Fig. 4: FTIR interferograms and spectra.

a, b FTIR interferograms obtained with the ppKTP and apKTP crystals, respectively. The inset shows the zoomed view of the interferogram shown in (a). OPD optical path-length differences, ppKTP and apKTP periodically and aperiodically poled potassium titanyl phosphate, respectively. c, d FTIR spectra obtained by Fourier-transforming the interferograms shown in (a) and (b), respectively.