Optical Characterization Methods Research Articles

Determination of Optical and Structural Parameters of Thin Films with Differently Rough Boundaries

The optical characterization of non-absorbing, homogeneous, isotropic polymer-like thin films with correlated, differently rough boundaries is essential in optimizing their performance in various applications. A central aim of this study is to derive the general formulae necessary for the characterization of such films. The applicability of this theory is illustrated through the characterization of a polymer-like thin film deposited by plasma-enhanced chemical vapor deposition onto a silicon substrate with a randomly rough surface, focusing on the analysis of its rough boundaries over a wide range of spatial frequencies. The method is based on processing experimental data obtained using variable-angle spectroscopic ellipsometry and spectroscopic reflectometry. The transition layer is considered at the lower boundary of the polymer-like thin film. The spectral dependencies of the optical constants of the polymer-like thin film and the transition layer are determined using the Campi–Coriasso dispersion model. The reflectance data are processed using a combination of Rayleigh–Rice theory and scalar diffraction theory in the near-infrared and visible spectral ranges, while scalar diffraction theory is used for the processing of reflectance data within the ultraviolet range. Rayleigh–Rice theory alone is sufficient for the processing of the ellipsometric data across the entire spectral range. We accurately determine the thicknesses of the polymer-like thin film and the transition layer, as well as the roughness parameters of both boundaries, with the root mean square (rms) values cross-validated using atomic force microscopy. Notably, the rms values derived from optical measurements and atomic force microscopy show excellent agreement. These findings confirm the reliability of the optical method for the detailed characterization of thin films with differently rough boundaries, supporting the applicability of the proposed method in high-precision film analysis.

Comparative analysis of optical gating time characterization methods for ultrafast gated image intensifiers with sub-nanosecond temporal resolution

Ultrafast gated image intensifiers are crucial for capturing high-resolution images with sub-nanosecond temporal resolution. This study compares two methods for characterizing optical gating time in these intensifiers: Method I, involving a “laser pulse walkthrough,” and Method II, utilizing a fiber optic delay array. Both methods were applied to the same intensifier, with results analyzed for consistency and discrepancies. The findings indicate that both methods yield consistent optical gating times within their standard deviations range, though Method II exhibited greater dispersion at the photocathode edges due to spatial non-uniformity. The study further reveals that while the optical gating time distribution across the photocathode is uniformly consistent, a constant delay exists between different regions. To enhance measurement precision it is recommended to subdivide the photocathode for individual calibration of gating time and delay. These insights are pivotal for improving the precision and reliability of optical gating time measurements in applications requiring sub-nanosecond or even picosecond temporal resolution.

Simulations of the optical diffraction patterns produced by the pressure field of a clinical shock wave source

Today, shock waves are used to treat a wide variety of ailments. Consequently, there is a need to develop efficient methodologies for comparing and evaluating the pressure fields generated by different equipment. Hydrophones are commonly utilized for accurate pressure measurements although they can be damaged by pitting due to acoustic cavitation. Furthermore, the range of measurement is limited by the position of the device. Optical methods have also been proposed since the presence of a disturbing device in the wave propagation medium is not necessary, and they provide a broader registering field. Nevertheless, these methods do not provide accurate measurements compared with those obtained with polyvinylidene difluoride or fiber-optic hydrophones. Herein, an optical method for shock wave characterization based on diffraction analysis, that can lead to more precise results, is proposed. The phase fluctuations of a light wave produced when it traverses the shock wave pressure field are calculated. The diffraction patterns produced by this perturbed wave at an observation plane at different propagation distances are presented. Considering the state of the art of high-speed cameras, we conclude that an experimental setup, based on the results reported here, can contribute to the evaluation and comparison of shock wave generators for medical applications.

(Keynote) Automated / Autonomous Platforms for Synthesis, Purification, and Characterization of Quantum Dots

Controlling composition, structure, and thus properties of nanomaterials continues to be of importance to numerous fields. For example, applications of quantum dots range from displays and bioimaging, to metal nanoparticles for (electro-) catalytic conversions. The vastness of synthesis parameter space, in combination with still limited understanding of the nucleation and growth mechanisms for many quantum dots systems still hampers progress in identifying nanomaterials with desired properties for existing and newly envisioned purposes. Furthermore, the identification of optimal synthesis recipes for these nanostructures remains a major hurdle.This presentation will report on the development and application a number of enabling capabilities for the synthesis and characterization of quantum dots, achieved through reactor engineering. We demonstrate how the use of automated flow reactors not only helps in run-to-run reproducibility but also in uncovering mechanistic information. Examples include reactors with dedicated nucleation, growth, and shell formation zones, and investigation that revealed mechanistic insight of how water concentration affects QD synthesis outcome. We also employed an autonomous flow reactor system to map synthesis parameter space of specific QD systems, a project that relied on fast, fully automated in-situ characterization using UV-vis or photo-luminescence in combination with multi-step machine learning workflows The utility of flow reactors, however, is limited when considering chemistries with longer reaction times. Hence, more recently we developed a fully automated batch reactor for parameter space mapping of QD synthesis via hot injection, the most frequently used method in both QD research and production at scale. This batch platform is being augmented with purification capabilities, to address some of the challenges of in-line, in-situ characterization of raw reaction mixtures, and to enable using advanced optical and structural characterization methods to provide insight into the actual structure of the QD materials.

Comparative study of laser-induced graphene fabricated in atmospheric and interface-confined environments

Laser-induced graphene (LIG) has found widespread applications in various carbon-based electronics, such as sensors, optoelectronics, and energy storage devices, due to its low cost, high efficiency, and large-scale fabrication potential. Here, two laser-induced carbonization techniques are investigated in terms of structural integrity (uniformity and densification) and electrical property by a combined experimental and theoretical approach; these techniques, namely interface and surface ablation, operate in confined and ambient environments, respectively. The property of LIG for each laser-induced carbonization technique is systematically studied using optical and electrical characterization methods. The results show that the naturally confined and oxygen-free environment of interface ablation can fabricate more compact and intact LIG than surface ablation in an open and oxygen environment, thus enabling a 7.6 times reduction of the square/sheet resistance. The underlying mechanism for these improved properties is the compressive thermal stress and higher carbonization efficiency of interface ablation. The better structural integrity and electrical property of interface-ablated graphene enable a four-fold improvement in the specific areal capacitance of supercapacitors. These findings present that interface ablation is a more effective and facile method to enable the scalable preparation of intact and highly conductive LIG for its potential use in high-performance carbon-based electronics.

Optical and electrical evaluation methods of plasma-induced damage in InP substrates

Indium phosphide (InP) has been focused on as one of the emerging materials that can be implemented in advanced semiconductor devices. We proposed optical and electrical characterization methods to evaluate plasma-induced physical damage (PPD)—ion bombardment damage—to InP substrates. By introducing a native oxide phase in an interfacial layer, we proposed an optical model of the damaged structure applicable for in-line monitoring by spectroscopic ellipsometry. Gas species dependence was obtained, which suggested that the H2 plasma exposure formed a thicker damaged layer than Ar. Impedance spectroscopy (IS) under various biases (V b) was implemented to reveal the nature of damaged structures. Capacitive and conductive components assigned by the IS were confirmed to depend on incident species from plasma, indicating the difference in the energy profile of created defects. The presented methods are useful to characterize and control PPD in designing future high-performance InP-based devices.

Characterizing MEMS Switch Reliability for Cryogenic Applications such as Quantum Computing

Micro Electromechanical Systems (MEMS) switches offer many advantages over conventional larger switches. One potential application we are exploring is the use of commercial radio frequency (RF) MEMS switches for quantum computing applications. However, it is well-documented that cryogenic temperatures can cause mechanical reliability issues for MEMS. Furthermore, commercial RF MEMS switches are designed for room temperature operation, thus their reliability at cryogenic temperatures is unknown. Commercial MEMS switches are also packaged inside sealed housings, which prevent the use of optical methods for characterization and inspection. We are therefore developing test methods to evaluate the reliability of commercial RF MEMS switches at cryogenic temperatures. We describe our test methods and preliminary reliability test data from room temperature down to 55 K.

Multispectral Holographic Intensity and Phase Imaging of Semitransparent Ultrathin Films.

In this paper, we demonstrate a novel optical characterization method for ultrathin semitransparent and absorbing materials through multispectral intensity and phase imaging. The method is based on a lateral-shearing interferometric microscopy (LIM) technique, where phase-shifting allows extraction of both the intensity and the phase of transmitted optical fields. To demonstrate the performance in characterizing semitransparent thin films, we fabricated and measured cupric oxide (CuO) seeded gold ultrathin metal films (UTMFs) with mass-equivalent thicknesses from 2 to 27 nm on fused silica substrates. The optical properties were modeled using multilayer thin film interference and a parametric model of their complex refractive indices. The UTMF samples were imaged in the spectral range from 475 to 750 nm using the proposed LIM technique, and the model parameters were fitted to the measured data in order to determine the respective complex refractive indices for varying thicknesses. Overall, by using the combined intensity and phase not only for imaging and quality control but also for determining the material properties, such as complex refractive indices, this technique demonstrates a high potential for the characterization of the optical properties, of (semi-) transparent thin films.

A Physical Method for Optical Characterization of Pollution in Industrial Wastewater Ponds Using Imaging Spectroscopy

A Physical Method for Optical Characterization of Pollution in Industrial Wastewater Ponds Using Imaging Spectroscopy

Simple method for optical characterization of surface agents on conjugated silver nanoparticles

In this study, a simple formula has been proposed to calculate the refractive index of surface agents of silver nanoparticles (Ag NPs) by using the plasmonic peak of the absorption spectra of dispersed Ag nano-colloids. The basis of the study is the shift in the localized surface plasmon resonance (LSPR) of Ag NPs upon alteration of surface agents. The color changes in a typical metal nano-colloid are mainly due to the shift in the LSPR, which is caused because of electrical interactions of surface agents on the particles. There are some theoretical models to simulate the absorption spectrum, but using these methods to evaluate the plasmonic peak is not facile for a wide range of users. Here, the required simulations were performed for different values of the refractive index of surface agents and particle sizes, and the absorption spectrum and dispersive curves were accordingly plotted. A simple formula was obtained between the wavelength of the plasmonic peak, the refractive index of the shell of surface agents, and the ratio of the hydrodynamic diameter to Feret size of the particles (R0). The refractive index of the surface agents can be calculated by n2=λmax−(202.3R02−748.6R0+947.4)−152.2R02+561.3R0−405.1, where λmax (in nanometer) is the wavelength of the absorption peak due to LSPR. This method can pave the way for experimenters to obtain the refractive index and consequently the type of surface agents around Ag NPs without the need for numerical or mathematical operations. It can also be useful in analyzing the spectral diagnosis of biological agents such as viral antibodies and antigens.

Antibacterial and Photocatalytic Activity of ZnO/Au and ZnO/Ag Nanocomposites

The use of a combination of nanoparticles as antimicrobial agents can be one strategy to overcome the tendency of microbes to become resistant to antibiotic action. Also, the optimization of nano-photocatalysts to efficiently remove persistent pollutants from wastewater is a hot topic. In this study, two composites ZnO/Au (1% wt.) and ZnO/Ag (1% wt.) were synthesized by simple aqueous solution methods. The structure and morphology of the r nanocomposites were analyzed by structural and optical characterization methods. The formation of AuNPs and AgNPs in these experiments was also discussed. The antimicrobial properties of ZnO, ZnO/Au, and ZnO/Ag nanomaterials were investigated against Gram-negative bacteria (Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus). The results showed an increase of 80% in the antimicrobial activity of ZnO/Au against Pseudomonas aeruginosa compared with 30% in the case of ZnO/Ag. Similarly, in the case of the S. aureus strain tests, ZnO/Au increased the antimicrobial activity by 55% and ZnO/Ag by 33%. The photocatalytic tests indicated an improvement in the photocatalytic degradation of methylene blue (MB) under UV irradiation using ZnO/Au and ZnO/Ag nanocomposites compared to bare ZnO. The photocatalytic degradation efficiency of ZnO after 60 min of UV irradiation was ∼83%, while the addition of AuNPs enhanced the degradation rate to ∼95% (ZP2), and AgNP presence enhanced the efficiency to ∼98%. The introduction of noble metallic nanoparticles into the ZnO matrix proved to be an effective strategy to increase their antimicrobial activity against P. aeruginosa and S. aureus, and their photocatalytic activity was evaluated through the degradation of MB dye. Comparing the enhancing effects of Au and Ag, it was found that ZnO/Au was a better antimicrobial agent while ZnO/Ag was a more effective photocatalyst under UV irradiation.

Research on Similarity Simulation Experiment of Mine Pressure Appearance in Surface Gully Working Face Based on BOTDA.

In order to study the mountain deflection characteristics and the pressure law of the working face after the mining of a shallow coal seam under the valley terrain, a geometric size of 5.0 × 0.2 × 1.33 m is used in the physical similarity model. Brillouin optical time domain analysis (BOTDA) technology is applied to a similar physical model experiment to monitor the internal strain of the overlying rock. In this paper, the strain law of the horizontal optical fiber at different stages of the instability of the mountain structure is analyzed. Combined with the measurement of the strain field on the surface of the model via digital image correlation (DIC) technology, the optical fiber strain characteristics of the precursor of mountain instability are given. The optical fiber characterization method of working face pressure is proposed, and the working face pressures at different mining stages in gully terrain are characterized. Finally, the relationship between the deflection instability of the mountain and the strong ground pressure on the working face is discussed. The sudden increase in the strain peak point of the horizontally distributed optical fiber strain curve can be used to distinguish the strong ground pressure. At the same time, this conclusion is verified by comparing the measured underground ground pressure values. The research results can promote the application of optical fiber sensing technology in the field of mine engineering.

Cylindrical Multimode Waveguides as Focusing Interferometric Systems.

Delivery and focusing of radiation requires a variety of optical elements such as waveguides and mirrors or lenses. Heretofore, they were used separately, the former for radiation delivery, the latter for focusing. Here, we show that cylindrical multimode waveguides can both deliver and simultaneously focus radiation, without any external lenses or parabolic mirrors. We develop an analytical, ray-optical model to describe radiation propagation within and after the end of cylindrical multimode waveguides and demonstrate the focusing effect theoretically and experimentally at terahertz frequencies. In the focused spot, located at a distance of several millimeters to a few centimeters away from the waveguide end, typical for focal lengths in optical setups, we achieve a more than 8.4× higher intensity than the cross-sectional average intensity and compress the half-maximum spot area of the incident beam by a factor of >15. Our results represent the first practical realization of a focusing system consisting of only a single cylindrical multimode waveguide, that delivers radiation from one focused spot into another focused spot in free space, with focal distances that are much larger than both the radiation wavelength and the waveguide radius. The results enable design and optimization of cylindrical waveguide-containing systems and demonstrate a precise optical characterization method for cylindrical structures and objects.

Simple Method for Optical Detection and Characterization of Surface Agents on Conjugated Gold Nanoparticles.

In this article, we propose a simple method to calculate electrical permittivity and refractive index of surface agents of gold nanoparticles (Au NPs), in which it is possible to find the refractive index of surface agents shell by using the absorption peak of the gold nano-colloid. One of the usual tests for detection of surface agents is colorimetric methods based on the change of color of Au NPs. The color change is mainly due to the shift of localized surface plasmon resonance which is related to electrical interactions of surface agents. Although there are many mathematical models for simulating the absorption spectrum and calculating the plasmonic peak, using them is not simple and possible for everyone due to the need for programming. Here, the necessary simulations have been performed for different values ​​of refractive index of surface agents and particle size, and absorption peaks have been obtained. Using numerical methods, a simple formula is obtained between the wavelength of plasmonic peak, the ratio of hydrodynamic diameter to Feret size of the particles, and the refractive index of the surface agents. This method can help researchers to obtain the refractive index and consequently the type or concentration of surface agents around Au NPs without the need for programming or complex mathematical operations. It can also open new horizons in analyzing colorimetric diagnosis of biological agents such as viral antibodies, antigens, and other biological agents.

Dipole doping effect in MoS2 field effect transistors based on phase transition of ferroelectric polymer dopant

Molybdenum disulfide (MoS2) has great potential for next-generation electronic devices. On the other hand, stable doping methods are required to adjust its physical properties so MoS2 can be utilized in practical applications, such as transistors and photodetectors. On the other hand, a conventional doping method based on ion implantation is incompatible with 2D MoS2 because of the damage to the lattice structures of MoS2. This paper presents an n-type doping method for MoS2 field-effect transistors (FETs) using a poly (vinylidene fluoride-co-trifluoroethylene) (P (VDF-TrFE)) and polar polymer. The dipole moment of P (VDF-TrFE) provides n-type doping on MoS2 FETs. The polar phase formation in dopant films enhances the doping effects, and the relationship between phase transition and n-type doping states was investigated using optical and electrical characterization methods. Under the optimal doping conditions, the doped MoS2 FET achieved an improved field effect mobility of 34.4 cm2 V−1s–1, a negative shift in the threshold voltage by −25.6 V, and a high on-current of 21 μA compared to the pristine MoS2 FET.

Recombination processes in MBE grown Al0.85Ga0.15As0.56Sb0.44

Quaternary AlGaAsSb alloys have exhibited low excess noise characteristics as gain regions in avalanche photodiodes. In this work, optical spectroscopy techniques are used to demonstrate the recombination dynamics in molecular beam epitaxy grown Al0.85Ga0.15As0.56Sb0.44 with temperature variation. Photoluminescence (PL) measurements at 8–50 K show that the bandgap varies from 1.547 to 1.527 eV. The radiative recombination processes in the alloy were found to be dictated by the complexities of antimony (Sb) incorporation during the growth. Time-resolved PL (TRPL) measurements show a change in initial carrier lifetimes of ∼3.5 µs at 8 K to ∼1 µs at 30 K. The knowledge of carrier dynamics from optical characterization methods such as PL and TRPL can be employed to contribute to shorter feedback loops for improvement of alloy fabrication in addition to enhancing growth processes.

Systematic correlation between ligand length, bandgap energy and Seebeck voltage of Fe-based spincrossover (SCO) metal complexes through optical characterization

Fe-based spincrossover (SCO) molecular complexes have shown to exhibit spincrossover behavior when subjected to stimuli such as heat, light and pressure. In a previous work, solutions of Fe-based spincrossover (SCO) molecular complexes with increasing ligand length, CnH2n+1NH2 (n = 12, 14, 16) of Fe(L12)2](BF4)2, [Fe(L14)2](BF4)2, and [Fe(L16)2](BF4)2, have shown to produce ultrahigh Seebeck coefficients when subjected to a temperature gradient. In this work, these three compounds are dissolved in dimethyl sulfoxide (DMSO) and subjected to temperature dependent Ultraviolet-visible (UV-vis) spectrometry. This optical characterization method was used to provide a correlation between the ligand length of the SCO complex and the bandgap energy measured. Subsequently, these findings were also triangulated with the effect of the ligand length on ionic conductivity and the Seebeck voltage. This work thus provides a systematic molecular understanding of the optical and electronic characteristics of SCO complexes, which paves the way for molecular design strategies in utilization of SCO for applications such as energy conversion and sensors.

Synchronous electro-optical method for studying mixed ionic–electronic conductors with hopping transport

Mixed ionic–electronic conductor materials are exploited in energy storage and conversion applications, and, more recently, in emerging ionic thin film devices with distinctive electronic and optical functionalities, that impels studying their properties and, in particular, electrical transport mechanisms. Electrical and optical characterization methods, for instance, relaxation experiments under a small step variation in the mobile ion activity, are routinely used to study transport properties of mixed ionic–electronic conductors. In this work, we develop a theoretical basis for a synchronous electro-optical characterization method that enables fast measurement of the ionic and electronic (small polaron) mobilities, absorption cross section, and defect concentration in mixed ionic–electronic conductors with hopping transport.

Quantitative phase imaging based on polarization encoding.

Most optical characterization methods rely on measuring the complex optical fields emerging from the interaction between light and material systems. Nevertheless, inherent scattering and absorption cause ambiguities in both interferometric and noninterferometric attempts to measure phase. Here we demonstrate that the complete information about a probe optical field can be encoded into the states of polarization, and develop a topography measurement method by blindly varying the ambient refractive index surrounding the sample in a wedged cuvette, which is capable of simultaneously measuring the thickness and the ambient refractive index of the sample in real time, as well as extending the measurement range of the sample thickness. With the method, we have successfully measured the topography of a 136.7 µm thick coverslip by blindly changing the ambient refractive index by 0.001246, resulting in the thickest sample characterization ever achieved by quantitative phase imaging, to the best of our knowledge. An efficient and complete characterization of optical fields is critical for any high-resolution imaging approach and the technique demonstrated here should prove attractive for applications ranging from microscopy to remote sensing. Thanks to the high precision and fast response speed, this method may pave a new way for measuring the topography of the thick samples, such as biological tissues.

Synthesis and ellipsometric characterizations of large-scale MoS2 sub-monolayer

Two-dimensional (2D) transitional metal dichalcogenides (TMDs) have drawn significantly interest motivated by their prominent properties for electronics beyond silicon. Despite extensive efforts, continuous TMDs monolayer films are still difficult to grow with high yield, and universal characterization methods potential for industrial mass production are lacking, which significantly hinder their applications. Here, we designed a controllable chemical vapor deposition method to directly synthesis high-quality and large-scale MoS2 monolayer with various surface coverage ratios (CRs), representing growth stages from nucleation to connected film. A series of optical characterization methods and dielectric analysis were conducted to systemically study the monolayer feature of MoS2. Then, we demonstrated an alternative characterization strategy by disclosing the relations of measured Spectroscopic Ellipsometry (SE) spectra ∆ and Ψ to the CRs of MoS2 sub-monolayer, achieving a direct model-free assessment. The SE spectra show extraordinary sensitivity to the sub-monolayer CRs. This work presents a paradigm to generally evaluate large-scale 2D materials and offers basic SE characterization toward industrial production.