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(a) Schematic diagram of the experimental setup. L1: optical lens; B: BBO; S: slit; L2: plastic lens; P: pellicle; D: balanced detector. (b) CCD images of plasmas taken at different focusing conditions. (c) Illustration of the simulated terahertz radiation generation concept. The terahertz field measured at point on the observing plane is contributed from each emission source along the plasma: , where is the length of the array and is the distance between the end of the array and the plane.
Profiles of the terahertz radiation (energy) (a) measured with full pulse energy and (b) simulated at the central frequency of , under different focal conditions. The focal lengths of the optical lenses used are 500, 300, 200, and , respectively. The dimensions of the emission sources in the simulation are estimated from a separate measurement in a similar manner as described in Ref. 13.
Terahertz pulse energy measured when moving a pinhole along the plasma created using a lens of focal length vs pinhole position. The input pump pulse energies are 200 and , respectively. The intensity distribution of plasma white light generation, fitted from the CCD images, is plotted under the normalized terahertz pulse energy curve, to give the corresponding location on the plasma. The dashed lines mark the beginning and end of the energy drop.
Profile of (a) measured and (b) simulated frequency components when using a lens of focal length with consideration of the self-phase modulation of the optical pulse.
Plasma size used in simulation ( is the length and is the radius), energy divergence full angle (FWHM) of terahertz radiation , and convergence angle of the fundamental beam at different focusing conditions. The input optical pulse energy is .
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