Strain can be added to each layer in the MQW stack. The MQW Solver Physics page details how this strain affects the MQW solver gain calculation.

## Methods for Defining Strain

There are two methods for defining the strain in the MQW stack layers: directly defined by the user or calculated with a reference material. The reference material calculation is based on the differences between the lattice constants of the different stack materials. In this calculation, the strain in each layer is defined as biaxial with the diagonal strain tensor components given as

$$\epsilon_{xx}=\epsilon_{yy}=\frac{a_0-a}{a}$$

$$\epsilon_{zz}=-\frac{2C_{12}}{C_{11}}\epsilon_{xx}$$

where \(\epsilon_{ij}\) are the components of the strain tensor, \(a_0\) is the reference lattice constant, \(a\) is the lattice constant of the layer material, and \(C_{11}\) and \(C_{12}\) are the elastic stiffness coefficients. This formulation covers one of the most important strained system: that obtained by a quantum well pseudomorphically grown on a (001)-oriented substrate. This means the Hamiltonion matrix terms \(R_\epsilon\) and \(S_\epsilon\) are equal to 0 (see MQW Solver Physics for more information on these terms in the Hamiltonian matrix).

## Directly Defined Strain

To directly define the strain:

- Turn off the MQW solver’s
**calculate strain**property on the**Layers**tab of the**Edit MQW Gain Solver**window. - Enter the normalized strain value epsilon_xx for each layer in the
**Strain**column of the layer table. A negative value represents compressive strain.

## Strain Calculated With a Reference Material

To have the strain calculated with a reference material:

- Turn on the MQW solver’s
**calculate strain**property on the**Layers**tab of the**Edit MQW Gain Solver**window**.** - Set the
**reference material**to your chosen reference material from the materials in the simulation.