Radiating pattern of surge-current-induced THz light in near-field and far-field zone

Area of Science:

• Terahertz (THz) science and technology
• Optics and photonics
• Semiconductor physics

Background:

• Terahertz (THz) wave generation using femtosecond laser pulses on semiconductor surfaces is a key technique.
• Understanding THz wave propagation is crucial for developing THz applications.
• The beam profile evolution from near-field to far-field is not always straightforward.

Purpose of the Study:

• To investigate the propagation characteristics of THz waves generated from an unbiased GaAs crystal.
• To analyze the THz beam profile in both near-field and far-field zones.
• To elucidate the physical mechanisms behind the observed far-field beam anisotropy.

Main Methods:

• Generation of THz waves by illuminating an unbiased GaAs crystal with femtosecond laser pulses at a 45° incidence angle.
• Comprehensive characterization of THz wave propagation using knife-edge scan measurements.
• Comparison of experimental results with simulations based on a dipole radiation model.

Main Results:

• In the near-field (540 μm), the THz beam exhibits a Gaussian shape consistent with paraxial wave equation predictions.
• In the far-field, the THz beam displays a highly anisotropic profile, maintaining Gaussian shape along the normal to the incidence plane but showing satellite peaks parallel to it.
• The anisotropic far-field pattern is attributed to position-dependent phase retardation and initial beam diffraction, leading to wave interference.

Conclusions:

• The THz beam profile transitions from Gaussian in the near-field to anisotropic and non-Gaussian in the far-field.
• The observed far-field beam characteristics are explained by the interplay of phase retardation and diffraction effects.
• This study provides a framework for understanding THz beam profile evolution in various experimental configurations.