Excitation, detection, and electrostatic manipulation of terahertz-frequency range plasmons in a two-dimensional electron system

www.nature.com/scientificreports OPEN received: 24 June 2015 accepted: 15 September 2015 Published: 21 October 2015 Excitation, detection, and electrostatic manipulation of terahertz-frequency range plasmons in a two-dimensional electron system Jingbo Wu, Alexander S. Mayorov, Christopher D. Wood, Divyang Mistry, Lianhe Li, Wilson Muchenje, Mark C. Rosamond, Li Chen, Edmund H. Linfield, A. Giles Davies & John E. Cunningham Terahertz frequency time-domain spectroscopy employing free-space radiation has frequently been used to probe the elementary excitations of low-dimensional systems. The diffraction limit, however, prevents its use for the in-plane study of individual laterally-defined nanostructures. Here, we demonstrate a planar terahertz frequency plasmonic circuit in which photoconductive material is monolithically integrated with a two-dimensional electron system. Plasmons with a broad spectral range (up to ~ 400 GHz) are excited by injecting picosecond-duration pulses, generated and detected by a photoconductive semiconductor, into a high mobility two-dimensional electron system. Using voltage modulation of a Schottky gate overlying the two-dimensional electron system, we form a tuneable plasmonic cavity, and observe electrostatic manipulation of the plasmon resonances. Our technique offers a direct route to access the picosecond dynamics of confined electron transport in a broad range of lateral nanostructures. Picosecond time-resolved measurements of low-dimensional semiconductors can reveal a diverse range of physical phenomena. Typically, a device containing a two-dimensional electron system (2DES) is subjected to free-space propagating terahertz (THz) radiation (100 GHz <  f <  5 THz), and either the transmitted THz response and/or the rectification response of the 2DES is then used to determine information about the system. Such experiments have provided information on, for example, coherent cyclotron resonance in a 2DES1,2, ultrastrong light-matter interactions between inter-Landau level transitions of a 2DES and the photonic modes of artificial resonators3,4, THz-wave modulation at room temperature5–7, and recently, the formation of plasmonic crystals using a gate-controlled 2DES8,9. The latter is particularly exciting, since the plasmonic cavity resonances in 2DESs on length scales of a few microns occur in the THz frequency range, offering the possibility of fabricating plasmonic circuits that can be used to manipulate THz signals. Another class of experiments involves the planar integration of a 2DES into THz waveguides, which allows pulses to be either directly coupled into the system by ohmic contacts, using a flip-chip arrangement10, or coupled by proximity to a nearby THz waveguide, where they are exposed to and interact with the evanescent THz electric field11. In both these cases, electrical pulses are usually guided along a lithographically-defined, sub-wavelength transmission line structure formed on a separate substrate, School of Electronic and Electrical Engineering, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom. Correspondence and requests for materials should be addressed to J.E.C. (email: j.e.cunningham@ leeds.ac.uk) Scientific Reports | 5:15420 | DOI: 10.1038/srep15420 1 www.nature.com/scientificreports/ Figure 1. Diagram of the THz 2D plasmonic circuit. (a) Schematic diagram of the THz plasmonic circuit and the measurement arrangement for gate-modulation signals. S1/S2 and S3/S4 are two pairs of PC switches formed on opposite sides of the 2DES mesa, which are used to generate or detect the THz pulses; pulses are generated by application of a bias while under illumination by a 800 nm pulsed Ti:sapphire laser, while detection is achieved by measuring the generated photocurrent as a function of optical path time delay. (b) The layer structure of the wafer monolithically integrating the LT-GaAs and GaAs/AlGaAs heterostructure containing the 2DES (red region). (c) Microscopic image of the 2DES mesa. A 4.4-μ m-long metallic gate was located on top of the 2DES mesa, and the widths of the ungated regions on either side of the gate were 19.7 μ m and 48.9 μ m. before interacting with the 2DES. The in-plane nature of these techniques provides an enhanced interaction between the THz signal and the low-dimensional system relative to that achieved through free-space coupling. Such techniques have allowed ultrafast ballistic picosecond transport10 and magnetoplamon resonances11,12 to be studied. Recently, we introduced an alternative technique in which growth-optimized LT-GaAs (providing THz-bandwidth pulse excitation and detection) and a high mobility 2DES channel are integrated in a single molecular beam epitaxy (MBE) wafer13. Here, we demonstrate that such integrated structures can be used to form broadband (up to ~400 GHz) on-chip plasmonic circuits capable of the in-plane excitation, detection, and electrostatic manipulation of 2D plasmons in quantum-confined 2DESs. The dynamic evolution of plasmon resonances in the gated 2DES region, controlled by an applied voltage, is recorded with a few-picosecond time resolution. Our methodology thus opens up a wide range of possible experiments in which broadband pulsed THz radiation is used to probe individual mesoscopic or nanoscale systems defined lithographically in a 2DES, rather than ensembles. Results Schematic and principle. A diagram of our THz 2D plasmonic circuit, in which the photoconductive material of LT-GaAs is monolithically integrated with 2DES, is shown in Fig. 1a. The device was fabricated from an MBE wafer (Fig. 1b), which comprised a layer of LT-GaAs along with a GaAs/AlGaAs heterostructure containing a 2DES (see further details in Methods). Two pairs of photoconductive (PC) switch contacts were then defined on the LT-GaAs layer, after it was subjected to a selective wet-etch to remove the 2DES and expose the underlying photoconductive LT-GaAs layer. A coplanar waveguide (CPW) guides THz pulses generated from, for example, PC switch S1, to an ohmic contact that is used to inject the picosecond pulses into the (73-μ m-long) 2DES mesa. When the propagating THz pulse arrives at this ohmic contact, a portion of the pulse energy is injected into the 2DES, transmitted through the 2DES, and then exits through a second ohmic contact, before being coupled into the adjacent section of CPW. The first ohmic contact also reflects a portion of the propagating pulse. The time-resolved reflected or transmitted signals are then sampled at S2 or S3/S4, respectively. As shown in Fig. 1c, a 4.4-μ m-wide metal gate was defined on the top of the 2DES mesa. A negative gate voltage (Vg) applied to this gate was used to deplete carriers and so tune the electron concentration (ns) in the 2DES underneath. The voltage (Vth) required to deplete carriers completely underneath the gate at 4 K after illumination was ~− 3.0 V (see Supplementary Note 1 and Figure S1). Our 2DES mesa supp (...truncated)