Correct way to write "to the power" with an indexed parameter

Question

Quick (and simple) question: I'm writing a formula that uses parameters with a subscript, and they are squared. What is the correct way to typeset that? See below.

I fear that in (1) the $^2$ might be seen as an additional characteristic and not as "squared".

1. $s_1^2$
2. ${s_1}^2$
3. $({s_1})^2$