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

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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. %%% ***