Stress modulation spectra

The discussion above illustrates how the application of hydrostatic or uniaxial stress can shift the exciton energy and features such as peaks A and A in the reflection spectrum. If such pertu2d>ations aire applied periodically and the reflected light analysed using a phase sensitive detector than various spectral features %diich are not obvious in the direct reflection spectrum can be enhanced (30). The reflectance modulation is given by... 

As with other modulation techniques, the idea is to measure the reflectance while some external parameter is varied periodically. The modulation of the reflected beam can be separated from the unmodulated background, and the resulting spectrum represents some kind of derivative of the original spectrum, showing up its singularities in fine detail. The external parameter may be electric field, pressure or other stress, temperature, or even rotation of the sample. Of these, the first has proven most useful. Electroreflectance is therefore currently the best method for picking out the singularities... 

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