Introduction to spectrum simulations

Reflectance, transmittance, ATR and related spectra can be computed using objects of type 'R,T,ATR'. Opening these objects with the Edit command in the spectra list you get access to a window like this:

You have to set several items to uniquely define the spectrum to be simulated:

Layer stack

If you work with several layer stacks you have to assign the layer stack for this spectrum. Open the list of layer stacks with Objects|Layer stacks in the main window:

Start a drag&drop operation from the layer stack you want to use. Drop it onto the white field below the label 'Layer stack' in the spectrum window. Convince yourself that the right layer stack has been assigned after this action.

If you want to compute reflectance and transmittance spectra for laterally structured samples you can use spectra of type 'Layer mix' instead of the type 'R,T,ATR'.

Range

The Range command lets you define the spectral range of the simulated spectrum. As in previous SCOUT versions this range is not required to be the same as the one used for the dielectric function definitions.

Spectrum type

Here you can choose between different quantities that are computed. At present the following options are possible:

Angle

This is the angle of incidence being zero at normal incidence of light and 90° at oblique incidence. As mentioned above the light is incident from the top halfspace of the layer stack.

You can also describe experiments where the illumination comes from the bottom of the the sample by setting the angle to a value between 90° and 180°. If you choose 172°, for example, the spectrometer beam comes from the sample's back with an angle of incidence of 180°-172°=8°.

Averaging over several angles of incidence

Sometimes it is important to average optical spectra for a distribution of angles of incidence. You can do this in SCOUT the following way. Open with Object|Angle of incidence distribution a list where you can define (by pushing the '+' button) as many angles of incidence as you like. Each angle gets a weight:

As soon as there is an entry in this list SCOUT ignores the angle setting in the spectrum window (see above) but computes for each angle in the list the spectrum. Each spectrum is multiplied by the weight of its angle of incidence. Then all spectra are added to the final spectrum which is displayed in the spectrum window. Note that the user, i.e. you, is responsible for the correct normalization of the weights (at least up to now). Of course, the sum of all the weight values should sum up to unity to represent a nice distribution function.

If you want to return to the normal setting of a single angle of incidence in the spectrum window you have to delete with the '-' button all entries in the list of angles of incdicence.

Polarization

For the polarization the following settings can be used:

Manipulation

After the computation of one of the spectrum types listed above you can apply a user-defined formula to finally modify the spectrum. By using a special keyword in the formula you can refer to the reference spectrum mentioned below. See the section on spectrum normalization for details.

ATR diel. const.

If you compute ATR spectra, the incident beam is incident from a material with an almost real refractive index which is higher than the  real part of the refractive index of the first material in the layer stack. You could replace the usual 'Vacuum' in the top halfspace of the layer stack definition by a suitable material and compute a regular reflectance spectrum. This is tedious and not very practical if you want to simultaneously analyze reflectance, transmittance and ATR spectra of one and the same sample. You would have to work with two different layer stacks in this case.

In order to avoid this complication, SCOUT lets you define the parameter ATR diel. const which is used only if you compute ATR spectra. In this case the top halfspace material assignment (usually Vacuum, as mentioned above) is ignored and a material with a constant and real dielectric function is used instead. The value of the constant real part of the dielectric function is given by the parameter ATR diel. const. The square root of this number is the refractive index of the ATR element.

Typical values are 11.7 for silicon and 16 for germanium in the infrared, or 2.25 for glass in the visible.

Import of experimental spectra

Experimental spectra can be imported using the Import command. After data import the experimental data are multiplied automatically by the spectrum defined in the Reference window.

Computing the deviation

The deviation of the simulated and measured spectrum is computed the following way: For each data point of the simulated spectrum the value of the measured spectrum is computed by linear interpolation. The difference of the simulated and measured value is squared. If a weight function (at the bottom of the window) is defined the value of the weight function at the spectral point is evaluated und used to multiply the squared difference. Finally the average of these numbers for all data points is computed. This is the deviation.

Special actions

Objects of type 'R,T,ATR' can perform some special actions which may be useful in some situations: