Photon-Assisted Tunneling through Self-Assembled InAs Quantum Dots in the Terahertz Frequency Range

We have investigated terahertz (THz) photon-assisted tunneling in single self-assembled InAs quantum dots (QDs). Two types of photon-assisted tunneling processes have been observed in the THz range: ground state resonance and photon-induced excited state resonance, depending on the relative magnitude between the orbital quantization energy of the QDs and the THz photon energy. Furthermore, we could realize a very high coupling efficiency between THz waves and QDs and observed multiphoton absorption up to the fourth-order during the tunneling process, resulting in almost complete lifting of the Coulomb blockade.

Figure 1

(a) Schematic illustration of the device structure and the experimental setup. (b) An SEM image of a fabricated QD sample with a bow-tie antenna structure. The inset shows a SEM image of the nanogap metal electrodes bridged by a single InAs QD. (c) Coulomb stability diagram for a sample with a QD of $\sim 60\times 60\mathrm{nm}$. The number of electrons, $N$, in each Coulomb diamond is also shown.

Figure 2

(a) Coulomb stability diagram without THz irradiation. (b) Coulomb stability diagram under THz ($h{f}_{\mathrm{THz}}=2.5\mathrm{THz}$, $P_{\mathrm{THz}}=5\mathrm{mW}$) irradiation. (c) Linear conductance spectra as a function of $V_{\mathrm{G}}$ with (red line) and without (black line) THz irradiation. $E_0$ and $E_1$ denote the first and second occupied energy levels of the dot, respectively. The inset shows the dependence of the source-drain current, $I_{\mathrm{SD}}$, at $V_{\mathrm{G}}=-1\mathrm{V}$ (i.e., at $E_0\mathrm{\text{−}}h{f}_{\mathrm{THz}}$) on the THz power. Panels (d) and (e) show energy diagrams showing the sequence of tunneling events that dominantly contribute to the side peaks for different gate voltages, $E_i-h{f}_{\mathrm{THz}}$ and $E_i+h{f}_{\mathrm{THz}}$ ($i=1,2$), respectively. A small dc bias raises the right Fermi level with respect to the left Fermi level of the leads.

Figure 3

(a) Coulomb stability diagram for a sample with a QD of $\sim 70\times 70\mathrm{nm}$. (b) Source-drain current $I_{\mathrm{SD}}$ as a function of $V_G$ at $V_{\mathrm{SD}}=1\mathrm{mV}$ without (black) and with (red) THz wave irradiation ($h{f}_{\mathrm{THz}}=2.5\mathrm{THz}$, $P_{\mathrm{THz}}=1.5\mathrm{mW}$). (c) An energy diagram showing the sequence of the tunneling processes which contribute to the side peak shown by the blue arrow.

Figure 4

Coulomb stability diagrams under THz irradiation (2.5 THz) for the THz power $P_{\mathrm{THz}}=0$ (a), 1 (b), and 5 mW (c), measured for a QD sample with a QD of $\sim 40\times 40\mathrm{nm}$. The dashed lines are guides for the eye. The number of electrons in each Coulomb diamond is also shown.

Figure 5

(a) Linear conductance spectra as a function of $V_G$ under several THz powers for $f_{\mathrm{THz}}=2.5\mathrm{THz}$. The inset shows the linear conductance spectra for $P_{\mathrm{THz}}=0$ and 1 mW on a logarithmic scale. (b) Linear conductance spectra for different THz frequencies of 2.5 and 3.1 THz. (c) The separation of the side peaks from the original Coulomb peak as a function of the photon energy of the THz wave plotted up to 2-photon absorption peaks. The dashed lines indicate $\alpha \Delta V_{\mathrm{G}}=h{f}_{\mathrm{THz}}$ and $\alpha \Delta V_{\mathrm{G}}=2h{f}_{\mathrm{THz}}$.