We demonstrate high-power, scalable terahertz generation by focusing two-color terawatt laser pulses in air with a cylindrical lens. This cylindrical lens focusing geometry creates a 2-D plasma sheet, which emits two diverging vertical terahertz lobes in the far field. By expanding the plasma source into 2-D plasma, cylindrical focusing can produce scalable THz radiation. This radiation is refocused to produce strong THz fields (>21 MV/cm) at the focus.
High power, broadband terahertz pulses can be produced via two-color laser mixing in air. In this scheme, an ultrashort laser pulse (ω) and its second harmonic pulse (2ω) are co-focused in air to ionize atoms and molecules at the focus. This ionization results in strong conical THz radiation in the forward direction. For strong THz generation, high-power lasers are necessary. However, the output THz does not scale with input laser energy, but rather saturates even with modest laser energy of ~10 mJ. This saturation effect is attributed to ionization-induced laser defocusing in tight focusing geometry. This results in inefficient laser-to-plasma coupling, limiting the output THz energy. This saturation can be mitigated by re-distributing the laser energy over a longer plasma length with weaker focusing. However, this scheme is limited by the group velocity walk-off between ω and 2ω pulses in air. We present an alternative method of increasing the plasma volume (cylindrical lens focusing) for scalable THz generation.
In our experiment, a terawatt (TW) laser pulse (800 nm, 50 fs, 50 mJ) is focused by a cylindrical lens of focal length f = 150 mm. A 100-μm thick BBO crystal is placed after the cylindrical lens to produce a 2ω pulse co-propagating with the ω pulse. These two pulses are focused in air and produce a 2D plasma sheet at the focus, emitting a broadband, single-cycle THz pulse in the forward direction. This focusing geometry yields two diverging vertical THz lobes. These diverging THz lobes are collimated and refocused to generate strong THz fields at the focus. Our experiment shows that the cylindrical focusing provides the maximum THz energy of ~31 µJ at EL = 45 mJ. Here, the conversion efficiency from laser to THz energy is 7 times higher than a typical value in two color laser mixing in air. The maximum THz field strength is expected to be ~21 MV/cm at the refocusing spot.
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