Patent ID: 7202751

Claim:
An apparatus comprising: an absorber comprising a material having first, second, and third energy states; a first radiation source that generates a first electromagnetic pump radiation having a frequency, ν L , that induces transitions between said first and third energy states, said first radiation source irradiating said absorber at a pump radiation intensity, said first and second energy states differing in energy by an amount that depends on said pump radiation intensity; a second radiation source that generates a second electromagnetic radiation having a frequency, ν M , that induces transitions between said first and second energy states, said second electromagnetic radiation source also irradiating said absorber; a first controller that sets ν M so as to maintain the absorption of said first electromagnetic radiation by said absorber at a target value; and a second controller that sets ν L to a value that is offset from ν MAX , where ν MAX is the value of ν L at which the absorption of said first electromagnetic radiation by said absorber is maximized, said offset being chosen such that the dependence of said difference in energy between said first and second energy states on said pump radiation intensity at ν L is less than the dependence of said difference in energy between said first and second energy states on said pump radiation intensity at ν MAX .