scholarly journals Extracting Thin-film Optical Parameters from Spectrophotometric Data by Evolutionary Optimization

2021 ◽  
Author(s):  
Rajdeep Dutta ◽  
Siyu Isaac Parker Tian ◽  
Zhe Liu ◽  
Selvaraj Venkataraj ◽  
Yuanhang Cheng ◽  
...  
Keyword(s):  
Thin Film ◽  
Inverse Problem ◽  
Refractive Index ◽  
Film Thickness ◽  
Incident Angle ◽  
Complex Refractive Index ◽  
Evolutionary Optimization ◽  
Optimization Techniques ◽  
Optical Parameters ◽  
Transmittance Spectra

Abstract Extracting optical parameters from spectrophotometric measurements is a challenging task. In a photometric setup, an unknown thin-film is subjected to an incident light beam for a range of admissible wavelengths, which outputs reflectance and transmittance spectra. The current work attempts to solve an inverse problem of extracting thin-film thickness and complex refractive index from reflectance and transmittance spectra for an incident angle of light. The film thickness is a scalar quantity, and the complex refractive index is composed of real and imaginary parts as functions of wavelengths. We leverage evolutionary optimization techniques to solve the underlying inverse problem, which determines the desired parameters associated with two optical dispersion models: ensemble of Tauc-Lorentz (TL) and ensemble of Gaussian oscillators, such that the generated spectra accurately fit the input data. The optimal parameters involved in the adopted models are determined using efficient evolutionary algorithms (EAs). Numerical results validate the effectiveness of the proposed approach in estimating the optical parameters of interest.

scholarly journals Retrieval of the tropospheric aerosol microphysical characteristics from the data of multifrequency lidar sensing

2018 ◽  
Vol 176 ◽  
pp. 05055 ◽  
Author(s):  
S. Samoilova ◽  
M. Sviridenkov ◽  
I. Penner ◽  
G. Kokhanenko ◽  
Yu. Balin
Keyword(s):  
Refractive Index ◽  
Distribution Function ◽  
Complex Refractive Index ◽  
Size Distribution Function ◽  
Coarse Fraction ◽  
Optical Parameters ◽  
Raman Lidar ◽  
Tropospheric Aerosol ◽  
Lidar Measurements ◽  
The Vertical Distribution

Regular lidar measurements of the vertical distribution of aerosol optical parameters are carried out in Tomsk (56°N, 85°E) since April, 2011. We present the results of retrieval of microphysical characteristics from the data of measurements by means of Raman lidar in 2013. Section 2 is devoted to the theoretical aspects of retrieving the particle size distribution function U(r) (SDF) assuming a known complex refractive index m (CRI). It is shown that the coarse fraction cannot be retrieved unambiguously. When estimating U(r) and m together (section 3), the retrieved refractive index is non-linearly related to the optical coefficients and the distribution function, which leads to appearance of different, including false values of m. The corresponding U(r) differs only slightly, so the inaccuracy in m does not essentially affect the retrieval of the distribution function.

Study on the influence of different aerosol mixing states on lidar ratio

2020 ◽  
Author(s):  
zhijie zhang
Keyword(s):  
Refractive Index ◽  
Extinction Coefficient ◽  
Scale Parameter ◽  
Complex Refractive Index ◽  
Scattering Coefficient ◽  
Optical Parameters ◽  
Backscatter Signal ◽  
Back Scattering ◽  
Lidar Ratio ◽  
Mixing States

<p>             After years of development, Mie lidar has become an important technical means to explore aerosol particles in the atmosphere, and has been widely used to explore the optical and physical properties of aerosols in atmosphere. By Using backscatter signal collecting by lidar, optical characteristics of aerosols can be qualitatively analyzed. However, in order to get the actual value of optical parameters, the accurate lidar ratio (LR) (the ratio of extinction coefficient to back-scattering coefficient) is needed in inversion.</p><p>            Using the Mie scattering theory, the key parameter of inversion: LR, can be measured out. The value of LR has been discussed in detail by changing complex refractive index, size parameter  and field angle of a single particle. It is found that when the scale parameter is greater than 0.6, the value of LR increases first and then decreases with the increasing scale parameter, and there are several extremums; the value of LR decreases with the increasing imaginary part of the complex refractive index; the value of LR increases with the increasing filed angle.</p><p>            To study the influence of different mixing states on optical parameters of aerosol clusters, a three-component optical equilibrium spherical aerosol model is assumed. The results shows that when the mixing states of aerosol are complete external mixture, complete uniform internal mixture and complete coated mixture, the value of LR appears to be: complete uniform internal mixture > complete external mixture > complete coated mixture.</p><p>            Assuming that the hygroscopic growth factor of aerosol is a constant which does not increase with the particle size and its value is GF = 1.5[p2] , the value of LR after hygroscopic growing is discussed. It is found that the value of LR will increase after hygroscopic growing, but it still follows the law that: complete uniform internal mixture > complete external mixture > complete coated mixture.</p><p>            By correcting the value of LR, accurate extinction coefficient and back-scattering coefficient can be measured out with inversion. The production of lidar will be quantified instead of qualitative after doing this.</p>

Determination of thin film refractive index and thickness by means of film phase thickness

Open Physics ◽  
2008 ◽  
Vol 6 (2) ◽  
Author(s):  
Milen Nenkov ◽  
Tamara Pencheva
Keyword(s):  
Thin Film ◽  
Thin Films ◽  
Refractive Index ◽  
Barium Titanate ◽  
Spectral Region ◽  
Complex Refractive Index ◽  
Refractive Index Dispersion ◽  
New Approach ◽  
Phase Thickness

AbstractA new approach for determination of refractive index dispersion n(λ) (the real part of the complex refractive index) and thickness d of thin films of negligible absorption and weak dispersion is proposed. The calculation procedure is based on determination of the phase thickness of the film in the spectral region of measured transmittance data. All points of measured spectra are included in the calculations. Barium titanate thin films are investigated in the spectral region 0.38–0.78 μm and their n(λ) and d are calculated. The approach is validated using Swanepoel’s method and it is found to be applicable for relatively thin films when measured transmittance spectra have one minimum and one maximum only.

Complex Refractive Index Dispersion of Strong Absorbing Material Determined Using Internal Reflectance Spectra Measurement

2018 ◽  
Vol 72 (9) ◽  
pp. 1349-1353 ◽  
Author(s):  
Zhichao Deng ◽  
Jin Wang ◽  
Zhixiong Hu ◽  
Jianchun Mei ◽  
Shike Liu ◽  
...  
Keyword(s):  
Refractive Index ◽  
Complex Refractive Index ◽  
Great Difficulty ◽  
Reflectance Spectra ◽  
Optical Parameters ◽  
Refractive Index Dispersion ◽  
Absorbing Material ◽  
Measuring Tool ◽  
Internal Reflectance

Complex refractive index dispersion (CRID) of offset inks is an important spectral property that affects the quality of printing. Due to the strong absorption of offset inks, great difficulty exists when measuring their CRID. In this study, a recently proposed apparatus that can detect the internal reflectance spectra was used to measure the CRID of three strong absorbing offset inks (magenta, yellow, and cyan). Both anomalous dispersion curve and extinction coefficient curve were well determined over the spectral range of 400–750 nm. This study experimentally proves that the apparatus and related method are feasible for the CRID measurement of strong absorbing materials and could serve as a powerful measuring tool for optical parameters.

DETERMINATION OF THE COMPLEX REFRACTIVE INDEX OF POROUS SILICON LAYERS ON CRYSTALLINE SILICON SUBSTRATES

2010 ◽  
Vol 24 (24) ◽  
pp. 4835-4850 ◽  
Author(s):  
M. C. ARENAS ◽  
HAILIN HU ◽  
R. NAVA ◽  
J. A. DEL RÍO
Keyword(s):  
Thin Film ◽  
Refractive Index ◽  
Porous Silicon ◽  
Extinction Coefficient ◽  
Near Infrared ◽  
Crystalline Silicon ◽  
Complex Refractive Index ◽  
Si Substrate ◽  
Free Standing ◽  
Analytical Relations

In this work, we show an algorithm to calculate the complex refractive index of porous silicon (PS) on its crystalline silicon (c-Si) substrate in UV-NIR range by means of the reflectance spectra only. The algorithm is based on the analytical relations established by Heavens to obtain both complex refractive index and thickness of an absorbing thin film on an absorbing substrate. Based on this model, some simplification is introduced at different wavelengths. We start with the NIR range (1000–2500 nm), where the c-Si substrate has a low extinction coefficient. Then, we continue with the near infrared to the optical range (300–1000 nm), where PS has a strong extinction coefficient and dispersion. The calculated n and k values are in agreement with those reported in the literature obtained from separated measurements of a free standing PS film. We consider that the algorithm can be applied to any thin film on a substrate with similar optical properties.

Optical Thin Films with Very Low Refractive Index and Their Application in Photonics Devices

2005 ◽  
Vol 901 ◽  
Author(s):  
Jingqun Xi ◽  
Jong Kyu Kim ◽  
Dexian Ye ◽  
Jasbir S. Juneja ◽  
T.-M. Lu ◽  
...  
Keyword(s):  
Thin Film ◽  
Thin Films ◽  
Refractive Index ◽  
Incident Angle ◽  
Columnar Structure ◽  
Bragg Reflector ◽  
Single Pair ◽  
Scanning Electron Micrographs ◽  
Distributed Bragg Reflector ◽  
Low Refractive Index

AbstractThe refractive index contrast in dielectric multilayer structures, optical resonators and photonic crystals is an important figure of merit, which creates a strong demand for high quality thin films with a very low refractive index. SiO2 nano-rod layers with low refractive indices n = 1.08, the lowest ever reported in thin-film materials, is grown by oblique-angle e-beam deposition of SiO2 with vapor incident angle 85 degree. Scanning electron micrographs reveal a highly porous columnar structure of the low-refractive-index (low-n) film. The gap between the SiO2 nano-rods is ≤50 nm, i.e. much smaller than the wavelength of visible light, and thus sufficiently small to make scattering very small. Optical micrographs of the low-n film deposited on a Si substrate reveal a uniform specular film with no apparent scattering. The unprecedented low index of the SiO2 nano-rod layer is confirmed by both ellipsometry measurements and thin film interference measurements. A single-pair distributed Bragg reflector (DBR) employing the SiO2 nano-rod layer is demonstrated to have enhanced reflectivity, showing the great potential of low-n films for applications in photonic structures and devices.

scholarly journals Satellite retrieval of aerosol microphysical and optical parameters using neural networks: a new methodology applied to the Sahara desert dust peak

2013 ◽  
Vol 6 (6) ◽  
pp. 10955-11010
Author(s):  
M. Taylor ◽  
S. Kazadzis ◽  
A. Tsekeri ◽  
A. Gkikas ◽  
V. Amiridis
Keyword(s):  
Remote Sensing ◽  
Neural Networks ◽  
Refractive Index ◽  
Size Distribution ◽  
Aerosol Optical Depth ◽  
Principal Components ◽  
Complex Refractive Index ◽  
Remote Sensing Data ◽  
Optical Parameters ◽  
Desert Dust

Abstract. In order to exploit the full-Earth viewing potential of satellite instruments to globally characterise aerosols, new algorithms are required to deduce key microphysical parameters like the particle size distribution and optical parameters associated with scattering and absorption from space remote sensing data. Here, a methodology based on neural networks is developed to retrieve such parameters from satellite inputs and to validate them with ground-based remote sensing data. For key combinations of input variables available from MODIS and OMI Level 3 datasets, a grid of 100 feed-forward neural network architectures is produced, each having a different number of neurons and training proportion. The networks are trained with principal components accounting for 98% of the variance of the inputs together with principal components formed from 38 AERONET Level 2.0 (Version 2) retrieved parameters as outputs. Daily-averaged, co-located and synchronous data drawn from a cluster of AERONET sites centred on the peak of dust extinction in Northern Africa is used for network training and validation, and the optimal network architecture for each input parameter combination is identified with reference to the lowest mean squared error. The trained networks are then fed with unseen data at the coastal dust site Dakar to test their simulation performance. A NN, trained with co-located and synchronous satellite inputs comprising three aerosol optical depth measurements at 470, 500 and 660 nm, plus the columnar water vapour (from MODIS) and the modelled absorption aerosol optical depth at 500 nm (from OMI), was able to simultaneously retrieve the daily-averaged size distribution, the coarse mode volume, the imaginary part of the complex refractive index, and the spectral single scattering albedo – with moderate precision: correlation coefficients in the range 0.368 ≤ R ≤ 0.514. The network failed to recover the spectral behaviour of the real part of the complex refractive index with only 39–45% of the data falling within the acceptable level of uncertainty relative to ground-truth data at the daily timescale. In the context of Saharan desert dust, this new methodological approach appears to offer some potential for moderately accurate daily retrieval of previously inaccessible aerosol parameters from space.

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