As far as I know (or I thought I knew), if we have an electric field
$$\mathbf{E}=\mathbf{E_0}\cos(\omega t – kx),$$
we can define it as the real part of
$$\mathbf{E}=Re(\mathbf{E_0}e^{i(\omega t – kx)}).$$
Introducing imaginary components to the electric field is only a matter of making the maths easier, in particular in the case of complex exponentials which are eigenfunctions of the differential operators.
Now.
In a Ph.D. thesis I was reading, about lasers and cavity misalignment (which doesn't really matter, my question does not require any knowledge of this), I came across something along the lines of:
The electric field to first order misalignment can be written as $$\mid E\rangle = | 00 \rangle + i | 01 \rangle, $$ where the two kets on the LHS are the fundamental and 1st excited Hermite-Gaussian modes.
But the physical part of the electric field is surely its real part? What purpose can the imaginary component above serve?
Best Answer
The $i$ in front of the $|01\rangle$ tells you that the first order mode is $90^\circ$ out of phase with the fundamental mode. The absolute phase generally has no meaning so you could just as well have put $-i$ in front of the fundamental mode.
The phase difference between the two has physical meaning. When you take the projection onto the real axis one will be acting as a cosine and the other as a sine which means that one will be maximum when the other is zero and vice versa. This phase difference is important e.g. in detecting the misalignment between a laser and an optical resonator.
It might help to think about it in the rotating phasor picture. In this picture you think about arrows rotating at the frequency of the optical field, and the physical portion is the projection onto the real axis. Figure 3 on this page has a good example of rotating phasors which are $\sim 30^\circ$ out of phase.