Tunable metamaterial
Hybrid metamaterial composites
Hybrid metamaterial composites
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Metamaterials were originally researched as a [passive response material](/wiki/Passivity_%28engineering%29 "Passivity (engineering)"). The passive response was and still is determined by the patterning of the metamaterial elements. In other words, the majority of research has focused on the passive properties of the novel transmission, e.g., the [size and shape](/wiki/Split-ring_resonator "Split-ring resonator") of the inclusions, the effects of metal film thickness, hole geometry, [periodicity](/wiki/Periodic_function "Periodic function"), with passive responses such as a negative electric response, negative index or gradient index etc. In addition, the resonant response can be significantly affected by depositing a [dielectric layer](/wiki/Dielectric "Dielectric") on metal hole arrays and by doping a [semiconductor](/wiki/Semiconductor "Semiconductor") substrate. The result is significant shifting of the [resonance](/wiki/Resonance "Resonance") frequency. However, even these last two methods are part of the passive material research.

Electromagnetic metamaterials can be viewed as structured composites with patterned metallic [subwavelength](/wiki/Subwavelength "Subwavelength") inclusions. As [mesoscopic](/wiki/Mesoscopic "Mesoscopic") physical systems, these are built starting from the unit cell level. These unit cells are designed to yield prescribed electromagnetic properties. A characteristic of this type of metamaterial is that the individual components have a resonant (coupling) response to the electric, magnetic or both components of the [electromagnetic radiation](/wiki/Electromagnetic_radiation "Electromagnetic radiation") of the source. The EM metamaterial as an artificially designed transmission medium, has so far delivered desired responses at frequencies from the microwave through to the near visible.

The introduction of a natural semiconductor material within or as part of each metamaterial cell results in a new design flexibility. The incorporation, application, and location of semiconductor material is strategically planned so as to be strongly coupled at the resonance frequency of the metamaterial elements. The hybrid metamaterial composite is still a passive material. However, the coupling with the semiconductor material then allows for external stimulus and control of the hybrid system as a whole, which produces alterations in the passive metamaterial response. External excitation is produced in the form, for example, photoconductivity, nonlinearity, or gain in the semiconductor material.


How were metamaterials originally researched?