Dataset for "Dense Arrays of Nanohelices: Raman Scattering from Achiral Molecules Reveals the Near-field Enhancements at Chiral Metasurfaces"

Data collection method:

The Raman spectra (with circularly polarised light and linearly polarised light) of crystal violet from the SERS substrates as shown in the main manuscript and supporting information with 532 nm. The data is provided in the form of WiRE files and text versions for ease of access. Where applicable, before and after images from the microscope are included. Also included are extracted Raman peak heights at 1177 cm-1 (crystal violet) before and after fluorescence background removal in a spreadsheet. In addition, the original data and images for the atomic force and scanning electron microscope images from the manuscript are included. The exported electric field distributions from simulations of the SERS substrates are also given. Each data folder contains a Metadata text file with explicit details about the nature, setup parameters and use of the data. SERS Substrate Characterization: The substrates were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM micrographs in Folder: “SEM_Data” were acquired with a Jeol JSM-7900F Schottky Field Emission SEM. The AFM micrographs in Folder: “AFM_Data” were acquire using a Multimode Scanning Probe Microscope (VEECO) operating in contact mode. The TEM images in Figure 2 were acquired using a JEOL JSM-2100PLUS. For TEM, a small square (~4 mm × 3 mm) of nanohelices on Si wafer were cut and sonicated in a 0.7 mL of solvent for 20 minutes before deposition (few µL) onto TEM grids – Au nanohelices: chloroform, formvar TEM grids; Ag nanohelices: ethanol, carbon coated Cu TEM grids. The Transmission electron microscopy data can be found in “TEM_Data”. Linearly polarised Raman Spectroscopy: Raman spectra were acquired using a Renishaw inVia Raman microscope (Folders: “532nm_Linear Pol Raman”). The incident light source for 532 nm was a continuous wave narrow bandwidth laser (Cobolt RL532-08; 50 mW). The irradiated light and epi-scattered Raman light were focused and collected through an N-plan 50× objective with a numerical aperture of 0.75. All spectra were averaged from a 40 µm × 40 µm square grid of 5×5 (25) uniformly distributed points; each separated by 10 µm. At each sample point, the spectrum acquisition was a total 10 seconds with an integration time of 1 second. The spectral resolution was 1.6 cm-1 for the spectra with 532 nm excitation. To establish the peak height relative to the baseline of the spectra in (File: “532nm_Linear Pol Raman/Peak Heights Crystal Violet_532nm.csv”), the fluorescence background was removed using Renishaw’s built-in 11th-order polynomial Intelligent Fitting™ algorithm (“subtract baseline” tool) in WiRE-version 5.3. The laser power at the sample was measured using a Thorlabs™ S175C - Microscope Slide Thermal Power Sensor (File: “532nm_Linear Pol Raman /Power_Readings_532nm_Laser.csv”). For experiments with the 532 nm continuous wave Cobolt laser, the laser power at the sample was varied between 80 µW and 25 mW using neutral density filters. The irradiance was computed by taking the measured power under the objective and dividing by the area of the spot size of the laser. The laser spot diameter was assumed to be equal to the diffraction limited size: 1.22 λ / NA; where NA = 0.75 is the numerical aperture of the objective. Circularly polarised Raman spectroscopy: Raman optical activity data were acquired using a modified Renishaw inVia Raman microscope (see Figure 1). The incident light source for 532 nm was a polarized continuous wave narrow bandwidth laser (Cobolt RL532-08; 50 mW). A Glan-laser polarizer was used prior to the Rayleigh filter to polarize the light and a λ/2-plate (not shown in Figure 1) was placed at the output of the laser to optimize power throughput. An achromatic λ/4-plate was placed after the Rayleigh filter to circumvent the retardance properties of the Rayleigh filter. The orientation of the λ/4-plate was coarsely optimized to mitigate ellipticity at the sample using a zero-order λ/4-plate, a wire-grid polarizer to filter circularly polarized light placed above a power meter. The analyzer, was a wire-grid achromatic polarizer with an achromatic λ/2-plate in tandem to optimize for the polarization sensitivity of the spectrometer; the orientation of the λ/2-plate was optimized with a Si sample using the linearly polarized light (no λ/4-plate). With these optics in, the orientation of the λ/4-plate was fine-optimized using a piece of polycrystalline ZnSe, an N-Plan 5× (NA: 0.12) objective and a wide spectrometer slit (150 µm) to ensure parity between left-handed and right-handed circularly polarized light. The optimization data can be accessed in folders: “Circular Polarisation_CoarseOptimise”, “Circular Polarisation_FineOptimise” and “Linear Polarisation_AnalyserOptimise”. Raman optical activity experiments were performed with an N-plan 50× objective with a numerical aperture of 0.75; however, the data presented in Figure S23 was collected using the N-plan 5× objective (NA: 0.12). The circular intensity sum and difference spectra in Figure 3 to Figure 5 (folder: “ChiroptiocalRaman_42kWcm “ and “ChiroptiocalRaman_4p2kWcm”) were averaged from three pairs of 60 µm × 60 µm square grids (13×13=169) uniformly distributed points; each point separated by 5 µm and each grid separated by approximately 100 µm. At each sample point, the integration time was a total of 2 seconds for data at 42 kW cm-2 and 1 second for data at 4.2 kW cm-2. The spectral resolution was 1.6 cm-1 for the spectra with 532 nm excitation. The peak height relative to the baseline of the spectra in Figure 5 was established using the same technique as for the linearly polarized light Raman spectroscopy. For all data except that shown in Figure S22 and Figure S23, the irradiance was computed by taking the measured power under the objective and dividing by the area of the spot size of a diffraction limited spot (see Raman spectroscopy – linearly polarized light section above). The irradiance for the spectra shown in Figure S22 and Figure S23 was computed using the measured laser spot-diameter. To measure the laser spot diameter at the image plane, a Raman line scan was taken over the sharp edge of a Si sample. Then, by interpolating a cubic spline function (scripted in Python) through the Raman peak height of the OΓ-point phonon in Si (at 520 cm-1) as a function of position across the edge, the diameter could be determined from the full-width at half-maximum of the first derivative of the intensity profile. This is illustrated in Figure S22a and Figure S23a for which the accompanying data can be found in folders: “ChiroptiocalRaman_SI_3step-50xObj” and “ChiroptiocalRaman_SI_3step-5xObj”. Simulations: Finite-difference time-domain simulations were performed in ANSYS-Lumerical™ to illustrate the electric field distributions, revealing the nature of the local field enhancements or hot-spots (Folder: “Simulations”). The simulation domain had periodic boundary conditions applied in the x and y directions to the edges of the unit cell (See Manuscript for dimensions). The domain in the vertical axis spanned -1.5 µm to 3µm depending on the size of the substrate model and had perfectly matched layer boundary conditions. The Eulerian mesh in the regions of interest was 5 Å for the nanohelices, 2.5 nm for the Au CNPs substrate and 2.5 nm for the Au G-Shaped motif nanostructures substrate. The optical properties of the Si wafer and SiO2 layer were modelled with an empirical based material model from Palik[53]. The nanohelix substrates had a 5 nm layer of SiO2; likewise 2 nm for the Au CNPs; and 100 nm for the G-Shaped Au nanostructures. The optical properties of the Au-based nanohelices were modelled using a 4:1 linear combination of the CRC[54] material models for Au and Cu. CRC based material models were also used for the optical properties of the Ag nanohelices and Au substrates. A pulsed plane wave source of light was incident on the models from 1 µm above the surface; the light was polarized parallel to the x-axis and had an amplitude of 1 V/m. For simulations with circularly polarized light, two orthogonally polarized planewave sources were superimposed with a 90° phase difference – this is contained in folder: “Simulations\Circ_Polarisation_Nanohelices”. The simulated wavelength range was 250 nm to 2.5 µm. The electric-field distributions were extracted at 532 nm and 785 nm from the cross-sectional planes indicated in the manuscript. To simulate the AFM data in the manuscript (Folder path: “Simulations\Full_Substrate\AFM_Based_Simulations”), the AFM data for the Au G-shaped motifs and Au conglomerate nanoparticles were exported into three column text data files (.txt) using Gwyddion. These x, y, z coordinates were then reshaped into a grid in Python and imported as a surface into ANSYS™ Lumerical. The Au surface topography was then superimposed onto the relevant Si/SiO2 layered substrate model. The models in folders “G-Shaped-motif_S6”, “LeftHand_Ag_Nanohelices” and “LeftHand_AuCu_Nanohelices” of folder path: “Simulations\Full_Substrate were generated using Autodesk Inventor™ based on dimensions extracted from the SEM micrographs in folder “SEM_Data”. The Ellipticity data were acquired through CD spectra and can be found in “CD_Spectra” folder. For each nanohelices SERS substrate, 200 spectra were acquired at rotational angles of 0° to 360° with intervals of 20°. These 200 spectra were averaged and exported into the _Average folder. The ellipticity was computed for each rotation for each sample and exported into _Ellipticity folder. These were then in tern averaged and presented in the paper. Plots of the ellipticity at every rotation and for each sample were generated and are found in the corresponding folders within the _Ellipticity folder. These plots were generated directly from the data found in the txt filed. The Python script used to analyze the data is in the subfolder “CD_Spectra_Analyzer”.