Ratio Of Optical Thickness Research Articles

Broadband Terahertz Absorption in Graphene-Embedded Photonic Crystals

The absorption in graphene is rather low at terahertz frequencies. Here, we present a graphene-embedded photonic crystal structure to realize broadband terahertz absorption in graphene. The approach provides absorption enhancement in the whole terahertz regime (from 0.1 to 10 THz). It is shown that the average absorption in the graphene-embedded photonic crystal can be enhanced in the multiple propagating bands of the photonic crystals. The absorption efficiency can be further improved by optimizing the characteristic frequency, optical thickness ratio in a unit cell, and the angle of incidence on the photonic crystals. A maximum broadband absorption factor of 28.8% was achieved for fixed alternative dielectric materials. The graphene-embedded photonic crystal is promising for terahertz functional devices with broadband response.

Satellite retrieval of the liquid water fraction in tropical clouds between −20 and −38 °C

Abstract. This study describes a satellite remote sensing method for directly retrieving the liquid water fraction in mixed phase clouds, and appears unique in this respect. The method uses MODIS split-window channels for retrieving the liquid fraction from cold clouds where the liquid water fraction is less than 50% of the total condensate. This makes use of the observation that clouds only containing ice exhibit effective 12-to-11 μm absorption optical thickness ratios (βeff) that are quasi-constant with retrieved cloud temperature T. This observation was made possible by using two CO2 channels to retrieve T and then using the 12 and 11 μm channels to retrieve emissivities and βeff. Thus for T < −40 °C, βeff is constant, but for T > −40 °C, βeff slowly increases due to the presence of liquid water, revealing mean liquid fractions of ~ 10% around −22 °C from tropical clouds identified as cirrus by the cloud mask. However, the uncertainties for these retrievals are large, and extensive in situ measurements are needed to refine and validate these retrievals. Such liquid levels are shown to reduce the cloud effective diameter De such that cloud optical thickness will increase by more than 50% for a given water path, relative to De corresponding to pure ice clouds. Such retrieval information is needed for validation of the cloud microphysics in climate models. Since low levels of liquid water can dominate cloud optical properties, tropical clouds between −25 and −20 °C may be susceptible to the first aerosol indirect effect.

Optimization of the inhibition of atmospheric window emission using photonic crystals

Transfer matrix method is used to discuss the effect inhibition of photonic crystals on atmospheric window emission (8–14 μm). According to the optical characteristics of the materials, germanium and zinc sulfide are used as the composing materials. The structure of the photonic crystals is optimized, and the optimal thickness values of the germanium and zinc sulfide are 0.63 and 1.11 μm respectively while the ratio of optical thickness is 1:1 and the period is 8. The photonic crystals are prepared by evaporation coating method, and the optical properties of the photonic crystals are measured, the inhibition of the photonic crystals to atmospheric window emission is verified.

Live Cell Refractometry Using Hilbert Phase Microscopy and Confocal Reflectance Microscopy

Quantitative chemical analysis has served as a useful tool for understanding cellular metabolisms in biology. Among many physical properties used in chemical analysis, refractive index in particular has provided molecular concentration that is an important indicator for biological activities. In this report, we present a method of extracting full-field refractive index maps of live cells in their native states. We first record full-field optical thickness maps of living cells by Hilbert phase microscopy and then acquire physical thickness maps of the same cells using a custom-built confocal reflectance microscope. Full-field and axially averaged refractive index maps are acquired from the ratio of optical thickness to physical thickness. The accuracy of the axially averaged index measurement is 0.002. This approach can provide novel biological assays of label-free living cells in situ.

One-Dimensional Photonic Crystal-Based Multichannel Filters Using Binary Phase-Only Sampling Approach

For one-dimensional (1-D) photonic crystals, the amplitude reversal in refractive index profile is defined as an equivalent pi phase shift. With these defined pi phase shifts, the binary phase-only sampling approach can be implemented to design 1-D photonic crystal-based multichannel filters. Such filters exhibit high-count channels and excellent channel uniformity, which is identical with the phenomena obtained in sampled fiber Bragg gratings (SFBGs). Simulations of a nine-channel filter are given out and the distribution of reflection peaks is consistent with that of SFBGs. Then, the impact of structural parameters on filtering characteristics (especially the peak-trough contrast ratio) is discussed, such as optical length of unit cell, optical thickness ratio of alternate dielectric layers, and apodization technique.

Determination of optimum aerosol optical thickness ratios for atmospheric correction of coastal zone color scanner data in the Georges Bank-Gulf of Maine Region

The influence of varying aerosol optical thickness ratios (AOTR) on the precision and accuracy of satellite-derived CZCS pigment was evaluated using 92 full-resolution CZCS scenes collected along the U.S. northeast coast (NEC) from 1979 to 1986. CZCS pigment was estimated using the standard CZCS two channel bio-optical algorithm and seven sets of AOTR values including individual ratios computed interactively for each NEC image using the “clear water” radiance technique (INDIVID); means of the individual NEC ratios (NECAVE, N = 92); means of individual ratios computed from Middle Atlantic Bight (MAB) and NEC scenes (NECMAB, N = 276); means of individual ratios computed from South Atlantic Bight (SAB), MAB and NEC scenes (ECOAST, N = 1091); “default” values for global marine aerosols (GLOBAL); and time-varying ratios derived from weak annual signals present in the individual ratios used to compute NECMAB and ECOAST means. Precision was estimated using geometric mean CZCS pigment values computed from the seven sets of images for three 70 × 70 km sub-domains located in the western Gulf of Maine, on southern Georges Bank and over the continental slope. The smallest root mean square difference (RMSD) for geometric mean CZCS pigment (<0.05 log 10 units) was obtained between images processed using means of individual ratios and time-varying ratios, while the largest RMSD (generally >0.1 log 10 units) was obtained between images processed using GLOBAL and INDIVID ratios. Accuracy was estimated using geometric mean CZCS pigment values computed from the seven sets of images and a sea truth data set. After removal of suspect CZCS pigment values, agreement between CZCS pigment and sea truth data was highest using ECOAST mean ratios and a spatial window diameter (temporal window) of 7 km (±0.5 days) for N = 546, displaying RMSD (log 10 units), linear regression slope and correlation coefficient of 0.19, 1.00 and 0.88, respectively. ECOAST “mean” AOTR values resulted in a superior atmospheric correction for computation of CZCS pigment values from the Georges Bank-Gulf of Maine region when compared with values computed using “default” global or individual “interactively determined” ratios. ECOAST mean AOTR values should provide a consistent regional atmospheric correction for all CZCS scenes from the Georges Bank-Gulf of Maine region, including scenes which lack low pigment water and therefore cannot make use of the “clear-water” radiance technique.

Infrared brightness temperature of Saturn's rings based on the inhomogeneous multilayer assumption

Models of Saturn's rings based on the classical multilayer assumption have been studied in the infrared. Thermal energy balance of Saturn's rings is treated rigorously by solving the infrared radiative transfer equations. It was found that a homogeneous multilayer model is incompatible with the observed infrared brightness variation of the A and B rings, although it can fit that of the C ring. The alternative inhomogeneous multilayer model with dark particles within a bright haze of small icy particles is presented in order to satisfy the available infrared data of the A, B, and C rings. The results based on the inhomogeneous multilayer model may be summarized as follows: The observed infrared brightness data of the three rings are explained in terms of the different optical thickness without having significant differences in the ring-particle properties, such as albedo, spin rate, and sizes. But each ring contains a different amount of bright haze particles and their concentration within the rings depends on whether or not dark particles emit radiation mostly from one hemisphere (slow rotator and/or low thermal inertia). If a dark particle is an isothermal radiator, the possible ranges of A 1 and A 2 for all three rings are given by 0.9 ≲ A 1 ≲ 0.95 and 0.0 ≲ A 2 ≲ 0.15, where A 1 and A 2 are the bolometric bond albedos of a bright haze and a dark particle, respectively. The possible ranges of the optical thickness ratio X of the dark particle layer to the total ring layer for the rings A, B, and C are given by 0.65 ≲ X ≲ 0.75, 0.8 ≲ X ≲ 0.9, and 0.8 ≲ X ≲ 1.0, respectively. If a dark particle is a slow rotator, we obtain 0.9 ≲ A 1 ≲ 0.95 and 0.0 ≲ A 2 ≲ 0.4 for all three rings. The ranges of X for the rings A, B, and C are given by 0.35 ≲ X ≲ 0.7, 0.65 ≲ X ≲ 0.9, and 0.35 ≲ X ≲ 1.0, respectively. In this paper, for the first time, a consistent model is presented which is applicable to all three rings from the multilayer point of view.