DATA REPOSITORY - DEEP STRONG LIGHT-MATTER COUPLING IN PLASMONIC NANOPARTICLE CRYSTALS

This data repository contains optical spectra and polariton dispersions of metallic nanoparticle crystals. The data are subdivided into experimental and simulated data. Each folder contains a readme file that describes the data format and how the data were obtained.

DATA FILE NAMES STARTING WITH EXP CONTAIN EXPERIMENTAL DATA. DESCRIPTION:

The experimental data include optical spectra and polariton dispersions that were obtained from measurements on layered gold nanoparticle (AuNP) films. The nanoparticle diameters and interparticle gaps were estimated from TEM images of monolayers and are given in the file name.

The optical spectra (transmittance, reflectance, absorbance) were measured with a micro absorbance spectrometer, as described in Mueller et al. ACS Photonics 5, 3962(2018). The spectra were measured on crystals of different thickness, which was determined by the optical contrast. The crystal thickness is given as the number of stacked layers (L) for each spectrum.

The polariton dispersions were obtained as follows: 
The absorption spectra were fit with a cubic background and a series of Lorentzians to obtain the polariton frequencies. The crystal forms an open cavity for polaritons creating standing waves with wavelength \lambda_pp = 2h/j, where j is integer and h the crystal thickness. The wave vectors in units of the \Gamma L distance (in fcc) are given by kpp = 2\pi/\lambda_pp = (j/N) (\Gamma L), where N is the layer number.

Format of the file names:

Exp_SpectrumType_AuNP_dnmDiameter_anmGaps.csv

Where 

SpectrumType = Absorbance, reflectance, or transmittance

d = nanoparticle diameter

a = gap size between two nanoparticles

Within each file the spectra are given for various layer numbers L as a function of excitation frequency

Exp_Polariton_Dispersions_from_Fits_of_Experimental_Spectra - experimental polariton dispersions extracted from the spectra. The NP diameter is specified in the files. Data points are given as column pairs of wavevector k_pp (in units of \Gamma L) and polariton frequency \omega_pp (in eV). 

DATA FILE NAMES STARTING WITH FDTD CONTAIN SIMULATED DATA. DESCRIPTION:

he simulated data include optical spectra and polariton dispersions that were obtained by finite-difference time-domain simulations of layered metallic nanoparticle films. The nanoparticle diameters, interparticle gaps, refractive index of the surrounding medium (need), type of nanoparticle material, layer number (L) and type of crystal stacking are given in the file names. 

Details about the simulations (see also Mueller et al. ACS Photonics 5, 3962-3969 (2018)):

We used the commercial finite-difference time-domain (FDTD) simulation package Lumerical FDTD solutions. We constructed the unit cell of a hexagonally close packed monolayer of metallic nanoparticles with diameters d, interparticle gap sizes a, and periodic boundary conditions along x and y. To obtain thin slabs of an fcc crystal we stacked nanoparticle layers along z in an abc stacking. 

We used a mesh-override region with 0.5 nm mesh size for a > 1.5 nm and a/3 mesh size for smaller gaps. The gold nanoparticles were modeled with the experimental dielectric function of gold from Olmon, R. L. et al. PRB 86, 235147 (2012). For aluminium, we used a fit of the experimental dielectric function from Raki­c, A. D. Appl. Opt. 34, 4755 (1995). For simulations within the Drude model we used the dielectric function \epsilon(omega)=1-\omega_p^2/(\omega^2+i\gamma_p) with \omega_p = 9.07 eV and \gamma_p = 40 meV. We modeled the surrounding medium with a dielectric constant of \epsilon_m = 1.96 mimicking the experiment where the gold nanoparticles are surrounded with polystyrene molecules.

To calculate the optical response of the crystal slab, we illuminated the structure with a broadband plane electromagnetic wave source along z with linear polarization along x. We verified that the direction of polarization in the xy plane has no influence on the calculated far-field spectra. The transmitted T and reflected R light were recorded with two-dimensional frequency-domain power monitors and the absorbance was calculated as A = 100% - T - R.

The polariton dispersions were obtained as follows: 

The simulated absorption spectra were analysed as described for the experimental data to obtain the simulated polariton dispersion (see readme file - experimental data). For spectra in which the absorbance peaks strongly overlapped we analyzed the internal current distribution as explained in Mueller et al. ACS Photonics 5, 3962 (2018). The polariton energies and wavelengths yield the polariton dispersions. 

List of data in the folder. If not otherwise stated, the simulaltions are for Au nanoparticles and for fcc crystals.

FDTD_PolaritonDispersion_fromFits_50nmDiameter_2nmGaps_nmed1p4_Au_DifferentLayerNumbers_fcc - Simulated polariton dispersion d=50nm, a=2nm, n_m=1.4

FDTD_PolaritonDispersions_fromFits_30nmDiameter_DifferentGaps_Different_nmed1_Drude_6Layers_fcc - Simulated polariton dispersion (fcc, d=30nm, a varies, n_m varies, see data). 

FDTD_PolaritonDispersions_fromFits_30nmDiameter_DifferentGaps_Different_nmed1_Drude_6Layers_fcc - Polariton dispersion, a=2nm, d varies, see data (Drude model)

FDTD_PolaritonDispersions_fromFits_50nmDiameter_DifferentGaps_nmed1p4_Au_fcc - Polariton dispersion d=50nm, a varies, n_m=1.4

FDTD_Spectra_30nmDiameter_DifferentGaps_Different_nmed1_Drude_6Layers_fcc - Simulated spectra, Drude model, d=30nm, a varies, n_m=1

FDTD_Spectra_30nmDiameter_DifferentGaps_nmed1p4_Al_6Layers_fcc - Simulated spectra, d=30nm, a varies, n_m=1.4, Aluminum.

FDTD_Spectra_50nmDiameter_2nmGaps_nmed1p4_Au_DifferentLayerNumbers_fcc - Simulated spectra, d=50nm, a=2nm, n_m=1.4

FDTD_Spectra_50nmDiameter_DifferentGaps_nmed1p4_Au_6Layers_fcc - Simulated spectra d=50nm, a varies, n_m=1.4

FDTD_Spectra_DifferentDiameters_2nmGaps_nmed1p4_Au_6Layers_fcc - Simulated spectra, d varies, a=2nm, n_m=1.4