二次谐波的产生是一个二阶非线性光学过程， 这个过程只发生在中心对称系数不为零的非中心对称区域。 利用二次谐波显微技术可以对各种具有内源性信号的生物组织进行无损伤实时成像， 如结缔组织的胶原纤维或肌细胞的肌动球蛋白。 在生物化学和结构生物学中， 虽然DNA是由脱氧核糖核苷酸构成而蛋白质是由氨基酸残基组成， 但是DNA和蛋白质大分子高级结构的形成机制是相似的。 利用光谱学成像技术对不同DNA样品进行检测， 获取DNA样品的SHG信号并进行高解析度成像。 这些DNA样品包括基因组DNA溶液、 细胞核提取物以及培养细胞的细胞核。 实验结果表明在常规条件下可以获得基因组DNA溶液和细胞核提取物的SHG信号， 但几乎观测不到来自培养细胞核区的SHG信号。 通过在培养基中添加少量无水乙醇（体积比小于5%）， 可以在培养细胞的细胞核区域检测到SHG信号。 推测在培养细胞中乙醇和DNA相互作用引起DNA分子构象发生变化， 这些变化可能导致了DNA分子非线性光学性质的改变。

Second harmonic generation (SHG) is a second-order nonlinear optical process that has symmetry constraints confining signal to regions lacking a center of symmetry. Using SHG microscopy, a variety of tissue structures have noninvasively been imaged by virtue of intrinsic signal generated by structured proteins such as collagen fibrils in connective tissues or the actomyosin lattice of muscle cells. In biochemistry and structure biology, the high-level structures of DNA and protein macro-molecules are similar in constructing mechanism, although DNAs consist of deoxynucleotides and proteins of amino acid residues. The principal purpose of present work is to detect the SHG signal from different DNA samples by spectral imaging technology based on two-photon excited fluorescence (TPEF) and SHG. These DNA samples include the solution of genomic DNA and extracted nuclei, and cultured living cells. Results show that we can obtain the SHG signal from solution of genomic DNA and extracted nuclei in routine condition, but nothing from cultured cell nuclei. After adding a little of absolute ethanol (less than 5% by volume) in culture medium, the SHG signal is detectable in the interested region of nuclei. The findings suggest that the interaction between ethanol and DNA in living cell gives rise to the shift of molecular conformation, and this shift changes some nonlinear optical properties of DNA molecules.