Pump-probe Method Research Articles

Chirped-Probe-Pulse Femtosecond Coherent Anti-Stokes Raman Scattering For Gas-Phase Temperature Measurements In High-Pressure Kerosene/Air Flames

Air traffic is increasing and its related emission is a major concern for the stability of the planet's climate. Innovative combustion systems need to be developed to continue to enable mobility while respecting the environment. The qualification of new propulsive systems requires the analysis of the associated combustion and one of the quantities of major interest is temperature. The present work exposes the development of the chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering (CPP-fs-CARS) thermometry at 1 kHz and its application in a semi-industrial test bench. Temperature measurements in a kerosene/air flame at a pressure of 0.75 MPa were carried out. CARS is a third order nonlinear optical diagnostic renown for high accuracy temperature determination. CPP-fs-CARS rely on the analyzis of the frequency-spread dephasing rate after the initial excitation of the Raman coherence on N2. This pump-probe method needs three input pulses. The two first excitation pulses are Fourier transform limited and present a temporal width of 100 fs at 800 nm and 675 nm. The frequency difference matches 𝑁2 molecular vibrational energy gap. Then the evolution of the coherence generated in the medium is probed with a delayed picosecond which encompass a sufficient part of the coherence evolution. In the CPP configuration, this longer probe pulse results from a 100 fs, 675 nm temporally stretched through a propagation in 30 cm glass rod. Following the energy conservation principle, the interaction of the probe with the excited medium results in the generation of a CARS signal, presenting a spectral shape sensitive to the temperature. Nevertheless, the spectral shape is also dependant on the incident pulses spectro-temporal features implying that spectral phases of the three input pulses need to be precisely evaluated. A polynomial fourth order function is then used to describe each pulse phase shape. Moreover, the molecular bath encompassing the nitrogen molecule contribute in a certain extend to the CARS spectrum. To deal with such a number of adjustable parameters, a genetic algorithm is used to tune those parameters. Finally, temperature is deduced from the single-shot CARS spectra. First results will report the application of the diagnostic in atmospheric pressure well known environments and the associated data processing. Such environments lead to found a temperature accuracy determined in a near-adiabatic laminar flame better than 1.5% at 2250 K. Those results demonstrate the applicability and usefulness of CPP-fs CARS and are pursued with the investigation of the high-pressure two-phase flames. The whole CPP-fs-CARS setup was then moved close to the representative aircraft combustor engine facility and adapted for pressurized environment. Measurements where performed at several locations in the plane orthogonal to the flame propagation and at several distances from the injection system. The data processing of the measurements in such harsh conditions was then adapted to disregard spectra resulting from the presence of liquid droplets. Finally, temperature evolution was extracted from single shot measurements and probability density function (PDF) are reported at several location of interest enabling to retrieve not only the mean temperature, but also important information on flame behaviour at different stages of its evolution within the combustion chamber length.

Ultrafast X-ray photoelectron diffraction from free molecules: Simulations of diffraction profiles from transient intermediates in the elimination reaction of C2H4I2

We have performed the simulations of C 1 s X-ray photoelectron diffraction (XPD) profiles from C2H4I2, bridged and classical anti-forms of C2H4I intermediates and C2H4 products to capture structures of transient intermediates in the elimination reaction of C2H4I2, under our ultrafast X-ray photoelectron diffraction (UXPD) scheme for free molecules using soft X-ray free-electron laser (SXFEL). In the UXPD scheme, the sample molecules are aligned in advance by near-infrared (NIR) laser with ns pulse duration before applying a pump probe method. Then, we have considered alignment control of C2H4I2 by using the elliptically polarized NIR laser to realize the UXPD experiments for the free molecules. As the results of simulations of XPD profiles from the laser-aligned C2H4I2 molecules, we have demonstrated the two-dimensional (2D) color maps of the C 1 s XPD profiles from C2H4I2, C2H4I, and C2H4. The 2D color maps have revealed that the transient C 1 s XPD profiles from the bridged-form and classical anti-form C2H4I intermediates exhibit remarkable differences, reflecting different intra-molecular scattering pathways of C 1 s photoelectrons within the intermediates. Thus, the present result has proved that UXPD for the free molecules has an advantage, compared with other traditional diffraction methods.

Influence of spherical anisotropy on optical mass sensing in plasmonic-molecular optomechanics

We use an all-optical pump-probe method to develop a mass sensing mechanism in a molecular plasmonic system at room temperature. The system consists of a double-clamped graphene nanoribbon that parametrically interacts with two types of isotropic and anisotropic spherical plasmonic cavities in the presence of a strong pump field and a weak probe pulse. Based on the mode-selective quantization scheme and analogy with the canonical model of the cavity optomechanics, we formulate the Hamiltonian of the system in terms of the electromagnetic Greens tensor. In this manner, we derive an explicit form of size-dependent optomechanical coupling function and plasmonic damping rate, which include the modal, geometrical, and material features of the plasmonic structure. Engineering material features of the plasmonic nanostructure, we find that the intensity of the probe field transmission spectrum for radially anisotropic spherical nanocavity enhances significantly compared to the silver sphere nanocavity due to the mode volume reduction. This scheme can provide to achieve the minimum measurable mass at room temperature.

Ultrafast Time-Resolved Pump-Probe Investigation of Nanosecond Extreme Ultraviolet-Light-Induced Damage Dynamics on B4C/Ru Nano-Bilayer Film.

An ultrafast time-resolved pump-probe setup with both high temporal and spatial resolution is developed to investigate the transient interaction between a nanosecond extreme ultraviolet (EUV) pulse and matter. By using a delayed femtosecond probe pulse, the pattern evolution of surface modification induced by an EUV pump at a wavelength of 13.5 nm can be imaged at different delay times, which provides deep insight into the EUV-induced damage dynamics and damage mechanisms. As a demonstration, single-shot EUV damage on a B4C(6.0 nm)/Ru(30.4 nm)/D263 nano-bilayer optical film is studied using this pump-probe method. A recoverable phenomenon is found during the evolution process of the dome-shaped damage region. This is explained by the elastic and plastic deformations resulting from the huge compressive stress difference at the Ru-substrate interface with the help of simulations on the thermal effects and mechanical responses. This damage mechanism is further proven by the complementary experiments at a higher EUV fluence at 13.5 nm.

Time-resolved photo-assisted Kelvin probe force microscopy on Cu(In,Ga)Se2 solar cells

Photo-assisted Kelvin probe force microscopy (P-KFM) is very useful for examining photovoltaic characteristics especially on solar cells, although a time resolution in the conventional P-KFM was not sufficient for investigating carrier dynamics in time domain. In this study, we have introduced both an intermittent bias application method and a pump-probe method into P-KFM to realize time-resolved measurements at μsec order. Then the photo-carrier dynamics in Cu(In,Ga)Se2 solar cells with different Ga compositions have been examined by our time-resolved P-KFM through direct observation of rising and decay waveforms of the photovoltage induced by the pulsed light illumination and through two-dimensional observation of photovoltage distributions at various time intervals between the optical pump and electrical probe pulses.

Signal Amplification of the Transient Response Measured by the Subnanosecond Pump-Probe Method Based on Surface Plasmon Resonance.

A pump-probe system with a subnanosecond pulsed laser is expected to be a compact and inexpensive transient spectroscopic system that enables nondestructive and noncontact evaluations of the physical properties. However, an improvement in the sensitivity and a theoretical model to complement the measurement signal are necessary to obtain the transient signal precisely because of the low sensitivity and large time resolution. We have developed a highly sensitive pump-probe system with a subnanosecond pulsed laser that combines signal amplification based on surface plasmon resonance (SPR) in this study. An integrated theoretical model of the transient response obtained by a subnanosecond pump-probe under the SPR condition was proposed. Our model consisted of the profile descriptions of the used pulse source, temperature change, generated thermoelastic stress, estimated permittivity change in the metal film, and estimated reflectivity change. The theoretical estimations in the time domain and the incident angle dependence were compared with those of the experimental results to verify our theory. As a result, the estimations were well in agreement with the experimental results. Moreover, the signal-amplification mechanism based on SPR was discussed using our theory. The amplification was caused by the broadening of the resonant curve of SPR and the shift of the resonant angle, which seemed to come from the increase in the electron-phonon scattering rate and the thermal expansion of the metal film, respectively. A clear mechanism of SPR-based signal amplification of the subnanosecond pump-probe was identified through experimental and theoretical approaches.

Synthesis, characterization and third-order nonlinear optical behaviour of two novel ethyne-linked polymers

Two novel ethyne-linked polymers (P1 and P2) were synthesized and characterized. Via femtosecond Z-scan, their nonlinear optical properties were investigated. Both polymers exhibit obvious reverse saturable absorption and self-defocusing. Using phase object pump-probe method, their transient absorption and refraction dynamic traces are measured and the mechanism of their optical nonlinearities is analyzed. The results indicate that two-photon absorption (TPA) mainly dominants nonlinear absorption of P1 and P2. The TPA coefficients are approximate 24.7 × 10−2 cm/GW for P1 and 3.12 × 10−2 cm/GW for P2. Through the extrapolation fitting, the estimated OL threshold for P1 is on the order of 10−2 J/cm2. On the other hand, both polymers display negative excited-sate refraction which is strong enough to dominate the transient nonlinear refraction over solvent effect. The refractive volumes of each effective state are determined. Our results indicate that ethyne-linked polymers have good nonlinear optical properties under femtosecond pulse excitation.

Ultrafast carrier and phonon dynamics in thin films of bismuth telluride on a flexible substrate

Thin films of topological insulators (TIs) possess exotic nonlinear optical properties such as strong light-matter interaction, broadband spectral sensitivity, thickness-dependent tunable bandgap, higher harmonic generation, etc. Due to the presence of metallic surface states with Dirac fermions and an insulating bulk band in TI, they are projected to be a viable material for studying novel physics, resulting in exciting new properties and technologies. The peculiar electron-phonon interactions at the surface have been linked to various unexpected physical features of topological insulators. Although electron behaviour in topological insulators has been extensively investigated on non-flexible substrates, electron-phonon interactions at TI bulk and surfaces states are less well known on a flexible substrate. Because of its potential uses in wearable devices, communications, sensors, and other fields, there is a significant need for the manufacture of high performance flexible optoelectronic responses employing novel exotic materials. In this paper, we preformed ultrafast pump-probe method to explore TI (Bi2Te3) thin films on a flexible PET (polyethylene terephthalate) substrate. We studied the dynamics of Bi2Te3 thin films' hot carrier relaxation progression and coherent phonon behaviour using transient absorbance measurements. Thickness-dependent low-frequency coherent acoustical phonon oscillations are observed in 10 nm thick films, and the changes vanish for 25 nm thick films, and high-frequency optical phonon oscillations are absent in our work. Subpicosecond range of time constant for the photon excitation, diffusion, carrier thermalization, and relaxation are reported. Longer characteristics time was observed for 25 nm film as compared to 10 nm film, and its variation has been discussed here. The thickness-dependent coherent acoustic phonon oscillating in the terahertz frequency range has been experimentally calculated. The film's terahertz frequency response varies with its thickness, allowing it to be employed in future terahertz applications based on flexible topological insulator thin films.

Structured illumination with thermal imaging (SI-TI): A dynamically reconfigurable metrology for parallelized thermal transport characterization

The recent push for the “materials by design” paradigm requires synergistic integration of scalable computation, synthesis, and characterization. Among these, techniques for efficient measurement of thermal transport can be a bottleneck limiting the experimental database size, especially for diverse materials with a range of roughness, porosity, and anisotropy. Traditional contact thermal measurements have challenges with throughput and the lack of spatially resolvable property mapping, while non-contact pump-probe laser methods generally need mirror smooth sample surfaces and also require serial raster scanning to achieve property mapping. Here, we present structured illumination with thermal imaging (SI-TI), a new thermal characterization tool based on parallelized all-optical heating and thermometry. Experiments on representative dense and porous bulk materials as well as a 3D printed thermoelectric thick film (∼50 μm) demonstrate that SI-TI (1) enables paralleled measurement of multiple regions and samples without raster scanning; (2) can dynamically adjust the heating pattern purely in software, to optimize the measurement sensitivity in different directions for anisotropic materials; and (3) can tolerate rough (∼3 μm) and scratched sample surfaces. This work highlights a new avenue in adaptivity and throughput for thermal characterization of diverse materials.

Magnetization Precession at Sub-Terahertz Frequencies in Polycrystalline Cu2 Sb-Type (Mn-Cr)AlGe Ultrathin Films.

A ferromagnetic metal nanolayer with a large perpendicular magnetic anisotropy, small saturation magnetization, and small magnetic damping constant is a crucial requirement for high-speed spintronic devices. Fabrication of these devices on Si/SiO2 amorphous substrates with polycrystalline structure is also strongly desired for the mass production industry. This study involves the investigation of sub-terahertz (THz) magnetization precessional motion in a newly developed material system consisting of Cu2 Sb-type MnAlGe and (Mn-Cr)AlGe films by means of an all-optical pump-probe method. These materials exhibit large perpendicular magnetic anisotropy in regions of a few nanometers in size. The pseudo-2D crystal structures are clearly observed in the high-resolution transmission electron microscopy (TEM) images for the film samples grown on thermally oxidized silicon substrates. The TEM images also show a partial substitution of Cr atoms for the Mn sites in MnAlGe. A magnetization precession frequency of 0.164 THz with a relatively small effective magnetic damping constant of 0.012 is obtained for (Mn-Cr)AlGe. Theoretical calculation infers that the modification of the total density of states by Cr substitution decreases the intrinsic magnetic damping constant of (Mn-Cr)AlGe.

Thermal model for time-domain thermoreflectance experiments in a laser-flash geometry

Time-domain thermoreflectance (TDTR) is a well-established pump–probe method for measuring thermal conductivity and interface conductance of multilayers. Interpreting signals in a TDTR experiment requires a thermal model. In standard front/front TDTR experiments, both pump and probe beams typically irradiate the surface of a multilayer. As a result, existing thermal models for interpreting thermoreflectance experiments assume that the pump and probe beams both interact with the surface layer. Here, we present a frequency-domain solution to the heat-diffusion equation of a multilayer in response to nonhomogeneous laser heating. This model allows analysis of experiments where the pump and probe beams irradiate opposite sides of a multilayer. We call such a geometry a front/back experiment to differentiate such experiments from standard TDTR experiments. As an example, we consider a 60nm amorphous Si film. We consider how signals differ in a front/front vs front/back geometry and compare thermal model predictions to experimental data.

Ultrafast disruptive probing: Simultaneously keeping track of tens of reaction pathways.

Ultrafast science depends on different implementations of the well-known pump-probe method. Here, we provide a formal description of ultrafast disruptive probing, a method in which the probe pulse disrupts a transient species that may be a metastable ion or a transient state of matter. Disruptive probing has the advantage of allowing for simultaneous tracking of the yield of tens of different processes. Our presentation includes a numerical model and experimental data on multiple products resulting from the strong-field ionization of two different molecules, partially deuterated methanol and norbornene. The correlated enhancement and depletion signals between all the different fragmentation channels offer comprehensive information on photochemical reaction pathways. In combination with ion imaging and/or coincidence momentum imaging or as complementary to atom-specific probing or ultrafast diffraction methods, disruptive probing is a particularly powerful tool for the study of strong-field laser-matter interactions.

Ultrafast manipulation of magnetic anisotropy in a uniaxial intermetallic heterostructure TbCo2/FeCo

We study experimentally and theoretically the dynamics of spin relaxation motion excited by a femtosecond pulse in the TbCo2/FeCo multilayer structures with different ratios of TbCo2 to FeCo thicknesses . The main attribute of the structure is in-plane magnetic anisotropy that is artificially induced during sputtering under a DC magnetic field. The optical pump-probe method revealed strongly damped high-frequency oscillations of the dynamical Kerr rotation angle, followed by its slow relaxation to the initial state. Modeling experimental results using the Landau–Lifshitz-Gilbert (LLG) equation showed that the observed entire dynamics is due to destruction and restoration of magnetic anisotropy rather than to demagnetization. For the pumping fluence of 7 mJ cm−2, the maximal photo-induced disruption of the anisotropy field is about 14% for the sample with and decreases when r d increases. The anisotropy relaxation is a three-stage process: the ultrafast one occurs within several picoseconds, and the slow one occurs on a nanosecond time scale. The Gilbert damping in the multilayers is found to be one order of magnitude higher than that in the constituent monolayers.

Optical Properties of V2O5 Thin Films on Different Substrates and Femtosecond Laser-Induced Phase Transition Studied by Pump-Probe Method.

Vanadium pentoxide can undergo a reversible phase transition by heating above 260 °C; its non-thermal phase transition, as well as ultrafast dynamical processes, is still not known. Here, femtosecond laser-induced phase transition properties in V2O5 thin films were first explored using femtosecond time-resolved pump–probe spectroscopy. The results show that the phase transient processes occur on a 10−15–10−13 temporal scale. The phase transition and recovery properties are dependent on both the substrates and pump laser energy densities. We propose the oxygen vacancies theory to explain the results, and we provide valuable insights into V2O5 films for potential applications.

Temperature and interlayer coupling induced thermal transport across graphene/2D-SiC van der Waals heterostructure

Graphene based two-dimensional (2D) van der Waals (vdW) materials have attracted enormous attention because of their extraordinary physical properties. In this study, we explore the temperature and interlayer coupling induced thermal transport across the graphene/2D-SiC vdW interface using non-equilibrium molecular dynamics and transient pump probe methods. We find that the in-plane thermal conductivity κ deviates slightly from the 1/T law at high temperatures. A tunable κ is found with the variation of the interlayer coupling strength χ. The interlayer thermal resistance R across graphene/2D-SiC interface reaches 2.71 times 10–7{text{Km}}^{2} /{text{W}} at room temperature and χ = 1, and it reduces steadily with the elevation of system temperature and χ, demonstrating around 41% and 56% reduction with increasing temperature to 700 K and a χ of 25, respectively. We also elucidate the heat transport mechanism by estimating the in-plane and out-of-plane phonon modes. Higher phonon propagation possibility and Umklapp scattering across the interface at high temperatures and increased χ lead to the significant reduction of R. This work unveils the mechanism of heat transfer and interface thermal conductance engineering across the graphene/2D-SiC vdW heterostructure.

Femtosecond laser-induced periodic structures: mechanisms, techniques, and applications

Over the past two decades, femtosecond laser-induced periodic structures (femtosecond-LIPSs) have become ubiquitous in a variety of materials, including metals, semiconductors, dielectrics, and polymers. Femtosecond-LIPSs have become a useful laser processing method, with broad prospects in adjusting material properties such as structural color, data storage, light absorption, and luminescence. This review discusses the formation mechanism of LIPSs, specifically the LIPS formation processes based on the pump-probe imaging method. The pulse shaping of a femtosecond laser in terms of the time/frequency, polarization, and spatial distribution is an efficient method for fabricating high-quality LIPSs. Various LIPS applications are also briefly introduced. The last part of this paper discusses the LIPS formation mechanism, as well as the high-efficiency and high-quality processing of LIPSs using shaped ultrafast lasers and their applications.

High-speed observation of damage generation during ultrashort pulse laser processing of dielectric materials

Ultrashort pulsed lasers enable microprocessing of dielectric materials, such as glass, sapphire and diamond, owing to the extremely high intensity. However, it is difficult to achieve precision processing, because damage is inevitably generated around the processed shape. To reveal the mechanism of the damage generation, we developed an imaging system, which combined the pump-probe imaging method and a high-speed camera. The system was used to capture the high-speed phenomena on a timescale ranging from picoseconds to milliseconds during the ultrashort pulsed laser processing. The observed phenomena and numerical analyses indicated that excitation of electrons and propagation of stress waves repeatedly occurred around the processed shape and resulted in damages. The revealed mechanism will contribute to developing a method to inhibit damage generation.

Layer-dependent photoexcited carrier dynamics of WS2 observed using single pulse pump probe method

Understanding the ultrafast carrier dynamics and the mechanism of two-dimensional (2D) transition metal dichalcogenides (TMDs) is key to their applications in the field of optoelectronic devices. In this work, a single pulse pump probe method is introduced to detect the layer-dependent ultrafast carrier dynamics of monolayer and few-layer WS2 excited by a femtosecond pulse. Results show that the ultrafast carrier dynamics of the layered WS2 films can be divided into three stages: the fast photoexcitation phase with the characteristic time of 2–4 ps, the fast decay phase with the characteristic time of 4–20 ps, and the slow decay phase lasting several hundred picoseconds. Moreover, the layer dependency of the characteristic time of each stage has been observed, and the corresponding mechanism of free carrier dynamics has been discussed. It has been observed as well that the monolayer WS2 exhibits a unique rising time of carriers after photoexcitation. The proposed method can be expected to be an effective approach for studying the dynamics of the photoexcited carriers in 2D TMDs. Our results provide a comprehensive understanding of the photoexcited carrier dynamics of layered WS2, which is essential for its application in optoelectronics and photovoltaic devices.

High thermal conductivity and ultrahigh thermal boundary conductance of homoepitaxial AlN thin films

Wurtzite aluminum nitride (AlN) has attracted increasing attention for high-power and high-temperature operations due to its high piezoelectricity, ultrawide-bandgap, and large thermal conductivity k. The k of epitaxially grown AlN on foreign substrates has been investigated; however, no thermal studies have been conducted on homoepitaxially grown AlN. In this study, the thickness dependent k and thermal boundary conductance G of homoepitaxial AlN thin films were systematically studied using the optical pump–probe method of frequency-domain thermoreflectance. Our results show that k increases with the thickness and k values are among the highest reported for film thicknesses of 200 nm, 500 nm, and 1 μm, with values of 71.95, 152.04, and 195.71 W/(mK), respectively. Our first-principles calculations show good agreement with our measured data. Remarkably, the G between the epilayer and the substrate reported high values of 328, 477, 1180, and 2590 MW/(m2K) for sample thicknesses of 200 nm, 500 nm, 1 μm, and 3 μm, respectively. The high k and ultrahigh G of homoepitaxially grown AlN are very promising for efficient heat dissipation, which helps in device design and has advanced applications in micro-electromechanical systems, ultraviolet photonics, and high-power electronics.

Quantitative Molecular Dynamics Characterization with Stimulated Emission and Photon Counting Based Pump-Probe Method

Quantitative Molecular Dynamics Characterization with Stimulated Emission and Photon Counting Based Pump-Probe Method