Low Optical Absorption Coefficient Research Articles

RF oscillator circuit for absorption coefficient measurement of piezoelectric materials.

We have introduced an approach for low optical absorption coefficient measurement of piezoelectric materials based on the piezoelectric resonance laser calorimetry (PRLC) method. An RF oscillator circuit provides a convenient way to track the piezoelectric resonance frequency during laser irradiation. In this work, the optical absorption coefficient of lithium triborate crystal under high-power IR laser irradiation was measured using the oscillator circuit and the conventional PRLC technique.

Precise measurements of the low optical absorption coefficients of massive lithium triborate crystal boules with untreated facets formed during the growth process.

A spatial diagnostic of optical quality of the massive lithium triborate crystal boule with untreated facets was performed by measuring optical absorption coefficients at 1070 and 1908nm wavelengths using the piezoelectric resonance laser calorimetry technique.

Photonic crystal based photoelectrochemical cell for solar fuels

AbstractThe solar fuel production efficiency of photoelectrochemical cells based on TiO2 is primarily limited by the low optical absorption coefficient and concomitant long penetration depth of visible light compared with the minority charge carrier diffusion length. A TiO2 photoelectrode consisting of a slanted conical‐pore photonic crystal immersed in water and integrated on a reflective back‐contact is proposed to reduce the light penetration depth by more than 10 times and branch the electron and hole transports into dimensions matching their diffusion lengths. This brings the photoelectrolytic process into the high‐efficiency operating regime. The maximum achievable photocurrent density for a lightly doped TiO2 with the imaginary part of the dielectric constant 0.01 in the visible range reaches 7.65 mA cm‐2.

Facile preparation of MgAl2O4/CeO2/Mn3O4 heterojunction photocatalyst and enhanced photocatalytic activity

Novel Mn3O4-promoted double p−n junction MgAl2O4/CeO2/Mn3O4 heterojunction photocatalyst was constructed by one-step synthesis method and two-step synthesis method. The X-ray powder diffraction, Fourier transform infrared spectrum, X-ray photoelectron spectroscopy, optical and photoluminescence demonstrated that the MgAl2O4/CeO2/Mn3O4 heterojunction photocatalyst was synthesized by the two-step synthesis method comprehends a high crystallinity, charge carrier migration and separation efficiency, and relatively low optical absorption coefficient. The MgAl2O4/CeO2/Mn3O4 heterojunction photocatalysts were efficiently used as simulated sunlight-driven n-n and p-n double junction photocatalyst for the simultaneous degradation of methylene blue (MB) dye. The continuous double p−n junction MgAl2O4/CeO2/Mn3O4 heterojunctions strengthened the function of single n-n or p-n junction and guided the charge carrier migration and separation direction; thus, the oxidation and reduction reactions occur at the active site of spatial separation and prevent the recombination of electrons and holes. The results suggest that the continuous double p−n junction MgAl2O4/CeO2/Mn3O4 heterojunctions are very promising candidate material for enhancing the photocatalytic activity in the photocatalytic degradation of MB dye.

Acoustic resonance laser calorimetry for measurements of low optical absorption.

A novel method of acoustic resonance laser calorimetry (ARLC) for the determination of low optical absorption coefficients of crystals and glasses is introduced. It is based on measurements of the kinetics of the equivalent temperature of the sample irradiated by the pulse laser radiation. The equivalent temperature of the sample is directly obtained by measuring the frequency changes of its temperature-calibrated acoustic resonances excited by the laser radiation with a corresponding pulse repetition rate. In contrast to the conventional resonant photoacoustic spectroscopy, where the resonance is used for the enhancement of the registered signal, in ARLC, the laser-excited acoustic resonance of the sample acts as a high-sensitivity temperature probe for measurements of its heating kinetics. The ARLC approach was verified by measuring the optical absorption coefficient of the potassium dihydrogen phosphate crystal (α=4.2⋅10-2cm-1) at 1064 nm wavelength.

(Invited) Insights into the UV-C Optical Absorption of AlN Substrates Grown by PVT

High temperature growth of AlN single crystals by physical vapor transport (PVT) was used to produce 2-inch diameter AlN substrates free of macrodefects and with average dislocation densities below 103 cm-2 [1, 2]. In spite of its high structural quality and uniformity, AlN grown by PVT has been characterized by high below-bandgap optical absorption in the ultraviolet C (UV-C) region, due to the presence of several deep-level, impurity-related absorption bands. UV-C light emitting diodes (LEDs) operating in the 260-280 nm range are being actively pursued for applications such as water purification and surface disinfection. These devices typically require light extraction through the substrate, for which high transparency in the emission wavelength range of interest is critical to high device performance. Adoption of AlN substrates for use in UV-C LEDs thus requires low optical absorption coefficients in the 260-280 nm emission range. However, several groups have identified an absorption band around 265 nm, which is associated with the carbon impurity, in these substrates [3]. Reducing this unwanted absorption band is critical to enhancing light extraction in UV-C LEDs grown on AlN substrates. In earlier work, co-doping with silicon was shown to reduce the unwanted 265 nm absorption band [4]. Formation of a carbon-silicon defect complex with an absorption band at 225 nm was demonstrated by optical absorption and photoluminescence measurements together with density functional theory calculations [4]. In this work, we have grown large diameter boules with varying concentrations of impurities in order to demonstrate 2-inch AlN substrates with low 265 nm absorption coefficients. The optical properties of double-side polished, c-plane substrates were measured using a high performance UV/Vis spectrophotometer (PerkinElmer Lambda 850). Transmittance and absolute reflectance spectra were collected using a 150 mm integrating sphere, and absorption coefficients were calculated with an approximation-free method [5] which accurately accounted for all reflectance losses. The absorption coefficients at 265 nm were correlated with impurity concentrations measured by secondary ion mass spectrometry (SIMS) of the three most prevalent electrically-active impurities, carbon, oxygen, and silicon, for various locations throughout the boule volume. Growth and characterization results of UV-C transparent 2-inch AlN substrates grown by PVT will be presented.

Phosphorus treatment to promote crystallinity of the microcrystalline silicon front contact layers for highly efficient heterojunction solar cells

The current loss is mainly due to the reflection and the parasitic absorption in the indium tin oxide (ITO) and amorphous silicon (a-Si:H) in the front side of silicon heterojunction (SHJ) solar cells. In this paper, we implemented n-type hydrogenated microcrystalline silicon oxide (n-μc-SiOx:H) as the front surface field (FSF) to improve the short-circuit current density (JSC) of SHJ solar cells. The advantage of employing n-μc-SiOx:H layer is due to its low optical absorption coefficient and tunable refractive index. However, the introduction of carbon dioxide increases light transmission but reduces the crystallinity of n-μc-SiOx:H layer. Meanwhile, inhibiting the incubation layer and increasing microcrystalline/amorphous mixture phase during the growth are critical to the solar cell performance. Therefore, we implemented a high phosphorus-doping seed layer to form a nucleation layer to improve the crystallinity of n-μc-SiOx:H layer. In addition, the plasma enhanced chemical vapor deposition (PECVD) process parameters of each layer were optimized to obtain good optical and electrical properties of n-μc-SiOx:H layer. Finally, a 242.5 cm2 solar cell had been fabricated with conversion efficiency of 23.87%, open-circuit voltage (VOC) of 739.8 mV, fill factor (FF) of 82.33% and JSC of 39.19 mA/cm2, which was 0.31 mA/cm2 higher than that of the conventional n type a-Si:H SHJ solar cells.

Pulsed laser-induced heating of mineral phases: Implications for laser-induced breakdown spectroscopy combined with Raman spectroscopy

Laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy are complementary techniques providing respectively chemical and structural information on the sample target. These techniques are increasingly used in Earth and Planetary sciences, and often together. LIBS is locally destructive for the target, and the laser-induced effects due to LIBS laser shots on the structure and on the Raman fingerprint of a set of geological samples relevant to Mars exploration are here investigated by Raman spectroscopy and electron microscopy. Experiments show that the structure of samples with low optical absorption coefficients is preserved as well as the structural information carried by Raman spectra. By contrast, minerals with high optical absorption coefficient can be severely affected by LIBS laser shots with local amorphization, melting and/or phase transformation. Thermal modeling shows that the temperature can reach several thousands of degrees at the surface for such samples during a LIBS laser shot, but decreases rapidly with time and in space. In 2020, NASA Mars 2020 mission will send a rover equiped with a combined LIBS/Raman instrument for remote analysis (SuperCam) as well as proximity science instruments at fine scale for X-ray fluorescence called PIXL for Planetary Instrument for X-ray Lithochemistry, and deep UV Raman spectroscopy called SHERLOC for Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals. We discuss the implications of our results for the operation of these instruments and show that (i) the SuperCam analytical footprint for Raman spectroscopy is many times larger than the LIBS crater, minimizing any effects and (ii) SHERLOC and PIXL analysis may be affected if they analyze within a LIBS crater created by SuperCam LIBS.

BaZrSe3: Ab initio study of anion substitution for bandgap tuning in a chalcogenide material

Recently, transition metal perovskite chalcogenide materials have been proposed as possible candidates for solar cell applications. In this work, we provide accurate theoretical calculations for BaZrS3 and two phases of SrZrS3, which have been recently synthesized and their optical properties elaborated. In this study, we consider the substitution of S in BaZrS3 with Se to form BaZrSe3. Evolutionary methods are used to find the optimal structure of this compound, and accurate calculations of its optoelectronic properties are presented. Using phonon frequency calculations and ab initio molecular dynamics, we assess the stability of this compound. We find that BaZrSe3 is likely to be stable under typical conditions, with a low bandgap and high optical absorption coefficients. This suggests that BaZrSe3 could be useful for solar cell applications.

Large Faraday effect of borate glasses with high Tb3+ content prepared by containerless processing

Borate glasses containing a large amount of Tb3+ ions have been prepared by containerless processing. The content of Tb2O3 reached 60 mol%. The glass bearing the highest content of Tb3+ ions showed a large Faraday effect; the Verdet constant was 234 rad/T m. Annealing of the glasses in H2/N2 atmosphere resulted in a low optical absorption coefficient, leading to an extremely large magneto-optical figure of merit that was ∼1.7 times higher than that of Tb3Ga5O12 single crystal.

Nonlinear absorption, nonlinear scattering, and optical limiting properties of carbon nanotube/molybdenum diselenide organic glass

Because MoSe<sub>2</sub> has broadband saturable absorption, and higher nonlinear refractive index. Compared with MoS<sub>2</sub>, thin-layered MoSe<sub>2</sub> possesses very attractive properties, including narrow bandgap, low optical absorption coefficient, and large spin-splitting energy at the top of the valence band. The narrow bandgap and low optical absorption coefficient could make MoSe<sub>2</sub> more applicable than MoS<sub>2</sub>. And the tunable excitation photoelectric effecthas great potential applications in the fields of photoluminescence, phototransistor, solar cells, nonlinear optics and other aspects. However, pure MoSe<sub>2</sub> has high photogenerated recombination rate, thus limiting its applications in some optical fields. By designing nanocomposites of MoSe<sub>2</sub>, the photogenerated recombination rate of these materials can be reduced and their application field can be broadened. In this work, MoSe<sub>2</sub> nanocomposites are prepared by simple methods. The two-dimensional layered MoSe<sub>2</sub> nanosheets are combined with nanorods. By integrating the surface effect, small size effect and interfacial effect of CNT, the optical nonlinearity and optical limiting performance of MoSe<sub>2</sub> composites are improved. The CNT/MoSe<sub>2</sub> composite nanomaterials are first synthesized based on narrower band gap and lower light absorption coefficient of MoS<sub>2</sub> than those of MoSe<sub>2</sub> by growing MoSe<sub>2</sub> nanoparticles on the surface of CNT through a solvothermal method, and then is dispersed in methyl methacrylate (MMA) to prepare an organic glass by a casting method, and the MMA is polymerized into poly (methyl methacrylate) (PMMA). The nonlinear absorption (NLA), nonlinear scattering (NLS) and optical limiting (OL) properties of the CNT/MoSe<sub>2</sub>/PMMA organic glass are studied by the modified Z-scan technique for the first time. The CNT/MoSe<sub>2</sub>/PMMA organic glass exhibits the saturable absorption (SA) and a changeover from SA to reverse saturable absorption by adjusting input energy. The experimental results show that the CNT/MoSe<sub>2</sub>/PMMA plexiglass exhibits better anti-saturation absorption and higher optical limiting properties than MoSe<sub>2</sub>/PMMA and CNT/PMMA plexiglass. Besides, the NLA and OL properties of the CNT/MoSe<sub>2</sub>/PMMA organic glass are enhanced compared with CNT/PMMA and MoSe<sub>2</sub>/PMMA organic glasses, which can be attributed to the existence of the C=C double bonds in CNTs, the layered structure of MoSe<sub>2</sub> nanosheets, and the interfacial charge transfer between CNTs and MoSe<sub>2</sub>. And the results demonstrate that the CNT/MoSe<sub>2</sub>/PMMA organic glass is very promising for optical devices such as optical limiters and mode-locked/<i>Q</i>-switched lasers.

Temperature-dependent birefringence of lithium triborate, LBO in the THz regime

Optical properties of lithium triborate (LBO) in the terahertz regime (0.2–2 THz) were characterized using broadband terahertz time-domain spectroscopy. The frequency dependence of refractive index and absorption coefficient of the LBO crystal was experimentally investigated over the temperature range of 77–297 K, which the experimental results indicated that LBO has very low optical absorption coefficient at terahertz frequencies especially for the beam polarization along the crystal’s principal dielectric axis X. Moreover, a giant birefringence was observed, and the refractive index difference between the axis X and Z gradually decreased with decreasing temperature, which is attributed to the behavior of the TO phonon modes of B1 and B2 symmetries at low frequencies at different temperatures. As potential applications, LBO can be exemplarily used as terahertz wave shapers, beam splitters, terahertz wave plates, circular polarizers and other polarization devices.

Structural, optical and mechanical properties of thin diamond and silicon carbide layers grown by low pressure microwave linear antenna plasma enhanced chemical vapour deposition

In this work, we detail the properties of thin silicon carbide and polycrystalline diamond layers grown by microwave plasma enhanced chemical vapour deposition with linear antenna delivery. Structural, mechanical and optical properties are compared for their potential use as transparent hard coatings. Silicon carbide layers exhibit mechanical properties comparable to thin diamond layers but with a significantly higher adhesion and lower optical absorption coefficient over a wide spectral range.

Voltage-controlled reactive magnetron sputtering of Nb-doped TiO2 films: electrical and optical properties

ABSTRACTNiobium-doped TiO2 films as highly transparent conducting oxidic electrodes were prepared by reactive magnetron sputtering from a titanium target in an argon-oxygen gas flow. As-deposited films were amorphous and exhibited high resistivities ranging from 10 to 1×105 Ω cm in dependence on the deposition parameters. We stabilized the reactive magnetron sputtering deposition by adjusting the magnetron discharge voltage at a constant oxygen gas flow. The precise process control during the preparation of the as-deposited films was essential to gain low resistivities (10-3 Ω cm) and low optical absorption coefficients (α550nm < 2×103 cm-1) after annealing. These polycrystalline TiO2:Nb films on borosilicate glass show a quite high electron concentration > 1×1020 cm-3 and a high carrier mobility (≈ 8 cm2 V-1 s-1).

The influence of multiple optical reflexions on the photoacoustic frequency response

The influence of multiple optical reflexions on optically generated surface temperature variations has scarcely been considered throughout the literature. In this work, a model of optically generated heat sources in the sample with low optical absorption coefficient is derived. Multiple optical reflexions are considered at both surfaces of the sample exposed to the intensity-modulated laser beam. The impacts of the reflexion coefficient, the absorption coefficient and the thickness of the sample on the amplitude characteristics of photothermal responses are discussed.

Reactive magnetron sputtering of Nb-doped TiO2 films: Relationships between structure, composition and electrical properties

Niobium-doped TiO2 films as highly transparent conducting oxides for electrical contacts were investigated. As-deposited films were amorphous and exhibited high resistivities ranging from 10 to 105Ωcm. A slight oxygen deficiency in as-deposited films was essential to gain low resistivities (10−3Ωcm) and low optical absorption coefficients (α550nm<2×103cm−1) in the annealed films. Therefore, we controlled the oxygen stoichiometry during the film deposition by adjusting the magnetron discharge voltage, while the oxygen gas flow was kept constant. The Hall mobility of degenerately doped films (electron concentration>1020cm−3) increased with decreasing substrate temperature owing to metal-like phonon scattering in these samples.

Investigation on growth behavior of multiphase silicon carbon film for front contact layer in a Si thin film solar cell

Boron doped hydrogenated multiphase silicon–carbon (multiphase silicon–carbon) film has been grown by a plasma enhanced chemical vapour deposition (PECVD) method to obtain the properties of high conductivity and a low absorption coefficient. It consists of amorphous carbon, amorphous silicon and a crystalline silicon-like clustering phase. It has the advantage of reducing optical loss due to the wider band gap of amorphous carbon compared to amorphous silicon carbide. The film was fabricated in conditions of low power density with a high hydrogen flow rate to increase the ratio of the amorphous carbon. This result is able to be achieved because the reaction of Si-based and C-based radicals is suppressed by the deposition condition of low electron temperature (Te) of the plasma and the short residence time of the gases. The multiphase silicon–carbon showed high electrical conductivity and a low optical absorption coefficient in the short wavelength region. Applying it for use as a front contact layer in a Si thin film solar cell, it showed an improvement in the conversion efficiency due to the increase in the quantum efficiency in the short wavelength region.

Enhancement of Photoaffinity and Charge Carrier Collection in Mixed Metal Oxides Photo-Anodes Implemented with Plasmonically Active Metal Nps

An effective solar light-driven water splitting in photo-electrochemical devices requires use of materials which combine both high photo-conversion efficiency and long term stability goals. For the latter reason the choice of the suitable systems is restricted to the semiconducting oxides that, in most cases, do not undergo photo-corrosion in aqueous solutions but which are also able to absorb efficiently the visible light. The low optical absorption coefficients near the fundamental band edge, which determine the extent of solar light absorption by many photo-anode materials, are the critical factors for a number of semiconductors characterized by an indirect optical transition. This is also the case of n-type semiconducting WO3 or Fe2O3 responding to the blue, respectively, green part of the solar spectrum for which the optical absorption depths exceed the minority charge carriers collection distances.Therefore, incoupling of light into semiconductor films by scattering from plasmonic nanostructures and/or resonant coupling of the plasmonic near field to the semiconductor, have a potential to improve the effectiveness of the photocurrent spectral response of the employed photoanode. The results regarding the improved light absorption and charge collection in doped and/or mixed semiconducting oxide photo-anodes will be presented.

Growth Mechanism and Characteristics of Semiconductor Nanowires for Photonic Devices

An overview on the growth mechanism of nanowire, fabrication and performance of selenide nanowires devices is presented. Binary, ternary and quaternary selenides have relatively low optical absorption coefficient and have wide transparency range. This combination of properties of selenides makes them very important materials for detector applications. Lead selenide (PbSe) nano particles and nanowires were grown by the physical vapor transport (PVT) method and zinc selenide (ZnSe) nanowire was grown by chemical vapor transport (CVT) methods. Observations on the growth of PbSe indicate that oriented attachment of nanocrystal building blocks produces nanowires. On silicon (111) substrates binding was observed to be on (001) and (111) faces. Zinc selenide nanowires were fabricated into nano Avalanche Photodiodes (Nano-APDs) structure. Experimental results on the fabricated devices of zinc selenide show significant enhancement in photoconductivity. Growth of nanowire and fabrication of devices has progressed significantly and has shown significant improvements in the performance, but fabrication still remains a big challenge.

Band gap and structure of single crystal BiI3: Resolving discrepancies in literature

Bismuth tri-iodide (BiI3) is an intermediate band gap semiconductor with potential for room temperature gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near ultraviolet (UV-Vis) range at room temperature yielded an indirect band gap of 1.67 ± 0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96 ± 0.05 eV and higher energy critical point features. The discrepancy between the UV-Vis and ellipsometry results originates from the low optical absorption coefficients (α ∼ 102 cm−1) of BiI3 that renders reflection-based ellipsometry insensitive to the indirect gap for this material. Further, electronic-structure calculations of the band structure by density functional theory methods are also consistent with the presence of an indirect band gap of 1.55 eV in BiI3. Based on this, an indirect band gap with a value of 1.67 ± 0.09 eV is considered to best represent the band gap structure and value for single crystal BiI3.