— journal of January 2014
physics pp. 59–70
Subwavelength propagation and localization of light using surface plasmons: A brief perspective
G V PAVAN KUMAR∗, DANVEER SINGH, PARTHA PRATIM PATRA and ARINDAM DASGUPTA
Photonics and Optical Nanoscopy Laboratory, Departments of Physics and Chemistry, Indian Institute of Science Education and Research, Pune 411 008, India
∗Corresponding author. E-mail: pavan@iiserpune.ac.in
DOI: 10.1007/s12043-013-0643-z; ePublication: 9 January 2014
Abstract. Surface plasmons at the metal–dielectric interface have emerged as an important can- didate to propagate and localize light at subwavelength scales. By tailoring the geometry and arrangement of metallic nanoarchitectures, propagating and localized surface plasmons can be obtained. In this brief perspective, we discuss: (1) how surface plasmon polaritons (SPPs) and local- ized surface plasmons (LSPs) can be optically excited in metallic nanoarchitectures by employing a variety of optical microscopy methods; (2) how SPPs and LSPs in plasmonic nanowires can be utilized for subwavelength polarization optics and single-molecule surface-enhanced Raman scat- tering (SERS) on a photonic chip; and (3) how individual plasmonic nanowire can be optically manipulated using optical trapping methods.
Keywords. Plasmons; surface-enhanced Raman scattering; microscopy; nanophotonics.
PACS Nos 73.20.Mf; 78.30.−j; 68.37.−d; 78.68.+m
1. Introduction
1.1 Nanophotonics with surface plasmons
Beating the diffraction limit of light has been one of the important challenges of nanophotonics [1]. Subwavelength propagation and localization of light has important consequences in optical communication, nano-optical sensors, quantum optics and many other scientific domains. In recent times, plasmons, which are coupled oscillations of light and free electrons of metals at metal–dielectric interface, have emerged as promis- ing candidates for subwavelength propagation and localization of light [2,3]. There are two fundamental surface excitations of plasmons: surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs). SPPs can propagate over a large distance (up to a centimetre) and can be harnessed for nano-optical waveguiding [1,4], whereas LSPs
are confined to small volumes (few nanometres) and can be utilized for nano-optical sensors [5].
1.2 Optical excitation of SPPs and LSPs
There are a variety of methods to excite SPPs and LSPs using laser beams [6,7].
Optical microscopy methods have emerged as a versatile, adaptable and effective way.
Using high-numerical aperture objective lenses, tight focussing of laser beams can be achieved, and this focussed beam can be further utilized to excite the defect and dis- continuities on the metal surface. This specific excitation configuration will convert the photon into plasmon waves, leading to either propagating optical fields or localized optical fields. Furthermore, the plasmon waves can be reconverted into photons when the plasmons scatter off a defect or discontinuity on the metal surface. This principle of converting photons to plasmons at subwavelength scales on a metal surface, and again reconverting them back to freely propagating photons is one of the important aspects of plasmon optics, and can be utilized for various optical functions at subwavelength scale.
Between SPPs and LSPs, the excitation of the former is slightly challenging because the phase matching conditions required are more stringent, whereas for laser excitation of LSPs, metallic nanostructures can be directly illuminated to obtain local optical fields.
In our laboratory, we have been developing novel optical microscopy and nanoscopy methods to excite, propagate and detect SPPs and LSPs.
2. Metallic nanowires for SPPs and LSPs
Among many plasmonic nanoarchitectures [8], silver nanowires (Ag NWs) have been extensively studied in the context of SPP waveguides due to their minimal propagation loss. Ag NWs have also been utilized to achieve enhanced light–matter interaction, such as surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence [9].
This indicates versatility of Ag NWs as excellent test platform for SPP- and LSP-based optics and spectroscopy.
Here we focus on plasmon-assisted light propagation and confinement in coupled geometries of Ag NWs (figure1a). Recently, we have shown that coupled Ag NWs form plasmonic interconnects that can be used to couple light from one location to another [10,11]. Interestingly, the coupling junction between the nanowires can also emit light making them confined light sources at nanoscale (figure1b). Some of the important ques- tions in the context of this geometry are: what is the effect of coupling angle between NWs on light propagation characteristics; how can polarization of excitation source alter the propagation characteristics; can the nanowire junctions and distal ends be harnessed as LSP-based light sources. Furthermore, important aspect of these coupled Ag NWs is that they can be used for remote excitation of surface-enhanced Raman scattering. In order to test such questions and capabilities, we performed measurements to resolve optical polarization and micro-Raman scattering and imaging which revealed strong polariza- tion anisotropy in the light propagation and SERS signal from the Ag NWs. We probed all the above-mentioned issues using optical methods based on a home-built multimodal optical microscope [10,11]. This microscope system was utilized to perform dark-field
Figure 1. End-to-end coupled plasmonic nanowires with laser illumination in (a) bright field, (b) dark-field image, (c) Raman-mapped image of the coupled nanowire with rhodamine 6G molecules deposited on the wire shown in (d) as optical image (reproduced with permission from ref. [10]).
plasmonic imaging [12], back-focal plane imaging, plasmon leakage radiation imaging, surface-enhanced Raman scattering [13] and optical trapping on a single platform. Such a versatile microscopy platform can be used to thoroughly characterize optical properties of isolated plasmonic nanosystems.
2.1 Light propagation and localization in end-to-end coupled nanowire
The end-to-end coupled silver nanowires are unique because they can facilitate propagat- ing and localized surface plasmons on a single nano-optical platform. The data in figure1 represent the light propagation and localization in the mentioned geometry. In this par- ticular experiment [10], a laser light of 633 nm wavelength was tightly focussed through a high numerical-aperture objective lens (0.9 NA, 100×), at one of the distal ends of the coupled nanowire. The incident photons were converted into SPPs and further localized at the junction of the coupled nanowires to emit light, as shown in figures1a and1b. The emission at the other distal end indicates that the light has been channelled from one end to the other end of the nanowire. To assure the presence of localized surface plasmons at the nanowire junction, we performed SERS imaging. The coupled wires were mapped with respect to Raman scattering signal arising out of the molecule (rhodamine 6G) deposited on the nanowires (see figure1c). The data indicate enhanced Raman signals from the nanowire junction as compared to other locations. Hence, the intense Raman signal from the junction can be considered as an indicator of the presence of intense optical fields due to localized plasmons. Such unique geometries with both SPPs and LSPs will have
implications in on-chip plasmonics, where subwavelength propagation and localization of light are to be achieved on a single platform.
2.2 Light propagation as a function of geometry and polarization
How does the geometrical arrangement, especially the angle between the coupled nanowires influence the plasmon propagation? How can we utilize the coupled nanowires for nanoscale polarization optics? These are interesting questions that we experimentally addressed using end-to-end coupled plasmonic nanowires [11]. Figures2a–2c indicate the modulation of light propagation as a function of angle between two coupled Ag NWs.
It was interesting to observe that obtuse angled nanowire pair exhibited light transport from one end to the other, whereas for the acute angled wires, the light is localized at the junction and does not propagate further into the second nanowire [11]. We also observed the polarization-dependent routing of light in end-to-end coupled nanowire. In this case, the laser light was illuminated at the junction, and as a function of incident polarization we could route the light toward the desired nanowire (figures2e–2g).
2.3 Evanescent-field based excitation and modulation of Rayleigh and Raman scattering from individual plasmonic nanostructures
The capability to modulate scattering light from individual plasmonic nanostructure is important to control light–molecule interaction at nanoscale. To achieve this we have developed evanescent-field microscopy method [12] to probe and modulate single nano- structures such as plasmonic nanowires and nanoparticles. Figure3a shows dark-field optical image of a single silver nanowire. Figures3b and3c show the same nanowire under evanescent field illumination with p and s polarizations, respectively. The evanes- cent field interacts with the single Ag NW and scattered the Rayleigh light into the detecting CCD camera. The important aspect of this experiment is that the whole nanowire is simultaneously illuminated at low laser powers. This prevents photo-damage of the wire and any other entity such as molecules placed on the nanowires. Furthermore, surface-enhanced Raman scattering signals of molecules deposited on the nanowire can be captured by evanescent field excitations, as shown in figure3d and the inset in figure3d.
Such dark-field illumination methods can also be employed to excite individual plas- monic nanoparticles. Figure4shows dark-field images of Ag nanoparticle deposited on a glass surface captured by illuminating a white-light source on the deposited samples.
Each point in this image indicates light emitted from single nanoparticle and this can be further routed towards the spectrometer to analyse the plasmon resonance of individual nanostructure. Such methods have implications in nano-optics and spectroscopy, and can be further harnessed to understand surface plasmon resonance and secondary emissions such as Raman scattering and fluorescence characteristics of individual nanoparticles [14].
3. Single-molecule SERS sensitivity of Ag-core Au shell nanoparticles
Single-molecule detection and characterization are very important for understanding fundamental molecular behaviour in various environments. SERS has emerged as a unique technique with single-molecule sensitivity [15]. The interaction of local optical
(d)
(a) (c)
(b)
(e)
(f) (g)
5µm
Figure 2. (a), (b) show light propagation through an acute and obtuse angle end-to- end coupled nanowire, (c) schematic illustration of propagation in acute and obtuse configuration systems, (d) an optical image of end-to-end connected Ag nanowire.
(e), (f) and (g) show polarization-dependent light routing (reproduced with permission from ref. [11]).
field of a plasmonic nanostructure with molecules in its vicinity is an important issue in single-molecule SERS. In the past, various approaches have been taken to probe single- molecule SERS sensitivity of various nanoplasmonic architectures, mostly based on silver
Figure 3. An optical image under evanescent field illumination in (a) bright field, (b) p-polarized illumination of 532 nm wavelength laser, (c) s-polarized illumination of 532 nm wavelength laser, (d) SERS spectra modulation (shown in the inset) under evanescent field (reproduced with permission from ref. [12]).
nanostructures [15–18]. It is known that silver is the best plasmonic metal at visible frequency, but the problem with silver is that it is toxic and it can be easily oxidized.
Alternatively, gold nanostructures are biocompatible and inert compared to silver in air and also SERS active at visible wavelengths, but the electromagnetic field enhancement
Figure 4. Dark field optical image of Ag nanoparticles deposited on a glass substrate.
is not good enough to detect single molecules. One such nanoarchitecture that can over- come this problem is silver-core gold-shell nanoparticles (Ag@Au NPs) [13,19,20]. The synthesis of Ag@Au nanoparticles is as facile as that of silver or gold nanoparticles.
Interestingly, Ag@Au NPs exhibit SERS enhancement comparable to Ag NPs.
Figures5a and5b show the transmission electron microscope (TEM) images of a sin- gle and a cluster of Ag@Au NP, respectively, prepared by seed-mediated growth method using citrate-reduced Ag colloidal nanoparticles as the seed [13]. The average size distri- bution of these nanoparticles is 60±20 nm and the thickness of the Au shell is∼6 to 8 nm which can be further tuned synthetically. Figure5c is the comparison of surface plasmon resonance (SPR) absorption spectra of Ag@Au NPs with that of Ag and Au NPs, pre- pared by Lee–Meisel (citrate reduction) method. The Ag@Au NPs that have been used for single-molecule SERS have the SPR absorbance maximum at 490 nm whereas the SPR absorbance maxima of Ag and Au NPs are at 420 nm and 523 nm, respectively.
We used Ag@Au NPs for single-molecule SERS detection [13] by employing bi- analyte method which is a contrast-based spectroscopy technique which includes statisti- cal probability calculation from a large number of spectra collected at a time. Here, Nile blue-chloride (NB) and Rhodamine 6G chloride (R6G), were used as analytes at 2 nM concentration for the experiment. The excitation wavelength was 532 nm at 7.8 mW power. The 592 cm−1and 612 cm−1Raman-vibrational peaks of NB and R6G, respec- tively, were monitored as the signatures of these two analytes. We collected 15,000 spectra with the accumulation time of 0.1 s for each spectrum and analysed by modified principal component analysis (MPCA).
Figure6a shows the crux of the bianalyte approach experiment, i.e. only four prob- able events can randomly occur throughout the 15,000 spectra. The bottom spectrum pertains to no signal from either of the molecules. The next one indicates the pure R6G event. Above this, is a mixed event which ascertains the presence of both the molecules in the probe-volume and the topmost is a signature of the pure NB event. Figures6b–6e are the results of modified principle component analysis (MPCA). Figure6b shows the two principal eigenvalues which carry most of the information of the 15,000 spectra. In figure6c the coefficient matrix (generated from the 15,000 spectra) is shown, where two
Figure 5. Ag core Au shell nanoparticles and their plasmon resonance. (a) TEM image of single Ag core Au shell nanoparticle (Ag@Au NP) with Au shell thick- ness approximately equal to∼8 nm, (b) TEM image of a cluster of Ag core Au shell nanoparticles used in this work, (c) plasmon resonance of Ag@Au NP nanoparticles compared with Ag and Au nanoparticle prepared by citrate reduction method. Ag@Au NP resonance is at 490 nm. Ag and Au NPs have resonance at 420 nm and 523 nm, respectively (reproduced with permission from ref. [13]).
axes refer to the two individual dye axes. Figure6d is the transformed and rescaled form of the coefficient matrix. Finally, in figure6e the probability histogram is shown which depicts the relative contribution of the number of the molecules to the total signal which is the central result of this experiment. The main contribution to the distribution in the
Figure 6. Analysis of the bianalyte single molecule surface-enhanced Raman scattering with Ag@Au NPs as substrates. (a) Four possible bianalyte SERS events from 15,000 spectra (acquisition time 0.1 s) for two dyes at 2 nM concentration with Ag@Au NPs at 532 nm excitation. (b) Eigenvalues (in decreasing order) obtained from PCA analysis of 15,000 spectra consisting of 34 wavelengths containing the 592 cm−1and 612 cm−1mode of NB and R6G, respectively. (c) Plot showing the dis- tribution of the covariant matrix coefficients obtained from PCA analysis. (d) Coef- ficient plot after linear transformation along the axes. Note that the axis is now orthogonal and rescaled. (e) Probability of NB events retained from 15,000 spectra after discarding the noisy events. The cut-off is indicated by a black square in (d) (reproduced with permission from ref. [13]).
histogram shown in figure6e is from NB events whose probability is either∼0 or∼1.
This portion of the histogram indicates single-molecule-detection sensitivity with the greatest probability. So this result concludes that Ag@Au NPs have excellent capability to detect single molecules through localized electromagnetic field.
4. Optical trapping and manipulation of plasmonic nanowires
Noble metal nanowires are very important because of the strong light–matter interac- tion originating from surface plasmons. Surface plasmons in these wires are capable of confining light laterally well below the diffraction limits. This confined optical fields can
be relayed over a long distance (∼10μm) using plasmonic nanowires. So, the nanowires have the potentials to be used as building blocks for plasmonic nanocircuits and hence these spatial manipulations are important. We employ optical trapping methods that use radiation pressure to trap single and transparent dielectric micro-objects. This is a very unique technique for all optical, noncontact manipulation of microparticles. It has been a very useful and powerful tool for physicists and biologists for many applications since its invention. In spite of being a very useful technique, it is very challenging to trap metallic particles because of their resonant scattering effects that repel them from a laser beam.
The problem becomes more complex when we need to trap anisotropic nanostructure, such as nanowire made of metal. However, it is now possible to manipulate and optically trap nanostructures on a two-dimensional substrates by employing holographic optical trap methods and off-resonant laser beams. In the following sections, we present optical trapping and rotation capabilities of single silver nanowire by changing the incident laser polarization.
The optical trapping system (figure7) was constructed around a dark-field microscope (Olympus B-51X). A 1064 nm laser was focussed onto a diffraction-limited spot using a 50×objective lens (Olympus, numerical aperture (NA)=0.8). The used incident power ranged from 100 to 200 mW. We used a dichroic mirror (Thorlabs, DMSP 1000) to route the laser into the microscope objective. The laser was expanded by 2×system to overfill the microscope back-aperture. The lenses used to build the beam expander system were of focal lengthsf1=30 mm andf2=60 mm. The white light illumination for dark-field imaging was provided by a dark-field condenser lens (Olympus, NA=0.9–1.4). After the dichroic mirror, we used a 750 nm short pass filter to ensure that no laser light goes into the imaging CCD. A half wave plate was used to rotate the polarization.
Chemically prepared silver nanowires were first diluted using milli-Q water. Then a very small amount of the suspension was added into the chamber made of two cover slips stuck to each other by a 100μm thick double-sided tape. Figure8shows dark-field time- series images of nanowire trapping. The wire was 5μm long. Three types of wire trapping were observed: first and the most probable was that the wire got trapped at one of its ends, aligning perpendicular to the incident laser polarization. Second, the wire trapped at the
Figure 7. Schematic diagram of optical tweezers set-up. M1, M2and M3are mirrors and L1,L2are lenses.
Figure 8. Dark-field time-series images of trapped 5μm long nanowire, showing its rotation with change in incident polarizations, denoted byθ. Vertical direction has been taken as reference (0◦)for incident polarization direction.
centre of its axis. Last and the least probable: the nanowire trapped in 3D by aligning itself in the direction of beam propagation to minimize scattering forces. In figure8, an example of the first type of trapping is shown. We have taken the vertical axis as the reference for incident polarization direction. Figure8a shows the case when incident polarization was 0◦. Now as the polarization was rotated, the nanowire rotated accordingly (figures8b–
8h), maintaining a 90◦angle with incident laser polarization direction. This perpendicular alignment to the incident polarization is expected because largest near-fields are created at the nanowire ends. As the polarization was rotated, the wire experienced a torque and aligned accordingly. Such optical alignment and rotation capabilities of plasmonic nanowries will have implications in nano- and biophotonics applications.
5. Conclusion
Thus we have discussed how geometrical arrangement of plasmonic nanostructures has important consequence on the propagation and localization of light at subwavelength scale. Furthermore, by using the principles of deterministic aperiodicity, where a spe- cific mathematical principle is used as a design rule to arrange nanostructures in two dimensions, one can also obtain novel optical properties at subwavelength scales. Such aperiodic arrangement leads to optical localization characteristics that are otherwise absent in periodic nanostructures. We have been looking into the geometrical tuning of near-field optical enhancement in plasmonic nanostructure, and have proposed a vari- ety of geometries such as Pascal triangles [21,22], nanocrescent [23], nanoantenna [24]
and gap plasmon structures [25]. Innovative plasmonic nanoarchitectures such as coupled nanowire systems and other emerging geometries can be excellent test-beds for nanoplas- monic circuitry and nano-optical sensors. By tailoring the geometry, morphology and illumination characteristics, plasmonic nanosystems can be used for nano-optics, quantum nanophotonics and biomolecular spectroscopy.
References
[1] P G Kik and M L Brongersma, Surface plasmon nanophotonics, in: Springer series in optical sciences (Springer, Dordrecht, 2007) Vol. 131, p. 1
[2] S A Maier, Plasmonics: Fundamentals and applications (Springer, New York, 2007) [3] L Novotny and B Hecht, Principles of nano-optics (Cambridge University Press, Cambridge,
UK, 2006)
[4] S I Bozhevolnyi, Plasmonic nanoguides and circuits (Pan Stanford Publishing Pte. Ltd, Singapore, 2009)
[5] G V P Kumar, J. Nanophoton. 6, 064503 (2012)
[6] A V Zayats, Smolyaninov II and A A Maradudin, Phys. Rep. – Rev. Sec. Phys. Lett. 408, 131 (2005)
[7] W L Barnes, A Dereux and T W Ebbesen, Nature 424, 824 (2003) [8] G V P Kumar, J. Nanophoton. 6, 06450301 (2012)
[9] S Lal et al, Accounts of Chemical Research 45, 1887 (2012)
[10] R Chikkaraddy, D Singh and G V P Kumar, Appl. Phys. Lett. 100, 043108 (2012) [11] D Singh, M Raghuwanshi and G V P Kumar, Appl. Phys. Lett. 101, 111111 (2012) [12] S Polali, D Singh and G V P Kumar, J. Phys. D: Appl. Phys. 46, 195107 (2013) [13] P P Patra and G V P Kumar, J. Phys. Chem. Lett. 4, 1167 (2013)
[14] S Nie and S R Emory, Science 275, 1102 (1997)
[15] E C Le Ru and P G Etchegoin, Ann. Rev. Phys. Chem. 63, 65 (2012) [16] K Kneipp et al, Phys. Rev. Lett. 78, 1667 (1997)
[17] J Steidtner and B Pettinger, Phys. Rev. Lett. 100, 236101 (2008) [18] H Xu et al, Phys. Rev. Lett. 83, 4357 (1999)
[19] G V P Kumar et al, J. Phys. Chem. C 111, 4388 (2007) [20] G V P Kumar, J. Raman Spectrosc. 40, 2069 (2009) [21] G V P Kumar, Int. J. Nanosci. 11 (2012)
[22] G V P Kumar, Appl. Opt. 49, 6872 (2010)
[23] G V P Kumar, J. Opt. Soc. Am. B: Opt. Phys. 29, 594 (2012) [24] A Dasgupta and G V P Kumar, Appl. Opt. 51, 1688 (2012) [25] N Mishra and G V P Kumar, Plasmonics 7, 359 (2012)
