A quantum cascade laser includes a semiconductor substrate and an active layer having a cascade structure, in which unit layered bodies, each composed of a quantum well light emitting layer and an injection layer, are stacked, wherein the unit layered body has a subband level structure having an upper laser level, a lower laser level, and a relaxation miniband composed of at least two energy levels with an energy spacing smaller than the energy difference (E.sub.UL) between the upper laser level and the lower laser level, the energy width of the relaxation miniband is smaller than the energy (E.sub.LOE.sub.UL) obtained by subtracting the energy difference (E.sub.UL) from the energy (E.sub.LO) of longitudinal optical phonons, and electrons subjected to the intersubband transition are relaxed in the relaxation miniband and are injected into a quantum well light emitting layer in a subsequent unit layered body.

Provided herein are systems and methods for switching the generation of light emissions using charge separation in a gain medium to manipulate carrier lifetimes. For a given output pulse energy, extended carrier lifetimes may allow carrier generation powers to be reduced and/or carrier generation times to be extended. L-switching of light output from a gain medium may be combined with other switching schemes utilizing different approaches to control lasing, such as Q-switching.

A method for electrical and optical bistable switching, including the following steps: providing a semiconductor device that includes a semiconductor base region of a first conductivity type between semiconductor collector and emitter regions of a second conductivity type, providing a quantum size region in the base region, and providing base, collector and emitter terminals respectively coupled with the base, collector, and emitter regions; providing input electrical signals with respect to the base, collector, and emitter terminals to obtain an electrical output signal and light emission from the base region; providing an optical resonant cavity that encloses at least a portion of the base region and the light emission therefrom, an optical output signal being obtained from a portion of the light in the optical resonant cavity; and modifying the input electrical signals to switch back and forth between a first state wherein the photon density in the cavity is below a predetermined threshold and the optical output is incoherent, and a second state wherein the photon density in the cavity is above the predetermined threshold and the optical output is coherent, said switching from the first to the second state being implemented by modifying the input electrical signals to reduce optical absorption by collector intra-cavity photon-assisted tunneling, and the switching from the second to the first state being implemented by modifying the input electrical signals to increase photon absorption by collector intra-cavity photon-assisted tunneling.

A method for electrical and optical bistable switching, including the following steps: providing a semiconductor device that includes a semiconductor base region of a first conductivity type between semiconductor collector and emitter regions of a second conductivity type, providing a quantum size region in the base region, and providing base, collector and emitter terminals respectively coupled with the base, collector, and emitter regions; providing input electrical signals with respect to the base, collector, and emitter terminals to obtain an electrical output signal and light emission from the base region; providing an optical resonant cavity that encloses at least a portion of the base region and the light emission therefrom, an optical output signal being obtained from a portion of the light in the optical resonant cavity; and modifying the input electrical signals to switch back and forth between a first state wherein the photon density in the cavity is below a predetermined threshold and the optical output is incoherent, and a second state wherein the photon density in the cavity is above the predetermined threshold and the optical output is coherent, said switching from the first to the second state being implemented by modifying the input electrical signals to reduce optical absorption by collector intra-cavity photon-assisted tunneling, and the switching from the second to the first state being implemented by modifying the input electrical signals to increase photon absorption by collector intra-cavity photon-assisted tunneling.