nf = 10; # number of frequencies to sweep
lambda = linspace(0.4e-6,0.7e-6,nf); # wavelength range
neff = matrix(nf);

for(i=1:nf){
select('source1');
set('wavelength start',lambda(i));
set('wavelength stop',lambda(i));
updatesourcemode;
n_effective=getresult("source1","neff");
neff(i)=n_effective.getattribute("neff");
}

plot(lambda*1e6,neff,'wavelength (um)','effective index','neff of mode1');
legend('real(neff)','imag(neff)');