Time-resolved Optical Measurements Research Articles

Carotenoid-to-Bacteriochlorophyll Energy Transfer in the LH1-RC Core Complex of a Bacteriochlorophyll b Containing Purple Photosynthetic Bacterium Blastochloris viridis.

Carotenoid-to-bacteriochlorophyll energy transfer has been widely investigated in bacteriochlorophyll (BChl) a-containing light harvesting complexes. Blastochloris viridis utilizes BChl b, whose absorption spectrum is more red-shifted than that of BChl a. This has implications on the efficiency and pathways of carotenoid-to-BChl energy transfer in this organism. The carotenoids that comprise the light-harvesting reaction center core complex (LH1-RC) of B. viridis are 1,2-dihydroneurosporene and 1,2-dihydrolycopene, which are derivatives of carotenoids found in the light harvesting complexes of several BChl a-containing purple photosynthetic bacteria. Steady-state and ultrafast time-resolved optical spectroscopic measurements were performed on the LH1-RC complex of B. viridis at room and cryogenic temperatures. The overall efficiency of carotenoid-to-bacteriochlorophyll energy transfer obtained from steady-state absorption and fluorescence measurements were determined to be ∼27% and ∼36% for 1,2-dihydroneurosporene and 1,2-dihydrolycopene, respectively. These results were combined with global fitting and target analyses of the transient absorption data to elucidate the energetic pathways by which the carotenoids decay and transfer excitation energy to BChl b. 1,2-Dihydrolycopene transfers energy to BChl b via the S2 → Qx channel with kET2 = (500 fs)(-1) while 1,2-dihydroneurosporene transfers energy via S1→ Qy (kET1 = (84 ps)(-1)) and S2 → Qx (kET2 = (2.2 ps)(-1)) channels.

Power-dependent spin amplification in (In, Ga)As/GaAs quantum well via Pauli blocking by tunnel-coupled quantum dot ensembles

Power-dependent time-resolved optical spin orientation measurements were performed on In0.1Ga0.9As quantum well (QW) and In0.5Ga0.5As quantum dot (QD) tunnel-coupled structures with an 8-nm-thick GaAs barrier. A fast transient increase of electron spin polarization was observed at the QW ground state after circular-polarized pulse excitation. The temporal maximum of polarization increased with increasing pumping fluence owing to enhanced spin blocking in the QDs, yielding a highest amplification of 174% with respect to the initial spin polarization. Further elevation of the laser power gradually quenched the polarization dynamics, which was induced by saturated spin filling of both the QDs and the QW phase spaces.

Influence of Self-Diffraction Effect on Femtosecond Time-Resolved Single-Shot Optical Kerr Measurements**Supported by the National Natural Science Foundation of China under Grant Nos 11304242, 11474078 and 61427816, the Natural Science Basic Research Plan in Shaanxi Province under Grant No 2014JQ1024, the Financial Support from China Postdoctoral Science Foundation under Grant Nos 2013M542335 and 2014T70924, and the Collaborative Innovation Center of Suzhou

Femtosecond time-resolved single-shot optical Kerr gating (OKG) measurements are performed by focusing the probe pulse and using a cylindrical lens to introduce a spatially encoded time delay with respect to the pump pulse. By measuring the pump power and polarization dependence of the OKG signals in CS2, the contribution of self-diffraction effect which is independent of the nonlinear response time of the material is directly observed on the rising edge of the time-resolved OKG signals. The influence of the self-diffraction effect on the optical Kerr signal could be controlled effectively by varying the polarization angle between pump and probe pulses.

Synthesis and Characterization of Mn:ZnSe/ZnS/ZnMnS Sandwiched QDs for Multimodal Imaging and Theranostic Applications.

In this work, a facile aqueous synthesis method is optimized to produce Mn:ZnSe/ZnS/ZnMnS sandwiched quantum dots (SQDs). In this core-shell co-doped system, paramagnetic Mn(2+) ions are introduced as core and shell dopants to generate Mn phosphorescence and enhance the magnetic resonance imaging signal, respectively. T1 relaxivity of the nanoparticles can be improved and manipulated by raising the shell doping level. Steady state and time-resolved optical measurements suggest that, after high level shell doping, Mn phosphorescence of the core can be sustained by the sandwiched ZnS shell. Because the SQDs are free of toxic heavy metal compositions, excellent biocompatibility of the prepared nanocrystals is verified by in vitro MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. To explore the theranostic applications of SQDs, liposome-SQD assemblies are prepared and used for ex vivo optical and magnetic resonance imaging. In addition, these engineered SQDs as nanocarrier for gene delivery in therapy of Panc-1 cancer cells are employed. The therapeutic effects of the nanocrystals formulation are confirmed by gene expression analysis and cell viability assay.

Influence of voltage pulse width on the discharge characteristics in an atmospheric dielectric-barrier-discharge plasma jet

We present here the analysis of the discharge characteristics of a He dielectric-barrier-discharge (DBD) plasma jet operated in the voltage duty ratio of 20 to 80% under the condition of driving voltage frequency 5 kHz. Discharge strength is sensitive to the voltage pulse width, and the pulse width of 70 µs, which corresponds to the duty ratio of 35%, leads to high O optical emission intensity. We also performed time-resolved optical emission measurements in a transient pulse discharge driven by various voltage duty ratios. Two distinct pulse discharges are observed in the rising and falling periods of the positive rectangular voltage, and the first and second discharges have a peak intensity of optical emission at different duty ratio. The observations indicate that an adequate voltage pulse width could ignite a strong discharge both in the rising and falling period of applied voltage, which could produce a large amount of reactive excited O atoms.

Coherent lattice dynamics in topological insulatorBi2Te3probed with time-resolved optical second-harmonic generation

The dynamics of phonon modes in thin flakes of topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ has been studied using transient optical second-harmonic spectroscopy. Using the surface sensitivity of second-harmonic generation, the excitation and dynamics of a family of optical phonon modes are observed with a response dependent on the excitation polarization. Three coherent phonon modes, inaccessible in previous linear time-resolved optical measurements, were detected in the nonlinear dynamic response. A symmetry-allowed surface optical phonon mode in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ has been identified using surface and symmetry selectivity of the second-order up-conversion process. Possible second-harmonic generation mechanisms in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ as well as the enhanced sensitivity of the nonlinear processes for the phonon mode detection in topological insulators are discussed.

Experimental Study of the Transient Response of Bunsen Flame to Nanosecond Pulsed Discharges

The transient processes associated with the interaction of a Bunsen flame and nanosecond pulsed discharges (NPD) are explored experimentally with two optical methods. A nanosecond-gated schlieren system is employed to image the shockwave propagation and the hydrodynamic response of the flame to NPD while the time-resolved optical emission spectroscopy measurements are carried out to determine active species and temperature in the plasma region created by the discharges. Therefore, the unsteady process of the interaction of the flame with the discharges is recorded in real-time by the combined measurements. Numbers of experimental evidences for understanding the dynamics of non-equilibrium plasma produced by NPD and performing further numerical simulation are offered.

Plasmon Dephasing in Single Gold Nanorods Observed By Ultrafast Time-Resolved Near-Field Optical Microscopy

We applied time-resolved near-field optical microscopic measurements with ultrashort light pulses of ∼16 fs duration to observe plasmon dephasing processes in single gold nanorods. The correlation widths of the time-resolved signals obtained at each position on the nanorods were broadened compared with the autocorrelation width of the pulse because of the plasmon lifetime. The correlation width maps of the rods showed spatially oscillating patterns that look similar to the plasmon mode structures observed in the static near-field optical images. The spatial variation of the correlation widths was explained as arising from the position-dependent contribution of the resonant plasmon excitation in the time-resolved signals relative to that of the nonresonant excitation. This finding indicates that the dephasing times of the resonant plasmon modes were constant regardless of the excitation position. This result is understood to be a consequence of the spatial coherence of the plasmon mode that causes the loca...

Ultrabright Luminescence from Gold Nanoclusters: Rigidifying the Au(I)-Thiolate Shell.

Luminescent nanomaterials have captured the imagination of scientists for a long time and offer great promise for applications in organic/inorganic light-emitting displays, optoelectronics, optical sensors, biomedical imaging, and diagnostics. Atomically precise gold clusters with well-defined core-shell structures present bright prospects to achieve high photoluminescence efficiencies. In this study, gold clusters with a luminescence quantum yield greater than 60% were synthesized based on the Au22(SG)18 cluster, where SG is glutathione, by rigidifying its gold shell with tetraoctylammonium (TOA) cations. Time-resolved and temperature-dependent optical measurements on Au22(SG)18 have shown the presence of high quantum yield visible luminescence below freezing, indicating that shell rigidity enhances the luminescence quantum efficiency. To achieve high rigidity of the gold shell, Au22(SG)18 was bound to bulky TOA that resulted in greater than 60% quantum yield luminescence at room temperature. Optical measurements have confirmed that the rigidity of gold shell was responsible for the luminescence enhancement. This work presents an effective strategy to enhance the photoluminescence efficiencies of gold clusters by rigidifying the Au(I)-thiolate shell.

Spectroscopic characterization of the plasma generated during the deposition of AlxGa1−xN films by pulsed laser co-ablation of Al and GaAs targets in electron cyclotron resonance nitrogen plasma

A nitrogen–aluminum–gallium–arsenic plasma is formed by pulsed laser co-ablation of an Al target and a GaAs target in electron cyclotron resonance discharge-generated nitrogen plasma for AlxGa1−xN film deposition. The formed plasma was characterized by time-integrated and time-resolved optical emission spectroscopy measurements and the process of AlxGa1−xN deposition was discussed. The plasma contains excited species originally present in the working N2 gas and energetic species ablated from the targets, and its emission is abundant in the emission bands of diatomic nitrogen molecules and molecular ions and the emission lines of monoatomic aluminum, gallium, and arsenic atoms and atomic ions. The temporal and spatial features of the plasma emission reveal that the nitrogen species in the electron cyclotron resonance nitrogen plasma experience additional excitations due to the expanding ablation plumes, and the ablated species are excited frequently when traveling with the expanding plumes in the nitrogen plasma, making the formed plasma very reactive, which is very important in the process of AlxGa1−xN film deposition. The deposited film was evaluated for composition analysis by energy-dispersive x-ray spectroscopy and structure characterization by x-ray diffraction. The AlxGa1−xN film is slightly nitrogen rich with an aluminum content x of about 0.6 and featured with hexagonal wurtzite crystal structure with preferred c-axis orientation.

Light-Controlled “Molecular Zippers” Based on Azobenzene Main Chain Polymers

Single strands of azobenzene main chain polymers exhibiting alkyl side chains can be largely and reversibly contracted and extended with light. We show that upon self-assembly in a thin layered film they act as “molecular zippers” that can be opened and closed with UV- and blue light, respectively. Simultaneously in situ recorded time-resolved X-ray diffraction and optical spectroscopy measurements, together with scanning force microscopy show that upon the light-induced E → Z isomerization of the main chain azobenzenes the layered film morphology remains, while the initially highly ordered alkyl side chains become disordered. Already the E → Z isomerization of about 20% of all azobenzene chromophores triggers a complete disorder of the alkyl chains. The kinetics of this partial amorphization of the film is about 18 times slower than the ensemble kinetics of the initial azobenzene photoisomerization. This is the first demonstration of a rigid main chain polymer film with reversibly photoswitchable side chain crystallinity.

Flow cytometry using Brillouin imaging and sensing via time-resolved optical (BISTRO) measurements.

A novel concept of Brillouin imaging and sensing via time-resolved optical (BISTRO) measurements is introduced for flow cytometry applications. The system affords robust, maintenance-free and high-speed elasticity-sensitive measurements.

Out-of-equilibrium dynamics of photoexcited spin-state concentration waves.

The spin crossover compound [FeIIH2L2-Me][PF6]2 presents a two-step phase transition. In the intermediate phase, a spin state concentration wave (SSCW) appears resulting from a symmetry breaking (cell doubling) associated with a long-range order of alternating high and low spin molecular states. By combining time-resolved optical and X-ray diffraction measurements on a single crystal, we study how such a system responds to femtosecond laser excitation and we follow in real time the erasing and rewriting of the SSCW.

Time-resolved spectroscopy using a chopper wheel as a fast shutter

Widely available, small form-factor, fiber-coupled spectrometers typically have a minimum exposure time measured in milliseconds, and thus cannot be used directly for time-resolved measurements at the microsecond level. Spectroscopy at these faster time scales is typically done with an intensified charge coupled device (CCD) system where the image intensifier acts as a "fast" electronic shutter for the slower CCD array. In this paper, we describe simple modifications to a commercially available chopper wheel system to allow it to be used as a "fast" mechanical shutter for gating a fiber-coupled spectrometer to achieve microsecond-scale time-resolved optical measurements of a periodically pulsed light source. With the chopper wheel synchronized to the pulsing of the light source, the time resolution can be set to a small fraction of the pulse period by using a chopper wheel with narrow slots separated by wide spokes. Different methods of synchronizing the chopper wheel and pulsing of the light sources are explored. The capability of the chopper wheel system is illustrated with time-resolved measurements of pulsed plasmas.

Heterogeneity in Dye–TiO2 Interactions Dictate Charge Transfer Efficiencies for Diketopyrrolopyrrole-Based Polymer Sensitized Solar Cells

Power conversion efficiency of a solar cell is a complex parameter which usually hides the molecular details of the charge generation process. For rationally tailoring the overall device efficiency of the dye-sensitized solar cell, detailed molecular understanding of photoinduced reactions at the dye-TiO2 interface has to be achieved. Recently, near-IR absorbing diketopyrrolopyrrole-based (DPP) low bandgap polymeric dyes with enhanced photostabilities have been used for TiO2 sensitization with moderate efficiencies. To improve the reported device performances, a critical analysis of the polymer–TiO2 interaction and electron transfer dynamics is imperative. Employing a combination of time-resolved optical measurements complemented by low temperature EPR and steady-state Raman spectroscopy on polymer–TiO2 conjugates, we provide direct evidence for photoinduced electron injection from the TDPP-BBT polymer singlet state into TiO2 through the C═O group of the DPP-core. A detailed excited state description of t...

Time-resolved optical measurement of thermal transport by surface plasmon polaritons in thin metal stripes

We investigate the thermal transport originating from the propagation of surface plasmon polaritons (SPPs) in a thin gold stripe. The SPPs are excited by a grating coupler on the Au stripe which was patterned onto a silicon membrane. The transmissivity changes of the Si membrane due to temperature-induced changes of the interference conditions enable measuring the temperature distribution with temporal and spatial resolution better than 1 μs and 1 μm. With this setup, we demonstrate that SPP excitation, propagation, and decay are accompanied by considerable heating and heat transport.

Effects of driving voltage frequency on the discharge characteristics of atmospheric dielectric-barrier-discharge plasma jet

We present here the analysis of the discharge characteristics of a He dielectric-barrier-discharge (DBD) plasma jet operated in the frequency range of 0.6 to 30 kHz under an open-air condition. Discharge strength is sensitive to driving voltage frequency, and an increasing driving frequency induces a weak pulse discharge with a small plume length. We also performed time-resolved optical emission measurements in a transient pulse discharge driven by various voltage frequencies. A strong optical emission from O atoms is observed near the quartz-tube outlet at a low driving voltage frequency of about 5 kHz, where more than 90% of the total O emission intensity is detected in the after-discharge period. The observations indicate that low-frequency discharge operation can generate a large number of reactive excited O atoms near the quartz-tube outlet, and this is ascribed to the chemical reactions in the after-discharge period.

Hierarchical synthesis of non-centrosymmetric hybrid nanostructures and enabled plasmon-driven photocatalysis.

Non-centrosymmetric nanostructures consisting of multiple functional subunits represents an emerging class of hybrid nanostructures that can possess dramatic difference in property and functionality from concentric core-shell configuration. Here we develop a general synthetic method to achieve hierarchical control of high-order non-centrosymmetric hybrid nanostructures. The key is to employ a common intermedium for sequential conversion to all distinct predesigned subunits under similar growth condition, thus facilitating manifold control of a hybrid nanostructure. This advancement leads to an optimally designed plasmon-mediated photocatalytic nanostructure with 14.8-fold enhancement of photocatalytic efficiency as compared with conventional photocatalysts. Mechanistic study involving theoretical modelling and ultrafast time-resolved optical measurement uncovers a hot plasmonic electron-driven photocatalysis mechanism with an identified electron transfer pathway. This study may represent an important step towards high-level control of artificial nanostructures with new horizons for fundamental and technological applications.

Energy Transfer from a Single Semiconductor Nanocrystal to Dye Molecules

In an energy transfer (ET) process, it is the optical responses of donor and acceptor materials on the single-particle level that ultimately determine its overall performance. Here we conduct time-tagged, time-resolved optical measurements to correlate the photoluminescence (PL) intensities and lifetimes of a donor semiconductor nanocrystal (NC) and acceptor dye molecules linked to its surface. We reveal that the PL intensity of dye molecules follows exactly the blinking behavior of the donor NC and shows a step-like quenching behavior due to the photobleaching effect. The corresponding recovery of the NC PL intensity has allowed us to realize the textbook definition of PL quantum efficiency measurement in dye molecules upon absorbing a single exciton. Our theoretical fitting of the lifetime data demonstrates that the buildup time of acceptor PL could be solely determined by the radiative lifetime of dye molecules when it is any shorter than the NC lifetime, thus confirming the long-existing Förster theory on ET dynamics.

Nanosecond Pulsed Electric Discharge Synthesis of Carbon Nanomaterials in Helium at Atmospheric Pressure from Adamantane

Nanosecond pulsed discharges in a helium-adamantane mixture at atmospheric pressure were used for the synthesis of carbon nanomaterials, using the smallest diamondoid, adamantane, as a precursor. The discharges were generated using a point-to-plane electrode with a gap distance of 1–2 mm, at room temperature, 373 and 473 K. Scanning electron microscopy of the electrodes showed the formation of carbonaceous deposits with different morphologies, while micro-Raman spectra of the deposits covering the electrodes contained features characteristic of carbon structures of varying order. To study the effect of the temperature conditions on the plasma characteristics, time-resolved optical emission spectroscopy measurements were carried out at room temperature and 456 K. The optical emission spectra showed that at room temperature the afterglow has a duration of about several microseconds, whereas at 456 K, the plasma decay is on the order of only 1 \({\mu }\) s. This may be an indication that the carbon nanomaterials are predominantly formed in the afterglow of the nanosecond pulsed discharge, with the presence of hydrogen radicals facilitating the formation of carbonaceous materials.