In a spin laser, the threshold current density for lasing to occur depends on the polarization of the current. The current density for electrons with spin up (red) and down (blue) is denoted by the rectangles’ heights; dashed line shows the threshold current required for lasing in a “spin-unpolarized” laser. (Left) The densities of spin-up and spin-down electrons are equal, , and none of the two subpopulations reaches the threshold. (Middle) If the same current density is injected, but the current contains spin-polarized electrons (), it is possible to reach a lasing threshold. This current is the first threshold. Injecting even more electrons with the same spin polarization (right), leads to the second threshold.