Hybrid photoexcitation in undoped diamond by mid-infrared femtosecond laser pulses

Direct interband and intragap photoexcitation by intense mid-infrared ($4.0$ and $4.7$ $\mu$m) femtosecond (fs) laser pulses was explored in ultrapure chemical-vapor deposited (CVD) diamond via acquisition of characteristic ultraviolet photoluminescence of free excitons and A-band photoluminescence of electrons anchored at deep donor–acceptor or dislocation-related traps, respectively. At lower laser intensities ($<10$ TW/cm$^2$) the excitonic photoluminescence yields exhibit highly nonlinear dependences with $I\lambda^2$-scaling and power slopes $N \approx 17$ ($4.0$  $\mu$m) and $14$ ($4.7$  $\mu$m), still insufficient to cross over the direct bandgap ($\geqslant6.5$ eV) by $\approx 1.2$ and $2.8$ eV, respectively. Similarly high slope of $\approx 9$ ($4.7$  $\mu$m) for intragap ($\geqslant 3.5$ eV) photo-population of donor–acceptor traps is still insufficient for their direct excitation by $\approx1$ eV. At the intermediate $I\lambda^2$-dependent values of the Keldysh parameter $\gamma\sim1$ such incomplete multiphoton excitation anticipates the hybrid total “multiphoton + tunneling” photoexcitation generally predicted by the Keldysh theory, but never unambiguously experimentally demonstrated. At higher laser intensities ($>10$ TW/cm$^2$) both the excitonic and A-band photoluminescence yields exhibit (sub)linear slopes, apparently, indicating formation of more strongly absorbing electron–hole plasma. These findings shed light on the hybrid multiphoton + tunneling character of Keldysh photoexcitation at intermediate values $\gamma$ and pave the way to defect/impurity band engineering of intragap nonlinear optical properties in bulk dielectrics for their precise fs-laser nanomodification.