TERS: beyond the classical limits of light!
Wave nature of light prevents it to efficiently interact with samples that are smaller than half of its wavelength. Optical microscopy, therefore, cannot achieve a spatial resolution better than about half of the wavelength of the probing light, a phenomenon known as the diffraction limit of light. However, near-field techniques that involve imaging of samples in close proximity of a metallic nanostructure have ways to overcome this diffraction limit. One of such techniques, known as tip-enhanced Raman scattering (TERS), utilizes a metallic nano-tip with sharp apex of the order of tens of nanometers as the imaging probe. The optical microscopy based on TERS goes beyond the classical limits not only in achieving super spatial resolution by jumping far beyond the diffraction limit of light, but also in achieving new exciting results by combining it with other analytical techniques, such as nonlinear spectroscopy or the application of unidirectional local pressure to the samples. When a metallic nano-tip is used for characterizing and imaging samples at molecular level in TERS microscopy, interesting near-field effects can be observed. Huge signal enhancement, along with nano-metric super-resolution, which come from the local confinement of photons in the vicinity of the metallic tip and the charge transfer in the metal-molecule complex, help not only in the successful imaging of molecular distribution but also in studying and investigating the samples at true nano scale. A spatial resolution as high as 15 nm and a signal enhancement as high as a million times can be achieved, which can reveal those details of the sample that no traditional optical microscopy can. Since TERS microscopy is a phenomenon based on the vibrational properties of the sample investigated, apart from the topographic imaging, it can also image various physical, chemical or biological properties of the sample that have their signatures in a vibrational spectrum. If these properties of the sample is color-coded, TERS can provide a very rich color image, with each color representing the distribution of one particular property of the sample at nanoscale.
Utilizing the same technique of tip-enhancement, imaging based on photoluminescence can also be benefited by the near-field effects at the sharp apex of a metallic nano-tip. We are also utilizing the microscopy based on tip-enhanced photoluminescence process, which can complement TERS.