**5.3.1 Without and with plasma in central default**

It has been studied that flat and forbidden bands at 18 GHz can be formed by MPC and defaulted MPC (Kumar 2011a, Kumar 2011b). Although this research work has potential to improve the tunability of PCs, it seems that enhancement in the tunability and controllability in this MPC is required because for tuning the MPC one needs to physically remove the metallic rods by mechanical effort (Kumar 2011a). For this concern, attention is paid to use a plasma column in the hole or default of MPC because plasma can be created and destroyed by switching ON and OFF. With the help of this approach, tunability of MPC can be increased as rapidly as the plasma can be formed and destroyed.

Hence experiments are carried out to measure the transmitted power at 18 GHz through MPC with and without plasma column. For this purpose electrodes of separation 20 mm are kept at the centre hole and well connected with the power-supplies. Finally, a plasma column of density of 5 x 1012 cm-3 and electron temperature of 2eV is formed around atmospheric pressure. Transmitted power of microwave is measured at different angles. Plasma is characterized [Kumar 2009b] as a collisional medium, which shows cut-off for 18 GHz microwave. A schematic of measurement method with electrode and with plasma in electrodes is shown in Fig. 8 (a) and (b) respectively.

Measurements of transmitted power of 18 GHz with electrodes and with plasma column are presented in Fig.9. Results of this figure show that transmitted power –38dBm at + 450 for

Plasma Photonic Crystal 291

next section, attention is paid to the study of microwave propagation when plasma column

with electroes

Fig. 9. Variation in the microwave transmitted power at different angles with electrodes

Angle (Degree)

with plasma

0 20 40 60 80 100

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

only and plasma column within electrodes at the centre hole or default.

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

out that position of plasma column can also control the propagation of microwave.

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

**6. Conclusion** 

is formed between transmitter and front row of MPC.


Transmitted Power (dBm)

electrodes at the separation of 20 mm are fixed and transmitted power becomes –48 dBm when plasma is formed between electrodes. Negative and positive refraction is also studied by forming the plasma in the left side and the right side default from the centre in front row. Hence by switching ON and OFF the plasma, flat and forbidden bands can be achieved at 450. Due to these strong evidences, it can be concluded that tunability and controllability of MPC over the PCs and PPCs can be enhanced by using a plasma column in a MPC.

Fig. 7. A schematic diagram of measurement method showing MPC in triangular shape inside a glass chamber for measuring microwave transmission through MPC with and without plasma columns.

Fig. 8. (a) Measurement method of transmission of 18 GHz with electrode only (b) with plasma column at the central default of MPC.

So far, it has been successfully demonstrated how forbidden bands and flat bands can be formed by hybrid plasma photonic crystal. Although this study reveals importance and applications of plasma to control the microwave propagation in PCs, some minor problems are realized during the experiment e.g. adjustment of the electrodes, initiation of breakdown for discharge and sustaining plasma for long time because plasma column is formed inside the MPC where it is surrounded by metallic rods, which creates capacitive effects. Here it may be quite interesting to use plasma column between transmitter and MPC. Hence in the

electrodes at the separation of 20 mm are fixed and transmitted power becomes –48 dBm when plasma is formed between electrodes. Negative and positive refraction is also studied by forming the plasma in the left side and the right side default from the centre in front row. Hence by switching ON and OFF the plasma, flat and forbidden bands can be achieved at 450. Due to these strong evidences, it can be concluded that tunability and controllability of

MPC over the PCs and PPCs can be enhanced by using a plasma column in a MPC.

+90<sup>0</sup>

Copper rod

Electrode

Front row

Plasma column

Fig. 7. A schematic diagram of measurement method showing MPC in triangular shape inside a glass chamber for measuring microwave transmission through MPC with and

Ga

MPC


Microwav e generator (2-28 GHz)

0

Vacuum

Glass chamber

Flange

**Spectr um analyz er** 

Fig. 8. (a) Measurement method of transmission of 18 GHz with electrode only (b) with

So far, it has been successfully demonstrated how forbidden bands and flat bands can be formed by hybrid plasma photonic crystal. Although this study reveals importance and applications of plasma to control the microwave propagation in PCs, some minor problems are realized during the experiment e.g. adjustment of the electrodes, initiation of breakdown for discharge and sustaining plasma for long time because plasma column is formed inside the MPC where it is surrounded by metallic rods, which creates capacitive effects. Here it may be quite interesting to use plasma column between transmitter and MPC. Hence in the

without plasma columns.

plasma column at the central default of MPC.

next section, attention is paid to the study of microwave propagation when plasma column is formed between transmitter and front row of MPC.

Fig. 9. Variation in the microwave transmitted power at different angles with electrodes only and plasma column within electrodes at the centre hole or default.
