DOI: 10.15125/bath-00592
title: The dataset for "Measuring chirality in the far-field from a racemic nanomaterial: diffraction spectroscopy from plasmonic nanogratings"

description: This dataset contains data on gold plasmonic nanogratings used for diffraction circular intensity difference spectroscopy. The data was collected to first characterise the various nanogratings studied using atomic force microscopy - the nanogratings show a lattice constant of 1.2 (square-ring and S-shaped) and 2.4 micrometres (L-shaped) respectively. All nanogratings are about 35 nm thick and have an arm width of 200 nm.  Second, circular intensity difference spectroscopy was used to study the chiroptical response of those nanogratings. Strong chiroptical responses were measured from the S- and L-shaped nanogratings. The experimental results were supported by rigorous numerical simulations carried out in Lumerical as well as a Fourier modal analysis. The users who would like to find out more in depth can then refer to the Documentation section
description: The atomic force microscopy micrograph in Figure 1d was acquired with a Multimode Scanning Probe Microscope (VEECO) operating in contact mode. The data was exported as a text file. 

The experimental setup consisted of a Fianium SC400-2 2 W laser source with a 1064 nm output wavelength and 20 MHz repetition rate and a 5 ps pulse spliced to an in-house fabricated supercontinuum fibre providing a spectrum between 450 and 1050 nm. Two achromatic linear Glan-Laser polarizers were used to control the power output and a remotely controlled quarter-waveplate (QWP) to selectively produce LCP and RCP light. The sample was mounted on an alignment disk, which in turn was mounted on x-y-translation stage placed in the centre of an optical breadboard. The breadboard was mounted on a remotely controlled rotation stage. The diffracted light from the sample was collected via a 200 µm core diameter multimode fibre (0.22 NA) mounted on a fibre launch system mounted on the breadboard at a distance of 15 cm and measured with an Ocean Optics QE Pro spectrometer. The light coupling into the fibre was optimized with a 20x microscope objective with 0.4 NA and 9 mm focal length. The automated setup used a step size of 0.5 °, the spectrometer used an integration time of 30 ms and was averaged over 50 scans. The data was saved as csv files, which were processed using the procedure described in "data processing".

The full-wave simulations were conducted by an FDTD solver, Lumerical. In the simulations, the incident LCP and RCP plane waves are generated by superposing two 90 degrees phased plane waves which are linearly polarized along the x- and y-directions. All layers in the sample have been taken into account. In detail, the Au layer is modelled by the material data from Johnson and Christy, the Cr layer and the Si layer is modelled by the material data from Palik and the reflective index of the SiO2 layer is assumed to be 1.5. It should be noted that instead of using the nominal thickness, the Au layer is assumed to be 50 nm. Two simulation regions are used. On the one hand, the FDTD simulation region is assumed to have periodic boundary conditions along the x- and y-directions and perfect matching layers (48 layers) along the z-direction. On the other hand, a mesh refinement region including both the Au and Cr layers is imposed to the S-shape structures, the L-shape structures, and the square-rings. The mesh steps along x, y and z directions are 10 nm, 10 nm and 1.5 nm, respectively. The data was saved as .mat files and the data processing of the data is described in the "data processing".
description: The data processing scripts have been included in the "plotting_scripts" folder. Move the individual scripts into the folders containing the data. The scripts are for the processing and plotting. 

For the experimental scripts to work, create a folder called "data" and move the individual data folders into it. Change the name of the global variable "MY_FOLDER" (line 31) to the target data folder and adjust the "LATTICE_CONSTANT" (line 23) - 2.4E-6 for L-shaped and 1.2E-6 for square-ring and S-shaped. The script was written with Python 3.6.

At the end of the script, various plotting options can be chosen (by commenting out / removing commenting of the required code-blocks)

The AFM data was plotted with Python 3.6 in a Jupyternotebook.

The Simulation data was plotted in MATLAB2016b.