The name is absent

41

size mismatch between micro-machined devices and THz wavelengths. THz modula-
tors based on quantum-well structures either require cryogenic cooling [82] or have a
poor modulation depth [83]. Other previous efforts in building THz modulators also
yield modulation of only a few percent [84] or require high operation voltage [85].

This chapter reports the construction of two THz SLMs based on the use of active
THz metamaterials [11,12,86,87]. This metamaterial device consists of a planar array
of sub-wavelength-sized split-ring resonator (SRR) elements fabricated on epitaxial
n-doped GaAs grown on a semi-insulating GaAs substrate [11] (see Figure 5.1(a)
and (b)). The control of the metamaterial resonance is realized by the depletion of
substrate charge carriers upon voltage bias which in turn changes the loss at the
capacitive split gaps and therefore the oscillator strength of all of the individual
SRR elements within a pixel. The device enables an amplitude modulation depth
of ^∙3 dB under a relatively small bias voltage (16 volts) at room temperature [12].
Moreover, fast modulation, in the megahertz range, is achievable [86]. The design
of such metamaterial devices is flexible because the resonant frequency can be tuned
by changing the geometry and dimensions of the SRR elements [88]. Compared to
existing THz modulators [82-85], the metamaterial-based devices are very promising
for the construction of a high-speed THz SLM.

5.1 MetamateriaIs

Metamaterials are artifically constructed materials which exhibit electromagnetic
properties not readily found in nature [89]. A metamaterial is usually a collection of
structures whose size and spacing are much smaller than the wavelength of the elec-
tromagnetic radiation it interacts with. The electromagnetic response of this artifical
material, or metamaterial, can be collectively characterized two macroscopic electro-

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