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Prabhat Verma
Prabhat Verma
Osaka University, Japan
Nano-optical imaging with Tip-Enhanced Raman Spectroscopy


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).

Prabhat Verma is currently a full professor at the department of Applied Physics and a member of Photonics Center at Osaka University. He is also an Executive Director of the Japan Society of Applied Physics (JSAP) and holds the positions of the Chair of the JSPS-Core-to-Core Program at Osaka University. He received his doctorate from IIT Delhi, India, and did post-doctoral research in Germany and Japan, before he moved to Osaka University. He is presently working in the fields of nanospectroscopy, nanoimaging, SERS, TERS, metamaterials, and related topics. He has co-authored a large number of papers in journals such as Nature Photonics, Nature Communications, PRL, ACS Nano, PRB, and other high impact journals. He is also involved in a number of activities in various international scientific societies and communities.
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