THz emission from coherent plasmons in InAs nanowires D. V. Seletskiy a , M.P. Hasselbeck a , M. Sheik-Bahae a , J.G. Cederberg b , and A.A. Talin c a Department of Physics and Astronomy, University of New Mexico,Albuquerque NM 87131,USA; b Sandia National Laboratories, Albuquerque NM 87185, USA; c Sandia National Laboratories, Livermore, CA 94551, USA; ABSTRACT We report the first observation of coherent plasmon emission of THz radiation from arrays of semiconductor nanowires. The THz signal strength from InAs nanowires is comparable to a planar substrate, indicating the nanowires are highly efficient emitters. This is explained by the preferential orientation of plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. Using a bulk Drude model, we identify the average donor density and mobility in the nanowires in a non-contact manner. Contact I- V transconductance measurements provide order of magnitude agreement with values obtained from the THz spectra. Keywords: coherent plasmons, nanowires, THz radiation, ultrafast spectroscopy, InAs 1. INTRODUCTION The development of electromagnetic radiation sources on the nano-scale is an area of current active research. The THz spectral region is particularly interesting because structures can be fabricated with sub-wavelength dimensions. Nanowires are receiving increasing attention for use as lasers, light-emitting diodes, waveguides, and inter-connects. The study of nanowire optics at far-infrared frequencies, however, has only just commenced. 1 We report what we believe is the first observation of THz emission from freestanding semiconductor nanowires. When excited with ultrashort laser pulses, high mobility InAs nanowires act as sub-wavelength antennas that can efficiently couple far-infrared radiation from coherent plasmons into free-space. Excitation of semiconductor surfaces by ultrashort laser pulses with photon energy above the band gap energy can produce THz emission through a variety of mechanisms. Radiation from optical rectification, 2 current surge, 3 coherent plasmons, 4 and coherent phonons 5 have all been observed. Ultrafast charge transport perpendicular to the surface, however, leads to macroscopic dipoles that are poorly oriented for efficient coupling of radiation to free space. The vast majority of this dipole radiation is trapped because of total internal reflection; the problem is illustrated schematically Fig. 1 (top). It has been shown that dc magnetic fields (∼ 1 T) can rotate charge motion via the Lorentz force and dramatically increase the emitted radiation. 6, 7 Another approach is to make the air-semiconductor interface perpendicular to the primary radiation lobe; this has been accomplished with a high-index prism. 8 A free-standing semiconductor nanowire, as we show here, addresses the trapping problem by having its primary surface parallel to the direction of charge transport with a radiating volume much smaller than the free-space wavelength, i.e. ≪ λ 3 . This results in an effective material refractive index of ∼ 1, which essentially eliminates total internal reflection. The effect is sketched in Fig. 1 (bottom). Figure 2 presents the calculated enhancement of radiation outcoupling of an InAs nanowire compared to a bulk substrate as a function of observation angle θ 0 . When illuminated with ultrashort near-infrared laser pulses, InAs will re-radiate a broadband THz pulse due to non-phasematched optical rectification of the excitation pulse spectrum. 9 At lower irradiance, nonlinear (χ 2 ) rectification is dramatically reduced and the radiation is primarily driven by ultrafast charge transport. This charge may be optically injected by the pump pulse (i.e. photocarriers that generate a current surge) or it may be the motion of a cold, quiescent charge distribution in the material. The former effect generally leads to a Corresponding author: Denis Seletskiy e-mail: denisel@unm.edu Invited Paper Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIII, edited by Kong-Thon Tsen, Jin-Joo Song, Markus Betz, Abdulhakem Y. Elezzabi, Proc. of SPIE Vol. 7214, 72140Y · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.810875 Proc. of SPIE Vol. 7214 72140Y-1