Twin pulses of THz generation from a nonlinear crystalline quartz by femtosecond laser pulse

We observed sub-picosecond terahertz (THz) pulses generated from an electro-optic quartz plate using femtosecond optical pulses. The time-resolved THz radiation signal clearly indicates two separated THz pulses with opposite polarity. Based on our results, a model based on optical-to-THz conversion via optical rectification is proposed to describe the twin pulsed THz emission mechanism of the quartz plate. Firstly, the two separated THz pulses are assigned to the forced and free THz pulses, respectively. With an optical pulse serving as an external source in the crystalline quartz, the forced THz pulse is a solution to linear Maxwell’s equations and propagates at the velocity of the pump laser pulse, while the free THz pulse propagates with the group velocity in the THz frequency range. Finally, as a non-centrosymmetry material, the THz amplitude exhibits perfect threefold symmetry with respect to the azimuthal angle and twofold symmetry with respect to the pump polarization angle.