Finite-difference time-domain and steady-state analysis with novel boundary conditions of optical transport in a three-dimensional scattering medium illuminated by an isotropic point source

We evaluate the numerical accuracy of finite-difference time-domain (FDTD) analysis of optical transport in a three-dimensional scattering medium illuminated by an isotropic point source. This analysis employs novel boundary conditions for the diffusion equation. The power radiated from an isotropic point source located at a depth equal to the reciprocal of the reduced scattering coefficient (1/μ'(s)) below the surface at the irradiated position is introduced to the integral form of the diffusion equation. Finite-difference approximations of the diffusion equation for a surface cell are derived by utilizing new boundary conditions that include an isotropic source even in a surface cell. Steady-state and time-resolved reflectances are calculated by FDTD analysis for a semi-infinite uniform scattering medium illuminated by an isotropic point source. The numerical results agree reasonably with the analytical solutions for μ'(s)=1-3 mm(-1) without resizing the mesh elements.     

Comprehensive two-dimensional gas chromatographic separations with a microfabricated thermal modulator

Rapid, comprehensive two-dimensional gas chromatographic (GC × GC) separations by use of a microfabricated midpoint thermal modulator (μTM) are demonstrated, and the effects of various μTM design and operating parameters on performance are characterized. The two-stage μTM chip consists of two interconnected spiral etched-Si microchannels (4.2 and 2.8 cm long) with a cross section of 250 × 140 μm(2), an anodically bonded Pyrex cap, and a cross-linked wall coating of poly(dimethylsiloxane) (PDMS). Integrated heaters provide rapid, sequential heating of each μTM stage, while a proximate, underlying thermoelectric cooler provides continual cooling. The first-dimension column used for GC × GC separations was a 6 m long, 250 μm i.d. capillary with a PDMS stationary phase, and the second-dimension column was a 0.5 m long, 100 μm i.d. capillary with a poly(ethylene glycol) phase. Using sets of five to seven volatile test compounds (boiling point ≤174 °C), the effects of the minimum (T(min)) and maximum (T(max)) modulation temperature, stage heating lag/offset (O(s)), modulation period (P(M)), and volumetric flow rate (F) on the quality of the separations were evaluated with respect to several performance metrics. Best results were obtained with a T(min) = -20 °C, T(max) = 210 °C, O(s) = 600 ms, P(M) = 6 s, and F = 0.9 mL/min. Replicate modulated peak areas and retention times were reproducible to <5%. A structured nine-component GC × GC chromatogram was produced, and a 21 component separation was achieved in <3 min. The potential for creating portable μGC × μGC systems is discussed.     

Combined Henyey-Greenstein and Rayleigh phase function

The phase function is an important parameter that affects the distribution of scattered radiation. In Rayleigh scattering, a scatterer is approximated by a dipole, and its phase function is analytically related to the scattering angle. For the Henyey-Greenstein (HG) approximation, the phase function preserves only the correct asymmetry factor (i.e., the first moment), which is essentially important for anisotropic scattering. When the HG function is applied to small particles, it produces a significant error in radiance. In addition, the HG function is applied only for an intensity radiative transfer. We develop a combined HG and Rayleigh (HG-Rayleigh) phase function. The HG phase function plays the role of modulator extending the application of the Rayleigh phase function for small asymmetry scattering. The HG-Rayleigh phase function guarantees the correct asymmetry factor and is valid for a polarization radiative transfer. It approaches the Rayleigh phase function for small particles. Thus the HG-Rayleigh phase function has wider applications for both intensity and polarimetric radiative transfers. For microwave radiative transfer modeling in this study, the largest errors in the brightness temperature calculations for weak asymmetry scattering are generally below 0.02 K by using the HG-Rayleigh phase function. The errors can be much larger, in the 1-3 K range, if the Rayleigh and HG functions are applied separately.     

Characterization of dentin and enamel by use of optical coherence tomography

Optical coherence tomographic images of human dentin and enamel are obtained by use of polarization-sensitive optical coherence tomography. A birefringence effect in enamel (lambda = 856 nm) and light propagation along dentinal tubules are observed. The group index of refraction for both dentin and enamel was measured at 1.50 +/- 0.02 and 1.62 +/- 0.02, respectively.     

Determination of optical coefficients and fractal dimensional parameters of cancerous and normal prostate tissues

Optical extinction and diffuse reflection spectra of cancerous and normal prostate tissues in the 750 to 860 nm spectral range were measured. Optical extinction measurements using thin ex vivo prostate tissue samples were used to determine the scattering coefficient (μ(s)), while diffuse reflection measurements using thick prostate tissue samples were used to extract the absorption coefficient (μ(a)) and the reduced scattering coefficient (μ'(s)). The anisotropy factor (g) was obtained using the extracted values of μ(s) and μ'(s). The values of fractal dimension (D(f)) of cancerous and normal prostate tissues were obtained by fitting to the wavelength dependence of μ'(s). The number of scattering particles contributing to μ(s) as a function of particle size and the cutoff diameter d(max) as a function of g were investigated using the fractal soft tissue model and Mie theory. Results show that d(max) of the normal tissue is larger than that of the cancerous tissue. The cutoff diameter d(max) is observed to agree with the nuclear size for the normal tissues and the nucleolar size for the cancerous tissues. Transmission spectral polarization imaging measurements were performed that could distinguish the cancerous prostate tissue samples from the normal tissue samples based on the differences between their absorption and scattering parameters.     

Temperature and intensity dependence of Yb-fiber laser light absorption in water

Absorption of CW Yb-fiber laser light of 1.07 μm wavelength in water has been measured at different water temperatures and laser intensities. The absorption coefficient was estimated to be 0.135 cm(-1) at 25 °C water temperature, and this was found to decrease with temperature at a rate of 5.7 × 10(-4) cm(-1) °C(-1). The absorption coefficient increased significantly when the laser beam was focused in water, and the increase depended on the distance of the focal point from the water surface. This has been attributed to the absorption and scattering losses of laser radiation in a cavity formed in water by the focused beam at laser intensities in the megawatts per square centimeter and higher range.     

Measurements and predictions of light scattering by clear coatings

Comparisons are made between calculated and measured angle-resolved light-scattering distributions from clear dielectric isotropic epoxy coatings over a range of rms roughness conditions, resulting in strongly specular scattering to diffuse scattering characteristics. Calculated distributions are derived from topography measurements performed with interferometric microscopes. Two methods of calculation are used. One determines the intensity of scattered light waves with a phase integral in the Kirchhoff approximation. The other is based on the reflection of light rays by locally flat surfaces. The angle-resolved scattering distributions for the coatings are measured with the spectral trifunction automated reference reflectometer (STARR) developed by the National Institute of Standards and Technology. Comparisons between measured and calculated results are shown for three surfaces with rms roughness values of approximately 3, 150, and 800 nm for an angle of incidence of 20 degrees .     

Evaluation of a microfabricated thermal modulator for comprehensive two-dimensional microscale gas chromatography

A microfabricated thermal modulator (μTM) designed for ultimate use in a comprehensive two-dimensional microscale gas chromatography (μGC × μGC) system is evaluated. The 2-stage device measures 13 mm (l) × 6 mm (w) × 0.5 mm (h) and consists of two interconnected serpentine etched-Si microchannels suspended from a thin Pyrex cap and wall-coated with PDMS (polydimethylsiloxane). The chip is mounted within a few tens of micrometers of a thermoelectric cooler that maintains both stages at a baseline temperature between -35 and -20 °C in order to focus analytes eluting from an upstream separation column. Each stage is heated to 210 °C sequentially at a rate as high as 2400 °C/s by independent thin-film resistors to inject the analytes in consecutive fractions to a downstream column, and then cooled at a rate as high as -168 °C/s. The average power dissipation is only ～10 W for heating and 21 W for cooling without using consumable materials. In this study, the outlet of the μTM is connected directly to a flame ionization detector to assess its performance. Following a demonstration of basic operation, the modulated peak amplitude enhancement (PAE) and full-width-at-half-maximum (fwhm) are evaluated for members of a series of n-alkanes (C(6)-C(10)) as a function of the rim and stage temperatures; modulation period, phase, and offset; analyte concentration; and carrier-gas flow rate. A PAE as high as 50 and a fwhm as narrow as 90 ms are achieved for n-octane under optimized conditions.     

Estimation of optical properties of turbid media: experimental comparison of spatially and temporally resolved reflectance methods

We compare two methods for the optical characterization of turbid media. The estimates of the absorption and reduced scattering coefficients (mu(a) and mu(')(s)) by a spatially resolved method and a time-resolved method are performed on tissue-like phantoms. Aqueous suspension of microspheres and Intralipid are used as scattering media with the addition of ink as an absorber. mu(')(s) is first measured on weakly absorbing media. The robustness of these measurements is then tested with respect to a variation of mu(a). The spatially resolved method gave more accurate estimates for mu(')(s) whereas the time-resolved method gave better results for mu(a) estimates.     

Multiple-scattering effects on static light-scattering optical structure factor measurements

We show that the extent and effect of multiple scattering on angularly resolved light-scattering intensity measurements, the optical structure factor, can be quantitatively described by a single parameter, the average number of scattering events along the scattering volume. This quantity is easily measured or calculated and hence provides a useful experimental indicator of multiple scattering, which is a hindrance to accurate structure factor measurements.     

Phase-shifting real-time holographic interferometry that uses bismuth silicon oxide crystals

A bismuth silicon oxide crystal is used in the diffusion regime as a dynamic recording medium in a real-time holographic interferometer based on anisotropic self-diffraction. This device is connected with an interferogram-analysis method that uses the phase-shifting technique for quantitative measurement of diffusive-reflecting object deformations. In addition to the usual error sources in phase shifting, the temporal interferogram erasure is studied and is found weakly perturbative for the measured phase. It is shown that quantitative measurements are possible for low-intensity object beams (8 μW/cm(2)) and a large observed area. Apractical situation of defect monitoring in a composite structure is presented.     

Portable Fourier transform infrared spectroradiometer for field measurements of radiance and emissivity

A hand-held, battery-powered Fourier transform infrared spectroradiometer weighing 12.5 kg has been developed for the field measurement of spectral radiance from the Earth's surface and atmosphere in the 3-5-μm and 8-14-μm atmospheric windows, with a 6-cm(-1) spectral resolution. Other versions of this instrument measure spectral radiance between 0.4 and 20 μm, using different optical materials and detectors, with maximum spectral resolutions of 1 cm(-1). The instrument tested here has a measured noise-equivalent delta T of 0.01 °C, and it measures surface emissivities, in the field, with an accuracy of 0.02 or better in the 8-14-μm window (depending on atmospheric conditions), and within 0.04 in accessible regions of the 3-5-μm window. The unique, patented design of the interferometer has permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters. The instrument has made field measurements of radiance and emissivity for 3 yr without loss of optical alignment. We describe the design of the instrument and discuss methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements. Examples of emissivity spectra are shown for both the 3-5-μm and 8-14-μm atmospheric windows.     

Structure and electrical properties of (111)-oriented Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 thin film for ultra-high-frequency transducer applications

Ternary lead magnesium niobate-lead zirconate titanate system 0.4Pb(Mg(1/3)Nb(2/3))O(3)-0.25PbZrO(3)-0.35PbTiO(3) (40PMN-25PZ-35PT) thin film with a thickness of 1.5 μm was grown on Pt(111)/Ti/SiO(2)/Si substrate via chemical solution deposition. X-ray diffraction and transmission electron microscopy results suggested the film obtained was highly (111)-oriented. The remanent polarization and coercive electric field of the film were found to be 25.5 μC/cm(2) and 51 kV/cm, respectively. In addition, at 1 kHz, the dielectric constant was measured to be 1960 and the dielectric loss 0.036. The film was observed to undergo a diffuse ferroelectric-to-paraelectric phase transition at around 209°C. The leakage current appeared to depend on the voltage polarity. If the Au electrode was biased positively, the leakage current was dominated by the Schottky emission mechanism. When the Pt electrode was biased positively, the conduction current curve showed an ohmic behavior at a low electric field and space-charge-limited current characteristics at a high electric field.     

Experimental and theoretical evaluation of a fiber-optic approach for optical property measurement in layered epithelial tissue

Improvements in measurement of epithelial tissue optical properties (OPs) in the ultraviolet and visible (UV-Vis) may lead to enhanced understanding of optical techniques for neoplasia detection. In this study, we investigated an approach based on fiber-optic measurement of reflectance to determine absorption and reduced scattering coefficients (μ(a) and μ(s)') in two-layer turbid media. Neural network inverse models were trained on simulation data for a wide variety of OP combinations (μ(a) = 1-22.5, μ(s)' = 5-42.5 cm(-1)). Experimental measurements of phantoms with top-layer thicknesses (D) ranging from 0.22 to 0.66 mm were performed at three UV-Vis wavelengths. OP estimation accuracy was calculated and compared to theoretical results. Mean prediction errors were strongly correlated with D and ranged widely, from 1.5 to 12.1 cm(-1). Theoretical analyses indicated the potential for improving accuracy with alternate probe geometries. Although numerous challenges remain, this initial experimental study of an unconstrained approach for fiber-optic-based OP determination in two-layer epithelial tissue indicates the potential to provide useful measurements.     

Extinction efficiency in the infrared (2-18 µm) of laboratory ice clouds: observations of scattering minima in the Christiansen bands of ice

Extinction measurements with a laser diode (0.685 μm) and a Fourier transform infrared spectrometer (2-18 μm) were performed on laboratory ice clouds (5 μm ≤ D ≤ 70 μm) grown at a variety of temperatures, and thus at a variety of crystal habits and average projected crystal area. Ice clouds were grown by nucleation of a supercooled water droplet cloud with a rod cooled with liquid nitrogen. The ice crystals observed were mainly plates and dendrites at the coldest temperatures (≈-15 °C) and were mainly columns and needles at warmer temperatures (≈-5 °C). The crystals were imaged with both a novel microscope equipped with a video camera and a heated glass slide and a continuously running Formvar replicator. The IR spectral optical-depth measurements reveal a narrow (0.5-μm-width) extinction minimum at 2.84 μm and a wider (3-μm-width) minimum at 10.5 μm. These partial windows are associated with wavelengths where the real part of the index of refraction for bulk ice has a relative minimum so that extinction is primarily due to absorption rather than scattering (i.e., the Christiansen effect). Bulk ice has absorption maxima near the window wavelengths. IR extinction efficiency has a noticeable wavelength dependence on the average projected crystal area and therefore on the temperaturedependent crystal properties. The average-size parameters in the visible for different temperatures ranged from 64 to 128, and in the IR they ranged from 2.5 to 44. The extinction efficiency and the single-scatter albedo for ice spheres as computed from Mie scattering also show evidence of the Christiansen effect.     

人体牙齿的显微组织及纳米力学性能

采用光学显微镜、能谱扫描电镜、高分辨透射电镜以及纳米力学探针等设备对人体牙齿的显微组织及纳米力学性能进行了研究。结果表明: 牙本质由规则排列的牙本质小管和基质组成,牙釉质由不同方向的釉柱及柱间质组成,牙本质与牙釉质之间有明显的界面;牙釉质中的Ca和P含量明显高于牙本质中的含量,而C含量明显低于牙本质中的含量。牙釉质中发现有大量的磷灰石晶体结构,而牙本质中则明显缺少。力学性能结果显示,牙釉质的纳米硬度平均为4.4 GPa,弹性模量为81 GPa,而牙本质的纳米硬度平均为1.0 GPa,弹性模量为26 GPa。牙釉质高的硬度和弹性模量与含有大量的羟基磷灰石晶体组织有关,Ca和P促进了该晶体组织形成。      

Accurate measurement of the radius of curvature of a concave mirror and the power dependence in a high-finesse Fabry-Perot interferometer

We describe the accurate measurement of the radius of curvature of a concave mirror in a Fabry-Perot interferometer with a finesse of 78,100. The radius of curvature of the concave mirror is determined by measuring the free spectral range and the transverse-mode range with the frequency response functions. The radii of curvature at two orthogonal (x and y) axes on the mirror surface resulting from the polishing nonisotropy were accurately measured to be r(x) = 1008.46 mm and r(y) = 1006.94 mm, respectively, with an accuracy of 8 × 10(-5). This accuracy is the best to our knowledge. The power dependence of the radii of curvature to the cavity internal intensity at a steady state was measured to be dr(x)/dI(c) = +60 μm/(MW/cm(2)) at the x axis and dr(y)/dI(c) = +47 μm/(MW/cm/(2)) at the y axis to an intensity of 2.1 MW/cm(2).     

In vivo local determination of tissue optical properties: applications to human brain

Local and superficial near-infrared (NIR) optical-property characterization of turbid biological tissues can be achieved by measurement of spatially resolved diffuse reflectance at small source-detector separations (<1.4 mm). However, in these conditions the inverse problem, i.e., calculation of localized absorption and the reduced scattering coefficients, is necessarily sensitive to the scattering phase function. This effect can be minimized if a new parameter of the phase function gamma, which depends on the first and the second moments of the phase function, is known. If gamma is unknown, an estimation of this parameter can be obtained by the measurement, but the uncertainty of the absorption coefficient is increased. A spatially resolved reflectance probe employing multiple detector fibers (0.3-1.4 mm from the source) is described. Monte Carlo simulations are used to determine gamma, the reduced scattering and absorption coefficients from reflectance data. Probe performance is assessed by measurements on phantoms, the optical properties of which were measured by other techniques [frequency domain photon migration (FDPM) and spatially resolved transmittance]. Our results show that changes in the absorption coefficient, the reduced scattering coefficient, and gamma can be measured to within +/-0.005 mm(-1), +/-0.05 mm(-1), and +/-0.2, respectively. In vivo measurements performed intraoperatively on a human skull and brain are reported for four NIR wavelengths (674, 811, 849, 956 nm) when the spatially resolved probe and FDPM are used. The spatially resolved probe shows optimum measurement sensitivity in the measurement volume immediately beneath the probe (typically 1 mm(3) in tissues), whereas FDPM typically samples larger regions of tissues. Optical-property values for human skull, white matter, scar tissue, optic nerve, and tumors are reported that show distinct absorption and scattering differences between structures and a dependence on the phase-function parameter gamma.     

Quantitative dental measurements by use of simultaneous frequency-domain laser infrared photothermal radiometry and luminescence

Modulated (frequency-domain) infrared photothermal radiometry (PTR) is used as a dynamic quantitative dental inspection tool complementary to modulated luminescence (LM) to quantify sound enamel or dentin. A dynamic high-spatial-resolution experimental imaging setup, which can provide simultaneous measurements of laser-induced modulated PTR and LM signals from defects in teeth, has been developed. Following optical absorption of laser photons, the experimental setup can monitor simultaneously and independently the nonradiative (optical-to-thermal) energy conversion by infrared PTR and the radiative deexcitation by LM emission. The relaxation lifetimes (tau1, tau2) and optical absorption, scattering, and spectrally averaged infrared emission coefficients (mu(alpha), mu(s), mu(IR)) of enamel are then determined with realistic three-dimensional LM and photothermal models for turbid media followed by multiparameter fits to the data. A quantitative band of values for healthy enamel with respect to these parameters can be generated so as to provide an explicit criterion for the assessment of healthy enamel and, in a future extension, to facilitate the diagnosis of the onset of demineralization in carious enamel.