A Terahertz Molecular Switch

We present time-dependent results describing the current through a molecular device, modeled as a complex with two active centers connected to leads under bias. We show that, at a properly adjusted external voltage, a passing terahertz electromagnetic pulse may cause a transition between states of finite and negligible current, suggesting that the system might be useful as a nanoscopic switch in the terahertz range. A phase diagram defining the bias region in which the transition takes place within a short time is given. As described, the physical processes involved are of an entirely different nature than those in ordinary photodetectors.

Figure 1

Scheme of the device: (a) The molecule is placed between leads at chemical potentials ${\mu }_L$ and ${\mu }_R$. It is characterized by two active sites with energies ${\epsilon }_0^o$, ${\epsilon }_1^o$, which under bias (b) change their relative positions allowing for the resonance condition to be established in a certain bias range. The shaded areas show the amount of electron charge at each site.

Figure 2

Current-voltage characteristic in the absence of external radiation. Parameter values are given in the text, and were chosen to fit the experimental data of Chen et al. [5] at $T=60\mathrm{K}$ shown as circles. The inset shows the charge in each molecule site at different bias values.

Figure 3

Time evolution of (a) the overall current and (b) the charge density in sites 0 and 1 at $\nu =10\mathrm{T}\mathrm{H}\mathrm{z}$. Parameters used are as in Fig. 2.

Figure 4

Boundary separating the region at which switching takes place within 2 ps of the arrival of an external pulse, labeled “YES,” from the region in which the well remains charged and current still flows beyond that time interval, labeled “NO.” $\delta V$ measures the external bias relative to the critical edge $V_c$, and $\delta E$ is the radiation strength, both in units of the lead hopping parameter $t$. Squares are for incoming radiation of frequency $\nu =10\mathrm{T}\mathrm{H}\mathrm{z}$. The filled circle and the triangle are for frequencies $\nu =20$ and 5 THz, respectively.