Laser-induced Transient Gratings Research Articles

Analysis of laser-induced transient population gratings by different types of exciting pulse

Motivated by atomic response to different initial coherent optical fields, we comparatively studied transient population grating (TPG) induced by successive pulse train. Time delay and pump pulse duration dependence of TPG is achieved by numerically solving the density matrix equations. Results reveal that the creation and erasure of TPG is possible by choosing the appropriate pulse parameters, which is illustrated by Bloch sphere model and quantitative validation. To obtain desired large grating amplitude for rectangular pulse, the allowed pulse duration can be extended to one order wider than that of Gaussian pulse. Population grating can be erased to near zero by a third pulse with time delay by an odd multiple of half the pulse width, and it also can be erased further to recover atom assembly back to the initial state by a fourth pulse with time delay equal to an integer multiple of pulse width. Atomic behaviors excited by different types of pulse presented here may be significant to manipulate TPG during coherent light-matter interaction.

A new concept for temporal gating of synchrotron X-ray pulses.

A new concept for temporal gating of synchrotron X-ray pulses based on laser-induced thermal transient gratings is presented. First experimental tests of the concept yield a diffraction efficiency of 0.18%; however, the calculations indicate a theoretical efficiency and contrast of >30% and 10-5, respectively. The full efficiency of the pulse picker has not been reached yet due to a long-range thermal deformation of the sample after absorption of the excitation laser. This method can be implemented in a broad spectral range (100 eV to 20 keV) and is only minimally invasive to an existing setup.

Transient grating spectroscopy of photocarrier dynamics in semiconducting polymer thin films

While charge carrier dynamics and thermal management are both keys to the operational efficiency and stability for energy-related devices, experimental techniques that can simultaneously characterize both properties are still lacking. In this paper, we use laser-induced transient grating (TG) spectroscopy to characterize thin films of the archetypal organic semiconductor regioregular poly(3-hexylthiophene) and its blends with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester on glass substrates. While the thermal responses from the thin film and the substrate cannot be distinguished due to their similar thermal diffusivities, we show that the recombination dynamics of photocarriers in the organic semiconductor thin films occur on a similar timescale and can be separated from the thermal response. Our measurements indicate that the photocarrier dynamics are determined by multiple recombination processes and our extracted recombination rates are in good agreement with previous reports using other techniques. We further apply TG spectroscopy to characterize another conjugated polymer and a molecular fluorescent material to demonstrate its general applicability. Our study indicates the potential of transient grating spectroscopy to simultaneously characterize thermal transport and photocarrier dynamics in organic optoelectronic devices.

Spatiotemporal Coherent Control of Thermal Excitations in Solids.

X-ray reflectivity measurements of femtosecond laser-induced transient gratings (TG) are applied to demonstrate the spatiotemporal coherent control of thermally induced surface deformations on ultrafast time scales. Using grazing incidence x-ray diffraction we unambiguously measure the amplitude of transient surface deformations with sub-Å resolution. Understanding the dynamics of femtosecond TG excitations in terms of superposition of acoustic and thermal gratings makes it possible to develop new ways of coherent control in x-ray diffraction experiments. Being the dominant source of TG signal, the long-living thermal grating with spatial period Λ can be canceled by a second, time-delayed TG excitation shifted by Λ/2. The ultimate speed limits of such an ultrafast x-ray shutter are inferred from the detailed analysis of thermal and acoustic dynamics in TG experiments.

Thermal transport in suspended silicon membranes measured by laser-induced transient gratings

Studying thermal transport at the nanoscale poses formidable experimental challenges due both to the physics of the measurement process and to the issues of accuracy and reproducibility. The laser-induced transient thermal grating (TTG) technique permits non-contact measurements on nanostructured samples without a need for metal heaters or any other extraneous structures, offering the advantage of inherently high absolute accuracy. We present a review of recent studies of thermal transport in nanoscale silicon membranes using the TTG technique. An overview of the methodology, including an analysis of measurements errors, is followed by a discussion of new findings obtained from measurements on both “solid” and nanopatterned membranes. The most important results have been a direct observation of non-diffusive phonon-mediated transport at room temperature and measurements of thickness-dependent thermal conductivity of suspended membranes across a wide thickness range, showing good agreement with first-principles-based theory assuming diffuse scattering at the boundaries. Measurements on a membrane with a periodic pattern of nanosized holes (135nm) indicated fully diffusive transport and yielded thermal diffusivity values in agreement with Monte Carlo simulations. Based on the results obtained to-date, we conclude that room-temperature thermal transport in membrane-based silicon nanostructures is now reasonably well understood.

Optical study of carrier diffusion and recombination in CVD diamond

We report on the measurements of nonequilibrium carrier dynamics in monocrystalline diamond prepared by the chemical vapor deposition (CVD) technique. Nonlinearly excited laser‐induced transient grating (LITG) and time‐resolved photoluminescence (PL) were used for investigation of carrier diffusion and recombination. Diffusion constant was observed to be strongly dependent on the sample temperature and carrier density (D = 2.5–35 cm2 s−1) what was explained in terms of the density dependent carrier‐carrier scattering. The new method based on the time‐resolved PL was proposed for measuring the diffusion and recombination dynamics of excitons. The measured exciton lifetime τFE = 700–800 ns was temperature independent between 100 and 295 K.

Macromolecular Crowding Effects on Reactions of TePixD (Tll0078)†

To reveal macromolecular crowding effects on a chemical reaction of a BLUF (sensors of blue light using FAD) protein (PixD from a thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 [TePixD, Tll0078]), the photoreaction was studied at various concentrations of the macromolecule Ficoll-70 by UV/Vis absorption spectroscopy and the pulsed laser-induced transient grating (TG) method. The absorption spectrum did not change with varying concentration of Ficoll-70. The crowding did not affect the quantum yield of the spectral red shift reaction, recovery rate of the product, rate constant of the volume change reaction and the magnitude of the volume change. However, the magnitude of the TG signal representing the diffusion-sensitive conformation change significantly increased on addition of Ficoll-70. This dependence was attributed to the crowding effect on the TePixD decamer-pentamer equilibrium in the solution. This result indicates that the TePixD reaction is more efficient in cellular than in in vitro conditions.

Diffraction of white-light supercontinuum by femtosecond laser-induced transient grating in carbon bisulfide

Experiments on fs laser-induced transient grating (LITG) in carbon bisulfide (CS2) are carried out to explore the chirp characteristics of a white-light supercontinuum (SC) generated by a 800-nm, 160-fs laser pulse in a 4-mm thick Al2O3 crystal. Two orders of diffraction signals of SC by fs LITG in CS2 are observed, demonstrating that both the third-order process and the fifth-order process are present simultaneously. The experimental results also imply that the formation of an fs transient refractive-index grating in CS2 is mainly due to the electronic polarization process.

Diffusion of gold ions and gold particles during photoreduction processes probed by the transient grating method

The translational diffusion of Au (gold) ions and Au particles during the photoreduction process from AuCl − 4 to metallic Au particles in aqueous ethanol solutions containing poly( N-vinyl-2-pyrrolidone) was investigated by using UV–vis absorption and the laser-induced transient grating (TG) methods. The TG signal of AuCl − 4 solution before photoirradiation was composed of three contributions; the thermal grating, the species grating due to the creation of AuCl − 2, and that due to the depletion of AuCl − 4. Upon photoirradiation, the species grating signal due to AuCl − 4 diminished rapidly and the TG signal due to Au particles appeared within a few minutes and became stronger at a very short time. The subsequent reduction of AuCl − 2 was concomitant with the formation of Au metal particles. The rapid growth of Au 0 atoms into Au particles took place in the short-duration photoirradiation. With the increase of the photoirradiation time, the TG signal was composed of two kinds of Au particle fragments possessing different diffusion coefficients. This is probably due to the pulse-laser induced fragmentation of the larger Au particles. Effects of the polymer on the particle formation were investigated by the concentration dependence of the polymer in the solution. The formation of Au particles by the photoreduction was also compared with that of Pt particles.

Finite element analysis of three-dimensional laser-induced transient thermal grating in diamond/ZnSe system

This paper uses finite element method to obtain the three-dimensional temperature field of laser-induced transient thermal grating (TTG) for two-layered structure of diamond film on ZnSe substrate. The numerical results indicate that unique two-times heating process is gradually experienced in the area between two adjacent grating stripes. However, there is a little change for the temperature field along the depth direction for the diamond film due to its great thermal conductivity. It further finds that the thickness of the diamond film has a significant influence on the temperature field in diamond/ZnSe system. The results are useful for the application of laser-induced TTG technique in film/substrate system.

Finite element modeling of laser-induced three-dimensional Transient Thermal Grating in two-layered systems

The three-dimensional (3D) temperature field of laser-induced transient thermal grating (TTG) for two-layered structures such as diamond/substrate is obtained by the finite element method (FEM). The numerical results indicate that the thickness of the diamond film has a significant influence on the temperature field. On the other hand, we further find that variation of the substrate significantly affects on the temperature distribution in the diamond film, which has been ascribed to the various thermal conductivities of the substrates.

Transient Dimerization and Conformational Change of a BLUF Protein: YcgF

The photochemical reaction dynamics of YcgF, a BLUF protein, were investigated by the pulsed laser-induced transient grating (TG) technique. The TG signal showed three reaction time constants: 2.7 micros, 13 micros, and 2 ms. The fastest was tentatively attributed to relaxation of the excited triplet state of the chromophore, flavin adenine dinucleotide (FAD), and the others represented conformational changes of the protein. The TG signal provided clear evidence that the diffusion coefficient (D) of the photoproduct (3.8x10(-11) m2 s-1) was significantly less than that of the reactant (8.3x10(-11) m2 s-1), with a time constant of 2 ms at a protein concentration of 700 microM. Interestingly, the rate constant increased in proportion to the concentration of the protein, indicating that protein dimerization was one of the main reactions occurring after photoexcitation. The significant reduction in D indicates that a conformational change leading to an increase in interactions with water molecules occurs upon formation of the signaling state. The 13 mus dynamics was attributed to the conformational change that induced transient dimerization. This conformational change might be an essential process for the creation of the signaling state. A detailed scheme for the photochemical reaction of YcgF is proposed.

Diffusion of Platinum Ions and Platinum Nanoparticles during Photoreduction Processes Using the Transient Grating Method

The photoreduction process of PtCl(6)2- to Pt nanoparticles in poly(N-vinyl-2-pyrrolidone) solutions upon UV light irradiation was investigated by monitoring the change in the diffusion coefficient (D). The D values of chemical species during UV irradiation was measured by the laser-induced transient grating (TG) method. The TG signal of the PtCl(6)2- solution before UV irradiation was composed of three kinds of contributions, the thermal grating, the species grating due to the creation of PtCl4(2-), and the species grating due to the depletions of PtCl6(2-). Upon UV irradiation of the solution, the species grating signal due to PtCl6(2-) diminished and then the TG signal of Pt nanoparticles gradually appeared. This result indicates that the gradual clustering of Pt0 atoms into Pt nanoparticles occurs after all PtCl(6)2- ions are photochemically reduced to PtCl(4)2- and subsequently transformed to Pt0 atoms with a short delay. With increasing time of the UV irradiation, the TG signal intensity increased, while D of the Pt nanoparticles did not change. This suggests that the number of Pt nanoparticles increases, but the size of the Pt nanoparticles with the polymer layer is unchanged, in the course of the UV irradiation.

Femtosecond laser-induced transient grating in CeO 2 -doped 75TeO 2 –20Nb 2 O 5 –5ZnO glass

Multiple order self-diffractions by femtosecond laser-induced transient grating in CeO2 doped 75TeO2–20Nb2O5–5ZnO glass are investigated by pump-probe technique. The glass structure and component are investigated by Raman and visible–NIR absorption spectra. It is showed that there exists Ce3+ ion in tellurite glass and the transient grating is created by the optical Kerr effects and improved by the excited particle grating from the resonant absorption of the 1S0→1F3 electronic energy transition of Ce3+ ion and the conversion efficiency of the first order diffraction signal reaches about 11%. The results indicate that CeO2 doped 75TeO2–20Nb2O5–5ZnO glass has potential applications in ultra-fast all-optical switching.

Excitation energy transfer in a weakly coupled system: Studies with time-resolved fluorescence microscopy and laser induced transient grating techniques

Excitation energy transfer in a new dye pair eosin (donor) to 1,1′,3,3,3′,3′-hexamethyl indotricarbocyanine iodide (HITCI) (acceptor) has been studied in solutions of dimethyl sulfoxide (DMSO) and polymer microparticles by picosecond time-resolved fluorescence microscopy and laser induced transient grating (LITG) techniques. There exits a large shift between the fluorescence maximum of the donor (552 nm) and the absorption maximum of the acceptor (747 nm) with a small spectral overlap resulting in a weak coupling between the two. On increasing the acceptor concentration, the fluorescence intensity and the fluorescence lifetime of the donor decreases in diffusion-controlled fashion. The fluorescence decay curves have been measured by time-correlated single photon counting (TCSPC) technique on ns time scale, while the LITG technique has been used to monitor the short-time decay dynamics. The critical transfer distance calculated from the overlap integral is 34 ± 1 Å. Förster theory of excitation energy transfer has been used to interpret the kinetics at higher acceptor concentrations. When the donor–acceptor system is embedded in single microspheres, while an enhancement in the radiative rate of the donor is observed, but no conclusive evidence of modification of the rate of the excitation energy transfer is found.

Optical nonlinearity of organic dyes as studied by Z-scan and transient grating techniques

The excited state absorption cross-section of 5,5′-dichloro-11-diphenyl-amino-3,3′-diethyl-10, 12-ethylinethiatricarbocyanine perchlorate (IR140) have been measured by using a single beam transmission technique. Z-scan experiments have been used to find out a few nonlinear parameters. The excited state relaxation times have also been measured by using laser induced transient grating (LITG) technique.

Theoretical and experimental studies on laser-induced transient gratings in laser dyes

Laser-induced transient grating technique has been used to measure the diffraction efficiency ( η) and calculate the third-order nonlinear susceptibility ( χ (3)) of some laser dyes. Theoretical simulations have been carried out on η and χ (3) as a function of wavelength covering the spectral range corresponding to the first excited singlet state of the dyes. Theoretically simulated values have been found in agreement to those observed experimentally. The decay profiles for these dyes have been measured by using diffraction of a delayed probe laser pulse to estimate the relaxation times in the excited state.

Denaturation of a Protein Monitored by Diffusion Coefficients: Myoglobin

Change of global protein structure as a function of concentration of a denaturant, guanidine hydrochloride (Gdn−HCl), was studied by using the laser-induced transient grating (TG) method for carboxymyoglobin (MbCO). From the time profile of the TG signal after the photodissociation reaction of the ligand, diffusion coefficients (D) of the protein were determined at various concentrations of the denaturant. The denaturation curve of MbCO monitored by D was compared with that monitored by the circular dichroism (CD) method. The −m value and Δ determined from the transition curve monitored by D are smaller than those obtained from the CD signal intensity. This noncoincidence of the two transition curves indicates that the global structure of Mb is still changing after the complete secondary structure (α-helices) deformation process. The smaller diffusion coefficient of unfolded MbCO compared to folded MbCO can be interpreted in terms of changes of the protein−water interaction and the surface roughness of th...

Theoretical and experimental studies on photophysical parameters of N, N′-bis(2,5,-di- tert-butylphenyl)-3,4:9-10-perylenebis(dicarboximide) (DBPI) by using transient grating technique

Diffraction efficiency of laser induced transient grating (LITG) for N, N ′-bis(2,5,-di- tert-butylphenyl)-3,4:9-10-perylenebis(dicarboximide) (DBPI) has been experimentally measured in solutions of benzene and dimethyl sulfoxide. Third-order nonlinear susceptibility ( χ (3)) values have been obtained from these measurements and have been compared to those obtained from theory. Further, the relaxation time of DBPI has been estimated by measuring the decay profiles of the LITG.

Themodynamic and transport properties of intermediate states of the photocyclic reaction of photoactive yellow protein.

Themodynamic and transport properties of intermediate states of the photocyclic reaction of photoactive yellow protein (PYP) were studied by a combination of the pulsed laser-induced transient grating (TG), transient lens (TrL), and photoacoustic (PA) spectroscopies from tens of nanoseconds to hundreds of milliseconds. The diffusion coefficients (D) of PYP in the ground state (pG) and of the second intermediate state (pB) were determined by the TG analysis, and it was found that D of pG is about 1.2 times larger than D of pB. At the same time, D at various denatured conditions were measured using guanidine hydrochloride as the denaturant. D of completely unfolded protein is about 0.4 times that of the native form. The enthalpy of pB is estimated to be 60 kJ/mol by the TrL method with an assumption that the volume change of pB is not sensitive to the temperature. Since the enthalpy of the first intermediate state (pR) is as high as 160 kJ/mol, it implies that most of the photon energy is stored as the strain of the protein in pR, and this may be the driving force for the successive reaction to pB. From the temperature dependence of the volume change, the difference in the thermal expansion coefficients between pG and pR was calculated. All of the characteristic features of PYP, the negative volume change, the larger thermal expansion coefficient, and the slower diffusion process, indicate that the intermediate pR and pB are reasonably interpreted in terms of the unfolded (loosened) protein structure.