Abstract
In this paper we study the effect of the length of dipole antennas on the spectrum of the radiated THz signal in pulse-excited opto-electronic terahertz systems. In particular, we investigate the origin of the commonly observed sharp dips that occur in the spectra of photoconductive dipole antennas, and explain them on the basis of reflections of the excitation current pulse that take place at the ends of the antenna. We develop a hybrid time-domain model for the system and show that the predictions of our model are in good agreement with experimental results.
Highlights
In recent years terahertz time-domain spectroscopy [1, 2] has gained popularity in basic research and in applications such as quality control [3]–[8] and national security [9]–[11]
We investigate the influence of the dipole structure on the radiated THz pulse by using Smith’s model for pulse-excited antennas and explain the origin of the sharp dips that are often observed in experimental measurements, see e.g. [12]–[15]
Note that the radiated electric field is proportional to the excitation current, not its time-derivative
Summary
In recent years terahertz time-domain spectroscopy [1, 2] has gained popularity in basic research and in applications such as quality control [3]–[8] and national security [9]–[11]. Most studies of pulse-excited terahertz systems focus on the short current pulse that is produced in the substrate as a result of the ultra-short optical excitation and treat this region as a point source for which the radiated electric field is directly proportional to the time-derivative of the current This approach is correct in the first approximation but it ignores the influence of the propagation of the pulse on the antenna and the presence of the striplines and the contact pads on the spectrum of the radiated THz signal. We combine the results of this model with results previously derived by Jepsen et al for the photocurrent produced by the ultrafast laser pulse excitation [21] and calculate the electric field after it has propagated through the emitter substrate and the hyper-hemispherical lens To our knowledge, this is the first analytical approach towards calculating the electric field radiated from pulse-excited THz antennas that takes into account the length of the dipole by incorporating the effects of reflections that take place at its ends
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