant I, Honerlage B, Astratov VN (1997) Enhancement of optical gain of semiconductors embedded in three-dimensional photonic crystals. Appl Phys Lett 71 1616-1618... 

Due to the small emission and absorption cross sections of Er +, a high Er density is needed to reach reasonable values of optical gain. Typically Er densities are between 0.1% and 1.0% (10 -10- Er/cm- ). These values are far beyond the equilibrium solubility limits of Er in silicon. Therefore, nonequilibrium methods have to be used, such as ion implantation. Er implantation in crystalline silicon leads to amorphization, and additional annealing (600°C) is required to... 

U.S. 5,307,363 Organic optical gain device and method of exciting the same... 

Some low-molecular dyes also have been investigated in inactive polymer matrices. For stilbene compounds in polystyrene, peak wavelengths in the green spectral region are obtained, with net optical gain coefficients of g = 15-20 cm-1 [187],... 

An important difference between luminescent conjugated polymers and laser dyes is that the former still exhibit strong optical gain in the solid state. This property means that luminescent conjugated polymers can be used to produce stimulated emission in thin films of these materials. 

The absence of optical gain for 3 and 11 in ACN can be explained by the relatively large excited state absorption of the emitted fluorescence photons from the singlet state Si >ctio(A,f), where cti (A,f) and aiolA-p) are... 

Hie dye-doped ORMOSIL materials exhibit much higher photostability than the reported polymer-doped materials. For example, optical gain was reported for several coumarin dyes in PMMA (13), It is significant to note that the best dyes reported could only be used to several hundred pulses and that most of the coumarin dyes could not even oscillate for 100 pulses. The ORMOSIL gels thus represent an improvement of at least 1-2 orders of magnitude in photostability. Optical gain was also reported for R6G doped into modified poly(methyl methacrylate) (14L These materials exhibited loss of 20% output after 180 pump pulses (1 J/cm ) at 532 nm. The reported plots indicate that these materials would have dropped by 90% after 275-300 pulses due to rapid steady state photodegradation once the pulse count exceeded the critical pulse number. In this case the R6G doped ORMOSIL gel laser offers a useful lifetime improvement by a factor of more than 15 over the reported polymer material. 

In Sect. II, a brief review of the fundamentals of the PR effect is provided. The energy transfer and light diffraction of the wave mixings in a PR medium is introduced, and the optical gain coefficient and diffraction efficiency are defined. The process of light-induced refractive index modulation is considered, and the main results of Kukhtarev s PR model (commonly used in inorganic and organic materials) are presented. 

Based on the PVK FDEANST system, the PR performance as a function of the photosensitizer has been studied [54], It is apparent that varying the sensitizer changes the photogeneration rate and greatly influences both optical gain and diffraction efficiency. By replacing TNF with TNFDM, the spectral sensitivity of composites can be extended to the near infrared (IR) region. 

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