Finite element method simulations have been carried out on the photonic nanojet (PNJ) mediated surface enhanced Raman scattering (SERS) technique for the first time, and this technique has been found to provide (i) better Raman scattering enhancement of single molecules and (ii) a long working distance between the microscopic objective lens and sample, as compared with the conventional SERS technique. A PNJ mediated surface enhanced fluorescence (SEF) technique has been proposed to enhance the fluorescence of single molecules using the combination of localized surface plasmons inside nanostructures and the PNJ of a dielectric microsphere (MS), and this technique is numerically proved to be efficient as compared with a conventional SEF technique. Moreover, the generation of a PNJ from single lollipop shaped microstructures and its applications in the above mentioned techniques have been reported.
© 2017 Optical Society of America
Recently, single molecule surface enhanced Raman spectroscopy has become an interesting tool for the real time detection of single molecules at their natural state and for studying the structural properties of single molecules with the help of efficient plasmon-active substrates [1,2]. This is extremely useful in the field of proteomics as the structural properties of protein molecules are heterogeneous . Even though it promises to provide valuable information about single molecules, obtaining single molecule Raman spectra is a tedious task. The reports on successful demonstration of the single molecule Raman scattering using a conventional surface enhanced Raman scattering (SERS) technique are scarce [4–9]. In the conventional SERS technique, the excitation laser was focused directly on the SERS substrates containing sample with the help of objective lens of a Raman microscope. Even if succeeded, the signal to noise ratio (S/N) of the experimental Raman data has been found to be poor.
According to electromagnetic theory, the SERS Stokes power (PSERS) originated from a molecule is proportional to the square of the incident light electric field intensity (Iinc) . Based on this, recently researchers were able to enhance the SERS signal of a few molecules by focusing the light through a dielectric microsphere (MS) placed on plasmonic nanosphere dimers and trimers dispersed on a dielectric substrate [11, 12] and it has been called as photonic nanojet (PNJ) mediated SERS technique. The enhancement of SERS signal in this technique is mainly due to the giant electric field intensity enhancement at the hotspots of plasmonic nanostructures in the location of PNJ of the dielectric MS. The PNJ is a narrow focused high intense and non-evanescent electromagnetic beam emerging from the shadow side surface of a lossless dielectric MS or microcylinder illuminated by a non-resonant plane wave or Gaussian wave [13–17].
On the other hand, researchers are also engaged in single molecule surface enhanced fluorescence (SEF) using nanoplasmonic structures for single protein molecule detection and protein dynamics visualization as well as for studying protein-protein interactions [18–20]. These will help in early disease detection, and to understand protein functions and networks in living cells. According to theory of SEF, the fluorescence enhancement (ξ) is given by [21,22]23–25] or by coating ultra-thin dielectric layer on all the nanostructures [26–28]. From Eqs. (1) & 2, it is apparent that the value of ξ depends upon the local electric field intensity enhancement which strongly depends upon the size, shape, relative electric permittivity of nanostructures, and polarization of the excitation light . Therefore, researchers have been trying to improve the enhancement with the help of different kinds of nanoplasmonic structures.
Herein, we report (i) Finite element method (FEM) simulations on PNJ mediated SERS technique for the first time and have estimated the enhancement of SERS signal of single molecule (ζ) residing in the nanogap of a gold nanosphere dimer, silver nanosphere dimer, and silica core-gold nanoshell dimer excited by PNJ of a dielectric MS, and (ii) The dependence of ζ on wavelength of the excitation light. Also, PNJ mediated SEF has been proposed as a new technique for enhancing SEF. With the help of FEM simulations, the enhancement of SEF of single molecules has been estimated and the dependence of enhancement value on excitation wavelength has been studied. Moreover, the generation of PNJ from single lollipop shaped microstructures and its application in above mentioned techniques have been explained.
2. Results and discussion
2.1. Numerical investigation on PNJ mediated SERS technique
In PNJ mediated SERS technique , the excitation light focuses through a lossless dielectric (silica or polystyrene) MS on Raman molecules adsorbed on metal nanoparticles dispersed on dielectric or semiconductor substrate (SERS substrate). The geometry used in modeling in COMSOL software (installed in computing server with an Intel Xeon, 2.67 GHz 64-bit processor, 42 GB RAM) for performing numerical investigation on PNJ mediated SERS technique is shown in Fig. 1. For all our FEM simulations, the values of size (d) and refractive index of a dielectric MS (ns) are fixed at 1.80 µm and 1.45, respectively. Similarly, diameter, thickness and refractive index of the cylindrical dielectric substrate (silica substrate) are fixed at 2 µm, 200 nm, and 1.45, respectively. Moreover, the focused Gaussian beam from objective lens is used for exciting the silica MS to generate PNJ from the shadow side of the MS. In the conventional SERS technique, this focused beam is used directly to excite localized surface plasmons in the SERS substrate.
In the arrangement shown in Fig. 1, the silica MS produces PNJ at its shadow side upon focused Gaussian beam illumination. The electric field distribution of PNJ emerging from this MS obtained using FEM is shown in Fig. 2. It is apparent that the electric field of the PNJ is nearly 4 times larger than the incident electric field, which is due to the constructive interference of incident and strong Mie scattered light at shadow side of the MS. The value of the electric field increases with size and refractive of the MS [14,15]. This PNJ propagates with little divergence and elongated shape for several wavelengths into the surrounding medium. Therefore, it interacts with nanoplasmonic particles located beneath the MS (inset of Fig. 1) and induces higher intensity hot spots on the surface of nanoparticles than that obtained for directly focused Gaussian beam excitation.
Using FEM, the enhancement of electric field and SERS signal of a molecule at hot spots of metal nanosphere dimers and dielectric core – metal nanoshell dimer excited with PNJ, have been numerically estimated and obtained results have been given below.
2.1.1. Enhancement of SERS signal of a molecule residing in nanogap of (i) nanosphere dimers and (ii) nanoshell dimers
Since the enhancement of SERS signal of a molecule is directly related to the local electric field enhancement of a nanoplasmonic structure, first we have estimated the electric field developed at the nanogap of a symmetric gold nanosphere dimer excited with a focused Gaussian beam and PNJ of a MS. Panel A shows the electric field distribution on the surface of nanosphere dimer for the excitation of focused Gaussian beam from the microscopic objective lens of NA: 0.90. It is apparent that the electric field is higher in between the nanospheres (nanogap) due to the efficient dipolar plasmon coupling [30, 31]. By using same objective lens, this local field has been improved when the MS is placed on the nanodimer due to the interaction of intense PNJ with the nanodimer (panel B). This enhancement has become significantly larger when the objective lens of NA: 0.4 was used (panel C). This is due to the efficient focusing of light on the nanodimer by the MS and this is possible when the focused Gaussian beam diameter is nearly equal to the microsphere diameter.
In order to investigate the variation of local field enhancement in the nanogap as a function of wavelength, the electric fields on the surface of the nanodimer have been calculated by varying the wavelength of the excitation light. Green curve in Fig. 3D shows the electric field values at nanogap of a symmetric gold nanosphere dimer placed on a silica substrate, illuminated with a focused Gaussian beam (of different wavelengths) from the microscopic objective lens of NA: 0.90. The blue curve in Fig. 3D represents the electric field values at the nanogap of same nanosphere dimer excited with the PNJ ofa MS generated by a focused Gaussian beam (of different wavelengths) from the microscopic objective lens of NA: 0.9. Here it is clear that the electric field in the nanogap is enhanced significantly for all excitation wavelengths as compared with green curve due to the efficient excitation of localized surface plasmons in the nanodimer by the PNJ of a MS. In addition, more than one peak has been observed in these spectra which indicate the splitting of the localized surface plasmon resonance (LSPR) of a nanostructure in the presence of a dielectric substrate [32,33]. This splitting predominates when the substrate dimensions are in the order of micrometers and it would disappear when the dimensions reduce down to a few tens of nanometers [34,35]. The red curve in Fig. 3(d) represents the electric field values at the nanogap of same nanosphere dimer excited with the PNJ of a MS generated by a focused Gaussian beam (of different wavelengths) from the microscopic objective lens of NA: 0.4. It is apparent that the electric field values are much larger in this case due to the reason mentioned in above paragraph.
After performing the numerical simulations on a gold nanosphere dimer, we emphasized on silver nanosphere dimer. Green curve in Fig. 4A represents the electric field values at nanogap of a symmetric silver nanosphere dimer placed on silica substrate, illuminated with a focused Gaussian beam (of different wavelengths) from the microscopic objective lens of NA: 0.90. The blue and red curves in Fig. 4A represent the electric field values at the nanogap of same nanosphere dimer excited with the PNJ of a MS generated by a focused Gaussian beam (of different wavelengths) from the microscopic objective lens of NA: 0.9 and 0.4, respectively. From this figure we can easily conclude that the electric field enhancements are much better when we use a MS and microscopic objective lens of lower NA: 0.4 as observed in the case of gold nanosphere dimer. Here it is worth to mention that the splitting of the LSPR is not visible due to the lower wavelength range and the peaks in blue & red curves are slightly shifted towards higher wavelength as compared to that in the green curve, which could be due to the additional dielectric perturbation introduced by the MS which is placed on this dimer. However, this red shift is not observed clearly in the case of gold nanosphere dimer due to more broadening of the peaks.
Recently, dielectric core-metal nanoshells have attracted the attention of researchers because of the larger local electric field and wide tunability in the LSPR wavelength. Therefore, numerical simulations on silica core-gold nanoshell dimers have been done. The pink curve in Fig. 4B shows the electric field values at nanogap of a symmetric silica core-gold nanoshell dimer placed on a silica substrate, illuminated with a focused Gaussian beam (of different wavelengths) from the microscopic objective lens of NA: 0.90. The orange and blue curves in Fig. 4B represent the electric field values at thenanogap of same nanosphere dimer excited with the PNJ of a MS generated by a focused Gaussian beam (of different wavelengths) from the microscopic objective lens of NA: 0.9 and 0.4, respectively. From this figure also we can easily conclude that the electric field enhancements are much better with the combination of a MS and the microscopic objective lens of lower NA: 0.4. The splitting of the LSPR of the nanoshell dimer in the presence of a dielectric substrate and red shift of the peaks in the presence of a silica MS have also been observed in this case.
Table 1 shows the enhancement of SERS signal of a molecule residing in the nanogap of gold nanosphere, silver nanosphere and nanoshell dimers excited with PNJ of a MS generated by using a focused Gaussian beam from different microscopic objective lens. It is apparent that the enhancement values are sensitive to the excitation wavelengths and more importantly for all the nanostructures the maximum SERS enhancement is in the order of 101 and 103 for the excitation of nanodimers using the objective lens of NA: 0.9 and 0.4, respectively. Since, better enhancements are obtained using lower NA objective lens, we can easily conclude that the PNJ mediated SERS technique allows us to get more SERS enhancement with the larger working distance which is not common in the conventional SERS technique.
2.2. PNJ mediated SEF technique
In the conventional SEF technique, a laser light of appropriate wavelength focuses directly on a plasmon-active substrate containing sample, by objective lens of a fluorescence microscope. In this arrangement, the electric field of the laser light excites localized surface plasmons (LSP) inside nanoplasmonic structures and the coupled oscillation of the LSP induces intense hot spots on the surface of the nanoplasmonic structures located at the focal point of the microscopic objective lens. The fluorescence photons from labeled protein molecules residing at hot spots are captured using backscattering geometry.
According to Eqs. (1) and (2), the SEF enhancement can be improved by enhancing the excitation enhancement and this could be achieved by improving the local electric field intensity of nanostructures. Therefore, in order to improve the sensitivity for many applications that relies on the use of fluorescence, a PNJ mediated SEF technique is proposed here for the enhancement of SEF by significantly improving the local electric field intensity. In this technique, a laser light focuses on a plasmon-active substrate with the help of a dielectric MS as shown in Fig. 5. As in the case of PNJ mediated SERS technique, here also, the intense PNJ generated from shadow side of the MS excites LSP inside nanoplasmonic structures placed in the location of PNJ as shown in the inset of Fig. 5. Due to this, higher local electric field intensity develops in the nanogaps of nanostructures as compared to that in the conventional SEF technique. Therefore, labeled protein molecules residing in these nanogaps emit more number of photons and these can be captured easily using a high resolution CCD camera and spectrometer in back scattering geometry.
Here, it is noteworthy that the metal core-dielectric nanoshell structures dispersed on a dielectric substrate could be preferred to avoid the direct contact between labeled proteins and metal nanostructures, which will help us to suppress the non-radiative rates. Fortunately, several wet chemistry methods have already been reported in the literature for synthesizing these kinds of nanoshell structures [26–28].
2.2.1. Numerical investigation on PNJ mediated SEF technique
In order to numerically prove that the PNJ mediated SEF technique is better than the conventional SEF technique, it is required to find the enhancement of local electric field intensity in the PNJ mediated technique. Therefore, FEM simulations have been carried out to estimate the local electric field intensity of a metal core-dielectric nanoshell excited directly by a focused Gaussian beam and PNJ of a dielectric MS. Panel A of Fig. 6 shows the electric field distribution of silver core – silica nanoshell dimer on a silica substrate, excited directly with a focused Gaussian beam of wavelength 394 nm. Panel B shows the electric field distribution of the same nanoshell dimer excited with PNJ of a dielectric MS, generated by a focused Gaussian beam from the microscopic objective lens of NA: 0.40. From Fig. 6, it is apparent that the local electric field in the nanogap isenhanced by nearly 8 times when it is excited with PNJ. Therefore, in the PNJ mediated SEF technique, the enhancement in SEF of any labeled protein molecules residing in the nanogap of this nanoshell dimer would be around 64.
To investigate the enhancement in SEF for different excitation wavelengths, the local electric field intensity distribution of nanoshell dimers for different excitation wavelengths have been estimated. The green curve in Fig. 7 show the numerical electric field intensity enhancement values at nanogap of the dimer excited with a focused Gaussian beam from the objective lens of NA: 0.90. The blue and red curves show the electric field intensity at the nanogap of the same nanodimer excited with PNJ of MS generated by a focused beam from different objective lens. As it can be seen in Fig. 7 that the electric field intensity in nanogap is larger in the case of PNJ excitation for all the wavelengths and the maximum enhancement is found to be ~64 (at resonance wavelength) in the case of lower NA objective lens. This means that SEF of single molecules enhances around 64 times in the PNJ mediated SEF technique and provides more working distance relative to the conventional SEF technique. Therefore, one can conclude that the PNJ mediated SEF technique is always better than the conventional SEF technique.
2.3. PNJ of single lollipop shaped microstructures
In 2004, Chen et al.  numerically discovered the nanojet phenomenon in the shadow side of a dielectric cylinder upon a non-resonant plane wave excitation and later, it also has been numerically observed in shadow side of single spherical microparticles [14–16]. Recently, researchers dispersed spherical microparticles on silicon wafers and thin films [36, 37] and demonstrated the enhancement of Raman scattering by focusing the laser light through microspheres dispersed on them. In addition, similar procedure has been used for enhancing the SERS signal of a few molecules using PNJ mediated SERS technique. However, the microspheres are dispersed on the substrates containing sample, are most often difficult to remove thereby making the substrates unusable. Also, the dispersed microspheres are not reusable for any other characterization. These problems can be overcome by using single lollipop shaped microstructures. Using a pulsed CO2 laser, these kinds of microstructures of various sizes (9 µm to 300 µm) could be prepared easily [38–40]. In order to verify that the PNJ still exists after the addition of a stem to MS while converting into a lollipop shape and to verify that the addition does not create any hindrance in the PNJ functioning, we have modeled lollipop shaped dielectric microstructure as shown panel A of Fig. 8 and performed FEM simulations on this microstructure illuminated with plane wave (instead of focused Gaussian beam for simplicity). Changing of the beam into focused Gaussian beam is not expected to change the functionality of lollipop shape.
Panel (b) of Fig. 8 shows the electric field distribution of single lollipop shaped microstructure of refractive index 1.45. From this figure, it is apparent that the PNJ is generated in the shadow side of the MS attached at tip of the stem. It is noteworthy that the PNJ can be generated from small (radius ~λ) as well as large microspheres (radius > 20 λ) . However, we have performed FEM simulations on smaller microstructures due to the limitation of the computing facility. Also, it can be noted that even though a dielectric microcylinder is also reusable, from FEM simulations it has been found that the electric field of PNJ generating from its shadow side is nearly two times lower than that obtained from a MS of same size. Hence, lollipop microstructures are preferable for enhancing SERS and SEF signals.
Numerical investigation has been done on PNJ mediated SERS technique for the first time. Using this technique, the maximum enhancement of SERS signal of single molecule residing in nanogap of nanosphere and nanoshell dimers has been found to be in the order of 103. In addition to the giant enhancement, this technique also provides a long working distance as compared with the conventional SERS technique. The observed enhancement factors could be improved further by using larger MS. The details about PNJ mediated SEF technique for enhancing SEF of single molecule are given. Numerical simulations carried out on this technique showed that the SEF signal could be enhanced at least one order of magnitude. Moreover, generation of PNJ at the shadow side of single lollipop shaped microstructures has been shown numerically. These microstructures are reusable and efficient for enhancing SERS and SEF of single molecules as compared with dielectric microcylinders. Since, PNJ mediated SERS and SEF techniques need only a single step of placing a suitable dielectric MS over a plasmon-active substrate, the findings reported here will enable research community to achieve comparatively higher single molecules Raman scattering and fluorescence enhancements with ease.
We thank Department of Science and Technology, India for financial support.
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