This section describes a number of techniques for calculating power absorption per unit volume (Pabs) due to material absorption. The files in this section were created using FDTD, but a similar approach can be applied for MODE Solution's propagator. Once Pabs is known, it is easy to calculate absorption in some volume by integrating the loss function over that volume.

Lumerical provides many built in analysis groups in our object library. Please press this button to open the online library of analysis groups and select optical power category to insert the analysis groups shown here.

The following sections provide three techniques for calculating power absorption per unit volume (Pabs).

## Simple method

This section describes the standard technique used to calculate power absorption per unit volume. Power absorption is proportional to the electric field intensity and the imaginary part of the permittivity. Therefore, the calculation requires field data from a frequency domain power monitor, and permittivity data from an index monitor.

## Advanced method

The advanced method is an extension of the simple calculation that minimizes interpolation errors near interfaces. This technique is appropriate for simulations with large field discontinuities at the structure interfaces. Such discontinuities are common in simulations involving metals.

## Divergence of Poynting vector

This section describes how to calculate the absorption per unit volume from the divergence of the Poynting vector. Divergence calculations tend to be very sensitive to numerical problems, which makes this technique inferior to the other techniques described in this section. This method is presented only because it was a recommended approach in earlier versions of FDTD.