A tip-enhanced Raman spectroscopy (TERS) based on plasmonic lens (PL) excitation is proposed in this work. A PL expected to realize a strong longitudinal electric field focus is designed. The focusing performance of the PL is calculated via finite-difference time-domain (FDTD) simulation and experimentally detected by a scattering-type scanning near-field optical microscope. The PL is introduced to a TERS system as a focusing device. Experimental results with carbon nanotube samples indicate that the Raman scatting signal is significantly enhanced. It proves experimentally that the combination of a PL focused excitation field with a metallic tip in a TERS system is a promising method.
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Tip-enhanced Raman spectroscopy (TERS) [1–4] has become a powerful and promising chemical and physical information detection method in the nanometer scale, due to its high spatial resolution and detection sensitivity [5–11]. The TERS technique uses a sharp metallic tip regulated in the near-field of a sample surface, which is illuminated with a certain incident beam meeting the excitation conditions of the wave-vector matching [12, 13]. The local electric field, and, consequently, the Raman scattering, from the sample in the vicinity of the tip apex are both greatly tip-enhanced owning to the excitation of localized surface plasmons (LSPs) and the lightning-rod effect. Typically, a TERS setup is composed of a scanning probe microscope, excitation and collection optical configurations, and a Raman spectroscope. In existing TERS setups, an objective lens or a parabolic mirror  is always used as the most important component in the illumination configuration, in order to focus the incident beam on the tip apex for excitation [15, 16].
In this work, a novel metallic plasmonic lens (PL), instead of conventional optical objective, is used in the illumination configuration. The PL not only focuses an incident beam to a sub-wavelength light spot, but also realizes a strong longitudinal electric field illumination, i.e. in the z-component of the electric-field (Ez) with linearly polarized incident beam. Using a PL in the illumination configuration of TERS improves the detection sensitivity of the setup in two aspects. First, the ability of the PL focusing the incident beam to a comparatively small volume. Second, the PL focusing the surface evanescent wave generates a longitudinal field which dominates the excitation field . Therefore, the PL is ideal for the excitation of tip-enhancement. Additionally, usage of a PL in TERS setup will be of benefit to an ultra-compact configuration and the lab-on-a-chip technique. The designed PL is a piece of gold film engraved with a semi-annular slit and three annular slits. It is suitable for focusing linearly polarized beams and generating effective excitation fields for TERS. Finite-difference time-domain (FDTD) simulations and experimental characterizations are presented. The theoretical and experimental results on the generation of the excitation field and the tip-enhancement based on the PL are discussed.
2. Theoretical design and physics analysis of PL
The Raman scattering signal from the nanometer volume is quite weak and susceptible to be drowned out by background noise. Therefore, the high enhancement of the local field is vital in TERS detection. Theoretical research suggests that contributions from the lightning-rod effect and excitation of localized surface plasmons depend primarily on the polarization state of the excitation field. It has indicated that for a sharp, conical tip, the longitudinal electric field, with a polarization direction parallel to the axis of the tip, is significantly effective for the concentration of free electrons at the tip apex . Consequently, it greatly enhances the optical field in the region close to the tip apex. In addition, the size of the focused illumination region on the sample should be as small as possible. The reason for this is the whole illuminated region will produce far-field scattering with average spectral information and bring about background noise to the TERS signal of each specific area on the nanometer scale. Thus, an illumination field spatially concentrated to a small region and with a strong longitudinal electric field component is desirable for high enhancement of TERS excitation. Aiming at obtaining an excitation field approaching the ideal one, a metallic PL is designed.
2.1 Design and analysis of the PL structure
PL is capable of focusing the evanescent components in the near-field region with sub-diffraction-limit resolution . Due to its unique features, a PL, instead of a conventional objective, is introduced to the illumination configuration of a TERS setup. It is designed for the purpose of focusing a linearly polarized incident beam to a concentrated light spot on the order of nanometers and creating strong longitudinal field components in the center of the structure.
According to the specifications of generating the desired excitation field, a PL with a concentric semi-annular and three annular nano-slits corrugated on a gold film supported by glass substrate is implemented. The schematic geometry of the PL is shown in Fig. 1 . The thickness of the gold film, h, is 150 nm and the width of slits, w, is 100 nm. The additional outer semi-annular slits with spacing, p, of 300 nm, approximately half a wavelength of the SPP wave propagating on the air-gold interface, behave as Bragg reflectors [19,20] for diminishing SPP waves propagating toward the outer side of the structure and intensifying the energy density at the focal region.
Since the film is much thicker than the skin depth, the incident light cannot penetrate through the film, except in the region where the slits are fabricated. The laser light is linearly polarized parallel to the x-axis direction, shown by the arrow in Fig. 1. To explain clearly the design ideas of the PL, we divide the structure geometry by the dashed line in Fig. 1(b) into two regions, the left side and the right side. Both the left and right side of the structures are composed of a group of semi-annular slits, three semi-annular slits on the left and four on the right. The sharp edges of sub-wavelength slits act as plasmonic launchers to couple light into surface plasmon-polaritons (SPPs) under appropriate illumination. Each semi-annular edge can be regarded as a series of SPP point sources. Incident polarization dictates only the portions of the semi-annular where a component of the incident electric field is perpendicular to the annular edge excites SPPs . When the linearly polarized excitation beam is normally incident on a semi-annular slit, the excited SPP waves will propagate on the film surface and concentrate with interference in the center of the semi-annular [21–23].
In order to improve the excitation efficiency, concentric semi-annular slits are set on the left and right side. Counter-propagating SPP waves are generated by the two sides, causing constructive or destructive interference to occur in the center depending on their phase difference. Under linearly polarized beam illumination, there is a π-phase difference in the initial phases of the longitudinal field between the counter-propagating SPP waves from the two sides. Via this mechanism, if SPP waves from the two sides meet after propagating an equal distance, they are out of phase and destructive interference will take place in the center [24, 25]. For this PL structure, the inner radius of the semi-annular slit on the right side (Rr) is 1.25μm and on the left side (Rl) is 1.55μm. The radius mismatch between the two sides (Rr - Rl) is approximately half a wavelength of a SPP wave. This results in a symmetry breaking in the structure . The symmetry breaking generates a π-phase shift in the propagating SPPs, regulating the total phase of the electric field in the center. Hence, the initial phase difference between the two sides is compensated by the symmetry breaking structure providing unequal SPP wave propagating lengths. With the phase compensation, constructive interference occurs in the center of the PL. Since the prominent component of electric field in SPPs is a longitudinal one [25, 27], a bright hot spot in the center of the PL pattern can be obtained by the constructive interference of the longitudinal field.
2.2 FDTD simulation of the optical field distribution from PL
The FDTD (FDTD Solution, Lumerical, Canada) method was employed to investigate the optical near-field distribution resulting from the light-nanostructure interaction. The illumination conditions and represented field distributions correspond to linearly polarized excitation and a 632.8 nm wavelength, which is consistent with the experimental conditions. The simulation results are shown in Fig. 2 .
The parameters and structure of the PL are as above. A linear polarization illumination beam along the x-axis is normally incident from the bottom as shown in Fig. 1. The calculated longitudinal electric field intensity distribution in xz plane, shown in Fig. 2(a), demonstrates that the induced electric field is confined in the x-direction and more extended in the z-direction. As shown in Fig. 2(b), the linearly polarized incident beam is focused into solid bright spot in the center. The full width at half maximum (FWHM) of the focusing spot is about 300 nm, less than half of the incident wavelength in the x-axis direction. The most energy is concentrated in this spot. Comparing the phase distributions on the two sides shown in Fig. 2(c), it can be seen that the longitudinal components are almost in phase in the center region and thus constructively interfere, contributing significantly to the enhanced field intensity.
3. Experimental Results and Discussions
3.1 Experiment on optical field detection of PL
A smooth gold film with 150 nm thickness was first evaporated onto a glass substrate. Patterns with a 100nm slit-width were then milled through the film using a focused ion beam.
In order to demonstrate the focusing performance of the PL, the near-field optical distribution from the PL was measured using a scattering-type scanning near-field optical microscope (s-SNOM). A collimated light beam, from a He-Ne laser of 632.8 nm, illuminates the bottom glass side on the PL structure. The near-field intensity was detected with a silver-coated apertureless tip on the opposite side, shown in Fig. 3(a) . The scattered light was collected with a parabolic reflector and detected with a photomultiplier tube (PMT). By raster scanning the tip over the central region of the PL surface, the near-field optical intensity distribution was obtained. The experimental setup is shown schematically in Fig. 3(d).
First, topography of the PL was imaged using an atomic force microscope (AFM) (NT-MDT, NTEGRA, Russia) and a scanning electron microscope (SEM) respectively, shown in Figs. 3(b) and 3(c). After that, the near-field optical distribution resulting from light-PL interaction was characterized by s-SNOM with a silver-coated apertureless scattering tip. The reason this procedure is used is because apertureless scattering tips are more sensitive and suited to the longitudinal field detection. They ensure the obtained signal is mainly from the longitudinal field. Aperture probes, in contrast, tend to be more sensitive to the transverse field whose electric field is perpendicular to the axis of the tip [28, 29]. The experimental result shown in Fig. 4(b) agrees well with theoretical calculation shown in Fig. 4(a). It verifies the focusing performance of the PL. The optical field distribution shows a bright elliptic spot in the lens center and several arc-like side-lobes on both sides. The intensity of the first left side-lobe is comparatively stronger than that of the right side, but is still much weaker than the intensity of the spot in the center.
3.2 TERS experiment with PL
After the focusing performance was experimentally verified, the PL was used as a focusing component in the excitation configuration of TERS setup to concentrate incident energy and generate a longitudinal optical field. The TERS setup was placed on the same platform of the s-SNOM shown in Fig. 3(d). A single-wall carbon nanotubes (SWCNTs) sample was placed in the center of the PL surface. The PL was used as a substrate at the same time. The incident light focused by the PL interacted with the tip apex and enhanced the Raman signal of the sample locally. The tip-enhanced Raman signal collected by the parabolic reflector was guided to the Raman spectroscope (Renishaw, InVia, UK) for detection and spectral analysis.
3.3 TERS results and discussions
Topography of the sample shown in Fig. 5(a) was characterized via tip scanning with atomic force feedback. When compared with the relatively smooth surface of the PL in the central region (Fig. 3(b)), the additional long-shaped bulges can be attributed to the SWCNTs sample. The silver-coated tip was regulated close to the sample surface and fixed above the center of the structure denoted with a black cross in Fig. 5(a). That was also the position where the tip-enhanced Raman spectrum, shown in Fig. 5(b), was detected. Figure 5(b) shows the Raman spectra of the SWCNTs with the tip approached (red curve) and retracted (black curve). For ease of viewing, the two Raman spectra were offset. The Raman shift in G-band (:1590 cm−1) is one of the feature Raman peak of carbon nanotubes originated from the planar vibrations of carbon atoms. It can be used to represent the distribution of SWCNTs and identify a SWCNT as a semiconductor or metallic type [30, 31]. It is clear that the Raman signal is effectively excited and enhanced with the PL and the metallic tip. The enhancement factor for the G-band intensity is calculated to be 4 × 103, taking into consideration tip-enhanced excitation area and PL focus. This is an indication of a strong □eld enhancement, resulting from the combination of a PL focused excitation field with a properly working tip.
In addition, this setup can be further improved in the following two ways. The first improvement would be to segregate the focusing PL and sample to reduce the direct influence of the sample on the excitation optical field. The second improvement would be to enlarge the spatial angle of the parabolic reflector to increase the signal collection efficiency.
In order to investigate the light-matter interaction and field enhancement in the vicinity of the tip apex and the PL’s focus, the FDTD simulation was applied to theoretically calculate the optical field distribution with the tip close to the near-field of the PL. The tip was set above the center of the PL with 10 nm separation between the tip apex and the PL surface. The calculation model is displayed in Fig. 6(d) with a white, dashed line. Figure 6(a) shows the optical field intensity distribution in xy plane and Fig. 6(b) shows the intensity curve along the dashed line. A strong, hot spot was tightly confined to a small region just beneath the tip apex. Within this region, the optical field could be effectively enhanced. Since in TERS the tip acts as an antenna to amplify both the incident and scattered light, the local enhancement of the Raman intensity is proportional to the fourth power of the electric field .
According to our knowledge, this work is the first to introduce a PL to a TERS system as a focusing excitation device and obtain experimental results with SWCNTs samples. A PL with symmetry breaking semi-annular slits corrugated on gold film for the purpose of generating concentrated sub-wavelength light spots with strong longitudinal electric field was designed and fabricated. The optical field distribution was characterized using an s-SNOM with silver-coated tip to demonstrate the focusing performance of the lens. The experimental result is in agreement with the theoretically calculated one. The designed PL has the advantages that it can focus the incident beam to a comparatively small volume and generate a longitudinal component that dominates the electric field with linearly polarized incident beam. Thus, the PL is suitable for generating excitation field for TERS excitation and ensures the effective enhancement and detection sensitivity of this new TERS technique. Additionally, using a PL as a focusing optical device and a substrate in a TERS setup is also one step further towards realizing an ultra-compact configuration and the lab-on-a-chip technique.
The authors are grateful to Zhendong Zhu for TERS tip preparation and helpful suggestions. This work was partially supported by the Nature Science Foundation of China (Grant No. 61177089 and No. 61227014), and the National Basic Research Program of China (Grant No. 2007CB936801) Research on Optical Detection in Nanometric Scale.
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