This thesis presents a study focused on interactions of femtosecond laser pulses
with graphene, a one atom thick carbon membrane.
Graphene, which exhibits exceptional electronic and optoelectronic properties,
could provide considerable advantage over current silicon-based electronics.
Graphene alone, being semi-metal, is not sufficient for electronic applications,
but requires modification. For this, a set of methods for modifying and
measuring the properties of graphene was developed.
With the perspective of making graphene a suitable component for electronics,
optoelectronics or photonics, ultrashort laser pulses were used for drawing
patterns on graphene. The procedure modifies graphene chemically by photo-
oxidation, and physically, by opening a gap to its electronic band structure,
changing graphene into a semiconductor. During the process, the band gap can
be increased to the extent, where the material becomes an insulator.
It was observed in topographic
studies that photo-oxidation begins at point-like
sources and expands into islands of oxidized graphene, which eventually merge
together. This is because the probability that new oxidation occurs in close
proximity to an already oxidized area is five orders of magnitude greater than
the probability of oxidation elsewhere on graphene. Also, accompanying the
oxidation process, a third-order nonlinear signal arising from the graphene,
diminishes, providing a contrast mechanism for optical imaging. Additionally,
the Raman spectrum shows notable changes in the position of the G-band and
increase in intensity of the D-band.
Further insight into the patterned structures was obtained with micro--X-ray
photoelectron spectroscopy. The initial steps of patterning only change the ratio
of sp2/sp3 carbons in the material but the degree of oxidation increases after the
islands coalesce. With higher irradiation doses the proportion of hydroxyl and
epoxide groups increases, finally reaching the level of ~65 %.
The Four-wave mixing (FWM) signal of graphene was monitored during the
oxidation process. By utilizing the extraordinarily strong non-linear optical
response of graphene FWM spectroscopy was combined with wide-field
microscopy, allowing the patterning process to be followed in real-time.
Femtosecond wide-field FWM microscopy was proven as fast large area
imaging technique for characterization of graphene and observing changes in
graphene in real-time. Time-resolved coherent anti-Stokes Raman scattering measurement (CARS)
was applied to graphene and a G-mode dephasing time was recorded.
Additionally, it was shown that by utilizing BOXCARS excitation geometry
various nonlinear optical processes could be unambiguously separated and
measured simultaneously. The short dephasing time (T2/2) of the G-mode (325
fs) was explained with dynamically changing G-mode frequency and width
accompanied with relaxation of excited charge carrier population, due to non-
adiabatic coupling between phonons and electrons in graphene.
...
