The frequency tuning of a quarter-wave resonator using an optoelectronic control is reported. Sharp notch characteristics with a small decibel-insertion loss and tunable frequency with matching better than 45 dB are obtained by varying both the optical power and the dc bias. The measured frequency shift is more than 60% below the dark resonant frequency and is carried out without altering the shape of the response. The biased photoinduced plasma (BPP) loading the open terminated microstrip line is then analyzed by comparing microwave simulations and measurements. The deduced complex load equivalent to this biased photoinduced plasma is then confirmed by semiconductor simulations. Results show the great possibilities offered by this BPP load (BPPL), which can be easily and widely tuned by means of a simple optoelectronic control. The frequency bandwidth of tuning is limited by the geometrical parameters and may be extended to millimeter-wave operation.
Figure 5 Simulated and measured absorption of a four-layer graded-chiral absorber
IV. CONCLUSIONIt was shown that the high loss associated with the chiral resonance can be used to improve the absorption characteristics of uniform and graded-dielectric Dallenbach absorbers. For this purpose, two design procedures were proposed. It was demonstrated theoretically that both procedures give good results, and allow us to improve the reflection coefficient in the low-frequency region. First experiments performed on an absorber designed according to the second method demonstrate the validity of the approach. Currently, the manufacturing procedures are being improved for optimized absorber performance.
REFERENCES1. V.K. Varadan, V.V. Varadan, and A. Lakthakia, On the possibility of designing antireflection coatings using chiral materials, J Ž .
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