Terahertz emission from lateral photo-Dember currents

Department of Physics and Center for Applied Photonics, University of Konstanz 78457, Germany.
Optics Express (Impact Factor: 3.49). 03/2010; 18(5):4939-47. DOI: 10.1364/OE.18.004939
Source: PubMed


The photo-Dember effect is a source of impulsive THz emission following femtosecond pulsed optical excitation. This emission results from the ultrafast spatial separation of electron-hole pairs in strong carrier gradients due to their different diffusion coefficients. The associated time dependent polarization is oriented perpendicular to the excited surface which is inaptly for efficient out coupling of THz radiation. We propose a scheme for generating strong carrier gradients parallel to the excited surface. The resulting photo-Dember currents are oriented in the same direction and emit THz radiation into the favorable direction perpendicular to the surface. This effect is demonstrated for GaAs and In(0.53)Ga(0.47)As. Surprisingly the photo-Dember THz emitters provide higher bandwidth than photoconductive emitters. Multiplexing of phase coherent photo-Dember currents by periodically tailoring the photoexcited spatial carrier distribution gives rise to a strongly enhanced THz emission, which reaches electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter.

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    • "In particular, over the roles of the metallic contact and the photo-excited charge density gradient . The work in [13] and [15] suggest that the metal layer is not required if a high gradient is present, whereas [16]–[18] suggest that the metal layer is crucial for emission. This has raised questions over the distribution of photo-current responsible for the lateral photo-Dember effect. "
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    • "In practice, the chance of short-cut increases by the number of interdigitated fingers during the fabrication process, or afterwards in device handling and test. For this reason, a bias-free THz emitter concept based on lateral photo-Dember current was introduced [11] [12] and as a result, the emitter stays functional even in the case of short-cut defects. In photo-Dember emitters, the radiating dipole current forms owing to the difference of mobilities or diffusion constants for holes and electrons. "
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    ABSTRACT: A new method for generating terahertz radiation by exposure to femtosecond laser pulses on the semiconductor surface is proposed. The essence of this method is that the exciting radiation intensity is subjected to spatial modulation by using a microlens array and by shading a part of the semiconductor surface by metal stripes. This gives rise to a concentration gradient of photo carriers along the surface at the sharp boundary of the metallic coating in the semiconductor (transverse Dember photoelectric effect), and its relaxation for times of ∼1 ps results in the emission of electromagnetic pulses of the terahertz range. A terahertz emitter model based on the proposed method was developed and designed, its efficiency was demonstrated, and methods for increasing its efficiency were considered.
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