**5.3.2 Plasma column between transmitter and metallic photonic crystal**

In this experiment, a plasma column of density around 1012 cm-3 is formed at 40 mm to 100 mm away from the MPC and transmitted power of microwave is measured at different angles. Measurements with and without plasma column are presented in Fig.10. Findings of this study suggest that transmitted power is –50dBm (forbidden band) at every angle around MPC when there is no plasma column within electrodes of separation 15 mm. When plasma is formed between electrodes, transmitted power –35 dBm (flat band) is received at +450. Thus, negative refraction can also be controlled using plasma column. During this experiment it is also noticed that when the plasma column is situated between 70 mm to 100 mm away from the front row of MPC towards the transmitter, flats bands are measured and if plasma column is situated at a distance of 10 mm to 40 mm from front row of MPC, forbidden bands are measured at same angle. With this experiment, it can also be pointed out that position of plasma column can also control the propagation of microwave.

#### **6. Conclusion**

Negative refraction by the photonic crystal has been achieved for microwave to optical range. Plasma photonic crystals are used to enhance the controllability and tunabelity of microwave propagation. Plasma crystals can be also used for microwave to terahertz frequency filter. However plasma in metallic photonic crystal is a suitable technique to control the microwave in such a way that negative and positive refraction can be achieved. For this purpose micro-discharged mechanism is used to form plasma column at atmospheric pressure in a metallic photonic crystal. Argon, helium, xenon and their mixtures are used as a background gas. Transmitter, MPC and plasma column are arranged in a glass chamber in such way that the experiments can be carried out for different positions of plasma columns. A 20 mm long plasma column of electron density around 1013 cm-3 is formed. Experiments are conducted to study the electromagnetic band gaps of X-band microwave through different configurations of triangular MPC with and without

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plasma column. Transmitted power of 18 GHz with electrodes (without plasma between electrodes) and with plasma in electrodes are measured. Results reveal that transmitted power of – 38 dBm is received at + 450 for electrodes at a separation of 20 mm which becomes -48 dBm when plasma is formed between electrodes. In another experiment, a plasma column is formed between MPC and transmitter and transmitted power is measured for every angle. Findings of the study suggest that when plasma column of length 15 mm is formed in electrodes, which are fixed 70 mm to 100 mm away from the MPC, flat band of power level -35 dBm is received at + 450 while forbidden band of power level – 50 dBm is noticed when plasma column is formed at a diastase of 10 mm to 40 mm from the MPC.

Angle (Degree)

Fig. 10. Variations in the transmitted microwave power at different angles in the presence of plasma column at different places in between transmitter and metallic photonic crystal.

Therefore, by switching ON and OFF the plasma column, propagation of microwave in metallic photonic crystal can be controlled in such a way that positive and negative refraction can be achieved. This chapter can be concluded from the fact that plasma can be used to form tunable / controllable photonic crystals.
