6.4 Characterizing SOI-DLSPP Waveguide Loss
6.4.3 Near-Field Microscopy Measurements of DLSPP Attenuation . 138
waveguide transmission measurements, we analyzed the same DLSPP devices using near-field scanning optical microscopy (NSOM). NSOM/AFM measurements were performed with a tuning-fork based Nanonics MultiView 2000 scanning probe micro- scope in contact mode using a 200-nm diameter aperture probe in collection mode.
Light was coupled into the SOI input waveguide of each device at a wavelength of 1520 nm using an identical lensed-fiber arrangement as used for the waveguide trans-
Figure 6.9. (a) Scanning electron micrograph of a 30-µm long DLSPP waveguide device. (b) AFM and (c) NSOM images collected simultaneously for a device with the same geometry, where light was coupled into the plasmonic section from a buried SOI waveguide lying to the left of the DLSPP device at λ = 1520 nm. (d) Total collected intensity from the NSOM image integrated along the y-direction as a function of position in the propagation direction. The measured intensity exhibits a characteristic decay length of 47µm.
mission measurements, and light collected by the scanning probe was detected using an InGaAs avalanche photodiode.
An NSOM scan for one of the 30-µm long DLSPP waveguides used for transmission measurements is shown in Fig. 6.9. The AFM and NSOM images in Fig. 6.9(b) and (c) were collected simultaneously using a high gain setting for the tip deflection signal, which minimized damage to the polymer waveguide but led to a noticeable increase in the noise associated with the measured topography. Qualitatively, there is significant collected intensity only along the length of the polymer waveguide in the NSOM image, indicating that optical power is preferentially coupled into the DLSPP mode as opposed to air-Au surface plasmons, which would be expected to spread out from the sides of the waveguide. We also see reduced intensity at the surface of the PMMA covering the SOI input/output waveguides because the SOI waveguide mode is largely confined to the buried Si layer and not accessible to the NSOM probe.
Comparing the scanning electron micrograph in Fig. 6.9(a) with the AFM image in Fig. 6.9(b), we observe that the size and shape of the NSOM tip affects the apparent width of the DLSPP waveguide in they-direction; however, we are primarily interested
in decay of the DLSPP mode along the propagation (x) direction. Consequently, the intensity in the NSOM image was integrated along the y-direction and plotted on a normalized logarithmic scale in Fig. 6.9(d) as a function of propagation distance along the x-direction.
Other than an initial jump in intensity at the input SOI-DLSPP transition due to scattering at the SOI-DLSPP junction, the collected intensity plotted in Fig. 6.9(d) decays exponentially, but with an even oscillation period. Fitting the intensity profile to a model for a Fabry-P´erot cavity with loss, we find the non-physical result that the oscillation period corresponds to an intracavity mode index significantly less than 1. If we instead consider the oscillations to be the result of two modes with different effective wavelengths beating against one another, the beat period, Lb = 6.7 µm, corresponds to an effective index difference of λ/Lb = 0.23 for λ = 1520 nm. This is very close to the difference between the DLSPP-mode effective index of 1.21 and the index of a radiative mode propagating in same direction in air (i.e. unity). It is therefore plausible that the observed oscillation is a result of near-field interference between the propagating DLSPP mode and scattered light propagating in free space along the x-direction.
Independent of the oscillation, the exponential decay in the intensity profile in Fig. 6.9(d) suggests a propagation length ofLprop = 47µm. This is in agreement with the DLSPP-mode propagation length of LSPP ≈ 50µm extracted from the variable- length waveguide transmission measurements and therefore supports our quantitative analysis of the SOI-DLSPP waveguide coupling loss.
6.4.4 Calculated SOI-DLSPP Insertion Loss
To further verify the measured coupling efficiency, the SOI-DLSPP waveguide struc- tures were modeled with three-dimensional full-field finite-difference time-domain (FDTD) calculations using the Lumerical software package. Figure 6.10(b) shows the simulated structure, including input/output SOI waveguides and a 20-µm long DLSPP section. At the waveguide transitions, we modeled the PMMA layer with a 2-µm long linear taper at the edge of the silicon layer, in accord with AFM mea-
surements of the fabricated topography. The calculated TM mode supported by the input SOI waveguide atλ = 1550 nm, plotted in Fig. 6.10(a), was used as the FDTD source, and we monitored the power transmitted through the output SOI waveguide 10 µm from the output transition. The monitor position was varied to ensure that it captured only power coupled into the (loss-less) output waveguide and not power scattered from the DLSPP-SOI transitions. The simulation boundaries were defined as perfectly matched layers and positioned far enough away from the waveguide so as to minimally impact the effective index of the calculated input mode. To ensure stability, the input source was defined temporally as a single pulse, and the field am- plitudes were allowed to decay to 0.001% of their initial values. Spectral filtering was used to extract the power transmission associated with the input wavelength.
In Fig. 6.10(c), we show the total transmission calculated for different values of the silicon-gold offset, doffset. The power profiles plotted in Fig. 6.10(b) indicate that light is coupled predominately into the DLSPP mode, supporting the assumption that light reaching the output monitor has traversed the gold region only through that mode. From FEM calculations at λ =1550 nm using the gold index from Johnson and Christy [139], the propagation loss for the DLSPP mode, plotted in Fig. 6.10(a), is 0.10 dB/µm. Given the total calculated transmission of 43.2% (3.65 dB loss) for doffset = 300 nm, we therefore estimate an attenuation of 2.0 dB forL= 20 µm. The modeled coupling loss is thus approximately 0.8 dB per SOI-DLSPP transition, just slightly lower than the experimentally measured value of 1.0±0.1 dB.