@acer making exponent smaller fullfills my requirement. Thankyou so much for your help. When I use labels=[Times, bold] I get a plot as below:

I wonder, if   font=[bold] in the  typesetting  allows making supperscript '2' bold (in the \theta^2  expression).?

Thanks,

@vv is it possible to reduce the size of sqrt? If i use font size 24, bold , sqrt size looks bigger compare to font cos .

@vv Thanks a lot! It really helps.

@Kitonum Thanks! It works.