Direct multiphoton femtosecond infrared laser excitation of a diamond lattice in the two-phonon region and modification of color centers

The nonlinear absorption of ultrashort laser pulses with intensities of $0.17$–$1.7$ TW/cm$^2$ at an intrinsic two-photon absorption wavelength of $4673$ nm in type IIb diamond has been studied experimentally. It has been shown that the main absorption mechanism in the studied sample is two-photon absorption with a coefficient of $\beta_2 = (72 \pm 7)$ cm/TW. Transmission microspectroscopy, visible photoluminescence, and infrared Fourier-transform microspectroscopy have demonstrated the possibility of laser-induced transformation of nitrogen impurity centers in synthetic type Ib diamond at higher radiation intensities.