Tunable-focus spherical lens

H. Ren, Y. H. Fan, S. Gauza, and S. T. Wu, Tunable-focus flat hquid crystal spherical lens, Appl. Phys. Lett. 84, 4789 (2004). 

Figure 2.32 shows a possible experimental arrangement for the observation of Doppler-free two-photon absorption. The two oppositely traveling waves are formed by reflection of the output beam from a single-mode tunable dye laser. The Faraday rotator prevents feedback into the laser. The two-photon absorption is monitored by the fluorescence emitted from the final state Ef into other states Em - From (2.66) it follows that the probability of two-photon absorption is proportional to the square of the laser intensity. Therefore, the two beams are focused into the sample cell by the lens L and the spherical mirror M. 

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