Graphene based materials for non-linear optical applications and ultrafast laser applications
Aidan Murray, Werner Blau
School of Physics and CRANN, Trinity College Dublin, Ireland
amurray8@tcd.ie
In very recent years, graphene has become the focus of significant research efforts.
Characteristics such as potential near-ballistic transport and high mobility make
graphene viable as a material for nanoelectronics. Not only this, but its mechanical,
electronic and thermal properties are also perfect for mirco- and nanoscale mechanical
systems, thin film transistors, and transparent and conductive composites and
electrodes. In this work, particular interest was afforded to the 2μm wavelength and to
the modelocking capabilities of graphene, exploiting its optoelectronic properties to
achieve this. Graphene is a prime candidate for several reasons, including its intrinsic
broadband operation capabilities due to the gapless linear dispersion of Dirac electrons.
Non-linear saturable absorption is required for materials used as a mode locker in lasers
to obtain light pulses of very short duration, in the order of femtoseconds.
High yields of graphene were prepared via liquid-phase
exfoliation of powdered graphite. This was achieved
through the use of methods devised by J. N. Coleman et
al.[1] and other groups[2]. These methods rely on the
exfoliation and stabilization of graphene using special
Fig.2. Graphene/THF dispersions
solvents or surfactants, combined with long sonication
times (~170 hours). Unfortunately, commonly used solvents, such as water, have strong
absorption peaks at 2μm. Therefore, initially potential solvents were tested for their
suitability, both for transparency at 2μm and for dispersions of graphene. A promising
solvent, tetrahydrofuran (THF), was the first to be tested, primarily due to its almost
complete transparency at 2μm and its ability to dissolve important polymers, such as
Poly(methyl methacrylate) (PMMA). Dispersions of graphene with N-Methyl-2pyrrolidone (NMP) as the solvent were also produced. While it does have slight
absorption at 2μm, it is proven to both provide efficient dispersions of graphene, and to
dissolve polymers such as PMMA. Afterwhich, the dispersions as well as thin films
were examined using various apparatus, including UV-Vis-IR spectrometer, raman
spectroscopy and z-scan techniques.
Nonlinear optical properties are routinely examined using the so-called z-scan method.
This set up consists of a thin sample being moved through the focus of a laser beam to
vary the light intensity on the sample. This allows for measurement of the non-linear
index 𝑛2 Kerr nonlinearity with the “closed” aperture method, and the non-linear
absorption coefficient ∆𝛼 via the “open” aperture method. Funding from the ISLA
project, which aims to develop a set of “building block” components for 2μm lasers, is
greatly acknowledged.
[1] Khan, U., O'Neill, A., Lotya, M., De, S. and Coleman, J. N., High-Concentration
Solvent Exfoliation of Graphene. Small, 6: 864–871 (2010)
[2] Bourlinos, A. B., Georgakilas, V., Zboril, R., Steriotis, T. A. and Stubos, A. K.,
Small 5, 184 (2009)