High index contrast photonic platforms for on-chip Raman spectroscopy

Abstract

Nanophotonic waveguide enhanced Raman spectroscopy (NWERS) is a sensing technique that uses a highly confined waveguide mode to excite and collect the Raman scattered signal from molecules in close vicinity of the waveguide. The most important parameters defining the figure of merit of an NWERS sensor include its ability to collect the Raman signal from an analyte i.e. "the Raman conversion efficiency" and the amount of "Raman background" generated from the guiding material. Here, we compare different photonic integrated circuit (PIC) platforms capable of on-chip Raman sensing in terms of the aforementioned parameters. Among the four photonic platforms under study, tantalum oxide and silicon nitride waveguides exhibit high signal collection efficiency and low Raman background. In contrast, the performance of titania and alumina waveguides suffers from a strong Raman background and a weak signal collection efficiency respectively.

Study on multiple waveguide platforms for waveguide integrated Raman spectroscopy

“…Thus, the calculated coupling efficiency (i.e., mode overlap calculation using the Lumerical MODE solutions software) was used for the normalization of the results. The normalization was followed by amplitude scaling based on the saturation length of the waveguides [39]. Although the length of all waveguides is equal (∼1.4 cm), the effective waveguide length for WGR can be different due to the different propagation loss of the waveguides.…”

“…where ρ is the molecular density, σ is the Raman cross-section of the molecule, η is the Raman conversion efficiency, γ in and γ out are the in-and out-coupling losses, α is the propagation loss of the waveguide [39]. In Eq.…”

“…Evans et al studied WGR in titanium oxide (TiO 2 ) waveguides [38]. Raza et al compared four high refractive index contrast materials typically used in integrated photonics, namely aluminum oxide (Al 2 O 3 ), silicon nitride (Si 3 N 4 ), titanium oxide (TiO 2 ) and tantalum oxide (Ta 2 O 5 ) [39]. In their study, the relatively low refractive index contrast of Al 2 O 3 led to poor Raman coupling efficiency, while the elevated background and high propagation losses of TiO 2 led to poor SNR.…”

Waveguide-based surface-enhanced Raman spectroscopy detection of protease activity using non-natural aromatic amino acids

“…In recent years, the silicon nitride (Si 3 N 4 ) platform was developed, which enables waveguide-based excitation and collection of Raman [19,20,21,22] and SERS spectra [23,24,25,26,27,28,29,30,31]. Despite the continuous improvement in the performance of the waveguide-based SERS platforms, they are limited by the background signal of the Si 3 N 4 [32,33]. Currently, free-space SERS substrates still slightly outperform waveguide-based SERS platforms [34].…”

Surface-Enhanced Raman Spectroscopy Based on Plasmonic Slot Waveguides With Free-Space Oblique Illumination

“…The spectrum of this background being itself a function of the mode shape, it depends slightly on the optical cladding (analyte) and therefore cannot be known perfectly for an unknown analyte. Moreover, the shot-noise associated with this background cannot be subtracted and constitute the greatest limiting factor to the signal-to-noise ratio [18]. The impact of this background is therefore detrimental [18]- [20], and its reduction has a high impact on the future prospects of on-chip Raman sensing.…”