White nano-light-source for optical nanoimaging with TERS

Authors

Prabhat Verma ⋅ JP Department of Applied Physics, Osaka University

Abstract

Visible light can efficiently interact with the electronic system of a sample and fetch rich information about its intrinsic properties, because visible light carries an energy that is comparable to the electronic energies of most of the naturally existing materials. Microscopic techniques based on optical spectroscopies, such as Raman spectroscopy, have therefore always been convenient tools for analyzing and imaging various materials. However, the spatial resolution in optical microscopy is restricted by the diffraction limit of the probing light, making it impossible to analyze materials smaller than about half of the wavelength. This restriction can be overcome by involving plasmonics with spectroscopy. When conventional Raman microscopy is combined with plasmonics, a technique known as tip-enhanced Raman scattering (TERS), one can perform Raman spectroscopy and imaging with extremely high spatial resolution down to about 10 nm [1-4]. In fact, under certain conditions, TERS can allow us to "see" a sample at an extremely high spatial resolution of about 4 nm [5].
The metallic nano-tip plays a very important role in confining the light field to the vicinity of the tip apex [6-7]. Generally, the apex of the metallic nano-tip used in TERS is illuminated with propagating light to confine the light field and to create a nano-light-source at the tip apex. However, since the illuminating light is focused to a diffraction-limited spot near the tip apex, a far-field scattering background from this spot is unavoidably associated with the near-field scattering signal. In order to avoid this far-field background, one can utilize the technique of plasmon-nanofocusing [8], wherein the illuminating light is focused on a coupler far from the apex, which couples with and excites the plasmons in the tip at the coupler. The plasmons then propagate and get adiabatically focused at the tip apex to create a nano-light-source at the apex. With this technique, ideally no far-field scattering background is generated since the incident laser is located far from the apex. The coupler is usually a grating, that efficiently couples a particular wavelength depending upon the parameters of the grating. However, since the process of plasmon nanofocusing is not a resonance phenomenon, it can be utilized to create a broadband (white) nano-light-source, if one can efficiently couple white light to excite the tip plasmons. Since superposition of multiple wavelengths can result in a sharp pulse-shaped function, we anticipated that a coupler made of a single slit instead of a grating can efficiently couple a broadband illumination with broadband plasmons, resulting in a white nano-light-source at the apex. Indeed, we demonstrated how we could create a white nano-light-source at the tip apex, that can be utilized for background-free TERS and NSOM measurements. It can be proved to be a powerful probing tool to evaluate samples with various frequency responses at nanoscale spatial resolution. We will discuss this in more details.

[1] P. Verma, T. Ichimura, T. Yano, Y. Saito, and S. Kawata, Laser & Photonics Rev. 4, 548 (2010).
[2] J. Yu, Y. Saito, T. Ichimura, S. Kawata and P. Verma, Appl. Phys. Lett. 102, 123110 (2013).
[3] Y. Okuno, Y. Saito, S. Kawata and P. Verma, Phys. Rev. Lett. 111, 216101 (2013).
[4] T. Yano, et al., Nature Commun. 4, 2592 (2013).
[5] T. Yano, P. Verma, Y. Saito, T. Ichimura, and S. Kawata, Nature Photon. 3, 473 (2009).
[6] T. Mino, Y. Saito and P. Verma, ACS Nano 8, 10187 (2014).
[7] I. Maouli, A. Taguchi, Y. Saito, S. Kawata and P. Verma, Appl. Phys. Exp. 8, 032401 (2015).
[8] T. Umakoshi, Y. Saito and P. Verma, Nanoscale 8, 5634 (2016).

About the Speaker

Prabhat Verma, Department of Applied Physics, Osaka University

Prabhat Verma is presently a Professor at the Department of Applied Physics and a member of Photonics Center at Osaka University. He is a Director of the Japan Society of Applied Physics, the Chair of the Asian Core Program and the Chair of the International Committee at Photonics Center, Osaka University. Professor Verma received his Master's degree form the Indian Institute of Technology (IIT), Kanpur and Doctorate degree from IIT, Delhi, after which he did his post-doctoral research at the Technical University of Freiberg, Germany and at the Kyoto Institute of Technology, Japan. He joined Osaka University in 2002 as a non-tenured Associate Professor, and subsequently became a tenured Associate Professor in 2006, and a full Professor in 2010. His current research interests are tip-enhanced Raman Spectroscopy (TERS), plasmonics, nanospectroscopy, and nanoimaging. He has also been working in the fields related to Raman spectroscopy, fluorescence, SERS, and metamaterials. He has co-authored a large number of papers in highly ranked journals such as Nature Photonics, Nature Communications, Physical Review Letters, ACS Nano, and Physical Review B. He has organized multiple international conferences and delivered about 50 invited talks in various international conferences. Professor Verma is also involved in a number of activities in various scientific communities including OSA, SPIE, JSAP, SPSJ, and OSJ.

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Issue

2016: Proceedings of the 34th Samahang Pisika ng Pilipinas Physics Conference

Article ID

SPP-2016-4C-01

Section

Invited Presentations

Published

2016-08-18

How to Cite

[1]

P Verma, White nano-light-source for optical nanoimaging with TERS, Proceedings of the Samahang Pisika ng Pilipinas 34, SPP-2016-4C-01 (2016). URL: https://proceedings.spp-online.org/article/view/SPP-2016-4C-01.