1. a) You are designing semiconductor quantum well lasers in both vertically-emitting and edge-emitting formats. Explain, with the assistance of labelled sketched graphs and diagrams as appropriate, why you would use strained quantum wells to minimize the laser threshold in each case and explain the resulting polarization of each laser. You may assume all other laser parameters are optimized.
b) Comment on the polarization of an edge-emitting laser using InGaAs quantum wells with GaAs barriers to provide the gain. [Hint: compare the quantum well lattice constant and the barriers lattice constant to determine if the quantum wells are strained].
2. a) Explain, with the use of graphs if necessary, the principle of operation of long-wavelength photodetectors based on intersubband transitions. Is doping of the semiconductor required? If yes, why and of which type (n or p)?
b) Calculate the wavelength of incident light for which peak intersubband absorption occurs in an n-type doped, 10-nm-thick GaAs quantum well. Consider an infinite quantum well and a transition between the first and second conduction band levels.
3. Explain, with the use of sketches, how the so-called 'W-shaped' band alignment of a quantum well can be used to extend the emission range of a material to longer wavelengths.