The invention provides a method for controlling the transmission of radiant energy through a light transmissive medium and an apparatus whereby the method can be effected. The method includes the steps of introducing a first flux of photons, or the evanescent field thereof, into said medium said photons having energy E, and said medium having energy level characteristics such that the energy, E, of said first flux of photons matches an energy gap between an energy level designated as energy level 2, and another energy level designated as energy level 3 of said medium, and said first flux is substantially unattenuated as said first flux of photons, or the evanescent field thereof, passes through said medium; and selectively introducing a second flux of photons, or the evanescent field thereof, into said medium to attenuate said first flux of photons, said second flux of photons causing the population of energy level two of said medium, wherein a portion of said first flux is absorbed, inducing a population in level 3, a portion of which population in level 3 returns to level 2 and is available for further attenuation of said first flux, whereby the control of the transmission of said first flux of photons by said second flux of photons is more efficient than if the return of excited states from level 3 to level 2 did not occur.