Optical Coefficient Research Articles

Growth, Structure, and Dielectric and Optics Properties of a Novel Optical Crystal with High Optical Nonlinearity

In this study, a new class of organic salt nonlinear optical (NLO) crystals, 2-HDSAI and HDSAI, were synthesized and grown by designing and adjusting the methyl position on the receptor. Single-crystal XRD analysis confirms that the crystal belongs to monoclinic group P21. Among them, the HDSAI crystal has second harmonic generation (SHG) activity, and the maximum powder SHG is about 0.7 times that of OH1. The maximum nonlinear optical coefficient of the crystal is d11 = 62 pm/V @1064 nm by theoretical calculation. The HDSAI single crystal was successfully grown by the slow cooling method. Through the study of the optical and dielectric properties of the crystal, the results showed that the newly designed crystal has high transmittance (>75%) and low dielectric constant (εr ≈ 2). The thermal property test shows that the crystal has high thermal stability.

Large Exciton-Driven Linear and Nonlinear Optical Processes in Monolayers of Nitrogen Arsenide and Nitrogen Antimonide

We demonstrate that atomically thin nitrogen-based binary group V wide band gap indirect semiconductors (β-NX (X = As, Sb)) embody 3-fold valley degenerated band structures and strong linear and nonlinear optical activities. Contrary to NAs, the fundamental optical absorption in NSb is quite resilient toward the temperature variations between 0 and 450 K. In the absence of lattice vibrations, exciton in NSb is, however, less strongly bound (1.30 eV) compared to the tighter case in NAs (1.62 eV), leading to a more delocalized Mott–Wannier-type texture. The inhomogeneous excitonic line widths for both monolayers within these temperatures are within the 100–400 meV range. The most significant nonlinear second harmonic optical coefficients (2ω resonances) in NAs and NSb monolayers are obtained as ∼636 and 270 pm/V at 3.2 eV (387 nm) and 2.6 eV (477 nm), respectively. We also present a detailed analysis of in-plane biaxial strain on these structures and found that tensile strain dynamically stabilizes structures with no loss in absorbance spectra (and does not influence exciton binding energy in NAs). The nonlinear coefficient also significantly improves at the two most important 810 and 1560 nm wavelengths compared to the cases offered by the monolayer transitional metal dichalcogenides. Our analyses originate from a fully ab initio G0W0+Bethe-Salpeter excited-state theory. The temperature-dependent linear absorption spectra are evaluated by including the electron–phonon self-energies, whereas the nonlinear spectra are treated using the modern theory of polarization within the same perturbative approach. Our reference findings provide important advanced characteristics for applications of two-dimensional β-NX (X = As, Sb) materials and should motivate further theoretical and experimental investigations of these interesting materials.

Two-Dimensional Hybrid Perovskitoid Micro/nanosheets: Colorful Ultralong Phosphorescence, Delayed Fluorescence, and Anisotropic Optical Waveguide.

Molecular persistent luminescence, such as room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), have attracted broad attention in the fields of biological imaging, information security, and optoelectronic devices. However, the development of molecular micro/nanostructures combining both RTP and TADF properties is still in an early stage. Herein, a new type of organic metal hybrid perovskitoid (OMHP) two-dimensional (2D) microcrystal has been fabricated through a facile solution method. The long-lived TADF-RTP dual emission can be highly tuned by changing the excitation wavelength, temperature, and decayed time. Moreover, the 2D OMHP microsheet exhibits an asymmetric and anisotropic optical waveguide with low optical loss coefficient, together with extremely high linearly polarized fluorescence-phosphorescence emission (anisotropy = 0.96), which is promising for the development of polarization-sensitive luminescent materials. Therefore, this work not only demonstrates new OMHP showing colorful persistent luminescence under different modes (such as excitation wavelength, temperature, polarization, lifetime, and dimension) but also takes advantage of the 2D micro/nanostructure to provide potential applications as optical logic gates and for delicate multiple information encryption.

Image reconstruction in non-reciprocal broken-ray tomography.

Optical methods of biomedical tomographic imaging are of considerable interest due to their non-invasive nature and sensitivity to physiologically important markers. Similarly to other imaging modalities, optical methods can be enhanced by utilizing extrinsic contrast agents. Typically, these are fluorescent molecules, which can aggregate in regions of interest due to various mechanisms. In the current approaches to imaging, the intrinsic (related to the tissue) and extrinsic (related to the contrast agent) optical parameters are determined separately. This can result in errors, in particular, due to using simplified heuristic models for the spectral dependence of the optical parameters. Recently, we have developed the theory of non-reciprocal broken-ray tomography (NRBRT) for fluorescence imaging of weakly scattering systems. NRBRT enables simultaneous reconstruction of the fluorophore concentration as well as of the intrinsic optical attenuation coefficient at both the excitation and the emission wavelengths. Importantly, no assumption about the spectral dependence of the tissue optical properties is made in NRBRT. In this study, we perform numerical validation of NRBRT under realistic conditions using the Monte Carlo method to generate forward data. We demonstrate that NRBRT can be used for tomographic imaging of samples of up to four scattering lengths in size. The effects of physical characteristics of the detectors such as the area and the acceptance angle are also investigated.

Bessel-Beam Interferometry for Turbidity With Refractive Index, Attenuation Coefficient, and Loss

This letter demonstrates a quasi-Bessel beam optical interferometry and fast Fourier transform (FFT) to estimate turbidity of liquid samples in terms of the refractive index (RI), optical attenuation coefficient, and loss. The device comprises a broadband superluminescent diode (SLD) source and a packaged deep-seated optical fiber negative axicon (DSNA) probe connected to the SLD source guiding light to the liquid sample trapped in between DSNA probe head and a glass slide. It generates a low-coherence interference spectrum as recorded in a spectrometer via an optical circulator. The FFT of the said spectrum gives power spectrum corresponding to the respective interfaces of the trapped sample. The power spectrum is used wisely to estimate turbidity in terms of RI, optical attenuation, and optical loss. The design is able to measure as low as 5 NTU (nephelometric turbidity units). This sensing platform may be useful in environmental protection, pharma, and processing industries.

Investigation of the nonlinear optical rectification coefficient in a multilayered spherical quantum dot

In the framework of the effective mass approximation, we have investigated the effects of hydrostatic pressure, temperature, nonparabolicity of the band, and polaronic mass on nonlinear optical rectification. Furthermore, the optical rectification coefficient's dependency on the inner dot radius and the barrier width is also investigated. The obtained results reveal that geometrical sizes, hydrostatic pressure, and temperature all have a significant impact on the nonlinear optical rectification. Moreover, the combined effects of the conduction band nonparabolicity and polaronic mass produce a redshift of the peak position and a reduction in the resonant peak magnitude.

Depth-resolved extraction of optical attenuation for glaucoma assessment in clinical settings: a pilot study.

We assessed the ability of the optical attenuation coefficient (AC) to detect early-stage glaucoma with two AC estimation algorithms: retinal layer intensity ratio (LIR) and depth-resolved confocal (DRC). We also introduced new depth-dependent AC parameters for retinal nerve fiber layer assessment. Optical coherence tomography B-scans were collected from 44 eyes of age-similar participants with eye health ranging from healthy to severe glaucoma, including glaucoma suspect patients. Mean AC values estimated from the DRC method are comparable to ratio-extracted values (p > 0.5 for all study groups), and the depth-dependent ACDRC parameters enhance the utility of the AC for detection of early-stage glaucoma.

Thermally stable gold nanorod dispersed silicone composite with plasmonic resonance in the optical communication window

Gold nanorods (AuNRs) possess a high optical nonlinear coefficient, ultrafast optical response speed and widely tunable localized surface plasmon resonance (LSPR) wavelength covering the visible and near infrared region. Therefore, they are extensively investigated for many optical applications. However, the poor thermal stability of the AuNRs seriously restricts their practical performance. In addition, for many applications, such as optical communication or laser modulation, AuNRs have to be combined with transparent solids, for example polymers, glass or crystals to make devices. Here, we report on the preparation of 0.23 mg AuNR dispersed methyl silicone resin (MSR) with longitudinal LSPR (L-LSPR) wavelength (1450 nm) in the optical communication window. We found that AuNR-silicone composites possess high thermal stability. After calcination in ambient environment at a temperature of 250 °C for 10 h, the L-LSPR peak of the sample can remain longer than 1380 nm, implying that the NR shape of the Au particles was well maintained. Using the open-aperture Z-scan technique, the nonlinear absorption coefficient of the composites was measured as −11.71 cm GW−1, higher than many nonlinear materials. Thus, the thermally stable AuNR@SiO2-MSR composite with high nonlinearity is promising for practical applications in the optical communication window.

Simultaneous Measurement of Strain and Temperature Based on Dual Cross-Axis Interference Polarization-Maintaining Fiber Interferometer

A dual cross-axis interference polarization-maintaining fiber interferometer (DCAI-PMFI) for simultaneous measurement of strain and temperature is proposed. By using the cross-interference of the fast and slow axes in the polarization-maintaining fiber interferometer with dual axis independent detection, common path dual cross-axis interferometers (DCAIs) are realized. Since the fast and slow axes are different in refractive index and thermo-optic coefficient, the DCAIs have different phase sensitivities to strain and temperature. A sensitivity matrix for simultaneous measurement of temperature and strain is constructed. The temperature phase sensitivities can be adjusted by changing the length ratio of two arms of the DCAI-PMFI to optimize the decoupling performance of temperature and strain. It is found that the temperature sensitivities are opposite signs when the length ratio of two arms is between the ratio of the temperature optical path coefficients of the fast and slow axis and its reciprocal, and the best measurement resolution can be obtained when two arms are equal in length. In the experimental setup of DCAI-PMFI with equal length of two arms, the strain sensitivities are 1.5648 rad/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μϵ</i> and 1.5790 rad/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μϵ</i> respectively at 0.35m strain sensing fiber, the temperature sensitivities are −25.4990 rad/°C and 24.6000 rad/°C respectively at 10m arm length, the resolutions are 2.4 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μϵ</i> in strain and 0.02°C in temperature. The experimental results show that the proposed sensor can solve the crosstalk between strain and temperature of interferometer, and can be applied to various interferometric fiber sensors without changing the topology.

A comprehensive investigation of structural and optical properties of the spray coated Nd-doped ZnO

We report on the synthesis and characterization of Nd3+ doped Zinc oxide (ZnO) thin films fabricated using the chemical spray pyrolysis technique at 723 K. A comprehensive investigation using a combination of experimental techniques is undertaken to unveil the structural, morphological, compositional, electrical, linear, and nonlinear optical characteristics of the Nd-doped (a higher ionic radius dopant) ZnO thin films. The X-ray diffraction reveals a hexagonal wurtzite structure of the polycrystalline deposits with preferred orientation change from (101) to (002) plane at dopant concentrations above 5%. The inclusion of Nd dopants causes the morphology to change from fibrous to needle-like. The oxidation states of the dopant elements were identified by using XPS spectra. The Raman spectroscopy elucidates the deterioration of crystallinity with the increase in the dopant concentrations. The additional absorption band at higher dopant concentrations in the Fourier transform infrared spectra points to a new phase. The optical bandgap and charge carrier concentrations decrease and increase, respectively, with dopant concentrations up to 5 at%, followed by a reversal in the trend at still higher doping. The optical refractive index and extinction coefficient of the doped samples show an increase compared to pure ZnO. The defect-mediated energy transfer from the host ZnO to Nd dopants was identified from photoluminescence spectra. The sign of the third-order nonlinear refractive index changes from negative for pure ZnO to positive with Nd doping.

Depth-resolved visualization and automated quantification of hyperreflective foci on OCT scans using optical attenuation coefficients.

An automated depth-resolved algorithm using optical attenuation coefficients (OACs) was developed to visualize, localize, and quantify hyperreflective foci (HRF) seen on OCT imaging that are associated with macular hyperpigmentation and represent an increased risk of disease progression in age related macular degeneration. To achieve this, we first transformed the OCT scans to linear representation, which were then contrasted by OACs. HRF were visualized and localized within the entire scan by differentiating HRF within the retina from HRF along the retinal pigment epithelium (RPE). The total pigment burden was quantified using the en face sum projection of an OAC slab between the inner limiting membrane (ILM) to Bruch's membrane (BM). The manual total pigment burden measurements were also obtained by combining manual outlines of HRF in the B-scans with the total area of hypotransmission defects outlined on sub-RPE slabs, which was used as the reference to compare with those obtained from the automated algorithm. 6×6 mm swept-source OCT scans were collected from a total of 49 eyes from 42 patients with macular HRF. We demonstrate that the algorithm was able to automatically distinguish between HRF within the retina and HRF along the RPE. In 24 test eyes, the total pigment burden measurements by the automated algorithm were compared with measurements obtained from manual segmentations. A significant correlation was found between the total pigment area measurements from the automated and manual segmentations (P < 0.001). The proposed automated algorithm based on OACs should be useful in studying eye diseases involving HRF.

Optical refraction and absorption spectra in perturbed monolayer PbBiI

We investigate the optical properties of a noncentrosymmetric quantum spin Hall insulator in the presence of electric and magnetic fields. We employ the Kubo formula to engineer the optical redshift and blueshift phenomena through the interplay between the bulk gap, the Rashba-like gap, and the external fields. Intriguingly, for a range of electric (magnetic) fields, the refracted and absorbed lights exhibit direct (indirect) interband optical transitions. Accordingly, for a critical electric field, optical refraction and absorption spectrum decreases gradually and becomes constant, respectively. While for a critical magnetic field, step-like jumps appear in the optical coefficients. All these findings are discussed in the context of symmetry breaking.

Workbench for a Parabolic Trough Solar Collector with a Tracking System.

Solar energy found abundantly in nature is considered a renewable energy source. It is also of great interest as an option for energy generation and CO2 emissions reduction. Several technologies of solar concentrating systems, known internationally as CSP (concentrated solar power), are found in the industrial and scientific environment. One of the most mature and internationally known technologies is the parabolic trough solar collector (PTSC), which has several applications, such as electricity generation, desalination, steam generation, and refrigeration systems, among others. However, more research and development (R&D) has been done to improve its performance, using new materials, absorber tube geometries, solar tracking systems, and work (thermal oils, nanofluids). Thus, the present work describes the development of a low-cost PTSC for academic and research purposes. The PTSC was built with an edge angle of 120°, an opening area of 2.2 m2, and a copper absorber tube of 42 mm in outer diameter without a glass envelope. The gutter structure is composed of wooden sheets cut in a parabolic shape, where a 1.2 mm-thick galvanized steel sheet coated with a reflective film is supported, thus functioning as the reflective surface of the PTSC. The solar tracking system is one of the active types with two axes containing photoresistive sensors, which are used to determine the solar position and electric actuators to correct the positioning of the gutter. The monitoring system was developed through an interactive panel to visualize the operating parameters of the sensing elements, thermocouples that measure the inlet and outlet temperature in the absorber tube, and the flow sensor to measure the flow of the heat transport fluid. Laboratory tests were performed with deionized water as a transport fluid, establishing two testing conditions. The first test condition analyzed the efficiency of the collector at different temperatures. Thus, the inlet temperature varied, between 30 and 70°C, presenting a flow of 0.020 kg/s. The second one evaluated the collector efficiency for different flows, subjecting the collector to flows from 0.002 to 0.030 kg/s. Thus, the proposed collector obtained an efficiency as a function of the temperature represented by the expression η = 0.324–2.47443 c′, where c′ is a parameter that relates the inlet temperature to the ambient temperature as a function of the solar radiation available. Yet, the efficiency in function of the flow became optimal when the flow regime became turbulent. It was concluded that the proposed solar collector obtained lower efficiency when compared with other collectors in the literature, which was assumed to be due to the diffusion losses of the parabolic trough reflector and thermal losses by convection in the parabolic trough absorber tube (optical efficiency, removal factor, and heat loss coefficient).

The study of acrylic bonding gap stress for the JUNO Central Detector

The Central Detector (CD) of Jiangmen Underground Neutrino Observatory (JUNO) has a huge acrylic spherical vessel, which will contain 20000 tons of liquid scintillator, being immersed in pure water, endure about 3 × 107 N of buoyancy. It is composed of 263 pieces of acrylic spherical panels bonded by the mass polymerization. Acrylic stress is critical to the structural safety of the JUNO CD, and it is required not to be greater than 3.5 MPa. The specialized measurement system for acrylic stress of the JUNO CD has been designed and realized, which supports non-destructive and quantitative measurement. The measurement system was applied to the main acrylic spherical panels without bonding gap first to verify stress level, which met JUNO's requirement. The bonding quality of acrylic spherical panels is critical, and the stress of bonding gap is concerned. The stresses of acrylic bonding gaps were measured using acrylic samples to help study the bonding techniques. The stress optical coefficient of acrylic bonding gap was calibrated and different bonding methods were studied and compared. We found that the objective function of numerical fitting for acrylic bonding gap needs modification when the measured optical path difference is large, which was analyzed and discussed in this article.

Ion-irradiation-induced effects on the surface characteristics of Ge–Bi–Se thin films

In this study, fabricated amorphous chalcogenide Ge10Se70Bi20 thin films were irradiated with nitrogen (N+), argon (Ar+), hydrogen (H+) and oxygen (O+) ion beams. The compositions of the pure and irradiated films were investigated using X-ray diffraction, which confirmed the amorphous structures of the pristine and the irradiated Ge10Se70Bi20 thin films. The optical parameters such as optical bandgap, absorption edge, Urbach energy, Tauc parameter and extinction coefficient of the unirradiated and irradiated films were determined using ultraviolet/visible spectroscopy. The energy gap was found to reduce from 1.355 eV for unirradiated Ge10Se70Bi20 to 1.02, 0.73, 0.60 and 0.51 eV after irradiation with nitrogen, argon, hydrogen and oxygen ion beams, respectively. Meanwhile, the band tail of Ge10Se70Bi20, which is 0.12 eV, increased to 0.16, 0.40, 0.45 and 0.48 eV after irradiation with nitrogen, argon, hydrogen and oxygen ion beams, respectively. In particular, the conductivity increased by two orders after the pristine film had been exposed to an oxygen beam. The direct current electrical conductivity of the pristine film increased from 1.5 × 10−7 to 1.4 × 10−5 Ω−1 cm−1 after irradiation with an oxygen ion beam. Furthermore, the activation energy and Mott’s parameters of the original and irradiated Ge10Se70Bi20 films were deduced. The reported modifications of the optical and electrical parameters suggest the use of irradiated Ge10Se70Bi20 thin films in important applications – for example, optical data storage and optoelectronic devices.

Diel changes of the optical backscattering coefficient of oceanic particulate matter determined from diel changes in apparent optical properties: a case study in the Mediterranean Sea (BOUSSOLE site).

Using in situ measurements of radiometric quantities and of the optical backscattering coefficient of particulate matter (bbp) at an oceanic site, we show that diel cycles of bbp are large enough to generate measurable diel variability of the ocean reflectance. This means that biogeochemical quantities such as net phytoplankton primary production, which are derivable from the diel bbp signal, can be potentially derived also from the diel variability of ocean color radiometry (OCR). This is a promising avenue for basin-scale quantification of such quantities because OCR is now performed from geostationary platforms that enable quantification of diel changes in the ocean reflectance over large ocean expanses. To assess the feasibility of this inversion, we applied three numerical inversion algorithms to derive bbp from measured reflectance data. The uncertainty in deriving bbp transfers to the retrieval of its diel cycle, making the performance of the inversion better in the green part of the spectrum (555 nm), with correlation coefficients >0.75 and a variability of 40% between the observed and derived bbp diel changes. While the results are encouraging, they also emphasize the inherent limitation of current inversion algorithms in deriving diel changes of bbp, which essentially stems from the empirical parameterizations that such algorithms include.

Physical properties’ temperature dynamics of GeTe, Ge2Sb2Te5 and Ge2Sb2Se4Te1 phase change materials

The work presents the results of comprehensive studies of the electrical resistivity and optical transmission coefficient temperature dynamics, phase transition heats, the phase composition, and Raman spectra of GeTe, Ge2Sb2Te5 (GST225), and Ge2Sb2Se4Te1 (GSST2241) samples obtained by vacuum thermal deposition. The phase transition from the amorphous to the crystalline state with intense crystallization for GSST2241 at Тα = 595 K which was first determined in our work. It was shown that the stepwise change in the electrical resistivity for a thin-film GeTe was 4.5 orders of magnitude and for GST225 – 5 orders and as for GSST2241 this value reached 6 orders of magnitude, the change in the relative optical transmittance ΔT/T reached almost 100%. The temperature regions with the maximum values of the derivatives of resistivity and optical transmittance are in good agreement with the temperatures of crystallization (phase transitions) determined by the DSC method.

Giant Nonlinear Optical Response of Graphene Oxide Thin Films Under the Photochemical and Photothermal Reduction

AbstractThin graphene oxide (GO) films have been widely explored for their outstanding optical properties, especially their large optical nonlinear coefficients, and high tunability of optical nonlinear coefficients under photoreduction. In this work, an experimental observation of giant nonlinear optical response in GO films under two different photoreduction mechanisms is reported. After ultraviolet (UV) light‐induced photochemical reduction and continuous wave (CW) laser‐induced photothermal reduction, the nonlinear optical response of reduced graphene oxide (rGO) via the Z‐scan method using a femtosecond laser beam at a wavelength of 800 nm is characterized. The rGO exhibits a giant optical nonlinearity, especially two‐photon absorption (TPA). A giant TPA coefficient of rGO on the order of ≈105 cm GW−1 is obtained under two photoreduction mechanisms (photochemistry and photothermal). This is the first time that a large enhancement effect of TPA has been observed in graphene‐based materials, and the TPA coefficient is an order of magnitude higher than the highest value previously reported. The versatile optical nonlinear properties imply a huge potential of the GO film in advanced photonic and optoelectronic devices such as broadband ultrafast optical switching and optical limiting.

A new measurement method for the optical feedback coupling factor and linewidth enhancement factor based on self-mixing interferometry

In this study, a practical and straightforward method is presented for measuring the optical feedback coefficient C and the linewidth enhancement factor α by means of the mode-hopping phenomena in the self-mixing interferometric system. A theoretical analytical model, in combination with the laser energy transition area of the phase jumping points and the C-parameter, is described deeply. In the experiment, a laser self-mixing vibration sensing system is fabricated to further verify this proposed theoretical model and simultaneously obtain the geometric area SR,F under different C values. From the actual measurements, the C-parameter is exactly measured by tracking the change of the geometric area SR,F, over the range of 1.0-4.5. Then, the α-factor of this self-mixing sensor is obtained as 2.41±0.15 with an accuracy of 6%. In addition, the absolute and relative error of the experimental results are less than 0.14 and 7.43%, respectively. In a word, the measured results agree well with the simulated values, of which validates the good applicability and practicability of this measurement method.

Nonlinear optical properties of lithium niobate crystals doped with alkaline earth and rare earth elements

Nonlinear optical properties of lithium niobate crystals doped with alkaline-earth (Zn2+) and rare-earth (Er3+) elements were quantitatively studied from the chemical bond viewpoint of crystal. The different influences of Zn and Er dopants on the dielectric response of LiNbO3 were compared. The value of nonlinear optical second-harmonic generation coefficients was determined mainly by the Li–O bonds both in LiNbO3:Zn and LiNbO3:Er crystals. We observed sharp changes in the second order nonlinear optical response of Zn-doped LiNbO3 single crystals at 1064 nm with increasing Zn concentration in the crystal. The calculated values of the nonlinear optical coefficients for the LiNbO3:Zn crystals exceed those for the LiNbO3:Er crystals. The LiNbO3:Zn crystal with dopant concentration 5.48 mol.% is the most suitable crystal for the second-harmonic conversion.