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Waveform, spectrum, and energy of backward terahertz emission from two-color femtosecond laser induced microplasma

We measured the key characteristics of the backward terahertz (THz) emission from a two-color femtosecond laser spark in air: waveform, spectrum, and energy. Through the controlled reflection of the forward THz pulse, we observed the backward and forward ones in the same waveform and hence directly evidenced the origin of the backward THz radiation from the plasma spark. The spectrum of the backward THz radiation is shifted to the low-frequency range as compared with the forward one. Measurements of the energy of the backward and forward THz pulses for different pump pulse energies provide their ratio to be ∼5.5%, which quantitatively agrees with the results of our numerical simulations. These results can be useful for the noninvasive monitoring of forward THz emission parameters by the backward one.We measured the key characteristics of the backward terahertz (THz) emission from a two-color femtosecond laser spark in air: waveform, spectrum, and energy. Through the controlled reflection of the forward THz pulse, we observed the backward and forward ones in the same waveform and hence directly evidenced the origin of the backward THz radiation from the plasma spark. The spectrum of the backward THz radiation is shifted to the low-frequency range as compared with the forward one. Measurements of the energy of the backward and forward THz pulses for different pump pulse energies provide their ratio to be ∼5.5%, which quantitatively agrees with the results of our numerical simulations. These results can be useful for the noninvasive monitoring of forward THz emission parameters by the backward one.