Scalable Microstructured Photoconductive Terahertz Emitters

Abstract

The development of scalable emitters for pulsed broadband terahertz (THz) radiation is reviewed. Their large active area in the 1 -100 mm 2 range allows for using the full power of state-of-the-art femtosecond lasers for excitation of charge carriers. Large fields for acceleration of the photogenerated carriers are achieved at moderate voltages by interdigitated electrodes. This results in efficient emission of single-cycle THz waves. THz field amplitudes in the range of 300 V/cm and 17 kV/cm are reached for excitation with 10 nJ pulses from Ti:sapphire oscillators and for excitation with 5 μJ pulses from amplified lasers, respectively. The corresponding efficiencies for conversion of near-infrared to THz radiation are 2.5×10 -4 (oscillator excitation) and 2×10 -3 (amplifier excitation). In this article the principle of operation of scalable emitters is explained and different technical realizations are described. We demonstrate that the scalable concept provides freedom for designing optimized antenna patterns for different polarization modes. In particular emitters for linearly, radially and azimuthally polarized radiation are discussed. The success story of photoconductive THz emitters is closely linked to the development of mode-locked Ti: sapphire lasers. GaAs is an ideal photoconductive material for THz emitters excited with Ti: sapphire lasers, which are widely used in research laboratories. For many applications, especially in industrial environments, however, fiber-based lasers are strongly preferred due to their lower cost, compactness and extremely stable operation. Designing photoconductive emitters on InGaAs materials, which have a low enough energy gap for excitation with fiber lasers, is challenging due to the electrical properties of the materials. We discuss why the challenges are even larger for microstructured THz emitters as compared to conventional photoconductive antennas and present first results of emitters suitable for excitation with ytterbium-based fiber lasers. Furthermore an alternative concept, namely the lateral photo-Dember emitter, is presented. Due to the strong THz output scalable emitters are well suited for THz systems with fast data acquisition. Here the application of scalable emitters in THz spectrometers without mechanical delay stages, providing THz spectra with 1 GHz spectral resolution and a signal-to-noise ratio of 37 dB within 1 s, is

Efficient metallic spintronic emitters of ultrabroadband terahertz radiation

“…4a) are operated under identical pump-beam and detection conditions. The THz amplitude obtained from the commercially available photoconductive switch (TeraSED3, based on interdigitated electrodes on a semi-insulating GaAs substrate 22 ) is maximized by setting the DC bias voltage to 12 V (20% above the maximum value recommended by the technical specifications). All measurements are performed at room temperature in a N 2 atmosphere.…”

“…For above-band-gap excitation, the response is dominated by a photocurrent 18,19,20,21,22,23,24 with a temporally step-like onset and, thus, generally smaller bandwidth than optical rectification 9 . Apart from rare exceptions 14 , however, most semiconductors used are polar 1,2,12,13,15,16,17,21,22 and strongly attenuate THz radiation around optical phonon resonances, thereby preventing emission in the so-called Reststrahlen band located between ~1 and 15 THz.The so far most promising sources covering the full THz window are photocurrents in transient gas plasmas 9,10,25,26,27,28,29 . The downside of this appealing approach is that the underlying ionization process usually requires amplified laser pulses with high threshold energies on the order of 0.1 mJ.…”

1550 nm ErAs:In(Al)GaAs large area photoconductive emitters

“…[10][11][12][13][14] In contrast to continuous-wave operated antenna-coupled emitters, pulsed LAEs do not require a short lifetime. 12,14,15 The ErAs recombination centers, however, assist to drastically increase the resistance of the sample by a factor of at least 45 compared to unintentionally doped intrinsic InGaAs. This allows for much higher biasing levels, resulting in a drastic increase of THz power.…”

“…Instead of using an antenna, 16 the carriers generated within the large area emit the THz radiation directly. 14,15 Photo-generated electrons and holes are accelerated by the external bias applied between the electrodes. The accelerated motion of the carriers generates THz radiation until they start scattering or are captured by trap states.…”

Carbon irradiated semi insulating GaAs for photoconductive terahertz pulse detection

“…There are only a few techniques for generation and detection of THz radiation. Among those, the photoconductive technique is the most efficient and popular one [5,6]. But the lack of suitable semiconductor materials represents a major restriction.…”