TERAHERTZ SOURCES: Large-area photoconductive emitters improve terahertz source efficiency

Biased large-area photoconductive emitters with high conversion efficiency and the generation of intense terahertz radiation without biasing based on the photo-Dember effect overcome the limits of currently available photoconductive-antenna-based terahertz sources.



Commercial terahertz spectrometers operating at frequencies from 0.1 to 3.0 THz are widely used in academic and industrial research labs. Most time-domain terahertz systems are based on terahertz generation via photoexcitation of photoconductive switches with femtosecond lasers and require highly efficient terahertz emitters in order to achieve good system performance. However, for many terahertz sources, long-term stability and high conversion efficiency is still lacking.

Two new concepts for terahertz generation overcome these problems. The first is a biased large-area photoconductive emitter with high conversion efficiency and the second concept is a new approach to the generation of intense terahertz radiation based on the photo-Dember effect—a technique that does not require a bias voltage. The large-area design of both concepts enables scalability toward high terahertz electric fields, enabling both high signal-to-noise ratios and short acquisition times.

Radiation from accelerated carriers
The common feature of both concepts is the emission of electromagnetic radiation from accelerated carriers in semiconductors like gallium arsenide (GaAs) or indium GaAs (InGaAs). Free carriers—electrons and holes—are generated at the surface of the semiconductor by femtosecond pulses with photon energy above the gap energy of the semiconductor (in our case we use 50 fs pulses at about 800 nm).

For conventional photoconductive switches, electrodes deposited on the substrate material are externally biased and hence an electric field occurs in between the electrodes. After photoexcitation, an electric dipole builds up when the photogenerated electrons and holes are separated in the electric field (see Fig. 1). Typically the electrons have a much higher mobility than the holes, so mainly the electrons are accelerated and contribute to the terahertz emission.

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FIGURE 1. In the case of a photoconductive switch, terahertz emission begins when an external applied bias generates an electric field Ebias between the two electrodes (a). A femtosecond laser pulse (red cone) creates photoexcited carriers in the semiconductor (grey). The electrons (blue spheres) are accelerated toward the positive electrode and the holes (red spheres) toward the negative electrode. In contrast, photo-Dember emitters do not require external bias (b). Here, photoexcitation of a partially metallized semiconductor surface with a femtosecond laser pulse results in a strong gradient in the carrier density at the edge of a metallized stripe. Since the photoinduced electrons diffuse much faster than the holes, this gives rise to a photo-Dember polarization PDember perpendicular to the edge of the metallized stripe. In both pictures, the green lobes indicate the dipole radiation patterns of the resultant terahertz radiation. (Courtesy of Universität Konstanz)

For photo-Dember emitters, no external bias is required. Instead, the driving force is a symmetry brake in the free-carrier densities achieved by depositing a metallic stripe on the semiconductor. Following photoexcitation near the edge of the metallic stripe, electron-hole pairs are generated with a strong spatial gradient. Here the gradient is parallel to the surface and perpendicular to the stripe edge. Due to their much higher mobility, electrons quickly diffuse underneath the metallic stripe, leading to a build-up of a dipole and the resulting emission of terahertz radiation.1 The strength of the terahertz emission in the photo-Dember emitters is determined solely by the achievable gradient of the carrier distribution and the intrinsic difference in the electron and hole mobilities.

A key feature of both terahertz-emitter concepts is the scalability of the active area. This is achieved by creating a metal-semiconductor-metal (MSM) structure processed by optical or electron beam lithography. For the photoconductive emitters, the electrode widths and spacings are chosen to be 5 µm; it is important that every second gap between two electrodes is covered with an additional metallization isolated from the electrodes.2 By doing so, a coherent superposition of the terahertz radiation in the far field is achieved due to unidirectional acceleration of the carriers over the entire active area. The size of the active area varies from several hundred microns up to 10 mm.

In conventional single photoconductive switches, high-voltage power supplies are needed to increase the terahertz emission. This problem is solved here by keeping the gap dimensions small such that typical values for the acceleration field on the order of several tens of kilovolts per centimeter are easily obtained with simple power supplies, while keeping the active area large.

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FIGURE 2. Multiplexed terahertz emitters can be based on the principle of a photoconductive switch (a) or lateral photo-Dember currents (b). For the photoconductive emitter, every second gap between the electrodes is masked to avoid destructive interference of the generated terahertz radiation (indicated by the green areas) in the far field. In an analogous manner for the photo-Dember emitters, every second carrier gradient is suppressed by wedged metal stripes to achieve unidirectional carrier gradients. (Courtesy of Universität Konstanz)

An analogous concept to the multiplexing of photoconductive switches is used also for the photo-Dember emitters. The deposition of simple metal stripes would lead to destructive interference of the terahertz radiation in the far field because the two carrier gradients of every stripe point in opposite directions. To achieve constructive interference in the far field, every second carrier gradient must be suppressed. There are several possibilities to realize unidirectional carrier gradients experimentally. We used wedged metal stripes to generate strong carrier gradients at their thick edge, while weak carrier gradients are obtained at their thin edges (see Fig. 2). For the emitters discussed here, the width of the stripes and their spacings was 4 µm and 2 µm, respectively.

Performance of high-efficiency terahertz sources
In combination with femtosecond laser sources, these two types of photoconductive antennas represent the central components of a powerful and easy-to-handle terahertz spectrometer. We compared the newly developed multiplexed photo-Dember emitters with a state-of-the-art, high-efficiency, large-area photoconductive emitter (see Fig. 3). Based on the amplitude of the Fourier spectra of both emitters, the multiplexed photo-Dember emitters are superior to the large-area photoconductive emitters biased with an accelerated electric field of 10 kV/cm. Surprisingly, the multiplexed photo-Dember emitter shows a broader spectrum and higher peak frequency than the large-area photoconductive emitter. The comparison of the two emitters was performed in our rapid-scanning terahertz precision spectrometer based on asynchronous optical sampling operating at 1 GHz repetition rate with an optical pulse energy limited to 1 nJ.3

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FIGURE 3. The spectra of a large-area photoconductive emitter operating at 10 kV/cm bias electric field are compared to a multiplexed photo-Dember emitter operating without bias voltage. Both spectra are normalized to the maximum spectral amplitude of the photo-Dember emitter.(Courtesy of Universität Konstanz)

The typical conversion efficiency of conventional photoconductive switches at high excitation fluence is rather low (10-5) due to various saturation effects occurring at high excitation densities. The large-area photoconductive emitter presented here is available with much larger active areas up to 1 cm2 (currently the size of the multiplexed photo-Dember emitters is limited by the fabrication process to 1 mm2). High pulse energy distributed over the large active area ensures high conversion efficiency even with amplifier-based systems. We have demonstrated that the emitted terahertz electric field can be increased by more than one order of magnitude by increasing the excitation spot size (see Fig. 4).4 Using microjoule laser pulses from an amplified laser system (repetition rate 250 kHz) at 800 nm, we achieved a record-high near-infrared-to-terahertz conversion efficiency for the terahertz emitter based on photoconductive switches on the order of 10-3. The maximum terahertz electric field of 36 kV/cm and a mean average power up to 1.5 mW were achieved. Interestingly, the phase front information of the excitation laser beam is conserved and therefore a small terahertz focus with 1 mm full width at half maximum could be generated after the emitter without the use of any additional focusing mirrors or lenses (see Fig. 5).

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FIGURE 4. The maximum emitted terahertz electric field is shown as a function of excitation fluence and excitation spot size radius on the photoconductive switch at an acceleration field of 20 kV/cm. The dashed lines (inset) correspond to the expected linear dependence without any saturation effects (that is, for low excitation densities). (Courtesy of Universität Konstanz)

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FIGURE 5. A schematic view of the excitation geometry of a large-area photoconductive emitter shows focusing of the output terahertz beam without any additional optics or lenses. Detection is achieved via electro-optic sampling using an electro-optic crystal, quarter-wave plate, and photodiodes D1 and D2. (Courtesy of Universität Konstanz)

Large-area terahertz emitters overcome the limitations of commercial sources such as low signal-to-noise ratios and slow acquisition times. And multiplexed photo-Dember emitters are promising candidates for passive high-efficiency terahertz emitters that will improve the capabilities of terahertz spectroscopy.

1. G. Klatt et al., Opt. Exp. 18, 4939 (2010).
2. A. Dreyhaupt et al., Appl. Phys. Lett. 86, 121114 (2005).
3. G. Klatt et al., Opt. Exp. 17, 22847 (2009).
4. M. Beck et al., Opt. Exp. 18, 9251 (2010).

Thomas Dekorsy is a professor, Jure Demsar is an assistant professor, Stephan Winnerl is a researcher, and Matthias Beck and Gregor Klatt are Dipl. Phys. at Universität Konstanz, Postbox M 700, Universitätstraße 10, 78457 Konstanz, Germany; e-mail: matthias.beck@uni-konstanz.de; www.uni-konstanz.de.

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