DOI: 10.15125/bath-00479
title: Dataset for "Circular dichroism in higher-order diffraction beams from chiral quasi-planar nanostructures"

description: This dataset  contains both experimental and numerical data relating to figures presented in the paper, "Circular dichroism in higher-order diffraction beams from chiral quasi-planar nanostructures". The experimental data contain measurements obtained from atomic force microscopy (AFM) and the spectra used to plot far-field circular dichroism (CD) diffraction patterns. The numerical data contain simulated spectra used to plot a corresponding set of far-field CD diffraction patterns, and electric near-field data corresponding to maximum and minimum CD responses in the far-field.
description: Sample fabrication:
10 mm x 10 mm x 525 µm single side polished Si(p-doped)-SiO2(300 nm) samples are sonicated in successive baths of acetone and IPA (Isopropyl alcohol) for 10mins, blow dried with compressed N2 and dehydrated on a hotplate (200°C, 20mins). PMMA (Polymethyl-methacrylate) A4 950k positive-tone photoresist is spin-coated (5,000 rpm, 45s) and baked (180°C, 2 mins), resulting in a final thickness of ~150 nm. 80 kV electron beam lithography (Nanobeam, nB-1) is used for the high-resolution patterning, with exposure conditions: area dose ~10 Cm-2, operating current 5 nAs-1 and main-field/sub-field apertures of 50/6 µm. Resist development is carried out in a 1:3 solution of MIBK (Methyl-isobutyl-ketone): IPA for 10 s. Deposition of Cr/Au (5/30 nm) is performed using a thermal evaporator at a base pressure ~1x10-6 mbar, at an evaporation rate ~0.1 nm s-1. Resist lift-off is carried out in NMP (N-Methyl-2-pyrrolidone) at an elevated temperature of 60°C for 4 hours, followed by fresh NMP sonication, acetone and IPA rinse. For nanoscale surface quality inspection, a Carl Zeiss Scanning Electron Microscope (SEM) operating at 3 keV is used. 

Sample characterisation :
AFM experiments were carried out using a Multimode Scanning Probe Microscope (Veeco, Plainview, NY) with a Nanoscope IIIA controller. Images were obtained in contact mode under ambient conditions. A Pointprobe-Plus® Silicon-SPM-Sensor AFM probe (PPP, NanosensorsTM, Neuchâtel, Switzerland) with a force constant of 0.039 N/m was used for imaging.
The additional SEM shown in Figure 1 (c) was taken with the JEOL SEM6480LV SEM operating in the backscattering mode at 10keV.

Experimental setup:
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, described in ref.[56] providing a spectrum between 450-1050 nm. We used a short-pass filter to only allow light in the spectral region between 450 – 750 nm. We used two linear Glan-Laser polarizers to control the power output and a remotely controlled quarter wave plate to selectively produce LCP and RCP light. The sample was mounted on an alignment disk, which in turn was mounted on an in-house designed adapter 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 400 µm diameter multimode fibre mounted on the edge of the breadboard at a distance of 25 cm and measured with an Ocean Optics QE Pro spectrometer. The automated setup used a step size of 0.5° and the spectrometer used an integration time of 250 ms and was averaged over 8 scans.

Simulations:
The optical near-field and the intensity of the diffracted beams have been computed numerically using the rigorous coupled-wave analysis (RCWA) method, implemented in Synopsis’ RSoft DiffractMOD, a commercially available software. In this method, both the distribution of the dielectric constant and electromagnetic field are decomposed in Fourier series, the corresponding Fourier coefficients being computed using the boundary conditions at the top and bottom of the structure. These coefficients are subsequently used to calculate the optical near-field and the intensities of the diffracted beams. The frequency dispersion of the permittivity of Au, Cr, and SiO2 has been fully incorporated in our simulations. Moreover, we used N=20 harmonics for each transverse dimension, which amounts to a total of (2N+1)^2=1681 harmonics.