This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. JSTQE-INV-NB-04992-2013 1 Plasmonics for Surface Enhanced Raman Scattering: Nanoantennas for Single Molecules Kenneth B. Crozier, Senior Member, IEEE, Wenqi Zhu, Dongxing Wang, Shiyun Lin, Michael D. Best, and Jon P. Camden compelling applications for plasmonics is surface enhanced Abstract— Surface enhanced Raman scattering (SERS) is Raman scattering (SERS). While the SERS phenomenon was undergoing a renaissance, spurred largely by developments in the reported some time ago [20, 21], the recent upsurge in burgeoning field of plasmonics. This paper reviews the current plasmonics has enabled exciting new capabilities. In this status and future directions in plasmonic nanostructures for paper, we first review the SERS phenomenon. We then outline SERS. We show that engineered plasmonic nanostructures enable exciting new functionalities, including beamed Raman how plasmonics and nanophotonics have enabled beamed scattering and highly reproducible chips for single molecule Raman scattering, single molecule SERS on a chip, and SERS SERS. We furthermore show that silicon photonics enables SERS with metal nanoparticles optically trapped by photonic to be performed using optically-trapped Ag nanoparticle clusters. resonators. These advances have been in part enabled by thinking about the plasmon structures as antennas [22-26]. Index Terms— Chemical and biological sensors, To understand SERS, we begin by considering the Raman nanofabrication, nanolithography, nanomaterials, nanophotonics, nanoscale devices, Raman scattering, surface process [27]. When light is scattered by a molecule (Fig. 1), plasmons (SPs). the process is generally elastic, a process is termed Rayleigh scattering. This can be modeled classically by considering the molecule to act as a dipole that oscillates at the same I. INTRODUCTION frequency as the incident electric field , with = 푝⃗ ECENT years have seen an explosion of interest in the where is the linear polarizability tensor. On the other hand, 퐿 �⃗ 퐿 �⃗ field of plasmonics, which has come to refer to the use of a molecule휈 containing atoms ( 2) 퐸will have푝⃗ 3훼� ∙ 퐸6 R 퐿 surface plasmon polaritons (SPPs) to route and manipulate vibrational훼� modes (3 5 for linear molecules). Each light on the nanoscale [1]. An exciting range of applications corresponds to a particular푁 vibrational푁 ≥ pattern, in which푁 −the have been enabled that are too diverse to fully review here, but atoms of the molecule푁 oscillate− at frequency , with measuring the oscillation amplitude and termed the normal include highly miniaturized optical devices, such as lasers [2, 푀 푘 3], waveguides [4], photodetectors [5] and modulators [6], mode coordinate. These perturbations modify th휈e molecule’s푄 biosensors [7], heat-assisted magnetic recording [8], polarizability. This can be expressed as follows: polarization elements [9-11], optical tweezers [12-18] and metamaterials [19]. We argue, however, that one of the most ( ) = (0) + + + (1) 2 휕훼�퐿 1 휕 훼�퐿 2 퐿 푘 퐿 푘 2 푘 Manuscript received August 13, 2013; revised September 8, 2013. First 휕푄푘 2 휕푄푘 훼 � 푄 훼� � �푄푘=0 푄 � �푄푘=0 푄 ⋯ published ZZ; current version published AA. This work was supported by the The perturbation caused by the vibration therefore modulates National Science Foundation (NSF, grant ECCS-0747560 and grant ECCS- 1201687), the Harvard Quantum Optics Center, and by the Center for the polarizability at frequency , meaning that the scattered Excitonics, an Energy Frontier Research Center funded by the U.S. field contains not only a component at , but also 푀 Department of Energy, Office of Science and Office of Basic Energy Sciences components at “beat” frequencies휈 and + . This under Award Number DE-SC0001088. This work was also supported by the 퐿 classical model therefore predicts the existence 휈of Stokes and UT/ORNL Joint Institute for Advanced Materials, and the U.S. Department of 퐿 푀 퐿 푀 Energy, Office of Basic Energy Sciences, under Award Number anti-Stokes Raman scattering (Fig.휈 − 휈1). The휈 model휈 has DESC0004792 (J.P.C.) and the National Science Foundation under Awards shortcomings [27], e.g. it does not correctly predict the CHE-0954297 and DMR-0906752 (M.D.B.). Fabrication work was carried out in the Harvard Center for Nanoscale Systems, which is supported by the magnitudes of the Stokes and anti-Stokes intensities, but NSF. nonetheless provides helpful insights, including the following Wenqi Zhu, Dongxing Wang, and Kenneth B. Crozier are with the School two. First, the model predicts that many of the vibrational of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 USA (emails: [email protected], [email protected], modes of molecules are Raman-inactive as, for these modes, and [email protected]). does not depend on . This is indeed the case. Second, an Shiyun Lin was with the School of Engineering and Applied Sciences, electronic resonance that results in an increase in should, Harvard University, Cambridge, MA 02138 USA. He is now with Oracle 훼�퐿 푄푘 (email: [email protected]). according to the model, result in an increase in 퐿 , and Michael D. Best and Jon P. Camden are with the Department of Chemistry, 훼� 휕훼�퐿 University of Tennessee, Knoxville, TN 37996, USA (emails: therefore also an increase in the Raman scattering.휕 푄This푘 is [email protected] and [email protected]). indeed seen in resonant Raman scattering (Fig. 1). Copyright (c) 2013 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing [email protected]. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. JSTQE-INV-NB-04992-2013 2 the mechanism of the first component (termed chemical enhancement) continues, it is generally agreed (see e.g. [33]) that it plays a far smaller role in the overall enhancement that the other two components (termed electromagnetic enhancement). The electromagnetic enhancement factor is the ratio between the Raman scattering from a molecule on 퐸퐹 an enhancing substrate to the Raman scattering that would퐸푀 be produced by the molecule in the absence of the enhancing substrate. It is commonly given by: | ( )| | ( )| = (2) | ( )| 2 | ( )| 2 퐸퐿푂퐶 휈퐿 퐸퐿푂퐶 휈푅 퐸퐹 2 2 퐸푀 퐼푁 퐿 퐼푁 푅 Fig. 1. Elastic (Rayleigh) and inelastic (Raman) scattering where and퐸 휈are the 퐸frequencies휈 of the excitation (laser) of light by a molecule (here taken as benzenethiol). and Raman scattered fields, respectively. | ( )| and 퐿 푅 | ( )휈| are the휈 magnitudes of the electric fields at the 퐿푂퐶 퐿 Raman spectroscopy is a powerful technique as it enables position of the molecule that would result for illumination퐸 휈 by 퐿푂퐶 푅 materials to be identified and characterized by their vibrational plane퐸 waves휈 with frequencies and , respectively. Note spectra, which can be thought of as “fingerprints”. In Fig. 2, that the illumination is along a specific direction, and with 퐿 푅 for example, the Raman spectrum of a 500 µm thick layer of polarization and amplitude specified휈 by휈 ( ) at the laser pure benzenethiol liquid is shown, measured using a confocal frequency and by ( ) at the Raman scattering frequency. 퐼푁 퐿 Raman system. From knowledge of the density of It is not always realized, however, that (2)퐸 is 휈only rigorously 퐼푁 푅 benzenethiol and the confocal collection volume of the Raman correct under the following퐸 휈 conditions [27]. Both illumination system (measured using method of [28, 29]), the number of and collection occur along a specific direction. The molecule molecules in the measurement was found to be ~3.7 × 10 . is assumed to have an isotropic Raman polarizability tensor When one considers the recent trend toward single molecule10 , which relates the Raman dipole = (modeling techniques in biology and chemistry [30], one realizes that this the Raman scattering) to the local field . Furthermore, (2) �푅 푅 �푅 �⃗퐿푂퐶 is a very large number. This underscores a difficulty in assume∝ s that the polarization of the푝 ⃗local∝ field∙ 퐸 is unchanged 퐿푂퐶 employing Raman spectroscopy in applications involving between laser and Raman frequencies.퐸�⃗ Lastly, (2) assumes small numbers of molecules, due to the fact that Raman polarized detection. Despite these limitations, (2) nonetheless scattering cross sections are comparatively small. in most cases provides a reasonable estimate, and emphasizes the importance of field enhancement. Metal nanoparticles are well-suited for this task, as the excitation of localized surface plasmons enables large field enhancement. This is especially true for nanoparticles separated by small gaps. In Fig. 3, for example, the intensity enhancement generated by a single gold sphere (60 nm diameter) can be seen to increase from ~24 to ~1686 times when, instead, a pair of spheres with a 5 nm gap is used. Coupling between the spheres red-shifts the resonance from λ0 = 522 nm (single sphere) to 550 nm (pair of spheres). Intensity enhancement -60 522 nm -40 Fig. 2. Measured Raman spectrum of benzenethiol liquid. -20 0 In the SERS phenomenon [20, 21], the Raman signals from y (nm) 20 40 molecules in close proximity to metallic nanostructures are 60 Au sphere (60 nm diam.) boosted significantly. The effect can be broken into three -100 -50 0 50 100 x (nm) components. First, the Raman cross section of the molecule can be modified upon its adsorption to the metal [31, 32], an -60 550 nm effect most often ascribed to charge transfer. Second, the -40 metallic nanostructure enhances the excitation electric field at -20 0 the molecule, thereby increasing the strength of the induced y (nm) 20 Raman dipole moment. Third, the metallic nanostructure 40 enables the induced Raman dipole moment to radiate more 60 5 nm gap -100 -50 0 50 100 power.