Normal Incidence Reflectance Spectra Research Articles

Tunable exciton-plasmon coupled resonances with Cu2+/Cu+ substitution in self-assembled CuS nanostructured films

Localized surface plasmon resonance (LSPR) coupled with exciton resonances in semiconductor nanostructures are crucial for extended spectral range of light matter interactions beyond diffraction limits. Here, we report the tunable coupling of exciton-plasmon resonances with Cu2+ substitution for Cu+ in pure covellite phase of CuS nanostructures by chemical bath deposition (CBD) method. The synthesized self-assembled multilayered nanostructures in shape of nanorods and nanoflakes are found to dramatically change their near field coupling resonances with Cu2+ substitution. The near normal incidence reflectance spectra is fitted by Drude-Lorentz oscillator model constrained with Kramers-Kronig relations to obtain various optical parameters. The substitution of Cu2+ ions found to significantly tune the complex refractive index nearby or below unity for wavelengths approaching plasmon frequency and correspondingly develop epsilon near zero (ENZ) modes in permittivity. The absorption spectrum measured with two different methods is used to distinguish the type of resonances in hybrid states. Raman spectra reveal the enhancement in intensity of A1g modes by four times. The carrier concentration and mobility were measured using Hall measurements. X-ray photoelectron spectroscopy (XPS) reveals the variable oxidation states of the constituent elements. This work demonstrates simplistic approach for efficient fabrication of quantum confined nanostructures for improved light-matter interactions.

Broadband normal-incidence mirrors for a range of 111–138 Å based on an a-periodic Mo/Be multilayer structure

Broadband Mo/Be multilayer structures were designed for maximum uniform normal-incidence reflectivity in a broad range of 111–138 Å, which lies near and beyond the L2,3 absorption edge of Si. A comparison was made of the capabilities of two classes of aperiodic structures and of so-called “stack” structures, which are composed of several periodic structures with different periods stacked one over the other. Six-stack Mo/Be 80-layer structures were synthesized on concave (R = 1 m) superpolished fused silica substrates. Their absolute reflectivity was measured at 13% – 14% in the 111–138 Å optimization range using a laboratory reflectometer with a laser-plasma radiation source. The normal-incidence reflection spectra of the mirrors were recorded in the configuration of a transmission-grating spectrograph using the slowly varying quasicontinuum of a laser-driven tungsten plasma. Comparing the reflectivity data with the reflection spectra recorded with a CCD permitted estimating a decrease in the detector responsivity beyond the Si L-edge. The broadband normal-incidence multilayer mirrors facilitate the development of a high-resolution imaging spectrograph covering a usable range about the Si L-edge to characterize, for instance, the L-edge fine structures and chemical states. These mirrors will also find use in imaging solar instruments with a high spectral resolution operating aboard a spacecraft and in laboratory instruments for plasma diagnostics.

Elevated surface plasmon resonance sensing sensitivity of Au-covered silica sphere monolayer prepared by Langmuir–Blodgett coating

The colloidal Langmuir–Blodgett coating process is used to fabricate Au-covered silica sphere monolayer (Au film over silica nanosphere (AuFON)) and study the effects of silica diameter on surface plasmon resonance (SPR) sensing sensitivity. The resulting hexagonal close-packed (HCP) monolayers are prepared with silica sphere diameters of 200, 400, 700, and 1000 nm. In SPR sensing applications, the optical properties of Au-covered silica sphere monolayer are evaluated by measuring normal-incidence reflection spectra and sensing tests. The high sensitivity (nm/RIU) is observed in silica sphere diameter (1000 > 700 > 400 > 200 nm) and plasmon mode (dipole > Fano resonance (FR) > and high order) while the highest sensitivity is 968 nm/RIU (dipole mode, 1000 nm of silica sphere diameter). 3-D Finite Difference Time Domain (FDTD) simulation shows a sensitivity trend similar to the experimental results.

Spectrophotometric Characterization of Thin Copper and Gold Films Prepared by Electron Beam Evaporation: Thickness Dependence of the Drude Damping Parameter

Copper and gold films with thicknesses between approximately 10 and 60 nm have been prepared by electron beam evaporation and characterized by spectrophotometry from the near infrared up to the near ultraviolet spectral regions. From near normal incidence transmission and reflection spectra, dispersion of optical constants have been determined by means of spectra fits utilizing a merger of the Drude model and the beta-distributed oscillator model. All spectra could be fitted in the full spectral region with a total of seven dispersion parameters. The obtained Drude damping parameters shows a clear trend to increase with decreasing film thickness. This behavior is discussed in the context of additional non-optical characterization results and turned out to be consistent with a simple mean-free path theory.

Gyroid Optical Metamaterials: Calculating the Effective Permittivity of Multidomain Samples

Gold gyroid optical metamaterials are known to possess a reduced plasma frequency and linear dichroism imparted by their intricate subwavelength single gyroid morphology. The anisotropic optical properties are, however, only evident when a large individual gyroid domain is investigated. Multidomain gyroid metamaterials, fabricated using a polyisoprene-b-polystyrene-b-poly(ethylene oxide) triblock terpolymer and consisting of multiple small gyroid domains with random orientation and handedness, instead exhibit isotropic optical properties. Comparing three effective medium models, we here show that the specular reflectance spectra of such multidomain gyroid optical metamaterials can be accurately modeled over a broad range of incident angles by a Bruggeman effective medium consisting of a random wire array. This model accurately reproduces previously published results tracking the variation in normal incidence reflectance spectra of gold gyroid optical metamaterials as a function of host refractive index and volume fill fraction of gold. The effective permittivity derived from this theory confirms the change in sign of the real part of the permittivity in the visible spectral region (so, that gold gyroid metamaterials exhibit both dielectric and metallic behavior at optical wavelengths). That a Bruggeman effective medium can accurately model the experimental reflectance spectra implies that small multidomain gold gyroid optical metamaterials behave both qualitatively and quantitatively as an amorphous composite of gold and air (i.e., nanoporous gold) and that coherent electromagnetic contributions arising from the subwavelength gyroid symmetry are not dominant.

Quantitative Correlations between the Normal Incidence Differential Reflectance and the Coverage of Adsorbed Bromide on a Polycrystalline Platinum Rotating Disk Electrode

Normal incidence reflectance spectra (λ = 635 nm) data were acquired from a Pt disk of a rotating Pt

Normal-incidence reflection spectra of the (111) photonic opal surface in the regions of the first and second photonic band gaps

We have studied the reflection spectra of opal photonic crystals with air-or ethanol-filled pores at different diameters of the silica spheres. An experimental technique has been proposed which enables identification of both the first and second photonic band gaps in the reflection spectrum of opal. The ability to observe the second band gap allowed us to derive a dispersion relation for the refractive index of the infiltrated substance. The calculations were performed using a model for a one-dimensional periodic layered medium with two refractive indices. We obtained ω(k) dispersion curves for electromagnetic waves in a photonic crystal (at normal incidence). The ω(k) dispersion law was used to find a dispersion relation for the reflectance of the photonic crystal.

Ferroelectric domains in epitaxial PbTiO3 films on LaAlO3 substrate investigated by piezoresponse force microscopy and far-infrared reflectance

Domain structures of 100–300 nm thin epitaxial films of ferroelectric PbTiO3 grown by metalorganic chemical vapor deposition technique on LaAlO3 perovskite substrate have been investigated by piezoresponse force microscopy and infrared reflectance spectroscopy techniques. Normal-incidence reflectance spectra reveal both E- and A1-symmetry modes of PbTiO3. The latter ones demonstrate the presence of a minor fraction of a-domains (with in-plane orientation of the spontaneous polarization) in the film. The piezoresponse force microscopy images allow to get local insight in the complex nanodomain architecture composed by few hundred nm large areas of primarily c/a/c/a but also a1/a2/a1/a2 domain pattern types, with participation of all six tetragonal ferroelectric domain states and both 180 ° and 90 ° ferroelectric walls.

Simple modeling of plasmon resonances in Ag/SiO_2 nanocomposite monolayers

Normal incidence transmittance and reflectance spectra of sputtered nanocomposite monolayer films of Ag in SiO2, buried and unburied, showed significant redshifted plasmon resonances from 410 to 455 nm, which could be well interpreted with a simple model that starts from the Maxwell Garnett theory and the Kreibig extension of the Drude-Lorentz equation, but with a further extension related to the dipolar interaction between the metal particles distributed on a surface.

Infrared thermal emission in macroporous silicon three-dimensional photonic crystals

In this paper we investigate the infrared thermal emission properties of macroporous silicon with modulated pore diameter. Samples with different pore modulation periodicities but fixed in-plane lattice constant are fabricated. Normal emission of these samples is measured between 373 and 673K. Room-temperature normal-incidence reflectance and transmission spectra are also measured and compared with the photonic band structure simulation. It is shown that thermal emission is suppressed due to photonic band gap effect along the pore axis in excellent agreement with the numerical calculations.

Generalized Fabrication of Two-Dimensional Non-Close-Packed Colloidal Crystals

In this paper we report a generalized templating approach for fabricating wafer-scale, two-dimensional, non-close-packed (ncp) colloidal crystals. Polymer nanocomposites consisting of monolayer ncp colloidal crystals prepared by a spin-coating process are used as sacrificial templates. After removal of the colloidal silica templates, the voids in the polymer matrix are infiltrated with other materials. By plasma-etching the polymer matrix, wafer-scale ncp colloidal crystals from a variety of functional materials can be made. This technique is scalable and compatible with standard microfabrication. Two-component colloidal arrays with complex micropatterns can also be fabricated by combining microfabrication with this templating approach. Normal-incidence reflectivity spectra of replicated titania ncp arrays agree well with theoretical prediction using Scalar Wave Approximation.

Two-dimensional nonclose-packed colloidal crystals formed by spincoating

We report a simple spin-coating technique for the production of monolayer nonclose-packed colloidal crystals. Dispersions of submicron silica spheres in triacrylate monomers are spincoated and polymerized to form two-dimensional colloidal crystal-polymer nanocomposites. By removing the polymer matrix, wafer-scale nonclose-packed colloidal crystals with high crystalline quality can be made. The technique is compatible with standard microfabrication and allows for the production of microstructures for potential devices. Normal-incidence reflectivity spectra in the visible and near-infrared regions show sharp peaks due to Bragg diffraction from the colloidal monolayers. The peak position matches with the theoretical prediction using scalar wave approximation.

In Situ, Time-Resolved Normal Incidence Reflectance Spectroscopy of Polycrystalline Platinum Microelectrodes in Aqueous Electrolytes

In situ normal incidence reflectance spectra of polycrystalline Pt microelectrodes have been monitored as a function of the applied potential in aqueous 0.5 M H(2)SO(4) using a He-Ne laser source (633 nm) and a beam splitter/microscope objective arrangement. Data recorded during voltammetric cycles in Ar-purged solutions revealed a linear correlation between the normalized change in reflectance, DeltaR/R = (R(s) - R(ref))/R(ref) (where R(s) and R(ref) are the light intensities measured by the detector at the sampling, s, and reference potentials, ref, respectively), and the extent of oxidation of the Pt surface over a wide coverage range. Reflectance spectra were also collected in CO-saturated 0.5 M H(2)SO(4) during chronocoulometric measurements involving judiciously selected limits for both the potential step and duty cycle parameters. Analysis of these results made it possible to extract contributions to the current derived from oxide formation during oxidation of adsorbed and bulk CO, based strictly on the optical response.

Optical Properties of Crystalline Pseudoisocyanine (PIC)

Normal incidence reflectance spectra of different crystal faces of 1,1‘-diethyl-2,2‘-cyanine (pseudoisocyanine) bromide (PIC-Br) and chloride (PIC-Cl) single crystals have been analyzed by a polariton model. The spectra are anisotropic with wide bands of quasimetallic reflectance for two orthogonal polarizations of light due to directional dispersion of polariton resonances. The measured spectra were analyzed by Kramers−Kronig transformation and compared with theoretical spectra obtained by solving the wave equation. Because of the herringbone-like packing of PIC molecules in single crystal, the π → π* transition is split into two Davydov components with energy separation of about 30 meV. Most of the oscillator strength is concentrated in the fundamental exciton peaks, while a fine structure related to molecular vibrations is also present. Structural isomorphism of PIC-Br and PIC-Cl single crystals is reflected in identical reflection spectra. Glassy PIC-Br films of thicknesses between 5 and 150 nm were prepared by spin-coating. The films are built up by a network of randomly distributed J-aggregates. By treatment with humid air they become highly ordered, showing three differently colored crystalline domains in polarized light. The absorption spectra of the domains coincide with that of the (100), (−101), and (101) faces of single crystal. Surface force microscopy (SFM) reveals heterogeneous layers composed of aligned aggregates of about 0.5 μm width.

Optical properties of a streptocyanine dye single crystal with tubelike aggregation: bis(dimethylamino)-heptamethinium tetrafluoroborate BDH+BF−4

Normal incidence reflectance spectra from different faces of an optically biaxial dye single crystal, bis(dimethylamino)-heptamethinium tetrafluoroborate, have been measured for two orthogonal polarizations of light. The spectra are very anisotropic with a wide band of quasi-metallic reflectance for one polarization of light. Strong coupling of light to electronic states leads to splitting of transverse and longitudinal excitons as large as 1.5 eV, resulting in strong directional dispersion of the spectra. The data were analyzed by Kramers-Kronig transformation of spectra from appropriate faces and checked by comparison of solutions of the wave equation and the experimental spectra. Electronic transition energies of two Davydov components were found at 2.07 and 2.18 eV shifted by 0.32 and 0.24 eV with respect to absorption spectra of molecules in solution. The results are compared with those obtained from the chloride of the same dye molecule.

Temperature dependence of the direct energy gap in a GaAs/AlAs superlattice.

Normal-incidence reflectance spectra have been used to study the thermal evolution of the direct energy gap in a superlattice composed of 120-times-repeated pattern of nominally 9 monolayers (ML) AlAs and 15 ML GaAs. Very low noise level in the spectra allows the structure of the heavy- and light-hole transitions to be enhanced by numerical differentiations. Excellent agreement of the observed temperature shifts in the 15--300-K range with the results of Kronig-Penney calculations is found when a temperature-dependent electron effective mass is taken into account. Two components of the lowest direct energy gap are resolved at low temperatures; we interpret this fact as due to the presence of two superlattice regions differing in the average composition by 1 ML of GaAs.

Exciton reflectivity of GaAs/Ga1−x Al x As multiple quantum wells

The normal incidence reflectivity spectrum of excitons in GaAs/Ga1−x Al x As multiple quantum wells is calculated within the local response approximation. It is shown that the reflectivity lineshape strongly depends on the sample geometry. Using realistic parameters of a multiple quantum well structure, we obtain an excellent fit of the experimental reflectivity curve, thus giving exciton energies, oscillator strengths and exciton broadening parameters.

Normal-incidence exciton transmission and reflection spectra of semiconducting plane-parallel slabs

Exciton transmission and reflection spectra of thin semiconducting crystals in the spectral region of excitonic polaritons are calculated by the Stahl's coherent wave approach. The influence of the crystal thickness and of the intrinsic damping is discussed. Calculations are performed for the lowest state of theA-exciton in CdS. Experimental results of Broseret al. (1985) are reproduced by inserting an appropriate energy-dependent damping.

Extensions of the Kramers–Kronig transformation that cover a wide range of practical spectroscopic applications

The Kramers–Kronig (KK) transformation has been widely used for the determination of the spectra of the refractive and absorption indices from the normal-incidence reflectivity spectrum of solids. This paper considers the extension of this technique to s-polarized light at nonnormal incidence with a transparent incident medium of arbitrary, constant refractive index. While the KK transformation is known to be inapplicable under certain circumstances, we show that in all cases satisfactory corrections can be applied, and readily applied in most cases. As an example, for the experimentally important case of attenuated total internal reflection from an organic liquid in the infrared, only one additional piece of information is required, namely, the refractive index at a frequency where the absorption is low. The procedures are demonstrated using model systems consisting of sets of damped harmonic oscillators. All the KK transformations were done using the FT software of a commercial Fourier transform infrared spectrometer in order to demonstrate the considerable potential of this approach to the routine determination of optical constants.

Determination of the morphology and effective optical constants of non-ideal thin films

Optical techniques are widely used as non-destructive methods for determining the thickness and effective optical constants of vacuum deposited thin films. Implicit inmany such analyses is the use of an ideal morphological model of a thin film. The morphology of vacuum deposited thin films, however, may be far from ideal. The influence of morphological features on normal incidence reflectance and transmittance spectra of thin films is discussed. In particular, the interpretation of the non-ideal surface of a thin film as either a region giving rise to scattering of incident light or, by applying effective medium theory, as a region of graded refractive index will be considered. The results of a comparative study of the normal incidence reflectance and transmittance spectra of a number of sputtered thin films using both surface scattering and surface region of graded refractive index models is presented. It is shown that the latter model is preferred for films with surface features with dimensions of up to a considerable fraction of wavelength. The implications of these normal incidence results for ellipsometric measurements are also discussed.