Electrical control of the light absorption in quantum-well functionalized junctions between thin metallic films

We use a time-dependent density functional theory approach to study the optical response of a hybrid nanostructure where the junction between thin metallic films is functionalized with a quantum well (QW) structure. We show that an unoccupied QW-localized electronic state opens the possibility of the active electrical control of the photoassisted electron transport through the junction and of the absorption at optical frequencies. Control strategies based on an applied bias or an external THz field are demonstrated.

Figure 1

(a) Geometry of the studied system. (b)–(e) Schematic electronic structure of the tunneling junction and the main optical absorption processes. The electronic levels (blue lines) of each slab are occupied up to the Fermi level $\left(E_F\right)$. The unoccupied electronic state of the QW $\left(E_{\mathrm{QW}}\right)$ is shown in red. When the bias $V_{\mathrm{bias}}$ is applied, the $E_F$ levels of each slab experience a corresponding energy shift. Excited states can be accessed by photon absorption. Blue arrows indicate the intraslab photon absorption processes. Red arrows indicate the transitions involving QW and leading to PAT.

Figure 2

(a) Ground-state potential of the metal-QW-metal nanostructure. Horizontal lines show the energies of the Fermi level of the metal slabs $E_F$ (blue) and QW-localized state $E_{\mathrm{QW}}$ (red). (b) Projected density of one-electron states at the $\overline{\mathrm{\Gamma }}$ point $\left(k_{\parallel }=0\right)$ calculated with the wave packet propagation technique for different model systems: Al slabs of thickness $L=5.5$ nm separated by a vacuum junction (blue line); Al slabs of thickness $L=5.5$ nm separated by a junction functionalized with a QW structure (red line); QW structure between semi-infinite slabs (black line). Artificial broadening of 0.025 eV has been introduced for the discrete states.

Figure 3

(a) Optical absorption cross section per unit area calculated with TDDFT for bare (no QW) and QW functionalized (QW) junctions without $\left(V_{\mathrm{bias}}=0\right)$ and with $(V_{\mathrm{bias}}=0.25$ V) an applied bias. The thickness of Al slabs is $L=5.5$ nm. (Inset) Optical absorption for a broader energy range, comprising a plasmon mode excitation. (b) Spectrum of the photon assisted current across the junction between the $L=5.5$ nm Al slabs. The color code is the same as in (a). (c) The same as (a) but for the $L=11$ nm thick Al slabs.

Figure 4

Electric field of the THz and fs laser pulses as a function of time. The time delay between the pulses is explicitly shown.

Figure 5

Energy transfer from the fs laser pulse to electron excitations in the $L=5.5\mathrm{nm}$ Al slab case. The change of the electron energy $\mathrm{\Delta }E$ as a result of the interaction with the fs laser pulse, per unit surface area, is calculated with TDDFT. Results are shown for the fs laser pulse durations ${\tau }_{\mathrm{fs}}=24.2$ fs [upper row, (a)–(c)] and ${\tau }_{\mathrm{fs}}=7.3$ fs [lower row, (d)–(f)]. [(a), (b), (d), and (e)] 2D maps of $\mathrm{\Delta }E$ as a function of the laser carrier frequency ${\omega }_{\mathrm{fs}}$ and of the time delay between the pump and probe pulses $T_D$. The results for metal-vacuum-metal [(a) and (d)] and metal-QW-metal [(b) and (e)] junctions are shown. [(c) and (f)] Cuts of the 2D maps showing $\mathrm{\Delta }E$ dependence on the time delay $T_D$ for several ${\omega }_{\mathrm{fs}}$ fixed close to the main absorption features.