Highly sensitive optical probe for uniaxial strain in GaN epitaxial films

We have previously demonstrated that a spectrally‐resolved four‐wave mixing highlights the anisotropically polarized excitons in the uniaxially strained GaN films [1], where the enhancement of sensitivity for an uniaxial strain was achieved by nonlinear response of the exciton polarization. In this work, we have applied this technique to the GaN films on various substrates including isotropic ones, and have examined the sensitivity of the technique. The results show distinct polarizations and energy variations originating from the uniaxial strain, the magnitude of which depends on the sample and its position. The minimum changes of the polarized FWM intensity and exchange energy splittings correspond to a uniaxial strain of 5.0 × 10–5, which currently gives a lower resolution limit of this technique. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)