Large-area laser-driven terahertz emitters

Abstract

Intense terahertz emitters are one of the most important components of terahertz (THz) time-domain spectroscopy systems. In this presented report, the development of THz emitters over the last two decades is reviewed, and an outlook for future THz emitters is given. The physical principle behind the THz generation process is discussed for two types of emitters: state-of-the-art large-area photoconductive emitters are compared to THz emitters based on the photo-Dember effect. The latter do not require an external bias voltage. This passive character of the photo-Dember emitters has several advantages which are outlined.The first photoconductive switch with electromagnetic waves in the picosecond regime was shown by Auston in 1984 [1]. The frequent employment of these 'Auston switches' then became established with the wide availability of femtosecond lasers such as the Kerr-lens modelocked titanium:sapphire in the 1990s. The most commonly used THz sources were subsequently based on either photoconductive switches [2] or the emission from semiconductor surfaces [3], in both cases irradiated with femtosecond laser beams. To date, these two methods introduced 20 years ago are still commonly employed for generating, by optical excitation, THz radiation in the frequency range 0.2-5 THz [4,5]. Advances have also been made in the generation of THz radiation via optical rectification or difference frequency mixing [6,7], but these will not be discussed further in this Letter.Typically, the peak frequency of photoconductive sources or bare semiconductor surfaces lies around 1 THz, and the spectral coverage is limited to 3 THz. Drawbacks of these sources are the lack of longterm stability, scalability and the low conversion efficiencies from the optical to the THz regime. Further research led to the development of large-area photoconductive emitters [8-10], which exhibit one order of magnitude larger conversion efficiency compared to the first sources. Furthermore, recently it has been shown that, with an improved excitation geometry, conversion efficiencies up to 2 × 10 23 are possible [11]. A new concept for the generation of intense THz radiation is based on the lateral photo-Dember effect [12]. Inspired by the large-area photoconductive emitters, these new emitters allow scalability, but do not require an external bias voltage.The common feature of these THz generation methods is the emission of electromagnetic radiation from accelerated carriers in semiconductors such as gallium arsenide (GaAs) or indium gallium arsenide (InGaAs) [13]. Free carriers -electrons and holes -are generated at the surface of the semiconductor by femtosecond pulses with a photon energy above the gap energy of the semiconductor. For conventional photoconductive switches, electrodes deposited on the substrate material are externally biased and hence an electric field exists between the electrodes. The accelerating electric field E ext is given by the externally applied voltage U ext divided by the electrode spacing d. After photoexcitation, a ti...

Review of terahertz photoconductive antenna technology

“…Periodic arrays of dipole electrodes have been proposed for improving various aspects of THz PCA performance. 124,[138][139][140][141] Early work by Froberg et al 124 fabricated a linear array of parallel microstrip dipole emitters on SI-GaAs. Each of the 64 electrodes was individually biased, while the entire array was illuminated with a train of 200-fs optical pulses.…”

“…It was shown that by controlling the bias of the individual electrodes, the direction and profile of the emitted THz pulses could be tuned. 124 Various works by Klatt et al 138,139 studied electrode arrays utilizing the photo-Dember effect for THz generation. The photo-Dember effect arises from the optically induced space-charge gradient in unbiased semiconductors due to the difference in electron and hole diffusion coefficients.…”

Scalable Microstructured Photoconductive Terahertz Emitters

“…Only recently THz emitters relying on lateral photo-Dember currents have been invented by the group of T. Dekorsy [115,116]. These photo-Dember currents are induced at the edge of metallization layers on the semiconductor surface.…”

“…Fig. 10) [115][116][117]. Strong THz wavelets are emitted at the sharp edge of each metallization stripe.…”

Multiple lateral photo-Dember terahertz emitters illuminated by a cylindrical micro-lens array

“…The photo-Dember (PD) effect is the emission of radiation resulting from the diffusion of carriers within a semiconductor without the need for an external bias. [1][2][3] When the semiconductor is illuminated by an ultrafast laser with an energy above that of the band gap, carriers are excited beneath the surface and then diffuse within the material. In materials such as gallium arsenide, the mobility of the electrons is typically in the range of an order of magnitude larger than that of the holes.…”