Source: http://aoot.osa.org/josab/abstract.cfm?uri=josab-36-4-1101
Timestamp: 2019-04-21 16:22:49+00:00

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We extend the theory of Cherenkov radiation from an ultrashort laser pulse in an electro-optic medium to the realistic case when the laser pulse propagates as a tightly focused Gaussian beam. By applying the developed theory to the situation of practical interest—a LiNbO3 crystal pumped by a near-infrared (Ti:sapphire or Yb) laser, we explore how the emitted terahertz energy and the terahertz spectrum depend on the focusing conditions for different laser pulse durations. The cases of focusing to a line and a spot are considered. It is shown that there is an optimal size of the beam waist, which maximizes the terahertz yield. The theoretical predictions are verified experimentally.
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Fig. 1. Ultrashort laser pulse propagating as a beam focused to (a) a line and (b) a spot.
Fig. 2. 2D case of focusing to a line. (a) Snapshot of the electric field Ey(t,x,z) for a0FWHM=10 μm, τFWHM=150 fs, and I0=100 GW/cm2. The orange region shows the laser beam. The dashed lines show the terahertz wavepaths from the laser beam to Points 1–4 at x=1 mm. (b) Terahertz waveforms at Points 1–4 depicted in (a). (c) Corresponding amplitude spectra.
Fig. 3. Terahertz energy W2D (normalized to the optical pulse energy Wopt=3.4 μJ/cm) as a function of a0FWHM for different τFWHM. The curves are plotted by using accurate approach with dispersion included (solid) and neglected (dashed) and in the adiabatic approximation with (stars) and without (dots) dispersion.
Fig. 4. 3D case of focusing to a spot. (a) Snapshots of the electric field Ey(t,x,z) in the planes z=0,1, and 2 mm (the laser pulse is at z=2.5 mm) for a0FWHM=10 μm, τFWHM=150 fs, and I0=100 GW/cm2. (b) Terahertz waveforms at Points 1–4 depicted in Fig. 2(a). (c) Corresponding amplitude spectra.
Fig. 5. Terahertz energy W3D (normalized to the optical pulse energy Wopt=1.1 nJ) as a function of the beam width a0FWHM for different τFWHM.
Fig. 6. Schematic of the experimental setup.
Fig. 7. Experimental dependence of the terahertz energy on the focal spot size (large dots), theoretical curve for τFWHM=100 fs reproduced from Fig. 5 (dashed), and modified theoretical curve (solid). Two other curves show the contributions from the terahertz absorption (dash-dotted) and terahertz beam divergence (dotted).

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