Infrared nano-spectroscopy via molecular expansion force detection
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Mid-infrared absorption spectroscopy in the “molecular fingerprint” region (λ = 2.5–15 μm) is widely used for in situ analysis of chemical and biological samples. Due to the diffraction limit, traditional far-field techniques such as Fourier-transform infrared spectroscopy cannot take sample spectra with nanometer spatial resolution. To conduct nanoscale infrared measurement, in photoexpansion nano-spectroscopy, an atomic force microscope cantilever is used as a light absorption detector, in the way that the cantilever is deflected proportionally by the localized sample heating and expansion induced by infrared pulses. Previous studies of this new opto-mechanical technique demonstrated its powerfulness and simplicity, but relied on using high-power laser pulses to produce detectable cantilever deflection signal and it was difficult to measure ultra-thin samples below ~100 nm. In addition, the spatial resolution, though improved, is limited by the thermal diffusion length inside samples. This dissertation presents a set of experiments which have substantially improved photoexpansion nano-spectroscopy in terms of sensitivity and spatial resolution, and have explored other aspects of this technique. For the first time, high-quality photoexpansion spectra have been obtained from molecular monolayers using low-power infrared pulses from a tunable quantum cascade laser. The orders of magnitude improvement in sensitivity is due to the two methods we implemented: mechanical enhancement by the cantilever resonance, and optical enhancement by the metalized cantilever tip. The spatial resolution is also improved and only determined by the locally enhanced field below the tip. After that, the dissertation shows the spectral background signal, which comes from infrared absorption by the substrate and tip, can be suppressed using a second laser. We have also investigated the nonlinearity of tip-sample interaction, and are able to detect sample photoexpansion force at heterodyne frequency. In the last part of this dissertation, we use our technique to image local optical energy distribution and ohmic heat dissipation of the metal nanoantennas.