Path Length Enhancement Research Articles

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.

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.

Study of the Absorption in Solar Cells with 2D Photonic Crystals

In this work, we design via numerical simulation the absorption properties of silicon nanohole arrays of vertically aligned square lattice arrays with lattice constants varying from 300 nm to 400 nm, using the nite di erence time-domain method (FDTD). The silicon (Si) is chosen as prototype material for this study due to better understanding of the material properties and widely available optical data. The parameters for photonic crystals are optimized through computer simulations to obtain the maximum absorption and path length enhancement. We investigated the performance of the considered structure and determined the geometrical parameters that allow a better absorption.

An improved retrieval of tropospheric NO<sub>2</sub> from space over polluted regions using an Earth radiance reference

Abstract. A novel tropospheric NO2 DOAS retrieval algorithm optimised for a nadir-viewing satellite instrument imaging polluted areas is proposed in this work. Current satellite DOAS retrievals have relied on using a solar reference spectrum to derive a total slant column, then using either model assimilation or spatial filtering to derive the tropospheric component. In the ERrs-DOAS (Earth radiance reference sector DOAS) algorithm, tropospheric NO2 slant columns are derived using spectra averaged from measurements over unpolluted regions, thus removing the need for post-hoc separation techniques, though some residual stratospheric biases may still remain. To validate the ERrs-DOAS algorithm, DOAS retrievals were performed on modelled spectra created by the radiative transfer model SCIATRAN, as well as L1B Earth radiance data measured by the NASA/KNMI Ozone Monitoring Instrument (OMI). It was found that retrievals using an Earth radiance reference produce spatial distributions of tropospheric NO2 over eastern China during June 2005 that highly correlate with those derived using existing retrieval algorithms. Comparisons with slant columns retrieved by the operational NO2 retrieval algorithm for OMI (OMNO2A) show that the ERrs-DOAS algorithm greatly reduces the presence of artificial across-track biases (stripes) caused by calibration errors, as well as the removal of path length enhancement in off-nadir pixels. Analysis of Pacific SCDs suggests that the algorithm also produces a 27% reduction in retrieval uncertainty, though this may be partially due to biases introduced by differences in the retrieval algorithm settings. The ERrs-DOAS technique also reveals absorption features over the Sahara and similar regions characteristic of sand and liquid H2O absorption, as first discovered in the analysis of GOME-2 NO2 retrievals.

Broadband light trapping in thin film solar cells with self-organized plasmonic nano-colloids

The intense light scattered from metal nanoparticles sustaining surface plasmons makes them attractive for light trapping in photovoltaic applications. However, a strong resonant response from nanoparticle ensembles can only be obtained if the particles have monodisperse physical properties. Presently, the chemical synthesis of colloidal nanoparticles is the method that produces the highest monodispersion in geometry and material quality, with the added benefits of being low-temperature, low-cost, easily scalable and of allowing control of the surface coverage of the deposited particles. In this paper, novel plasmonic back-reflector structures were developed using spherical gold colloids with appropriate dimensions for pronounced far-field scattering. The plasmonic back reflectors are incorporated in the rear contact of thin film n-i-p nanocrystalline silicon solar cells to boost their photocurrent generation via optical path length enhancement inside the silicon layer. The quantum efficiency spectra of the devices revealed a remarkable broadband enhancement, resulting from both light scattering from the metal nanoparticles and improved light incoupling caused by the hemispherical corrugations at the cells’ front surface formed from the deposition of material over the spherically shaped colloids.

Nanoporous Silicon Produced by Metal-Assisted Etching: A Detailed Investigation of Optical and Contact Properties for Solar Cells

We present a detailed investigation of the optical and contact quality of silicon wafers textured using metal-assisted etching (MAE) to produce a nanoporous silicon (nSi) surface. We show that the solar weighted reflectance ( $R_{{\rm w}}$ ) of the bare optimized MAE nSi structure is only ∼8%, and this is further reduced to 3% with the addition of an Al2O 3/TiO2 stack. We also show that the optical path length enhancement can be >20 near the band edge of Si. The contact resistivity measurements of phosphorus-doped MAE nSi samples show that good contact can be made even on lightly doped samples, and we find no significant difference in contact quality between surfaces with different surface area enlargements (between ∼2.2 and 2.7). This loosens constraints on finger width, helping to achieve reduced shading losses.

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.

Two-tone frequency-modulation spectroscopy in off-axis cavity

As opposed to a conventional optical resonator, an off-axis-aligned cavity is able to transmit without distortion radiation modulated at a frequency even far above the cavity bandpass. This allows us to implement a simple spectroscopic technique that combines the cavity path-length enhancement of integrated cavity output spectroscopy (ICOS) and the noise reduction associated with radio-frequency modulation (FM). An FM-ICOS spectrometer is demonstrated for the first time using a two-tone modulation technique. The performance is compared to the traditional ICOS by examining the acetylene absorption at 1543.77 nm. A signal-to-noise ratio improvement by a factor 3.5 is found with our proof-of-concept setup. Larger improvements are expected in a more optimized setup.

COMPARISON BETWEEN PATH LENGTHS TRAVELED BY SOLAR ELECTRONS AND IONS IN GROUND-LEVEL ENHANCEMENT EVENTS

We have examined the Wind/3DP/SST electron and Wind/EPACT/LEMT ion data to investigate the path length difference between solar electrons and ions in the ground-level enhancement (GLE) events in solar cycle 23. Assuming that the onset time of metric type II or decameter-hectometric (DH) type III radio bursts is the solar release time of non-relativistic electrons, we have found that within an error range of plus or minus 10% the deduced path length of low-energy (approximately 27 keV) electrons from their release site near the Sun to the 1 AU observer is consistent with the ion path length deduced by Reames from the onset time analysis. In addition, the solar longitude distribution and IMF topology of the GLE events examined are in favor of the coronal mass ejection-driven shock acceleration origin of observed non-relativistic electrons.We have also found an increase of electron path lengths with increasing electron energies. The increasing rate of path lengths is correlated with the pitch angle distribution (PAD) of peak electron intensities locally measured, with a higher rate corresponding to a broader PAD. The correlation indicates that the path length enhancement is due to the interplanetary scattering experienced by first arriving electrons. The observed path length consistency implies that the maximum stable time of magnetic flux tubes, along which particles transport, could reach 4.8 hr.

Intracavity trace molecular detection with a broadband mid-IR frequency comb source

Ultrasensitive detection of methane, isotopic carbon dioxide, carbon monoxide, formaldehyde, acetylene and ethylene is performed in the spectral range 2.5 - 5 \mu m using intracavity spectroscopy in broadband optical parametric oscillators (OPOs). The OPOs were operated near degeneracy and synchronously pumped either by a mode-locked erbium (1560 nm) or thulium (2050 nm) fiber laser. A large instantaneous bandwidth of up to 800 cm$^{-1}$ allows for simultaneous detection of several gases. We observe an effective path length enhancement due to coherent interaction inside the OPO cavity and achieve part-per-billion sensitivity levels. The measured spectral shapes are in good agreement with a model that takes into account group delay dispersion across the broad OPO frequency band.

Enhanced light trapping in the silicon substrate with plasmonic Ag nanocones

Ag nanocone enhanced light trapping in the silicon substrate is numerically investigated. For a wide range of the dielectric spacer thickness, the normalized scattering cross section of the rear located particles is higher than that of the front located particles, which is contrary to previous reports. This design not only avoids the conflict with the detrimental Fano effect but is also beneficial to the rear located particles. The fraction of the incident light scattered into silicon is calculated. The path length enhancement is assessed. The Ag nanocone shows highly competitive light-trapping potential.

Ultralight-Trapping Filters With Volume Reflection Holograms

Light trapping is a useful approach for increasing the absorption of thin film photovoltaic (PV) cells. Simple light trapping can be achieved by incorporating a scattering layer on the top and bottom surface of cells and can increase absorption by a factor of 4n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Recently, ultralight trapping using Rugate and 1-D photonic bandgap filters have been proposed to increase light trapping by a factor of 4n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /sin <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> θ, where θ is half of the acceptance angle. In this paper, we present the design of a holographic ultralight trapping filter. The holographic filter can be implemented in large areas at a low cost, which makes it scalable for PV applications. A design is presented that increases the optical path length for near bandgap wavelengths in a thin-film silicon PV cell. The optical path length enhancement is converted to electrical output using the PC-1D simulation software. The short-circuit current for a 10-μm-thick silicon PV cell increases by nearly 14.7% relative to a cell without light trapping.

Multimode diode laser correlation spectroscopy using gas-filled porous materials for pathlength enhancement

A novel pathlength enhancement approach has been applied in multimode diode laser correlation spectroscopy measurements of gas concentrations. Reference and sample gas cells made of porous polystyrene foam and alumina ceramic, respectively, were employed in proof-of-principle measurements of molecular oxygen. Equivalent pathlengths of 164 and 52 cm were obtained for the reference and sample cells with physical pathlengths of 4 and 1 cm, respectively. With a measurement time of 60 s, a physical-pathlength-integrated sensitivity of 15 ppm⋅m was achieved with an enhancement of more than one order of magnitude compared with an open-air setup. Practical application aspects were investigated including gas cell size and light alignment tolerance. This generic approach for pathlength enhancement provides advantages in terms of compactness and robustness.

Broadband laser enhanced dual-beam interferometry

We demonstrate a dual-beam, balanced detector approach, compatible with commercial Fourier transform infrared spectrometers that provide a single modulated output. Implemented with a near-IR mode-locked fiber laser source and an external broadband polarizing beamsplitter, the dual-beam method provides relative intensity noise reduction and real-time baseline drift cancellation. Noise levels within a factor of three above the shot noise limit (using 0.6 mW of optical power) are demonstrated for the weak second overtone of CO. The method should be particularly well suited for applications like broadband spectroscopy using a large fraction of the supercontinuum generated in a highly nonlinear fiber, and attenuated reflection spectroscopy, for which extreme pathlength enhancement is challenging.

Cavity-Enhanced Parity-Nonconserving Optical Rotation in Metastable Xe and Hg

We propose the measurement of cavity-enhanced parity-nonconserving (PNC) optical rotation in several transitions of metastable Xe and Hg, including Xe (2P(3/2)(o))6s(2)[3/2](2)(o)→(2P(1/2)(o))6s(2)[1/2](1)(o) and Hg 6s6p (3)P(2)(o)→6s6p (1)P(1)(o), with calculated amplitude ratios of E(1)(PNC)/M1=11×10(-8) and 10×10(-8), respectively. We demonstrate the use of a high-finesse bow-tie cavity with counterpropagating beams and a longitudinal magnetic field, which allows the absolute measurement of chiral optical rotation, with a path length enhancement of about 10(4), necessary for PNC measurement from available column densities of 10(14) cm(-2) for metastable Xe or Hg. Rapid PNC-signal reversal, allowing robust background subtraction, is achieved by shifting the cavity resonance to an opposite polarization mode or by inverting the magnetic field. The precise measurement of isotope and nuclear-spin dependent E(1)(PNC) amplitudes provides a sensitive low-energy test of the standard model.

Diffractive Backside Structures via Nanoimprint Lithography

For decreasing thicknesses of wafer based silicon solar cells, photon management structures to maintain high quantum efficiencies will gain importance. Diffractive gratings on the wafer back side can be designed to achieve very high path length enhancements, especially for weakly absorbed infrared radiation. This technologically demanding concept has to be realised using processes with upscaling potential. Therefore, we present a fabrication process for producing photonic structures in silicon based on interference lithography and nanoimprint lithography (NIL).We realised linear as well as crossed gratings of different depths, which were etched into the wafer back side. Polarisation dependent reflection measurements were made to get information about potential absorption enhancement as well as the occurrence of parasitic absorption in the metal reflector. This is conducted for a PECVD silicon oxide buffer layer between grating and reflector as well as a spin coated silicon oxide layer. Besides these optical characterisations, we further investigated the electrical properties of the back surface, where we applied a concept in which electrical and optical properties are decoupled. This is realised by a layer stack on the wafer back side, consisting of a thin Al2O3 passivation and a doped amorphous silicon layer.

Disordered, Strongly Scattering Porous Materials as Miniature Multipass Gas Cells

We investigate the interaction of light and gas in strongly scattering nano- and macroporous media. Manufacturing and structural characterization of ZrO(2), Al(2)O(3) and TiO(2) ceramics with different pore sizes, measurements of optical properties using photon time-of-flight spectroscopy, and high-resolution laser spectroscopy of O(2) at 760 nm are reported. We show that extreme light scattering can be utilized to realize miniature spectroscopic gas cells. Path length enhancement factors up to 750 are reached (5.4 m path through gas for light transmitted through a 7 mm ZrO(2) with 49% porosity and 115 nm pores).

Measuring Photochemical Kinetics in Submonolayer Films by Transient ATR Spectroscopy on a Multimode Planar Waveguide

Understanding the kinetics of reactions in molecular thin films can aid in the molecular engineering of organic photovoltaics and biosensors. We have coupled two analytical methods, transient absorbance spectroscopy (TAS) and attenuated total reflectance (ATR), in a relatively simple arrangement when compared with previous TAS/ATR instruments to interrogate molecular structure and photochemistry at interfaces. The multimode planar waveguide geometry provides a significant path length enhancement relative to a conventional transmission geometry, making it feasible to perform measurements on low-surface-coverage films. The performance of the instrument was assessed using a thin film composed of purple membrane (PM) fragments containing bacteriorhodopsin deposited onto PDAC, a positively charged polymer. The surface coverage of retinal chromophore in this film is ∼0.1 monolayer and its orientation distribution is anisotropic, with a mean tilt angle of 68° from surface normal. After photoinduced formation of the transient M state, the chromophore decays to the ground state in 4.4 ± 0.6 ms, equivalent to the decay of suspended PM fragments, which shows that deposition on PDAC does not alter M-state photokinetics. The surface coverage of the M state is calculated to be 2 pmol/cm(2), which is ∼1% of a close-packed monolayer. This work demonstrates that TAS/ATR can be used to probe structure and photochemical kinetics in molecular films at extremely low surface coverages.

How front side plasmonic nanostructures enhance solar cell efficiency

Dielectric roughness on the front surface enhances significantly solar cell efficiency by light trapping in the absorbing layer. However in plasmonic assisted thin silicon solar cells we show, by a detailed analysis of the various mechanisms, that front-surface plasmonic structures enhance the efficiency by a different mechanism—namely the effective broadband forward scattering into the silicon, while trapping (path length enhancement) is relatively small for these structures. The plasmonic local field enhancement contribution is even smaller in this configuration. Based on our study we optimized this “anti-reflection mechanism” by tuning the plasmonic structure such that the spectral location of the more efficiently scattering dipole and quadrupole resonances fit correctly the visible and NIR sun spectrum.