How to get modeshapes from fft

Hi all,

I have a vector of length 1502 rows x 1 column, Flux. I am combining all the flux values from a simulation on a matrix of dimensions 1502 rows x 20 columns, as 20 are the samples. How can I use 'fft' as a means of getting the modeshapes for each harmonic of each sample? Is there or package I could use for good visualisation?