Multiple-layer reflector

Modem design of efficient surface emitting semiconductor lasers implies monolithic solid state stmctures with an active layer and periodic multilayer stacks comprising Distributed Bragg Reflectors (DBR). The latter provides reflection band at the emission wavelength due to multiple reflection/interference in a complex medium with periodically graded refraction index of the layers [1], The larger is the refraction index difference A n = U/ - between a couple of materials chosen to... 

Comaish (1968) has applied the technique of multiple internal reflection (MIR) to intact human skin in vivo. Using a germanium reflector plate he obtained spectra (in a Wilks Skin Analyzer ) of normal skin (Fig. 18.7) and various dermatoses (e.g., Fig. 18.8) and found gross changes from the normal in the lesions of psoriasis, eczema, and other disorders. He has also demonstrated abnormalities in the clinically normal skin of psoriatic patients. The technique measures surface changes, so that only the outer layers of stratum corneum, sweat, sebum, and extraneous materials will affect the results obtained. Lactic acid, lactate, and ammonia from sweat probably contribute substantially to the bands at 2935, 1465, 1125, and 1045 cm . After the skin was washed and dried these bands were diminished or disappeared entirely. Any contributions the stratum corneum makes to the infrared spectrum are at 2870, 1650, and 1550 cm . ... 

The Virus House was finished in October. Besides a laboratory the structure contained a special brick-lined pit, six feet deep, a variant of Fermi s water tank for neutron-multiplication studies. By December Heisenberg and von WeizsScker had prepared the first of several such experiments. With water in the pit to serve as both reflector and radiation shield they lowered down a large aluminum canister packed with alternating layers of uranium oxide and paraffin. A radium-beryllium source in the center of the canister supplied neutrons, but the German physicists were able to measure no neutron multiplication at all. The experiment confirmed what Fermi and Szilard had already demonstrated that ordinary hydrogen, whether in the form of water or paraffin, would not work with natural uranium to sustain a chain reaction. 

In a distributed Bragg reflector, as shown in Fig. la, multiple alternating layers of material with contrasting physical properties give a high reflectivity for certain wavelengths due to the net effect of the multiple partial reflections at the interfaces between layers. The reflector consists of AA repeats of an [a, b layer pair. 

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