Terahertz Field Enhancement and Photon-Assisted Tunneling in Single-Molecule Transistors

We have investigated the electron transport in single-${\mathrm{C}}_{60}$-molecule transistors under the illumination of intense monochromatic terahertz (THz) radiation. By employing an antenna structure with a sub-nm-wide gap, we concentrate THz radiation beyond the diffraction limit and focus it onto a single molecule. Photon-assisted tunneling (PAT) in the single molecule transistors is observed in both the weak-coupling and Kondo regimes. The THz power dependence of the PAT conductance indicates that when the incident THz intensity is a few tens of mW, the THz field induced at the molecule exceeds $100\mathrm{kV}/\mathrm{cm}$, which is enhanced by a factor of $\sim {10}^5$ from the field in the free space.

Figure 1

(a) Schematic illustration of the sample structure. (b) Scanning electron microscopy (SEM) image of the fabricated sample with a bowtie antenna structure. The scale bar corresponds to $10\mu \mathrm{m}$. The inset shows an SEM image of a metal nanocontact. The scale bar corresponds to 200 nm. (c) Schematic illustration of the fabricated sample mounted on a hemispherical silicon lens. (d) Coulomb stability diagram of a single ${\mathrm{C}}_{60}$-molecule transistor. The yellow dashed lines indicate the excited states that correspond to the excitation of the internal vibrational mode ($H_g(1)$ mode) of the ${\mathrm{C}}_{60}$ molecule.

Figure 2

(a)–(c) Coulomb stability diagrams measured under the illumination of 2.5 THz radiation for $P_{\mathrm{THz}}=0$ (a), 16 (b), and 55 mW (c). The black dotted lines represent the boundaries between the transport and Coulomb blockaded regions. The red dotted lines show the differential conductance peaks generated by the THz radiation. (d), (e) Energy band diagrams when the photon-assisted tunneling process occurs at the bias points indicated by the black dots in (b). The dotted lines denote photon sidebands created by the THz radiation.

Figure 3

$P_{\mathrm{THz}}$ dependence of $G_1/G_0$ (black dots) and $G_2/G_0$ (red dots). The black and red curves indicate ${J_1}^2(\alpha )/{J_0}^2(\alpha )$ and ${J_2}^2(\alpha )/{J_0}^2(\alpha )$ as a function of $\alpha$, respectively.

Figure 4

(a) Coulomb stability diagram of a ${\mathrm{C}}_{60}$ SMT in the strong coupling regime. (b) Differential conductance spectra measured at 5, 18, and 29 K (top to bottom). (c) Temperature dependence of the Kondo resonant peak height. The red solid line is a fit to the experimental data using the semiempirical functional form for the spin-$1/2$ quantum dot with $T_K=65\mathrm{K}$. (d) Differential conductance spectra measured at 5 K under the illumination of 2.5 THz radiation for $P_{\mathrm{THz}}=0$, 30, and 56 mW (top to bottom). (e) Differential conductance spectrum measured at 5 K under the illumination of 2.5 THz radiation at $P_{\mathrm{THz}}=56\mathrm{mW}$ (black). The red, green, and blue curves are the fitting curves for the main Kondo peak, the satellite Kondo peaks on the negative- and positive-bias sides, respectively. The violet curve is the resulting sum of the three fitting curves. (f) $P_{\mathrm{THz}}$ dependence of $G_1/G_0$. ${J_1}^2(\alpha )/{J_0}^2(\alpha )$ is plotted as a function of $\alpha$ by a black curve.