Influence of Substrate Material on Radiation Characteristics of THz Photoconductive Emitters

Jens Klier,Nina Susan Schreiner,Emilien Peytavit,Wissem Zouaghi,Jean-François Lampin,Daniel Molter,Frank Ellrich,Garik Torosyan,René Beigang,Joachim Jonuscheit,Georg Von Freymann

doi:10.1155/2015/540175

Abstract

We present in this paper spectral and spatial characteristics of terahertz emission from standard dipole antenna structures used as emitters depending on the substrate material. All antenna structures were lithographically fabricated on low-temperature (LT) grown, few-micrometers-thick gallium arsenide (GaAs) layers. To investigate the effect of the substrate material on the radiation pattern of terahertz beams, either semi-insulating gallium arsenide or high-resistivity silicon substrate wafers have been used. As detector a standard 40 µm long dipole antenna on a semi-insulating GaAs substrate with a low-temperature grown gallium arsenide layer on it has been employed; this configuration allows for broadband detection and is still efficient enough for the characterization purpose. Strong dependence of the radiation pattern on the substrate used for the terahertz source is demonstrated. The measured patterns and differences between the two cases of substrates are well explained by means of classical diffraction.

Highlights

In the electromagnetic spectrum, the region between roughly 1011 Hz and 1013 Hz is referred to as the terahertz (THz) region: 1 THz = 1012 Hz
We have presented radiation pattern measurements of dipole antennas fabricated using two different emitter substrates
For LT-gallium arsenide (GaAs) on SI-GaAs as emitter substrate strong interference effects and higher-order diffraction effects are observed due to the refractive index step between the emitter substrate and the Si lens. These effects can be avoided by using lowtemperature grown gallium arsenide (LT-GaAs) on high-resistivity silicon (HR-Si) substrate resulting in almost interference-free beam propagation

Summary

Introduction

The operation principle of the photoconductive switch used as an antenna for THz radiation is the following: Two symmetric strip lines are arranged on a semiconducting substrate with the symmetrically positioned dipole having a 5 μm long gap in the center (see Figure 1). The shape and the duration of this transient depend on the laser pulse itself and the semiconductor’s physical properties, that is, the lifetime of the generated free charge carriers. According to Maxwell’s theory the current transient induces a pulsed electromagnetic wave of corresponding duration to be emitted into the free space. In this particular case it is the THz pulse through which all the spectroscopic measurements are performed.

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