Paper 1.16: "Successive Oscillatory Fields at Radio to Optical Frequencies," N. F. Ramsey, Appl. Phys. B60, 85–88 (1995)

After the invention of the methods of separated and successive oscillatory fields, their uses were initially limited to radio and microwave frequencies, because electromagnetic radiations at higher frequencies were too incoherent. This barrier was eliminated with the development of lasers. But, even with suitable lasers, it was not obvious that sharp separated oscillatory fields resonances would be obtainable, since atoms move several optical wavelengths between successive pulses, thus destroying the coherency. In Paper 1.16, I review different ways in which this problem can be overcome.

One way is to eliminate the effect of velocity by using coherent two-photon Doppler-free excitations in each of the two pulses. Another is to use three or four successive coherent pulses with the phase difference between the first two pulses being of opposite sign to that between the last two, in which case the motion-induced phase shift of the first pulse is opposite that of the second. Yet another is to use extremely short pulses and such short intervals of time between the first and the final pulse that the atoms move transversely less than a wavelength. As short femtosecond laser pulses have become available, this method has become increasingly popular. Examples of these different applications are given in Paper 1.16.