在聚焦光场矢量衍射理论的基础上，利用贝塞尔函数的微分递推关系，推导了聚焦光场的梯度变化表达式。根据中心对称材料二次谐波产生的唯象模型，研究了单个中心对称球形纳米颗粒在聚焦光场激发下的二次谐波产生，考察了表面响应和体效应产生的二次谐波辐射分布随着数值孔径增大的变化规律，并利用聚焦光场及其梯度场分布特性讨论差异产生的原因。研究结果表明，激发光场的聚焦程度对体响应的影响更为显著。

Based on vectorial diffraction theory, an analytical integral representation to calculate the field gradient of focused electromagnetic fields is derived by use of the differential recursion formula of the Bessel functions with different order. Within the phenomenological model for second harmonic generation (SHG) of centrosymmetric material, second harmonic (SH) generation of single centrosymmetric spherical particle excited by a focused beam is investigated. With increasing numerical aperture, the SH radiation distributions of the surface and bulk responses are compared and discussed in term of the properties of the intensity and gradient distributions of the focused field. The results indicate that the influence of the variation of the focused field on the bulk response is more remarkable.