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We report on the coherent control of boosted ultrashort terahertz pulse emitted from air plasma pumped by a femtosecond sawtooth pulse composed of three optical harmonic fields. With an in-line optical setup, the relative phases between the three optical components of the pump pulse can be varied independently with attosecond precision without the complexity of external phase stabilization. We observe that the amplitude of the terahertz emission pumped by the phase-optimized sawtooth wave is enhanced by 1.8 times compared to the widely used two-color scheme. Moreover, by manipulating the relative phases between the three colors at attosecond precision, coherent control of the azimuthal angle, ellipticity, and polarity of THz radiation is achieved. A local current model reproduces well the coherent control observations.

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The processes leading to the N2+ lasing are rather complex and even the population distribution after the pump laser excitation is unknown. In this paper, we study the population distribution at electronic and vibrational levels in N2+ driven by ultra-short laser pulse at the wavelengths of 800 nm and 400 nm by using the quantum-mechanical time-domain incoherent superposition model based on the time-dependent Schrödinger equation and the quasi-classical model assuming instantaneous ionization injection described by density matrix. It is shown that while both models provide qualitatively similar results, the quasi-classical instantaneous ionization injection model underestimates the population inversions corresponding to the optical transitions at 391 nm, 423 nm and 428 nm due to the assumption of quantum mixed states at the ionization time. A fast and accurate correction to this error is proposed. This work solidifies the theoretical models for population at vibrational states in N2+ and paves the way to uncover the mechanism of the N2+ lasing.

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In this paper, the electrical and ultraviolet optoelectronic properties of the interdigitated finger geometry β-Ga2O3 photodetector were investigated before and after 1 MeV electron irradiation. Under the dark condition, the voltage at which the minimum current was located shifted from 0 V to −9.5 V after the electron irradiation. As the fluence increased from 5.0 × 1013 cm−2 to 1.0 × 1015 cm−2, the current at the voltage of 3 V of the β-Ga2O3 photodetector increased from 0.047 nA to 0.121 nA. The negative deviation of the minimum current was related to the positive charge trap caused by electron irradiation, while the improvement of the current was related to the fact that the electron irradiation produced a large number of electron -hole pairs. Under 365-nm illumination, the current at the voltage of 3 V of the β-Ga2O3 photodetector increased from 0.199 nA to 0.898 nA, as the fluence increased from 5.0 × 1013 cm−2 to 5.0 × 1014 cm−2; then, it dropped to 0.779 nA when the fluence reached 1.0 × 1015 cm−2. The increase of the current was due to the increase of defects generated by the electron irradiation under 365-nm illumination. Also, the decline of the current at the fluence of 1.0 × 1015 cm−2 may be caused by the quenching effect. Under 254-nm illumination, the current at the voltage of 3 V of the β-Ga2O3 photodetector dropped from 78.566 nA to 19.362 nA after the electron irradiation. This change was due to the lattice distortion and the reduction of the defect energy. However, as the fluence of the irradiation increased, the current increased gradually. This may be related to the increase of defects excited by electrons.