Surface-Enhanced Raman and Fluorescence Spectroscopy with an All

Subscriber access provided by - Access paid by the | UCSB Libraries

C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Surface-Enhanced Raman and Fluorescence Spectroscopy With an All-Dielectric Metasurface Silvia Romano, Gianluigi Zito, Stefano Managò, Giuseppe Calafiore, Erika Penzo, Stefano Cabrini, Anna Chiara De Luca, and Vito Mocella J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03190 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Surface-Enhanced Raman and Fluorescence Spectroscopy with an All-Dielectric Metasurface Silvia Romano⊥ ,† Gianluigi Zito⊥ ,∗,‡ Stefano Managó,‡ Giuseppe Calafiore,¶ Erika Penzo,¶ Stefano Cabrini,¶ Anna Chiara De Luca,‡ and Vito Mocella† †National Research Council IMM, Via Pietro Castellino, Naples, 80131, Italy ‡National Research Council IBP, Via Pietro Castellino, Naples, 80131, Italy ¶Molecular Foundry, Berkeley, CA 94720, USA E-mail: [email protected]

⊥

These authors contributed equally to this work Abstract Plasmonic substrates play a crucial role in the confinement and manipulation of localized electromagnetic fields at the nanoscale. The large electromagnetic field enhancement at metal/dielectric interfaces is widely exploited in surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS) spectroscopies. Despite the advantage of near field enhancement, unfortunately, in metals, the large absorption at optical frequencies induces local heating of the analyte fluid with possible damage of the biological material. In addition, in SEF plasmonic substrates, spacer layers are necessary to minimize undesired fluorescence quenching due to non-radiative decay, which strongly depends on the distance between molecules and metallic substrate. Therefore, the possibility of managing surface electromagnetic states mimicking surface-plasmon

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

resonances in terms of spatial localization, high-field intensity and dispersion characteristics while avoiding metallic losses is of great interest. However, dielectric nanoantennas can currently provide limited possibilities in the visible range of optical frequencies. We present the realization of all-dielectric metasurfaces made of nanostructured transparent silicon nitride supporting bound states in the continuum (BICs). We show that this special kind of Fano resonances can be effectively used in standard microscopy for practical applications. We achieved concurrent enhancements of ∼ 103 fold of fluorescence emission and Raman scattering far field intensities of molecules dispersed on these metasurfaces. In addition, we demonstrate that the gain of conventional SERS signals can be increased of more than one order of magnitude by resonant matching of the localized surface plasmon resonance with the BIC field. Our results can find significant applications for enhanced sensing, Raman imaging and non linear processes.

Introduction The local amplification of the electromagnetic field is key to enhance light-matter interaction in a plethora of applications that include optical detection, 1 energy conversion, 2 non-linear phenomena, 3 bio-sensing in surface-enhanced fluorescence (SEF) 4 and surface-enhanced Raman scattering (SERS) spectroscopies. 5,6 Metallic nanoantennas represent a widely used approach to localize the electromagnetic field down to the molecular scale. 3 Nowadays plasmonic devices have reached a mature degree of engineering, igniting the paradigm of flat optics with metal metasurfaces. 7,8 However, the advantages provided by plasmonic nanostructures come at the price of high optical losses caused by light absorption of metal electrons. Although advantageous for several applications including photothermal cancer therapy, 9 local heating causes damages to the biological material under investigation and to the nanostructure itself by altering its photonic properties. In other words, the plasmonic route of optical energy localization and storage limits real-world applications. 10 In addition, in SEF, in order to provide a significant amount of fluorescence amplification, it

2


Page 2 of 27

Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is necessary to reduce the distance-dependent non-radiative decay channel with a complex architecture, typically a nanometer-range spacing layer. However, this strategy is detrimental to SERS detection because molecules are too far from the so-called hot-spots. For this reason, SEF and SERS are typically incompatible with the same nanostructure architecture. Recently, several steps have been made to move beyond the aforementioned limits of plasmonic nanoantennas, basically taking advantage of high refractive index dielectrics applied in NIR applications (such as silicon, germanium or gallium phosphide), unfortunately with limited quality factors (Q < 100). 11 An alternative class of optical resonances that can be highly promising for increasing lightmatter interaction at the nanoscale is represented by so-called optical Bound states in the Continuum (BICs), 12–15 recently observed in photonic crystal metasurfaces (PhCMs) with engineered geometries. 16–25 These modes can be seen as radiationless and localized electromagnetic waves that are embedded in the continuum of radiative modes carrying energy away from the structure. 22 As such, BICs show infinite radiative Q−factors. In real experimental structures characterized by finite sizes, scattering losses, material density fluctuations and unavoidable material absorption, including also the finite collimation of the excitation beam, the leaky-modes that approch the BIC point in the space of parameters may possess arbitrarily large radiative Q−factors. 24,26–28 These modes can be completely decoupled from the continuum of propagating free-space radiation mainly because of two mechanisms: i ) mode symmetry incompatibility with the symmetry of propagating fields - these modes are termed symmetry-protected; and ii ) destructive modal interference occurring in the space out of the confinement region, which is known as resonance-trapping mechanism. 19,24,25,27 BIC modes can be engineered by standard scalable technology with materials that do not suffer from absorption losses and can be readily employed in the visible range. 19,20 Herein, we demonstrate that resonance-trapped BICs are highly robust against input beam collimation and extension and can be used for real world applications in standard optical microscopy. We present - for the first time to the best of our knowledge - three major results

3



obtained with a microscopy configuration for resonance-trapped BICs. (i ) We experimentally demonstrate that BIC optical field enhancement in proximity of the PhC metasurface can boost the fluorescence emission of probe molecules of ∼ 103 -fold with a beam waist of solely 1.5 µm. (ii ) Then, we observe a significant amplification of the Raman scattering on the same nanostructure, very useful for label-free spectroscopic imaging in bio-medicine. (iii ) Finally, we demonstrate a novel synergistic approach based on the BIC pre-amplification of the surface plasmon gain provided by a model plasmonic system placed on the PhCM. All the results were obtained using the same platform, i.e. a large-area PhCM resonator made of silicon nitride, thus free from absorption losses in all the visible and infrared range. In contrast to current dielectric nanoantennas technology based on Si, 11 our large-area, cavity-free approach with a transparent medium and optical-resonance engineering facilitates the microscopy application towards cell imaging, which would be very challenging with Si nanoantennas non-transparent to visible radiation. Our study represents a crucial step for technological implementation of BIC-enhanced spectroscopic microscopy for dual labeled and label-free imaging at the single-cell scale, which can find application in a large variety of bio-imaging techniques including super-resolution microscopies. 29

Results and Discussion Design and Fabrication. Figure 1a shows a schematic of the photonic crystal structure implemented in this work. The PhC metasurface is made of a two dimensional square lattice of cylindrical holes etched on a silicon nitride thin film of ∼ 50 nm deposited on a commercial glass coverslip (Fig. 1b). All details of the pattern design, resulting from extensive numerical simulations, can be found in Materials and Methods. Ellipsometric measurements of as-deposited silicon nitride films are shown in Fig. S1 in the Supporting Information. All the PhC patterns were optically characterized by measuring their transmission spectra as a function of the incidence angle (Materials and Methods). The experimental wavelength-

4


Page 4 of 27

Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


angle (wavevector) dispersion diagram was accordingly reconstructed. In Fig. 2a, we show the transmission TE band for a PhCM with thickness 54 nm. When a mode is excited at a specific angle, a very narrow dip appears in the transmission, which progressively vanishes approaching the diverging Q-factor BIC mode. 19,24 Symmetry-protected BICs have been used for light emission enhancement. 30 However, a stringent bottleneck of this approach is due to the necessity of large area, collimated illumination and precise optical alignment at the normal incidence. The Q-factor depends on the illuminated area of the PhCM, which must have macroscopic sizes in order to support high Q. This issue poses a significant fabrication challenge and hampers microscopic applications, 31 while it would be highly advantageous to use BIC-based field enhancement for microscopy applications. More recently, supercavity lasing 24 and structured light emission 32 have been demonstrated with resonance-trapped BICs. This kind of BICs provide a larger versatility in the pattern design and excitation conditions. 27 In our PhC metasurface, three dispersion bands are experimentally evident (modes 1-3) (Fig. 2a). The first one corresponds to a singly degenerate mode (simmetry-protected mode). 13 Ideally, the mode is characterized by arbitrarily large Q-factor at θ = 0◦ . However, its quality factor decreases dramatically away from the normal incidence (Γ point, θ = 0◦ ). The top inset of Fig. 2a shows the expected symmetry-protected BIC (simulations details in Materials and Methods, further notes in Supporting Information, Figs. S1-S3). The intensity profile in the unit cell (x, y)-plane is rendered with a blue-to-red colormap, whereas the superimposed arrow map shows the distribution of the electric field having a vortex character with a 4-fold rotational symmetry and consequent decoupling from the continuum. The field is evanescent in the normal direction zˆ (Fig. S3, SI). The bottom dispersion band in Fig. 2a splits in two for non-normal incidence and corresponds to a doubly degenerate mode, i.e. the trapped resonance of interest to our work. 24 The bottom inset depicts the calculated mode showing an intensity profile revealing a symmetry compatible with radiative fields. Yet, the mode is trapped in the structure as evidenced by the arrow map of the electric field showing

5



its vorticity and the evanescent decay in both air and quartz (Fig. S3, SI). This kind of BICs is produced by destructive interference among modes 2 and 3 and can occur at any wavevector - thus also at θ = 0◦ - depending on the pattern parameters. 24 They depend on the geometry of the unit cell, but as experimentally demonstrated for lasing action, 24 an interesting advantage of these BICs consists in the weaker dependence on the sizes of the finite experimental PhCM. Recently, this less restrictive size effect has been further strengthened with the observation of structured vortex lasing from metasurfaces with side length of only 8 unit-cells. 24,32 In our sample geometry, a width of solely 8 unit cells corresponds to approximately 2.7 µm (Fig. 1b). In addition, resonance-trapped BICs preserve a very high Q-factor within a much broader angular range of incident excitation. 24 Both conditions provide the opportunity for microscopy-like interrogations with an objective lens of large numerical aperture (NA), since despite the integration over the aperture cone, the integrated Q-factor is still large-enough to induce a significant near-field amplification as demonstrated in this work. In order to determine its robustness against the spread of wavevectors of the input excitation, we measured the linewidth of the resonance-trapped mode as a function of the numerical aperture of the illuminating objective lens. Results are shown in Fig. 2b. In this case, the Q-factor must be considered integrated over the solid angle of excitation. It is worth stressing that for increasing NA the excited area of the metasurface becomes progressively smaller, from a laser beam waist of ∼ 1 mm for a collimated beam (NA = 0) to ∼ 2 µm for NA = 0.8, respectively. The experimental Q-factor (integrated over the NA cone) decreases only of a factor ∼ 2. It continues to be large despite the microscale area of interrogation and the wide solid angle of illumination, which demonstrates the robustness of the resonance-trapped BIC mode.

BIC-enhanced Fluorescence. We investigated the amplification of the fluorescence

6


Page 6 of 27

Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1: Design and fabrication. a Schematic layout of the two-dimensional square PhCM. b Scanning electron micrographs of a PhCM sample, top-view, side-view and tiltedview. The inset shows a photograph of the large-area PhCM. signal of probe molecules on our photonic crystal metasurface with a microscope setup. Measurements were carried out with an inverted Raman microscope (details in Materials and Methods). The excitation wavelength was at 532 nm. The fluorescence spectrum was acquired in backscattering using the same excitation water-immersion objective having a numerical aperture NA = 1.2 and magnification 60×. This allowed us to study the microscopic spatial distribution of the signal. Figure 3a shows the optical image of the sample. We first measured the blank signal from the bare PhCM sample, which showed the auto-fluorescence spectrum reported in Fig. 3b. Strong peaks with asymmetric Fano-like profiles were observed in the collected backscattered signal. The set of PhCM resonances was observed in the range 540 - 570 nm (see also Fig. 2a). No signal was observed outside the PhCM area. The Fano-like profile is due to the coupling with continuum that any real structure shows and cannot be avoided because of the deviation from the ideal BIC point. Since the symmetry-protected mode was not identifiable in the transmission spectrum upon convergent illumination (Fig. 2b), we ascribed the Fano-like resonance to the resonance-trapped BIC mode. The resonant mode, exponentially decaying in air, produces a significant near field enhancement in a region close to the surface. Figure 4a shows a cross-section of the mode intensity inside the unit cell. It is worth mentioning that when the refractive index of the material

7



Figure 2: Experimental optical characterization of the PhCM. (a) Dispersion band diagram from the TE-transmission spectra along the ΓX direction close to the normal incidence: the dips in the normalized transmission indicate three resonant modes. The dark red-to-white colormap indicate the relative transmittance. The top right inset is an experimental zoom of the region around θ = 0◦ in the rectangle, showing the vanishing character of all the modes towards zero. The upper band is characterized by a vanishing linewidth towards θ = 0◦ . The first singly-degenerate mode is a symmetry-protected BIC. The top inset shows the normalized intensity of the calculated mode profile (blue-to-red colormap). The lower doubly-degenerate band instead shows avoided crossing resonances at Γ splitting for larger wavevectors (kx = (ω/c) sin θ) and is associated to a resonance-trapped BIC. The bottom inset shows the normalized intensity of the calculated (degenerate) mode profile. (b) Experimental measurement of the Q-factor of the symmetry portected BIC (singly-degenerate mode) and resonance-trapped BIC (doubly-degenerate mode) as a function of the NA of the excitation objective lens: for NA > 0.1, the first becomes no more identifiable in the spectrum. 8


Page 8 of 27

Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3: (a) Bright field image of the sample (left side: region outside the PhCM; right side: region inside the PhCM, sketch superimposed). (b) Autofluorescence signal corresponding to the regions inside and outside the PhCM. No relevant autofluorescence was observed out of the PhCM area.

9



over the PhCM increases, the resonant wavelength may shift to longer wavelength, which could be actually used for sensing. 33,34 Generally speaking, a different band structure can appear. In our case, the transmission spectrum at normal incidence confirmed the same band structure. A variation of the refractive index using matching liquids (Cargille oils) revealed a linear variation of the resonant peak wavelength, as reported in ref. 33 Defining the effective interaction distance lint as the length at which the field intensity decays of a factor 1/e2 with respect to the value on the top pattern surface, we estimated a decay length lint ∼ 90 nm in air. Within this range, a strong interaction with probe molecules is expected. In case of a small perturbation of the surface area, the decay length is supposed not to be affected, thus we assumed no variation in the decay length in the case of a molecular layer deposition. In the case of fluorescence, we studied the emission of fluorophores dispersed on the metasurface into a PMMA polymer coating (n = 1.5) of thickness 3 µm, with a concentration of 10 µM (Materials and Methods). In this case, the decay length slightly increases, lint ∼ 150 nm, because of the more symmetric configuration of the sandwiched PhCM system, but still remains of the same order of magnitude. The fluorescent rate depends of the product between the excitation enhancement, depending on the local electromagnetic field enhancement, and the intrinsic quantum yield that can be modified/increased by the actual local density of optical states radiating in the far field. In this work, we focused our attention mainly to the excitation enhancement. For this reason, we used commercially available molecules of rhodamine-6G (R6G) as flurophores (sketch in Fig. 4b), having a quantum yield > 0.7, and a strong absorption band in the range 440 570 nm - peaked at ∼ 530 nm - with fluorescence emission in the range 510 - 650 nm peaked at ∼ 552 nm (Fig. S1, SI). Considering that the PhCM resonance wavelengths were in the range 540 - 570 nm (depending on the excitation angle) (Fig. 2a) and the fact that we used a wide-solid angle excitation, a good matching between absorption and emission of R6G is ensured. R6G molecules were deposited on both the PhC metasurface and surrounding unpatterned

10


Page 10 of 27

Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface. Inside the PhCM the fluorescence emission intensity was amplified up to a factor ∼ 40 with respect to the surrounding unpatterned Si3 N4 film (Fig. 4b). This was repeatedly verified with multiple raster scans on an area as large as the whole PhCM. Following the normalization of the signal to the effective near-field volume (determined by the molecules subject to the near field amplification, i.e. within lint ∼ 150 nm) we derive a BIC-based fluorescence enhancement factor of FBIC−F = 790, i.e. of the order of ∼ 103 (Materials and Methods). We ascribe the observed amplification of the emitted fluorescence photons mainly to the resonant field at the pump wavelength within a certain cone of angles around the normal to the PhCM, in other words to the integrated Q-factor near the pump wavelength. This is consistent with the good amplification of the excitation rate that we observed, that likely can be improved upon optimization of the system. Also a partial increase of the radiation efficiency of the fluorophores can be responsible for the observed amplification. The above value is comparable with state-of-the-art SEF enhancement factors expected for gold plasmonic systems 35 and is also comparable with what reported for non-plasmonic Si nanoantennas in the NIR. 11 In our case though, besides the transparency of the PhCM to the visible range and the fabrication ease, a remarkable advantage is in that while the field is confined along the z-axis (Fig. 3a), it is delocalized in the PhCM plane over a large area, which increases the sensing area and ease of use. The PhCM can then be used as a platform to implement more sophisticated devices. An example is illustrated in the last section where we take advantage of the synergy between the BIC mode and the localized surface plasmon resonance (LSPR) of gold nanoparticles (Au-NPs).

BIC-enhanced Raman effect. Raman spectroscopy has important biological applications, 36–38 in particular as non-invasive label-free microscopy technique alternative or complimentary to fluorescence microscopy. However, Raman scattering is a weak process that often requires amplification by means of SERS in suitable metal nanostructures. 5,6 Our second investigation aimed at demonstrating the enhancement of the spontaneous Raman scattering

11



Figure 4: (a) Numerical simulation showing the electric field intensity inside the unit cell in resonance condition: the local field is mainly confined at the PhCM/SiO2 interface but also at the Air/PhCM interface the enhancement can induce strong light matter interaction. (b) (Left) Schematic layout of the PhCM with the R6G fluorophores deposited over the whole sample surface. (Right) Fluorescence emission spectra of R6G molecules in and out of the PhCM. (c) (Left) Schematic layout of the PhCM with the CV Raman analytes deposited over the whole sample surface. (Right) Raman scattering spectra of CV molecules inside and outside of the PhCM.

12


Page 12 of 27

Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5: (a) Hyperspectral SERS map of a characteristic CV band obtained by raster scanning a region outside and inside the PhC metasurface. (b) SERS spectra from positions 1-2 and BIC-SERS spectra from positions 3-4 as depicted in (b).

13



provided by the local near-field amplification of a large-area and transparent all-dielectric structure at the BIC. Since the Raman scattering process is proportional to the near-field intensity of the pump laser, for efficient Raman yield amplification, the BIC resonance must match the pump wavelength at 532 nm. Therefore, we scaled the lattice constant of the PhCM of about 5% to have a resonance at 532 nm. For this experiment we used molecules of crystal violet (CV) dye as probe Raman analytes (Fig. 4c). This choice was prompted by the fact that CV molecules have a fluorescence emission range that is far enough from their spectral range of Raman fingerprint. Thus the CV Raman scattering signal is not completely embedded in a strong fluorescence background as instead occurs for the Raman spectrum of R6G. To this end, 40 µl of a 10-µM solution of CV in ethanol was drop-casted over an area of 4 mm2 covering both the patterned and unpatterned areas of the silicon nitride (Materials and Methods). In order to maximize the amplification of the Raman signal we slightly tilted the sample angle of approximately one degree in order to have a better resonance matching. Only upon close matching between pump wavelength and BIC wavelength attained close to the normal, we were able to observe a good amplification. Figure 4c (right panel) shows the BIC-enhanced Raman spectrum excited on the PhCM, and for comparison, the spectrum outside the structure. No appreciable Raman scattering on the unpatterned silicon nitride was detectable. We can consider that, outside the PhCM area, any Raman peak amplitude is not detectable because masked by the background noise. In order to evaluate conservatively the Raman yield amplification provided by the BIC field we considered the noise amplitude as nonamplified reference value. By doing so, we achieve a BIC enhancement factor FBIC of the Raman signal FBIC−R > 103 . Such an amplification means that in principle is possible to amplify the Raman signal of biological membranes without complex plasmonic nanostructures, thus avoiding any photodegradation of the probed molecules.

14


Page 14 of 27

Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BIC-LSPR hybrid effect. Studying the interaction of a BIC-resonant field with the LSPRs of a model plasmonic system is significant because the BIC resonances may be conveniently exploited to push further the limit of amplification of resonant plasmonic nanostructures. As a proof of principle, we used chemically synthesized AuNPs of diameter d ' 40 nm that have the dipole-LSPR peaked at λres = 530 nm. A single AuNP with d = 40 nm induces, at resonant excitation, a local gain of the incident electric field |E/E0 | = |(Es + E0 )/E0 | ' 6, which provides only a minimal plasmonic amplification or SERS enhancement factor. 5 If the isolated NP is excited by a BIC mode (Fig. 4a) at the frequency matching the NP plasmonic resonance, the expected amplification in the NP near-field increases in cascade. In this work, AuNPs of 40 nm were dispersed randomly over the PhC metasurface and over the surrounding unpatterned area (Methods). The initial solution of sodium citratestabilized colloids was clearly red, hence with no evidence of aggregation. AuNPs were deposited following the deposition of CV molecules, as done for the Raman measurements described above. The presence of NPs on the surface did not destroy the BIC mode being it a large-area mode very robust against external perturbations. However, we measured a broadening of the resonance linewidth up to 4 nm, i.e. of factor 16 with respect to the pristine sample. A representative hyperspectral map of the Raman band intensity of CV at 1620 cm−1 is shown in Fig. 5a. Mapping the SERS intensity as a function of the spatial coordinates allowed us to better identify the amplification of the AuNPs and the additional boost provided by the PhCM. The correlation of the signal boost within the PhCM was verified repeatedly on large areas. The SERS spectra reported in Fig. 5b have been extracted from positions 1 4 as indicated in Fig. 5a, outside and inside the PhC metasurface. Outside the PhCM area, the SERS signal was only due to the LSPR of AuNPs. In contrast, we observed a significant amplification of a factor 13 of the SERS signal as a result of the BIC-LSPR synergistic amplification on the patterned region of the PhCM (FBIC−S ∼ 13). This result points out the remarkable possibility of engineering a mutual response between

15



the PhCM at the BIC point of the parameters space and other dielectric/plasmonic nanostructures (or emitters like quantum dots). For what concerns our simple experiment, a straightforward consequence is that the input power on the sample can be even 13 times less intense to provide the same scattering signal that AuNPs would provide alone, reducing heating due to the metal absorption. To confirm that the PhCM influence on the amplification was not due to a diffractive effect, we carried out scan measurements on the same PhCM off-resonance at the second wavelength of our Raman setup, i.e. at 638 nm. Remarkably, in this case there was no enhancement on the PhCM. The SERS signal was random over both patterned and unpatterned area with no correlation with the PhCM nanostructure.

Conclusion In this work, we have engineered an all-dielectric, nanoscale-thick metasurface of high refractive index, transparent to visible radiation, having a photonic lattice supporting bound states in the continuum. We have demonstrated, with three proof-of-principle experiments, that standard optical microscopy interrogation of this BIC-based open cavity resonator enables fluorescence rate and Raman scattering enhancement. In contrast to plasmonic nanostructures, which are characterized by detrimental losses, our metasurface is a loss-free, transparent and extended dielectric platform. The structure can be readily employed for any kind of biological investigation because it is simply based on a suitably designed large-area structured glass. The strongly enhanced field close to the metasurface and the easy access to it provides a new platform for sensing applications. The BIC mode is tightly confined along the normal direction but is delocalized along the periodicity plane, which allows large area sensing. Our approach can find important applications in many kinds of labelling-based fluorescence spectroscopies 29 and concurrently label-free Raman spectroscopic imaging. In

16


Page 16 of 27

Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


addition, we have also demonstrated that conventional plasmon-enhanced Raman scattering can be amplified above one order of magnitude by resonant matching of the LSPR of a model plasmonic system with the BIC field. This synergistic metal-dielectric BIC-LSPR approach may be advantageous for improving plasmonic-based systems.

Materials and Methods Fabrication. Si3 N4 was deposited by plasma-enhanced chemical vapor deposition (PECVD) (1% SiH4 /Ar = 100 sccm and N2 = 50 sccm). The PhC metasurface consisted of a twodimensional square lattice of cylindrical air holes in a silicon nitride layer deposited on a SiO2 coverslip (120 µm of thickness) (Fig. 1b). Si3 N4 covered all the coverslip area. State-ofthe-art fabrication methodologies were used to produce large-area, square PhC metasurfaces as large as 1 - 9 mm2 of Si3 N4 . The PhCM pattern was realized by high-voltage electron beam lithography (Vistec VB300UHR EWF) followed by coupled plasma etching process using CHF3 and O2 (Oxford Instruments PlasmaLab 80 Plus tool) at room temperature. The geometric parameters of the square were optimized in order to excite the BIC resonance in the visible range of 530-560 nm (lattice constant a = 355 nm, hole radius r = 89 nm, hole depth h = 54 nm) (Fig. 1b). Several PhCs were fabricated with tuned etching parameters by changing the exposure dose and lattice constant in order to finely tune the PhCM resonances. Numerical Simulations. Numerical simulations of the PhCM optical modes were carried out by using a full three-dimensional rigorous coupled wave approach (RCWA) 14,39 based on a Fourier modal expansion and finite difference time domain (FDTD) simulations, using commercial software FULLWAVE (https://optics.synopsys.com/rsoft/rsoft-passive-devicefullwave.html). Finite element method-based simulations, carried out with Comsol Multiphysics 5.2, were finally used to verify the consistence of all simulations performing eigenmode calculations and computing the mode profiles reported in Fig. 2a. The computational domain was limited to one unit cell of the PhCM with Bloch periodic boundary condition

17



to surfaces along x-, y-directions (Fig. S3, SI). On top and bottom surfaces, normal to the z-direction, and far enough from the metasurface, we imposed perfectly-matched-layer absorbing boundary conditions. 14,40,41 The adapted mesh along z had a size-step of 3 nm inside the PhCM and increased outside, up to a value of 20 nm. The local amplification from FEM simulations of the near-field appeared to be limited by the numerical precision of the simulation, reaching values as large as 108 Ei , with Ei input field amplitude (Fig. 4a), as a result of a diverging Q-factor. RCWA simulations still predicted a huge field amplification (given the resolution imposed in the frequency scan) with a near-field amplitude of 8.5 × 104 Ei . These values are orders of magnitude larger than what expected in engineered plasmonic nanostructures. Of course, real-world PhCM structures are expected to be characterized by lower amplifications. We investigated the possible PhCM modes as a function of the thickness h, fixed the ratio a/r. We determined two values of h for which resonance trapped BICs can occur close to the normal incidence (additional notes can be found in the Supporting Information), i.e. for h = 54 nm and for h = 87 nm. We imposed a refractive index n = 2.2 as resulting from ellipsometric measurements (Fig. S2, SI), however, even for large deviation from this value, these modes still occurred at different free-space wavelengths (within tens of nanometers) (Fig. S3). For both values of h, two kinds of modes occurred, a symmetry-protected BIC and a resonance-trapped BIC. Experimentally, in this work, we focused our attention to the case h = 54nm in order to simplify the analysis. Indeed, for larger h, other leaky modes with lower Q−factor exist in the same spectral region of interest. Optical characterization. The sample was illuminated with a collimated supercontinuum laser source (NKT Photonics, SuperK EXTREME, 400-2400 nm single mode output). The collimated beam incident on the sample had a beam waist of ∼ 0.5 mm and was polarized by a Glan-Thompson polarizer. TE amd TM resonant bands in transmission were acquired through the transparent sample. For measurements reported in Fig. 2b, the input beam was focused on the sample through several air-immersion microscope objective lenses

18


Page 18 of 27

Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of progressively larger NA. The transmitted signal was filtered in crossed-polarized configuration by using a Glan-Taylor polarizer in order to suppress the supercontinuum background out of resonance. The transmitted light was collected through a multimode optical fiber connected to an Ocean Optics USB4000 spectrometer (resolution 0.25 nm, operating range 350 - 800 nm). Angle-resolved measurements with a resolution of 0.01◦ were carried out by means of a computer-controlled rotational stage. 42

BIC-enhanced Fluorescence Measurements. For fluorescence enhancement measurements, 950PMMA A (MicroChem) was mixed with the fluorescent dye (R6G, Sigma Aldrich) to reach a final fluorophore concentration of 10 µM. The substrates were dip-coated in the solution then baked for 5 min above glass-transition temperature. The coating thickness t was ∼ 3 µm. We used a confocal inverted Raman miscroscope Horiba-Jobin Yvonne XploRA INV (equipped with three solid state laser sources at 532, 638, and 785 nm) using 532 nm as pump wavelength in backscattering configuration. 43 The objective lens was a Nikon Plan Apo VC 60×A, NA = 1.2, WD = 300 µm. The slit aperture of the spectrometer was 100 µm and confocal pinhole was 100 µm. Pump laser backscattering was filtered-out with a notch-filter. The beam radius was measured with a knife-edge technique using the Raman peak at 514 cm−1 of a Si wafer blade and gave the value of 1.5 µm at 532 nm. The PhCM sample was mounted on a servo-motorized xy stage with spatial resolution of 0.5 µm, controlled via computer and synchronized with spectrometer acquisition. This allowed us to scan the sample area inside and outside the PhCM. An image of the PhCM border is shown in Fig. 3a. For spectra acquisition, the incident power on the sample was set to 60 µW with a dwell time of 20 ms (Fig. 4b). The BIC-enhancement of the fluorescence yield, FBIC−F , was evaluated by the following relation 30

FBIC−F =

t lint

I IN − I OUT I OUT

19


,


since the estimated interaction length of the BIC field is lint t, being t the thickness of the molecular layer contributing to the non-amplified signal. I IN,OUT were respectively the intensities of the signal in and out of the PhCM at the peak (563 nm). BIC-enhanced Raman Measurements. For Raman measurements, 40 µl of 50 µM ethanol solution of CV was dispersed over a large area of 4 mm2 using a parafilm well to select the area of interest. The CV resulting layer had a thickness below lint . We used the same optical configuration described in the section above. Optical setup was not changed but this time a microscope objective with a longer working distance was adopted (Nikon CFI S Fluor 10×, NA = 0.5, WD = 1.2 mm). This allowed us to use a custom tilting setup to adjust the sample orientation along the ΓX direction in order to maximize the experimental Raman signal collected from the PhCM area. For spectra acquisition, the incident power on the sample was set to 1.5 mW with a dwell time of 10 s. Spectra were background corrected with a polynomial fit to the fourth degree (Fig. 4c). The BIC-enhancement of the Raman IN OUT since t < lint . yield, FBIC−R , was evaluated by FBIC−R = I I−I OUT BIC-LSPR hybrid effect. For BIC-LSPR effect, citrate-stabilized AuNPs of 40 nm (Sigma Aldrich) were dispersed over an area of 1 cm2 by drop-casting 40 µl solution at a concentration of 1.75 × 1010 NPs/mL, which produced a surface coverage of approximately 9%, i.e. ensuring minimal aggregation effects. Then, 40 µl of 10 µM ethanol solution of CV was drop-casted over same area. The optical setup is the same, with the objective lens Nikon Plan Apo VC 60×A, NA = 1.2, WD = 300 µm. For spectra acquisition, the incident power on the sample was set to 700 µW with a dwell time of 1 s. The hyperspectral map of Fig. 5a is reconstructed by integrating the Raman intensity of CV band centered at 1620 cm−1 as a function of the coordinates of the scanned area. Spectra were not background corrected. The representative image in Fig. 5a shows an area of 300 × 40 µm2 , rasterscanned with step of 3 µm. The BIC-enhancement of the SERS yield, FBIC−S , was evaluated IN OUT by FBIC−S = I I−I since t < lint . OUT

20


Page 20 of 27

Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Associated Content The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021 ... Ellipsometric measurements for determination of optical constants; further details on numerical simulations.

Author Information Corresponding Author *E-mail: [email protected] (G.Z.) ORCID Gianluigi Zito: 0000-0003-2376-0080 Silvia Romano: 0000-0002-8432-7611 Anna Chiara De Luca: 0000-0002-3696-8465

Author Contributions S.R. and G.Z. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Acknowledgments We are grateful to Dr. Mario Iodice, IMM-CNR Naples, for assistance with ellipsometric measurements. This work was partially supported by a grant from the Italian Association for Cancer Research-AIRC (Start-up Grant 11454).

21



References (1) Maier, S. A.; Atwater, H. A. Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures. J. Appl. Phys. 2005, 98, 10. (2) Atwater, H. A.; Polman, A. Plasmonics for improved photovoltaic devices. Nat. Mater. 2010, 9, 205–213. (3) Schuller, J. A.; Barnard, E. S.; Cai, W.; Jun, Y. C.; White, J. S.; Brongersma, M. L. Plasmonics for extreme light concentration and manipulation. Nat. Mater. 2010, 9, 193–204. (4) Lakowicz, J. R. Plasmonics in biology and plasmon-controlled fluorescence. Plasmonics 2006, 1, 5–33. (5) Yang, S.; Dai, X.; Stogin, B. B.; Wong, T.-S. Ultrasensitive surface-enhanced Raman scattering detection in common fluids. Proc. Natl. Acad. Sci. USA 2016, 113, 268–273. (6) Zito, G.; Rusciano, G.; Pesce, G.; Dochshanov, A.; Sasso, A. Surface-enhanced Raman imaging of cell membrane by a highly homogeneous and isotropic silver nanostructure. Nanosc. 2015, 7, 8593–8606. (7) Yu, N.; Capasso, F. Flat optics with designer metasurfaces. Nat. Mater. 2014, 13, 139–150. (8) Lin, D.; Fan, P.; Hasman, E.; Brongersma, M. L. Dielectric gradient metasurface optical elements. Science 2014, 345, 298–302. (9) Gobin, A. M.; Lee, M. H.; Halas, N. J.; James, W. D.; Drezek, R. A.; West, J. L. Nearinfrared resonant nanoshells for combined optical imaging and photothermal cancer therapy. Nano Lett. 2007, 7, 1929–1934. (10) Khurgin, J. B. How to deal with the loss in plasmonics and metamaterials. Nat. Nanotech. 2015, 10, 2–6. 22


Page 22 of 27

Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Caldarola, M.; Albella, P.; Cortés, E.; Rahmani, M.; Roschuk, T.; Grinblat, G.; Oulton, R. F.; Bragas, A. V.; Maier, S. A. Non-plasmonic nanoantennas for surface enhanced spectroscopies with ultra-low heat conversion. Nat. Comm. 2015, 6 . (12) Friedrich, H.; Wintgen, D. Interfering resonances and bound states in the continuum. Phys. Rev. A 1985, 32, 3231. (13) Fan, S.; Joannopoulos, J. Analysis of guided resonances in photonic crystal slabs. Physical Review B 2002, 65, 235112. (14) Mocella, V.; Romano, S. Giant field enhancement in photonic resonant lattices. Phys. Rev. B 2015, 92, 155117. (15) Bulgakov, E. N.; Sadreev, A. F. Bloch bound states in the radiation continuum in a periodic array of dielectric rods. Phys. Rev. A 2014, 90, 053801. (16) Marinica, D.; Borisov, A.; Shabanov, S. Bound states in the continuum in photonics. Phys. Rev. Lett. 2008, 100, 183902. (17) Moiseyev, N. Suppression of Feshbach resonance widths in two-dimensional waveguides and quantum dots: a lower bound for the number of bound states in the continuum. Phys. Rev. Lett. 2009, 102, 167404. (18) Plotnik, Y.; Peleg, O.; Dreisow, F.; Heinrich, M.; Nolte, S.; Szameit, A.; Segev, M. Experimental observation of optical bound states in the continuum. Phys. Rev. Lett. 2011, 107, 183901. (19) Hsu, C. W.; Zhen, B.; Lee, J.; Chua, S.-L.; Johnson, S. G.; Joannopoulos, J. D.; Soljačić, M. Observation of trapped light within the radiation continuum. Nature 2013, 499, 188–191. (20) Zhen, B.; Hsu, C. W.; Lu, L.; Stone, A. D.; Soljačić, M. Topological nature of optical bound states in the continuum. Phys. Rev. Lett. 2014, 113, 257401. 23



(21) Monticone, F.; Alú, A. Embedded photonic eigenvalues in 3D nanostructures. Phys. Rev. Lett. 2014, 112, 213903. (22) Hsu, C. W.; Zhen, B.; Stone, A. D.; Joannopoulos, J. D.; Soljačić, M. Bound states in the continuum. Nat. Rev. Mater. 2016, 1, 16048. (23) Rivera, N.; Hsu, C. W.; Zhen, B.; Buljan, H.; Joannopoulos, J. D.; Soljačić, M. Controlling directionality and dimensionality of radiation by perturbing separable bound states in the continuum. Sci. Rep. 2016, 6 . (24) Kodigala, A.; Lepetit, T.; Gu, Q.; Bahari, B.; Fainman, Y.; Kanté, B. Lasing action from photonic bound states in continuum. Nature 2017, 541, 196–199. (25) Gomis-Bresco, J.; Artigas, D.; Torner, L. Anisotropy-induced photonic bound states in the continuum. Nat. Phot. 2017, 11, 232–236. (26) Yoon, J. W.; Song, S. H.; Magnusson, R. Critical field enhancement of asymptotic optical bound states in the continuum. Sci. Rep. 2015, 5, 18301. (27) Yuan, L.; Lu, Y. Y. Bound states in the continuum on periodic structures surrounded by strong resonances. Phys. Rev. A 2018, 97, 043828. (28) Bulgakov, E. N.; Maksimov, D. N. Light enhancement by quasi-bound states in the continuum in dielectric arrays. Opt. Exp. 2017, 25, 14134–14147. (29) Huang, B.; Bates, M.; Zhuang, X. Super-resolution fluorescence microscopy. Annual review of biochemistry 2009, 78, 993–1016. (30) Zhen, B.; Chua, S.-L.; Lee, J.; Rodriguez, A. W.; Liang, X.; Johnson, S. G.; Joannopoulos, J. D.; Soljačić, M.; Shapira, O. Enabling enhanced emission and low-threshold lasing of organic molecules using special Fano resonances of macroscopic photonic crystals. Proc. Natl. Acad. Sci. USA 2013, 110, 13711–13716.

24


Page 24 of 27

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Lee, J.; Zhen, B.; Chua, S.-L.; Qiu, W.; Joannopoulos, J. D.; Soljačić, M.; Shapira, O. Observation and differentiation of unique high-Q optical resonances near zero wave vector in macroscopic photonic crystal slabs. Phys. Rev. Lett. 2012, 109, 067401. (32) Bahari, B.; Vallini, F.; Lepetit, T.; Tellez-Limon, R.; Park, J.; Kodigala, A.; Fainman, Y.; Kante, B. Integrated and Steerable Vortex Lasers. arXiv preprint arXiv:1707.00181 2017, (33) Romano, S.; Zito, G.; Torino, S.; Calafiore, G.; Penzo, E.; Coppola, G.; Cabrini, S.; Rendina, I.; Mocell, V. Label-free sensing of ultralow-weight molecules with all-dielectric metasurfaces supporting bound states in the continuum. Photonics Research 2018, 6, 726–733. (34) Romano, S.; Lamberti, A.; Masullo, M.; Penzo, E.; Cabrini, S.; Rendina, I.; Mocella, V. Optical biosensors based on photonic crystals supporting bound states in the continuum. Materials 2018, 11, 526. (35) Eustis, S.; El-Sayed, M. A. Why gold nanoparticles are more precious than pretty gold: noble metal surface plasmon resonance and its enhancement of the radiative and nonradiative properties of nanocrystals of different shapes. Chem. Soc. Rev. 2006, 35, 209–217. (36) Cotton, T. M.; Kim, J.-H.; Chumanov, G. D. Application of surface-enhanced Raman spectroscopy to biological systems. J. Ram. Spec. 1991, 22, 729–742. (37) Movasaghi, Z.; Rehman, S.; Rehman, I. U. Raman spectroscopy of biological tissues. Appl. Spec. Rev. 2007, 42, 493–541. (38) Petry, R.; Schmitt, M.; Popp, J. Raman spectroscopy - a prospective tool in the life sciences. Chem. Phys. Chem. 2003, 4, 14–30.

25



(39) Liu, V.; Fan, S. S4: A free electromagnetic solver for layered periodic structures. Comp. Phys. Comm. 2012, 183, 2233–2244. (40) Zito, G.; Rusciano, G.; Sasso, A. Dark spots along slowly scaling chains of plasmonic nanoparticles. Opt. Exp. 2016, 24, 13584–13589. (41) Zito, G.; Rusciano, G.; Sasso, A. Enhancement factor statistics of surface enhanced Raman scattering in multiscale heterostructures of nanoparticles. J. Chem. Phys. 2016, 145, 054708. (42) Romano, S.; Cabrini, S.; Rendina, I.; Mocella, V. Guided resonance in negative index photonic crystals: a new approach. Light: Science & Applications 2014, 3, e120. (43) Managò, S.; Migliaccio, N.; Terracciano, M.; Napolitano, M.; Martucci, N. M.; De Stefano, L.; Rendina, I.; De Luca, A. C.; Lamberti, A.; Rea, I. Internalization kinetics and cytoplasmic localization of functionalized diatomite nanoparticles in cancer cells by Raman imaging. J. Bioph. 2017, 10.1002/jbio.201700207.

26


Page 26 of 27

Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic

27


Surface-Enhanced Raman and Fluorescence Spectroscopy with an All

Recommend Documents