[2] [2] Marcband E W. Gradient index optics. Academic Press, 1978

[5] [5] Kenichi Lga. Evaluation and reduction of aberrations in distributed-index lens.Applied Optics, 1982,21(6): 1024～1030

[6] [6] Yamashi T. Gradient-index rod with high N.A.Applied Optics, 22(3): 400～403

[7] [7] Betzig E,Trauman J K,Harris T D,et al.Breaking the diffraction barrier:Optical microscopy on a nanometric scale.Science, 1991,251: 1468～1470

[8] [8] Stockl R,Fokas C,Deckert V,et al.High-quality near-field optical probes by tube etching.App Phys Lett, 1992, 75(12): 160～162