Axis help in plotting

[Rony150640]

Anyone here knows how to add a superscript to an axis label? For example like in the photo. I can't find these commands anywhere
I want to add them all at once but I don't know-how.

[technocrat]

library(ggplot2)
p <- ggplot(mtcars,aes(mpg,wt))
x_lab <- bquote("chl-a(" ~ mgm^-3 ~ ")")
y_lab <- bquote("POE(" ~ mgm^-3 ~ ")")
p + xlab(x_lab) + ylab(y_lab) + theme_minimal()

^{Created on 2020-03-14 by the reprex package (v0.3.0)}