Phase shifting a Pulse

Hi everyone,

please i need some help in my programme, i want to do the FFT of a pulse and then phase shifting every spectrum with his frequency multiplied by 2, i get a forme that its not correct as the teacher showed us last time.

here is my program :

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%PULSE%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
fe=1e+8;
te=1/fe;
subplot(2,1,1);
t=[0:te:(1e-6)-te]; 
A=2;                    
x=A*trapmf(t,[100e-9 105e-9 205e-9 210e-9]); 
plot(t,x);
xlabel ('Temps(s)');
ylabel ('Amplitude(V)');
title ('Pulse de tension');
grid on;
%%%%%%%%%%%%%%%%%%%%%%%%%%%**FFT**%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
subplot(2,1,2);
Xf=fftshift(fft(x));
df=fe/length(x)
f=[-fe/2:df:fe/2-df];
subplot(2,1,2);
stem(f,abs(Xf),'r');
grid on;
xlabel('frequence');
ylabel('X(f)');
%%%%%%%%%%%%%%%%%%%%%%%**Phase shifting**%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
ind = 1:numel(Xf)/2; %// lower half (negative frequencys)
phi = 2*f(ind); %/ desired phase shift (i used only the positive frequency, cause with all the vector F i get a matrix deminsions error !!)
Xff(ind) = abs(Xf(ind)).*exp(1i*angle(Xf(ind))+1i*phi); %// add 1i*phi
ind = numel(Xf)/2+1:numel(Xf); %// upper half (positive frequencys)
Xff(ind) = abs(Xf(ind)).*exp(1i*angle(Xf(ind))-1i*phi); 
%%%%%%%%%%%%-- IFFT---%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
y=ifft(ifftshift(Xff));
figure(2);
plot(t,real(y),'r','linewidth',2);
hold on;
plot(t,x,'linewidth',2);
grid on;

Thank youu!