Laurent Pilon

Optical constants of silica glass from extreme ultraviolet to far infrared at near room temperature

We thoroughly and critically review studies reporting the real (refractive index) and imaginary (absorption index) parts of the complex refractive index of silica glass over the spectral range from 30 nm to 1000 microm. The general features of the optical constants over the electromagnetic spectrum are relatively consistent throughout the literature. In particular, silica glass is effectively opaque for wavelengths shorter than 200 nm and larger than 3.5-4.0 microm. Strong absorption bands are observed (i) below 160 nm due to the interaction with electrons, absorption by impurities, and the presence of OH groups and point defects; (ii) at aproximately 2.73-2.85, 3.5, and 4.3 microm also caused by OH groups; and (iii) at aproximately 9-9.5, 12.5, and 21-23 microm due to Si-O-Si resonance modes of vibration. However, the actual values of the refractive and absorption indices can vary significantly due to the glass manufacturing process, crystallinity, wavelength, and temperature and to the presence of impurities, point defects, inclusions, and bubbles, as well as to the experimental uncertainties and approximations in the retrieval methods. Moreover, new formulas providing comprehensive approximations of the optical properties of silica glass are proposed between 7 and 50 microm. These formulas are consistent with experimental data and substantially extend the spectral range of 0.21-7 microm covered by existing formulas and can be used in various engineering applications.

The pyroelectric energy harvesting capabilities of PMN–PT near the morphotropic phase boundary

This paper reports on direct thermal to electrical energy conversion by performing the Olsen cycle on pyroelectric materials. The energy harvesting capability of commercially available [001] oriented 68PbMg1/3Nb2/3O3?32PbTiO3 (PMN?32PT) single crystal capacitors was measured experimentally. An energy density of 100? mJ?cm?3/cycle, corresponding to 4.92?mW?cm?3, was obtained by successively dipping the material in oil baths at temperatures 80 and 170??C and cycling the electric field between 2 and 9? kV?cm?1. Similarly, an energy density of 55?mJ?cm?3/cycle was obtained between 80 and 140??C. An estimated 40% of this energy resulted from the strain polarization due to the rhombohedral to tetragonal phase transition. The strain from this transition disappeared when the maximum operating temperature exceeded the Curie temperature of about 150??C. The optimal low electric field used in the Olsen cycle maximizing the energy harvested was found to be around 2?kV?cm?1. In addition, the material suffered from (i)?dielectric breakdown for electric fields larger than 9? kV?cm?1 and (ii)?cracking from thermal stress for operating temperature differences in excess of 90??C. A physical model predicting the total amount of energy harvested was also derived, accounting for thermal expansion as well as temperature dependent dielectric constant and spontaneous polarization. The model predictions fell within 20% of the experimental results in the temperature range between 80 and 170??C and electric fields ranging from 2 to 9?kV?cm?1.

Pyroelectric energy harvesting using Olsen cycles in purified and porous poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] thin films

This paper is concerned with the direct conversion of heat into electricity using pyroelectric materials. The Olsen (or Ericsson) cycle was experimentally performed on three different types of 60/40 poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer samples, namely commercial, purified, and porous films. This cycle consists of two isoelectric field and two isothermal processes. The commercial and purified films were about 50 μm thick and produced a maximum energy density of 521 J l −1 and 426 J l −1 per cycle, respectively. This was achieved by successively dipping the films in cold and hot silicone oil baths at 25 and 110 ◦ C under low and high applied electric fields of about 200 and 500 kV cm −1 , respectively. The 11 μm thick porous films achieved a maximum energy density of 188 J l −1 per cycle between 25 and 100 ◦ C and electric field between 200 and 400 kV cm −1 . The performance of the purified and porous films suffered from their lower electrical resistivity and electric breakdown compared with commercial thin films. However, the energy densities of all 60/40 P(VDF-TrFE) films considered matched or exceeded those reported recently for 0.9Pb(Mg1/3Nb2/3)O3‐0.10PbTiO3 (0.9PMN‐0.1PT) (186 J l −1 )a nd Pb(Zn1/3Nb2/3)0.955Ti0.045O3 (243 J l −1 ) bulk ceramics. Furthermore, the results are discussed in light of recently proposed figures of merit for energy harvesting applications.

Hyperspectral imaging in diabetic foot wound care.

Diabetic foot ulceration is a major complication of diabetes and afflicts as many as 15 to 25% of type 1 and 2 diabetes patients during their lifetime. If untreated, diabetic foot ulcers may become infected and require total or partial amputation of the affected limb. Early identification of tissue at risk of ulcerating could enable proper preventive care, thereby reducing the incidence of foot ulceration. Furthermore, noninvasive assessment of tissue viability around already formed ulcers could inform the diabetes caregiver about the severity of the wound and help assess the need for amputation. This article reviews how hyperspectral imaging between 450 and 700 nm can be used to assess the risk of diabetic foot ulcer development and to predict the likelihood of healing noninvasively. Two methods are described to analyze the in vivo hyperspectral measurements. The first method is based on the modified Beer-Lambert law and produces a map of oxyhemoglobin and deoxyhemoglobin concentrations in the dermis of the foot. The second is based on a two-layer optical model of skin and can retrieve not only oxyhemoglobin and deoxyhemoglobin concentrations but also epidermal thickness and melanin concentration along with skin scattering properties. It can detect changes in the diabetic foot and help predict and understand ulceration mechanisms

Purified and porous poly(vinylidene fluoride-trifluoroethylene) thin films for pyroelectric infrared sensing and energy harvesting

This paper aims at improving the performance of the poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer for pyroelectric infrared detection and direct thermal to electrical energy conversion. Three different types of samples were prepared and examined: commercial, purified and porous films. Here, full characterization of the thermophysical and electrical properties relevant to pyroelectric infrared detection and energy conversion of both purified and porous P(VDF-TrFE) thin films is presented. Properties measured include (1) density, (2) ferroelectric to paraelectric phase transition temperature, (3) enthalpy of change of phase, (4) electrical resistivity and (5) ferroelectric hysteresis, as well as (6) specific heat, (7) dielectric constant, (8) loss tangent and (9) pyroelectric coefficient as a function of temperature. The figures of merit for infrared detection FV , FI and FD were improved by 47.0, 59.6 and 51.6%, respectively, for the purified films while the porous films with a porosity of 33% showed an improvement of 52.8, 66.3 and 62.6%, respectively, when compared to those of dense commercial P(VDF-TrFE) films. In addition, figures of merit for energy harvesting, FE and k^2, indicate that the purified and porous films are attractive for thermal to electrical energy conversion as well.

Rapid and accurate estimation of blood saturation, melanin content, and epidermis thickness from spectral diffuse reflectance.

We present a method to determine chromophore concentrations, blood saturation, and epidermal thickness of human skin from diffuse reflectance spectra. Human skin was approximated as a plane-parallel slab of variable thickness supported by a semi-infinite layer corresponding to the epidermis and dermis, respectively. The absorption coefficient was modeled as a function of melanin content for the epidermis and blood content and oxygen saturation for the dermis. The scattering coefficient and refractive index of each layer were found in the literature. Diffuse reflectance spectra between 490 and 620 nm were generated using Monte Carlo simulations for a wide range of melanosome volume fraction, epidermal thickness, blood volume, and oxygen saturation. Then, an inverse method was developed to retrieve these physiologically meaningful parameters from the simulated diffuse reflectance spectra of skin. A previously developed accurate and efficient semiempirical model for diffuse reflectance of two layered media was used instead of time-consuming Monte Carlo simulations. All parameters could be estimated with relative root-mean-squared error of less than 5% for (i) melanosome volume fraction ranging from 1% to 8%, (ii) epidermal thickness from 20 to 150 mum, (iii) oxygen saturation from 25% to 100%, (iv) blood volume from 1.2% to 10%, and (v) tissue scattering coefficient typical of human skin in the visible part of the spectrum. A similar approach could be extended to other two-layer absorbing and scattering systems.

Effective optical properties of absorbing nanoporous and nanocomposite thin films

This paper aims at developing numerically validated models for predicting the through-plane effective index of refraction and absorption index of nanocomposite thin films. First, models for the effective optical properties of such materials are derived from previously reported analysis applying the volume averaging theory (VAT) to Maxwell’s equations. The transmittance and reflectance of nanoporous thin films are computed by solving Maxwell’s equations and the associated boundary conditions at all interfaces using finite element methods. The effective optical properties of the films are retrieved by minimizing the root mean square of the relative errors between the computed and theoretical transmittance and reflectance. Nanoporous thin films made of SiO2 and TiO2 consisting of cylindrical nanopores and nanowires are investigated for different diameters and various porosities. Similarly, electromagnetic wave transport through dielectric medium with embedded metallic nanowires are simulated. The numerical r...

Pyroelectric waste heat energy harvesting using relaxor ferroelectric 8/65/35 PLZT and the Olsen cycle

Waste heat can be directly converted into electrical energy by performing the Olsen cycle on pyroelectric materials. The Olsen cycle consists of two isothermal and two isoelectric field processes in the electric displacement versus electric field diagram. This paper reports on the electrical energy generated by lanthanum-doped lead zirconate titanate (8/65/35 PLZT) subjected to the Olsen cycle. The material was alternately dipped into a cold and a hot silicone oil bath under specified electric fields. A maximum energy density of 888 J l−1/cycle was obtained with a 290 µm thick 8/65/35 PLZT sample for temperatures between 25 and 160 °C and electric fields cycled between 0.2 and 7.5 MV m−1. To the best of our knowledge, this is the largest pyroelectric energy density experimentally measured with multiple cycles. It corresponded to a power density of 15.8 W l−1. The electrical breakdown strength and therefore the energy and power densities of the material increased as the sample thickness was reduced from 720 to 290 µm. Furthermore, a physical model for estimating the energy harvested by ferroelectric relaxors was further validated against experimental data for a wide range of electric fields and temperatures.

Use of Mie theory to analyze experimental data to identify infrared properties of fused quartz containing bubbles

An improved method used to determine the absorption and scattering characteristics of a weakly absorbing substance containing bubbles is suggested. The identification procedure is based on a combination of directional-hemispherical measurements and predictions of Mie-scattering theory including approximate relations for a medium with polydisperse bubbles. A modified two-flux approximation is suggested for the calculation of directional-hemispherical transmittance and reflectance of a refracting and scattering medium. The complete identification procedure gives not only the spectral radiative properties but also the volume fraction of bubbles and the characteristics of possible impurity of the medium. This procedure is used to obtain new data on near-infrared properties of fused-quartz samples containing bubbles.

Radiation and optical properties of Nannochloropsis oculata grown under different irradiances and spectra

This paper reports accurate measurements of the radiation characteristics and optical properties of Nannochloropsis oculata in the photosynthetically active radiation (PAR) region. These marine microalgae were grown in 2 cm thick culture bottles with vented caps exposed, on one side, to either white fluorescent light bulbs or red LEDs emitting at 630 nm. The illuminance varied from 2000 to 10,000 lux. The microalgae average equivalent diameter ranged from 2.52 to 2.63 μm. Their radiation characteristics and optical properties were statistically identical over most of the PAR region. Other N. oculata grown with 2 vol.% CO2 injection in 1cm thick flat bottles exposed to light from both sides reached a significantly larger mass concentration and featured lower pigment concentration and smaller absorption cross-sections. This was due to nutrient limited growth conditions. The refraction index was independent of illuminance, spectrum, and growth conditions and featured resonance at wavelengths corresponding to absorption peaks.