Variable Angle Spectroscopic Ellipsometer Research Articles

Study of Ion-Assisted E-Beam Evaporation Coating Process on Chirped Mirrors

A chirped mirror designed with 60 fs2 in group delay dispersion and reflectance large than 97.5% in the bandwidth between 700 and 900 nm by open-sourced thin film filter design software is presented. The chirped mirrors are then fabricated by an ion-assisted deposition coater. First, the optical constants of the thin films deposited by specific process parameters are calibrated by trial runs with single-layer samples measured by variable-angle spectroscopic ellipsometer; then factors are entered to compensate for the settings on film thickness. Afterward, the spectral phase of the coated mirrors is measured by polarization gating frequency-resolved optical gating whereas the reflectance is measured by photo-spectrometer. In conclusion, the measured results indicate a reasonable match with the design goals to show the effectiveness of the proposed method. It is noted that ion-assisted electron gun evaporation coating systems are applicable to the mass production of chirped mirrors employed for ultrafast lasers.

Optical properties of large-size and damage-free polished Lu2O3 single crystal covering the ultraviolet-visible-and near-infrared (UV–VIS–NIR) spectral region

Lu2O3 has attracted widespread attention in high-power lasers, scintillators, and semiconductors due to its high thermal conductivity, high stability, and large band gap. Due to the limitations of the growth process, high-precision characterization of polished quality and optical properties for large-size Lu2O3 single crystal is extremely scarce, which hinders the design and optimization of optics based on Lu2O3. In this study, we design a systematic machining scheme for large-size Lu2O3 single crystal thick chip to obtain polished crystal with virtually no surface and subsurface damage. The surface/subsurface quality was evaluated utilizing optical profilometer, scanning electron microscope, and transmission electron microscope. Variable-angle spectroscopic ellipsometer was employed to measure the optical constants over wide spectral region (210–1690 nm), removing the lack of optical constants of Lu2O3 single crystal in the ultraviolet region. The band gap is analyzed based on the optical constants results. The consistency of the measured data with the relevant literature verifies the reliability and accuracy of the measurement method along with the data. The surface/subsurface damage inspection method provides an effective approach for ultra-precision machining mechanism research and process optimization of Lu2O3 and other hard and brittle materials. The band gap and the optical constants in wide spectral region provide critical fundamental data for the design and performance optimization of related optics.

Optical Constant and Conformality Analysis of SiO2 Thin Films Deposited on Linear Array Microstructure Substrate by PECVD

SiO2 thin films are deposited by radio frequency (RF) plasma-enhanced chemical vapor deposition (PECVD) technique using SiH4 and N2O as precursor gases. The stoichiometry of SiO2 thin films is determined by the X-ray photoelectron spectroscopy (XPS), and the optical constant n and k are obtained by using variable angle spectroscopic ellipsometer (VASE) in the spectral range 380–1600 nm. The refractive index and extinction coefficient of the deposited SiO2 thin films at 500 nm are 1.464 and 0.0069, respectively. The deposition rate of SiO2 thin films is controlled by changing the reaction pressure. The effects of deposition rate, film thickness, and microstructure size on the conformality of SiO2 thin films are studied. The conformality of SiO2 thin films increases from 0.68 to 0.91, with the increase of deposition rate of the SiO2 thin film from 20.84 to 41.92 nm/min. The conformality of SiO2 thin films decreases with the increase of film thickness, and the higher the step height, the smaller the conformality of SiO2 thin films.

Optical Parameters of Both As2S3 and As2Se3 Thin Films from Ultraviolet to the Near-Infrared via Variable-Angle Spectroscopic Ellipsometer

In the UV-visible-near infrared regions from 245 to 1000 nm, variable-angle spectroscopic ellipsometer (VASE) was used to investigate optical functions of As2S3 and As2Se3 thin films. In the entire measured spectral range, data were analyzed by assembly from several dispersion models. These assemblies comprise individual Tauc–Lorentz supplemented by several Lorentz (TL-group) or single Cody–Lorentz with several Lorentz (CL-group) models. For As2S3 and As2Se3 thin films, the optical parameters were quantified. The model parameters, such as the Lorentz amplitude, resonance frequency, oscillator width, extinction coefficients, refractive indices, and Urbach and optical band energy of both films were obtained. The band gap energy Eg was experimentally determined using the obtained data of CL-group from (αhν)1/2 vs. hν plots. It is found that the band gap energies of As2Se3 and As2S3 were 1.796 and 2.349 eV, respectively. The Eg values for the films were theoretically investigated by the bond statistics of the random covalent network model (CRNM) with the aid of Manca’s relation. Plausible agreement between the experimental and calculated Eg values for both samples was obtained.

Optical properties of ultra-thin silver films deposited by thermal evaporation and its application in optical filters

The ultra-thin silver films with different thickness are deposited by resistance thermal evaporation at room temperature, and the optical constants n and k are obtained by using variable angle spectroscopic ellipsometer (VASE) in the spectral range 380–1600 nm. The thickness range of ultra-thin silver film is 5.3–24.9 nm. SEM images on the surface morphology of ultra-thin silver films show that the ultrathin silver films have discontinuous island structure. The volume fraction of solid silver materials in ultra-thin silver films are simulated by Brugeman approximation theory. Dielectric-metal-dielectric (D-M-D) sandwich structure ZnS/Ag/ZnS optical filter is designed with the optical constant of the modified ultra-thin silver films. The average transmittance of the fabricated D-M-D optical filter is as high as 88.5% and peak transmittance is above 90.5% in the wavelength range between 400 and 700 nm. The maximum reflectance was 94.0% in the region of 2000–12000 nm.

Vanadium oxide thin films for optical readout-based thermal imager

In this study, optical characterization of vanadium oxide (VOx) thin film is performed using a variable angle spectroscopic ellipsometer (VASE). The characterization is performed at four different room temperatures, 25.5–28.5 °C, in steps of 1 °C. The best fitting results of the ellipsometric parameters φ and Δ are obtained by using a GenOsc layer with the Lorentz oscillator. Four different thermo-optic coefficients (TOCs) are obtained from the modelled data. The TOC value with the least extinction coefficient is chosen, which occurs at 26.5 °C. The effect of the TOC value on shifting the Fabry-Perot cavity filter (FPCF) tuning frequency is examined on all designs (2, 4, and 6 stacks) to determine the sensitivity of the FPCF sensor in scene temperature variations. We verify the performance of the sensor as a long-wave infrared radiation absorber and as a visible light band-pass filter. This sensor is designed to have a low thermal conductivity and a low mechanical stress.

Single-incidence polarimetry for optical characteristics

In this paper, we propose a simplified measurement called single-incidence polarimetry measurement (SPM) to obtain optical characteristics such as di-attenuationP, variation of phase difference Δ and depolarization index d on material surface. Compared with tradition Mueller matrix polarimetry which needs multiple (at least 4) Stokes vectors as incidences in experiments, SPM only needs single polarized light as incidence, which simplifies experimental procedures tremendously and increases experimental efficiency. In addition, SPM is suitable for complicated materials such as thick lump samples, uneven surfaces, powder and liquid which are not available on traditional ellipsometers, e.g., the variable-angle spectroscopic ellipsometer (VASE) produced by J. A. Woollam Company. The objectivity and lower complexity of SPM are verified by the contrast experiments with Mueller matrix polarimetry. The advantage that SPM is more suitable for complicated experimental condition is demonstrated by the contrast experiments with VASE. All the analyses and discussions are realized by the root mean squared error (RMSE) between referenced values and experimental results. In conclusion, SPM can realize low-cost and easy-assembled measurements on complicated and depolarized surfaces rather than obtaining higher precision data on smooth and thin surfaces. In the aspect of efficiency and convenience, SPM is useful to analyze optical characteristics qualitatively.

Optical and structural characterization of ultrananocrystalline diamond/hydrogenated amorphous carbon composite films deposited via coaxial arc plasma

Ultrananocrystalline diamond/hydrogenated amorphous carbon composite films were deposited in the ambient of hydrogen by coaxial arc plasma deposition. The film compositions and chemical bonding structures were investigated by X-ray diffraction, X-ray photoemission and hydrogen forward scattering spectroscopies. The sp3/(sp2+sp3) ratio and hydrogen content in the film were estimated to be 64% and 35 at.%, respectively. The optical parameters and the optical dispersion profile were determined by using a variable angle spectroscopic ellipsometer at 55°, 65° and 75° angle of incidence in the photon energy range of 0.9–5 eV. Combinations of multiple Gaussian, and Tauc-Lorentz or Cody-Lorentz dispersion functions are used to reproduce the experimental data. Results of ellipsometry showed a refractive index of approximately 2.05 (at 2eV) and optical band gap of 1.63 eV. The imaginary part of dielectric function exhibited a peak at 3.8 eV, which has assigned to π-π* electron transitions. Furthermore, Electron spin resonance measurements implied the existence of dangling bonds, which might have a partial contribution to the optical absorption properties of the deposited films. A correlation between optical parameters and structural profile of the deposited films is discussed.

Influence of Deposition Conditions on the Characteristics of Luminescent Silicon Carbonitride Thin Films

The influence of the substrate temperature and argon gas flow on the compositional, structural, optical, and light emission properties of amorphous hydrogenated silicon carbonitride (a-SiCxNy:H) thin films were studied. Thin films were fabricated using electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) at a range of substrate temperatures from 120 to 170°C (corresponding to deposition temperatures of 300 to 450°C) in a mixture of SiH4, N2, and CH4 precursors. Variable angle spectroscopic ellipsometer (VASE), elastic recoil detection (ERD), and Rutherford backscattering spectrometry (RBS) verified optical bandgap widening, layer densification, and an increase of the refractive index at higher substrate temperatures. The microstructure of a-SiCxNy:Hz thin films was determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. The substrate temperature strongly affected the binding state of all atoms, and in particular, carbon atoms attached to silicon and nitrogen, as well as hydrogen-terminated bonds. We correlated the films' microstructural changes to a higher species' mobility arriving on the growin layer at higher temperatures. Photoluminescence (PL) measurements showed that the total intensity of visible light emission increased. A systematic blueshift of the centroid of the wide PL peak was observed following the increase of optical gap.

Optical parameters of Ge15Sb5Se80 and Ge15Sb5Te80 from ellipsometric measurements

The optical properties of Ge15Sb5Se80 (GSS) and Ge15Sb5Te80 (GST) films prepared by thermal evaporation method were investigated in the photon energy range from 0.9 eV to 5 eV by using a variable-angle spectroscopic ellipsometer. Combinations of multiple Gaussian, and Tauc–Lorentz or Cody-Lorentz dispersion functions are used to fit the experimental data. The models' parameters (Lorentz oscillator amplitude, resonance energy, oscillator width, optical band gap, and Urbach energy) of both GSS and GST films were calculated. Refractive indices and extinction coefficients of the films were determined. Analysis of the absorption coefficient shows that the optical absorption edge of GSS and GST films to be 1.6 eV and 0.89 eV, respectively. Manca's relation based on mean bond energy and the bond statistics of chemically ordered model (COM) and random covalent network model (CRNM) is applied for the estimation of the optical band gap (Eg) of the investigated films. A good agreement between experimental and calculated Eg is obtained.

Annealing of silicon carbonitride nanostructured thin films: interdependency of hydrogen content, optical, and structural properties

Silicon carbonitride (SiCxNy) materials, inspired by their outstanding multifunctional properties, are finding increasing applications in a variety of fields, including as next generation solar cells and hard coatings. Hydrogenated SiCxNy thin films, along with binary submatrices of stoichiometric SiC and SiN3 as a reference, were deposited using electron cyclotron resonance plasma-enhanced chemical vapor deposition. We described a comparative study of the effects of post-deposition thermal annealing, from 300 to 1200 °C, on the evolution of hydrogen-rich a-SiC1.2N0.7:H1.4 thin films. Concurrently, two featured annealing temperatures (500 and 900 °C) were found to have the significant influence on the morphology, optical, and microstructural properties of the films. During annealing the amorphous phase of SiC1.2N0.7:H1.4 thin films was fully maintained according to the transmission electron microscopy and X-ray diffraction analyses. The hydrogen density was quantitatively analyzed employing two different experimental techniques, elastic recoil detection and Fourier transform infrared spectroscopy, showing the associate annealing temperatures of hydrogen desorption, breaking of all hydrogen-terminated bonds, and depletion of all hydrogen content. The refractive indices and optical band gap of the films were calculated using variable angle spectroscopic ellipsometer. Thermal annealing resulted in hydrogen desorption and consequently layer densification along with an increase in the refractive index. During the annealing process, first the optical band gap widened, and then narrowed due to hydrogen out-diffusion or chemical bond restructuring, depending on the annealing temperature. In addition, Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy were performed. These findings are discussed in the context of the interdependency of the hydrogen desorption and thermally induced changes in chemical bonds, mass density, and optical properties.

Sol-gel derived single-layer zeolite-based coatings on glass for broadband antireflection properties

In this study, a simple and effective sol-gel method was used to generate antireflection coatings on borosilicate glass substrates. Commercially available zeolite 4Å powder and tetraethyl orthosilicate were used as the starting materials to synthesize the sol. Single layer antireflection coatings were generated on borosilicate glass substrates using dip coating technique and densified in air at 450°C. Optical properties were analyzed using UV–Vis-NIR transmittance/reflectance spectrophotometer and variable angle spectroscopic ellipsometer. Mechanical properties were analyzed using adhesion, pencil scratch hardness testing as well as using crockmeter tests. The antireflection coatings were found to exhibit excellent broad band transmittance over the visible wavelength range. An average transmittance of 96.8±1% could be obtained for the wavelength range 400–800nm. The reflectance of the bare borosilicate glass substrate was found to decrease from 8.6±0.1% to 2.5±0.05% after deposition of the antireflection coating. The scratch hardness of the coatings was determined to be 6H and adhesion tests showed the adhesion to be of rank 5B. The transmittance loss was 0.9±0.1% after 1000cycles of crockmeter test using a cotton fabric as the abrading medium. The single layer zeolite-based antireflection coatings were found to exhibit good mechanical properties and could be used for a variety of applications.

Hole-dominated transport in InSb nanowires grown on high-quality InSb films

We have developed an effective strategy for synthesizing p-type indium antimonide (InSb) nanowires on a thin film of InSb grown on glass substrate. The InSb films were grown by a chemical reaction between Sb2S3 and In and were characterized by structural, compositional, and optical studies. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies reveal that the surface of the substrate is covered with a polycrystalline InSb film comprised of sub-micron sized InSb islands. Energy dispersive X-ray (EDX) results show that the film is stoichiometric InSb. The optical constants of the InSb film, characterized using a variable-angle spectroscopic ellipsometer (VASE) shows a maximum value for refractive index at 3.7 near 1.8 eV, and the extinction coefficient (k) shows a maximum value 3.3 near 4.1 eV. InSb nanowires were subsequently grown on the InSb film with 20 nm sized Au nanoparticles functioning as the metal catalyst initiating nanowire growth. The InSb nanowires with diameters in the range of 40–60 nm exhibit good crystallinity and were found to be rich in Sb. High concentrations of anions in binary semiconductors are known to introduce acceptor levels within the band gap. This un-intentional doping of the InSb nanowire resulting in hole-dominated transport in the nanowires is demonstrated by the fabrication of a p-channel nanowire field effect transistor. The hole concentration and field effect mobility are estimated to be ≈1.3 × 1017 cm−3 and 1000 cm2 V−1 s−1, respectively, at room temperature, values that are particularly attractive for the technological implications of utilizing p-InSb nanowires in CMOS electronics.

Thermo-Optical Properties of Thin-Film TiO2–Al2O3 Bilayers Fabricated by Atomic Layer Deposition

We investigate the optical and thermo-optical properties of amorphous TiO2–Al2O3 thin-film bilayers fabricated by atomic layer deposition (ALD). Seven samples of TiO2–Al2O3 bilayers are fabricated by growing Al2O3 films of different thicknesses on the surface of TiO2 films of constant thickness (100 nm). Temperature-induced changes in the optical refractive indices of these thin-film bilayers are measured by a variable angle spectroscopic ellipsometer VASE®. The optical data and the thermo-optic coefficients of the films are retrieved and calculated by applying the Cauchy model and the linear fitting regression algorithm, in order to evaluate the surface porosity model of TiO2 films. The effects of TiO2 surface defects on the films’ thermo-optic properties are reduced and modified by depositing ultra-thin ALD-Al2O3 diffusion barrier layers. Increasing the ALD-Al2O3 thickness from 20 nm to 30 nm results in a sign change of the thermo-optic coefficient of the ALD-TiO2. The thermo-optic coefficients of the 100 nm-thick ALD-TiO2 film and 30 nm-thick ALD-Al2O3 film in a bilayer are (0.048 ± 0.134) × 10−4 °C−1 and (0.680 ± 0.313) × 10−4 °C−1, respectively, at a temperature T = 62 °C.

Mg thin films with Al seed layers for UV plasmonics

We demonstrate that the inclusion of aluminum (Al) seed layers allows for the growth of finer grain, smoother magnesium (Mg) films, allowing for improved plasmonic response in the UV spectral range. We deposit Al seed layers and Mg films using DC/RF magnetron sputtering. For Al seed layer thicknesses of 2, 5, 10 and 15 nm, we measure the corresponding optical constants over the wavelength range of 250 to 600 nm using a variable angle spectroscopic ellipsometer and compare the results to a reference 100 nm thick Mg film deposited without a seed layer. The optical constants of Mg depend on the seed layer thicknesses and the surface morphology of Mg films. The surface morphology of the Mg films are characterized using scanning electron microscopy, atomic force microscopy and x-ray diffraction and the surface oxide layer on Mg is examined using x-ray photoelectron spectroscopy. We observe that the presence of the seed layer improves the LSP figure of merit in the UV spectral range of 250–400 nm. From the perspective of plasmonics applications, we find that the best localized surface plasmon resonance (LSP) figures of merit are observed for the 10 nm thick Al seed layer. In fact, the LSP figure of merit of the 100 nm Mg film with and without the Al seed layer is found to be greater than Al films in the spectral range of 250–400 nm, confirming earlier findings with thicker Mg films.

Effect of Calcination on Properties of ZnO Nanoparticles

ZnO nanoparticles have gained much interest in optoelectronics due to their unique and novel properties. Zinc acetate is used as precursor. Sol-gel method is used to synthesize ZnO nanoparticles. Calcination at three different temperatures (100-500°C) is carried out in order to study the effect of calcination on their structural and optical properties. Structural and optical properties are studied by using X-ray diffractometer and Variable Angle Spectroscopic Ellipsometer. XRD results indicate the formation of pure phase hexagonal wurtzite ZnO. Crystallite size decreases (24-17nm) with increase in calcination temperature to 300°C. With further increase in calcination temperature, crystallite size increases to 19.5nm. High transmission values are observed by increasing the calcination temperature. Band gap values increase with the increase in calcination temperature.

Ultraviolet–Visible Plasmonic Properties of Gallium Nanoparticles Investigated by Variable-Angle Spectroscopic and Mueller Matrix Ellipsometry

Self-assembled, irregular ensembles of hemispherical Ga nanoparticles (NPs) were deposited on sapphire by molecular beam epitaxy. These samples, whose constituent unimodal or bimodal distribution of NP sizes was controlled by deposition time, exhibited localized surface plasmon resonances tunable from the ultraviolet to the visible (UV/vis) spectral range. The optical response of each sample was characterized using a variable-angle spectroscopic ellipsometer, and the dielectric response of the ensemble of NPs on each sample was parametrized using Lorentz oscillators. From this, a relationship was found between NP size and the deduced Lorentzian parameters (resonant frequency, damping, oscillator strength) for most unimodal and bimodal samples at most frequencies and angles of incidence. However, for samples with a bimodal size distribution, Mueller matrix ellipsometry revealed nonspecular scattering at particular frequencies and angles, suggesting a resonant interparticle coupling effect consistent with recently observed strong local field enhancements in the ultraviolet.

Structural and Optical Properties of Luminescent Silicon Carbonitride Thin Films

A series of silicon carbonitride (SiCN) thin films, a relatively unstudied material, were fabricated by electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR PECVD) located at McMaster University. For comparison, a set each of silicon nitride and silicon carbide samples were also fabricated. To study the effect of introducing rare earth ions into SiCN host matrix, we synthesised a series of SiCN co-doped with Ce and Tb. This work investigates the influence of film composition and annealing conditions on the luminescence and structure of rare earth doped and undoped SiCN thin films through photoluminescence (PL), X-ray emission (XES), X-ray absorption near edge structure (XANES), and X-ray diffraction (XRD) experiments. The XES measurements were measured on Bl. 8.0.1 at the Advanced Light Source in Berkeley, USA. XANES has been carried out on the high resolution spherical grating monochromator (SGM) 11ID-1, soft X-ray microcharacterization beamline (SXRMB) 06B1-1, and variable line spacing plane grating monochromator (VLS PGM) beamline at the Canadian Light Source (CLS) synchrotron facility. PL emission was measured at the room temperature using a 17 mW 325 nm HeCd laser as the excitation source. For both doped and un-doped thin films, the XANES measurements were recorded at the N K-edge, C K-edge, and Si L3,2-edge while total fluorescence yield (TFY), total electron yield (TEY), and partial fluorescence yield (PFY) were simultaneously measured. Both the XES and XANES spectra were calibrated using the same reference materials. For doped sample the XANES spectra of Ce N5,4-edge and Ce M5,4-edge are also measured. The XES measurements included the Ce N5,4, Si L3,2, C K, and N K spectra. XRD also was used to determine the local bonding coordination of rare earth ions when incorporated in amorphous as well as crystalline structures. Film composition was determined using Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD) analysis carried out at the Tandetron Laboratory at Western University. The thickness of film varying from 170 to 200 nm allowed the conduction of ion beam and X-ray absorption spectroscopy measurements without interference from the substrate. The variable angle spectroscopic (VASE) ellipsometer was employed to investigate the thickness and optical constants of the films.The PL spectra of SiCN samples annealed at the lower temperatures consist of a broad peak ranging from 400 to 700 nm. Bright Ce3+ and Tb3+ related emission lines are observed for the doped films annealed in nitrogen at 1200°C for one hour. TEM analysis reveals the formation of randomly distributed nanocrystals in doped SiCN films following 1200°C annealing, while the undoped SiCN samples did not appear to show any crystalline structure at any temperature. Comparisons of the XANES and XES results indicated that in a the doped SiCN films, structural re-ordering occurs at the higher annealing temperatures leading to the formation of Si clustering along with a decrease in Si-N bonding. Conversely, little to no difference was observed in the electronic structure of undoped SiCN films following annealing at different temperatures. This suggests that the electronic structure of the un-doped SiCN films may not change with respect to the Si and N atoms.

Low-loss magnesium films for plasmonics

The optical properties of pure nanostructured magnesium films deposited on glass, Si and GaAs substrates using DC magnetron sputtering are examined in this work to determine the potential of Mg as plasmonic material in the ultraviolet (UV) to near-infrared (NIR) spectral range. Using film thicknesses ranging from 210 to 910 nm, the corresponding optical constants over the wavelength range of 250–1700 nm were measured using a variable angle spectroscopic ellipsometer and these results are compared to reference Al thin films. The optical constants of Mg depend on the film thickness. From the perspective of plasmonics applications, we find that the best values are observed in this work for the 910 nm thick Mg film on glass substrates. The film morphology and structure of the Mg films were characterized using atomic force microscopy, scanning electron microscopy, stylus profilometry and X-ray diffraction. The optical dielectric constants and plasmonic figures of merit are strongly influenced by the film structure. The results show that smooth and [0 0 0 2] textured magnesium films have extremely low loss, superior or comparable to aluminum films in portions of the UV spectrum.

Complex Refractive Indices of Thin Films of Secondary Organic Materials by Spectroscopic Ellipsometry from 220 to 1200 nm

The complex refractive indices of three different types of secondary organic material (SOM) were obtained for 220 to 1200 nm using a variable angle spectroscopic ellipsometer. Aerosol particles were produced in a flow tube reactor by ozonolysis of volatile organic compounds, including the monoterpenes α-pinene and limonene and the aromatic catechol (benzene-1,2-diol). Optically reflective thin films of SOM were grown by electrostatic precipitation of the aerosol particles onto silicon substrates. The ellipsometry analysis showed that both the real and imaginary components of the refractive indices decreased with increasing wavelength. The real part n(λ) could be parametrized by the three-term form of Cauchy's equation, as follows: n(λ) = B + C/λ(2) + D/λ(4) where λ is the wavelength and B, C, and D are fitting parameters. The real refractive indices of the three SOMs ranged from 1.53 to 1.58, 1.49-1.52, and 1.48-1.50 at 310, 550, and 1000 nm, respectively. The catechol-derived SOM absorbed light in the ultraviolet (UV) range. By comparison, the UV absorption of the monoterpene-derived SOMs was negligible. On the basis of the measured refractive indices, optical properties were modeled for a typical atmospheric particle population. The results suggest that the wavelength dependence of the refractive indices can vary the Angstrom exponent by up to 0.1 across the range 310 to 550 nm. The modeled single-scattering albedo can likewise vary from 0.97 to 0.85 at 310 nm (UV-B). Variability in the optical properties of different types of SOMs can imply important differences in the relative effects of atmospheric particles on tropospheric photochemistry, as well as possible inaccuracies in some satellite-retrieved properties such as optical depth and mode diameter.