Fig. 1. The schematic diagram of a heterostructure δ doped AlGaAs in a single cycle

Fig. 2. （a）The evolution of the carrier density produced by laser pulses with different pulse durations and（b）the partial enlarged profiles，the dashed lines are the laser envelopes

Fig. 3. Photo currents produced by laser pulses with different pulse durations

Fig. 4. （a）Time-domain terahertz pulses produced by different laser pulses，and（b）their frequency spectra

Fig. 5. The full-width at half-maximum（FWHM）of terahertz pulses produced by different laser pulses

Fig. 6. The energy of the terahertz pulses produced by the laser pulses with different pulse durations