凝聚态光物理学5课件.ppt

5;5.1 Light emission in solids;5.1 Light emission in solids;The PL was excited by absorption of 4.9 eV photons from a frequency doubled copper vapour laser. The spectrum consist of a narrow emission line at 3.5 eV close to the band gap energy, while the absorption shows the usual threshold at Eg with continuous absorption for h? Eg.;5.3 Photoluminescence 5.3.1 Excitation and relaxation ;5.3.2 Low carrier densities ;5.3.3 Degeneracy;5.3.4 Photoluminescence spectroscopy ;5.4 Electroluminescence;The main factors that determine the choice of the material: (1) The size of the band gap: Eg ? ?; kBT ? ? ?; Constraints relating to lattice matching; the ease of p-trpe doping. ;Grow thin ultra-pure layers on the top of a substrate by epitaxy.;5.4.3 Diode lasers;5.4.3 Diode lasers;5.4.3 Diode lasers;1. The band gap of the III-V semiconductor alloy AlxGa1-xAs at k=0 varies with composition according to Eg(x)=(1.420+1.087x+0.438x2)eV. The material is direct for x ? 0.43, and indirect for larger values of x. Light emitters for specific wavelengths can be appropriate choice of the composition. (i). Calculate the composition of the alloy in a device emitting at 800 nm. (ii). Calculate the range of wavelengths than can usefully be obtained from an AlGaAs emitter. 2. A very short laser pulse at 780 nm is incident on a thick crystal which has an absorption coefficient of 1.5?106 m-1 at this wavelength. The pulse has an energy of 10 nJ and is focused to a circular spot of radius 100 ?m. (i) Calculate the initial carrier density at the front of the sample. (ii) If the radiative and non-radiative lifetimes of the sample are 1 ns and 8 ns respectively, calculate the time taken for the carrier density to drop to 50 % of the initial value. (iii) Calculate the total number of luminescent photons generated by each laser pulse. (提示: 利用dI = -?· dz · I(z) ) 3. Explain why the emission probability for a