Biography
Visible light can interact directly with the electronic or vibronic system of a sample and can extract rich information related to the intrinsic properties of the sample. This is the reasons why optical techniques, such as Raman spectroscopy, have always been convenient tools for analyzing and imaging various materials. However, Raman microscopy in its conventional form is not suitable for analyzing and imaging nanomaterials due to two major reasons. First, the poor spatial resolution restricted by the diffraction limits of the probing light, makes it impossible to analyze materials smaller than about half of the wavelength (about 200-300 nm for visible light). And second, due to the extremely small volume of nanomaterials, Raman scattering intensity is extremely weak for such samples. However, when conventional Raman microscopy is combined with the near-field techniques, it achieves new and exciting features as it goes beyond the conventional limits of optical microscopy, in terms of both the spatial resolution and scattering intensity. This can be done by utilizing the technique of tip-enhanced Raman spectroscopy (TERS), which is based on plasmonic enhancement and confinement of light field near the apex of a sharp metallic nanotip for characterizing and imaging samples at nanoscale. This plasmonics-based technique allows us to have a spatial resolution down to about 10 nm in optical nanoimaging [1-4].
Here, I will show how such a high spatial resolution in TERS is obtained and how it can be useful in various applications. The spatial resolution, however, can be further improved if we combine TERS with some other mechanism. One of such examples is the inclusion of tip-applied pressure in TERS, which distorts the sample locally, where we have shown that a spatial resolution better than 4 nm can be achieved [5]. Further, I will discuss some techniques to obtain background-free nanoimaging in TERS.
[1] S. Kawata, Y. Inouye, and P. Verma, Nature Photon. 3, 388 (2009).
[2] P. Verma et al., Laser & Photon. Rev. 4, 548 (2010).
[3] Y. Okuno, Y. Saito, S. Kawata and P. Verma, Phys. Rev. Lett. 111, 21601 (2013).
[4] J. Yu et al., Appl. Phys. Lett. 102, 123110 (2013).
[5] T. Yano et al., Nature Photon. 3, 473 (2009).