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Diffraction enhanced x-ray imaging (DEI) has simultaneous contrast sources from absorption, x-ray refraction gradients, and scatter-rejection (extinction). The combination of these contrast mechanisms generally allows many more features in objects to be observed compared to conventional radiography. In some instances or in specially prepared systems it is possible to eliminate one of the contrast mechanisms so as to create features that arise from a single contrast mechanism. With the DEI technique it is most interesting to either eliminate the absorption contrast of an object or conversely eliminate the refraction gradient contrast. We have explored both extremes of this contrast scale in order to better understand the DEI contrast mechanisms and to exploit the absence of a contrast mechanism for technical purposes. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have observed an extremely narrow absorption spectrum due to bound-to-continuum transition in GaAs/Al[sub x]Ga[sub 1-x]As multiple quantum wells (MQWs). Its linewidth is only about one tenth of the values reported previously. Our calculation indicates that the broadening of the excited state in the continuum has little contribution to the absorption linewidth. We have grown a sample whose MQW region contains two kinds of wells with a minor thickness inhomogeneity. Its resultant absorption linewidth is six times as large as that of homogeneous well sample, which is in good agreement with our theoretical analysis. Thus we can suggest that the wider absorption spectra reported by many authors may be due to the well width inhomogeneity. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]