NOVELTY - The device (100) has a luminous element (10) i.e. LED, formed with a light emitting surface (S). A gradient-index nanoparticle layer (20) comprises a set of dielectric layers (21, 22) with different refractive index of nanoparticles, where diameter of the nanoparticle is about 10-100 nm. The dielectric layers are stacked upward from the light emitting surface to allow the gradient-index nanoparticle layer to cover the light emitting surface, where the refractive indexes of the dielectric layers decrease bottom up. A set of scattering particles is arranged on the nanoparticle layer.    USE - Optical device i.e. top emission light emitting device. Can also be used for an organic light emitting device, a thin-film solar cell and an organic solar cell.    ADVANTAGE - The gradient-index nanoparticle layer covered on the light emitting surface of the device ensures anti-reflection effect so as to reduce reflection on the light emitting surface effectively. The device adopts simple spin coating technique to arrange the dielectric layers on the light emitting surface of the luminous element so as to ensure low-cost and compatible light extraction efficiency (LEE) structure on the luminous element.    DETAILED DESCRIPTION - The LED is selected from one of top emission LED and flip chip LED. The nanoparticle is selected from group consisting of titanium dioxide nanoparticle, zirconium dioxide nanoparticle, indium tin oxide nanoparticle, zinc oxide nanoparticle, dysprosium oxide nanoparticle, yttrium oxide nanoparticle, tantalum pentoxide, aluminum oxide nanoparticle and silicon dioxide nanoparticle. The scattering particle is selected from a group consisting of titanium dioxide, zirconium dioxide, indium tin oxide, zinc oxide, dysprosium oxide, yttrium oxide, tantalum pentoxide, aluminum oxide and silicon dioxide.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of an optical device.    Light emitting surface (S)    Luminous element (10)    Gradient-index nanoparticle layer (20)    Dielectric layers (21, 22)    Optical device (100)