Pump absorption coefficient

This derivation highlights several key features of solid-state laser refrigeration. First, Eq. (12) shows that the pump absorption coefficient diminishes exponentially as the temperature is lowered, which is a result of the thermal depletion of the initial state II). This reduces the net power density that can be extracted from the solid (Eq. 11) and ultimately sets a lower limit for the temperature that can be reached by laser cooling. Finding materials with a small ground-state crystal-field sphtting 5Eg) is advantageous in this respect. 

Applications Involving Nonlinear Absorption Phenomena. Saturable absorption (hole-burning) is a change (typically a decrease) in absorption coefficient which is proportional to pump intensity. For a simple two level system, this can be expressed as... 

Reverse saturable absorption is an increase in the absorption coefficient of a material that is proportional to pump intensity. This phenomenon typically involves the population of a strongly absorbing excited state and is the basis of optical limiters or sensor protection elements. A variety of electronic and molecular reorientation processes can give rise to reverse saturable absorption many materials exhibit this phenomenon, including fuUerenes, phthalocyanine compounds (qv), and organometaUic complexes. 

To check whether the parameters of our system satisfy coherent three-photon purity , we carried out two calculations with pump pulse duration FWHM = 2ps. The first calculation based on input data from Table la corresponds to the suppression factors FjjT 10 — 10 (depending on F ,). In the second calculation the suppression factors were decreased by three orders of magnitude by decreasing all Fjj in 10 times. Both calculations gave almost the same three-photon absorption coefficient, y (46) (the relative difference Ay/y 7%). This is a direct evidence of negligibly small step-wise absorption processes compared to the coherent three-photon absorption. 

Fig. 12.4 Photoluminescence of host materials and absorption spectrum of DCM2 (a), and absorption coefficients of the host materials (b). The vertical line indicates the pump wavelength.

Alternatively one may modulate the sample concentration by periodically removing it from and replacing it within the absorption cell as done, albeit rather slowly, in pulsed supersonic jet absorption spectroscopy. The sample absorption coefficient can be modulated by double resonance using a second microwave source to pump a transition that has one energy level in common with the... 

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