  Although diamond photonics has driven considerable interest and useful
applications, as shown in frequency generation devices and single photon
emitters, fundamental studies on the third-order optical nonlinearities of
diamond are still scarce, stalling the development of an integrated platform
for nonlinear and quantum optics. The purpose of this paper is to contribute to
those studies by measuring the spectra of two-photon absorption coefficient
($\beta$) and the nonlinear index of refraction (n$_2$) of diamond using
femtosecond laser pulses, in a wide spectral range. These measurements show the
magnitude of $\beta$ increasing from 0.07 to 0.23 cm/GW, as it approaches the
bandgap energy, in the region from 3.18 to 4.77 eV (390 - 260 nm), whereas the
n$_2$ varies from zero to 1.7E-19 m$^2$/W in the full measured range, from 0.83
- 4.77 eV (1500 - 260 nm). The experimental results are compared with
theoretical models for nonlinear absorption and refraction in indirect gap
semiconductors, indicating the two-photon absorption as the dominant effect in
the dispersion of the third-order nonlinear susceptibility. These data,
together with optical Kerr gate measurements, also provided here, are of
foremost relevance to the understanding of ultrafast optical processes in
diamond and its nonlinear properties.
