Collaborative Research: Nanoprobes for mapping the spatiotemporal evolution of ultrafast optical vector near field

光学纳米技术的进步使其得到了广泛的应用，例如提高了对具有等离子体纳米粒子的少数分子的检测灵敏度。为了量化光学器件的性能和开发新的性能，必须用纳米级的空间分辨率和飞秒级的时间分辨率来测量超快光场的行为。为了应对这一挑战，该计划产生了一种新型的纳米探测器，与定制的近场扫描显微镜系统集成在一起，用于全面表征超快光学近场。该研究项目将通过结合三所大学不同研究领域的三名研究人员的专业知识来增进理解。超快光学和纳米光学的激动人心的领域将为研究生和本科生以及K-12学生在实验室、课堂和外展活动中提供极好的教育机会。研究生和本科生将通过纳米探针制造、近场扫描光学显微镜系统的应用、光学测量以及数值模拟和检索等研究活动在三所大学的协作环境中进行培训。结果将被纳入课程。为了与研究人员之前的项目保持一致，将鼓励女性和未被充分代表的群体参与该计划。该计划的目标是开发一种基于纳米探测器的表征方法，可以在纳米-飞秒尺度上绘制超快光矢量近场的时空演变图。纳米探头由放置在纳米线上的二阶非线性纳米晶体或近场扫描光学显微镜(NSOM)探头组成，将与定制的NSOM系统集成，以实现样品-探头距离控制和纳米级空间分辨率。利用纳米晶体的非线性响应(即二次谐波产生)，可以通过共线倍频分辨光学门控(FROG)全息术来表征局域超快场的振幅和相位分布以及时空演化。由于存在较强的本振和对零差探测的依赖，青蛙全息术也将提高测量灵敏度。最后，利用纳米探针产生的偏振倍频信号探测局域超快光场的偏振特性。由于二次谐波信号具有不同的波长，因此它对基场的反射或散射产生的任何背景噪声都不敏感。此外，二阶非线性张量由晶体结构等材料性质决定，并且在很大程度上与颗粒形态无关，从而产生了更可控的纳米探针传感器。对局部时空场的了解提高了从等离子体结构中量化光谱信号的能力，这是纳米光谱学中的一个长期挑战。

Advances in optical nanotechnology have enabled a wide range of applications, such as increased sensitivity for the detection of just a few molecules with plasmonic nanoparticles. To quantify the performance of optical devices and develop new capabilities, it is essential to measure the behaviors of ultrafast optical fields with nanoscale spatial resolution and femtosecond scale temporal resolution. To address the challenge, this program engenders a novel type of nanoprobe, integrated with a custom near-field scanning microscope system, for comprehensive characterization of the ultrafast optical near field. The research program will enhance understanding by combining the expertise of three researchers in different research areas at three universities. The exciting areas in ultrafast optics and nano-optics will provide excellent education opportunities for graduate and undergraduate as well as K-12 students in the lab, in the classroom, and through outreach activities. Graduate and undergraduate students will be trained through the research activities such as nanoprobe fabrication, application of near-field scanning optical microscope system, optical measurements, and numerical simulation and retrieval in a collaborative setting across the three universities. Results will be incorporated into courses. In keeping with prior projects of the researchers, women and underrepresented groups will be encouraged and expected to participate in the program.The goal of this program is to develop a nanoprobe based characterization method that can map the spatiotemporal evolution of ultrafast optical vector near field in nanometer-femtosecond scale. A nanoprobe, which consists of a second order nonlinear nanocrystal perched on a nanowire or a near-field scanning optical microscope (NSOM) probe, will be integrated with a custom-built NSOM system to achieve sample-probe distance control and nanoscale spatial resolution. The nonlinear response of the nanocrystal (i.e., second harmonic generation -SHG) can be exploited to characterize both the amplitude and the phase profiles of the local ultrafast field as well as the spatiotemporal evolution through the collinear SHG frequency resolved optical gating (FROG) holography. Due to the presence of a strong "local oscillator" and the reliance on homodyne detection, FROG holography will also improve the measurement sensitivity. Finally, polarized SHG from the nanoprobe is utilized to probe the polarization of the local ultrafast optical field. Since the second harmonic signal has a distinct wavelength, it is insensitive to any background noise generated by the reflection or scattering of the fundamental field. Further, the second order nonlinear tensor is determined by the material properties such as the crystal structure and is largely independent of the particle morphology, leading to a more controllable nanoprobe sensor. Knowledge of the local spatiotemporal fields enhances the capability to quantify spectroscopic signals from plasmonic structures, a long-standing challenge in nanospectroscopy.