Gbps Terahertz External Modulator Based on a ... - ACS Publications

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Gbps Terahertz External Modulator Based on a Composite Metamaterial with a Double-Channel Heterostructure Yaxin Zhang,† Shen Qiao,*,† Shixiong Liang,‡ Zhenhua Wu,† Ziqiang Yang,*,† Zhihong Feng,‡ Han Sun,† Yucong Zhou,† Linlin Sun,† Zhi Chen,§ Xianbing Zou,§ Bo Zhang,∥ Jianhao Hu,§ Shaoqian Li,§ Qin Chen,⊥ Ling Li,† Gaiqi Xu,† Yuncheng Zhao,† and Shenggang Liu† †

Terahertz Science Cooperative Innovation Center, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China ‡ National Key Laboratory of Application Specific Integrated Circuit, Hebei Semiconductor Research Institute, Shijiazhuang 050000, China § National Key Laboratory of Communication, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China ∥ School of Electronics Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China ⊥ Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China S Supporting Information *

ABSTRACT: The past few decades have witnessed a substantial increase in terahertz (THz) research. Utilizing THz waves to transmit communication and imaging data has created a high demand for phase and amplitude modulation. However, current active THz devices, including modulators and switches, still cannot meet THz system demands. Double-channel heterostructures, an alternative semiconductor system, can support nanoscale two-dimensional electron gases (2DEGs) with high carrier concentration and mobility and provide a new way to develop active THz devices. In this Letter, we present a composite metamaterial structure that combines an equivalent collective dipolar array with a double-channel heterostructure to obtain an effective, ultrafast, and all-electronic grid-controlled THz modulator. Electrical control allows for resonant mode conversion between two different dipolar resonances in the active device, which significantly improves the modulation speed and depth. This THz modulator is the first to achieve a 1 GHz modulation speed and 85% modulation depth during real-time dynamic tests. Moreover, a 1.19 rad phase shift was realized. A wireless free-spacemodulation THz communication system based on this external THz modulator was tested using 0.2 Gbps eye patterns. Therefore, this active composite metamaterial modulator provides a basis for the development of effective and ultrafast dynamic devices for THz wireless communication and imaging systems. KEYWORDS: Terahertz, external modulator, double-channel heterostructure, composite metamaterial

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photoinduced THz modulator based on a metamaterial−bulk semiconductor hybrid structure with phase modulation was developed.19,20 Later, the tunable characteristics of phase transition materials such as VO2 films and organic ferroelectrics were applied to thermally and optically control THz active devices with a modulation depth of greater than 80% and several hundred kilohertz (kHz) speed.21−24 Graphene− metamaterial and GaAs−metamaterial composite THz tunable devices with remarkable modulation depth and fast speeds were proposed in 2011.25−36 These outstanding achievements have improved the speed of THz modulators from several kHz to 10 MHz. Although many publications predicted the possibility of

erahertz (THz) science and technology is rapidly becoming a notable area of scientific research for potential applications in security, wireless communication, imaging and other areas.1−7 The past few decades have witnessed a substantial increase in THz research. A wide variety of THz sources and detectors have been developed, which has accelerated THz radiation applications.8−11 Utilizing THz waves to transmit communication and imaging data creates a high demand for phase and amplitude modulation.12−16 Therefore, active devices such as modulators and switches have been studied intensively in the THz regime. In recent decades, a considerable effort has been made to develop active THz devices using different advanced semiconductor systems.17−36 Combining metamaterials with doped semiconductors was proposed in 2006 to achieve electric control of THz wave transmissions, which is one of the earliest methods used to construct THz modulators and switches.17 In 2008, a © XXXX American Chemical Society

Received: March 4, 2015 Revised: April 21, 2015

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Figure 1. MMTM structures. (a) Schematic of a unit cell. (b) Sectional axonometric drawing of a unit cell. (c) Image of the fabricated MMTM. (d) Photomicrograph of a portion of the fabricated MMTM. (e) Unit cell in the fabricated sample.

equivalent collective dipolar array that is combined with a DC heterostructure to construct a THz modulator. The MMTM is schematically depicted in Figure 1. Each unit in the array consists of two plates butted together with a split gap and three electrodes (Figure 1a,b), which can reduce the resistor−capacitor (RC) parameters and simplify fabrication. The DC heterostructure contains a 15 nm-thick Al0.82In0.18N barrier layer, 2 nm-thick AlN layer, 7 nm-thick GaN layer, 1 nm-thick AlN layer, and 1.2 μm-thick undoped GaN layer. This InAlN/AlN/GaN/AlN/GaN heterostructure has a DC between the AlN and GaN layers that provides approximately 10 nm-thick 2DEG layers with a sheet carrier concentration of 1.13 × 1013 cm−2 and electron mobility of 1570 cm2/V·s at room temperature, which corresponds to a sheet resistance of 212 Ω/□. Two condenser plates are integrated with these nanoscale 2DEG DC heterostructure layers via an ohmic contact between the source and drain electrodes. These two electrodes and a Schottky gate between them form a single unit with a 500 nm gate length (for a detailed description, see Supporting Information part I). Applying an external voltage across the three electrodes can dynamically control the carrier concentration in the 10 nm-thick 2DEG. Figure 1c shows a 10 mm × 10 mm MMTM attached to a printed circuit board (PCB). The fabricated array and MMTM unit are shown in Figure 1d,e. A dipolar resonance is always induced in a metallic wire based on the wire length. Therefore, when the source and drain in this structural unit are connected by the 10 nm-thick 2DEG of the DC heterostructure, the induced THz wave will excite a dipolar resonance in the long central wire. Depleting the 2DEG disconnects the source from the drain such that there is another dipolar resonance in the short wires. Therefore, there will be a frequency shift that could modulate the amplitude at a certain frequency. THz static and dynamic test systems were used to characterize the MMTM performance. First, the THz-wave amplitude and phase modulation of the MMTM were characterized using two static test systems: THz time-domain spectroscopy (THz-TDS) in the transmission mode and a continuous wave (CW) test system (see Supporting Information part III for detailed information regarding the

higher modulation speeds with deeper modulation depths via these means, the modulation speed is still the most important obstacle to ultrafast THz active device applications, and no device has exhibited an experimental speed over 100 MHz. Recently, double-channel (DC) heterostructures have garnered more attention as a promising material for semiconductor devices.37−40 The DC heterostructure channel being split in two decreases the polarized carrier concentration in both channels, which can reduce hot phonon scattering. Thus, DC heterostructures can support a nanoscale two-dimensional electron gas (2DEG) layer with high concentration and mobility. More importantly, the low sheet resistance can ensure that DC heterostructure active devices exhibit high linearity. In this Letter, we present a composite metamaterial THz modulator (MMTM) that combines an equivalent collective dipolar array with an InAlN/AlN/GaN/AlN/GaN DC heterostructure to address modulation speed and depth issues. In the MMTM, the electron concentration in the 10 nmthick 2DEG layers is controlled by an external electrical signal and yields a resonant mode conversion between two dipolar resonances, which generates remarkably fast modulation of both the amplitude and phase in the transmitted THz wave. The most important finding in this Letter is that such devices can achieve on−off modulation speeds of greater than 1 GHz and data transmission rates of 0.2 Gbps, which are crucial for developing high-speed, wireless, free-space THz communication systems based on external modulators. The electromagnetic structure of a metamaterial can be designed to yield various resonant modes, such as the inductance−capacitance (LC) resonance, dipole resonance, quadrupole resonance, and Fano resonance. Most previously published papers have studied LC resonance and electromagnetically induced transparency (EIT) effects in dynamic devices using different means of controlling the resonance intensity to achieve THz-wave amplitude modulation.27,28 Fundamental LC resonance structures are usually made of typical split-ring resonators (SRRs) that contain an equivalent circuit that includes an inductor and capacitor. However, for rapid modulation, this LC structure yields relatively large, parasitic capacitances and inductances that affect the modulation speed. Therefore, this Letter proposes a simple B

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Figure 2. Static THz experimental results versus the voltage. (a) Experimental schematic diagram. (b) Frequency-dependent transmission of THzTDS at different voltages. (c) Frequency-dependent phase shift of THz-TDS at different voltages. (d) Transfer characteristics for a single unit. (e) Transmission power change for a single-frequency THz wave at different voltages characterized by a CW system. (f) Phase shift for a singlefrequency THz wave at different voltages characterized by a CW system.

Figure 3. Simulation results for different carrier concentrations. (a) Electric field and surface current distributions for the on- and off-modes. (b) Transmission spectra for different carrier concentrations.

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Figure 4. Modulated THz waves with different modulation speeds detected by a receiver during the dynamic THz experiments.

long, central wire with fields focused at the edge of each unit cell, and the surface current on the central wire was uniform (ideal off-mode in Figure 3a). The resonant frequency for this off-mode mainly depended on the distance between the up and down long wires. Applying a reverse gate-voltage increased the depletion. The 2DEG depletion in the split gap separated the source and drain, which suppressed the equivalent collective dipolar resonance on the central long wire. Instead, a new dipolar resonance was induced on the short cut wires. The surface current distribution indicates that the current oscillated on the short cut wires with the field focused on the central split gap plates (on-mode in Figure 3a). The simulated THz wave transmission spectra (Figure 3b) indicate that such dipolar resonance mode conversions caused a large resonant frequency blueshift. A blueshift from 0.18 THz (ideal off-mode) to 0.352 THz (on-mode) with a 91% modulation depth was obtained, as shown by the black dashed and red solid lines in Figure 3b. A carrier concentration of 4.85 × 1012 cm−2 with a mode conversion between off- and on-modes could yield an 88% simulated modulation depth at 0.352 THz (corresponding to the green and red solid lines in Figure 3b). Most importantly, the MMTM resonant mode conversion exhibited a reduced parasitic capacitance and inductance relative to the equivalent LC-circuit resonance for conventional SRR structures,16−20 which ensures a high modulation speed. A dynamic experiment was performed to further test the realtime modulation speed for our device by applying a sinusoidal voltage to the MMTM gate to modulate the THz carrier wave (see Supporting Information part IV for details regarding this dynamic test system and the driving circuit). A THz receiver was used to detect the modulated amplitude signal. The dynamic test results for 50 MHz−1 GHz modulation speeds are shown in Figure 4. Up to 1 GHz, all of the detected signals were standard sinusoidal waveforms rather than the triangular waveforms caused by the response time. Therefore, this modulator can load up to 1 Gbps data on the carrier wave. The modulation depth results as a function of frequency are shown in Figure 5. Even at 1 GHz, the modulation depth was almost 30%. However, increasing the modulation speed,

static test system). During the static test, a normal THz incident wave was projected onto the modulator surface with the electric field polarized perpendicular to the connecting wires (Figure 2a), while a direct current voltage was applied to the MMTM. The THz-TDS experimental results (Figure 2b, c) shown in Figure 2b indicate that the resonant frequency was approximately 0.27 THz when no voltage was applied. Increasing the voltage gradually depleted the 2DEG in the DC heterostructure and yielded a resonant frequency blueshift. The 2DEG was completely depleted at approximately 6 V, with a frequency near 0.351 THz. This blueshift was due to the resonant mode conversion caused by changing the 2DEG electron concentration via the gate-voltage, which yielded a significant amplitude modulation. At approximately 0.351 THz, the modulation depth was as high as 85% (η = (T1 − T2)/T1). The applied gate voltage also changed the equivalent dielectric permittivity, thereby yielding a THz wave phase shift, as shown in Figure 2c. At approximately 0.3 THz, the THz transmission varied in phase from 0.18 rad at 0 V to −1.01 rad at 6 V; the phase was shifted by 1.19 rad (approximately 68°) with a small variation in the transmission amplitude. The modulation depth and phase shift for the THz-CW test system were tested further using a single-frequency THz wave, as shown in Figure 2e,f. The transmission power and phase shift changes with respect to the voltage agreed well with the transfer characteristics for a single unit, as shown in Figure 2d, which indicates that the MMTM operating characteristics could be strictly controlled by varying the single-unit performance. To illustrate the dynamic electrical-modulation mechanism, a finite difference time domain (FDTD) code and dispersive Drude model (see Supporting Information part VI) were used to analyze the MMTM electromagnetic properties at different carrier concentrations (equivalent to the applied gate-voltage variations in the experiments). Without an applied voltage, the source and drain were connected by the high nano-2DEG layer concentration in the DC heterostructure via the ohmic contacts. The resonance induced by the incident THz waves arose from the equivalent collective dipolar resonance in the D

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modulator to achieve a 1 GHz modulation speed and 85% modulation depth. The wireless free-space-modulation THz communication system based on this external THz modulator was tested using 0.2 Gbps eye patterns, which was the first demonstration that such an external modulator could be a dynamic component for THz communication systems. More importantly, this device could be adapted to various THz sources, including Watt-level CW sources, which demonstrates the enormous potential for long-distance, high-speed wireless free-space-modulation THz communication systems.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed descriptions of the unit cell, MMTM fabrication processes, static experiment system setup, dynamic experiment system setup, MMTM semiconductor test, and 2DEG simulation model. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b00869.

Figure 5. Modulation depth as a function of frequency.

especially above 200 MHz, via the modulator package and impedance mismatch, increased the reflection coefficient for the driving circuits, which reduced the modulation depth, as shown in Figure 5. According to the method presented in ref 25, the expected modulator speed could be evaluated from the inverse of the calculated RC time constant as approximately 1.3 GHz (the relevant parameters are shown in the Supporting Information part V). More importantly, the extreme modulation speed for a single unit with a 500 nm grid length was approximately 20 GHz, and this modulator array had 42 × 42 units. Therefore, there is significant room to improve the modulation speed. Based on these dynamic experimental results for the MMTM external THz modulator, a wireless free-space THz communication system was modulated using eye patterns with different data transmission speeds, as shown in Figure 6. At 0.1 and 0.15 Gbps, the modulation data rate for the eye patterns was good; once the speed reached 0.2 Gbps, the error rate increased because of the impedance mismatch between the modulator and baseband signal input circuit. Other possible reasons may include the nonlinear characteristics of the modulator. Therefore, linear and precisely matched circuits are especially important at increased modulation speeds (especially above 0.2 GHz). Even so, these data transmission experimental results are the first evidence that such modulators can be applied in Gbps THz wireless communication systems. In conclusion, combining an equivalent collective dipolar array with an InAlN/AlN/GaN/AlN/GaN DC heterostructure yielded a Gbps THz modulator based on dipolar resonance conversion. The superior nano-2DEG performance in the DC heterostructure and high dipolar resonant intensity allowed this

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Weili Zhang and Prof. G.-F. Yu for discussions about this manuscript. This work was supported by the National Natural Science Foundation of China (Contract Nos. 61370011 and 61072036), the National High-tech Research and Development Projects of China (2011AA010204), the National Key Program of Fundamental Research of China (Contract No. 2014CB339806), the Program for New Century Excellent Talents from the Ministry of Education of China (Grant No. NECT_13_0100), and the Sichuan Province Youth Science and Technology Foundation (No. 2014JQ0013).

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ABBREVIATIONS THz, terahertz; 2DEG, two-dimensional electron gas; DC, double channel; MMTM, metamaterial THz modulator; LC, inductance-capacitance; EIT, electromagnetically induced transparency; RC, resistor−capacitor; PCB, printed circuit board; THz-TDS, THz time-domain spectroscopy; CW, continuous wave; FDTD, finite difference time domain

Figure 6. Eye patterns for 0.1, 0.15, and 0.2 Gbps data transmission speeds used to test the wireless free-space-modulation THz communication system based on the external THz modulator, MMTM. E

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