It is unclear what you are asking.
If you are asking about raising a positive number to negative power, things are simple.
[MATH]a,\ b \in \mathbb R, \ a > 0, \text { and } b > 0 \implies a^{-b} = \dfrac{1}{a^b} = \left ( \dfrac{1}{b} \right )^b \in \mathbb R.[/MATH]
If you are asking about raising negative numbers to a power, that is not a problem if you are talking about integer powers, but rational powers take you outside the real number system.
[MATH]a \in \mathbb R,\ a < 0, \text { and } b \in \mathbb Z \implies[/MATH]
[MATH]a^{(2b)} = (-\ a)^{2b} \in \mathbb R \text { and } a^{(2b-1)} = -\ (-\ a)^{(2b-1)} \in \mathbb R.[/MATH]