9.2 solid-state lasers and semicondutcor lasers.pptVIP

Injection pumping Injecting carriers across the junction under a sufficiently large forward bias → population inversion Homojunction laser diode Homojunction: uses the same direct bandgap semiconductor material( GaAs) 31Ga (gallium, 镓) Optical cavity: the ends of the crystal cleaved to be flat and optically polished to provide reflection Wavelength of radiation built up in the cavity: m: integer; λ: wavelength; n: refractive index; L: length of the cavity m → mode → resonant frequency: Main problem of homojunction laser diode 1. The threshold current density Jth is too high for practical uses, in the order of ~ 500 A mm-2 for GaAs at room temperature → can only be operated at very low temperature 2. Methods to reduce Jth: (1) operate at very low temperature for homojunction semeconductor, not practical; (2) use heterostructured semiconductor laser diode Heterostructure laser diodes What are required to reduce Ith to a practical value? (1) improving the rate of stimulated emission; (2) improving the efficiency of the optical cavity. How to reduce Ith? (1) carrier confinement: confine the injected carriers to a narrow region around the junction → less current is needed to establish the necessary concentration of carriers for population inversion. (2) photon confinement: build a dielectric wave-guide around the optical gain region to increase the photon concentration and the probability of stimulated emission. Why use heterostructure? To reduce Ith. DH – double heterostructure Two junctions: n-AlGzAs ~ p-GaAs; p-GaAs ~ p-AlGaAs Eg(AlGaAs) ≈ 2 eV, Eg(GaAs) ≈ 1.4 eV p-GaAs: active layer of 0.1μm ~ 0.2 μm p-GaAs and p-AlGaAs: heavily p-type doped and are degenerate with EF in VB Even moderate forward currents can inject sufficient number of electrons into CB of p-GaAs and confined there to establish the necessary electron concentration for population inversion in this layer Photons are confined in the