Vibrational spectroscopy is widely used for a large variety of sensing and imaging applications. Nonlinear optical interactions using ultrashort laser pulses can facilitate selective coherent excitation of molecular vibrational modes, such as stretching and bending C-H and O-H vibrational modes. However, such spectroscopic probes are often based on the assumption that the energy distribution is static throughout the system, neglecting complex molecular dynamics and the effects of local nano- and microenvironments. To tackle this challenge, we developed a coherent control technique with a special focus on the temporal evolution of molecular vibrational modes. By utilizing a high-dynamic range detection, we demonstrate molecular vibrational dynamics and the environmental effects with multidimensional spectroscopic sensing. Such capability promises a broad range of sensing and imaging applications in biology, materials, and chemical sciences.
Infrared (IR) spectroscopy depicts molecular structure and dynamics based on vibrational absorption of chemical bonds. Spatially resolved IR spectroscopy, i.e. IR imaging, further enabled label-free in situ chemical imaging for dynamics in complex systems. However, IR imaging suffers from low spatial resolution at a few micrometers due to diffraction limit, thus having difficulty in applications such as sub-cellular imaging. Recently, by visible light probing of the photothermal effect of vibrational absorption, mid-infrared photothermal imaging (MIP) overcomes the limitations of conventional IR microscopy and has achieved sub-micron resolution. In this work, we built an optimized MIP system to boost the spatial resolution and sensitivity, and demonstrated MIP imaging of nanometer-sized polymeric microspheres and living cells with a high spatial resolution of 200 nm.
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