Self-induced transparency

These experiments give information on the polarizability of excited molecules (from the amplitude of the oscillation), on the transverse phase relaxation times T2 (which depend on the cross sections of phase-perturbing collisions, Vol.l, Sect. 3.3), and on the population decay time T. For more details see [911, 913]. 

One characteristic feature of optical pulses is the pulse area 

Here Dik is the matrix transition moment, E is the electric field vector, Qq the Rabi-frequency in case of resonance o = coq and r is the pulse length. For A = tt the pulse is called a r-pulse. When optical r-pulses with a wavelength A, matching the energy difference AEik between two atomic levels i) and ) pass through a system of two-level atoms with the absorption coefficient a, they produce a population inversion. This causes for a following weak optical pulse amplification instead of absorption without the r-pulse. 

An incident pulse with the pulse area A = 2n propagates over considerable distances without noticeable attenuation. The time profile of a pulse with length x approaches after a pathlength Az = m/a (m = 1 2 3 ...) a hyperbolic secant profile with the electric field amplitude 

Here Vg is the group velocity, zo = rp/Dy and Dy is the group-velocity dispersion. For zo 00 i.e. Dy 0 the pulse form remains unchanged. Such pulses are called solitons (Fig. 7.25). 

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The discovery of self-focussing of intense laser beams 318 ) and the phenomena of self-induced transparency 318b) have stimulated a whole new class of experimental investigations which give information about the transient interaction between light and matter 318c)... 

It is expected within the framework of the theory of self-induced transparency that the pulse duration is either small or of the order of the relaxation time T2 in... 

Summary Self-induced transparency has 3 main features ... 

In Fig. 7.26 a comparison between absorption, stimulated emission and self-induced transparency is illustrated. More information about self-induced transparency can be found in [914-916]. 

Fig. 7.26 Comparison of absorption, stimulated emission and self-induced transparency...

Self-induced transparency is different from ordinary saturated absorption. In saturated absorption, the energy that is taken from the pulse to maintain the medium in a partial state of excitation is permanently lost to the radiation field. In self-induced transparency, the energy given to the medium is lost from the radiation field only temporarily and is eventually returned to it. 

The formation of self-guided filaments in the air at the laboratory scale has been known for almost ten years [12, 15-17]. Filamentation occurs when high-power laser pulses propagate in a highly non-linear way in transparent media (Fig. 14.2). This high non-linearity induces a self-action, which... 

Photo-oxidative self-cleaning and antifogging effects of transparent titanium dioxide films has attracted considerable attention for the past decade [334, 335]. In order to understand the photo-induced hydrophilic conversion on titanium dioxide coatings in details, it is inevitably necessary to understand the relationship between the photo reaction and the surface crystal structure this can be done, for example, by an evaluation of the photo-induced hydrophilic conversion on the different crystal faces of rutile single crystals and also... 

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