However, before we define our first SPRAY interface we need to have at least two different materials (or refractive indices). Up to now our list of dielectric functions contains vacuum only. Hence we have to create a new entry. Setting up new dielectric functions is not easy since you have to now a lot about materials and their optical constants. For the present exercise, however, a very simple model will do the job. For the material the cylinder is made of we will use a constant and real dielectric function, which means that there is no dispersion and no absorption. In this case the refractive index (which is one of the complex square roots of the dielectric function in general) is also real.
Open the list of dielectric functions by the 'Dielectric functions' button in the main window. Select the object type 'Dielectric function model' and press the '+' button to create a new dielectric function object. Overwrite 'dummy name' by 'Fiber core' and open the new object by Edit. Select the Range command and set a spectral range 500 ... 5000 1/cm with 100 data points. Then press the 'a' key on the keyboard for an automatic scaling of the graph. You should have arrived here:
Use the command Susceptibilities to open another list which contains the susceptibility contributions to the dielectric function. Select there the new type 'Dielectric background' and create an entry of this type by pressing '+'. Move the cursor to the '1.000' cell and change it to '2.25':
Close this list and select 'Recalc' in the 'Fiber core' window, followed by another 'a' for automatic re-scaling:
The blue line shows the real part of the dielectric function (=2.25) whereas the red line displays the imaginary part (=0). The corresponding refractive index of this fiber core material is 1.5, like that of glass in the visible spectral range. Later on we will create more complex optical constants but for the moment this one will be good enough.
We can now define the first interface that we will use for the cylinder walls in the next section. Go to the main SPRAY window and open the list of interfaces by the 'Interfaces' button.
Select the new object type 'Layer stack' and create an item of this type ('+' button). Rename the new entry (initially called 'Dummy name') to 'Vacuum-fiber'. The interface list should now look like this:
Open the layer stack window by Edit (or a click with the right mouse button). You are now here:
The layer stack defines an interface between the top halfspace and the bottom halfspace. Later we will add a thin film (representing a coating of the cylinder) but at present we just want to replace the vacuum entry of the bottom halfspace. Open (from the main window) the list of dielectric functions and drag with the mouse the row named 'Fiber core' to the 'Vacuum' entry of the bottom halfspace (i.e. line 2 in the current layer stack). You must place both the list of dielectric functions and the 'Vacuum-fiber' layer stack on the screen to perform this drag&drop operation. Your layer stack should now look like this:
Close all windows except the main window. We are now ready to 'coat' a geometric object like a cylinder with the interface we just defined.
(The present status can be found in the configuration file tu1_step6.s99).