Optical Reflectance Technique Research Articles

Real‐Time 3D Visualization of the Formation of Micrograting Structures Upon Direct Laser Interference Patterning of Ge

AbstractDirect Laser Interference Patterning (DLIP) is a versatile technique that enables the fabrication of periodic micro‐ to nanometric scale structures over large areas in a variety of materials. The periodically modulated excitation pattern can be exploited to trigger a range of complex processes, including heating, melting, ablation, and matter reorganization.In this work, a novel strategy is developed to combine deep‐ultraviolet (UV) DLIP (λ = 193 nm, τ = 23 ns) and real‐time optical reflectivity and diffraction techniques to unravel the formation dynamics of grating structures with periods down to Λ = 740 nm. Applied to crystalline Ge wafers, single‐pulse topography modulation profiles with amplitudes up to 85 nm can be imprinted. Moreover, the dynamics of the melting and solidification processes, as well as the surface topography deformation, can be followed in real‐time. Combined with a model, changes in topography over time with ns resolution can be obtained. The results unambiguously reveal that the formation process of the 3D structures can only be understood when taking both Marangoni convection and thermocapillary waves into account. The technique presented here has the potential to unravel the formation dynamics of a wide range of periodic structures in other materials.

Ultrafast generation and detection of coherent acoustic phonons in SnS0.91Se0.09

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS0.91Se0.09 can obtain a high average thermoelectric figure of merit (ZTave≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS0.91Se0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS0.91Se0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced.

A fast approach to measuring the thickness uniformity of a homoepilayer grown on to any type of silicon wafer

Optical reflection spectroscopy techniques offer a non-destructive and fast method of measuring the thickness of silicon (Si) epilayers, enabling very fast thickness uniformity mapping across the full surface of epiwafers up to 450 mm in diameter. However, their use for undoped or low doped epilayers has traditionally been constrained by a dependence on high levels of substitutional doping in the Si wafer, at values of approximately 5 × 1019 cm−3. Whilst the high dopant concentration of this wafer creates the necessary reflectance boundary for optical reflection, their commercial availability is mainly limited to the (001) surface orientation only. Optical reflectance techniques are therefore also limited in use to this orientation. In this article, an approach to measure the thickness of a Si epilayer on any Si wafer, independent of its crystallographic orientation, doping type and value, using Fourier transform infrared reflection spectroscopy is proposed and demonstrated. Because the use of non-destructive optical reflection spectroscopy is already common and well-understood within both industry and academia, this technique could easily be implemented within existing industrial and research fabrication facilities. Furthermore, this approach could be adapted, with further work, to suit other semiconductor materials and other optical reflection techniques.

Vibronic States and Edge-On Oriented π-Stacking in Poly(3-alkylthiophene) Thin Films

The influence of alkyl side-chain length [CnH2n+1 (A), where n = 6 (hexyl), 8 (octyl), 12 (dodecyl)] and thermal annealing (TA) and the definite roles of these in the quantity and quality of π-stacked crystallinity and edge-on orientated (EO) ordering of spin-coated poly(3-alkylthiophene) [P3AT] thin films, which are of massive significance in their optoelectronic properties, were investigated using complementary optical absorption spectroscopy and X-ray reflectivity (XR) techniques. The energy-band diagram, corresponding to the vibronic levels, obtained from the optical absorption spectrum provides information about the percentage, planarity, local order, and average conjugation length of the crystalline aggregates, while the electron density profile obtained from the XR provides unique information about the EO ordering and its variation along the depth. A prominent EO ordering near the substrate with a gradually decreased ordering toward the top surface is observed in each film. An as-cast P3HT film of shorter side-chain (having a natural nucleation tendency and rigidity) shows a slightly higher crystallinity and weaker excitonic coupling, while as-cast P3DDT film (having longer side-chain flexibility) shows a slightly better EO ordering. After the TA, the planarity of the unfolded backbone improves in each film; however, the effect is maximum in the P3DDT film. The EO ordering also improves throughout the film (by a thermal energy-induced reorientation of crystallites) but more toward the top surface (by increasing the decay length), preserving the exponential decay nature. The P3HT film (with the better initial crystallinity and planarity) shows an appreciable improvement, while the P3DDT film (where the crystallinity increases and crystallites have a better reorientational ability) shows the maximum improvement in the EO ordering. The improvement of the crystallinity (in quantity and quality) and the EO ordering of P3AT in thin films (as active layers), especially near the film–substrate interfaces, are of immense importance for their better in-plane charge carrier mobility in a thin-film transistor.

Imaging layer thickness of large-area graphene using reference-aided optical differential reflection technique.

Transparent layers are critical for enhancing optical contrast of graphene on a substrate. However, once the substrate is fully covered by large-area graphene, there are no accurate transparent layer and reference for optical contrast calculations. The thickness uncertainty of the transparent layer reduces the analytical accuracy of graphene. Thus, in this Letter, we propose a reference-aided differential reflection (DR) method with a dual-light path. The accurate thickness of the transparent layer is obtained by improving the DR spectrum sensitivity using a designable reference. Hence, the analytical accuracy of graphene thickness is guaranteed. To demonstrate this concept, a centimeter-scale chemical-vapor-deposition-synthesized graphene was measured on a SiO2/Si substrate. The thickness of underlying SiO2 was first identified with the 1 nm resolution by the DR spectrum. Then, the thickness distribution of graphene was directly deduced from a DR map with submonolayer resolution at a preferred wavelength. The results were also confirmed by ellipsometry and atomic force microscopy. As a result, this new method provides an extra degree of freedom for the DR method to accurately measure the thickness of large-area two-dimensional materials.

Transformer Winding Deformation Detection Based on BOTDR and ROTDR.

In order to realize distributed measurement of transformer winding temperature and deformation, a transformer winding modification scheme with a built-in distributed optical fiber was designed. By laying a single-mode fiber and a multi-mode fiber on the transformer winding, the Brillouin optical time domain reflection technique (BOTDR) and the Raman optical time domain reflection technique (ROTDR) are used to measure the strain and temperature of the winding to complete the more accurate winding deformation detection. The accuracy of strain and temperature sensing of this scheme was verified by simulation. Then, according to the scheme, a winding model was actually wound, and the deformation and temperature rise tests were carried out. The test results show that this scheme can not only realize the deformation detection and positioning of the winding, but can also realize the measurement of the winding temperature; the temperature measurement accuracy reached ±0.5 °C, the strain measurement accuracy was 200 με, and spatial resolution was up to 5 m. In this experiment, the deformation location with the precision of 2 turns was realized on the experimental winding.

Solvent Dependent Spin-Crossover and Photomagnetic Properties in an Imidazolylimine FeII Complex.

The synthesis and physico-chemical characterization of an FeII complex [Fe(L1)3 ](ClO4 )2 ⋅CH3 CN⋅0.5H2 O, 1, incorporating a bidentate imidazolylimine-based ligand are reported. Complex 1 crystallises as the mer-isomer and the crystal lattice is replete with hydrogen bonding interactions between ClO4 - anions, solvent molecules and imidazole N-H groups. Variable-temperature structural, magnetic, photomagnetic and optical reflectivity techniques have been deployed to fully characterise the spin-crossover (SCO) behaviour in 1 along with its desolvated phase, 1⋅desolv. Variable-temperature (1.8-300 K) magnetic-susceptibility measurements reveal a broad two-step full SCO for 1 (T1/2 =158 and 184 K) and photomagnetic experiments at 10 K under white-light irradiation revealed complete photo-induced SCO. 1⋅desolv displays considerably different magnetic behaviour with sharp single-step SCO accompanied by a thermal hysteresis (T1/2↑ =105 K, T1/2↓ =95 K) in addition to full photo-induced SCO at lower temperatures.

Layer-number dependent reflection spectra of MoS2 flakes on SiO2/Si substrate

MoS2 flakes have attracted much attention due to their attractive properties. Optical reflectance techniques can prove to be very powerful techniques to study some of the thickness dependent physical properties, e.g. A and B excitonic peaks. Here, we measured reflection spectra of MoS2 flakes on SiO2/Si substrate in the broad wavelength range of 400-800 nm and studied the emission wavelength of A and B excitons as a function of the layer number. Moreover, we calculated the optimized SiO2 thickness to avoid the substrate-related interference effect influencing the investigation of exciton properties.

Physico-chemical characterization, density functional theory (DFT) studies and Hirshfeld surface analysis of a new organic optical material: 1H-benzo[d]imidazol-3-ium-2,4,6-trinitrobenzene-1,3 bis(olate)

A novel organic crystal, 1H-benzo[d]imidazol-3-ium-2,4,6-trinitrobenzene-1,3 bis(olate) (BITB), was synthesized. Single crystals of BITB were harvested by solution growth-slow evaporation technique. 1H and 13C NMR spectroscopic techniques were utilized to confirm the presence of various types of carbons and protons in BITB. Single crystal XRD confirms that BITB crystallizes in monoclinic system with a space group of P21/n. The suitability of this material for optical applications was assessed by optical absorption, transmittance, reflectance and refractive index spectroscopic techniques. Gaussian 09 program at B3LYP/6–311++G(d,p) level of basis set as used for the optimization of molecular structure of BITB. Greater first order hyperpolarizability value of BITB is due to intensive hydrogen bond network in the crystal. The value is 15 times greater than that of Urea, a reference standard. Computation of frontier molecular orbitals and electrostatic potential surface helped to understand the electron density and reactive sites in BITB. The material was thermally stable up to 220 °C. Hirshfeld surface analysis was performed to quantify the covalent and non covalent interactions.

A two-level detection algorithm for optical fiber vibration

Optical fiber vibration is detected by the coherent optical time domain reflection technique. In addition to the vibration signals, the reflected signals include clutters and noises, which lead to a high false alarm rate. The “cell averaging” constant false alarm rate algorithm has a high computing speed, but its detection performance will be declined in nonhomogeneous environments such as multiple targets. The “order statistics” constant false alarm rate algorithm has a distinct advantage in multiple target environments, but it has a lower computing speed. An intelligent two-level detection algorithm is presented based on “cell averaging” constant false alarm rate and “order statistics” constant false alarm rate which work in serial way, and the detection speed of “cell averaging” constant false alarm rate and performance of “order statistics” constant false alarm rate are conserved, respectively. Through the adaptive selection, the “cell averaging” is applied in homogeneous environments, and the two-level detection algorithm is employed in nonhomogeneous environments. Our Monte Carlo simulation results demonstrate that considering different signal noise ratios, the proposed algorithm gives better detection probability than that of “order statistics”.

In vivo measurements of cutaneous melanin across spatial scales: using multiphoton microscopy and spatial frequency domain spectroscopy.

The combined use of nonlinear optical microscopy and broadband reflectance techniques to assess melanin concentration and distribution thickness in vivo over the full range of Fitzpatrick skin types is presented. Twelve patients were measured using multiphoton microscopy (MPM) and spatial frequency domain spectroscopy (SFDS) on both dorsal forearm and volar arm, which are generally sun-exposed and non-sun-exposed areas, respectively. Both MPM and SFDS measured melanin volume fractions between (skin type I non-sun-exposed) and 20% (skin type VI sun exposed). MPM measured epidermal (anatomical) thickness values ~30-65 μm, while SFDS measured melanin distribution thickness based on diffuse optical path length. There was a strong correlation between melanin concentration and melanin distribution (epidermal) thickness measurements obtained using the two techniques. While SFDS does not have the ability to match the spatial resolution of MPM, this study demonstrates that melanin content as quantified using SFDS is linearly correlated with epidermal melanin as measured using MPM (R² = 0.8895). SFDS melanin distribution thickness is correlated to MPM values (R² = 0.8131). These techniques can be used individually and/or in combination to advance our understanding and guide therapies for pigmentation-related conditions as well as light-based treatments across a full range of skin types.

New approach for sensitive photothermal detection of C60 and C70 fullerenes on micro-thin-layer chromatographic plates

In this paper the pulse thermovision (photothermal) detection and quantification methods of C60 and C70 fullerenes are presented. Quantification results are compared with optical and fluorescence measurements. Target components were separated under isothermal conditions (30°C) on micro-TLC plates (RP18WF254S) using n-hexane as the mobile phase. The principle of described analytical protocol is based on sensitive measurement of the temperature contrast generated within TLC stationary phase and fullerenes spots after white light pulse excitation. It has been demonstrated that observed temperature contrast is mainly driven by the optical properties of fullerenes (UV–vis absorption spectra). Contrary to the commonly applied optical reflection or transmission techniques the proposed thermovision method involves dissipated light. The results of presented experimental work have revealed that both types of quantitative measurements provide similar outcome despite the key differences in the signal origin. However, it has been found that thermovision method was characterized by smaller value of LOD, particularly for C60 molecule. We demonstrated that application of correlation technique to post-acquisition analysis of the sequence of temperature contrast images significantly increase detection limits of fullerenes, even in comparison to fluorescence quenching detection mode. Moreover, the thermal contrast images and particularly, computed correlation image, allow detection of stationary phase layer nonuniformities, including changes in the adsorbent thickness and thermal conductivity. Therefore, invented pulsed thermovision methodology can be additionally used for fast quality screening of home made and commercially available TLC plates.

Speed limit of the insulator–metal transition in magnetite

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase. Here we investigate the Verwey transition with pump-probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.

Ultrafast dynamics in NiFe alloy thin films by two times of transient reflection

The femtosecond optical pumped transient reflection technique is an effective tool to investigate the ultrafast dynamics in metal thin films. In this paper, both the transient reflectivity response and nonequilibrium relaxation time of electron–phonon interaction in NiFe alloy thin films are studied by using femtosecond laser pump–probe transient reflection experimental technology. The result shows that the heating time of NiFe thin films with different thickness is independent of the thickness of the films, while the thermalization time by electron–phonon relaxation from nonequilibrium state to equilibrium state is proportional to the thickness of the films. In addition, two transient reflectivity signals on glass substrate films are found which is different with previous researches.

Optical and x-ray time resolved study of the structural transition in mixed valence manganites

Time resolved optical reflectivity and x-ray diffraction techniques are employed to study the laser-induced structural response in two charge and orbitally ordered manganites. Optical data indicate a non-thermal nature of the laser-triggered phase transition via the disappearance of an optical phonon related to the charge and orbitally ordered phase. The x-ray diffraction measurements on superlattice reflections confirm the non-thermal time scale of the initial step of this phase transition but also show that the complete change of structural symmetry is not instantaneous.

Evaluation of optical reflectance techniques for imaging of alveolar structure

Three-dimensional (3-D) visualization of the fine structures within the lung parenchyma could advance our understanding of alveolar physiology and pathophysiology. Current knowledge has been primarily based on histology, but it is a destructive two-dimensional (2-D) technique that is limited by tissue processing artifacts. Micro-CT provides high-resolution three-dimensional (3-D) imaging within a limited sample size, but is not applicable to intact lungs from larger animals or humans. Optical reflectance techniques offer the promise to visualize alveolar regions of the large animal or human lung with sub-cellular resolution in three dimensions. Here, we present the capabilities of three optical reflectance techniques, namely optical frequency domain imaging, spectrally encoded confocal microscopy, and full field optical coherence microscopy, to visualize both gross architecture as well as cellular detail in fixed, phosphate buffered saline-immersed rat lung tissue. Images from all techniques were correlated to each other and then to corresponding histology. Spatial and temporal resolution, imaging depth, and suitability for in vivo probe development were compared to highlight the merits and limitations of each technology for studying respiratory physiology at the alveolar level.

Brewster-Angle Variable Polarization Spectroscopy of Colloidal Au-Nanospheres and -Nanorods at the Silicon Surface

Colloidal Au-nanospheres and -nanorods were chemically synthesized from chloroauric acid containing solutions and adsorbed to hydrogen-terminated p-Si(111) surfaces. A comparative analysis of the respective plasmon resonance modes, in solution and at the silicon surface, was carried out by in situ optical transmission and surface reflection techniques. In solution, the absorbance was determined by VIS transmission spectroscopy and compared to Mie theory as well as finite difference time domain calculations. At the p-Si(111) surface, the reflectance was analyzed for the first time by Brewster-angle variable polarization spectroscopy (BA-VPS) and discussed in terms of anisotropic uniaxial thin-film properties. With BA-VPS, the strengths of parallel and orthogonal electric field components of the incident wave can be varied relative to the surface plane. Consequently, opposite changes of transverse and longitudinal resonance strengths are detected upon gradually incremented light polarization angles. Model considerations confirm that spherical and elongated particles can thereby be distinguished. The influence of particle–surface interaction and the dielectric environment is finally discussed.

Optical properties of solution-processable semiconducting TiOx thin films for solar cell and other applications

The optical properties of solution-processable semiconducting titanium suboxide (TiOx) thin films were investigated as a function of wavelength (350-800 nm) using ellipsometric and optical reflection technique. The variation of refractive index under different thermal annealing conditions (room temperature to 900 °C) was studied. The increase in refractive index with high-temperature thermal annealing process was observed, allowing the opportunity to obtain refractive index values from 1.77 to 2.57 at a wavelength of 600 nm. The x-ray diffraction and atomic force microscopy studies indicate that the index variation is due to the TiOx phase, density, and morphology changes under thermal annealing. The TiOx thin films have applications in organic and inorganic solar cells as well as other optical and photonic devices. We show that TiOx thin films can be used as an effective antireflection layer for Si solar cells.

Determination of Lysozyme Using Microcantilever Sensor Based on Atomic Force Microscopy

Abstract A microcantilever sensor was developed to detect lysozyme in a pharmaceutical formulation based on the principle of atomic force microscopy. By measuring the cantilever bending and deflection using the optical reflection technique, the lysozyme adsorption on a dodecanethiol-modified cantilever surface was detected. Under optimal conditions, a good linear relationship between the deflection of the microcantilever and the logarithm of the lysozyme concentration in the range from 10 ng L−1 to 0.1 mg L−1, with a correlation coefficient of 0.998, was established. The limit of detection was 5.0 ng L−1. The lysozyme in the pharmaceutical formulation sample was determined with the desirable results by using the present method.

Electrothermal theory of photomodulated optical reflectance on active doping profiles in silicon

The electrical characterization of the source and drain extension regions of complementary metal oxide semiconductor (CMOS) transistors is highlighted in the international technology roadmap for semiconductors (ITRS) as a major challenge for future technology nodes. In practice, there is a clear need for techniques which are simultaneously accurate, nondestructive, fast, local, and highly reproducible. The photomodulated optical reflectance (PMOR) technique has shown to be a very promising candidate to solve this need. However, even though this technique has been widely studied on homogeneous bulk material and on as-implanted (i.e., unannealed) doping profiles, the extension toward active doping profiles requires a detailed investigation (due to the presence of a built-in electric field). In this paper, after performing an in-depth investigation of the optical and transport models involved in a PMOR experiment, we derive an analytical theory to explain the PMOR signal behavior observed on active doping profiles. In the optical model, we show that only the electrorefractive Drude and thermorefractive effects are to be considered for red and near-infrared wavelengths on Si. In the transport model, we begin the discussion with the study of homogeneous Si substrates. We show that, due to the high carrier injection induced by the lasers, the only important effects are, for the free carriers, the Auger recombinations, the (ambipolar) diffusion and the bandgap narrowing-induced quasidrift; the thermoelectric effects being negligible. Based on the results on homogeneous substrates and on the assumption that the quasi-Fermi levels are flat through the space-charge region, we derive an analytical formula for PMOR signals on active doping profiles. We discuss this formula based on experimental PMOR data measured on active doping profiles with a simple boxlike shape. This formula proves to be in good qualitative agreement with the experimental data both when the power of the pump laser is varied (power curves) and when the distance between the lasers is changed (offset curves). We also show that, for the formula to be quantitative, a very good knowledge of the bandgap profile throughout the sample would be required.