Optical Characterization of Ultra-Thin Films of Azo-Dye-Doped Polymers Using Ellipsometry and Surface Plasmon Resonance Spectroscopy

Najat Andam,Zouheir Sekkat,Siham Refki,Yasushi Inouye,Hidekazu Ishitobi

doi:10.3390/photonics8020041

Abstract

The determination of optical constants (i.e., real and imaginary parts of the complex refractive index (nc) and thickness (d)) of ultrathin films is often required in photonics. It may be done by using, for example, surface plasmon resonance (SPR) spectroscopy combined with either profilometry or atomic force microscopy (AFM). SPR yields the optical thickness (i.e., the product of nc and d) of the film, while profilometry and AFM yield its thickness, thereby allowing for the separate determination of nc and d. In this paper, we use SPR and profilometry to determine the complex refractive index of very thin (i.e., 58 nm) films of dye-doped polymers at different dye/polymer concentrations (a feature which constitutes the originality of this work), and we compare the SPR results with those obtained by using spectroscopic ellipsometry measurements performed on the same samples. To determine the optical properties of our film samples by ellipsometry, we used, for the theoretical fits to experimental data, Bruggeman’s effective medium model for the dye/polymer, assumed as a composite material, and the Lorentz model for dye absorption. We found an excellent agreement between the results obtained by SPR and ellipsometry, confirming that SPR is appropriate for measuring the optical properties of very thin coatings at a single light frequency, given that it is simpler in operation and data analysis than spectroscopic ellipsometry.

Highlights

Surface plasmons (SPs) are electromagnetic waves that are bound to metal/dielectric interfaces and are capable, among other things, of yielding the optical properties of very thin films deposited on metal layers [1,2,3]
For the DR1/PMMA material studied in this paper, we found that BR’s theory better fits our observations compared to Maxwell– Garnett (MG)
We determine the real and imaginary parts of nc for Disperse Red One (DR1), an azo dye that is well known for its photoisomerization and nonlinear optics (NLO) properties [50], incorporated into a poly-methyl-methacrylate (PMMA) matrix at different dye/polymer concentrations by using surface plasmon resonance (SPR) and spectroscopic ellipsometry (SE), and we show that there is an excellent agreement between SPR and SE in the determination of the real and imaginary parts of nc for quite thin (i.e., 58 nm) DR1/PMMA films

Summary

Introduction

Surface plasmons (SPs) are electromagnetic waves that are bound to metal/dielectric interfaces and are capable, among other things, of yielding the optical properties of very thin films (i.e., down to few angstroms) deposited on metal layers [1,2,3]. Besides SPs, other techniques can be applied to thin films, such as spectroscopic ellipsometry (SE) and photothermal deflection (PTD) spectroscopy The latter can be applied to dye-doped polymers to measure optical absorption, requiring optical modeling such as Kramer–Kronig to get the refractive response. We determine the real and imaginary parts of nc for Disperse Red One (DR1), an azo dye that is well known for its photoisomerization and NLO properties [50], incorporated into a poly-methyl-methacrylate (PMMA) matrix at different dye/polymer concentrations by using SPR and SE, and we show that there is an excellent agreement between SPR and SE in the determination of the real and imaginary parts of nc for quite thin (i.e., 58 nm) DR1/PMMA films For this range of thicknesses of the DR1/PMMA dielectric layer, only SP is observed (vide infra), and waveguides occur for thicker films [51]

Materials and Methods

SPR Spectroscopy