Time-resolved Reflectivity Measurements Research Articles

Robust Ordered Cubic Mesostructured Polymer/Silica Composite Films Grown at the Air/Water Interface

Polymer/silica composite films, stable to calcination, were produced using catanionic surfactant mixtures (hexadecyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS)) and polymers (polyethylenimine (PEI) or polyacrylamide (PAAm)) at the air/water interface. Film formation processes were probed by time-resolved neutron reflectivity measurements. Grazing incidence X-ray diffraction (GID) measurements indicate that the mesophase geometry of the interfacial films could be controlled to give lamellar, 2D hexagonal, and several cubic phases (Pn3¯m, Fm3¯m, and Im3¯m) by varying the polyelectrolyte molecular weight, polyelectrolyte chemical nature, or the cationic:anionic surfactant molar ratio. On the basis of GID results, a phase diagram for the catanionic surfactant/polyelectrolyte/TMOS film system was drawn. These films can be easily removed from the interface and mesoporous silica films which retain the film geometry can be obtained after calcination; moreover, this film preparation method provides a simple way to impart polymer functionality into the mesostructured silica wall, which means these films have potential applications in a variety of fields such as catalysis, molecular separation, and drug delivery.

Dynamics of optical phonons in Bi2Se3crystal studied using femtosecond time-resolved reflection measurement

The dynamics of coherent optical phonons (A 1g 1 , A 1g 2 and E g 2 ) in Bi 2 Se 3 single crystal have been observed by using femtosecond time-resolved reflection measurement. The phonons with the higher frequency have the faster damping rate (decay faster). The A 1g 2 phonons show cosine-like oscillation, while the A 1g 1 and E g 2 phonons show cosine-like oscillation. The difference in the initial phase suggests that the potential energy surface may shift significantly along the A 1g 2 eigenvector and little along the A 1g 1 and E g 2 eigenvectors in the excited state.

Experimental estimation of the photons visiting probability profiles in time-resolved diffuse reflectance measurement

A time-gated intensified CCD camera was applied for time-resolved imaging of light penetrating in an optically turbid medium. Spatial distributions of light penetration probability in the plane perpendicular to the axes of the source and the detector were determined at different source positions. Furthermore, visiting probability profiles of diffuse reflectance measurement were obtained by the convolution of the light penetration distributions recorded at different source positions. Experiments were carried out on homogeneous phantoms, more realistic two-layered tissue phantoms based on the human skull filled with Intralipid–ink solution and on cadavers. It was noted that the photons visiting probability profiles depend strongly on the source–detector separation, the delay between the laser pulse and the photons collection window and the complex tissue composition of the human head.

Time-resolved detection of fluorescent light during inflow of ICG to the brain—a methodological study

It was reported that time-resolved reflectance measurements carried out during inflow and washout of an optical contrast agent may provide information on the blood supply to the brain cortex of human adults. It was also shown that a measurement of fluorescence excited in the dye circulating in the brain is feasible. Unfortunately, patterns of time-resolved fluorescence signals observed during in vivo measurements are difficult to interpret. The aim of this study was to analyze the influence of several factors on the fluorescence signals measured during in vivo experiments. A laboratory instrument for recording the distributions of arrival of fluorescence photons was constructed and optimized for measurements on humans. Monte Carlo simulations and laboratory measurements on liquid phantoms as well as in vivo measurements on healthy volunteers were carried out. An influence of source–detector separation, position of the source–detector pair on the head, as well as a dose of the injected indocyanine green (ICG) on the fluorescence signals were studied in detail. It was shown that even for a small dose of ICG (0.025 mg kg−1) the time-resolved signals can be successfully detected on the surface of the head. Strong influence of the studied factors on the fluorescence signals was observed. It was also noted that the changes in moments of distributions of arrival times of fluorescence photons depend on the anatomical structure of the tissues located between the source and the detector.

Solid phase epitaxy of silicon thin films by diode laser irradiation for photovoltaic applications

High temperature solid phase epitaxial crystallization of amorphous silicon layers prepared by electron beam evaporation is investigated. By using a continuous wave diode laser for heating the films rapidly (in milliseconds to seconds) this method is suitable on glass substrates with low temperature resistance. Therefore, the method is an economically advantageous technique of producing absorber layers for thin film solar cells. For the experiments 500nm of amorphous silicon was deposited on two different configurations of substrates. In the first one monocrystalline wafers of three different crystallographic orientations were used. In the second one a polycrystalline seed layer prepared on borosilicate glass served as substrate. The crystallization process was monitored in situ by time resolved reflectivity measurements. Depending on the crystal orientation 2s to 3s was needed for complete solid phase epitaxial crystallization of the amorphous films. The evolution of temperature during crystallization was simulated numerically.

Epitaxial thin films of topological insulator Bi2Te3 with two-dimensional weak anti-localization effect grown by pulsed laser deposition

We have grown high quality epitaxial topological insulator Bi2Te3 thin films on silicon (111) substrates by pulsed laser deposition. Systematic structural characterization of the films using X-ray diffraction and transmission electron microscopy has demonstrated that a low laser pulse rate is the key to achieving epitaxial films. The films show n-type metallic behavior, consistent with Te deficiency as determined by Rutherford backscattering spectrometry measurements. The A1g longitudinal optical phonon mode of Bi2Te3 was detected by time-resolved reflectivity measurements. A 2-dimensional (2-D) weak-antilocalization effect was also observed at low temperatures, which indicates the existence of topologically protected 2-D surface states in our thin films. This growth and characterization effort paves the way to fabricate multi-layer heterostructures of topological insulators along with ferromagnetic oxides and high temperature superconductors by the same growth technique in the search for physics arising from their interfacial couplings.

Exciton and carrier spin relaxations in InGaAs lattice-matched to off-cut Ge substrates

We have investigated the exciton and carrier spin relaxations in InGaAs lattice-matched to Ge substrates. Time-resolved spin-dependent pump and probe reflectance measurements revealed a spin relaxation behavior between 10 and 300 K. The presence of the carrier density dependence of spin relaxation time at 10-200 K implies that the Bir-Aronov-Pikus process is effective. At 250-300 K, the strong temperature and weak carrier density dependences of spin relaxation time show that the D’yakonov-Perel’ process is dominant. The longest observed spin relaxation time of 2.6 ns at 77 K is explained by the decrease in the spatial overlap of electrons and holes.

Determination of the melting threshold of TiO2 thin films processed by excimer laser irradiation

Processing surfaces by laser needs an understanding of the mechanisms generated by irradiation. In this work, to gain understanding of the mechanisms occurring during irradiation of TiO2 thin films by means of KrF excimer laser, we have performed infrared time resolved reflectivity measurements. This experimental investigation revealed modifications of the heating/cooling cycle as a function of the fluence (F). These modifications start appearing for a fluence value of about ∼0.25 J/cm2 which is associated with the melting threshold of the film. Additionally, we have solved numerically the heat equation of the system with specific boundary conditions. From these calculations, we have established the thermal history of the film during the 25 ns irradiation pulse. The data reveal that a part of the medium liquefies around a fluence of 0.23 J/cm2 in good agreement with the experimental data.

Ultrafast carrier and coherent phonon dynamics in semi-insulated and n-type 4H-SiC

We investigated ultrafast carrier and phonon dynamics in semi-insulated (SI) and n-type 4H-SiC using time-resolved reflectivity measurements. In the n-type 4H-SiC, carriers are excited by an inter-conduction band transition, and thermalized by electron-electron scattering within 20 fs. We observed coherent phonons of folded phonon modes, which become Raman active as a result of the zone folding, and those of A1-symmetry longitudinal optical (A1-LO) phonon mode. In the n-type SiC, the A1-LO coherent phonon forms a coupled mode with a plasmon, resulting in an asymmetrically broadened Fourier transform spectrum. The polarization dependence and the sine-type initial phase indicate that the impulsive stimulated Raman scattering is the mechanism generating coherent phonons of E2-symmetry transverse optical and A1-LO modes in both the SI and n-type 4H-SiC.

Rheological modeling of the diffusion process and the interphase of symmetrical bilayers based on PVDF and PMMA with varying molecular weights

The diffusion process in the molten state at a polymer/polymer interface of symmetrical and model bilayers has been investigated using a small-amplitude oscillatory shear measurement. The polymers employed in this study were poly (vinylidene fluoride) (PVDF) and poly (methyl methacrylate) (PMMA) of varying molecular weights and polydispersities. The measurements were conducted in the linear viscoelastic regime (small deformations) so as to decouple the effect of flow from the diffusion. The focus of this paper has been to investigate the effects of healing time, angular frequency (ω), temperature, and molecular weight on the inter-diffusion and the triggered interphase between the neighboring layers. The kinetics of diffusion, based on the evolution of the apparent diffusion coefficient (D a) versus the healing time, was experimentally obtained. The transition from the non-Fickian to the normal Fickian region for the inter-diffusion at the interface was clearly observed, qualitatively consistent with the reptation model, but it occurred at a critical time greater than the reptation time (τ rep). In non-Fickian region, effects of frequency and temperature were studied with regard to the ratio of the apparent diffusion coefficient to the self-diffusion coefficient (D a/D s). The D s determined in the Fickian region was found to be consistent with Graessley’s model as well as with the literatures. And the dependence of the Ds on the frequency agreed well with the Doi–Edwards theory, in particular, scaling as $D_{\rm s} \sim \omega^{1/2}$ at ω > 1/τ e and $D_{\rm s} \sim \omega^{0}$ at ω < 1/τ rep. Our experimental results also confirmed that the dependence of the D s on the temperature for PMMA and PVDF can be well described by the Arrhenius law. Moreover, blends of PMMAs have been proposed in order to be able to change the $\overline M_\emph{w} $ . The rheological investigations of these corresponding bilayers rendered it possible to monitor the effect of $\overline M_\emph{w} $ on the diffusion process. The obtained results gave $D_{\rm s} \sim \overline M_\emph{w}^{-1}$ , thus corroborating some earlier studies and some experimental results recently reported by Time-Resolved Neutron Reflectivity Measurements. Lastly, the thickness of the interphase and its corresponding viscoelastic properties could be theoretically determined as a function of the healing time.

Observation of coherent higher frequency [formula omitted] phonons in Bi2Se3 using femtosecond time-resolved reflection measurement

The dynamics of phonons in a Bi2Se3 crystal has been studied by using femtosecond time-resolved reflection measurements. Two coherent optical phonons with A1g1 and A1g2 modes are observed at 2.14 and 5.09 THz at room temperature, respectively. The initial amplitude of the generated coherent A1g2 phonons is higher than that of the A1g1 phonons, but those are quickly damped with a rate of 1.28 THz.

Comparison of measurement techniques for electron spin relaxation time in a GaAs/AlGaAs multiple quantum well

We compared the performance of three measurement techniques for electron spin relaxation time, i.e., polarization- and time-resolved photoluminescence (PTRPL) measurement, time-resolved reflectance (TRR) measurement, and time-resolved Kerr rotation measurement. With all three techniques similar electron spin relaxation times (0.83–1.2ns) were obtained in a (110) GaAs/AlGaAs multiple quantum well with excitation wavelengths near the heavy-hole exciton resonance at 77K. We extracted the spin-independent contribution of the Coulomb interaction which obscures TRR signals as an offset without limiting the excitation wavelength to heavy-hole exciton resonance, using the degree of electron spin polarization simultaneously obtained by the PTRPL measurement. It was found that the Coulomb interaction contribution is dominant in TRR signals over both heavy- and light-hole exciton resonances.

Precise Analyses of Short-Time Relaxation at Asymmetric Polystyrene Interface in Terms of Molecular Weight by Time-Resolved Neutron Reflectivity Measurements

The short-time mutual diffusion at an interface of linear polystyrene/linear deuterated polystyrene with different molecular weights was examined by time-resolved neutron reflectivity (TR-NR) measurements. The model scattering length density (b/V) profiles obtained by solving a partial differential equation for the diffusion process, using the segmental diffusion coefficients as the only fitting parameter were used to analyze the TR-NR data. In short-time diffusion, the asymmetric (b/V) profiles derived from a segmental relaxation model were able to model the data much better than those obtained by center-of-mass diffusion based on reptation model and by a simple error function. These analyses clearly indicate that even if the molecular weights of both components are larger than the critical molecular weight for entanglement, the initial interfacial broadening of bilayer films with different molecular weight proceeds with asymmetric mobility being proportional to N–1. Time dependence of the interfacial po...

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The photoinduced charge transfer mechanism in aligned and unaligned carbon nanotubes

Using time-resolved reflectivity measurements on unaligned and aligned bundled single-wall carbon nanotubes with a pump energy of 1.55 eV, quasi-resonant with the second Van Hove singularity of semiconducting tubes, a positive sign of the transient reflectivity is detected in unaligned nanotubes. In contrast a negative sign is detected in aligned nanotubes. This discovery addresses a long-standing question showing that in unaligned nanotubes the stronger intertube interactions favor the formation of short-lived free charge carriers in semiconducting tubes. A detailed analysis of the transient reflectivity spectral response shows that the free carriers in the photo-excited state of semiconducting tubes move towards metallic tubes in about 400 fs.

Non-invasive determination of the absorption coefficient of the brain from time-resolved reflectance using a neural network

We investigated the performance of a neural network for derivation of the absorption coefficient of the brain from simulated non-invasive time-resolved reflectance measurements on the head. A five-layered geometry was considered assuming that the optical properties (except the absorption coefficient of the brain) and the thickness of all layers were known with an uncertainty. A solution of the layered diffusion equation was used to train the neural network. We determined the absorption coefficient of the brain with an RMS error of <6% from reflectance data at a single distance calculated by diffusion theory. By applying the neural network to reflectance curves obtained from Monte Carlo simulations, similar errors were found.

Picosecond laser pulse-driven crystallization behavior of SiSb phase change memory thin films

Transient phase change crystallization process of SiSb phase change thin films under the irradiation of picosecond (ps) laser pulse was studied using time-resolved reflectivity measurements. The ps laser pulse-crystallized domains were characterized by atomic force microscope, Raman spectra and ellipsometrical spectra measurements. A reflectivity contrast of about 15% can be achieved by ps laser pulse-induced crystallization. A minimum crystallization time of 11 ns was achieved by a low-fluence single ps laser pulse after pre-irradiation. SiSb was shown to be very promising for fast phase change memory applications.

Doping-type dependence of phonon dephasing dynamics in Si

We studied the dephasing dynamics of coherent phonons in n-type, p-type, and intrinsic Si using time-resolved reflectivity measurements with sub-10 fs laser pulses. The dephasing time of the coherent phonons increases (decreases) for n-type (p-type) doping compared with that of intrinsic Si, while the frequencies of the coherent phonons exhibit a redshift for both types of doping. These doping-induced changes in the coherent phonon dynamics are observed when the carrier concentration exceeds 1019 cm−3. The doping-type dependent changes in the dephasing time are attributed to the interconduction and intervalence band transitions in n-type and p-type Si, respectively.

Coherent Optical Phonons in the Iron Oxypnictide SmFeAsO1-xFx(x=0.075)

Coherent optical phonons with the Fe-related mode B 1 g (Fe) in the iron oxypnictide SmFeAsO 1- x F x ( x = 0.075) are observed using femtosecond time-resolved reflection measurements with electrooptic sampling, in addition to the A 1 g (Sm) and A 1 g (As) modes. The time evolution of the frequency and amplitude of optical phonons is studied and the lifetimes of phonons are estimated. The frequency softening and positive frequency chirp related to the strong carrier–phonon coupling are found for the A 1 g (Sm) mode.

Advanced Neutron Reflectometer for Investigation on Dynamic/Static Structures of Soft-Interfaces in J-PARC

A novel neutron reflectometer with horizontal geometry will be established at BL16 in Materials and Life Science Experimental Facility (MLF) of Japan Proton Accelerator Research Complex (J-PARC) as a successor of a reflectometer ARISA-II. ARISA-II corresponding to a single neutron beam downward at 2.22 deg has achieved off-specular and time-resolved reflectivity measurements. The novel reflectometer is designed so as to receive two tilted neutron beams (2.22 & 5.71 deg), which gives us an opportunity in investigation on a free liquid surface. The reflectometer can provide a micro-sized beam by slit collimation and obtain a fair reflectivity with small sample area. Also, T0 chopper and neutron focusing mirror are newly introduced. The T0 chopper can suppress the background due to fast neutrons. The focusing mirror produces further reduction of measurement time not only for specular reflection by focusing neutrons on a sample, but also grazing incidence small-angle neutron scattering (GISANS) measurements by focusing on a detector.