In this study, we have prepared aluminum oxide (Al 2 O 3 nanoparticles) NPs with size ranging from 50 to 90 nm by laser ablation of aluminum target in ethanol. The effect of laser fluence on the structural, morphological and optical properties of Al 2 O 3 was demonstrated and discussed. X-ray diffraction XRD results confirm that the synthesized Al 2 O 3 NPs are crystalline in nature. The sample prepared at 3.5 J/cm 2 /pulse exhibits single phase of c-Al 2 O 3 , while the XRD patterns of the nanoparticles synthesized at 5.3 and 7.5 J/cm 2 /pulse show the co-existence of the a-Al 2 O 3 and c-Al 2 O 3 phases. Nanostructured Al 2 O 3 films have been used as anti-reflecting coating and surface passivation layer to improve the photoresponse characteristics of silicon photodiode. The experimental data showed that the optical energy gap decreases from 5.3 to 5 eV as the laser fluence increases from 3.5 to 7.3 J/cm 2 . The lowest optical reflectivity was found for silicon photodiode deposited with a single layer of Al 2 O 3 prepared at 3.5 J/cm 2 /pulse. The effect of laser fluence on the refractive index and extinction coefficient of the nanostructured Al 2 O 3 film was studied. The photosensitivity of the silicon photodiode increased from 0.4 to 1.4 AW -1 at 800 nm after depositing Al 2 O 3 prepared at 3.5 J/cm 2 /pulse, followed by rapid thermal annealing at 400°C for 60 s.