Jarrett Jeremy W, Herbert Patrick J, Dhuey Scott, Schwartzberg Adam M, Knappenberger Kenneth L
Department of Chemistry and Biochemistry, Florida State University , Tallahassee, Florida 32306-4390, United States.
J Phys Chem A. 2014 Sep 18;118(37):8393-401. doi: 10.1021/jp501488k. Epub 2014 Mar 11.
The Nonlinear Optical Localization using Electromagnetic Surface fields (NOLES) imaging technique was used to generate optical images in which the position of a chiral object could be determined with nanometer precision. Asymmetric gold bowtie nanostructures were used as a model system with 2D chirality. The bowties functioned as a chiral nonlinear medium that converted the fundamental of a Ti:sapphire laser to its second harmonic frequency. The bowties consisted of two lithographically prepared equilateral triangles (base = 75 nm, height = 85 nm, thickness = 25 nm) separated by a 20 nm gap. Asymmetric bowties were formed by lateral displacement of one triangle by 10 nm, yielding C2 point group symmetry. The chirality of the bowtie nanostructures was confirmed via nonzero second-harmonic generation circular dichroism (SHG-CDR) ratios, which came from single-particle SHG measurements. The SHG-CDR ratios were validated using numerical finite difference time domain simulations that quantified the relative magnitudes of gap-localized electromagnetic fields at the harmonic frequency resulting from excitation by left and right circularly (LCP and RCP) and linearly polarized fundamental waves. The relative electric dipolar and magnetic dipolar contributions to the SHG responses were determined using single-particle continuous polarization variation (CPV) SHG measurements. The spatial localization precision obtainable for individual chiral nanostructures was determined by statistical analysis of the SHG image point spread function. Our results demonstrated that both the chiral image contrast, which resulted from LCP and RCP excitation, and the corresponding localization precision was dependent upon the relative magnetic dipole/electric dipole ratio (G/F). A localization precision of 1.13 ± 0.13 nm and left-to-right image enhancements of 400% were obtained for bowties with the highest G/F ratios using 5 s frame exposure times. The polarization dependence and magnetic dipole amplification confirmed here demonstrate that the NOLES imaging technique is a powerful method for studying chiral specimens with high spatial precision.
利用电磁表面场的非线性光学定位（NOLES）成像技术被用于生成光学图像，其中手性物体的位置可以精确到纳米级来确定。不对称金蝴蝶结纳米结构被用作具有二维手性的模型系统。这些蝴蝶结起到手性非线性介质的作用，将钛宝石激光器的基频转换为其二次谐波频率。蝴蝶结由两个光刻制备的等边三角形组成（底边 = 75 纳米，高 = 85 纳米，厚度 = 25 纳米），中间间隔 20 纳米的间隙。通过将一个三角形横向位移 10 纳米形成不对称蝴蝶结，产生 C2 点群对称性。蝴蝶结纳米结构的手性通过非零二次谐波产生圆二色性（SHG - CDR）比率得到证实，该比率来自单粒子 SHG 测量。SHG - CDR 比率通过数值有限差分时域模拟进行验证，该模拟量化了由左旋和右旋圆偏振（LCP 和 RCP）以及线偏振基波激发在谐波频率下间隙局部电磁场的相对大小。使用单粒子连续偏振变化（CPV）SHG 测量确定了对 SHG 响应的相对电偶极和磁偶极贡献。通过对 SHG 图像点扩散函数的统计分析确定了单个手性纳米结构可获得的空间定位精度。我们的结果表明，由 LCP 和 RCP 激发产生的手性图像对比度以及相应的定位精度均取决于相对磁偶极/电偶极比率（G/F）。使用 5 秒帧曝光时间，对于具有最高 G/F 比率的蝴蝶结，获得了 1.13 ± 0.13 纳米的定位精度和 400%的左右图像增强。此处证实的偏振依赖性和磁偶极放大表明，NOLES 成像技术是一种用于以高空间精度研究手性标本的强大方法。
