We report on photocurrent and photoconductance processes in a freely suspended p-doped single GaAs nanowire. The nanowires are grown by molecular beam epitaxy, and they are electrically contacted by a focused ion beam deposition technique. The observed photocurrent is generated at the Schottky contacts between the nanowire and metal source-drain electrodes, while the observed photoconductance signal can be explained by a photogating effect induced by optically generated charge carriers located at the surface of the nanowire. Both optoelectronic effects are sensitive to the polarization of the exciting laser field, enabling polarization dependent photodetectors. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3193540͔Semiconductor nanowires have attracted considerable attention for the past few years because of their compelling electronic, mechanical, and optical properties. 1-10 A very suitable and versatile technique for nanowire growth is the direct synthesis on a substrate. [11][12][13][14] The fabrication of III-V semiconductor nanowire based devices by such a bottom-up approach ensures the rational use of materials, as the nanowires can be obtained in principle on any substrate. Here, we report on the optoelectronic properties of photodetectors based on single p-doped GaAs nanowires grown by molecular beam epitaxy ͑MBE͒ with the so-called vapor-liquid-solid method using Ga droplets as self-catalysts. [15][16][17] The nanowires are electrically contacted by metal electrodes using a focused ion beam ͑FIB͒ deposition technique. 18,19 We experimentally identify two dominating optoelectronic processes in the metal-GaAs nanowire-metal photodetectors. On the one hand, there is a photocurrent generated at the Schottky contacts between the GaAs nanowires and the metal sourcedrain electrodes, as recently shown for CdS nanowires. 8 On the other hand, we observe a photoconductance effect, when illuminating the GaAs nanowire far away from the contacts. We interpret the photoconductance effect to arise from band bending effects caused by surface states on the nanowire surface. In particular, optically generated excess electrons are trapped at the surface, where they act as a negative gating voltage on the p-doped nanowires ͑photogating effect͒. At the same time, the optically excited free excess holes raise the Fermi energy of the hole gas within the nanowires ͑photodoping effect͒. Both effects can raise the conductance of the semiconductor circuits. 20,21 We demonstrate that both photocurrent and photoconductance effects are sensitive to the orientation of linear polarized light. The photoconductance ͑-current͒ varies by ϳ35% ͑ϳ15%͒ for the photon polarization being parallel or perpendicular to the direction of the GaAs nanowires. Hereby, the metal-GaAs nanowiremetal circuits act as polarization-sensitive photodetectors, 2 which can be integrated into electronic circuits by the FIBdeposition technique in a very versatile way.Starting point are GaAs nanowires, which are grown by MBE on a SiO 2 covered ͑111͒-oriente...

A phase-stable superposition of femtosecond pulses from a compact erbium-doped fiber source and their second harmonic is shown to induce ultrashort approximately microA current bursts in single unbiased GaAs nanowires. Current injection relies on a quantum interference of one- and two-photon absorption pathways. The vector direction of the current is solely dictated by the polarization and relative phase of the harmonically related light components while its power dependence is consistent with a third order optical nonlinearity.

Femtosecond ω/2ω pulse pairs derived from a compact Er:fibre source induce coherently controlled currents in low temperature grown GaAs. They are characterized by analyzing charge accumulation at contacts closeby. We focus on the photoresponse of bowtie optical antennas integrated into such metal-semiconductor-metal structures. Antennas are designed to enhance the ω field and to confine it into the 50 nm antenna gap. The coherently controlled current is markedly enhanced by the plasmonic nanostructure. However, we find an only unpronounced dependence on the antenna length which is probably related to the large refractive index of GaAs and intricate resonance conditions for ultrabroadband excitation light.

We propose and demonstrate an ultrabroadband concept to characterize amplitude and phase changes of femtosecond pulses. The radiation is superimposed with the first subharmonic spectral components from the same laser source. This effective ω/2ω pulse pair induces a coherently controlled charge current in a time-integrating semiconductor detector. An interferometric variation of the time delay between the harmonically related components then reveals the electric field of the 2ω part. This method is realized with the second harmonic of a compact Er:fiber source centered at 390 THz and a GaAs-based detector. Most strikingly, it is sensitive to ∼π/20 phase changes and can be utilized to analyze femtojoule pulses.