Non-linear interactions of femtosecond laser pulses with graphene : photo-oxidation, imaging and photodynamics

Abstract

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.