Plasmonic slanted slit gratings for efficient through-substrate light-plasmon coupling and sensing

We present an experimental study of plasmonic slanted slit gratings (PSSGs) designed to achieve directional coupling between an incident light beam and surface plasmon polaritons (SPPs) propagating along the surface of the structure. We also investigate mirrored PSSG pairs interconnected by a plasmonic slab waveguide. The structures are fabricated using direct milling by a gallium focused ion beam (FIB). In a mirrored pair arrangement, the first PSSG couples a perpendicularly-incident light beam to SPPs propagating in one direction along the waveguide, while the second PSSG decouples SPPs to perpendicularly-emerging light. This configuration shows promise for sensing applications due to the high sensitivity of the excited SPPs to changes in the refractive index of the bounding medium, and the separation of the optics from the fluidics by the substrate. The design also exhibits robustness to fabrication tolerances. The optical characteristics and sensing potential are investigated theoretically and experimentally, highlighting its potential for a wide range of applications.


Fabrication Methodology
We employed focused ion beam (FIB) milling using the Zeiss Orion Nanofab to fabricate ordered arrays of slanted gratings.The gallium ion beam source was tilted 46 o with respect to the imaging helium-source axis.By default, a substrate was located along the middle of the gun-column at an eccentric position towards the imaging and milling ion sources.Before FIB milling, 30 nm of Cr and 30010 nm of Au were sequentially deposited on a fused silica substrate by e-beam evaporation at deposition rates of 0.1 A/s and 0.5 A/s, respectively.The deposited Au/Cr bilayers are shown in Fig. S3.
A FIB Ga probe was set at an acceleration voltage of 30 kV and a probe current of 100 pA to synchronize the designed and milled patterns.The milling parameters used to define the slanted gratings are: (i) a dwell time of 1 μs, (ii) a beam step size of 2 nm, and (iii) a total dose of 1.26 nC/m 2 for the ion beam exposure.The number of milling patterns was carefully chosen to achieve appropriate areal coverage of the gratings.For the air-optimized PSSG, there are 10 slits, while for the water-optimized PSSG, there are 16 slits, ensuring that the length of the gratings falls within the range of 15 ± 1 μm.The width of the PSSGs is 8 μm.
The milling procedure was performed in two steps, with one step for the input grating and another for the output grating, necessitated by the fixed nature of the ion source.Once the input grating milling was completed, the substrate was rotated 180°, and the same pattern was aligned away from the input grating to fabricate the output grating.This process resulted in a 40 ± 2 μm distance between the input and output gratings.Consistent with our theoretical design in Table 1, we observe that when ff is set at 60%, the coupling efficiency between the incident light and SPPs reaches its maximum, resulting in higher transmittance levels.However, the structure is robust against slight deviations from the optimal ff, leading to no significant changes in the coupling efficiency.An additional practical consideration pertains to variations in the depth profile resulting from the angled milling process, potentially leading to the partial removal of the quartz substrate, as illustrated in Fig. S1(d).Even with a partial etch of up to 200 nm, the structure's behavior remains stable.These results show an excellent robustness of the device to fabrication imperfections.The successful deposition of Au/Cr materials is another important aspect to be considered.

Figure S1 |
Figure S1 | The effect of variations in the design parameters and fabrication imperfections on the response of the air-optimized PSSG structure.Simulated transmittance of the air-optimized PSSG structure while changing (a) the fill factor and (b) angle of slants.The effect of (c) non-parallel slants and (d) milling into the substrate.The effect of (e) overcut and (f) undercut of slants on the structure performance.

Fig
Fig. S1(b) demonstrates the effect of variations in the slant angle () on the structure

Figure S2 |
Figure S2 | Effect of Au and Cr thicknesses on the response of the air-optimized PSSG structure.Simulated transmittance of the air-optimized structure while varying the thickness of (a) Au (HAu) and (b) Cr (HCr).

Figure S3 |
Figure S3 | Analysis of the deposited Au/Cr bilayers.(a) Helium ion microscope image of a trench milled with gallium beam in a region devoid of gratings to analyze the Au/Cr layer deposition.(b) Helium ion microscope cross-sectional image of the Au/Cr bilayers within the trench shown in (a).

Fig
Fig. S2(a) and (b) illustrate the impact of variations in the Au and Cr thicknesses, respectively,

Figure S4 |
Figure S4 | Light intensities coupled out through the output grating, extracted using a frame grabber card for DI water at different wavelengths.

Figure S5 |
Figure S5| Light intensities coupled out through the output grating, extracted using a frame grabber card for IPA at different wavelengths.