Electrified graphene modulates terahertz light

The electromagnetic spectrum from 0.1 to 10 terahertz (THz) presents challenges and opportunities for the study of physical phenomena, with potential payoff in a variety of areas ranging from homeland security to pharmacology. Two seemingly different solid-state phenomena—electrical transport and optical transitions—merge in this frequency range to create an exciting field of research. Technologically, however, this regime is poorly developed compared to those of electronics (<100GHz) and photonics (>10THz). Despite the recent impressive development of THz sources and detectors as well as advanced THz spectroscopy and sensing techniques,^{1, 2} mature solid-state technologies for manipulating THz waves are still lacking. By judiciously combining THz technology and nanotechnology, we can significantly advance our understanding of THz physics, while improving existing THz devices.

Low-dimensional carbon nanostructures, such as carbon nanotubes and graphene, offer new opportunities for THz science and technology.³ For example, aligned single- and multiwall carbon nanotubes have been shown to be excellent THz polarizers.^4–6 Graphene is attractive because it is extremely conductive, more conductive even than silver at room temperature. As a result, the DC and linear electrical characteristics of graphene have been extensively studied in the last decade. However, its unusual finite-frequency, nonlinear, and nonequilibrium properties are largely unexplored, although they are predicted to be useful for new THz device applications. In the near-IR and visible range, single-layer graphene absorbs only 2.3% of incident light through interband absorption.^7–9 However, in the THz range, free-carrier intraband absorption dominates⁷ and can be significantly larger than 2.3%.^{10, 11} This strong THz absorption is promising for controlling THz waves by controlling the free-carrier density.

Here, we describe fabrication of a centimeter-size, single-layer graphene device with a gate electrode (to apply voltage) that modulates the transmission of THz and mid-IR waves.¹² Using time-domain THz spectroscopy and Fourier transform IR spectroscopy in a wide frequency range (10–10,000cm⁻¹), we measured the dynamic conductivity change induced by electrical gating and thermal annealing. Both methods effectively tuned the Fermi energy, E_F, which in turn modified the free-carrier intraband absorption in the THz as well as the ‘2E_F onset’ for interband absorption in the mid-IR. These results not only provide fundamental insight into the electromagnetic response of Dirac fermions (massless electrons) in graphene but also demonstrate the key functionalities of large-area graphene devices that are desirable for components in THz and IR optoelectronics.

Because of the characteristically long wavelengths of THz radiation, large-area samples are required for this type of application, as opposed to more commonly used small flakes of graphene obtained through exfoliation of graphite. We grew the sample used for this study from a solid-state carbon source.¹³ As shown in Figure 1, to electrically tune the Fermi level of graphene by applying a gate voltage, we employed a substrate consisting of a lightly p-doped silicon wafer (5–10Ω-cm) with a thickness of 440μm and a surface area of ∼1.5×1.5cm. We covered the top of the wafer with a 300nm-thick silicon dioxide layer. The THz beam was incident normal to the center of the graphene, and we recorded the THz waves transmitted at different gate voltages.

Figure 1. Experimental configuration of gate-voltage-dependent terahertz (THz) transmission measurements on large-area single-layer graphene. SiO₂: Silicon dioxide. p-Si: p-Doped silicon. ΔV : Change in voltage. ΔI: Change in current. Au: Gold. V_g: Gate voltage. CB: Conduction band. VB: Valence band. E_f: Fermi energy. e: Electrons. h: Holes (positive charge carriers).

Figure 2 shows the gate-voltage-dependent transmitted THz waveforms. At +30V, which is the gate voltage at which E_F is at the Dirac point, the highest THz transmission is obtained, while at all other voltages above and below +30V, THz transmission decreases (or absorption increases) monotonically with the voltage change. After Fourier transforming this time-domain data, we obtained the corresponding transmission spectrum in the frequency domain and calculated the dynamic conductivity change induced by electrical gating.¹²

Figure 2. Gate-voltage-dependent THz wave transmission through single-layer graphene. The colors represent data taken at different voltages. The top trace corresponds to +30V, and the lowest to –120V. a.u.: Arbitrary units.

In summary, by applying an external gate voltage to graphene, we were able to electrically tune its Fermi level, which in turn modulated the transmission of THz and IR waves. The intraband conductivity in the THz range exhibited free-carrier frequency dependence with Fermi-energy-dependent magnitude. These results not only provide fundamental insight into the dynamic properties of Dirac fermions in graphene but also promise future applications of this unusual 2D material in THz and IR optoelectronics. Our next step will be to increase the efficiency and degree of modulation.

We thank our collabrators on this work: J. Yao, Z. Sun, Z. Yan, Z. Jin, R. Kaneko, I. Kawayama, M. Tonouchi, and J. M. Tour. This work was supported by the Department of Energy (grant DE-FG02-06ER46308), the National Science Foundation (grant OISE-0530220), the Robert A. Welch Foundation (grant C-1509), and the Japan Society for the Promotion of Science Core-to-Core Program.