all the substrings of a given string S

Howdy everyone, I need to select all the substrings available from a given string S. By substring I mean any S(i:j), with j <= length(S) and i<=j. I wrote the following working code:

if true
voc{1}='';
cont=2;
for i=1:length(string)
  for j=i:length(string)
      voc{cont}=string(i:j);
      cont=cont+1;
  end
end
voc=unique(voc);
end

The variable voc contains all the substrings plus the element '' in the first position. I would need something faster than this. Any suggestion is appreciated. Thanks in advance