What is an advantage of using a double heterostructure (DH) semiconductor laser instead of a homojunction semiconductor laser? (There are two correct answers here - just pick one!) a. The DH laser has better photon confinement than the homojunction laser. b. The DH laser can be made from less expensive indirect bandgap materials. c. The DH laser has a lower threshold current density than the homojunction laser. d. The DH laser uses phonon interactions to improve quantum efficiency.