Single mode waveguide platform for spontaneous and surface-enhanced on-chip Raman spectroscopy

Abstract

We review an on-chip approach for spontaneous Raman spectroscopy and surface-enhanced Raman spectroscopy based on evanescent excitation of the analyte as well as evanescent collection of the Raman signal using complementary metal oxide semiconductor (CMOS)-compatible single mode waveguides. The signal is either directly collected from the analyte molecules or via plasmonic nanoantennas integrated on top of the waveguides. Flexibility in the design of the geometry of the waveguide, and/or the geometry of the antennas, enables optimization of the collection efficiency. Furthermore, the sensor can be integrated with additional functionality (sources, detectors, spectrometers) on the same chip. In this paper, the basic theoretical concepts are introduced to identify the key design parameters, and some proof-of-concept experimental results are reviewed.

Keywords: Raman spectroscopy; laboratory on a chip; silicon photonics; surface-enhanced Raman spectroscopy; waveguides.

Figure 1.

(a) Schematic of the beam geometry for a confocal microscope. Only signal from confocal volume contributes significantly to the signal. (b) Schematic of the waveguide-based evanescent Raman sensors. A long interaction length leads to a very large detection volume.

Figure 2.

(a) Normalized power P/P₀ coupled to the fundamental TE (solid) and TM (dashed) modes of a slab waveguide as a function of normalized waveguide width k₀d, for Si (blue) and Si₃N₄ (red) cores. The powers are calculated for dipoles on the core surface and oriented vertical (V) and horizontal (H) to it. Inset: a generic slab waveguide (adapted from [39]). (b) Theoretical η₀ curves for strip waveguides as a function of the normalized waveguide width w for Si (blue) and Si₃N₄ (red) waveguide systems for fixed h = 220 nm. Solid lines: TE polarized excitation and collection. Dashed lines: TM polarized excitation and collection. Only the curve segments corresponding to a well-defined single mode operation are shown. Inset: a general strip waveguide (adapted from [25]).

Figure 3.

Theoretical conversion efficiency η₀ for slotted waveguides for SOI (blue) and Si₃N₄ (red) waveguide systems. (a) Fixed r = 350 nm, h = 220 nm, as a function of slot width s for fundamental TE mode. (b) Fixed s = 150 nm, h = 220 nm, as a function of waveguide width w = s + 2r. Solid lines: TE-polarized excitation and collection. Dashed lines: TM-polarized excitation and collection. (Adapted from [25].)

Figure 4.

Si₃N₄ waveguide functionalized with an array of N bowtie antennas. The top inset depicts the geometrical parameters of a bowtie antenna (gap Δ, length L and apex angle α). The bottom inset shows an SEM image of an integrated bowtie antenna (adapted from [27]).

Figure 5.

Comparison of the antenna extinction E (dB), η_A and FOM(N) for four different bowtie antennas with fixed α = 60° and Δ = 10 nm but varying length L (70, 90, 110, 130 nm). The red and cyan line mark the respective pump and Stokes wavelength (adapted from [26,27]).

Figure 6.

Experimental set-up. Ti : saph, tunable Ti : sapphire laser emitting the pump beam at λ_P = 785 nm; PM, power meter, SR 303i and iDus 416, spectrometer and cooled CCD detector from ANDOR; BS, beamsplitter; λ₀/2, half-wave plate; LLF, laser line filter for 785 nm; P, polarizer; M, fixed mirror; OBJ, objective (50×, NA = 0.9); ASPH, aspheric lens (NA = 0.5); S, sample stage; DM, dichroic mirror; PMC, parabolic mirror collimator (EFL = 15 mm, NA = 0.2); FS, fibre splitter (adapted from [27]).

Figure 7.

(a) Raman spectra measured from a 1.6 cm waveguide (w = 700 nm) without IPA (blue) and with IPA (red) on top. The spectrum with IPA is shifted vertically with zeros at the dashed red line (adapted from [24]). (b) Evanescently measured Raman spectra of IPA after background subtraction. The spectra are normalized to the 819 cm⁻¹ peak obtained from the slot waveguide (w = 700 nm, s = 150 nm). The blue spectrum (with blue axes) is obtained by using a 700 nm wide Si₃N₄ strip (adapted from [25]). (c) Evanescently measured Raman spectra of 1 M glucose solution before and after background subtraction obtained from the slot waveguide (w = 700 nm, s = 150 nm).

Figure 8.

The theoretical and experimental values of η₀ obtained for (a) TE modes and (b) TM modes of Si₃N₄ waveguides. The markers with error bars represent the estimated experimental errors [24]. The lines represent theoretical curves. The red solid lines are the theoretical curve for slot waveguides with s = 150 nm. The blue and black dashed lines are the theoretical curves for TE and TM polarizations, respectively, for strip waveguides. Circle, TE polarization, strip waveguides. Square, TE polarization, slot waveguides. Left-handed triangles, TM polarization, strip waveguides. Right-handed triangles, TM polarization, slot waveguides (adapted from [25]).

Figure 9.

Raman spectra, before and after NTP coating, of a reference waveguide (N = 0) and waveguides functionalized with N = 10, 20, 30, 40 antennas (adapted from [27]).

Figure 10.

Signal and SNR of the 1340 cm⁻¹ peak. The solid lines and shaded areas represent a fit to our on-chip SERS model (adapted from [27]).