**5. Conclusion**

than that of the Si substrate-based rigid graphene modulators (~5 dB) [52]. The extremely low loss can be attributed to the small refractive index of PET (1.65) as compared to that of Si (3.42). Our results indicate that employing a substrate with low refractive index is beneficial

**Figure 7.** Transmittance and attenuation as a function of frequency for flexible graphene modulator at the gate voltage

Modulation depth is one of the crucial parameters that determine the real applications of THz modulators. The typical MD of existing GFET modulators is only 20%. High MD has been achieved by a complex and exquisite integration of GFET with THz quantum cascade lasers

**Figure 8.** Overview of the cascaded THz modulators. (a) Schematic illustration of the device with cascaded two GFETs on a single PET substrate. (b) Photograph of the devices. The boundaries of graphene channel and ion-gel layer are marked with dark dotted lines and red solid lines, respectively. (c) Optical image showing the flexible nature of the

for obtaining a THz modulator with low insertion loss.

128 Design, Simulation and Construction of Field Effect Transistors

of 1 V (Dirac point) and PET.

device.

Field-effect transistors are one of the most widely discussed applications of graphene in microelectronics and opto-electronics. However, graphene is intrinsically a zero-band-gap semiconductor, which is believed to be unsuitable for use in an electronic transistor. Fortunately, graphene FET finds its potential usage as THz modulators since THz wave is highly sensitive to the free carrier concentration, which can be effectively tuned by electrical gating in a graphene FET. In this chapter, the physics principle, device structure, and the modulation characteristics of GFET-based THz modulators, both rigid and flexible, are discussed and experimentally demonstrated. It shows that THz modulators can be easily realized with graphene FET, and highly desired properties can be obtained such as the large modulation depth, high speed, broad width band, and insert loss.

[4] Ho L, Pepper M, Taday P. Terahertz spectroscopy: Signatures and fingerprints. Nature

Graphene Field-Effect Transistor for Terahertz Modulation

http://dx.doi.org/10.5772/intechopen.76744

131

[5] Chen HT, Padilla WJ, Zide JMO.Active terahertz metamaterial devices. Nature. 2006;**444**:

[6] Rahm M, Li JS, Padilla WJ. THz wave modulators: A brief review on different modulation techniques. Journal of Infrared, Millimeter, and Terahertz Waves. 2013;**34**:1-27

[7] Ostmann TK, Dawson P, Pierz K. Room temperature operation of an electrically driven

[8] Shrekenhamer D, Rout S, Strikwerda A C. High speed terahertz modulation from metamaterials with embedded high electron mobility transistors. Optics Express. 2011;

[9] Chan WL, Chen HT, Taylor AJ. A spatial light modulator for terahertz beams. Applied

[10] Chen HT, Padilla WJ, Cich MJ. A metamaterial solid-state terahertz phase modulator.

[11] Chen HT, Padilla WJ, Zide JMO. Ultrafast optical switching of terahertz metamaterials fabricated on ErAs/GaAs nanoisland superlattices. Optics Letters. 2007;**32**:1620-1622

[12] Chen HT, Yang H, Singh R. Tuning the resonance in high temperature superconducting

[13] Jin BB, Zhang CH, Engelbrecht S. Low loss and magnetic field-tunable superconducting

[16] Alius H, Dodel G. Amplitude-, phase-, and frequency modulation of far-infrared radia-

[17] Vogel T, Dodel G, Holzhauer E. High-speed switching of far-infrared radiation by pho-

[19] Cheng LJ, Liu L. Optical modulation of continuous terahertz waves towards cost-effective reconfigurable quasi-optical terahertz components. Optics Express. 2013;**21**:28657-28667

[18] Okada T, Tanaka K. Photo-designed terahertz devices. Scientific Reports. 2011;**1**:121

[20] Xie ZW, Wang XK, Ye JS. Spatial terahertz modulator. Scientific Reports. 2013;**3**:3347

cut-wires for thermal

thin films on silicon

as buffer layers. Journal of Physics D: Applied Physics.

terahertz metamaterials. Physical Review Letters. 2010;**105**:247402

terahertz metamaterial. Optics Express. 2010;**18**:17504-17509

[14] Wen QY, Zhang HW, Yang QH. Terahertz metamaterials with VO2

[15] Xiong Y, Wen QY, Chen Z. Tuning the phase transitions of VO2

O3

tion by optical excitation of silicon. Infrared Physics. 1991;**32**:1-11

toionization in a semiconductor. Applied Optics. 1992;**31**:329-337

tenability. Applied Physics Letters. 2010;**97**:021111

terahertz modulator. Applied Physics Letters. 2004;**84**:3555-3557

Photonics. 2008;**2**:541-543

597-600

**19**:9968-9975

Physics Letters. 2009;**94**:213511

Nature Photonics. 2009;**3**:148-151

substrates using ultrathin Al2

2014;**47**:455304
