An anomalous absorption band was observed around 1.2 eV in Si1-xGex alloy-based structures by photoluminescence excitation spectroscopy and photocurrent measurement. From spectral changes after mechanical lapping and subsequent air-borne annealing, it is found that the 1.2 eV absorption band develops only after oxidation of the alloy surface. The 1.2eV band survived up to 240 K in photocurrent measurements, which promises a wavelength selective detector operating near room temperature.