Exponential Weight Volatility Average

to write a function that will exponentially weight a series of returns by a certain decay factor.

I have excel spreadsheets that can do this over several columns, like this

series of returns... rtns squared
2.50% =2.50%*2.50%
3.50% =3.50%*3.50%
5.00% =5.00%*5.00%
1.01% =1.01%*1.01%
0.75% =0.75%*0.75%
4.01% =4.01%*4.01%
-2.78% =-2.78%*-2.78%