Time-resolved Transient Grating Research Articles

Carrier Dynamics in InGaN/GaN on the Basis of Different In Concentrations

InGaN/GaN samples grown on c-plane sapphire substrate with different In concentrations by metal organic chemical vapor deposition are demonstrated. The subsequent capping GaN layer growth opens a possibility for dislocation reduction due to the lateral strain relaxation in growth geometry. We present the further growth optimization and innovative characterization of InGaN layers overgrown on different structures with varying In concentrations. The photoelectrical and optical properties of the InGaN layers with/without capping GaN layer are investigated by time-resolved picosecond transient grating and temperature dependence photoluminescence. We note a 10-fold increase in carrier lifetime in the InGaN layers when the sample structure changed from PIN to single InGaN layer.

Nonlinear XUV-optical transient grating spectroscopy at the Si L2,3–edge

Time-resolved transient grating (TG) spectroscopy facilitates detailed studies of electron dynamics and transport phenomena by means of a periodic excitation of matter with coherent ultrashort light pulses. Several current and next generation free-electron laser (FEL) facilities provide fully coherent pulses with few femtosecond pulse durations and extreme ultraviolet (XUV) photon energies. Thus, they allow for transient grating experiments with periodicities as small as tens of nanometers and with element specific photon energies. Here, we demonstrate the element specificity of XUV TG (X-TG) experiments by tuning the photon energy across the Si L2,3-edge of Si3N4. We observe a shortening of the signal decay when increasing the XUV photon energy above the absorption edge. The analysis of the wavelength dependent signal shows that the faster decay is driven by the increase in the charge carrier density. From the decay constants the interband Auger coefficient at elevated temperatures and high electron densities has been determined.

Time-Resolved Study of Interprotein Signaling Process of a Blue Light Sensor PapB-PapA Complex.

PapB is a short BLUF protein from the purple bacterium Rhodopseudomonas palustris, and it has the function of regulating the phosphodiesterase activity of its partner protein PapA. Because PapA is an enzymatic protein containing an EAL domain, which hydrolyzes a cyclic-dimeric-GMP, the PapB-PapA complex regulates, for example, biofilm formation in a light-dependent manner. The reaction dynamics of PapB, as well as dynamics of signal transduction in the PapB-PapA complex, were studied mostly by time-resolved transient grating and CD methods. It was found that although PapB exists as a monomer in both dark and light states, the diffusion coefficient slightly increases and partial molar volume changes upon light irradiation with time constants of 80 μs, 3.2 ms, and 24 ms. The last conformational change leads to a signaling state that is attributed to the α-helix unfolding in the C-terminal region of the BLUF domain. Interestingly, when PapA is added to a PapB solution to form a PapB-PapA complex, the diffusion coefficient decreases significantly, with a time constant of 26 ms, which is similar to the rate of the last conformational change of PapB. It was found that this change is due to the conformational change of PapA, and the light signal from PapB is transmitted through the conformational change of PapB. Compared with another BLUF protein, BlrP1, which possesses the EAL effector domain and requires photoexcitation of two monomer units of the dimer for a major conformational change, the conformational change of the PapB-PapA complex is induced by excitation of one monomer PapB among the heterotetramer.

Transient grating study of the intermolecular dynamics of liquid nitrobenzene**Project supported by the National Natural Science Foundation of China (Grant Nos. 11304058 and 11404307) and NSAF (Grant No. U1330106).

Femtosecond time-resolved transient grating (TG) technique is used to study the intermolecular dynamics in liquid phase. Non-resonant excitation of the sample by two crossing laser pulses results in a transient Kerr grating, and the molecular motion of liquid can be detected by monitoring the diffraction of a third time-delayed probe pulse. In liquid nitrobenzene (NB), three intermolecular processes are observed with lifetimes of 37.9±1.4 ps, 3.28±0.11 ps, and 0.44±0.03 ps, respectively. These relaxations are assigned to molecular orientational diffusion, dipole/induced dipole interaction, and libration in liquid cage, respectively. Such a result is slightly different from that obtained from OKE experiment in which the lifetime of the intermediate process is measured to be 1.9 ps. The effects of electric field on matter are different in TG and optical Kerr effect (OKE) experiments, which should be responsible for the difference between the results of these two types of experiments. The present work demonstrates that TG technique is a useful alternative in the study of intermolecular dynamics.

Tracking the photodissociation dynamics of liquid nitromethane at 266 nm by femtosecond time-resolved broadband transient grating spectroscopy

Femtosecond time-resolved transient grating (TG) technique was employed to get insight into the photodissociation mechanism of liquid nitromethane (NM). Broadband white-light continuum was introduced as the probe to observe the evolution of electronic excited states of NM molecules and the formation of photodissociation products simultaneously. The reaction channel of liquid NM under 266nm excitation was obtained that NM molecules in excited state S2 relax through two channels: about 73% relax to low lying S1 state through S2/S1 internal conversion with a time constant of 0.24ps and then go back to the ground state through S1/S0 internal conversion; the other 27% will dissociate with a time constant of 2.56ps. NO2 was found to be one of the products from the experimental TG spectra, which confirmed that C–N bond rupture was the primary dissociation channel of liquid NM.

Photo-induced inter-protein interaction changes in the time domain; a blue light sensor protein PixD

ABSTRACT : For understanding molecular mechanisms of photochemical reactions, in particular reactions of proteins with biological functions, it is important to elucidate both the initial reactions from the photoexcited states and the series of subsequent chemical reactions, e.g., conformation, intermolecular interactions (hydrogen bonding, hydrophobic interactions), and inter-protein interactions (oligomer formation, dissociation reactions). Although time-resolved detection of such dynamics is essential, these dynamics have been very difficult to track by traditional spectroscopic techniques. Here, relatively new approaches for probing the dynamics of protein photochemical reactions using time-resolved transient grating (TG) are reviewed. By using this method, a variety of spectrally silent dynamics can be detected and such data provide a valuable description about the reaction scheme. Herein, a blue light sensor protein TePixD is the exemplar. The initial photochemistry for TePixD occurs around the chromophore and is detected readily by light absorption, but subsequent reactions are spectrally silent. The TG experiments revealed conformational changes and changes in inter-protein interactions, which are essential for TePixD function. The TG experiments also showed the importance of fluctuations of the intermediates as the driving force of the reaction. This technique is complementary to optical absorption detection methods. The TG signal contains a variety of unique information, which is difficult to obtain by other methods. The advantages and methods for signal analyses are described in detail in this review. ultraviolet

Photoinduced intermolecular electron transfer and off-resonance Raman characteristics of Rhodamine 101/N,N-diethylaniline

The ultrafast photoinduced intermolecular electron transfer (PIET) reaction of Rhodamine 101 (Rh101+) in N,N-diethylaniline (DEA) was investigated using off-resonance Raman, femtosecond time-resolved multiplex transient grating (TG) and transient absorption (TA) spectroscopies. The Raman spectra indicate that the CC stretching vibration of the chromophore aromatic ring is more sensitive to ET compared with the CC stretching mode. The ultrafast photoinduced intermolecular forward ET (FET) from DEA to Rh101+∗ occurs on a time scale of τFET=425–560fs. The backward ET (BET) occurs in the inverted region with a time constant of τBET=46.16–51.40ps. The intramolecular vibrational relaxation (IVR) process occurs on the excited state potential energy surface with the time constant of τIVR=2.77–5.39ps.

Photoinduced electron transfer of Rhodamine 6G/N,N-diethylaniline revealed by multiplex transient grating and transient absorption spectroscopies

The dynamics and spectroscopic characteristics of the ultrafast photoinduced electron transfer (ET) of Rhodamine 6G (Rh6G+) in N,N-diethylaniline (DEA) were studied using femtosecond time-resolved multiplex transient grating and transient absorption spectroscopies. The ultrafast photoinduced forward ET from DEA to the Rh6G+* cation radical excited state has a time constant of τFET = 219–318 fs. The much slower backward ET from the neutral radical Rh6G· to DEA+ with a time constant of τBET = 22.76–42.31 ps occurs in the inverted region. Intramolecular vibrational relaxation of the excited state takes place in τIVR = 2.18–6.91 ps.

Dynamics of the Amino-Terminal and Carboxyl-Terminal Helices ofArabidopsisPhototropin 1 LOV2 Studied by the Transient Grating

Recently, conformational changes of the amino-terminal helix (A'α helix), in addition to the reported conformational changes of the carboxyl-terminal helix (Jα helix), have been proposed to be important for the regulatory function of the light-oxygen-voltage 2 domain (LOV2) of phototropin 1 from Arabidopsis. However, the reaction dynamics of the A'α helix have not been examined. Here, the unfolding reactions of the A'α and Jα helices of the LOV2 domain of phototropin 1 from Arabidopsis thaliana were investigated by the time-resolved transient grating (TG) method. A mutant (T469I mutant) that renders the A'α helix unfolded in the dark state showed unfolding of the Jα helix with a time constant of 1 ms, which is very similar to the time constant reported for the wild-type LOV2-linker sample. Furthermore, a mutant (I608E mutant) that renders the Jα helix unfolded in the dark state exhibited an unfolding process of the A'α helix with a time constant of 12 ms. On the basis of these experimental results, it is suggested that the unfolding reactions of these helices occurs independently.

Carrier lifetime, diffusion length and mobility in (100) CVD diamond samples pre-treated in an O2/H2-plasma

ABSTRACTIn this article, (100) Ib diamond substrates are pre-treated for different durations in an O2/H2-plasma, influencing the etching of defects like unepitaxial crystals, flat top hillocks, and pyramidal hillocks [1-3]. Such procedure is used to prevent those to incorporate into the ~ 100μm thick CVD diamond film, which is subsequently grown on top. While the surface morphology and structure of substrates and films are studied by SEM, the time-resolved Light-Induced Transient Grating (LITG) technique provides information on the excess carrier parameters close to the front surface of the grown layers. This technique is particularly useful as it does not requires a separation of the CVD film from its substrate. It will be shown that O2/H2-plasma treatments of more than 150 minutes but less than 240 minutes largely reduce the incorporation of defects in the bulk of the grown film. This, in turn, influences the carrier dynamics as measured by LITG, but also the surface roughness and growth rate as shown by SEM.

Carrier dynamics in coalescence overgrowth of GaN nanocolumns

Coalescence overgrowth of pattern-grown GaN nanocolumns on c-plane sapphire substrate with metal organic chemical vapor deposition is demonstrated. The subsequent coalescence overgrowth opens a possibility for dislocation reduction due to the lateral strain relaxation in columnar geometry. We present further growth optimization and innovative characterization of metal organic chemical vapor deposition layers, overgrown on the columnar structure with varying diameters of columns. Nano-imprint lithography was applied to open circular holes of 250, 300, 450, and 600 nm diameter on the SiO 2 layer, deposited on the GaN layer on the c-plane sapphire template. After the growth of ~ 1 μm high GaN nanocolumns, the further coalescence conditions led to an overgrown layer ~2 μm thickness. Photoelectrical and optical properties of the overgrown layers and a reference sample were investigated by time-resolved picosecond transient grating and time-integrated photoluminescence. We note a 3–4 fold increase in carrier lifetime in the overgrown epilayers when the diameter of columns increased from 250 to 450 nm. This feature is a clear indication of an ~4-fold reduced defect density.

On the Correlation of the Structural Perfection and Nonequilibrium Carrier Parameters in 3C SiC Heterostructures

Photoelectric properties of 3C sublimation-grown epitaxial layers with different structural quality were studied by using time-resolved picosecond transient grating and free carrier absorption techniques. The layer quality was described by a parameter LTW which gives the total length of twin boundaries in a layer. Optical measurements of diffusion coefficients and carrier lifetimes in wide excess carrier density (N >1018 cm-3) and temperature range (10 K to 300 K) revealed the twin defect density dependent ambipolar mobility value at RT as well as essentially different temperature dependences of mobility of the layers. The larger value of absorption cross section in more defective layer at 1064 nm wavelength pointed out to contribution of defect-assisted absorption, which gradually vanished after the filling defect states by free carriers.

Interfacial Effects in the Relaxation Dynamics of Silver Nanometal-Glass Composites Probed by Transient Grating Spectroscopy

Relaxation dynamics of silver nanoparticles (NPs) embedded in glass have been studied by picosecond time-resolved transient grating (TG) spectroscopy. Phosphate-based glasses were prepared by melting and heat-treatment processes by which two different nanometal-glass composites were produced. The first is a glass system containing silver and tin in which Ag NPs are embedded in the matrix upon heat treatment. The second is a heat-treated silver-doped glass with spectroscopic indications of Ag+–Ag0 pairs located at or near the surface of the NPs. The time evolution of the light-induced TG for the Ag/Sn-doped glass shows an uncommon relaxation on the nanosecond time scale. Such behavior is explained in terms of energy transfer processes between polaronic and/or excitonic states in the near-interface region of the glass matrix and the NPs. In contrast, a faster monotonic relaxation is observed for the Ag-doped nanocomposite. This result is attributed to Ag NP → Ag+–Ag0 plasmon resonance energy transfer.

Photochemical reaction and diffusion of caged calcium studied by the transient grating

A photoreaction of caged calcium (Ca 2+) was investigated by using the time-resolved transient grating (TG) method. The q 2-dependent feature of the TG signal was analyzed based on a model that there are two parallel dissociation steps with different rates and Ca 2+ is released promptly after the fracture of the caged compound. The TG signal representing the presence of Ca 2+ was appeared by the volume contribution. Although the diffusion coefficients of Cg and the decomposed product are different without Ca 2+, only one diffusion component was observed after the dissociation of CgCa 2+, indicating that the caged compounds and Ca 2+ diffuse together.

Photochemical Intermediates of Arabidopsis Phototropin 2 LOV Domains Associated with Conformational Changes

The photochemical reactions of Arabidopsis phototropin 2 light- oxygen-voltage domain 2 (LOV2) with the linker region (LOV2-linker), without the linker (LOV2), and LOV1 were studied using the time-resolved transient grating (TG) and transient lens (TrL) methods. Although the absorption spectra did not change after the formation of the adduct species, a small volume expansion process with a time constant of 9 ms was observed for LOV2. For the LOV2-linker, at 293 K, a volume contraction process with a time constant of 140 μs was observed in addition to a volume expansion process with 9 ms and the diffusion coefficient change with 2 ms. The reaction intermediate species were characterized on the basis of their thermodynamic properties, such as changes in enthalpy, thermal expansion, and heat capacity. For the first intermediate (S 390), the values of these properties were similar to those of the ground state for both LOV2 and LOV2-linker. A relatively large thermal expansion volume (0.09 cm 3mol − 1 K − 1 ) and a positive heat capacity change (4.7 kJ mol − 1 K − 1 ) were detected for the intermediates of LOV2-linker. These characteristic features were interpreted in terms of structural fluctuation and exposure of hydrophobic residues in the linker domain, respectively. The enthalpy change of S 390 of the LOV1 domain was significantly greater than changes for the LOV2 or LOV2-linker samples. Data from this study support a major conformational change of the linker region in the photochemical reaction of phototropin.

Dynamics of Conformational Changes of Arabidopsis Phototropin 1 LOV2 with the Linker Domain

Conformational changes of Arabidopsis phot1-LOV2 with the linker (phot1-LOV2-linker) were investigated from the viewpoint of the changes in molecular volume and molecular diffusion coefficient (D) by time-resolved transient grating (TG) and transient lens (TrL) methods. Although the absorption spectrum change completes within a few microseconds, the D-value detected by the TG method decreased drastically with a time constant of 1.0 ms from 9.2(±0.4)×10−11 m2/s to 5.0(±0.3)×10−11 m2/s. This time-dependent D was interpreted in terms of the unfolding of α-helices in the linker region. The change of the α-helices was confirmed by observing the recovery of the circular dichroism intensity. The TrL signal showed that the molecular volume decreases with two time constants; 300 μs and 1.0 ms. The former time constant is close to the previously observed photo-dissociation reaction rate of the phot1-LOV2 (without the linker) dimer, and the latter one agrees well with the rate of the D-change. Considering a similar time constant of the dissociation reaction of the LOV2 dimer, we interpreted these kinetics in terms of the dissociation step of the linker region from the LOV2 domain (T390pre state). After this step, the protein volume and D are decreased significantly with the lifetime of 1.0 ms. The D decrease indicates the increase of the intermolecular interaction between the protein and water molecules. On the basis of these observations, a two-step mechanism of the linker unfolding is proposed.

Photochemical Reaction of 2-Nitrobenzaldehyde by Monitoring the Diffusion Coefficient

The photochemical reaction of 2-nitrobenzaldehyde (NBA) in various electrolyte solutions as well as in a solution with a protein is investigated from the viewpoint of the diffusion coefficient by using the time-resolved transient grating (TG) method. For studying reaction with optically silent materials such as electrolytes, we found that the diffusion coefficient is useful for identifying chemical species involved in the reaction. The TG signal that represents the diffusion process was dramatically changed by adding electrolytes such as Cl- or Br-, and this result indicates that these electrolytes should participate in the photochemical reaction of NBA. It was found that the time profile of the grating signal also depends on the addition of guanidine−HCl or apomyoglobin. The structural change of the protein associated with the NBA reaction is discussed.

Photoreaction of caged ATP studied by the time-resolved transient grating method.

The photoreaction of caged ATP, P3-[1-(2-nitrophenyl)ethyl]adenosine-5'-triphosphate, has been investigated using the time-resolved transient grating (TG) method. We found that a feature of the TG signal time profile depends sensitively on the grating wavenumber (q) after the photoexcitation of caged ATP. This q-dependent feature of the TG signal was interpreted based on a model where the ATP release rate is comparable to the molecular diffusion process. We found that the TG signals at various q can be consistently analyzed based on this model and the ATP release rate determined. The enthalpy and volume changes of the reaction have been determined by quantitative measurement of the grating and photoacoustic signals.

Energy and volume changes induced by photoinitiated proton releasing reaction with apomyoglobin

The photodissociation reaction of o-nitrobenzaldehyde (NBA) aqueous solution and of the NBA solution with a protein, apomyoglobin (ApoMb), were studied by the time-resolved transient grating (TG) technique. The amount of released thermal energy and the volume change accompanied with this reaction were determined by the TG and photoacoustic method. Without the protein, the photoproduct of NBA releases a proton in the solution (pH jump reaction). The time profile of the grating signal of NBA and ApoMb shows two diffusing species with diffusion coefficients of 1.39±0.15 and 0.14±0.02×10−9 m2 s−1, respectively. From the diffusion coefficients, we suggest that the photoproduct of NBA induces a molecular recombination reaction between ApoMb and another small molecule or ion in the solution.

Protein dynamics detected by the time-resolved transient grating technique

Abstract Dynamics of two proteins [myoglobin and photoactive yellow protein (PYP)] initiated by photoexcitation were studied by the time-resolved transient grating (TG) and photo-acoustic methods. For both proteins, the temporal profiles of the TG signals showed new features that have not been detected before by the transient absorption method. These dynamics were attributed to the volume change, which is caused by the protein dynamics apart from the chromophores.