A metamaterial solid-state terahertz phase modulator

Over the past two decades, terahertz time-domain spectroscopy1 and quantum-cascade lasers2 have been two of the most important developments in terahertz science and technology. These technologies may contribute to a multitude of terahertz applications that are currently under investigation globally3. However, the devices and components necessary to effectively manipulate terahertz radiation require substantial development beyond what has been accomplished to date. Here we demonstrate an electrically controlled planar hybrid metamaterial device that linearly controls the phase of terahertz radiation with constant insertion loss over a narrow frequency band. Alternatively, our device may operate as a broadband terahertz modulator because of the causal relation between the amplitude modulation and phase shifting. We perform terahertz time-domain spectroscopy, in which our hybrid metamaterial modulator replaces a commercial mechanical optical chopper, demonstrating comparable broadband performance and superior high-speed operation. The controllable properties of engineered metamaterials facilitate novel opportunities for manipulating electromagnetic radiation4. Electromagnetic phenomena achieved with metamaterials include negative index of refraction5,6, super-resolution in optical imaging7,8, and electromagnetic invisibility9. The resonant electromagnetic response originates from patterned metallic subwavelength structures, in which the dimensions can be appropriately scaled to operate at terahertz frequencies10,11 where natural material response is somewhat rare. As such, metamaterials provide the basis for the construction of novel terahertz devices. Terahertz metamaterial devices have been demonstrated as state-of-the-art frequency-agile far-infrared filters12,13, all-optical switches and modulators14,15, and perfect absorbers16,17. Room-temperature, voltage-controlled metamaterial devices consisting of a single unit cell layer in the propagation direction have also been shown and are of particular interest here18,19. Despite the rapid progress in terahertz technology generally, a component largely unavailable as yet is an efficient terahertz phase shifter. Its counterparts at microwave and optical frequencies have many important applications, including, for example, phased array antennas and high-speed Mach–Zehnder modulators. However these configurations are, in general, difficult to extend to the terahertz regime. There have been a few attempts to demonstrate terahertz phase shifters, including using semiconductor quantumwell structures at cryogenic temperatures20,21, or liquid crystals with very low speed22. The single-layer planar hybrid metamaterial phase modulator presented in this paper overcomes these shortcomings. Although phase shifting of terahertz radiation may be inferred from our earlier work13,18, where frequency tuning of a metamaterial resonance and amplitude modulation were reported, phase modulation was not explicitly discussed or explored. Here we present the first experimental demonstration of a room-temperature solidstate phase modulator at terahertz frequencies as well as an investigation of its potential applications. Our new device achieves a voltage-controlled linear phase shift of p/6 radians at 16 V. Moreover, the causal relation between amplitude switching and phase shifting enables broadband modulation. A single unit cell of the device is illustrated schematically in Fig. 1a. Metallic electric split-ring resonators (SRRs)23 were patterned to form a square array and connected by metal wires. They were fabricated on a 1-mm-thick epitaxial n-doped GaAs layer with an electron density of 2 1016 cm23 grown on an intrinsic GaAs wafer. The SRRs and semiconductor form the Schottky diode structure that can, upon application of an external voltage, actively modify the depletion zone (see Supplementary Information). The control of the carrier density in the depletion zone permits tuning of the local dielectric properties near the gaps of the SRRs. This results in changes of the transmission (amplitude, phase, or both) of the metamaterial device. The micro-fabrication of the device has been described previously18,19; an optical microscopy image is shown in Fig. 1b. For this design, the SRR gaps are located at the four outer corners and are directly connected to the ohmic contact through the n-GaAs epilayer. This maximizes depletion of electrons near the metamaterial gaps upon application of bias voltage, which is essential to control the metamaterial resonances. In an earlier metamaterial switch18, the split gap was located at the centre of an electric SRR and was surrounded by a closed outer ring. This reduced the voltage available for effective charge depletion within the split gap, which limited device performance. (See Supplementary Information for further details of the device.) Conventional terahertz time-domain spectroscopy24 (see Supplementary Information) was used to characterize the device. Resonances at 0.81 and 1.7 THz (driven by the electrical component of the terahertz radiation as indicated in Fig. 1b) are obtained as shown in Fig. 2a and b. The resonance at 0.81 THz arises from the individual SRRs and is due to the inductive–capacitive coupling of the circulating currents (Fig. 1c), while the resonance at 1.7 THz originates from a collective dipolar resonance (Fig. 1d), where the resonance frequency also depends on the SRR periodicity25. At zero voltage bias the resonances are weak because carriers in the substrate shunt the metamaterial capacitive gaps, thereby damping the response. Under reverse bias voltage an increase in depletion occurs, reducing the damping and causing an increase in oscillator strength for both resonances. The depletion near the split gaps plays