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The excitation of nonlinear materials by femtosecond lasers is one of the key technologies for terahertz generation at present. Owing to its advantages such as ultrashort time resolution and ultrabroad frequency spectrum, it has been widely applied in the characterization and measurement, sensing and imaging of terahertz waves. However, the methods of controlling terahertz waves through microstructures can only regulate the transmission process of terahertz waves and face obstacles such as diffcult design and complex process, making it hard to be widely used in industry. In this paper, by introducing a pulse shaping system to change the time dispersion of femtosecond laser pulses, the interaction process between femtosecond laser and lithium niobate crystals can be directly regulated, thereby directly controlling the terahertz generation process. Taking the second-order time dispersion as an example, using the pump-probe phasecontrast imaging system, we detected the terahertz signals generated by pump light with different second-order time dispersions in lithium niobate. Meanwhile, by using the impact stimulated Raman scattering model and Huang-Kun equation, we simulated the generation process of terahertz waves, proving the feasibility of regulating terahertz waves by using the time dispersion of femtosecond laser pulses. The experimental and simulation results show that when the time dispersion of femtosecond laser causes the pulse width to increase, the time that the lithium niobate lattice is subjected to the impact stimulated Raman scattering force is prolonged, and the macroscopic polarization of the lithium niobate lattice is correspondingly extended. On the one hand, the longer duration of polarization leads to a wider terahertz signal in the time domain and a narrower one in the frequency domain. On the other hand, since the impact stimulated Raman scattering force is proportional to the intensity of the pump light and is in the same direction throughout the interaction time, when the Raman scattering force ends, the lattice reaches the maximum displacement. The longer Raman scattering force causes the lattice to move to one side for a longer time, and correspondingly, the time required for the subsequent vibration of one period is longer, eventually resulting in a lower center frequency. In addition, this paper also points out that the modulation of terahertz signals by pump light pulse width may be affected by the thickness of the wafer, and the modulation effect may be more obvious on thinner media. This result is of great reference significance for the active regulation of on-chip terahertz sources based on lithium niobate crystals in the future.
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Keywords:
- Temporal Dispersion /
- Terahertz Generation /
- On-Chip Integration
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