Greek letters and Latex formula in R (to be continuted)

I tried to add $\omega_s$ as axis label. It seems some extra work need to be done.

plot(1:3, ylab = expression("Diameter of apeture (" * mu ~ "m)"),
  , xlab = expression("Force spaces with ~" ~ mu ~ pi * sigma ~ pi)
  , main = expression("This is another Greek character with space" ~ sigma))

http://stackoverflow.com/questions/6044800/adding-greek-character-to-axis-title

http://stackoverflow.com/questions/1395105/getting-latex-into-r-plots