Abstract
We generate the THz wave on the surface of an unbiased GaAs crystal by illuminating femtosecond laser pulses with a 45° incidence angle, and investigate its propagation properties comprehensively both in a near-field and in a far-field zone by performing a knife-edge scan measurement. In the near-field zone, i.e. 540 μm away from the generation point, we found that the beam simply takes a Gaussian shape of which width follows well a behavior predicted by a paraxial wave equation. In the far-field zone, on the other hand, it takes a highly anisotropic shape; whereas the beam profile maintains a Gaussian shape along the normal to the plane of incidence, it takes satellite peak structures along the direction in parallel to the plane of incidence. From the comparison with simulation results obtained by using a dipole radiation model, we demonstrated that this irregular beam pattern is attributed to the combined effect of the position-dependent phase retardation of the THz waves and the diffraction-limited size of the initial beam which lead to the interference of the waves in the far-field zone. Also, we found that this consideration accounting for a crossover of THz beam profile to the anisotropic non-Gaussian beam in the far-field zone can be applied for a comprehensive understanding of several other THz beam profiles obtained previously in different configurations.
Highlights
Over the last few decades, an electromagnetic wave at the THz frequency has received a significant attention as it can provide a useful spectroscopic access to unveil the collective behaviors of electrons entangled with other degrees of freedom, i.e., orbital, spin, and lattice[1,2,3,4,5,6,7]
Before we discuss the THz beam profiles in the near- and far-field zone, let us first examine the radiation pattern of the THz wave at the generation point which will be used for further discussions about the wave propagation
As we do not have an experimental access to the beam characterization at the generation point, we provide only the computed intensity profile of the surge-current-induced THz wave upon its generation
Summary
Before we discuss the THz beam profiles in the near- and far-field zone, let us first examine the radiation pattern of the THz wave at the generation point which will be used for further discussions about the wave propagation. We discuss how the THz wave evolves further during its propagation in the far-field zone In this far-field characterization, we locate a conventional knife-edge at the distance L apart from the generation point (Fig. 2(a)), and vary its position (φi=x,y) along both x- and y-directions to obtain the two-dimensional profile of the beam intensity. It should be noted that this situation contrasts to the grazing incidence reflection process; the point dipoles formed at the medium interface oscillate following the incident wave, and the phase relationship in the wavefront is readily recovered during the wave propagation after the reflection This conclusion is supported by the observation of the Gaussian THz beam profile along the y-axis at the same value of L. With a common understanding of the interference effect for the wave, we consider that a diffraction-limited size of Φ0THz is another major factor leading to the non-Gaussian THz beam profile in the far-field regime
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