  The degree of laser-induced graphitisation from a sp3-bonded to a sp2-bonded
carbon fraction in a single crystal chemical vapour deposited (CVD) diamond
under a varying fluence of an ultrashort pulsed laser (30 fs, 800 nm, 1 kHz)
irradiation has been studied. The tetrahedral CVD sp3-phase was found to
transition to primarily an sp2-aromatic crystalline graphitic fraction below
the critical fluence of 3.9 J/cm2, above which predominantly an amorphous
carbon was formed. A fractional increase of fluence from 3.3 J/cm2 to 3.9 J/cm2
(~ 20 %) resulted in a substantial (~ three-fold) increased depth of the
sp2-graphitised areas owing to the non-linear interactions associated with an
fs-laser irradiation. Additionally, formation of C=O carbonyl group was
observed below the critical threshold fluence; the C=O cleavage occurred
gradually with the increase of irradiation fluence of 30 fs laser light. The
implications for these findings on enhancement of fs-driven processing of
diamond are discussed.
