Near-field investigation of THz surface-wave emission from optically excited graphite flakes

THz emission from an unbiased graphite flake after optical sub-ps pulse inter-band excitation is measured using a novel micro-machined photoconductive probe-tip. Oscillatory behavior of the measured THz near-field emission is shown to originate from electromagnetic eigenmode resonances of the laterally limited graphite flake. The excitation efficiency of the lowest order eigenmode resonances strongly dependents on optical pump location. From this data a radial symmetric charge carrier translocation at the pump location is revealed as the dominating THz surface-wave emission effect. Measurements show good agreement with numerical field simulations of the eigenmode center frequencies and the spectral response expected for THz emission through basal plane oriented radial current surge excitation.