| 摘要 |
PROBLEM TO BE SOLVED: To efficiently obtain a Fresnel lens having a large numerical aperture(NA), to efficiently condense incident light to a small spot diameter and to enhance resolution by forming a diffraction grating on the surface of a substrate comprising a material having a specified refractive index. SOLUTION: A diffraction grating 17 is formed on one of parallel principal surfaces of a flat substrate 11 comprising an optical material having a refractive index (n) of >=1.5 to obtain the objective optical element 10. The optical material is, e.g. a material based on GaP(gallium phosphorus, n=3.3), GaN(gallium nitrogen, n=3) or SrTiO3(strontium titanium oxide, n=2.8). The substrate 11 preferably comprises GaP, GaN or SrTiO3, in particular GaP. Parallel light beams Le incident on the optical element 10 are efficiently diffracted by the diffraction grating 17 and emitted as parallel light beams Ld.
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