Multiple Quantum Well

A quantum well is a potential well with only discrete energy values.

Multiple quantum well

This Figure shows a typical structure of multiple quantum well (MQW). Compared with traditional active layer, MQW has step-like electronic density of states which is easy to obtain narrow linewidth lasing. Compared with single quantum well, the active layer enhances its optical confinement factor and ability to collect unbalanced carriers. The TE mode in MQW, it is should be noted, has larger gain than TM mode due to more heavy holes than light holes (1).
Due to quantum effect, in simplified model, the optical absorption in a quantum well is a series of steps. And as showed in figure below, it is the same rule as that in electron density. Further more, in the quantum wire is a series of peaks, and in the quantum dot in a series of line.

Density of states for electron in quantum structure

Optical Confinement Factor

Because the effective length of fundamental mode light field is much greater than thickness of MQW layer, the $\Gamma_M$ in quantum well can be consider as:

$$\Gamma_W \approx \Gamma_{MQW} \frac{N_W d_W}{N_W d_W + N_B d_B}$$

$$\Gamma_{MQW} = 2\pi^2 \left(N_Wd_W + N_Bd_B\right)^2 \frac{n^2 - n_e^2}{\lambda}$$

$$n = \frac{N_W d_w n_w + N_B d_b n_B}{N_W d_W + N_B d_B}$$

Where $N_W$ and $N_B$ are number of quantum well and its barrier, $d_W$ and $d_B$ are thickness of them, $n$ is average refractive index of MQW layer, $n_e$ is effective refractive index.

Reference

1. Optical physics of quantum wells
2. 量子阱半导体激光器_阿阳955的博客-CSDN博客