1. Introduction

Surface-enhanced Raman scattering (SERS) can help to increase the Raman signal and achieve even up to the limit of single-molecule detection where huge electromagnetic fields are generated at the narrow nanogaps between sharp corners and edges of nanostructure plasmonic nano-antennas [1–3]. Compared with free-space sensing, waveguide based Raman spectroscopy can strongly increase the collected Stokes scattering, where lithography patterned nano-antennas, colloidal gold or silver nanoparticles could be integrated [4–10]. However, nano-antennas usually require a critical pattern alignment and complicated fabrication process [4–7], while colloidal particles lack the reproducibility of top-down fabricated SERS substrates [8–10]. Free standing nanoporous gold (NPG) film can generate large local electromagnetic field due to the positive and negative curve ligaments and the coupling effect between neighbors of the bicontinuous nano-scaled structure [11], making it a good candidate for SERS detection. It is easy to fabricate and has a good reproducibility, but a waveguide integrated NPG film has not been applied for SERS application to the best of our knowledge.

Silicon nitrides (Si₃N₄) based photonic integrated circuits have aroused much attention due to its compatibility with the complementary-metal-oxide-semiconductor (CMOS) technology and negligible nonlinear (two-photon) absorption [12], which can offer a large fabrication tolerance, superior performance in the coupling and propagation loss [13], and favorable ability for three-dimensional vertical integration [14]. Many Si₃N₄ integrated devices such as high-quality resonator filters [15,16], polarizing beam splitters [17,18], and bio-photonic sensors [19–21] have been developed. Due to its wide transparent bandwidth from visible to near-infrared wavelength, Si₃N₄ can also enable on-chip fluorescence [22] and Raman spectroscopy [23–27]. To avoid mechanical rupture caused by a high tensile stress, Si₃N₄ waveguide thickness is usually chosen to be around 200 nm, which can usually guide a transverse electric (TE) light in a suspended case without SiO₂ cladding layer [20]. It is also difficult to propage light with a narrow thin waveguide. Till now, polarization and the waveguide structure dependent properties have not been studied for most of the waveguide-based Raman spectroscopy [27]. In the following, a series of thick Si₃N₄ waveguide are fabricated and integrated with NPG film, in order to investigate the polarization-dependent SERS detection. In order to enhance interaction with the surrounded environment and increase fabrication volume with a reduced product size, a T-shape suspended waveguide structure is applied. In this case, a thick waveguide would also benefit the wet etching and have a low insertion loss between un-suspended and suspended section.

2. Device fabrication

Figure 1(a) shows a schematic of the chip fabrication process. A 3-μm SiO₂ buffer layer was deposited on silicon wafer by plasma enhanced chemical vapor deposition (PECVD) at 350°C. Then electron cyclotron resonance (ECR) plasma-enhanced sputtering was used to form the Si₃N₄ guiding layer at room temperature. A 580-nm-thick Si₃N₄ layer was deposited with optimized ECR deposition condition, in order to avoid a high tensile stress. By using electron beam lithography, Cr film deposition, lift-off, and reactive ion etching, Si₃N₄ photonic circuits can be formed with Cr as an etching mask. After removing the Cr mask, the sample was cleaned using oxygen plasma and a wet-chemical processing. A 2-μm-thick SiO₂ top-cladding layer was deposited by PECVD for better input/output optical coupling. A 400-μm-wide wet etching window was then prepared by mask aligner with photoresist AZ1500. The sample was put into the buffered HF solution to form the desired T-shape suspended structure. Microscope image of the fabricated chip is shown in Fig. 1(b).

 

Fig. 1 (a) Schematic illustration of the fabrication process. (b) Microscope image of the fabricated chip with suspended waveguide. (c) Microscope image of the NPG film integrated chip under test (bright spot at chip edge indicates the input light).

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For the suspended waveguide fabrication, if there is little or no SiO₂ pedestal remaining after wet etching, the mechanical stability of the Si₃N₄ waveguide would be deteriorated, thus the waveguide cannot maintain its original position and would be easily deformed. Thus a key point for the fabrication is to control the remaining SiO₂ pedestal width. And some straight waveguides with small width are placed to monitor the wet etching process [20]. Center part of Fig. 1(b) is the wet etching area for suspended waveguide. One curved line near edge is the narrow waveguide for monitoring.

Then the chip was placed into the solution with floating NPG film to realize the waveguide and NPG integration. The gold-silver alloy was corroded by concentrated nitric acid and then soaked into crystal violet (CV) solution at a concentration of 10 μM for two hours to yield NPG film of 100 nm with CV. Finally the sample was cut by dicing saw for the following measurement, as shown in Fig. 1(c). Figure 2 is the SEM image of the fabricated chip without and with NPG film, which shows that the surrounding SiO₂ cladding layer was etched away and the waveguide was successfully covered with NPG film.

 

Fig. 2 SEM images of the fabricated chip (a) before and after (b) covering the NPG film.

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3. Device characterization and discussion

Free-space and waveguide-excited Raman spectroscopy were measured with the same input laser power of 0.78 mW at a 633-nm wavelength. An Ador Raman spectrometer was used, as illustrated in Fig. 3. For the waveguide excited SERS, a half-wave plate was adopted to adjust the input light polarization. Waveguide coupled SERS spectra were collected top-down across the same waveguide using a Nikon TU PLAN ELWD objective (50 × /0.6 NA), with the chip placed horizontally under microscope. This is close to the highest possible etendue using an air objective, and thus can result in the strongest possible SERS spectrum collected in free space for this particular SERS substrate.

 

Fig. 3 Schematic of the confocal microscope used for collecting the scattered light from both waveguide and free-space coupled NPG film.

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The measured spectra for free-space and waveguide-coupled SERS for crystal violet are shown in Fig. 4, where a Si₃N₄ waveguide without NPG film was also presented for comparison. The waveguide width is 2.0 μm. There are 6 peaks positioned at 441, 524, 801, 916, 1173 and 1375 cm⁻¹ for CV Raman emission from 200 to 1500 cm⁻¹. For the reference Si₃N₄ waveguide without plasmonic antenna, there is no obvious peak, which confirms that the CV peaks are originated from gold-bound molecules on the NPG film. The CV characteristic peak intensities are 5876, 7615 and 9047 for free-space, waveguide-coupled TE and TM mode at 441-cm⁻¹ Raman shift, respectively. Obviously, the Raman peak intensity for waveguide coupled TM mode is stronger than that for waveguide-coupled TE mode and free-space coupled case.

 

Fig. 4 SERS spectra for the reference Si₃N₄ waveguide (black), free-space coupled NPG film (red), and waveguide coupled case at TE (blue) and TM (green) polarization.

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Figure 5(a) shows TM light excited SERS spectra for waveguides with varying width. At 441-cm⁻¹ Raman peak, the intensities are 6632, 7238, 8590 and 9047 for the mesa width of 0.5, 1.0, 1.5 and 2.0 μm, respectively. The intensity of Raman spectra can be enhanced with the increase of mesa width. For TE polarized case, the result is similar and shown in Fig. 5(b). The corresponding peak intensities are 6022, 6266, 7017 and 7615, respectively. It can also be seen that the peak intensity for TM mode is slightly higher than that for TE mode at the same waveguide width.

 

Fig. 5 Measured spectra for waveguide coupled SERS with different width at (a) TM and (b) TE polarization.

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To investigate the potential physical mechanism, mode profile for the integrated waveguide with varying mesa width is calculated with the finite element method using the software Rsoft. For the thick waveguide case here, the waveguide is multimode. If the free-space light is input to the center of waveguide, most of the energy could be transferred to the fundamental waveguide mode. Figure 6(a) shows the simulated fundamental mode coupling efficiency with assuming the input light having a 900-nm-wide Gaussian profile. It is also shown that almost no first order mode can be excited due to the different mode profile distribution. To ensure the fundamental mode condition for the waveguide coupled SERS spectra measurement, we carefully adjusted the waveguide position with an electric stage to make the light input to the mesa center and realize maximum light output intensity.

 

Fig. 6 (a) Simulated light coupling efficiency from free-space to waveguide. (b) Corresponding mode confinement for the suspended waveguide with varying width. Inset shows the mode profiles of a 2-μm-wide waveguide with 100-nm NPG for TE and TM mode.

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For a 2-μm-wide mesa covering with NPG film, corresponding mode profiles for TE and TM light are shown in the inset of Fig. 6(b). It can be seen that the mode profile is highly confined in the waveguide area for TE mode, but it is more close to the interface between NPG film and waveguide for TM mode, which is consistent with the surface plasmonic effect [28]. Thus the corresponding Raman peak intensity is higher under TM light excitation. Besides, the mode confinement factor in the waveguide area can also be calculated as in Fig. 6(b). It can be seen that the mode will be confined more in the waveguide for TE light, which would also induce a lower Raman peak intensity. For a wider mesa, mode will also be confined more in the waveguide area, which is unfavorable for Raman peak intensity increase. Since the waveguide propagation loss is around 0.1 dB/mm [20] and light insertion loss from the un-suspended to suspended waveguide is not so high, the coupling efficiency may be the main factor accounting for the Raman signal enhancement. Considering the propagation and insertion loss as well as the coupling loss in Fig. 6(a), the recorded Raman spectra can be normalized as in Fig. 7. The normalized relative spectrum for a 500-nm-wide waveguide is stronger than that of 1-μm case, showing that a weak mode confinement will benefit SERS. But it is also comparable with that of 1.5 μm and weaker than the 2-μm case, which may be caused by the different area and nonuniform NPG film distribution as in Fig. 1(c). Actually the NPG film will absorb the propation light in the waveguide and its surface topography will influence the finally performance. To exclude the NPG film distribution influence, more work should be done to optimize the NPG film processing and its integration with Si₃N₄ chip. Anyhow, it can be confirmed that there is a trade-off between the mode confinement and coupling related loss for waveguide coupled SERS. A higher Raman peak intensity can be obtained for a wider waveguide in practical application.

 

Fig. 7 Normalized relative spectra with considering the coupling loss, insertion loss and the propagation loss for (a) TM and (b) TE mode, respectively.

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4. Conclusion

To summarize, a hybrid integration of nanoporous gold with silicon nitride waveguide was realized for surface-enhanced Raman spectroscopy at a 633-nm wavelength. The localized surface plasmon resonance of the nanoporous gold can result in a waveguide and polarization-dependent SERS substrate enhancement. The SERS signal was excited through 580-nm-thick, T-shape suspended waveguides with varying width and collected through an objective lens. Both TE and TM light can excite Raman scattering but TM wave would result in a better enhancement, while a wider waveguide would also benefit the SERS detection. The presented miniaturized chips for SERS would have a great potential to be applied for portable Raman spectrometer.