Real Part Of Refractive Index Research Articles

All-dielectric optical metasurfaces as platforms for sensing different analytes with identical real parts of refractive index

All-dielectric optical metasurfaces as platforms for sensing different analytes with identical real parts of refractive index

Simple retrieval of the real part of the refractive index for ambient aerosols using extinction coefficient analysis via camera and volume size distribution

This study explored a novel approach for estimating the variations in the refractive index (RI) of aerosols, which is a crucial optical parameter that contains information about particle scattering and absorption. RI is often subject to constraints related to observation time, wavelength selection, and analysis methods. To overcome these limitations, we employed real-time continuous measurements using a camera and optical particle counter (OPC) equipment and applied optimization techniques based on the Mie theory to reverse-calculate changes in the refractive index. Our methodology uses a camera to measure the aerosol extinction coefficients at three different wavelengths within the RGB spectrum (597 nm, 534 nm, and 459 nm), whereas OPC observations provide information on particle size distribution (0.127–17.58 μm as radius). Because the extinction efficiency at known wavelengths varies with the refractive index according to the Mie theory, we retrieved the refractive index, which is an unknown variable, from the extinction coefficient equation. We focused on the real part of the RI because it is challenging to discern the imaginary part of the RI from the visible wavelengths. Our observations were conducted in areas near Busan Harbor and in mixed residential regions. The real part of the RI was determined to be 1.53 ± 0.12. During periods of observed dust events, the refractive index's real part was 1.62 ± 0.05, while during periods anticipated to have emissions of fine aerosols, it was 1.51 ± 0.10. The retrieved RI fell within a range similar to that in other studies, and the changes in the refractive index showed meaningful patterns when analyzed for variations in the particle size distribution and changes in the scattering coefficient. This approach has the potential to advance our understanding of aerosol optical properties and can be applied to real-time analyses with a relatively simple equipment setup, thereby opening new possibilities for its use as a critical variable in optical measurements and environmental models.

Phase ratio in single-scattering dust particles: Implication to future DESTINY+ study of asteroid (3200) Phaethon

The phase ratio is defined as a ratio of reflectivity of object at two different phase angles α. This light-scattering characteristic has revealed high sensitivity to physical parameters of a planetary regolith. We investigate the phase ratio of single-scattering irregularly shaped particles with different morphology by means of the discrete dipole approximation. The phase ratio defined at α = 0° and 30° appears to be highly sensitive to the imaginary part of refractive index and less sensitive to the size distribution of target particles; whereas, it is not sensitive to their real part of refractive index and irregular morphology when it attains a high level of disorder. Further, the phase ratio defined at α = 30° and 60° is almost fully insensitive to microphysics of target particles. This feature might help to enhance the scientific return from the future DESTINY+ mission to asteroid (3200) Phaethon.

О ВЛИЯНИИ ДИСПЕРСИИ ПОГЛОЩЕНИЯ НА ВРЕМЕННУЮ ЗАВИСИМОСТЬ ГОЛОНОМНОГО УЗКОПОЛОСНОГО СИГНАЛА ВДАЛИ ОТ ТОЧКИ ИЗЛУЧЕНИЯ

It is well known that when a narrow-band signal propagates in any medium with a sufficient length of the path, the amplitude-frequency spectrum of the signal inevitably begins to deform due to the dispersion of the absorption coefficient of the medium, that is, due to the dependence of the absorption coefficient on frequency. Therefore, the time dependence of the signal «at infinity» significantly depends not only on the dispersion of the real part of refractive index, but also on the dispersion of the absorption. In this work, we consider the propagation and distortion of a narrow-band holonomic signal in a homogeneous dispersive medium (by the example of a weakly collisional plasma) taking into account the absorption dispersion. It turns out that the absorption dispersion manifests itself differently for different types of holonomic signals – for holonomic signals with a limited spectrum, the spectrum of which is identically equal to zero outside a certain spectral interval, and for holonomic signals with an unlimited spectrum, the spectrum of which does not vanish with increasing frequency. It is shown that for holonomic signals with an unlimited starting spectrum, the absorption dispersion with increasing path length leads to a shift in the carrier frequency towards lower absorption and to normalization of the spectrum intensity, and hence to normalization of the time dependence of the signal, which becomes Gaussian at infinity. At the same time the signal continues to remain narrow-band. For holonomic signals with a limited spectrum, the absorption dispersion leads to a narrowing of the spectrum, which «presses» it against the boundary of the original signal spectrum. This time, as in the case of signals with an unlimited spectrum, the “standardization” of the time dependence of the signal also occurs, but now the time dependence of an arbitrary signal turns out to be not Gaussian, but “Lorentzian”.

A Simple Expression for the Refractive Index of Distilled Water

Water is conceivably the most important material in the universe and most essential to the functioning of all the known life-forms. A simple expression for the real part of refractive index of water was investigated and proposed a new equation as a function of temperature between 0 oC and 100 oC and also wavelength in the range of 200 to 1100 nm. Water is transparent in the visible light and has a complex optical absorption property in the infrared and ultraviolet ranges. The refractive index highly depends on wavelength and temperature. The expression for refractive index is useful for different applications in biomedical optics. The proposed sample has accurate expression, has good agreement, as well as demonstrates increased performance with experimental measurements for calculations of knowledge of the refractive index of water at given ranges.

Optimal photonic crystal slabs for modulators based on transitions between photonic bands

The real part of refractive index is important for the phase shift of light propagating in a dielectric medium. Modulation of the refractive index can be used for electro-optic modulators based on Mach–Zehnder interferometer. In this study we simulate Si-based 2D photonic crystal slabs and by comparing the performance of different lattice types and structure of holes, in respect to the value of effective refractive index change when modulating the refractive index of Si, we find the most promising structures for Si photonics electro-optic modulators based on transition between photonic bands.

The influence of surface roughness to particle optical characteristics by a study of Chebyshev particles

In this work, Chebyshev particles are used to study the influence of surface roughness and nonsphericity to the optical characteristics of particles by exploiting the parameters of Chebyshev particles, namely the waviness parameter and the deformation parameter. This is analyzed by comparing the scattering, extinction, and absorption cross sections, asymmetry parameter, single-scattering albedo, phase function, and linear polarization degree of different types of particles, including spheres and several types of Chebyshev particles. To calculate the optical characteristics, Extended Boundary Condition Method is utilized. The results show that there are some deviations affected by nonsphericity or surface roughness of particles, especially in relatively large size parameter, real part of refractive index is larger than 1, and imaginary part of refractive index is equal to 0. By approximating the surface roughness of particles by Chebyshev particles, larger waviness and deformation parameters tend to give larger difference from ideal particles in optical characteristics, so the optical characteristics could be controlled by modifying these parameters.

Ellipsometric investigation of AlN thin films - Effect of temperature and doping with Ti,Cr

Spectroscopic Ellipsometer is employed to study the optical properties (including complex refractive index and bandgap) of pristine and doped AlN (Ti and Cr) films as a function of doping concentration and temperature (273 K–573 K). Pristine AlN, Al1−xTixN (x = 1.5, 3 and 4 at%) and Al1−xCrxN (x = 2, 4 and 6 at%) films are grown on Si(100) substrates using magnetron sputtering technique. The crystal structure and chemical composition of the as grown films are analyzed using Grazing Incidence X-ray Diffraction (GIXRD) and X-ray Photoelectron Spectroscopy (XPS) techniques, respectively. The optical properties of Ti, Cr doped AlN films at a wider range of energies as well as different ambient temperatures are reported. To determine the complex refractive indices, the measured ellipsometric parameters Is and Ic are fitted using a five layer model with Lorentz oscillator dispersion relation. As far as the bandgap region is concerned, both the real and imaginary parts of the refractive index show significant changes near the bandgap. It is observed that, the real part of refractive index of AlN increases monotonically with the increase in Ti doping and opposite in Cr doping. The extinction coefficient is relatively larger for Cr-doped AlN compared to Ti-doped AlN films. With the increase in doping concentration, the bandgap is found to decrease rapidly for Cr-doped AlN in comparison to Ti-doped AlN films. Additionally, a significant change in the complex refractive index of doped AlN films is observed with temperature.

Au@Ag Dendritic Nanoforests for Surface-Enhanced Raman Scattering Sensing.

The effects of Au cores in Ag shells in enhancing surface-enhanced Raman scattering (SERS) were evaluated with samples of various Au/Ag ratios. High-density Ag shell/Au core dendritic nanoforests (Au@Ag-DNFs) on silicon (Au@Ag-DNFs/Si) were synthesized using the fluoride-assisted Galvanic replacement reaction method. The synthesized Au@Ag-DNFs/Si samples were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, reflection spectroscopy, X-ray diffraction, and Raman spectroscopy. The ultraviolet-visible extinction spectrum exhibited increased extinction induced by the addition of Ag when creating the metal DNFs layer. The pure Ag DNFs exhibited high optical extinction of visible light, but low SERS response compared with Au@Ag DNFs. The Au core (with high refractive index real part) in Au@Ag DNFs maintained a long-leaf structure that focused the illumination light, resulting in the apparent SERS enhancement of the Ag coverage.

Effect of chemical structure on optical properties of secondary organic aerosols derived from C12 alkanes

With the development of the economy, anthropogenic emissions in the atmospheric environment increases, and air pollution has caused wide public concern. Vehicle exhaust is an important emission source in the atmosphere, and alkanes are the representative components in it. In this study, the optical properties of secondary organic aerosol (SOA) derived from several C12 alkanes (2-methylundecane, hexylcyclohexane, and cyclododecane) in the absence of NOX were determined. Absorption (imaginary part of the refractive index (RI), k) at 532 nm was negligible for all the derived SOA, and the scattering (real part of RI, n) of the SOA at 532 nm followed the order of cyclododecane SOA < hexylcyclohexane SOA < 2-methylundecane SOA, at both room- (25 °C) and low- (5 °C) temperature. The chemical compositions of the SOA formed were analyzed with an electrospray ionization time-of-flight mass spectrometer (ESI-TOF-MS). The mass spectra showed that the oligomers were generated in the reactions. It was shown that the different reaction pathways (due to various alkane structures) leaded to the difference in SOA chemical composition, which changed the RI values. The low-temperature condition promoted the progress of the oligomerization reaction so that the final RI values also changed. This work suggested that when estimating the radiative forcing of SOA using regional or global models, the structure of the precursors and reaction conditions should be taken into account.

Extraordinary propagation characteristics of electromagnetic waves in one-dimensional anti-PT-symmetric ring optical waveguide network**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674107, 61475049, 11775083, 61875057, 61774062, and 61771205), the Natural Science Foundation of Guangdong Province, China (Grant No. 2015A030313374), and the Special Funds for the Cultivation of Guangdong College Students’ Scientifific and Techonlogical Innovation, China (Grant No.

In this paper, we design a one-dimensional anti-PT-symmetric ring optical waveguide network (1D APTSPROWN). Using the three-material network equation and the generalized Floquet–Bloch theorem, we investigate its photonic mode distribution, and observe weak extremum spontaneous anti-PT-symmetric breaking points (WBPs) and strong extremum spontaneous anti-PT-symmetric breaking points (SBPs). Then the transmission spectrum is obtained by using the three-material network equation and the generalized eigenfunction method. The 1D APTSPROWN is found to generate ultra-strong transmission near SBPs and ultra-weak transmission near WBPs and SBPs, with the maximal and minimal transmissions being 4.08× 1012 and 7.08× 10−52, respectively. The maximal transmission has the same order of magnitude as the best-reported result. It is not only because the distribution of photonic modes generated by the 1D APTSROWN results in the coupling resonance and anti-resonance, but also because the 1D APTSROWN composed of materials whose real parts of refractive indices are positive and negative has two kinds of phase effects, which results in the resonance and anti-resonance effects in the same kind of photonic modes. This demonstrates that the anti-PT-symmetric and PT-symmetric optical waveguide networks are quite different, which leads to a more in-depth understanding of anti-PT-symmetric and PT-symmetric structures. This work has the potential for paving a new approach to designing single photon emitters, optical amplifiers, and high-efficiency optical energy saver devices.

Low-loss composite photonic platform based on 2D semiconductor monolayers

Two dimensional materials such as graphene and transition metal dichalcogenides (TMDs) are promising for optical modulation, detection, and light emission since their material properties can be tuned on-demand via electrostatic doping. The optical properties of TMDs have been shown to change drastically with doping in the wavelength range near the excitonic resonances. However, little is known about the effect of doping on the optical properties of TMDs away from these resonances, where the material is transparent and therefore could be leveraged in photonic circuits. Here, we probe the electro-optic response of monolayer TMDs at near infrared (NIR) wavelengths (i.e. deep in the transparency regime), by integrating them on silicon nitride (SiN) photonic structures to induce strong light$-$matter interaction with the monolayer. We dope the monolayer to carrier densities of ($7.2 \pm 0.8$) $\times$ $10^{13} \textrm{cm}^{-2}$, by electrically gating the TMD using an ionic liquid. We show strong electro-refractive response in monolayer tungsten disulphide (WS$_2$) at NIR wavelengths by measuring a large change in the real part of refractive index $\Delta$n = $0.53$, with only a minimal change in the imaginary part $\Delta$k = $0.004$. The doping induced phase change ($\Delta$n), compared to the induced absorption ($\Delta$k) measured for WS$_2$ ($\Delta$n/$\Delta$k $\sim 125$), a key metric for photonics, is an order of magnitude higher than the $\Delta$n/$\Delta$k for bulk materials like silicon ($\Delta$n/$\Delta$k $\sim 10$), making it ideal for various photonic applications. We further utilize this strong tunable effect to demonstrate an electrostatically gated SiN-WS$_2$ phase modulator using a WS$_2$-HfO$_2$ (Hafnia)-ITO (Indium Tin Oxide) capacitive configuration, that achieves a phase modulation efficiency (V$_\pi$L) of 0.8 V $\cdot$ cm with a RC limited bandwidth of 0.3 GHz.

Convolutional neural networks for particle shape classification using light-scattering patterns

Particle shapes can be distinguished by the properties of optical light-scattering patterns. We investigate the feasibility of convolutional neural networks for shape-based classification using light-scattering patterns by mimicking a real application scenario of shape classification of micro-sized phytoplankton. Six different particle shapes were chosen. The volume-equivalent size parameter ranged from 2.98 to 14.89. The real part of refractive index was n = 1.06 ± 0.04. The particles were random oriented with respect to the direction of propagation of the incident light. The angular range of the light-scattering pattern in the data set was limited to 89–152° for the scattering angle and 0–63° for the azimuthal angle. Using a convolutional neural network trained with a pre-simulated database as a classifier, an average of above 97% classification accuracy is achieved. The results demonstrate the potential of convolutional neural networks to distinguish particle shape using light-scattering patterns.

Polystyrene Microsphere Optical Properties by Kubelka–Munk and Diffusion Approximation with a Single Integrating Sphere System: A Comparative Study

The optical properties of 1 μm polystyrene in the wavelength range of 500–750 nm were estimated by using a white light spectrophotometric transmittance spectroscopy and a single integrating sphere system. To retrieve the optical characteristics, two analytical methods, namely, diffusion approximation and Kubelka–Munk were used, and then their results were compared with Mie theory calculations. The correspondence of the Kubelka–Munk scattering coefficient with Mie was obvious, and relative errors varied between 6.73% and 2.66% whereas errors varied between 6.87% and 3.62% for diffusion theory. Both analytical methods demonstrated the absorption property of polystyrene over the abovementioned wavelength range. Although absorption coefficient turned out to be much lower than scattering, constructing a realistic optical phantom requires taking into account absorption property of polystyrene. Complex refractive index of polystyrene based on these two methods was determined. Inverse Mie algorithm with scattering coefficient was also used to retrieve the real part of refractive index and absorption coefficient for calculating the imaginary part of refractive index. The relative errors of the real part did not exceed 2.6%, and the imaginary part was in consistence with the prior works. Finally, the presented results confirm the validity of diffusion theory with a single integrating sphere system.

Structural, optical and electronic properties of CuO and Zn doped CuO: DFT based First-principles calculations

Density functional theory based First-principles calculations have been performed to investigate the structural, optical and electronic properties of CuO and Zn doped CuO and compared with experimental results. Calculations are demonstrated by Cambridge Serial Total Energy Package. Calculated values of lattice parameters matched 80% with experimental data of CuO and for Zn doped CuO, there was a 55% match. Figures of electronic band structure, TDOS and PDOS have been computed from the electronic structure of CuO and Zn doped CuO. Significant transition occurs in band gap after Zn doping. Optical properties showed that CuO and Zn doped CuO were transparent, having a small energy gap and maximum reflectivity at infrared region. The real part of refractive index was higher at lower energy region and imaginary part of refractive index was zero at 28 eV photon energy. The calculated value of band gap was in good agreement with the experimental value.

Method to measure the size-resolved real part of aerosol refractive index using differential mobility analyzer in tandem with single-particle soot photometer

Abstract. Knowledge on the refractive index of ambient aerosols can help reduce the uncertainties in estimating aerosol radiative forcing. A new method is proposed to retrieve the size-resolved real part of the refractive index (RRI). The main principle of deriving the RRI is measuring the scattering intensity by a single-particle soot photometer (SP2) of size-selected aerosols. This method is validated by a series of calibration experiments using the components of the known RRI. The retrieved size-resolved RRI covers a wide range, from 200 to 450 nm, with uncertainty of less than 0.02. Measurements of the size-resolved RRI can improve the understanding of the aerosol radiative effects.

Oblique absorption effects of the D region during HF waves heating

With the development of heating research, the oblique heating effects of the D region are becoming increasingly important. For the vertical heating, the larger the beam width of heating wave is, the more evident the oblique effects are; For the oblique heating, the D region which also exhibits evident heating effects has received less attention than the F region in previous studies; Furthermore, the oblique heating effects of the D region should be considered for the studies on multi-beam modulation heating that has arisen recently. Therefore, the numerical model for oblique heating needs to be established to develop the heating study in the D region. In this paper, the classical model for vertical heating is improved by further considering the lossy characteristic of the D region. An added factor, the real part of the refractive index, is introduced to estimate the power density of heating wave. Based on this improved model, the oblique heating model is presented by applying the oblique extension of Wentzel-Kramers-Brillouin (WKB) method and “real pseudo ray” approximation. Considered that the D region presents horizontally discrete strata, oblique absorption effects are quantitatively studied by powerful high-frequency wave heating. Notably, the electron energy absorption occurs only along the vertical direction at the different incident angles due to the special spatial characteristic of the D region. Results show that with the increase of incident angle, the absorption index increases whereas the effective power density of oblique heating wave has a considerable reduction. Therefore, the larger the incident angle is, the less the electron temperature increases. Moreover, the electron temperature in the upper region is more sensitive to variations in incident angles than that in the lower region. In the upper D region, two additional factors severely restrict the enhancement of electron temperature. The self-absorption of the heating wave, well-known as one factor, decreases with the increase of incident angle. Another factor, the real part of refractive index which is neglected in past studies also restricts the increase of electron temperature in the upper D region.

Low-loss photonic-like guided mode in metal-supported optical nanofibers

Subwavelength-diameter optical nanofibers have been proved to be a versatile platform in fields from nanophotonics to cold atom physics. Practical applications of these dielectric nanofibers usually demand a substrate for either functional operation or mechanical support. However, substrate-induced leakage, which always leads to high propagation loss and a cutoff diameter larger than the vacuum wavelength (λ), makes the nanofiber difficult to be operated as a subwavelength waveguide. Here, we demonstrate a low-loss photonic-like guided mode in a metal-supported nanofiber. Owing to its smaller-than-one real part of refractive index of the substrate and the small surface plasmon polariton fraction in the photonic-like mode, this guided mode can be propagated with a subwavelength cutoff diameter and low propagation loss. Using an Au substrate, our calculations show that the propagation length is about 400 μm at a 1.55-μm wavelength in a nanofiber with a diameter of 0.48 λ, which is confirmed by our experimental results (437 μm). We also show that, for nanofibers with higher-index materials, the cutoff diameter can go down to λ/4. Our results may open an opportunity for circuiting and manipulating optical fields in subwavelength-diameter optical nanofibers for a variety of applications.

Alpha-factor in multimode lasers

A theory of the spectral linewidth for a multi-mode laser is presented and used to explain experiment results. It turned out that linewidth is much greater than predicted by Schawlow-Townes theory. Henry attributed this enhancement to variation of real part of refractive index with carrier density. Spontaneous emissions cause changes to imaginary part of refractive index, which is coupled to changes in a real counterpart. This approach is expanded to cover multi-mode lasers and interactions between modes.

Black carbon aggregates: A database for optical properties

Black carbon (BC) aerosol plays an important role in the atmosphere as an absorber of solar radiation and contributes significantly to global warming. The optical properties of BC are fundamental for both direct radiative effect studies and remote sensing of BC properties. However, due to the complex aggregation structures of BC particles, accurate simulations of their optical properties are limited and computationally expensive, while equivalent spherical models or approximate methods could easily introduce relative errors up to 30%. This study generates a comprehensive database to calculate aggregate optical properties efficiently and accurately. The database covers a wide range of BC properties including aggregation structure, refractive index, and particle size (both monomer size and number of monomers). An accurate numerical model, i.e. the multiple-sphere T-matrix method, is used to calculate the single-scattering properties of BC clusters in the form of fractal aggregates. By interpolation, the database can be used to give the scattering properties of aggregates with monomer number from 1 up to 3000, monomer size parameter from 0.05 to 0.5, real part of refractive index from 1.2 to 2.0, and imaginary part from 0.2 to 1.0. The relative errors caused by the interpolation are much less than 1% for the single scattering properties. The pre-calculated database and the implementation used to calculate the required aggregate optical properties are publicly available for users interested in remote sensing of BC aerosols and performing radiative effect calculations.