Light can serve as a probe of material structure, sometimes as a result of nonlinear effects. Scientists at the University of Toronto (Toronto, ON, Canada) are using optical second-harmonic generation (SHG) by graphene to determine whether the graphene is single- or multilayered. The technique is useful to physicists, material scientists, and nanotechnologists, and also can potentially study other effects on graphene such as effects of external electric fields and of adsorbates.
A titanium:sapphire laser provided 1.0 nJ, 150 fs pump pulses at an 800 nm wavelength; the pulses were attenuated to 0.06 nJ so as not to damage the graphene. Layers of graphene were mounted on a 300 nm film of silicon dioxide on a silicon substrate, which was in turn mounted on rotation and translation stages. The p-polarized light was focused on the graphene at a 60° angle, producing a 7 × 10 µm spot. P-polarized 400 nm SHG light was optically filtered and detected by a photon-counting photomultiplier tube. When the signal as a function of azimuth angle showed a four-lobed pattern characteristic of the substrate, the researchers knew the graphene was single layer; if the pattern was three-lobed (characteristic of graphene), then more than one layer was present. Contact Jesse Dean at [email protected].
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