Scattering-Scanning Near-Field Optical Microscopy with Femtosecond Laser Pulses.

摘要

The macroscopic properties of materials we observe emerge from the collective structural configuration and dynamical behavior of the atomic or molecular constituents. Therefore, in order to fully characterize and understand these properties, it is necessary to develop measurement techniques capable of probing at these scales. Such a technique is that of combining scattering scanning near-field optical microscopy with ultrafast spectroscopy. Traditional, far-field microscopy is limited by diffraction, making it impossible to resolve details smaller than approximately half the wavelength of the illuminating light. However, the electromagnetic field that is produced when an object is illuminated is not simply characterized by the light that carries energy, radiating to the far-field, but also consists of a more structured rapidly decaying evanescent field. The structure of this evanescent, nonradiating near-field is not limited by diffraction, and so in measuring this field, it is possible to resolve the microstructure of matter in a way that is independent of the illuminating wavelength. By placing a metallic tip close enough to the surface to be within this evanescent field, the electric near-field is then scattered off of it, allowing it to be observed. Through the measurement of backscattered light off of nanometer scale probes developed for the use in scanning probe microscopy, near-field detection was realized in the form of scattering scanning near-field optical microscopy, allowing wavelength independent but spectroscopically sensitive imaging with nanoscale resolution. However, background interference from stray reflections usually overwhelm the back-scattered signal. By oscillating the tip at its mechanical resonant frequency, the near-field component of the signal can be detected in the anharmonic response to this modulation. This anharmonicity appears in the Fourier components of frequencies at integer multiples of the modulation frequency, a type of demodulation done through the use of a lock-in amplifier. However, a lock-in amplifier continuously samples the detector, while to obtain temporal resolution, a pulsed laser sources are necessary. By measuring the response of a sample to an initial excitation with a time delayed secondary laser pulse in pump-probe spectroscopy, the time dependence of an excitation can be measured. However, if the repetition rate of this pulsed laser is close to the modulation frequency of the tip, a lock-in amplifier will have difficulty demodulating the near-field signal.;As an alternative to lock-in detection, we developed a method which synchronized data acquisition with the repetition rate of the laser pulse source, acquiring data only when the signal at the photodetector is at its maximum. In doing so, we were not only able to improve the signal quality as compared to that of a lock-in amplifier, as measured in the noise of approach curves over a gold surface and in a raster scan of a gold-silicon step edge, we were also able to apply an alternative method of near-field detection whereby we compared the curvature of the tip-scattered signal at the top and bottom of the tip-oscillation. This was similarly demonstrated in an approach curve over gold and in a raster scan over the step edge. Also, since the data were saved onto a computer, we were able to apply post-processing, enabling us to improve this method of curvature comparison, and create an approach curve showing a very clear near-field signal with little far-field interference. Therefore, in developing this data collection technique, we have demonstrated both an improvement to traditional lock-in detection when applied to pulsed laser sources along with a capability of implementing near-field reconstruction beyond that of harmonic demodulation.