Restoration of collection mode scanning near-field optical microscope image

Recently we have derived the angular spectrum transfer function(ASTF) for photon scanning tunneling microscope and collection mode scanning near field optical microscope(collection-SNOM) under dark illumination (Proc.SPIE, Vol.3467,23 similar to 33). The present paper discussed image restoration using the ASTF. It was found that the key to achieve better restoration is to find a ASTF as close to that of the system as possible, which then depends on the determination of tip size, tip sample distance, incident angle of illuminating laser beam, etc. When low values are used for tip diameter and tip sample distance improvement of image quality is obvious after restoration. But when the value adopted for tip sample distance is 0.05 lambda higher than real one, high frequency oscillation in the restored image become intolerable, which we call over restoration. Slight over restoration was also observed when the value employed for tip diameter is 0.02 lambda larger than real one. The appearance of over restoration on the other hand can be utilized for the estimation of tip diameter and tip sample distance of the system. Generally the image seems more sensitive to tip sample distance than to tip size.