Efficient terahertz emission from InAs nanowires

We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The terahertz radiation power efficiency of these structures is ∼15 times higher than a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast terahertz spectroscopy, and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.

Figure 1

(a) Low-magnification cross-sectional SEM image of the InAs nanowire ensemble, highlighting the nanowires’ high aspect ratio and their taper. The scale bar is 3 μm across. (b) Higher-magnification SEM image showing the base dimension and existence of a planar wetting layer formed by vapor–solid epitaxy. The scale bar is 1 μm across. (c) Sketch (not to scale) of the three regions of the sample that are potential sources of THz radiation transients. Our experiments show that the signal from the InAs nanowires dominates the signal from the InAs wetting layer or the Si-doped GaAs substrate.

Figure 2

Autocorrelation signals (a) and corresponding Fourier spectra (b) for an InAs nanowire sample (top), an InAs substrate (middle), and control sample (bottom), as described in the text. The top two signals are plotted on the same relative scale; the bottom signals are magnified 50 times. Curves are offset vertically for clarity.

Figure 3

THz power changes linearly as excitation intensity is varied (via a variable neutral density filter) over a range of 150. The inset shows the normalized spectra of THz transients from InAs nanowires as the intensity varies by a factor of 25. Absence of a spectral shift is consistent with radiation emanating from coherent plasma.

Figure 4

(a) SEM image depicting the probing of an individual nanowire on the growth substrate. The scale bar corresponds to 1.5 μm. (b) I-V curve for the InAs nanowire measured in (a). In the inset, the same data plotted in the formalism of the SCLC enable us to extract the ohmic (intercept) and SCLC (slope) contributions to the total current and hence the crossover voltage.

Figure 5

(a) Source-drain current vs source-drain voltage sweeps of a representative FET at different fixed gate biases showing the n-type depletion mode behavior. (b) Transfer characteristic of the FET shown in (a), which enables determination of the mobility and carrier concentration by way of the transconductance and threshold voltage, respectively. The extracted device parameters are $n_e$ = 4.8 × 10${}^{17}$ cm${}^{-3}$ and μ = 101 cm${}^2$/V·s. Nanowire dimensions are R = 34 nm and L = 3 μm.

Figure 6

(a) Dispersion of the longitudinal bulk and surface plasmon modes. (b) Normalized measured emission spectrum of the InAs nanowire ensemble. (c) Theoretical spectra of the lowest-order axial surface acoustic plasmon modes of the nanowire, with $n_e$ = 4.8 × 10${}^{17}$ cm${}^{-3}$, R = 34 nm, L = 3 μm, and plasmon broadening of $0.05{\omega }_p$.

Figure 7

(a) Radiation pattern from a plasma dipole (oscillation direction indicated by bold double arrow) beneath the surface of a planar semiconductor. The shaded region within the lobes depicts the radiation escape cone; only light within this cone (ray 1) can escape into free space. The remaining radiation (ray 2) is trapped due to total internal reflection. (b) Schematic of coherent plasmon motion and light propagation with a freestanding semiconductor nanowire.