Discussion Forum

Dear all,

I would like to compute the transmission through a phononic crystal supercell with periodicity a=500nm in silicon in 2D (for now, later slab) using comsol 4.4 and the Structural Mechanics module only. The type of structures I would like to study are found here arxiv.org/pdf/1003.5265v2.pdf .
I have previously worked on FDTD simulations of photonic crystals but I am new to comsol and phononics.
I believe I need to create a plane wave pulse ideally with a Gaussian profile so that I have a central frequency f (~10GHz) and a frequency width df (~10GHz).
As I understand it I need to apply a time varying boundary load to one edge of the domain, low reflecting boundary (or would pml be better) on the opposite and periodic boundaries on the adjacent sides.

There are several things I am unsure about:

1) How do I define my pulse as a force time function. The Gaussian pulse time function
g(t)=I_0*exp(-i*w*t)*exp(-((t-t_0)^2)*(dw^2)*0.5) should correspond to the displacement field itself. Would I get the correct force function by taking the second derivative? What load type do I have to choose?

2) How do I take the second derivative in comsol? I have tried d(d(g,t),t) but that gives an error about complex numbers and if I use real(d(d(g,t),t)) the simulation seems to converges much too slowly.

3) If I want to define my function as a global function should I define g(t) as an analytic or piecewise equation (both give errors), so far I have used it by writing it out explicitly in the boundary load.

5) What do I have to do to make it an S or P wave.

Thank you very much.