Development of a Cryogenic Femtosecond Aptureless Near-Field Scanning Optical Microscope for Nanostructure Research

9802784 Levy该奖项提供部分支持，以开发一种可变温度的“无孔”近场扫描光学显微镜（ANSOM），用于研究纳米结构光电材料中的局部电子和晶格动力学。 该仪器将具有独特的和前所未有的能力，结合近原子空间分辨率，飞秒时间分辨率，宽工作温度范围（1.6K-400 K），并在高达10特斯拉的磁场中工作。 该仪器特别适用于研究与折射率变化相关的材料特性。 ANSOM的对比度机制来自于测量来自原子或磁力显微镜尖端的散射光中的小相移。 通过调制的尖端样品分离，它是可能的，以测量局部样品的极化率（电或磁）的长度尺度显着小于传统的光学或光纤为基础的近场扫描光学显微镜（NSOM）。 该仪器的设计也将允许它被用作共焦扫描光学显微镜，它可以聚焦或收集光与高效率和衍射有限的空间分辨率。 目前有几个低温NSOM在世界各地的实验室中运行。 这些仪器的亚波长空间分辨率通过扫描靠近样品的锥形光纤以形成图像来获得。 然而，无论是实际的和基本的限制，阻止光学分辨率接近原子力显微镜（AFM）或扫描隧道显微镜（STM）。 该仪器的独特功能将来自于将光学方法的传统优势（例如，时间分辨率或能量选择性）与原子力显微镜的空间分辨率。 使用ANSOM可以实现低于10 A的空间分辨率。 空间和时间分辨率的结合将为纳米尺度凝聚态系统的动力学过程开辟许多新的研究途径。 直接的科学应用有三个方面：（1）半导体量子点中激子退相的研究，（2）单共轭聚合物链中中性激发的光谱和时间分辨研究，以及（3）铁电体和量子顺电体中的晶格动力学和畴壁运动。 更长远的目标包括研究磁性薄膜中的磁化动力学，生物学中的应用（例如，DNA测序）和光学检测的磁共振。 该仪器的建设将受益于低温扫描探针显微镜领域的早期先驱的设计原则。 确保充分隔振、粗接近机制等的原则，已纳入拟议设计中。 许多关键的设计参数已被评估从一个室温的原型，这是目前被用来研究铁电薄膜中的畴动力学的建设。 该仪器的建设将为研究生和本科生提供宝贵的经验。 该项目将由首席研究员监督，他将指导一名博士后研究员，一名研究生和几名本科生。 研究生将与本科生密切合作，完成与构建拟议仪器的更大目标相关的各种子任务，例如为锁定放大器和温度控制器等仪器编写软件驱动程序。 项目的总体目标将在小组会议期间讨论，以便本科生（和研究生）可以看到他们的项目如何符合更大的目标。 对研究生和本科生的支持来自NSF CAREER奖DMR-9701725。 %%% ***

9802784 Levy This award provides partial support to develop a variable- temperature "apertureless" near -field scanning optical microscope (ANSOM) for studying local electronic and lattice dynamics in nanostructured optoelectronic materials. The instrument will have unique and unprecedented capabilities, combining near-atomic spatial resolution, femtosecond temporal resolution, a wide operating temperature range (1.6K-400K), and operation in magnetic fields up to 10 Tesla. The instrument is specifically suited for the study of material properties which couple to refractive index changes. The contrast mechanism for ANSOM comes from measuring small phase shifts in scattered light from an atomic or magnetic force microscope tip. By modulating the tip-sample separation, it is possible to measure local sample polarizabilities (electric or magnetic) at length scales significantly smaller than those attainable by either conventional optical or fiber-based near-field scanning optical microscopes (NSOMs). The instrument design will also allow it to be used as a confocal scanning optical microscope, which can focus or collect light with high efficiency and diffraction-limited spatial resolution. There are presently several low-temperature NSOMs operating in laboratories around the world. Sub-wavelength spatial resolution for these instruments is obtained by scanning a tapered optical fiber close to the sample to form an image. However, both practical and fundamental constraints prevent the optical resolution from approaching that of atomic-force microscopy (AFM) or scanning tunneling microscopy (STM). The unique power of this instrument will come from the ability to combine traditional strengths of optical methods (e.g., time resolution or energy selectivity) with the spatial resolution of atomic force microscopy. It is possible using ANSOM to achieve spatial resolution below 10 A. The combination of spatial and temporal resolution will open many new research avenues related to dynamical processes in nanometer-scale condensed matter systems. The immediate scientific applications are threefold: (1) the study of exciton dephasing in semiconductor quantum dots, (2) spectroscopic and time-resolved studies of neutral excitations in single conjugated polymer chains, and (3) lattice dynamics and domain wall motion in ferroelectrics and quantum paraelectrics. Longer-range goals include the study of magnetization dynamics in ultrathin magnetic films, applications in biology (e.g., DNA sequencing), and optically detected magnetic resonance. The construction of this instrument will benefit from the design principles of earlier pioneers in the area of low-temperature scanning probe microscopy. Principles that ensure sufficient vibration isolation, coarse approach mechanisms, etc., have been incorporated into the proposed design. Many of the critical design parameters have been assessed from the construction of a room-temperature prototype, which is currently being used to study domain dynamics in ferroelectric thin films. The construction of this instrument will provide valuable experience for students at both the graduate and undergraduate level. The project will be overseen by the principal investigator, who will direct a postdoctoral researcher, a graduate student, and several undergraduate students. The graduate student will work closely with undergraduates on various subtasks related to the larger goal of building the proposed instrument, such as writing software drivers for instruments such as lock-in amplifiers and temperature controllers. The overall goals of the project will be discussed during group meetings so that undergraduates (and graduate students) can see how their project fits in with the larger goal. Support for both the graduate and undergraduate students comes from an NSF CAREER award DMR-9701725. %%% ***