  Femtosecond micromachining offers a contact-free and mask-less technique for
material patterning. With ultrafast laser irradiation, permanent modifications
to the properties of single layer graphene through material ablation or defect
introduction can be induced. Multiple femtosecond pulse interactions with a
single layer graphene are studied and a low laser ablation threshold ~9.2
mJ/cm2 is reported for a 15 second illumination time. Clean ablated structures
are generated in such a multi pulse irradiation configuration at low pulse
energies as an attractive alternative to ablation with single femtosecond, high
energy pulses. For a fully ablated graphene hole, a radially symmetric region
extending around 2 um from the ablated edge is characterized by strong defect
generation. Average distances between point-defects down to ~58 nm are derived
and Raman spectroscopy implies that overall there is a strong resemblance to
amorphous structures. For fluence values around 75% of the ablation threshold,
modification with defect generation down to ~48 nm average defects lengths is
reported, while the underlying graphene structure is maintained. Thus,
depending on the laser parameter choice, the same laser configuration can be
used to ablate graphene or to primarily introduce defect states. The presented
findings offer interesting insights into femtosecond induced structural
modifications of graphene that can lead to improved precision ablation and
patterning of single-layer materials at the micro- and nano-scale. Further,
this can be attractive for graphene or carbon-based device fabrication as well
as sensor and transistor applications, where regions of varying carrier
concentrations and different electrical, optical or physical properties are
desired.

Materials Science;Optics