Double-pulse Excitation Research Articles

Killer Phonon Caught: Femtosecond Stimulated Raman Spectroscopy Identifies Phonon-Induced Control of Photophysics in Rubrene Derivatives.

Molecular reaction coordinates are defined by the interplay of a number of orthogonal nuclear coordinates and are inherently multidimensional for large molecules. Identifying how specific nuclear motions along these reaction coordinates can be used to drive and control chemical processes is a promising approach for the optimization of chemical outcomes and targeted synthetic design. Here, we used femtosecond stimulated Raman spectroscopy (FSRS) to quantify the effects of individual phonon nuclear motions on singlet fission in rubrene derivatives. Rubrene readily undergoes singlet fission and is amendable to chemical derivatization, yet the factors that impact the singlet fission yield are not fully understood. Crystal packing is known to play a significant role in both fission and carrier transport, and thus, we focused on the impact of phonon nuclear motions on the photophysics. We used four halogen-substituted rubrene crystals and successfully identified one specific phonon mode that suppresses singlet fission in these crystals. We used FSRS with single-pulse excitation and double-pulse excitation to coherently amplify each phonon mode and quantify its effects on the excited-state process. We found that coherent amplification of the specific phonon vibration involving twisting of the peripheral phenyl rings and tetracene core motions resulted in less ground-state depletion and fewer triplet state absorption. Our study demonstrated that it is possible to use coherent phonon excitation to influence the photophysical outcome, while also showing that FSRS with double-pulse excitation can be a successful tool for quantifying mode-selective contributions to photophysics.

Single- and Double-Pulse Laser Kinetic Spectroscopy of Indoline Spiropyran

Phototransformations of 6-nitro-8-methoxy-substituted indoline spiropyran in acetonitrile have been studied by laser kinetic spectroscopy (laser photolysis) with single- and double-pulse excitation. The role of the triplet state in the kinetics of photocoloration of spiropyran has been revealed. It has been shown that photochromic transformations of spiropyran involve unstable trans-isomers (conformers) of the open merocyanine form of spiropyran. Data on the transformation kinetics of the trans-isomers are presented.

Stimulated Raman blockade in coherent phonon generation

We theoretically investigate the behavior of coherent optical phonons in an electron–phonon coupled system at moderate laser intensity: below GW/cm2. We calculate the coherent phonons from impulsive stimulated Raman scattering (ISRS) and impulsive absorption (IA) mechanisms with a nonperturbative model. The generation efficiency of coherent phonons in single-pulse excitation exhibits gradual nonlinear saturation with increasing optical intensity in a nonperturbative regime. The ISRS mechanism exhibits substantial saturation at relatively low pulse intensity, whereas the IA mechanism remains roughly linear. We further examine the influence of double-pulse excitation on quantum path interference. The ISRS mechanism exhibits a pronounced saturation effect near 0 fs of pump–pump delay, whereas the IA process exhibits negligible saturation behavior. These results reproduce well experimental data obtained by double-pulse excitation and support our proposal that ISRS is the primary quantum path for the observed coherent phonons in gallium arsenide.

Phase control of thermally excited spin precession in ferromagnetic thin films

The precession phase of spins in magnetic medium is a key parameter for encoding and transferring information in spintronic devices. A remaining challenge is the flexible control of the spin precession phase in magnetic materials responding to thermal excitation. Here, we demonstrate ultrafast phase control of a spin precession induced by laser heating in ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (Py) thin films with double-pulse excitation. A continuous phase tuning is realized by changing the arrival time of the second pulse, where the coverage of phase variation can reach \ensuremath{\sim}2\ensuremath{\pi} by increasing the laser fluence of the second pulse. The origin of this phase tuning is determined by the combined effects of the orientation of the magnetization at the arrival of the second pulse as well as the direction shift of the effective magnetic field induced by laser heating of the second pulse. Our findings suggest a different pathway for phase tuning of spin precessions under thermal excitation in ferromagnetic materials, which is of great importance for further spintronic applications.

Ultrafast quantum path interferometry to determine the electronic decoherence time of the electron-phonon coupled system in n -type gallium arsenide

Quantum coherence is the key to new quantum technologies, although the details of decoherence processes and times are still not well understood in solids. We demonstrate how to measure the decoherence time of electronic excited states in $n$-type gallium arsenide ($n$-GaAs) crystals using a relative-phase locked double pulse excitation method and quantitatively evaluate with a simple model including temperature dependence. The interference patterns of both electronic and phonon coherence are observed in $n$-GaAs using ultrafast quantum path interferometry with $\ensuremath{\pi}/4$ polarized femtosecond pulses, via the amplitude of longitudinal optical phonons as a function of pump-pump delay. The electronic coherence shows uplifting and splitting in the collapse and revival of the electronic interference, which is sensitive to temperature, and these features are well reproduced using a simple quantum mechanical model. The decoherence time is determined using quantum calculations, from the splitting and uplifting in the interference shape, to be $27.8\ifmmode\pm\else\textpm\fi{}0.5, 23.0\ifmmode\pm\else\textpm\fi{}0.3$, and $12.9\ifmmode\pm\else\textpm\fi{}0.3$ fs at 10, 90, and 290 K, respectively. The temperature dependence of the decoherence time is well reproduced by the density of electrons in the conduction band with the group velocity of the photoinduced electrons.

Dynamic Characterization of Thermocouples under Double-Pulse Laser-Induced Thermal Excitation.

This study investigated the dynamic characteristics of thermocouples by using double-pulse laser excitation for dynamic temperature calibration under extreme conditions. An experimental device was constructed for double-pulse laser calibration; the device uses a digital pulse delay trigger to precisely control the double-pulse laser to achieve sub-microsecond dual temperature excitation with adjustable time intervals. The time constants of thermocouples under single-pulse laser excitation and double-pulse laser excitation were evaluated. In addition, the variation trends of thermocouple time constants under different double-pulse laser time intervals were analyzed. The experimental results indicated that the time constant increases and then decreases with the decrease in the time interval of the double-pulse laser. A method for dynamic temperature calibration was established for the evaluation of the dynamic characteristics of temperature sensors.

Controlling High-Frequency Spin-Wave Dynamics Using Double-Pulse Laser Excitation

Manipulating spin waves is highly required for the development of innovative data transport and processing technologies. Recently, the possibility of triggering high-frequency standing spin waves in magnetic insulators using femtosecond laser pulses was discovered, raising the question about how one can manipulate their dynamics. Here we explore this question by investigating the ultrafast magnetization and spin-wave dynamics induced by double-pulse laser excitation. We demonstrate a suppression or enhancement of the amplitudes of the standing spin waves by precisely tuning the time delay between the two pulses. The results can be understood as the constructive or destructive interference of the spin waves induced by the first and second laser pulses. Our findings open exciting perspectives towards generating single-mode standing spin waves that combine high frequency with large amplitude and low magnetic damping.

Traps in the spotlight: How traps affect the charge carrier dynamics in Cs2AgBiBr6 perovskite

Suitable optoelectronic properties of lead halide perovskites make these materials interesting semiconductors for many applications. Toxic lead can be substituted by combining monovalent and trivalent cations, such as in Cs 2 AgBiBr 6 . However, efficiencies of Cs 2 AgBiBr 6 -based photovoltaics are still modest. To elucidate the loss mechanisms, in this report, we investigate charge dynamics in Cs 2 AgBiBr 6 films by double-pulse excitation time-resolved microwave conductivity (DPE-TRMC). By exciting the sample with two laser pulses with identical wavelengths, we found a clear photoconductance enhancement induced by the second pulse even 30 μs after the first laser pulse. Modeling the DPE-TRMC results, complemented by photoluminescence and transient absorption, we reveal the presence of deep emissive electron traps, while shallow hole trapping is responsible for the long-lived transient absorption signals. These long-lived carriers offer interesting possibilities for X-ray detectors or photocatalysis. The DPE-TRMC methodology offers unique insight into the times involved in charge trapping and depopulation in Cs 2 AgBiBr 6 . • Double-pulse excitation TRMC is developed to study trapping processes • Cs 2 AgBiBr 6 shows high photoconductance up to 30 μs after the first excitation • A high yield of free charges results from populated trap states by the first pulse • Emission of holes residing in shallow states extends over many microseconds Trap-assisted electron recombination in Cs 2 AgBiBr 6 thin films, combined with low hole mobility and shallow trapping, can be a major drawback of Cs 2 AgBiBr 6 in photovoltaic devices. Caselli et al. reveal that the long-lived charge carriers in Cs 2 AgBiBr 6 can offer a suitable, lead-free alternative for other applications, such as X-ray detectors or photocatalysis.

Theory for coherent control of longitudinal optical phonons in GaAs using polarized optical pulses with relative phase locking

A theoretical model for the coherent control of longitudinal optical phonons in GaAs (001) by double-pulse excitation was derived using a simplified band model and the allowed and forbidden Raman scattering. The time evolution of the electron-phonon states was calculated with the density-matrix formalism and second-order perturbation. The amplitude of the longitudinal optical phonons controlled by the two pulses was obtained as a function of the delay between the pulses for several polarization conditions. For parallel-polarized pulses, electronic and phonon interference fringes were predicted, which were independent of the crystal orientation and the ratio between the allowed and forbidden Raman scattering intensities. For orthogonally polarized pulses, only phonon interference fringes were obtained at an angle of $\ensuremath{\pi}/4$ from the [100] direction. When one of the pulses was along the [100] direction, electronic interference fringes were induced by the allowed Raman scattering, although the pump pulses do not interfere themselves.

Coherent Interference Fringes of Two-Photon Photoluminescence in Individual Au Nanoparticles: The Critical Role of the Intermediate State.

The interaction between light and metal nanoparticles enables investigations of microscopic phenomena on nanometer length and ultrashort timescales, benefiting from strong confinement and enhancement of the optical field. However, the ultrafast dynamics of these nanoparticles are primarily investigated by multiphoton photoluminescence on picoseconds or photoemission on femtoseconds independently. Here, we presented two-photon photoluminescence (TPPL) measurements on individual Au nanobipyramids (AuNP) to reveal their ultrafast dynamics by double-pulse excitation on a global timescale ranging from subfemtosecond to tens of picoseconds. Two orders of magnitude photoluminescence enhancement, namely, coherent interference fringes, has been demonstrated. Power-dependent measurements uncovered the transform of the nonlinearity from 1 to 2 when the interpulse delay varied from tens of femtoseconds to tens of picoseconds. We proved that the real intermediate state plays a critical role in the observed phenomena, supported by numerical simulations with a three-state model. Our results provide insight into the role of intermediate states in the ultrafast dynamics of noble metal nanoparticles. The presence of the intermediate states in AuNP and the coherent control of state populations offer interesting perspectives for imaging, sensing, nanophotonics, and in particular, for preparing macroscopic superposition states at room temperature and low-power superresolution stimulated emission depletion microscopy.

Great enhancement on two-photon photoluminescence imaging contrast of Au nanoparticles via double-pulse femtosecond laser excitation with controlled phase differences.

Au nanoparticles are attractive contrast agents for noninvasive living tissue imaging with deep penetration because of their strong two-photon photoluminescence (TPPL) intensity and excellent biocompatibility. However, the inevitable phototoxicity and huge auto-fluorescence are consistently associated with laser excitation. Therefore, enhancement of TPPL intensity and suppression of backgrounds are always highly desired under the demand of reducing excitation powers. In this work, we develop a double-pulse TPPL (DP-TPPL) scheme with controlled phase differences (Δφ) between the double pulses to significantly improve the signal-to-noise ratio (SNR) of TPPL imaging. Under the modulated phase (Δφ periodically varying between 0-2π), our results show that SNR can be improved from 4.3 to 1715, with an enhancement of up to 400 folds at the integration of 50 ms. More importantly, this enhancement can be unlimitedly lifted by increasing the number of photons or integration times in principle. Further boosting has been achieved by reducing the magnitude of background noises; subsequently, SNR is improved by more than 104 times. Our schemes offer great potential for reducing phototoxicity and extracting extremely weak signals from huge backgrounds and open up a new possibility for a rapid, flexible, and reliable medical diagnosis by TPPL imaging with diminished laser powers.

Laser Atomic Emission Spectrometer with Achromatic Optical System

A laser atomic emission multichannel spectrometer (LAEMS) with an achromatic optical configuration is developed for research and educational purposes. The new spectrometer fits all the requirements for equipment for laser atomic emission spectroscopy, and has a number of advantages owing to the features of the design and technical characteristics of its components. The excitation source for the spectrometer includes two Nd:YAG lasers pumped by semiconductor laser diode arrays with a controllable energy (from 0 to 100 mJ) and interval between between pulses (from 1 to 100 μs), and an average pulse duration of ≈15 ns. The LAEMS provides for studies in both a single pulse and a double pulse laser ablation regime and excitation of emission spectra. Double pulse laser ablation on the LAEMS provides a substantial (up to a factor of 10) increase in the analytic signal with a negligible (1.5–2 times) increase in surface damage.

Accessory laboratory measurements to support quantification of hydrogen isotopes by in-situ LIBS from a robotic arm inside a fusion vessel

In order to try to improve spectral separation for hydrogen isotope detection in plasma facing components, a flow of noble gas at reduced pressure is proposed as alternative to atmospheric pressure experiments in air during maintenance of fusion vessels. To this aim the already realized compact LIBS probe has been equipped with a sealable tip where a noble gas could flow. The tip was in contact with the selected point of the target and its proper positioning was guaranteed by an adjustable metallic cone. Laboratory measurements were carried out by flowing either He or Ar on ITER-like tiles. A positive effect of Ar flow on Hα signal intensity was observed, while a significant narrowing of the Hα band was obtained upon He addition.Additional measurements on deuterium doped samples simulating the tile's composition in ITER divertor region were performed by applying a local vacuum in the point of analysis or utilizing the set-up in double pulse (DP) mode. These experiments were aimed at optimizing the acquisition parameters of the LIBS setup (e.g, single or double pulse excitation, gate delay, gate width, interpulse delay) when operated in different conditions (vacuum, atmospheric pressure, under gas flow) in order to resolve the two nearby emission bands Hα and Dα, still preserving a satisfactory Signal-to-Noise-Ratio (SNR). Finally, considerations on applicability of Calibration Free (CF) method for the quantification of hydrogen isotopes are presented.

Analysis of the temperature- and fluence-dependent magnetic stress in laser-excited SrRuO3.

We use ultrafast x-ray diffraction to investigate the effect of expansive phononic and contractive magnetic stress driving the picosecond strain response of a metallic perovskite SrRuO3 thin film upon femtosecond laser excitation. We exemplify how the anisotropic bulk equilibrium thermal expansion can be used to predict the response of the thin film to ultrafast deposition of energy. It is key to consider that the laterally homogeneous laser excitation changes the strain response compared to the near-equilibrium thermal expansion because the balanced in-plane stresses suppress the Poisson stress on the picosecond timescale. We find a very large negative Grüneisen constant describing the large contractive stress imposed by a small amount of energy in the spin system. The temperature and fluence dependence of the strain response for a double-pulse excitation scheme demonstrates the saturation of the magnetic stress in the high-fluence regime.

Liquid jet-based broadband terahertz radiation source

The features of a liquid jet-based broadband terahertz (THz) generator that includes an optical part and system for a liquid jet formation are described. This system is based on the trapezium-shaped slit-type nozzle, the output of which is limited by two parallel blades. The applications of this generator for studying the optical-to-THz conversion efficiency for various liquids and THz energy temperature dependence are demonstrated. A trend for a decrease in the optical-to-THz conversion efficiency when using high-temperature water is revealed. This trend is interpreted through the increase in the THz wave absorption of the water jet. This system allows to achieve the optical-to-THz conversion efficiency up to 0.1% in the case of α-pinene double-pulse excitation.

Varying pre-plasma properties to boost terahertz wave generation in liquids

Laser-driven nonlinear phenomena can both reveal the structural features of materials and become the basis for the development of various translated technologies, including highly intense terahertz sources. Here we realize a modified single-color double-pulse excitation scheme for enhancing the terahertz wave generation in flat liquid jets, and we show that the pre-ionization effect is crucial for finding the optimal input conditions. The experimental results, being supported by numerical simulations, reveal the preference for longer pre-pulses to induce the effective ionization process and shorter signals for the strong laser-plasma interaction. In addition to the identified features of the terahertz wave energy enhancement with respect to the duration change for both pulses and their ratio variation, we state the possibility of achieving the optical-to-THz conversion efficiency value up to 0.1% in the case of double-pulse excitation of an α-pinene jet.

Ultrafast coherent control of higher-order spin waves in a NiFe thin film by double-pulse excitation

By using a double-pulse excitation scheme, we demonstrate the optical control of higher-order spin waves in a ferromagnetic permalloy thin film. Three spin precession modes are observed in the film under single pulse excitation. Based on a theoretical calculation, these spin precession modes are assigned to the fundamental Kittel mode and the first- and second-order perpendicular standing spin-wave (PSSW) modes. In order to excite the first-order PSSW mode selectively, we use double-pulse excitation at 400 nm. We show that, by choosing an appropriate time interval between the two pump pulses, it is possible to obtain a relatively large amplitude of the first-order PSSW mode while the amplitudes of the other two modes are almost completely suppressed. By analyzing the spin dynamics, it is found that the spin precession motion under the double-pulse excitation condition can be explained by the interference between the spin waves that have been induced by the first and second pulses. Our finding indicates that the selective excitation of a spin precession mode with a high precession frequency in a ferromagnetic metal may be realized by a multiple-pump pulse excitation scheme with more than two pulses.

Coherent control of a surface structural phase transition.

Active optical control over matter is desirable in many scientific disciplines, with prominent examples in all-optical magnetic switching1,2, light-induced metastable or exotic phases of solids3-8 and the coherent control of chemical reactions9,10. Typically, these approaches dynamically steer a system towards states or reaction products far from equilibrium. In solids, metal-to-insulator transitions are an important target for optical manipulation, offering ultrafast changes of the electronic4 and lattice11-16 properties. The impact of coherences on the efficiencies and thresholds of such transitions, however, remains a largely open subject. Here, we demonstrate coherent control over a metal-insulator structural phase transition in a quasi-one-dimensional solid-state surface system. A femtosecond double-pulse excitation scheme17-20 is used to switch the system from the insulating to a metastable metallic state, and the corresponding structural changes are monitored by ultrafast low-energy electron diffraction21,22. To govern the transition, we harness vibrational coherence in key structural modes connecting both phases, and observe delay-dependent oscillations in the double-pulse switching efficiency. Mode-selective coherent control of solids and surfaces could open new routes to switching chemical and physical functionalities, enabled by metastable and non-equilibrium states.

UV post-ionization laser ablation ionization mass spectrometry for improved nm-depth profiling resolution on Cr/Ni reference standard.

Laser ablation combined with mass spectrometry forms a promising tool for chemical depth profiling of solids. At irradiations near the ablation threshold, high depth resolutions are achieved. However, at these conditions, a large fraction of ablated species is neutral and therefore invisible to the instrument. To compensate for this effect, an additional ionization step can be introduced. Double-pulse laser ablation is frequently used in material sciences to produce shallow craters. We apply double-pulse UV femtosecond (fs) Laser Ablation Ionization Mass Spectrometry to investigate the depth profiling performance. The first pulse energy is set to gentle ablation conditions, whereas the second pulse is applied at a delay and a pulse energy promoting the highest possible ion yield. The experiments were performed on a Cr/Ni multi-layered standard. For a mean ablation rate of ~3 nm/pulse (~72 nJ/pulse), a delay of ~73 ps provided optimal results. By further increasing the energy of the second pulse (5-30% higher with respect to the first pulse) an enhancement of up to 15 times the single pulse intensity was achieved. These conditions resulted in mean depth resolutions of ~37 and ~30 nm for the Cr and Ni layers, respectively. It is demonstrated on the thin-film standard that the double-pulse excitation scheme substantially enhances the chemical depth profiling resolution of LIMS with respect to the single-pulse scheme. The post-ionization allows for extraordinarily low ablation rates and for quantitative and stoichiometric analysis of nm-thick films/coatings.

Giant Enhancement of THz Wave Emission under Double-Pulse Excitation of Thin Water Flow

Simultaneous measurements of THz wave and hard X-ray emission from thin and flat water flow when irradiated by double femtosecond laser pulses (800 nm, 35 fs/transform-limited, 0.5 kHz, delay times up to 15 ns) were carried out. THz wave measurements by time-domain spectroscopy and X-ray detection by Geiger counters were performed at the transmission and the reflection sides of the flow. THz wave emission spectra show their dynamic peak shifts toward the low frequency with the highest intensity enhancements more than 1.5 × 10 3 times in |E| 2 accumulated over the whole spectrum range of 0–3 THz at the delay time of 4.7 ns between the two pulses. On the other hand, X-ray intensity enhancements are limited to about 20 times at 0 ns under the same experimental conditions. The mechanisms for the spectral changes and the intensity enhancements in THz wave emission are discussed from the viewpoint of laser ablation on the water flow induced by the pre-pulse irradiation.