cw Measurement of the Optical Nonlinearity of Ammonium Dihydrogen Phosphate

We have measured the value of the element $d_{36}$ of the nonlinear dielectric tensor of ammonium dihydrogen phosphate for the doubling of the 6328-Å line of the He-Ne laser. Two measurements were made. The first used a collimated single-transverse-mode laser beam ($\mathrm{focal}\mathrm{spot}\mathrm{radius}\approx 0.2$ cm) containing several longitudinal modes. The second used a weakly focused single-mode and single-frequency laser ($\mathrm{focal}\mathrm{spot}\mathrm{radius}\approx 0.03$ cm). In both experiments the crystal was slowly rotated through the index-matching angle. The value of $d_{36}$ obtained by either method is 1.36×${10}^{-9\mathrm{}}$(±12%) in cgs esu. The experiments also verify the factor $\frac{(2n-1)}{n}$ for the enhancement of second-harmonic generation when $n$ independent longitudinal modes are present.