Laser Pump-probe Technique Research Articles

Au ion irradiation induces ultralow thermal conductivity in GaN

Gallium nitride (GaN) is widely considered as a crucial semiconductor for the nuclear industry and space explorations due to its superior radiation hardness. Despite extensive studies of the electronic and optical properties of irradiated GaN, the effects of particle irradiation on the thermal properties remain largely unexplored. Here, we begin with single-crystalline GaN and employ an accelerator equipped with heavy gold ions (Au2+) as the radiation source in order to imitate extreme environments and maximize lattice damages. Eight different irradiated samples are prepared with the fluence of Au2+ spanning four orders of magnitude from 1011 to 1015 cm−2. The thermal conductivity (κ) of the ion-affected regions is measured using the laser pump–probe technique of frequency-domain thermoreflectance. We find that κ decreased consistently and notably with increasing irradiation fluence and observe a transition from crystal to glass-like thermal transport. Remarkably, the room-temperature κ of the GaN sample with the highest Au2+ fluence of 1 × 1015 cm−2 reaches about 1 Wm−1 K−1, which is two orders of magnitude lower than the κ of pristine GaN and approaches the theoretical minimum. A Callaway-type model captures the phonon–point defect scattering in samples with relatively low ion fluences. At higher fluences, the increased defect types and densities, together with the formation of nitrogen bubbles, further suppress phonon transport. Our findings are instrumental in fundamentally understanding the impact of heavy-ion irradiation on thermal transport and may prove useful for the application of GaN-based devices in radiation-intense environments.

A Photoswitchable Metallocycle Based on Azobenzene: Synthesis, Characterization, and Ultrafast Dynamics.

The novel photoswitchable ligand 3,3'-Azobenz(metPA)2 (1) is used to prepare a [Cu2(1)2](BF4)2 metallocycle (2), whose photoisomerization was characterized using static and time-resolved spectroscopic methods. Optical studies demonstrate the highly quantitative and reproducible photoinduced cyclic E/Z switching without decay of the complex. Accordingly and best to our knowledge, [Cu2(1)2](BF4)2 constitutes the first reversibly photoswitchable (3d)-metallocycle based on azobenzene. The photoinduced multiexponential dynamics in the sub-picosecond to few picosecond time domain of 1 and 2 have been assessed. These ultrafast dynamics as well as the yield of the respective photostationary state (PSSZ = 65 %) resemble the behavior of archetypical azobenzene. Also, the innovative pump-probe laser technique of gas phase transient photodissociation (τ-PD) in a mass spectrometric ion trap was used to determine the intrinsic relaxation dynamics for the isolated complex. These results are consistent with the results from femtosecond UV/Vis transient absorption (fs-TA) in solution, emphasizing the azobenzene-like dynamics of 2. This unique combination of fs-TA and τ-PD enables valuable insights into the prevailing interplay of dynamics and solvation. Both analyses (in solution and gas phase) and quantum chemical calculations reveal a negligible effect of the metal coordination on the switching mechanism and electronic pathway, which suggests a non-cooperative isomerization process.

Direct Observation on H-Elimination Enhancement from C2H4 through Non-Adiabatic Process by Femtosecond Laser Induced Coulomb Explosion

Ethylene, the simplest model of a carbon-carbon double bond system, is pivotal in numerous chemical and biological processes. By employing intense infrared laser pump-probe techniques alongside coincidence measurements, we investigate the ultrafast non-adiabatic dynamics involved in the breakage of carbon-carbon double bonds and hydrogen elimination in dissociation of ethylene. Our study entails analyzing the dynamic kinetic energy release spectra to assess three bond-breaking scenarios, movements of nuclei, and structural changes around the carbon atoms. This allows us to evaluate the relaxation dynamics and characteristics of various dissociative states. Notably, we observe a significant rise in the yield of fragments resulting from C–H bond breakage with the delay time extended, suggesting non-adiabatic coupling through conical intersections from C–C bond breakage as a probable cause.

Study of the mercury chalcogenide single crystals by means of a combination of laser pump-probe thermoreflectance technique with Fabry–Perot interferometer

Near-surface layers of the mercury chalcogenide (HgSe and n-HgTe) single crystals with the electron concentration in the range of 7 × 1014–4.2 × 1018 сm−3 were studied using the laser pump-probe thermoreflectance (TR) technique combined with the Fabry–Perot (FP) interferometer in the wide temperature interval of 10–300= K. The TR–FP measurements were carried out toward samples with a freshly etched reflective surface. As a result, a minimum in the time dependence of the TR–FP signal and sign inversion of the TR–FP signal were revealed both for HgSe and n-HgTe within the specified temperature range. Noticeably, both anomalies were observed for n-HgTe at substantially lower temperatures than that for HgSe. The proposed qualitative interpretation of the observed anomalies is based on the hypothesis of two types of Weyl nodes located in the Brillouin zone of mercury chalcogenides involving specific for the Weyl semimetals energy barriers, separating the bulk Weyl fermion helical states from the topologically protected Fermi-arc surface states.

Estimation of Magnetic Gilbert Damping at High Temperature: An Approach of Ferromagnetic Resonance Study

High-temperature time-resolved magneto-optical Kerr effect microscopy, an updated version of the pump-probe laser technique, enables one to measure the magnetization dynamics at elevated temperatures and is established for the first time. An understanding of the damping mechanism with its dependence on temperature for highly anisotropic ferromagnetic materials is a crucial step toward the developmental approach for spintronic devices. So, in this work, we studied the magnetization dynamics of tetragonally distorted, highly anisotropic ultrathin Fe0.5Co0.5 films at elevated temperatures. A clear dynamic response of magnetization was observed at elevated temperature and the damping decreased with increasing sample temperature. A monotonic decrease in resonance frequency and damping constant as a function of temperature can be ascribed due to the extrinsic contribution mechanism. The thermal dependence of the damping constant and effective anisotropic field measured from this ferromagnetic methodology is very important for such highly anisotropic materials for the development of memory devices. Based on the first-principle study and ferromagnetic resonance technology, we predict that the reduction of damping constant at elevated temperature might be influenced by two-magnon scattering instead of structural defects (tetragonal distortion), which can act as a potential origin.

Direct observation of ultrafast carrier coupling dynamics in monolayer graphene/metal system

• Femtosecond laser pump-probe technique measures the reflectivity changes of graphene/copper system. • A model assuming phonon-phonon coupling at graphene/copper interface is built and validated by the experiment. • Establish a new research paradigm to directly study the carrier coupling dynamics between 2D materials and substrates. For promising graphene-based devices, graphene/metal contacts are inevitable. When they operate at high frequencies, it is essential to explore the ultrafast carrier dynamics between graphene and metal, which has been rarely studied. Owing to the special monolayer graphene/copper structure we designed, that is, the graphene is grown on copper, we have directly observed the ultrafast carrier dynamics between monolayer graphene and copper by using femtosecond laser pump-probe technique for the first time. The measured optical reflectivity changes of graphene/copper system are significantly different from that of bare copper, which is attributed to the ultrafast carrier dynamics between graphene and copper, and graphene optical properties. A model considering carriers coupling in graphene and copper, including electron and phonon, has been built and validated by the experiment. Present study provides direct observation of the ultrafast carrier dynamics between graphene and metal, which is of great significance for graphene-based device applications.

Picosecond Acoustics Technique to Measure the Sound Velocities of Fe-Si Alloys and Si Single-Crystals at High Pressure

We describe here a time resolved pump-probe laser technique—picosecond interferometry—which has been combined with diamond anvil cells (DAC). This method enables the measurement of the longitudinal sound velocity up to Mbar pressure for any kind of material (solids, liquids, metals, insulators). We also provide a description of picosecond acoustics data analysis in order to determine the complete set of elastic constants for single crystals. To illustrate such capabilities, results are given on the pressure dependence of the acoustic properties for prototypical cases: polycrystal (hcp-Fe-5 wt% Si up to 115 GPa) and single-crystal (Si up to 10 GPa).

RETRACTED: Thermal transport across graphene-mediated multilayer tungsten nanostructures

Abstract The thermal conductivity of monolayer graphene/β-phase tungsten (β-W) periodic stack nanostructure and the interfacial thermal resistance induced by graphene have been measured by a modified two-color femtosecond laser pump-probe technique. The thickness of β-W films is 15, 30, 40 nm respectively, and the total thickness of periodic stack nanostructures is about 120 nm ignoring the thickness of graphene. The cross-plane thermal conductivity (k) of β-W film is determined as 7.58 W/m K which is two orders of magnitude smaller than that of α-phase bulk tungsten. The small value is attributed to the vacancies in β-W and the small grain size of β-W, which can suppress the mean free path of hot carriers. The cross-plane thermal conductivity of periodic stack nanostructure is smaller than the value of pure W film and gradually decreases with the increasing number of graphene layers. The interfacial thermal resistance between the monolayer graphene and the tungsten films ranges from 4 × 10−9 to 8.15 × 10−9 m2 K/W. The results predicted by the diffuse mismatch model are smaller than the experimental results, indicating that the phonon inelastic scattering plays an important role in the heat transport of W/graphene periodic stack nanostructure.

Ultrafast dynamics of acetone photooxidation on TiO2(110).

Using light energy to drive chemical reactions on semiconductor surfaces is the basis for technological applications ranging from the removal of organic pollutants to the generation of renewable solar fuels, yet our understanding of the mechanisms has been hindered by the multistep nature of the process and the wide range of time scales over which it occurs (femtoseconds to seconds). In this work, we use ultrafast laser pump-probe techniques to follow the time evolution of substrate-induced photooxidation of acetone on a titania surface. A UV light at 260 nm initiates carrier-induced fragmentation of adsorbed acetone on a TiO2(110) surface that was pretreated with oxygen. The photoreaction results in the ejection of methyl radicals into the gas-phase that are detected by the probe pulse via resonant multiphoton ionization. The time evolution of the methyl radicals leaving the surface exhibits ultrafast rise times, 300-700 fs, followed by a more gradual rise that plateaus by 10 ps, with faster rates at a low acetone coverage. These results are interpreted in terms of a time-dependent rate expression and a mechanism in which the fragmentation of the acetone surface species is driven by interactions with nonequilibrium, "hot" holes.

Time-resolved molecular dynamics of single and double hydrogen migration in ethanol

Being the lightest, most mobile atom that exists, hydrogen plays an important role in the chemistry of hydrocarbons, proteins and peptides and most biomolecules. Hydrogen can undergo transfer, exchange and migration processes, having considerable impact on the chemical behavior of these molecules. Although much has been learned about reaction dynamics involving one hydrogen atom, less is known about those processes where two or more hydrogen atoms participate. Here we show that single and double hydrogen migrations occurring in ethanol cations and dications take place within a few hundred fs to ps, using a 3D imaging and laser pump-probe technique. For double hydrogen migration, the hydrogens are not correlated, with the second hydrogen migration promoting the breakup of the C–O bond. The probability of double hydrogen migration is quite significant, suggesting that double hydrogen migration plays a more important role than generally assumed. The conclusions are supported by state-of-the-art molecular dynamics calculations.

Structure Function Analysis of Temperature-Dependent Thermal Properties of Nm-Thin Nb2O5

A 166-nm-thick amorphous Niobium pentoxide layer (Nb2O5) on a silicon substrate was investigated by using time domain thermoreflectance at ambient temperatures from 25 °C to 500 °C. In the time domain thermoreflectance measurements, thermal transients with a time resolution in (sub-)nanoseconds can be obtained by a pump-probe laser technique. The analysis of the thermal transient was carried out via the established analytical approach, but also by a numerical approach. The analytical approach showed a thermal diffusivity and thermal conductivity from 0.43 mm2/s to 0.74 mm2/s and from 1.0 W/mK to 2.3 W/mK, respectively to temperature. The used numerical approach was the structure function approach to map the measured heat path in terms of a RthCth-network. The structure function showed a decrease of Rth with increasing temperature according to the increasing thermal conductivity of Nb2O5. The combination of both approaches contributes to an in-depth thermal analysis of Nb2O5 film.

Measurements of sound velocity in iron–nickel alloys by femtosecond laser pulses in a diamond anvil cell

By comparing the seismic wave velocity profile in the Earth with laboratory data of the sound velocity of iron alloys, we can infer the chemical composition of materials in the Earth’s core. The sound velocity of pure iron (Fe) has been sufficiently measured using various techniques, while experimental study on the sound velocity of iron–nickel (Fe–Ni) alloys is limited. Here, we measured longitudinal wave velocities of hexagonal-close-packed (hcp) structured Fe up to 29 GPa, Fe–5 wt% Ni, and Fe–15 wt% Ni up to 64 GPa via a combination of the femtosecond pulse laser pump–probe technique and a diamond anvil cell at room temperature condition. We found that the effect of Ni on the sound velocity of an Fe-based alloy is weaker than that determined by previous experimental study. In addition, we obtained the parameters of Birch’s law to be VP = 1146(57)ρ − 3638(567) for Fe–5 wt% Ni and VP = 1141(45)ρ− 3808(446) for Fe–15 wt% Ni, respectively, where VP is longitudinal wave velocity (m/s) and ρ is density (g/cm3).

Femtosecond Time-Resolved Photofragment Rotational Angular Momentum Alignment in Electronic Predissociation Dynamics.

This Letter presents an experimental and theoretical study of femtosecond time-resolved vector correlations in methyl iodide (CH3I) electronic predissociation via the second absorption B-band at 201.2 nm. The time evolution of the phenomenological anisotropy parameters βl was determined from time-resolved photofragment angular distributions obtained by means of the femtosecond laser pump-probe technique coupled with velocity map imaging detection of vibrational ground-state CH3(ν = 0) fragments and spin-orbit excited I*(2P1/2) atoms. Theoretical interpretation of the experimental results was performed on the basis of a fitting procedure using quasiclassical theory, which elucidates vector correlations in photodissociation of symmetric top molecules. The results of the fitting are in very good agreement with the experimental data and demonstrate the important role of molecular excited-state lifetimes, parent molecule and methyl fragment rotations, and methyl fragment angular momentum alignment on the time-dependent electronic predissociation dynamics.

Electron impact transfer rates between metastable and resonant states of argon investigated by laser pump–probe technique

The laser pump–probe technique is used to study the electron impact transfer between the 1s5 and 1s4 states of argon (in Paschen’s notation) belonging to the 2P3/2 ion core for electron temperatures in the range of 1–2 eV. A rate coefficient of m3s−1 is determined for the transfer from 1s5 to 1s4 state. Different pumping schemes between the 1s and 2p states but also 3p states are used to verify the obtained value but also to probe the transfers with ion-core change toward the 2P1/2 ion-core. Our results show the presence of an important transfer channel between 1s2 and 1s4 states, and a rate coefficient of m3 s−1 is estimated for this transfer. The present results confirm that recent quantum mechanical calculations by Zatsarinny et al [] underestimate significantly the cross sections for electron impact transfers between 1s states of argon.

Nanoscale nuclei in phase change materials: Origin of different crystallization mechanisms of Ge2Sb2Te5 and AgInSbTe

Phase change memory devices are based on the rapid and reversible amorphous-to-crystalline transformations of phase change materials, such as Ge2Sb2Te5 and AgInSbTe. Since the maximum switching speed of these devices is typically limited by crystallization speed, understanding the crystallization process is of crucial importance. While Ge2Sb2Te5 and AgInSbTe show very different crystallization mechanisms from their melt-quenched states, the nanostructural origin of this difference has not been clearly demonstrated. Here, we show that an amorphous state includes different sizes and number of nanoscale nuclei, after thermal treatment such as melt-quenching or furnace annealing is performed. We employ fluctuation transmission electron microscopy to detect nanoscale nuclei embedded in amorphous materials, and use a pump-probe laser technique and atomic force microscopy to study the kinetics of nucleation and growth. We confirm that melt-quenched amorphous Ge2Sb2Te5 includes considerably larger and more quenched-in nuclei than its as-deposited state, while melt-quenched AgInSbTe does not, and explain this contrast by the different ratio between quenching time and nucleation time in these materials. In addition to providing insights to the crystallization process in these technologically important devices, this study presents experimental illustrations of temperature-dependence of nucleation rate and growth speed, which was predicted by theory of phase transformation but rarely demonstrated.

Determination of electron-impact transfer rate coefficients between argon 1s2 and 1s3 states by laser pump–probe technique

In a microwave argon plasma, the electron-impact population transfers between the first four excited states of argon are studied by time-resolved laser pump–probe technique. Metastable atoms in the 1s5 state (in Paschen's notation) are selectively pumped up to the 2p3 state, with a nanosecond pulsed dye laser tuned to the 706 nm argon transition and the temporal response of the densities in the 1s3, 1s4 and 1s5 states are monitored by time-resolved laser diode absorption. The electron density and temperature are also measured by Thomson scattering along the plasma column for different pressures. The rate coefficient measured for the 1s3 to 1s2 state transfer, for which only rough estimations exist in the literature is found to be 9 × 10−13 m3 s−1, almost five times larger than the value commonly assumed.

Strain characterisation in nanocrystalline ceramics: the laser pump-probe technique

Strain characterisation in nanocrystalline ceramics: the laser pump-probe technique

Strain characterisation in nanocrystalline ceramics: the laser pump-probe technique

Strain characterisation in nanocrystalline ceramics: the laser pump-probe technique

Photolyses of mammalian carboxy-hemoglobin studied by photoacoustic calorimetry

The enthalpy and conformational volume changes in the photolyses of carboxy-hemoglobin (HbCO) of human, bovine, pig, horse and rabbit are investigated by photoacoustic calorimetry. Considering the time scales of the exciting laser pulse and the receiving ultrasound transducers (PVDF films and PZT ceramics), as well as the reaction lifetimes in the photolysis processes of HbCO, the measured results are related to the geminate recombination and tertiary relaxation in photolyses of HbCO. Moreover, the quantum yields of the five mammals are also measured by laser pump-probe technique. The results show that the dynamic parameters, such as enthalpy and conformational volume changes, differ between the processes of the geminate recombination and tertiary relaxation. Also, the dynamic parameters differ among the five mammals although some of them may be consistent with each other.

Final state distributions of methyl radical desorption from ketone photooxidation on TiO2(110)

In this work, we report on product energy distributions for methyl radicals produced by UV photooxidation of a set of structurally related carbonyl molecules, R(CO)CH(3) (R = H, CH(3), C(2)H(5), C(6)H(5)), adsorbed on a TiO(2)(110) surface. Specifically, laser pump-probe techniques were used to measure the translational energy distributions of methyl radicals resulting from α-carbon bond cleavage induced by photoexcited charge carriers at the TiO(2) surface. Photoreaction requires the presence of co-adsorbed oxygen and/or background oxygen during UV laser (pump) exposure, which is consistent with the formation of a photoactive oxygen complex, i.e., η(2)-bonded diolate species (R(COO)CH(3)). The methyl translational energy distributions were found to be bimodal for all molecules studied, with "slow" and "fast" dissociation channels. The "fast" methyl channel is attributed to prompt fragmentation of the diolate species following charge transfer at the TiO(2) surface. The average translational energies of the "fast" methyl channels are found to vary with R-substituent and correlate with the mass of the remaining surface fragments, RCO(x) (x =1 or 2). By comparison, the average energies of the "slow" methyl channels do not show any obvious correlation with R-substituent. The apparent correlation of the "fast" methyl translation energies with surface fragment mass is consistent with a simple two-body fragmentation event isolated on the diolate molecule with little coupling to the surface. These results also suggest that the total available energy for methyl fragmentation does not vary significantly with changes in R-substituent and is representative of exit barriers leading to "fast" methyl fragments.