lambda=532
w0 = 1; % beam waist
z = 0; % propagation distance
k = 2*pi/lambda; % wave number
l = 1; % topological charge
x = linspace(-5*w0,5*w0,256); % x-axis grid
y = linspace(-5*w0,5*w0,256); % y-axis grid
[X,Y] = meshgrid(x,y); % create meshgrid
R = sqrt(X.^2 + Y.^2);
phi = atan2(Y,X) + l*atan2(R,z);
u = exp(-(X.^2 + Y.^2)/w0^2) .* exp(1i*phi);
figure;
angu = angle(u);
imagesc(x,y,angu);
colormap('hsv');
axis equal tight;
xlabel('x');
ylabel('y');
caxis
[min(angu(:)), max(angu(:))]
cb = colorbar;
cb.Ticks = [0 0.5 1];
cb.TickLabels = {'0', 'pi/2', '2pi'};
Your actual data being drawn ranges from pretty much 
to 
. imagesc() leaves the data alone but changes the CLim (color axes limits) to match the actual range of data. You then ask for tick marks at the absolute positions 0, 0.5, and 1, rather than at some kind of relative position.
to . imagesc() leaves the data alone but changes the CLim (color axes limits) to match the actual range of data. You then ask for tick marks at the absolute positions 0, 0.5, and 1, rather than at some kind of relative position. You also label with 0 to 
even though the values are 
to 
even though the values are to I would suggest that what you want is
w0 = 1; % beam waist
z = 0; % propagation distance
k = 2*pi/lambda; % wave number
l = 1; % topological charge
x = linspace(-5*w0,5*w0,256); % x-axis grid
y = linspace(-5*w0,5*w0,256); % y-axis grid
[X,Y] = meshgrid(x,y); % create meshgrid
R = sqrt(X.^2 + Y.^2);
phi = atan2(Y,X) + l*atan2(R,z);
u = exp(-(X.^2 + Y.^2)/w0^2) .* exp(1i*phi);
figure;
angu = angle(u);
imagesc(x,y,angu);
colormap('hsv');
axis equal tight;
xlabel('x');
ylabel('y');
caxis([-pi pi])
cb = colorbar;
cb.Ticks = [-pi 0 pi];
cb.TickLabels = {'-\pi', '0', '\pi'};
