Brewer, Mooradian, and Stoicheff reply: We attributed the first demonstration of laser emission in organic dyes to the two independent work groups led by Peter Sorokin and Fritz Schäfer (reference 7 in our article).
We also erroneously attributed tunability to them. Although Schäfer and coauthors noted that the laser wavelength of each dye was tunable over a great part of its fluorescence band with changing dye concentration, that was not the narrowband tunability suitable for sweeping over an atomic or molecular spectrum.
Indeed, Bernard Soffer and B. B. McFarland first showed that pulsed dye laser radiation could be tuned with a reflective diffraction grating at one end of the laser cavity; that generally more useful method provided tuning over 400 Å in rhodamine 6G with the requisite narrow output of 0.6 Å (50 GHz).
Later, others replaced the grating with an echelon or Lyot filter to generate much narrower output. However, the technique used most frequently for precision spectroscopy was the extremely monochromatic (MHz) and tunable dye laser emission produced by using the grating at grazing incidence, a method still in use. Also, continuous-wave and broadly tunable dye lasers with linewidths as low as 50 kHz have been observed by frequency-offset locking the dye frequency to a stable reference.
Currently, broad tunability in the near-IR and visible regions with narrow linewidths between 50 and 300 kHz is obtained with a continuous-wave diode laser—a semiconductor chip—using an external cavity that consists of a diffraction grating and a retroreflector. Resolution of this magnitude has permitted precision measurements in fundamental investigations.
