Time-resolved Pump-probe Technique Research Articles

Laser-pump-resistive-probe technique to study nanosecond-scale relaxation processes

Standard optical pump-probe methods analyze a system’s temporal response to a laser pulse within sub-femtoseconds to several nanoseconds, constrained by the optical delay line’s length. While resistance is a sensitive detector in various fields, its measurements are typically slow (>µs) due to stabilization requirements. We suggest here a time-resolved pump-probe technique that combines an optical pump pulse and a rectangular electrical probe pulse through the sample, measuring transmission in a 50 ohm matched circuit with a digital oscilloscope. This allows electrically driven delays from nanoseconds to seconds. Demonstrations include studying heat-induced changes in a thin amorphous VO x film and carrier relaxation in a CdS photoresistor, showcasing potential applications in heat transfer, biochemical reactions, and gradual electronic transformations.

Tunneling-Driven Marcus-Inverted Triplet Energy Transfer in a Two-Dimensional Perovskite.

Quantum tunneling, a phenomenon that allows particles to pass through potential barriers, can play a critical role in energy transfer processes. Here, we demonstrate that the proper design of organic-inorganic interfaces in two-dimensional (2D) hybrid perovskites allows for efficient triplet energy transfer (TET), where quantum tunneling of the excitons is the key driving force. By employing temperature-dependent and time-resolved photoluminescence and pump-probe spectroscopy techniques, we establish that triplet excitons can transfer from the inorganic lead-iodide sublattices to the pyrene ligands with rapid and weakly temperature-dependent characteristic times of approximately 50 ps. The energy transfer rates obtained based on the Marcus theory and first-principles calculations show good agreement with the experiments, indicating that the efficient tunneling of triplet excitons within the Marcus-inverted regime is facilitated by high-frequency molecular vibrations. These findings offer valuable insights into how one can effectively manipulate the energy landscape in 2D hybrid perovskites for energy transfer and the creation of diverse excitonic states.

Large scale purification in semiconductors using Rydberg excitons

Improving the quantum coherence of solid-state systems is a decisive factor in realizing solid-state quantum technologies. The key to optimize quantum coherence lies in reducing the detrimental influence of noise sources such as spin noise and charge noise. Here we demonstrate that we can utilize highly-excited Rydberg excitons to neutralize charged impurities in the semiconductor Cuprous Oxide - an effect we call purification. Purification reduces detrimental electrical stray fields drastically. We observe that the absorption of the purified crystal increases by up to 25% and that the purification effect is long-lived and may persist for hundreds of microseconds or even longer. We investigate the interaction between Rydberg excitons and impurities and find that it is long-ranged and based on charge-induced dipole interactions. Using a time-resolved pump-probe technique, we can discriminate purification from Rydberg blockade, which has been a long-standing goal in excitonic Rydberg systems.

Coherent Spin Dynamics of Electrons in CsPbBr3 Perovskite Nanocrystals at Room Temperature.

Coherent spin dynamics of charge carriers in CsPbBr3 perovskite nanocrystals are studied in a temperature range of 4-300 K and in magnetic fields of up to 500 mT using time-resolved pump-probe Faraday rotation and differential transmission techniques. We detect electron spin Larmor precession in the entire temperature range. At temperatures below 50 K, hole spin precession is also observed. The temperature dependences of spin-related parameters, such as Landè g-factor and spin dephasing time are measured and analyzed. The electron g-factor increases with growing temperature, which can not be described by the temperature-induced band gap renormalization. We find that photocharging of the nanocrystals with either electrons or holes depends on the sample cooling regime, namely the cooling rate and illumination conditions. The type of the charge carrier provided by the photocharging can be identified via the carrier spin Larmor precession.

Single-shot switching in Tb/Co-multilayer based nanoscale magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) are elementary units of magnetic memory devices. For high-speed and low-power data storage and processing applications, fast reversal of the magnetization by an ultrashort laser pulse is extremely important. We demonstrate single-shot switching of Tb/Co-multilayer based nanoscale MTJs by combining the optical writing and the electrical read-out methods. A 90-fs-long laser pulse switches the magnetization of the storage layer (SL). The change in the tunneling magnetoresistance (TMR) between the SL and a reference layer (RL) is probed electrically across the oxide barrier. Single-shot switching is demonstrated by varying the cell diameter from 300 nm to 20 nm. The anisotropy, magnetostatic coupling, and switching probability exhibit cell-size dependence. By suitable association of laser fluence and magnetic field, successive commutation between high-resistance and low-resistance states is achieved. The nature of the magnetization reversal of both SL and RL in a continuous film is probed with a depth-resolved magneto-optical Kerr effect (MOKE) magnetometry. The ultrafast dynamics in the continuous full-MTJ stack is investigated with the time-resolved pump–probe technique. Our experimental findings provide strong support for the growing interest in ultrafast spintronic devices.

Dynamics of filamentary damage induced by nanosecond laser in fused silica

The phenomena and mechanisms of energy deposition and plasma shielding during laser breakdown of transparent dielectrics deserve further discussion. In this work, the formation characteristics of 1064 nm ns pulsed laser-induced fused silica filamentary damage behavior during the bulk damage process were studied using the time-resolved pump-probe technique. The role of plasma shielding in the damage process was also further studied. The results indicate that the filamentary damage formed before the core damage. With the enhancement of laser energy, the filamentary damage length is stable in the initial stage and then begins to grow dramatically, and the core damage location moves towards the front surface of the fused silica. However, the plasma scattering and shielding effects in air and fused silica severely affect the damaged region when the laser energy is further enhanced. It causes rapid decay of the filamentary damage length, while the core damage moves along the optical axis to the rear surface of the fused silica. It was also found that the diameter of the filamentary damage was not significantly affected by the relatively small laser energy.

High-performance antireflection nanostructure arrays on aluminum-doped zinc oxide film fabricated with femtosecond laser near-field processing

Transmittance is critical to the performance of optoelectronic devices, and the fabrication of nanostructure arrays with sizes and periods similar to wavelengths on optical surfaces can effectively reduce reflectance and increase transmittance. A method combining a femtosecond laser and the near-field effect of polystyrene (PS) microspheres to prepare integrated high-performance anti-reflective nanostructure arrays on aluminum-doped zinc oxide (AZO) film was proposed in this study. The near-field effect of microspheres with different diameters on a laser light field for several laser wavelengths was simulated with the finite-difference time-domain (FDTD) method. The simulation results showed that a 10-fold improvement of the intensity of a focused laser beam could be obtained with the combination of an 800-nm laser and 1-μm PS microspheres. Nano-craters on the surface of the AZO film were obtained with femtosecond laser irradiation. The depth and the area of these nano craters both increased with the increase of the laser fluences and tended toward saturation when the fluence went from 0.67 to 0.83 J/cm2. Facilitated by the gradient refractive index formed by the nano-craters, the transmittance of the sample treated with 0.83 J/cm2 increased by 8% in the 400–1100 nm band and even by 15% in the 400–600 nm band. The ablation mechanism was revealed by the time-resolved pump-probe technique, which proved that the nano-craters were formed by the phase explosion, and the smooth bottom with fine cracks was induced by the inhomogeneous resolidification of the superheated liquid phase. Our method could endow a photoelectric device with high-performance integrated anti-reflection capability, eliminating the need for additional anti-reflection coating.

Progress and Perspectives of Spectroscopic Studies on Carbon K-Edge Using Novel Soft X-ray Pulsed Sources

The development of novel coherent and brilliant sources, such as soft X-ray free electron laser (FEL) and high harmonic generation (HHG), enables new ultrafast analysis of the electronic and structural dynamics of a wide variety of materials. Soft X-ray FEL delivers high-brilliance beams with a short pulse duration, high spatial coherence and photon energy tunability. In comparison with FELs, HHG X-ray sources are characterized by a wide spectral bandwidth and few- to sub-femtosecond pulses. The approach will lead to the time-resolved reconstruction of molecular dynamics, shedding light on different photochemical pathways. The high peak brilliance of soft X-ray FELs facilitates investigations in a nonlinear regime, while the broader spectral bandwidth of the HHG sources may provide the simultaneous probing of multiple components. Significant technical breakthroughs in these novel sources are under way to improve brilliance, pulse duration, and to control spectral bandwidth, spot size, and energy resolution. Therefore, in the next few years, the new generation of soft X-ray sources combined with novel experimental techniques, new detectors, and computing capabilities will allow for the study of several extremely fast dynamics, such as vibronic dynamics. In the present review, we discuss recent developments in experiments, performed with soft X-ray FELs and HHG sources, operating near the carbon K-absorption edge, being a key atomic component in biosystems and soft materials. Different spectroscopy methods such as time-resolved pump-probe techniques, nonlinear spectroscopies and photoelectron spectroscopy studies have been addressed in an attempt to better understand fundamental physico-chemical processes.

Core and filamentary damage of fused silica induced by nanosecond laser at 1064 nm

The behavior of 1064 nm laser-induced bulk damage (including core damage and filamentary damage) of fused silica was studied using a time-resolved pump-probe shadowgraph technique. The results indicate that the enhancement of incident laser energy only affects the area and location of core damage and the length of filamentary damage, while the diameter and the rightmost position of the filament are not affected. Based on the self-focusing effect and the temporal profile of the laser pulse, a discrete focus hypothesis is proposed, which can effectively elaborate the damage process.

Time-resolved imaging of CaF2 poly-crystal response following 355 nm nanosecond laser irradiation

The evolution of 355 nm nanosecond laser-induced damage to C a F 2 poly-crystals is investigated by using the time-resolved pump-probe shadowgraph technique. The damage morphologies of the front surface, rear surface, and interior of the C a F 2 crystal are imaged by optical microscopy. When the rear surface is focused by one laser pulse throughout the front surface, three shock waves (SWs) and one SW are observed in the air beside the front and rear surfaces, respectively. When the laser energies are 40 mJ and 60 mJ, at delay time of 1000 ns the radii of SW fronts beside the front surface are 2569.8 µm and 2831.7 µm, while those beside the rear surface are 1012.9 µm and 1078.1 µm, respectively. The filamentary channels inside the C a F 2 crystal are established before the end of a laser pulse at energies of 25, 40, or 60 mJ. The average propagation velocities of SWs along the filamentary channel are approximately 8.2 µm/ns. The maximum diameters of channels can reach approximately 53 µm and 128 µm for 25 mJ energy and 40 mJ energy, respectively. The experimental results help to explore the mechanism of laser-induced C a F 2 poly-crystal damage in nanosecond regime.

Photometric sensing of heavy metal ions using a naphthoquinodimethyl-bis-thioamide dye: Selectivity & photophysics of the metal organic complexes

As Mother Nature is experiencing catastrophic environmental pollutions from both natural sources and anthropogenic activities, many scientists have been working around the clock to develop environmentally benign and cost-effective, yet sensitive, detection techniques for pollutants, especially heavy metal species (e.g. lead, cadmium, mercury, among others) that are deleterious to human health. Herein, we report a novel sulfur-containing small organic dye/sensor naphthoquinodimethyl-bis-thioamide (QDM), which was found to be particularly selective toward mercury ion (Hg2+). Using a combination of UV–vis absorption, photoluminescence, and time-resolved pump–probe techniques, we established that QDM and Hg2+ can form stable complex(es) due to the strong affinity of sulfur toward mercury. In this investigation, while a higher ratio of QDM:Hg2+ was necessary to fully quench the fluorescence emission of QDM, only 1 equiv of the Hg2+ ion was necessary to observe the sensing effect on the excited state photo-behavior(s) of QDM. The present results highlight a synergy between molecular sensors’ selectivity/sensitivity and sensor-analytes dynamics.

Ultrafast Dynamics of Optical Nonlinearities in β-Ga2O3

We report the different nonlinear optical mechanisms and defect-related carrier dynamics in Sn-doped β-Ga2O3 crystal by utilizing time-resolved pump-probe technique based on phase object under UV excitation. The obtained nonlinear optical parameters arise from bound electron can be well explained by the theoretical calculation of two-band model and Kramers-Kronig transformation. By tuning the probe wavelength, the carrier nonlinearity can be modulated greatly due to additional absorption of defects within the bandgap. The results reveal that by choosing a proper probe wavelength that matches the defect state to the valence band, the nonlinear absorption and refraction of the carriers can be greatly enhanced, which provides an important reference for the design of gallium oxide-based waveguide materials and all-optical switching materials in the future.

Filamentary damage of fused silica irradiated by a 532 nm nanosecond laser

In this study, we use a time-resolved pump-probe shadowgraph technique to investigate the evolution characteristics of filamentary damage in bulk fused silica induced by a nanosecond pulse at 532 nm. The pump laser focuses on the front surface of sample and filamentary damage appears independently in the middle of sample. The whole damage process can be divided into single filament (SF), double filaments (DFs), and long filament (LF) successively. At the same time, the improved moving focus model is proposed by taking into account the temporal shape of the laser pulse and the laser is blocked and reflected by plasma at the critical density. It is in good agreement with the experimental result of filamentary damage process and helps to explore the mechanism of laser-induced filamentary damage in nanosecond regime.

Mode-Specific Autodetachment Dynamics of an Excited Non-valence Quadrupole-Bound State.

The autodetachment dynamics of vibrational Feshbach resonances of the quadrupole-bound state (QBS) for the first time has been investigated in real time for the first excited state of the 4-cyanophenoxide (4-CP) anion. Individual vibrational resonances of the cryogenically cooled 4-CP QBS have been unambiguously identified, and their autodetachment rates state-specifically measured using the picosecond time-resolved pump-probe technique employing the photoelectron velocity-map imaging method. The autodetachment lifetime (τ) is found to be strongly dependent on mode, giving τ values of ∼56, ∼27, and ≤2.8 ps for the 12'1 (Evib = 406 cm-1), 12'2 (Evib = 806 cm-1), and 21'1 (Evib = 220 cm-1) modes, respectively. The striking mode-specific behavior of the QBS lifetime has been invoked by the physical model in which the loosely bound electron falls off by the dynamic wobbling of the three-dimensional quadrupole moment ellipsoid associated with the corresponding vibrational motion in the autodetachment process.

One-Step Fabrication Method of GaN Films for Internal Quantum Efficiency Enhancement and Their Ultrafast Mechanism Investigation.

The third-generation semiconductors are the cornerstone of the power semiconductor leap forward and have attracted much attention because of their excellent properties and wide applications. Meanwhile, femtosecond laser processing as a convenient method further improves the performance of the related devices and expands the application prospect. In this work, an approximate 3 times improvement of the internal quantum efficiency (IQE) and a 5.5 times enhancement of the photoluminescence (PL) intensity were achieved in the GaN film prepared using a one-step femtosecond laser fabrication method. Three types of final micro/nanostructures were found with different femtosecond laser fluences, which could be attributed to the decomposition, melting, bubble nucleation, and phase explosion of GaN. The mechanisms of the microbump structure formation and enhancement of IQE were studied experimentally by the time-resolved reflection pump-probe technique, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Simulations for the laser-GaN interaction have also been performed to ascertain the micro/nanostructure formation principle. These results promote the potential applications of femtosecond lasers on GaN and other wide band gap semiconductors, such as UV-light-emitting diodes (LEDs), photodetectors, and random lasers for use in sensing and full-field imaging.

Ultrafast laser-induced guided elastic waves in a freestanding aluminum membrane

Ultrafast laser-induced guided acoustic waves in thin, freely suspended films are important for many applications adopting the laser-ultrasonics technique. These waves show unique dispersion relations, leading to minimal propagation losses and acoustic energy confinement. While this principle has been known, the separation of various physical effects in the formation of measured signals involving these guided acoustic waves has not been clearly elaborated. Here, we present a combined experimental and theoretical study on all-optical excitation and detection of these waves in a thin, freestanding aluminum membrane. The acoustic dynamics is excited and measured by using a femtosecond time-resolved pump-probe technique with controlled probing position, enabling spatially resolved detection. The measured signals are compared with an advanced numerical model that we developed earlier [H. Zhang et al., Phys. Rev. Appl. 13, 014010 (2020)], showing excellent agreement. The combination of experiment and simulation allows us to decode various physical effects in the signal formation, including different acoustic field components. Unknown material properties, such as acoustic attenuation coefficients, and the two complex photoelastic constants are quantitatively retrieved by fitting the measured signals. Furthermore, we provide evidence of nonlinear propagation of the excited guided acoustic waves.

Exchange stiffness and damping constants of spin waves in CoFeB films

Exchange stiffness and damping of spin waves in a soft ferromagnet of CoFeB are investigated using a time-resolved pump-probe technique. A pump pulse excites coherent spin waves in CoFeB via optical spin-orbit torque from Pt, and a probe pulse detects the phase of the spin waves via magneto-optic Kerr effect. From the frequency of spin waves, exchange stiffness is determined to be 13 pJ m−1, which is nearly independent of the CoFeB thickness from 6 to 16 nm and the annealing temperature of 400 °C. Damping constant of spin waves is determined from the relaxation of spin waves. Importantly, the spin wave damping increases with decreasing the CoFeB thickness, suggesting the spin pumping effect at the interface between CoFeB and Pt. The thickness dependence of damping becomes stronger after annealing at 400 °C, and the spin wave damping of >0.08 is obtained at a CoFeB thickness of 6.3 nm. Such large damping of spin waves would have an important effect on the operation of the CoFeB-based memory devices.

Attosecond light source in material science investigation

Since the dawn of the 21st century, attosecond (1 as =10–18 s) science has been explored and is now fast developing. It brought us the unprecedented time-resolved spectroscopy of quasiparticle dynamics in materials. To date, femtosecond (1 fs =10–15 s) time-resolved pump-probe technology is widely used to study the ultrafast quasiparticle dynamics of quantum materials, such as strong correlated quantum materials, topological materials, superconductors, and semiconductors. In a typical pump-probe experiment, the laser source will be divided into two beams, namely the pump and the probe beams. The pump beam excites the quasiparticles in target materials, and the time-delayed probe beam scans the ultrafast dynamics of the excited state carriers and the coherent lattice vibrations. By employing this method, we are able to observe the lifetimes due to various interactions, such as the electron-electron scattering, electron-phonon coupling, phonon-phonon scattering, etc. By manipulating the various laser pulse characteristic parameters, such as the photon energy, polarization, pulse duration and laser fluence, femtosecond ultrafast spectroscopy excels in investigating the physical properties of materials, which are largely determined by the outer electrons. To investigate the excited states of various materials, femtosecond pump-probe spectroscopy is often conducted under extreme conditions, such as low temperature, strong magnetic field, and high pressure. As a comparison, attosecond ultrafast spectroscopy can not only get access to exploring even faster ultrafast processes, but also probe the inner shell electrons of various types of materials. Attosecond pulses are usually prepared by high-order harmonic generation (HHG). The most common way of generating HHG is in an atomic gas. The excited unbound electrons are accelerated to the nuclei, and the electron-core collisions lead to multiple ionizations. The photons are emitted as trains of ultra-shot pulses. Recently, HHG has also been realized in liquids and solids. The separation of isolated attosecond pulses from trains of attosecond harmonics can be achieved by manipulating the polarization gating, the ultrashort few-cycle driving lasers, or both. Both attosecond pulse trains and isolated attosecond pulses can be applied to the investigation of quantum materials. With the aid of attosecond time-resolved ultrafast pump-probe spectroscopy, more and more fundamental physical properties and relevant applications will be revealed, which will greatly lead the development of condensed matter physics, material sciences, and their applications. Conversely, the attosecond science and technology can also benefit from the deeper understanding of the physical properties of solid state materials. In this article, we first introduce two types of attosecond time-resolved pump-probe experimental techniques, attosecond transient absorption spectrum (ATAS) and attosecond time-resolved angle-resolved photoemission spectroscopy (atto-ARPES). We review typical experiments carried out using such facilities in dielectrics, semiconductors, layered materials, transition metals, etc., and make an outlook for the potential applications of attosecond laser spectroscopy in this field of material science. Furthermore, as the attosecond pulses are often generated from the HHG of mid-infrared laser beams, the mid-infrared ultrafast spectroscopy itself can also be applied to material investigations. Moreover, we review the applications of mid-infrared ultrafast spectroscopy in condensed matter physics, with intriguing laser-induced novel phenomena.

Thermal coupling parameters between electron, phonon, and magnon of Nickel

Thermal interaction in an electron-phonon-magnon system in a ferromagnetic Ni film is investigated using a time-resolved pump-probe technique with thermoreflectance and magneto-optical Kerr effect. A nanoscale Au/Ni bilayer is used to induce thermal excitation of Ni in two ways: direct heating of Ni and indirect heating of Ni via Au. By comparing experiments and thermal modeling, we obtain the coupling parameters for electron-phonon, electron-magnon, and phonon-magnon pairs: 8.55 × 1017, 1.0–1.3 × 1017, and 0.8–1.1 × 1017 W m−3 K−1, respectively. In particular, we find that the phonon-magnon coupling parameter plays a significant role as a fast heat dissipation channel from magnon to phonon during the initial few picoseconds. In addition, both direct and indirect heating can be explained by similar coupling parameters with a variation less then 30%. Consequently, we conclude that the heat exchange between electron, phonon, and magnon is the dominant mechanism for the ultrafast demagnetization.

Plasmon charge transfer dynamics in layered Au–ZnO nanocomposites

Understanding the charge transfer dynamics at the interface of metals and semiconductors has received much attention in efficient plasmonic induced photonic devices. Here, we present ultrafast charge transfer dynamics in Au–ZnO nanocomposite systems by exciting them in interband and intraband levels of Au with pump energies higher (2.48 eV) and lower (1.96 eV) than the threshold energy for interband transition (2.4 eV), using the femtosecond time-resolved pump-probe technique. The spectral responses for both the excitations exhibit different behaviors, and these variations are quantitatively interpreted in terms of pump-induced changes in the dielectric constant of Au. It is found from the temporal dynamics that the electron–phonon component in the Au–ZnO system decays relatively faster (∼3 ps in the multilayer Au–ZnO) than that observed in the Au sample (∼7 ps in Au) for both the pump energies. The transfer of highly energetic hot electrons from Au nanoparticles to ZnO across the Schottky barrier results in an accessible optical response for wide bandgap ZnO in the visible to infrared range via plasmon charge collection. The calculated charge transfer rate in the Au–ZnO system is found to be (>1011 s−1). Our results demonstrate the pump excitation dependent ultrafast plasmon charge behavior in an optically active Au–ZnO system that can be attractive for efficient plasmonic-based hybrid photonic devices.