THz Waveforms and Polarization from Laser Induced Plasmas by Few-Cycle Pulses

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Terahertz (\(\mathop{\mathrm{THz}}\nolimits\)) emission from air plasmas induced by intense few-cycle laser fields is investigated to achieve controllable waveforms and polarizations. Linearly and circularly carrier envelope phase (CEP) stabilized few-cycle laser pulses are produced using a home-built three-stage optical parametric amplifier (OPA) system and a hollow fiber compressor. Focusing the linearly polarized few-cycle pulses into air, \(\mathop{\mathrm{THz}}\nolimits\) waveform varies by changing the detected filament length and the CEP of driving pulses. Simulation using the photocurrent model including the propagation effects reveals the phase evolution inside the filament. The ellipticity and conversion efficiency of \(\mathop{\mathrm{THz}}\nolimits\) radiation are found dependent on the pulse duration of circularly polarized few-cycle pulses. Both the asymmetry and ellipticity of the driving pulses significantly affect the polarization of the generated \(\mathop{\mathrm{THz}}\nolimits\) waves. The direction of the elliptically polarized \(\mathop{\mathrm{THz}}\nolimits\) radiation rotates by varying the CEP of few-cycle pulses. Such waveform and polarization controllable \(\mathop{\mathrm{THz}}\nolimits\) emission is of great importance due to its potential application in \(\mathop{\mathrm{THz}}\nolimits\) sensing and coherent control of quantum systems. The evolution of \(\mathop{\mathrm{THz}}\nolimits\) waveform provides a sensitive probe to the variation of the CEP of propagating intense few-cycle pulses. The number and positions of the inversions of \(\mathop{\mathrm{THz}}\nolimits\) polarity are dependent on the initial CEP, which is constantly near \(0.5\uppi\) under varied driving pulse energies as two inversions become one. This provides a method of measuring the initial CEP at an accuracy that is only limited by the stability of the driving few-cycle pulses.