The present invention relates to a high-density optical disk having thin film layer made from a thermochromic polymer with a red-shift which exhibits self-focusing and optical bleaching. After an incident light beam passes through the polymer medium, not only can the spot size of the beam be reduced but also transmittance can be enhanced due to these kinds of third-order nonlinear optical response. Thus, a high-density disk structured according to the present invention is capable of the signal reproduction resolution beyond the diffraction limit with an efficient reflectivity. Accordingly, data recording density of a high-density disk can be improved greatly.