Effective Absorption Coefficient Research Articles

Porous nano-grained cuprous selenide (Cu2Se) for mid-infrared photonics

Recently, mid-infrared photonics has attracted tremendous attention for various applications. To address the dilemma of the limited variety of materials for mid-infrared photonics, the mid-infrared optical modulation characteristics of the porous nano-grained cuprous selenide (Cu2Se) were investigated for the first time. The large effective nonlinear absorption coefficient, large modulation depth, and low saturation absorption intensity at 2.3 μm indicate that the porous nano-grained Cu2Se were promising saturable absorbers for the generation of Q-switched pulsed laser in the mid-infrared regime. As an application, the Tm:GdVO4 passively Q-switched laser operating on the 3H4 → 3H5 Tm3+ transition was achieved for the first time by using the prepared porous nano-grained Cu2Se as the saturable absorber. The minimum pulse duration and the maximum repetition rate of the stable Q-switched pulse train were 440 ns and 76.9 kHz, respectively. Our results demonstrated that the porous nano-grained Cu2Se was a promising candidate for the mid-infrared photonic applications.

On the area-averaged effective sound absorption coefficient of porous materials excited by a monopole

The area-averaged effective sound absorption coefficient (SAC) of a rigid-backed homogeneous porous material subjected to a monopole excitation is calculated as the absorbed-to-incident sound power ratio. Using Allard's model to describe the sound propagation above the porous material, an analytical model for this power-based SAC is proposed and proves to give a good approximation of the sound absorption performance under monopole excitation of sufficiently large areas of material. The impact of factors on the power-based SAC, such as monopole height, material radial dimension used to calculate the sound powers, and material properties is discussed. The power-based SAC frequency-dependent behavior is analyzed through sound intensity field assessments at the material surface and is compared to normal incident plane wave and diffuse field SACs. The sound absorption behavior of sound absorbers under monopole excitation exhibits notable distinctions and peculiar results compared to those observed under plane wave and diffuse fields, particularly at low frequencies and for sources close to the material.

Determination of the sound absorption coefficient of ETFE cushions in-situ using the genetic material optimizer in Odeon®

ETFE cushions are used as cladding in predominantly mid to large size architectural projects. Smaller cushions are about 10 m2 in size and the biggest single cushions can be several hundreds of square metres large. They are composed of ETFE membranes a few hundred of micrometres thick. From a point of view of room acoustics, their acoustic transparency, which increases with decreasing frequency, makes them to effectively act like sound absorbers. Their voluminous nature and transparency make it challenging to measure the sound absorption coefficient of ETFE cushions in a laboratory. Using the genetic material optimizer of Odeon® software, the effective sound absorption coefficient of ETFE cushions in four rooms was extracted from room acoustic parameters, which were in turn derived from measured room impulse response data. The results were compared and interpreted by the help of theoretical modelling and measurements on small cushions in a reverberant room. There is a relatively good agreement between the three types of results, possible causes for the differences are explained in the paper.

Modifying optical nonlinearities of ionic liquid crystal glass by adding gold and carbon nanoparticles

Future disruptive photonics technologies depend on the development of new types of nonlinear-optical glass composites containing nanoparticles. In this paper, we report the synthesis, structural characterization, spectral and nonlinear-optical properties of overlooked yet very promising glass nanocomposites made of glass forming ionic liquid crystals (cadmium caprylate) and two types of nanoparticles (gold and carbon). Nonlinear-optical properties of the prepared nanocomposites are studied by means of a standard Z-scan method and using visible (532 nm) nanosecond laser pulses. By exciting glass nanocomposites within their absorption band, an interplay between several nonlinear-optical mechanisms can be revealed. A simultaneous use of two types of nanoparticles (i.e. carbon and gold) enables a better control over effective optical nonlinearities of nanocomposites. More specifically, by changing an intensity of a laser beam, it is possible to vary the magnitude of the effective nonlinear refractive index and of the effective nonlinear absorption coefficient. In addition, even the sign of the effective nonlinear absorption coefficient can be changed. The proposed strategy to create novel nonlinear-optical materials using metal-alkanoates based glass-forming ionic liquid crystals and several types of nanoparticles will benefit rapidly growing nanophotonics and plasmonics technologies.

AlSb/ZrS2 heterojunction: A direct Z-scheme photocatalyst with high solar to hydrogen conversion efficiency and catalytic activity across entire PH range

Using the First principles method, the electrical, optical, and photocatalytic characteristics of AlSb/ZrS2 van der Waals heterojunctions (vdwH) with direct Z-scheme photogenerated electron transfer mechanism were calculated in this study. The inherent type-II band structure, the built-in electric field that facilitates photogenerated carrier separation, and the high mobility of electrons and holes along the x and y axes all contribute to the strong photocatalytic activity. Furthermore, the AlSb/ZrS2 van der Waals heterojunction has better light absorption performance than monolayers, with a light absorption coefficient of 4.7 × 105 cm−1 at 500 nm and a solar to hydrogen efficiency of 25.81%. Meanwhile, the breakdown reaction of heterojunctions for water can be spontaneously reacted under the light, according to the free energy calculations of the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) processes. Unexpectedly, adding biaxial strain to the heterojunction allows for effective band gap and light absorption coefficient adjustments. and the photolysis reaction of water can be realized in the whole PH range within a suitable strain range. The AlSb/ZrS2 van der Waals heterojunction is expected to be a promising candidate for Z-scheme heterojunction photocatalysts for the whole hydrolysis in the future.

Critical review and experimental validation of radiative transfer models for semitransparent media containing large gas bubbles

Many materials manufacturing, photobiochemical, and photoelectrochemical processes involve radiation transfer through foams and bubbly fluids. Controlling and optimizing these processes require accurate predictions of radiation transfer through semitransparent media. Previously developed models treated the heterogeneous media as homogeneous plane-parallel slabs with some effective radiation characteristics while radiation transfer was governed by the one-dimensional radiative transfer equation (RTE). Unfortunately, their accuracy and range of validity remains unclear for lack of comparison with one another or against the same set of experimental data. This study aims to critically review and assess the validity of these models both numerically and experimentally. First, predictions of the transmittance and reflectance of heterogeneous slabs containing large randomly distributed bubbles were compared with those obtained from Monte Carlo ray-tracing (MCRT) method based on geometric optics. A new hybrid model was proposed that predicts the effective scattering coefficient and asymmetry factor using the Lorenz–Mie theory and the effective absorption coefficient as the volume-weighted sum of the bubbles and medium absorption coefficients and solves the RTE using the Monte Carlo method. Its predictions showed excellent agreement with those by the MCRT method for a wide range of bubble volume fractions (0%–30%) as well as slab thicknesses (2–40 mm) and medium absorption coefficients (0.1–100 m−1). Second, microcomputed X-ray tomography scans were performed on a fused silica sample containing bubbles with mean radius of 480 μm to retrieve the exact locations, diameters, and total volume fraction of bubbles. Here also, predictions of the hybrid model using the retrieved data agreed well with experimental measurements of the spectral normal-hemispherical reflectance and transmittance of the sample for wavelengths between 0.4 and 3 μm when silica ranges from weakly absorbing to absorbing.

Study of Effective Atomic Number, Mass Energy Absorption Coefficient and Absorbed Dose Rate in Human Organ and Tissue Substitutes

Abstract: The effective atomic number and mass energy absorption coefficient are two essential parameters used to investigate the radiation response of composite materials of dosimetric interest in many medical applications. The effective atomic number of nine selected human tissues and eleven samples of tissue substitute materials were computed using two methods, the interpolation method and the direct method in the energy range used in brachytherapy applications (0.1-2.0) MeV. The applicability of using the effective atomic number values to investigate scatter and absorption properties of some tissue substitutes, against that of the corresponding human tissues, to validate their tissue-equivalency, is examined. The effect of partial interaction cross sections is also explicitly discussed. Further, the absorbed dose rate, for an isotropic point source, in the selected tissue samples was computed using their estimated mass energy absorption coefficient values. The obtained data are analyzed and differences in sample dose rate relative to water, for photon energies of interest, are evaluated. The results indicate that numbers of tissue substitute samples yield estimates of dose rate to within 5% of dose rate of their corresponding human tissues. The obtained results are expected to be useful in improving dose calculation accuracy in brachytherapy treatment planning or dose evaluation after treatment. Keywords: Dosimetry, Human organs and tissues, Tissue equivalent materials, Equivalent atomic number, Attenuation and absorption coefficients.

NANOCOMPOSITE MATERIALS BASED ON GRAPHENE, GRAPHENE OXIDE, AND SILVER NANOPARTICLES

In this work, plasmon characteristics of nanocomposite materials based on graphene, graphene oxide, and silver nanoparticles have been studied. The effective dielectric constant and absorption coefficient of the nanocomposites based on graphene-silver and graphene oxide – silver depending on the concentration and size of nanoparticles have been calculated. A change in the silver nanoparticles filling factor by 5 percent leads to significant changes in both the real and imaginary parts of the effective dielectric constant of the nanocomposite material. A pronounced absorption peak is observed in the case of graphene-based nanocomposite with a silver filling factor of 0.2. At the same time, the absorption peak can be indicated at a silver filling factor of 0.1 for the graphene oxide-based nanocomposite. The maximum absorption is observed for the nanocomposite material with nanoparticles having a radius of 5 nm in both cases. The researched nanocomposite materials can be successfully used for various organic electronics applications.

一种逐级超分子组装策略用于显著提高簇合物的三阶非线性光学响应

Developing practical optical materials with strong third-order nonlinear optical (NLO) responses attracts increasing attention. Efforts in this area extend to the activation of appropriate materials and the magnification of their low NLO responses. Herein, we propose a stepwise assembly-amplification strategy to enhance the NLO responses of low NLO-active metal sulfide clusters. Through a self-assembly process, W/Cu/S cluster-based tetrahedral and octahedral cages are successfully obtained, both exhibiting weak NLO responses in solution that are not apparent for the unassembled components. Their incorporation into polyvinyl alcohol (PVA), followed by spin-coating, results in the formation of cluster@PVA composite membranes with controlled thicknesses. Remarkably, the effective nonlinear absorption coefficients β for 12-layered membranes are 104 times higher than those of the respective cage solutions. The octahedral cage delivers a stronger response than the tetrahedral cage, which likely reflects the higher degree of aggregation of the clusters. This work showcases a new approach for amplifying NLO responses, greatly facilitating the development of integrated photonic devices.

Bias Reconfigurable Photoresponse of an Infrared Nanocrystal Film Integrated into a Coupled Fabry-Perot Resonator

The coupling of a photonic structure to a nanocrystal film enables light focusing over a distance compatible with the charge diffusion length. This strategy has been implemented in infrared sensors based on nanocrystal films to maximize their absorption. Here, we demonstrate that the photonic structure can be further exploited to generate a bias-reconfigurable photoresponse. To reach this goal, we use a coupled Fabry-Perot structure that can be seen as a periodic grating with two different cavities per period. The two cavities are coupled and a resonance appears, increasing the effective absorption coefficient by a factor of 30 compared to standard interdigitated electrodes. When connected in the appropriate configuration, this device enables the turning-on and -off of part of the spectral response (over a factor of 25 in magnitude) simply by tuning the applied bias (<1 V). Finally, we also demonstrate that this resonator geometry is promising for integration at the focal plane array level since the resonance is maintained for a pixel size as small as 10 μm.

Au–WS2 Nanohybrids with Enhanced Optical Nonlinearity for Optical Limiting Applications

Utilizing plasmonic effects of metal nanostructures on tailoring the properties of semiconducting materials has gained substantial research attention in the past 10 years. Here, we report the plasmon resonance-induced enhanced optical nonlinearity of Au–WS2 nanohybrids prepared via the blending of liquid phase exfoliated few-layer WS2 nanosheets with chemically synthesized spherical gold nanoparticles (NPs). Nonlinear optical properties were studied using the open aperture Z-scan technique with nanosecond laser pulses of wavelength 532 nm, which lie in the plasmonic band of Au NPs (480–550 nm) and B-excitonic transition range of WS2 nanosheets (520–535 nm). Au–WS2 nanohybrids exhibited an enhanced nonlinear absorption and optical limiting activity with an effective nonlinear absorption coefficient of 182 cm/GW at an on-axis input intensity of 0.14 GW/cm2, which is 3.87 times higher than that of bare WS2 nanosheets. The improved NLO response of nanohybrids is due to the plasmon resonance-induced local field effects, which significantly enhance B-excitonic transitions in WS2 nanosheets. The simulated field intensity enhancement profile also confirmed the effect of formation, where a strong interaction of plasmonic fields with WS2 nanosheets is clearly visible. The enhancement observed in simulated absorption density profiles indicated improved B-excitonic transitions in WS2 nanosheets due to the resonant plasmonic field. Au–WS2 nanohybrids exhibited a low limiting threshold of 0.55 J/cm2, which is better than that of many eminent nanohybrids reported so far. The strategy followed in this work for attaining superior nonlinear optical responses in Au–WS2 nanohybrids will be useful to develop novel nonlinear optical materials for photonic device applications.

Third-order optical nonlinearity and passively Q-switching operation with BiOCl nanosheets at 1.3 µm.

In this contribution, we measured the third-order nonlinear optical response of bismuth oxychloride (BiOCl) nanosheets with the open-aperture (OA) and the closed-aperture (CA) Z-scan techniques with a variable excitation intensity at 1.34 µm. The effective nonlinear absorption coefficient βeff and the nonlinear refractive index n2 of the prepared BiOCl nanosheets with abundant oxygen vacancies were obtained under the excitation intensity. The third-order nonlinear optical susceptibility |χ(3)| was 1.64 × 10-9 esu. The nonlinear optical features of BiOCl enabled it as a superb saturable absorber for pulse laser generation. As a consequence, we demonstrated the first passively Q-switched Nd:GdVO4 laser with the BiOCl saturable absorber, producing a shortest pulse duration of 543 ns and a highest repetition rate of 227 kHz, leading to a maximum pulse energy of 74 nJ. Our findings show that BiOCl nanosheets with oxygen vacancies have large nonlinear optical sensitivities and can be exploited to generate optical pulses.

Modulating the nonlinear absorption response of SnOx thin films via phase engineering.

Phase (composition) is known to play a key role in determining the electronic and optical properties of amorphous oxide semiconductors. In this work, modulating the ultrafast nonlinear optical (NLO) response of SnO2 and SnO thin films by tuning oxygen partial pressure during film sputtering is explored. Femtosecond Z-scan results demonstrate that intermediate phases have no profound impact on the two-photon absorption (TPA) response of SnO2 and SnO films. Interestingly, the magnitude of the effective nonlinear absorption coefficient (βeff) of both intermediate SnO2-x and SnOx are enhanced after the change of Sn2+/Sn4+ composition ratio, as measured by picosecond Z-scan technique. Femtosecond degenerate pump-probe measurements show that intermediate phases accelerate the carrier trapping and improve the defect-related carrier absorption in SnOx (SnO-rich) film, while intermediate phase suppress the TPA response of SnO2-x (SnO2-rich) films, therefore carrier-induced absorption dominates the NLO behavior of SnO2-x film on picosecond regime. Our results indicate a simple and effective way to modulate the NLO response of transparent conductive oxide SnO2 and SnO.

Multilayer structures for high-intensity sound energy absorption in low-frequency range

Although small-amplitude sound absorption problems have been comprehensively studied over the past decades, the high-intensity sound absorption problem in the low-frequency range remains unresolved. This paper is concerned with the design of a multilayer structure consisting of a main pore and hierarchical absorption layers to realize perfect absorption of high-intensity sound energy in the low-frequency range. The multilayer structure has enhanced energy dissipation ability in the high-intensity noise excitation environment due to vortex shedding in the sub-slits of the structure. An analytical model based on the equivalent fluid method is established for computing the nonlinear sound absorption coefficient of the structure. Effects of geometrical parameters on the sound absorption coefficient of the multilayer structure are investigated. It is found that under high-intensity sound excitation (e.g., 140 dB), the resistance of the multilayer structure grows along with the sound amplitude and renders a higher absorption coefficient, leading to great sound energy attenuation for high-intensity acoustic waves. The effective sound absorption coefficient of the designed structure is also studied by experiments based on an impedance tube. Noise reduction of a scaled-down payload fairing equipped with multilayer structures is investigated by experiments in a high-intensity sound pressure reverberation chamber in order to confirm the great noise control performance of the proposed structure. The results show that the sound pressure levels inside the fairing can be significantly suppressed by the multilayer structure in broadband, and the maximum reduction of the average sound pressure level is more than 17.5 dB.

Metalated porphyrin-napthalimide based donor-acceptor systems with long-lived triplet states and effective three-photon absorption

• Excited state electron dynamics of novel Metalated porphyrins-napthalimide molecules was investigated by femtosecond transient absorption spectroscopy (fs-TAS). • The rate constants of excited states are estimated using the global analysis of the obtained fs-TAS data. • Long-lived triplet state (lifetime ∼10 μ s for PN-Zn) formed via intersystem crossing. • Open aperture z-scan at non-resonant excitation of 800 nm revealed strong three-photon absorption by these molecules. • The experimental second hyperpolarizability was estimated to be ∼ 10 -31 esu and verified by DFT calculations. Porphyrin applications are primarily related to electronic transitions across a broad spectrum. In this context, understanding the excited state electron dynamics is necessary. Herein, we report the ultrafast photophysical characterization of one free base and three transition metallated porphyrin - napthalimide based donor - acceptor systems ( PN-Fb , PN-Ni , PN-Cu , and PN-Zn ) in dichloromethane (DCM) solution using transient absorption spectroscopy (TAS) in the spectral range of 430-780 nm by the Soret band excitation at 400 nm. The different rate constants were extracted using a target model analysis of the collected transient absorption spectra and established a consistent photophysical model of molecular excited state population relaxations. The photophysical model consisted of the following various processes: (a) internal conversion in the range of 215-400 fs, (b) vibrational relaxation in the range of 1.35-62 ps, and (c) singlet state relaxation times in the range of 0.17-2.06 ns, and finally (d) 0.03-10 μs was attributed to the triplet state lifetimes. We have also measured the nonlinear optical properties of these molecules by the open and closed aperture Z-scan studies, which was carried out by 800 nm, 70 fs photoexcitation. The theoretical fittings to the experimental data resulted in an effective three-photon absorption coefficient and positive nonlinear refractive index. Further, from the Z-scan data, the second-order hyperpolarizability of the molecules was calculated, and the values were 10 - 31 esu. Additionally, complete optimization of individual structures and construction of these PN systems was achieved using density functional theory (DFT) calculations and the obtained results were found to be consistent with the experimental observations.

Entanglement negativity in a fermionic chain with dissipative defects: exact results

We investigate the dynamics of the fermionic logarithmic negativity in a free-fermion chain with a localized loss, which acts as a dissipative impurity. The chain is initially prepared in a generic Fermi sea. In the standard hydrodynamic limit of large subsystems and long times, with their ratio fixed, the negativity between two subsystems is described by a simple formula, which depends only on the effective absorption coefficient of the impurity. The negativity grows linearly at short times, then saturating to a volume-law scaling. Physically, this reflects the continuous production with time of entangling pairs of excitations at the impurity site. Interestingly, the negativity is not the same as the Rényi mutual information with Rényi index , in contrast with the case of unitary dynamics. This reflects the interplay between dissipative and unitary processes. The negativity content of the entangling pairs is obtained in terms of an effective two-state mixed density matrix for the subsystems. Criticality in the initial Fermi sea is reflected in the presence of logarithmic corrections. The prefactor of the logarithmic scaling depends on the loss rate, suggesting a nontrivial interplay between dissipation and criticality.

The Influence of Temperature on Frequency Modulation Spectroscopy in Atom Gravimeter.

Atom gravimeters use locked lasers to manipulate atoms to achieve high-precision gravity measurements. Frequency modulation spectroscopy (FMS) is an accurate method of optical heterodyne spectroscopy, capable of the sensitive and rapid frequency locking of the laser. Because of the effective absorption coefficient, Doppler broadening and susceptibility depend on temperature, and the signal-to-noise ratio (SNR) of the spectroscopy could be affected by temperature. We present a detailed study of the influence of the temperature on FMS in atom gravimeters, and the experimental results show that the SNR of the spectroscopy is dependent on temperature. In this paper, the frequency of the reference laser is locked by tracking the set point of the fringe slope of FMS. The influence of the frequency-locking noise of the reference laser on the sensitivity of the atom gravimeter is investigated by changing the temperature of the Rb cell without extra operations. The method presented here could be useful for improving the sensitivity of quantum sensors that require laser spectroscopic techniques.

Nonlinear optical properties of porous monoclinic BiVO4 at 2.3 µm for the passively Q-switching laser

In this work, we report the nonlinear optical response of porous monoclinic bismuth vanadate (m-BiVO4) at 2.3 µm for the first time. We adopted the conventional open/closed aperture (OA/CA) Z-scan techniques to systematically investigate the nonlinear optical (NLO) behavior. The effective nonlinear absorption coefficient βeff was −6.4 ± 0.02 cm MW−1, the nonlinear refractive index n2 was 9.88 ± 0.08 × 10-1 cm2 GW−1, and the NLO third-order susceptibility |χ(3)| was 2.69 × 10-7 esu. These NLO properties allowed it to be used as a fantastic saturable absorber for the generation of mid-infrared (MIR) pulses. As an application, we implemented a passively Q-switched Tm:YAP laser with the prepared porous m-BiVO4 saturable absorber for the first time. A minimum pulse duration of 349 ns and a maximum pulse repetition frequency of 193 kHz were achieved, resulting in a maximum pulse energy of 1.18 µJ. Our findings suggest that the porous m-BiVO4 could provide a route to the MIR optoelectronic applications.

Intensity-dependent optical nonlinearities of composite materials made of ionic liquid crystal glass and bimetallic nanoparticles

ABSTRACT Although isotropic glass materials are ubiquitous in everyday life, the properties and applications of anisotropic liquid crystal glasses stable at room temperature are still insufficiently explored. In this paper, nonlinear-optical properties of unconventional glass nanocomposites made of mesogenic cadmium octanoate and bimetallic nanoparticles are reported. Two types of bimetallic nanoparticles (Ag/Au core – Au shell and Ag/Au homogeneous alloy) are synthesised utilising an ionic liquid crystal phase of cadmium octanoate as a nanoreactor. A nonlinear-optical characterisation of the studied samples is performed by means of a Z-scan technique and using nanosecond laser pulses at 1064 nm excitation wavelength. The studied nanocomposites exhibit a strong nonlinear-optical response with the magnitude of the third-order nonlinear optical susceptibility within a 2.81 × 10−8 - 2.19 × 10−7 esu range. The measured values of the effective nonlinear refractive indices and nonlinear absorption coefficients depend on the laser beam intensity. The physical mechanisms of the observed intensity-dependent optical nonlinearities are discussed.

The role of defects on the transition from saturable absorption to nonlinear absorption of Bi12GeO20 single crystal under increasing laser excitation

This work reports defect and input intensity dependent nonlinear optical behaviors of Bi12GeO20 (BGO) single crystal. Open aperture (OA) Z-scan experiments were performed with 532 nm excitation wavelength under 4 ns and 100 fs pulsed laser irradiation. Obtained data were fitted with a theoretical model considering one-photon, two-photon and free carrier absorption contributions to nonlinear absorption due to longer lifetime of localized defect states than that of used laser pulse durations. At low input intensities, the BGO single crystal showed saturable absorption (SA) behavior and transition to nonlinear absorption (NA) behavior observed with further increase of the input intensities both of pulse durations. At low input intensity, the OPA mechanism is dominant and results in SA by filling of the defect states due to defect state at around one photon energy (2.32 eV). At higher input intensity, multi-photon, two-photon and free carrier absorption become dominant mechanisms, and nonlinear absorption behavior was observed. The lowest saturation threshold was found as 1.36 × 1010 W/cm2 with nanosecond pulses. We have revealed the mechanisms contributing both SA and NA, and determined saturation intensity threshold and effective nonlinear absorption coefficients. Our findings indicate that the tails of defect states overlap in the energy band gaps especially in sufficiently disordered crystal. With this way, the spectral range for saturable absorption and nonlinear absorption can be broadened.