Time-resolved Transmittance Measurements Research Articles

Electron Density Changes and Ultrafast Carrier Dynamics of the Antiferromagnetic Semiconductor MnPS3 through Magnetic Phase Transition.

Studying ultrafast dynamics provides us with a way to modify materials from the timescale of particle interaction, and the related research on antiferromagnetic semiconductors is still inadequate. Based on the electron density reconstruction, we achieve the visualization of magnetic interactions of bulk antiferromagnetic MnPS3 in the ground state, reveal the role of two atomic site occupations of S atoms in different magnetic phase transitions, and provide the theoretical and experimental support for modifying magnetic properties by selectively replacing the S atom. The ultrafast carrier dynamics can provide information from the excited state to the ground state. Based on time-resolved transmittance measurements, ultrafast carrier dynamics of MnPS3 are reported. The phonon-assisted gap transition driven by the electronic structure is characterized. The coupling relationship among electrons, spin, and phonons is established. Furthermore, the spin orientations within different phases are confirmed.

Temporal-Spatial Measurement of Surface Morphological Evolution Time in Nanosecond Laser Irradiation on the Copper Film

In this research, the pump-probe imaging technology and the time-resolved transmittance measurements are used to investigate the transient time scale of the oxidation area, the dewetted melt pool, and the ablation-induced mass transport, and their dependence on the incident laser. Moreover, the evolution time of various morphologies have been studied through a certain delay time from 10 to 30 ns with different dynamic processes. Meanwhile, the underlying formation mechanism and the phase change pathway have been studied. In this regard, a nanosecond (ns) pulsed laser with 532 nm wavelength was used to fabricate nanostructures on the surface of the thin Copper (Cu) films. The observations confirmed that the oxidation process of Cu plays a significant role in the mechanism of laser fluence-dependent oxidation, melting, and ablation dynamics. The results revealed that the different characteristics of the Cu film are related to the incident laser pulse energy. Furthermore, the experimental evidence was in good agreement with the SEM images and the Raman spectra. It is found that the Cu2O surface-rim with a central hole could be fabricated with the laser fluences above the ablation threshold. Although, for the laser fluences below the ablation threshold, the Cu2O phase can be formed in the central region.

Optical Characterization of Homogeneous and Heterogeneous Intralipid-Based Samples

Different scattering processes take place when photons propagate inside turbid media. Many powerful experimental techniques exploiting these processes have been developed and applied over the years in a large variety of situations from fundamental and applied research to industrial applications. In the present paper, we intend to take advantage of Static Light Scattering (SLS), Dynamic Light Scattering (DLS), and Time-Resolved Transmittance (TRT) for investigating all the different scattering regimes by using scattering suspensions in a very large range of scatterer concentrations. The suspensions were prepared using Intralipid 20%, a material largely employed in studies of the optical properties of turbid media, with concentrations from 10−5% to 50%. By the analysis of the angular and temporal dependence of the scattered light, a more reliable description of the scattering process occurring in these samples can be obtained. TRT measurements allowed us to obtain information on the reduced scattering coefficient, an important parameter largely used in the description of the optical properties of turbid media. TRT was also employed for the detection of inclusions embedded in Intralipid suspensions, by using a properly designed data analysis. The present study allowed us to better elucidate the dependence of scattering properties of Intralipid suspensions in a very large concentration range and the occurrence of the different scattering processes involved in the propagation of light in turbid media for the first time to our knowledge. In so doing, the complementary contribution of SLS, DLS, and TRT in the characterization of turbid media from an optical and structural point of view is strongly evidenced.

Estimate of tissue composition in malignant and benign breast lesions by time-domain optical mammography

The optical characterization of malignant and benign breast lesions is presented. Time-resolved transmittance measurements were performed in the 630-1060 nm range by means of a 7-wavelength optical mammograph, providing both imaging and spectroscopy information. A total of 62 lesions were analyzed, including 33 malignant and 29 benign lesions. The characterization of breast lesions was performed applying a perturbation model based on the high-order calculation of the pathlength of photons inside the lesion, which led to the assessment of oxy- and deoxy-hemoglobin, lipids, water and collagen concentrations. Significant variations between tumor and healthy tissue were observed in terms of both absorption properties and constituents concentration. In particular, benign lesions and tumors show a statistically significant discrimination in terms of absorption at several wavelengths and also in terms of oxy-hemoglobin and collagen content.

An Edge-Heating Device for Optical Measurement of Thermochromic Glazing Materials and Recommended Test Procedure

Abstract Thermochromic materials have optical properties that vary with temperature. To simulate energy performance of such materials, it is important to have spectral data in the solar range, 300–2500 nm, for each temperature that the material will have in the simulation. This paper describes a temperature control strategy that allows for measurement of reflectance and transmittance at a fixed temperature using a commercial spectrophotometer. A specimen frame is used to clamp heating strips to the surface at the edge of the sample that is being tested. Multiple thermocouples are used to monitor the temperature gradient over the sample as the center is cooler than the edge. Verification using an infrared (IR) camera and time-resolved transmittance measurements show that the center sample temperature is stable and how long it takes to achieve equilibrium. An interpolation method is described and verified to reduce the number of states that need to be measured. A recommended test procedure is described and used on two different materials.

Temperature-jump investigations of the kinetics of hydrogel nanoparticle volume phase transitions.

The dynamics of the deswelling and swelling processes in thermoresponsive poly-N-isopropylacrylamide (pNIPAm) hydrogel nanoparticles have been studied by using time-resolved transmittance measurements, in combination with a nanosecond laser-induced temperature-jump (T-jump) technique. A decrease in the solution transmittance associated with deswelling of the particles has been observed as the solution temperature traverses the volume phase transition temperature of the particles. Upon inducing the T-jump, the deswelling transition only occurs in a small percentage (<10%) of the particle volume, which was found to be a thin periphery layer of the particles. The particle deswelling occurs on the microsecond time scale, and as shown previously, the collapse time can be tuned via adding small amounts of hydrophobic component to the particle shell. In contrast, the reswelling of the particles was thermodynamically controlled by bath equilibration, and only small differences in particle reswelling kinetics were found due to sluggish heat dissipation (millisecond time scale) from the sample cell.

Inclusion characterization in a scattering slab with time-resolved transmittance measurements: perturbation analysis

A procedure for the time-domain optical characterization of an inclusion in a scattering slab is investigated theoretically and experimentally. The method relies on the measurement of a contrast function, which is defined as the time-dependent relative change in the transmitted signal resulting from the presence of the inclusion. Analytical expressions for the contrast functions of absorptive and diffusive inclusions are obtained through a perturbation solution of the diffusion equation. This procedure is used successfully to determine the optical properties of absorptive, diffusive, and mixed inclusions located at midplane in a scattering slab by use of time-resolved transmittance measurements.

Time-Resolved Studies of Light Propagation in Crassula and Phaseolus Leaves.

Abstract- Time-resolved transmittance was used to extract in vivo optical properties of leaves of green plants experimentally. In time-resolved transmittance measurements an ultrashort light pulse is directed onto the surface of the object and the transmitted light is measured with a time resolution in the range of picoseconds. A table-top terawatt laser was used to generate 200 fs light pulses at 790 nm with a repetition rate of 10 Hz. The light pulses were focused through a cuvette filled with water to produce white light pulses and optical filters were placed in the beam path to select the wavelength of the light focused onto the leaf surface. The time profiles of the light transmitted through the leaves was recorded with a streak camera having a time resolution of about 2.5 ps. Results from Crassula falcata and Phaseolus vulgaris studied at 550, 670 and 740 nm are reported. The three selected wavelength regions represent medium, high and a low absorption of light, respectively. A library of curves was generated using Monte Carlo simulation, and the absorption and scattering coefficients were extracted by comparison of experimental curves with this library. Our results suggest that in the case of the thin (200 μm) Phaseolus leaves and certainly in the case of the thick (4 mm) Crassula leaves, light scattering plays an important role in light transport through the leaf and should also affect light flux in these leaves.

Time-Resolved Studies of Light Propagation in Crassula and Phaseolus Leaves

Abstract— Time-resolved transmittance was used to extract in vivo optical properties of leaves of green plants experimentally. In time-resolved transmittance measurements an ultrashort light pulse is directed onto the surface of the object and the transmitted light is measured with a time resolution in the range of picoseconds. A table-top terawatt laser was used to generate 200 fs light pulses at 790 nm with a repetition rate of 10 Hz. The light pulses were focused through a cuvette filled with water to produce white light pulses and optical filters were placed in the beam path to select the wavelength of the light focused onto the leaf surface. The time profiles of the light transmitted through the leaves was recorded with a streak camera having a time resolution of about 2.5 ps. Results from Crassula falcata and Phaseolus vulgaris studied at 550, 670 and 740 nm are reported. The three selected wavelength regions represent medium, high and a low absorption of light, respectively. A library of curves was generated using Monte Carlo simulation, and the absorption and scattering coefficients were extracted by comparison of experimental curves with this library. Our results suggest that in the case of the thin (200 μm) Phaseolus leaves and certainly in the case of the thick (4 mm) Crassula leaves, light scattering plays an important role in light transport through the leaf and should also affect light flux in these leaves.

A solid tissue phantom for photon migration studies

A solid tissue phantom made of agar, Intralipid and black ink is described and characterized. The preparation procedure is fast and easily implemented with standard laboratory equipment. An instrumentation for time-resolved transmittance measurements was used to determine the optical properties of the phantom. The absorption and the reduced scattering coefficients are linear with the ink and Intralipid concentrations, respectively. A systematic decrease of the reduced scattering coefficient dependent on the agar content is observed, but can easily be managed. The phantom is highly homogeneous and shows good repeatability among different preparations. Moreover, agar inclusions can be easily embedded in either solid or liquid matrixes, and no artefacts are caused by the solid - solid or solid - liquid interfaces. This allows one to produce reliable and realistic inhomogeneous phantoms with known optical properties, particularly interesting for studies on optical imaging through turbid media.

Imaging of optical inhomogeneities in highly diffusive media: Discrimination between scattering and absorption contributions

An effective projection imaging technique is proposed that allows not only detection of realistic optical inhomogeneities in highly diffusive media, but also the discrimination between scattering and absorption contributions. The method is based on information derived from time-resolved transmittance measurements. Scattering inhomogeneities are discriminated using maps of the transport scattering coefficient, as provided by best fitting of the diffusion theory to experimental data. Absorption variations are effectively classified by time gating on the tail of the transmittance pulse.

Reconstruction of diffuse photon-density wave interference in turbid media from time-resolved transmittance measurements

We demonstrate an improved technique to precisely localize inhomogeneities in turbid media by means of reconstructing diffuse photon-density wave interference from time-resolved transmittance measurements applying the Fourier transform. This interference can also be obtained in the reverse mode, that is using a single source and combining the signals detected at several locations. This increases the collection efficiency and the possibility for postprocessing and allows one to evaluate the data from one measurement in different ways to make the analysis more robust.