On-Chip Surface-Enhanced Raman Spectroscopy using Nanosphere-Lithography Patterned Antennas on Silicon Nitride Waveguides

A hybrid integration of nanoplasmonic antennas with silicon nitride waveguides enables miniaturized chips for surface-enhanced Raman spectroscopy at visible and near-infrared wavelengths. This integration can result in high-throughput SERS assays on low sampling volumes. However, current fabrication methods are complex and rely on electron-beam lithography, thereby obstructing the full use of an integrated photonics platform. Here, we demonstrate the electronbeam-free fabrication of gold nanotriangles on deep-UV patterned silicon nitride waveguides using nanosphere lithography. The localized surface-plasmon resonance of these nanotriangles is optimized for Raman excitation at 785 nm, resulting in a SERS substrate enhancement factor of 2.5 × 105. Furthermore, the SERS signal excited and collected through the waveguide is as strong as the free-space excited and collected signal through a high NA objective. © 2017 Optical Society of America OCIS codes: (130.0130) Integrated optics; (130.6010) Sensors; (240.6695) Surface-enhanced Raman scattering; (220.4241) Nanostructure fabrication. References and links 1. K. De Vos, I. Bartolozzi, E. Schacht, P. Bienstman, and R. Baets, “Silicon-on-Insulator microring resonator for sensitive and label-free biosensing,” Optics Express 15, 7610–7615 (2007). 2. E. Ryckeboer, R. Bockstaele, M. Vanslembrouck, and R. Baets, “Glucose sensing by waveguide-based absorption spectroscopy on a silicon chip.” Biomedical Optics Express 5, 1636–48 (2014). 3. A. Z. Subramanian, P. Neutens, A. Dhakal, R. Jansen, T. Claes, X. Rottenberg, F. Peyskens, S. Selvaraja, P. Helin, B. Dubois, K. Leyssens, S. Severi, P. Deshpande, R. Baets, and P. 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Introduction
Silicon photonics based on high index-contrast silicon-on-insulator (SOI) waveguides has proven to be an excellent platform for biosensing.SOI enabled, amongst others, the development of sensitive and compact refractive index [1] and absorption [2] sensors across the telecom and mid-infrared wavelength range.The success of the SOI platform is largely due to the use of standard CMOS-fabrication techniques, which ensures high-volume manufacturing, reproducible components and potential CMOS compatibility.This inspired the development of a silicon nitride (SiN) counterpart, paving the way for deep-UV fabricated integrated photonics at visible wavelengths.The ongoing improvement of different optical components such as low-loss waveguides [3], spectrometers [4,5] and hybrid integrated lasers [6] is leading to a maturation of this SiN platform [7].Being transparent at near-infrared and visible wavelengths, SiN enables on-chip fluorescence [8] and Raman spectroscopy [9], as these scattering processes scale with λ −4 .Even though waveguide-based Raman spectroscopy strongly increases the collected Stokes scattering as compared to free-space sensing [10][11][12][13], it remains limited by both the low Raman cross-section of most molecules and the intrinsic background Raman scattering of the waveguide itself [10].Surface-enhanced Raman scattering (SERS) allows to dramatically increase the Raman signal from molecules in the close vicinity of plasmonic nano-antennas, even up to the limit of single-molecule detection [14].Recent efforts succeeded in combining plasmonic antennas with Si [15] and SiN [16][17][18] waveguides, providing proof-of-concept experiments for integrated localized surface plasmon resonance (LSPR) sensing [18] and even waveguide-excited and -collected SERS [17].However, all these approaches rely on multiple electron-beam lithography steps with critical alignment for writing both the waveguides and the nano-antennas.Apart from being time-consuming and resource-intensive, the use of e-beam lithography inhibits exploiting the full functionality of a mature integrated photonics platform.Alternative approaches combine a waveguide platform with colloidal gold or silver nanoparticles [19][20][21].However, colloidal particles lack the reproducibility of top-down fabricated SERS substrates.In this work, we develop a nanosphere-lithography [22] (NSL) based technology for patterning deep-UV fabricated SiN waveguides with gold nanotriangles.We characterize the plasmonic resonance of these nanotriangles through their absorption spectra and demonstrate efficient waveguide-based excitation and collection of surface-enhanced Raman spectra at a pump wavelength of 785 nm.Furthermore, for an equal input power we experimentally find the Stokes scattered power to be approximately equal when exciting and collecting through the waveguide versus a high numerical aperture objective.To the best of our knowledge, this is the first demonstration of an e-beam free platform for on-chip SERS, an important step towards a complete on-chip SERS-platform combining the sensing area, filters, spectrometer and even the laser on a single chip.

Results
Fabrication of integrated nanotriangles Fig. 1. schematically shows how a deep-UV patterned SiN chip is decorated with nanotriangles.The fabrication process is described in more detail in the methods section.First, a 2.5 − 15 µm wide window is defined across the waveguide using photoresist.This window is thinned down using oxygen plasma to avoid stacking of the polystyrene beads.Next, polystyrene beads with a 448 nm diameter are spin-coated on the chip to form a self-assembled, hexagonally close packed monolayer in this window.These beads act as a mask for gold deposition.After lifting of the beads and resist, the waveguide is covered with a repetitive pattern of nanotriangles.Fig. 2. shows SEM images of the monolayer of beads and resulting gold nanotriangles on the waveguide.Because of the 220 nm height difference between the waveguide and the substrate, the beads form a double layer on the latter.Openings in the double HCP-layer result in the smaller nanodots visible next to the waveguide in Fig. 2(b).

Spectral properties of the localized surface plasmon resonance
The nanotriangles and waveguides are designed for a Raman pump laser of 785 nm and Stokes emission at 877 nm, corresponding to the 1339 cm −1 symmetric stretching mode (ν s ) of NO 2 in the gold-binding molecule para-nitrothiophenol (NTP) [23].A maximal overlap of the LSPR with the wavelength region of the pump laser and Stokes emission is necessary to achieve a high SERS enhancement factor.The spectral location of this resonance strongly depends on geometrical parameters such as the size and aspect ratio of the triangles, as well as the refractive index of the surrounding materials.For uncoupled gold nanostructures, implying a large separation between the antennas, the extinction spectrum provides a reasonable indication for wavelength-dependent enhancement factor [24].With a gap of approximately 75 nm (Fig. 2(d).), this condition is met for the nanotriangles.The optimal geometry was experimentally found by tuning the radius of the polystyrene beads, thinning down the beads with O 2 plasma (Fig. 1., between step (2) and ( 3)) and adjusting the thickness of the evaporated gold.Optimal values of respectively 224 nm, 15 s and 70 nm were determined from UV-Vis absorption and SERS spectra measured on different geometries using a top-down, free-space excitation (data not shown).These parameters are  used for fabricating nanotriangles on a SiN waveguide.Fig. 3(a).shows the spectral absorption from different lengths of these nanotriangle patterns on waveguides when excited by a TE polarized supercontinuum source.As expected, the absorbance increases with increasing number of nanotriangles.Furthermore, the 785 nm pump wavelength and 1339 cm −1 Stokes shift overlap well with this spectrum, indicating that these structures are suited for exciting surface-enhanced Raman spectra in the near-infrared.From these data we extract an absorption coefficient for the gold nanotriangles α p of 1.7 ± 0.5 dB/µm at the 785 nm pump and α s of 2.7 ± 0.5 dB/µm at the 877 nm Stokes wavelength.From α s , α p we calculate the total collected Stokes power as function of the length of the nanotriangle section: Here, σ is the Raman cross-section of the molecule P 0 is the laser power incident on the entrance facet of the chip.The losses when coupling pump light in and the Stokes light out of the chip are respectively given by γ in and γ out .Because of the strong absorption of pump and Stokes light, after only 2.5 µm of nanotriangles the SERS signal reaches 93% of the maximum value (solid blue curve in Fig. 3(c).).Hence making the nanotriangle stretch longer does not contribute to a stronger signal.Fig. 3(c).also shows the experimentally measured intensity of the 1339 cm −1 SERS peak on 8 waveguides each for lengths of 2.5 µm, 5 µm, 10 µm and 15 µm, indeed showing no significant differences in signal strength with increasing length.Surface-enhanced Raman scattering of waveguide-coupled versus free-space coupled nanotriangles SERS-spectra of the same nanotriangle patterns are measured using both free-space and waveguide-based excitation.A commercial Raman microscope is used in both cases, with the chip placed respectively perpendicular or parallel to the beam path, as shown in Fig. 4(a).This allows for an accurate comparison between the total free-space-and waveguide-collected Stokes scattered power for the same input power.before encountering the nanotriangle section.Along this propagation, a strong Raman signal of the SiN core is generated [10].As a consequence, the Raman spectra collected through the waveguide consist of two major contributions, as highlighted in Fig. 5(a).One is the SERS spectrum of NTP, which in the rest of this work will be quantified by its most prominent peak at 1339 cm −1 (ν s NO 2 ).The other is the SiN spectrum, which for our particular SiN consists of a broad background that decreases monotonically towards longer relative frequency shifts [10] and has a characteristic peak at 2330 cm −1 .Note that the shape and strength of this background scales with the waveguide length and depends not only on the waveguide material but also on the exact deposition parameters.Fig. 5(a).shows a raw waveguide-excited SERS spectrum, as well as a SiN background spectrum collected from a reference waveguide without plasmonic antennas.The latter is rescaled for equal peak intensity at 2330 cm −1 (Fig. 5(a)., inset) because of differences in propagation length and absorption.The lack of NTP peaks on the reference spectrum also proves that the NTP peaks indeed originate from gold-bound molecules on the nanotriangles.To compare the waveguide-excited SERS signals to a free-space excitation, we subtract the SiN background and rescale the spectrum to compensate for a total coupling loss (γ in γ out ) of 8 dB.The rationale behind compensating for these coupling losses is a future integration of the spectrometer [5] and light source [6] on the chip, eliminating the need for edge-coupling and thus minimizing losses in between components.On a shorter term it is possible to substantially reduce coupling losses by optimizing the waveguide design for a specific coupling geometry.Free-space excited SERS spectra are collected top-down across the same waveguide using a 100x/0.9NA objective, with the chip placed horizontally under the microscope.This is close to the highest possible etendue using an air objective, and thus results in the strongest possible SERS spectrum collected in free space for this particular SERS substrate.An average spectrum is calculated over the first 2.5 µm of nanotriangle pattern on the waveguide, as this part contributes to over 90% of the waveguide-coupled Raman signal.Fig. 5(b).shows both free-space and waveguide excited SERS spectra for a typical waveguide, both having the same shape and approximately equally magnitude.This comparison was made for multiple waveguides on two different chips.The integrated SERS intensity of the 1339 cm −1 peak for each individual nanotriangle-patterned waveguide is shown in Fig. 5(c).for both free-space and waveguide-based excitation.From this data, we conclude that exciting and collecting surface-enhanced Raman spectra through the waveguide is at least as efficient as through the high NA objective.When the guided power in the waveguide equals the power through the objective, an equal amount of Stokes scattered photons is collected by both systems.Note that this does not imply that both the field intensity and collection efficiency are individually identical in both systems.A difference in mode volume and the asymmetry of the nanotriangles on the SiN waveguide can lead to a different excitation or collection efficiency in both systems [25,26].(c) Fig. 5. (a) 4-NTP SERS signal acquired through the waveguide (solid blue) and the SiN background spectrum on a blank reference waveguide (dashed green).The 1339 cm −1 is used for quantifying the enhancement factor.The inset shows a characteristic peak for our SiN at 2330 cm −1 .(b) Waveguide collected SERS spectrum (solid blue) after subtracting the SiN background and scaling with the coupling losses, compared to a free-space collected SERS spectrum (dashed red) acquired on the same nanotriangle section.(c) SSEF for free-space excitation and collection compared to the signal strength using a waveguide-based measurement, acquired on multiple waveguides on two different chips.
To enable a quantitative comparison with other (future) integrated SERS platforms, we calculate the SERS substrate-enhancement factor for the gold nanotriangle patters as described by Le Ru et al. [27].The SSEF is defined as the ratio between the average signal per molecule on the SERS substrate and that in a bulk, spontaneous Raman measurement: In this definition, the intensity of the Raman and SERS signals are scaled to the same excitation power and integration time.The ratio between the number of molecules in a bulk measurement and those adsorbed on the gold surface can be calculated from the bulk molecule density ρ (6.022 × 10 7 molecules/µm 3 ), the effective height H e f f of the collection volume (150 µm), the density of the 4-NTP molecules on the gold µ NT P (4.4 × 10 6 molecules/µm 2 ) [28], the surface area of a single nanotriangle A m (0.023 µm 2 ) and the density of the metal nanostructures µ Au (11.5 triangles/µm 2 ).It is important to remember that, although this calculation assumes the use of exactly similar conditions (objective, microscope, pinhole etc.) for acquiring SERS and spontaneous Raman spectra, the final enhancement factor is independent of the optical setup.An exact translation of this free-space enhancement factor to a waveguide-coupled number is not straightforward because of the intrinsic differences in both geometries, and therefore outside the scope of this paper.Qualitatively, we conclude that SERS signals are in the same order of magnitude while the number of molecules excited in both cases is similar.

Conclusion
In conclusion, we have demonstrated a relatively easy process for patterning individual dies of deep-UV waveguides with gold nanotriangles in a post-processing step.The absorption spectrum of this nanopattern is well suited for Raman spectroscopy in the near-infrared, a wavelength region interesting because of its low absorption by biological specimen, reasonably strong scattering and low background fluorescence.SERS data show that the SSEF of this pattern is 2.5 × 10 5 , and that excitation and collection of SERS spectra through the waveguide is at least as efficient as using the best possible air objective.Although the enhancement factor of the nanotriangles presented in this manuscript is not among the highest reported in literature, nanosphere-lithography has also been used to fabricate high-performance SERS substrates, at least under free-space excitation [23,28,29].Variations of these can possibly be combined with photonic waveguides.The hybrid integration of nano-plasmonic antennas with a mature integrated photonics platform is a prerequisite for fully integrated spectroscopic sensors.We are convinced that further technological developments on this topic will lead to breakthrough applications that are currently limited to free-space SERS sensing.

Fabrication
As shown in Fig. 1., we start from a deep-UV patterned die containing 220 nm high by 1.4 µm wide SiN waveguides.After cleaning the chips, a 2.5 − 15 µm wide open window is patterned in AZ MiR 701 positive photoresist using UV-contact lithography (SÜSS MA6), resulting in a resist thickness of approximately 700 nm.This resist is thinned down to less than 400 nm using O 2 plasma (Vision 320 RIE, 50 sccm O 2 , 75 W, 100 mTorr, 235 s), after which the polystyrene beads are spin-coated on the chip.The resist must be thinner than the size of the polystyrene beads to avoid stacking of the beads at the edges of the window.Typically, 40 µl of a 3.75 w% in 2/1 methanol/water mixture of 448 nm polystyrene beads (Microparticles GMBH) was used for a two-step spin coating.First a HCP monolayer is formed at a speed of 2100 rpm and an acceleration of 1600 rpm/s during 45 seconds, followed by the removal of excessive beads and solvents at 5000 rpm and 1000 rpms during 40 seconds.Note that these parameters should be slightly changed depending on the chip size (4 cm 2 ), humidity and temperature in order to acquire an optimal arrangement of the beads into an HCP-monolayer.After spin-coating, an additional 15 s of O 2 plasma is applied.This reduces the size of the beads such that the resulting nanotriangles will have the optimal dimensions for a LSPR around 785-870 nm.The O 2 plasma also improves the adhesion of metals to the SiN surface.First a 1-4 nm Ti adhesion layer is sputtered, followed by the thermal evaporation of a 70 nm Au layer (Leybold Univex 450).Finally, the polystyrene beads are removed in dichloromethane and the resist is lifted off in acetone.The chips were characterized through scanning electron microscopy on a FEI Nova 600 Nanolab Dual-Beam FIB system, using a voltage of 18 kV and a through the lens (TLD) detection.
Transmission measurements Nanotriangle extinction spectra were measured as described by Peyskens et al. in the supplementary information of [17].Light from an NKT EXR-4 supercontinuum source is filtered through a near-IR transmitting filter and coupled in a fiber.This fiber is plugged into a fiberbench consisting of 3 parts: an achromatic fiber collimator which converts the fiberized light to a free-space beam, a free-space broadband polarizer which

Fig. 2 .
Fig. 2. SEM image of (a) a hexagonally close-packed monolayer of polystyrene beads in photoresist across the SiN waveguide and (b-d) a typical gold nanotriangle pattern on the waveguide.

Fig. 3 .
Fig. 3. (a) Absorption spectra of waveguides decorated with increasing lengths of nanotriangle patterns.The LSPR-absorption shows a good overlap with a 785 nm pump laser and a 1139 cm −1 Stokes shift, respectively green and red shaded.(b) Absorption versus nanotriangle length at pump and Stokes wavelength measured on four different waveguides for each length.A linear fit to the data gives an absorption of 2.7 ± 0.5 dB/µm at Stokes and 1.7 ± 0.5 dB/µm at the pump wavelength.(c)SERS signal strength in function of the length of the nanotriangle section, showing that the maximum signal is already reached for a 2.5 µm length.The orange bars give the average of at least 8 different measurements.The solid blue curve shows the calculated signal strength versus length when collecting the backwards Stokes scattered light in a reflection mode, as used in this work.The red curve shows the forward collected Stokes power in a transmission mode.

Fig. 4 (Fig. 4 .
Fig. 4. (a) Schematic of the confocal microscope used for collecting Stokes scattered light from both waveguide-and free-space coupled nanotriangles.(b) Camera image of the chip in bright field (left) and dark field (right).Light is coupled from the top into the vertically oriented waveguides, and propagates along the waveguide (green arrow) until the nanotriangle section.Stokes scattered light is collected in reflection (red-dashed arrow).Scattering of the pump laser can be seen at the entrance facet and nanotriangle section.
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