Abstract
Surface enhanced Raman spectroscopy (SERS) is a widely known sensing technique that uses a plasmonic enhancement to probe analytes in ultra-small volumes. Recently, the integration of plasmonic structures with photonic integrated waveguides promised the full integration of a SERS system on a chip. Unfortunately, the previously reported sensors provide modest overall SERS enhancement resulting in a limited signal to noise ratio. Here, we report a photonic waveguide interfaced SERS sensor that shows an order of magnitude higher pump to Stokes conversion efficiency and lower background than previous realizations. Moreover, the plasmonic structure is fabricated without the use of e-beam lithography but rather using a combination of atomic layer deposition and deep UV photolithography. We investigate numerically the performance of the sensor in terms of Raman conversion efficiency for various design parameters. The experimental results are presented via the acquisition of SERS spectra that show a conversion efficiency of 10−9 for a monolayer of 4-nitrothiophenol. To explore the broadband characteristic of our sensor in the therapeutic spectral window, two different pump wavelengths, i.e., 632 and 785 nm, are used. To the best of our knowledge, this is the first ever broadband SERS demonstration of an on-chip Raman sensor. We further study the reproducibility of our SERS sensor, reaching a relative standard deviation of the acquired spectra (RSD) < 5%.
Highlights
The combination of integrated photonics with plasmonic structures can provide a strong field enhancement over an increased interaction length resulting in a strong light-matter interaction.[10,11,12,13]
Raman spectra are measured from both 100 μm long gold coated and 0.6 mm long bare Si3N4 slot waveguides, the latter serving as a reference
We have reported a SERS sensor suitable for on-chip integrated Raman spectroscopy, fabricated using ALD assisted DUV lithography and compatible with the back-end CMOS fabrication. We have reported both the experimental and numerical characterization of our sensor demonstrating capabilities that have so far been difficult to combine in a single sensor: (1) a large Raman conversion efficiency (≈1 × 10−9), thanks to simultaneous long interaction length, high confinement, and plasmonic enhancement (SMEF = 1.5 × 107), (2) a low background due to the reduced overlap of the field with the core of the waveguide, (3) a good reproducibility of the Raman spectra (RSD < 5%) across different chips, thanks to the nanometer fabrication accuracy provided by atomic layer deposition, (4) a good tolerance to rather large input power due to a moderate local field enhancement, and (5) a broadband enhancement, thanks to the use of propagating plasmons rather than localized plasmon
Summary
The integration of plasmonic structures on photonic platforms has attracted a lot of attention.[1,2,3,4,5,6,7,8,9] The combination of integrated photonics with plasmonic structures can provide a strong field enhancement over an increased interaction length resulting in a strong light-matter interaction.[10,11,12,13] One of the key applications in this context is surface enhanced Raman spectroscopy (SERS). Many SERS substrates have been developed[14,15,16] providing more reproducible Raman spectra In parallel to those developments of SERS, Raman spectroscopy enhanced by waveguides was introduced to increase the interaction length between light and analyte while keeping a
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