Frequency up-conversion of infrared laser radiation in the human retina

Experimental observation is reported of conversion in a human retina of infrared laser radiation (with a wavelength of 1.06 mu m) into the visible upon scattering by a solid target. The wavelength of this visible radiation, as estimated by several independent observers, is around 0.557 mu m. Frequency up-conversion is observed down to a peak power on the retina of only 150 W/cm(2), while, for other laser sources, this threshold is as low as 1 W/cm(2). It is suggested that the observed conversion is due to the second-harmonic generation in the periodic structure of the retina of a human eye. Deviation of the observed wavelength from that of the second harmonic in vacuum is ascribed to the spectral dependence of the refractive index of rhodopsin molecules within the retina.