Disordered, strongly scattering porous materials as miniature multipass gas cells

Linear Probing of Molecules at Micrometric Distances from a Surface with Sub-Doppler Frequency Resolution

We report on precision spectroscopy of subwavelength confined molecular gases. This was obtained by rovibrational selective reflection of NH_{3} and SF_{6} gases using a quantum cascade laser at λ≈10.6  μm. Our technique probes molecules at micrometric distances (≈λ/2π) from the window of a macroscopic cell with submegahertz resolution, allowing molecule-surface interaction spectroscopy. We exploit the linearity and high resolution of our technique to gain novel spectroscopic information on the SF_{6} greenhouse gas, useful for enriching molecular databases. The natural extension of our work to thin cells will allow compact frequency references and improved measurements of the Casimir-Polder interaction with molecules.

Pore size assessment using gas in scattering media absorption spectroscopy and gas adsorption

Here, we investigate effects of the size of pores in porous alumina powders on the broadening of the oxygen gas absorption line. The line broadening is caused by collisions of oxygen molecules with the pore walls and is extracted using gas in scattering media absorption spectroscopy (GASMAS), while the average pore size is determined using the gas adsorption technique. The average pore size of the samples studied lies within the range 10-40 nm. In this range, the contribution of the wall collision broadening is found to be approximately inversely related to the average pore diameter. Furthermore, the confined oxygen gas absorbance measured by GASMAS is found to be linearly correlated with the effective porosity evaluated by the saturation method.

Optical porosimetry of weakly absorbing porous materials

The physical porosity ${\Phi}$Φ of a porous material determines most of its properties. Although the optical porosity ${\Phi} _{\textrm {opt}}$Φ_opt can be measured, relating this quantity to ${\Phi}$Φ remains a challenge. Here we derive relationships between the optical porosity, the effective refractive index $n_{\textrm {eff}}$n_eff and the physical porosity of weakly absorbing porous media. It introduces the absorption enhancement parameter ${B}$B, which quantifies the asymmetry of photon path lengths between the solid material and the pores and can be derived from the absorption coefficient $\mu _a$μ_a of the material. Hence ${\Phi}$Φ can be derived from combined measurements of $n_{\textrm {eff}}$n_eff and $\mu _a$μ_a. The theory is validated against laboratory measurements and numerical experiments, thus solving a long-standing issue in optical porosimetry. This suggests that optical measurements can be used to estimate physical porosity with an accuracy better than 10$\%$%.

Investigating the Relation between Absorption and Gas Concentration in Gas Detection Using a Diffuse Integrating Cavity

The relationship between absorption and gas concentration was studied using a diffuse integrating cavity as a gas cell. The light transmission process in an arbitrary diffuse cavity was theoretically derived based on a beam reflection analysis. It was found that a weak absorption condition must be satisfied to ensure a linear relationship between absorbance and gas concentration. When the weak absorption condition is not satisfied, a non-linear relation will be observed. A 35 × 35 × 35 cm diffuse integrating cavity was used in the experiment. Different oxygen concentrations were measured by detecting the P9 absorption line at 763.8 nm, based on tunable diode laser absorption spectroscopy. The relationship between the absorption signals and oxygen concentration was linear at low oxygen concentrations and became non-linear when oxygen concentrations were higher than 21%. The absorbance value of this transition point was 0.17, which was considered as the weak absorption condition for this system. This work studied the theoretical reason for the non-linear phenomenon and provided an experimental method to determine the transition point when using a diffuse integrating cavity as a gas cell.

Random Multiple Scattering Enhanced Photoacoustic Gas Spectroscopy with Disordered Porous Ceramics

Light-gas interaction can be enhanced by using disordered porous materials because multiple random scattering increases light intensity near the surface of the material. Here we report signal enhancement of photoacoustic gas spectroscopy with disordered porous ceramics. The amplitude and frequency characteristics of photoacoustic signal due to gas absorption in disordered materials are modeled theoretically. Experiment with a porous Al₂O₃ sample demonstrates photoacoustic signal enhancement of ∼4 times at 5 kHz.

Measurement of the effective optical path length of diffusing integrating cavities using time-resolved spectroscopy

A method to measure the effective optical path length (EOPL) of a diffusing cavity using time-resolved spectroscopy is presented. A 710 nm short-pulsed laser was employed as the light source with a FWHM of 0.7 ns. From the temporal response curves, the time to establish a uniform optical field was obtained as approximately 4 ns for this cavity. Correspondingly, light was scattered 5-6 times by the inner coating. EOPLs with different port fractions were obtained by the selection of an appropriate initial time. Moreover, we verified a previously reported theoretical EOPL law using time-resolved spectroscopic results. This work provides a method to measure the EOPL of a diffusing cavity at different wavelengths.

Fabrication of a Combustion-Reacted High-Performance ZnO Electron Transport Layer with Silver Nanowire Electrodes for Organic Solar Cells

Herein, a new methodology for solution-processed ZnO fabrication on Ag nanowire network electrode via combustion reaction is reported, where the amount of heat emitted during combustion was minimized by controlling the reaction temperature to avoid damaging the underlying Ag nanowires. The degree of participation of acetylacetones, which are volatile fuels in the combustion reaction, was found to vary with the reaction temperature, as revealed by thermogravimetric and compositional analyses. An optimized processing temperature of 180 °C was chosen to successfully fabricate a combustion-reacted ZnO and Ag nanowire hybrid electrode with a sheet resistance of 30 Ω/sq and transmittance of 87%. A combustion-reacted ZnO on Ag nanowire hybrid structure was demonstrated as an efficient transparent electrode and electron transport layer for the PTB7-Th-based polymer solar cells. The superior electrical conductivity of combustion-reacted ZnO, compared to that of conventional sol-gel ZnO, increased the external quantum efficiency over the entire absorption range, whereas a unique light scattering effect due to the presence of nanopores in the combustion-derived ZnO further enhanced the external quantum efficiency in the 450-550 nm wavelength range. A power conversion efficiency of 8.48% was demonstrated for the PTB7-Th-based polymer solar cell with the use of a combustion-reacted ZnO/Ag NW hybrid transparent electrode.

Maximal absorption regime in random media

Efficient optical energy transfer is key to many technologies, ranging from biosensing to photovoltaics. Here, for the first time we show that by introducing a random medium with appropriate filling factor, absorption in a specific volume can be maximized. Using both numerical simulations and an analytical diffusion model, we identify design rules to maximize absorption in the system with different geometrical and scattering properties. By combining a random medium with an open photonic cavity, we numerically demonstrate a 23-fold enhancement of the absorbed energy. We also show how absorption as high as 99% can be reached in a device as thin as 500 μm for normal incidence illumination. Finally, our data indicate that introducing a non-absorbing random medium into a light trapping system for thin solar cells can enhance absorption of energy by a factor of 2.2. This absorption enhancement, caused by the random medium, is broadband and wide-angle and can help design efficient solar cells, light trapping devices, biosensors and random lasers.

Laser spectroscopic studies of gas diffusion in alumina ceramics

Experiments for measuring gas diffusion through porous alumina ceramics are described. With the gas in scattering media absorption spectroscopy (GASMAS) technique, gaseous oxygen signals are measured continuously during the gas diffusion process. It is experimentally demonstrated that the time-dependence of the transient oxygen signal is described by an exponential curve. Moreover, the effect on gas diffusion of material porosity and whether water is present or not is also experimentally investigated.

Path length enhancement in disordered media for increased absorption

We theoretically and numerically investigate the capability of disordered media to enhance the optical path length in dielectric slabs and augment their light absorption efficiency due to scattering. We first perform a series of Monte Carlo simulations of random walks to determine the path length distribution in weakly to strongly (single to multiple) scattering, non-absorbing dielectric slabs under normally incident light and derive analytical expressions for the path length enhancement in these two limits. Quite interestingly, while multiple scattering is expected to produce long optical paths, we find that media containing a vanishingly small amount of scatterers can still provide high path length enhancements due to the very long trajectories sustained by total internal reflection at the slab interfaces. The path length distributions are then used to calculate the light absorption efficiency of media with varying absorption coefficients. We find that maximum absorption enhancement is obtained at an optimal scattering strength, in-between the single-scattering and the diffusive (strong multiple-scattering) regimes. This study can guide experimentalists towards more efficient and potentially low-cost solutions in photovoltaic technologies.

Development of a compact multipass oxygen sensor used for gas diffusion studies in opaque media

A highly scattering porous ceramic sample is employed as a miniature random-scattering multipass gas cell for monitoring of oxygen content in opaque media, that is, wood materials in the present work. Gas in scattering media absorption spectroscopy is used by employing a 760 nm near-infrared laser diode to probe the absorption of molecular oxygen enclosed in the pores of the ceramic material working as the multipass gas cell, with a porosity of 75%. A path length enhancement of approximately 26 times and a signal-to-noise ratio of about 60 were obtained for the ceramic sample used in this work. The gas sensor was then used in a case study of the gas diffusion in wood materials, namely, oak, spruce, and mahogany samples. Differences depending on whether gas diffusion was studied longitudinal or radial to the tree annual rings are demonstrated, with very little gas diffusing in the radial direction. We can also observe that the gas diffusion for the densest material-oak-had the fastest diffusion time, and mahogany, which had the lowest density, showed the slowest diffusion time.

Multiscattering-enhanced absorption spectroscopy

An original scheme for sensitive absorption measurements, particularly well-suited for low analyte concentrations, is presented. The technique is based on multiscattering-enhanced absorption spectroscopy (MEAS) and benefits from the advantages of conventional absorption spectroscopy: simplicity, rapidity, and low costs. The technique relies on extending the optical path through the sensing volume by suspending dielectric beads in the solution containing the analytes of interest, resulting in multiple scattering of light, which increases the optical path length through the sample. This way, a higher sensitivity and lower limit of detection, compared to those of conventional absorption spectroscopy, can be achieved. The approach is versatile and can be used for a broad variety of analytes. Here, it is applied to the detection of phenol red, 10 nm gold nanoparticles, and envy green fluorescence dye; the limit of detection is decreased by a factor of 7.2 for phenol red and a factor of 3.3 for nanoparticles and dye. The versatility of this approach is illustrated by its application in increasing the sensitivity of colorimetric detection with gold nanoparticle probes and a commercially available hydrogen peroxide bioassay. The influence of different parameters describing the scattering medium is investigated in detail experimentally and numerically, with very good agreement between the two. Those parameters can be effectively used to tailor the enhancement for specific applications and analytes.

Pathlength determination for gas in scattering media absorption spectroscopy

Gas in scattering media absorption spectroscopy (GASMAS) has been extensively studied and applied during recent years in, e.g., food packaging, human sinus monitoring, gas diffusion studies, and pharmaceutical tablet characterization. The focus has been on the evaluation of the gas absorption pathlength in porous media, which a priori is unknown due to heavy light scattering. In this paper, three different approaches are summarized. One possibility is to simultaneously monitor another gas with known concentration (e.g., water vapor), the pathlength of which can then be obtained and used for the target gas (e.g., oxygen) to retrieve its concentration. The second approach is to measure the mean optical pathlength or physical pathlength with other methods, including time-of-flight spectroscopy, frequency-modulated light scattering interferometry and the frequency domain photon migration method. By utilizing these methods, an average concentration can be obtained and the porosities of the material are studied. The last method retrieves the gas concentration without knowing its pathlength by analyzing the gas absorption line shape, which depends upon the concentration of buffer gases due to intermolecular collisions. The pathlength enhancement effect due to multiple scattering enables also the use of porous media as multipass gas cells for trace gas monitoring. All these efforts open up a multitude of different applications for the GASMAS technique.

Laser absorption spectroscopy of oxygen confined in highly porous hollow sphere xerogel

An Al2O3 xerogel with a distinctive microstructure is studied for the application of laser absorption spectroscopy of oxygen. The xerogel has an exceptionally high porosity (up to 88%) and a large pore size (up to 3.6 µm). Using the method of gas-in-scattering media absorption spectroscopy (GASMAS), a long optical path length (about 3.5m) and high enhancement factor (over 300 times) are achieved as the result of extremely strong multiple-scattering when the light is transmitted through the air-filled, hollow-sphere alumina xerogel. We investigate how the micro-physical feature influences the optical property. As part of the optical sensing system, the material's gas exchange dynamics are also experimentally studied.

Light diffusion in quenched disorder: role of step correlations

We present a theoretical and experimental study of light transport in disordered media with strongly heterogeneous distribution of scatterers formed via nonscattering regions. Step correlations induced by quenched disorder are found to prevent diffusivity from diverging with increasing heterogeneity scale, contrary to expectations from annealed models. Spectral diffusivity is measured for a porous ceramic where nanopores act as scatterers and macropores render their distribution heterogeneous. Results agree well with Monte Carlo simulations and a proposed analytical model.

Sensing the dynamics of oxidative stress using enhanced absorption in protein-loaded random media

Reactive oxygen species play a key role in cell signalling and oxidative stress mechanisms, therefore, sensing their production by living organisms is of fundamental interest. Here we describe a novel biosensing method for extracellular detection of endogenous hydrogen peroxide (H2O2). The method is based on the enhancement of the optical absorption spectrum of the hemoprotein cytochrome c when loaded into a highly scattering random medium. Such a configuration enables, in contrast to existing techniques, non-invasive and dynamic detection of the oxidation of cyt c in the presence of H2O2 with unprecedented sensitivity. Dynamic information on the modification of the cell oxidative status of Chlamydomonas reinhardtii, an aquatic green algae, was obtained under oxidative stress conditions induced by the presence of trace concentrations of Cd(II). Furthermore, the dynamics of H2O2 production was investigated under different lighting conditions confirming the impact of Cd(II) on the photosynthetic activity of those phytoplanktonic cells.

Rb optical resonance inside a random porous medium

We studied resonant laser interaction with Rb atoms confined to the interstitial cavities of a random porous glass. Due to diffusive light propagation, the effect of atomic absorption on the light scattered by the sample is almost entirely compensated by atomic fluorescence at low atomic densities. For higher densities, radiation trapping increases the probability of nonradiative decay via atom-wall collisions. A simple connection of the fluorescence/absorption yield to the sample porosity is given.

Absorption spectroscopy of powdered materials using time-resolved diffuse optical methods

In this paper a novel method, based on time-resolved diffuse optical spectroscopy, is proposed to measure the absorption of small amounts of nanostructured powder materials independent of scattering. Experimental validation, in the visible and near-infrared spectral range, has been carried out on India Ink<sup/>particles. The effectiveness of the technique to measure scattering-free absorption is demonstrated on carbon nanotubes. The comparison between the absorption spectra acquired by the proposed method and conventional measurements performed with a commercial spectrophotometer is discussed.

Spectral control of broadband light through random media by wavefront shaping

A random medium can serve as a controllable arbitrary spectral filter with spectral resolution determined by the inverse of the interaction time of the light in the medium. We use wavefront shaping to implement an arbitrary spectral response at a particular point in the scattered field. We experimentally demonstrate this technique by selecting either a narrow band or dual bands with a width of 5.5 nm each.

Time-resolved diffuse optical spectroscopy up to 1700 nm by means of a time-gated InGaAs/InP single-photon avalanche diode

We present a new compact system for time-domain diffuse optical spectroscopy of highly scattering media operating in the wavelength range from 1100 nm to 1700 nm. So far, this technique has been exploited mostly up to 1100 nm: we extended the spectral range by means of a pulsed supercontinuum light source at a high repetition rate, a prism to spectrally disperse the radiation, and a time-gated InGaAs/InP single-photon avalanche diode working up to 1700 nm. A time-correlated single-photon counting board was used as processing electronics. The system is characterized by linear behavior up to absorption values of about 3.4 cm(-1) where the relative error is 17%. A first measurement performed on lipids is presented: the absorption spectrum shows three major peaks at 1200 nm, 1400 nm, and 1700 nm.

Observation of anisotropic diffusion of light in compacted granular porous materials

By analyzing spatio-temporal characteristics of short optical pulses diffusively transmitted through compacted granular materials, we reveal that powder compaction can give rise to strongly anisotropic light diffusion. Our disclosure represents a revision of the understanding of optics of powder compacts. Routes to material characterization and investigation of compression-induced structural anisotropy are opened, and the falsification of isotropic models have implications for quantitative spectroscopy of powder compacts (e.g., pharmaceutical tablets).