If exit is a function of of transition, how do you make a laser with tunable frequency?
The discussion today described lasers for only the simplest case of a transition between two energy levels: real life is more complicated, as always. Lasers with tunable frequencies, such as dye lasers, have molecular lasing materials (typically organic molecules). Molecules have many degrees of freedom, and many near-continuous allowed vibrational energy levels (we didn’t cover molecular energy levels, but there’s material about it your text in Chapter 11). Lasing in such a material can actually happen for a range of frequencies, corresponding to lots of closely spaced levels, rather than for a single pair of energy levels. You can pick the frequency you want with a monochromator-essentially a diffraction grating to spread out and select frequencies so you get only the one you need.
