vector values substitution in an expression using label_bquote in ggplot()

Dear Sir

I am using label_bquote() for labeling facet plots in ggplot(). I need to
write the expression a_[12], a_[13] etc for each plot. I am writing as

ggplot() + facet_grid(..., labeller=label_bquote(rows = a[1, 2:3])

The above exemplary code writes only a_[12] for each facet plot.

Could anyone please guide me about that?
Thank you

Shaami