Different materials have been used as the medium to form a laser structure, and semiconductor is a popular material. Explain why and how. Based on the threshold condition for a laser, write down the relation between the threshold gain, the absorption loss per unit length of the medium, and the losses due to reflections at the two mirrors. If a GaAs laser has an absorption loss of 10 cm, a refractive index of 3.5, and the length is 300 um, estimate the threshold gain. For a photodetector, its internal quantum efficiency is 80% under incident laser light at 900 nm wavelength, and the reflection and transmission are 10% and 30%, respectively. Calculate its photoresponsivity