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Since these materials can be influenced by an external applied voltage, the metamaterial resonance can be switched from active and passive and back again by tuning the sublayer conductivity.25 A hybrid semiconductor formed of these materials consists of SRR electronic plates fabricated on a 1-μm Si-GaAs layer [Fig. 8(a)]. By connecting all SRRs, the structure resembles a Schottky gate in function.25 The carrier electron density can be completely controlled by a reverse voltage, making control of metamaterial resonance and terahertz operation possible [Fig. 8(b)]. This structure can operate with 50% modulation depth at less than 15 V applied bias voltage.26 These metamaterial modulators offer potential as high-speed modulators for imaging and communications applications.27

A spatial light modulator (SLM), which can operate in the terahertz region, is capable of electronic and optical control of transmission and reflection of an optical beam and as a result change of direction and advance coding. These devices are essential for many optical and electronic applications, including imaging and spectroscopy.28 By introducing this technology for use in the terahertz region, it can play an important role in spectroscopy and communication applications.29 As an example, in recent terahertz single pixel imaging, an SLM served as a key component for decoding to terahertz waves.29

9. This schematic diagram shows the components in an amplifier designed for use from 160 to 180 GHz.

Figure 9 shows an amplifier consisting of five 150-GHz stages designed to operate between 160 and 180 GHz. It includes three cascaded stages and two common emitters. The final stage consists of two devices in parallel to achieve high output power. The amplifier is optimized by tuning its resistors, capacitors, and inductors for operation between 140 and 170 GHz. For operation at 170 GHz, capacitors with values of less than 30 fF are required. For such small capacitance values, a metal-oxide-metal structure was developed. Figure 10 offers a MMIC multiplier and amplifier for terahertz use; the MMIC amplifier consists of five simple stages.

10. These photographs offer a multiplier (a) and amplifier (b) designed to produce 125-GHz output signals.

Both photonic and electronic means are capable of reaching terahertz frequencies. As a comparison, Table 2 shows specifications for a 125-GHz transmitter realized by means of photonic (UTC-PD) and electronic (InP HEMT MMIC) approaches.2,4 Each method has advantages and disadvantages, with electronic approaches being somewhat limited in usable output-power levels and photonic approaches also allowing fiber connections from point to point in a system. The technologies are steadily improving, making terahertz applications more feasible in the near future.


A. Mohammadi, Researcher

Faculty of Electrical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran; e-mail:

M. Danaeifar, Researcher

Faculty of Electrical and Computer Engineering, K.N. Toosi University of Technology, Tehran, Iran.


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