Laser-induced Phase Transition Research Articles

Controlling Drug Partitioning in Individual Protein Condensates through Laser-Induced Microscale Phase Transitions.

Gelation of protein condensates formed by liquid-liquid phase separation occurs in a wide range of biological contexts, from the assembly of biomaterials to the formation of fibrillar aggregates, and is therefore of interest for biomedical applications. Soluble-to-gel (sol-gel) transitions are controlled through macroscopic processes such as changes in temperature or buffer composition, resulting in bulk conversion of liquid droplets into microgels within minutes to hours. Using microscopy and mass spectrometry, we show that condensates of an engineered mini-spidroin (NT2repCTYF) undergo a spontaneous sol-gel transition resulting in the loss of exchange of proteins between the soluble and the condensed phase. This feature enables us to specifically trap a silk-domain-tagged target protein in the spidroin microgels. Surprisingly, laser pulses trigger near-instant gelation. By loading the condensates with fluorescent dyes or drugs, we can control the wavelength at which gelation is triggered. Fluorescence microscopy reveals that laser-induced gelation significantly further increases the partitioning of the fluorescent molecules into the condensates. In summary, our findings demonstrate direct control of phase transitions in individual condensates, opening new avenues for functional and structural characterization.

Speed limits of the laser-induced phase transition in FeRh

We use ultrafast x-ray diffraction and the polar time-resolved magneto-optical Kerr effect to study the laser-induced metamagnetic phase transition in two FeRh films with thicknesses below and above the optical penetration depth. In the thin film, we identify an intrinsic timescale for the light-induced nucleation of ferromagnetic (FM) domains in the antiferromagnetic material of 8ps, which is substantially longer than the time it takes for strain waves to traverse the film. For the inhomogeneously excited thicker film, only the optically excited near-surface part transforms within 8ps. For strong excitations, we observe an additional slow rise of the FM phase, which we experimentally relate to a growth of the FM phase into the depth of the layer by comparing the transient magnetization in frontside and backside excitation geometry. In the lower lying parts of the film, which are only excited via near-equilibrium heat transport, the FM phase emerges significantly slower than 8ps after heating above the transition temperature.

VO2 Films Decorated with an MXene Interface for Decreased-Power-Triggered Terahertz Modulation.

VO2, which exhibits semiconductor-metal phase transition characteristics occurring on a picosecond time scale, holds great promise for ultrafast terahertz modulation in next-generation communication. However, as of now, there is no reported prototype for an ultrafast device. The temperature effect has been proposed as one of the major obstacles. Consequently, reducing the excitation threshold for the phase transition would be highly significant. The traditional strategy typically involves chemical doping, but this approach often leads to a decrease in phase transition amplitude and a slower transition speed. In this work, we proposed a design featuring a highly conductive MXene interfacial layer between the VO2 film and the substrate. We demonstrate a significant reduction in the phase transition threshold for both temperature and laser-induced phase transition by adjusting the conductivity of the MXene layers with varying thicknesses. Our observations show that the phase transition temperature can be decreased by 9 °C, while the pump fluence for laser excitation can be reduced by as high as 36%. The ultrafast phase transition process on a picosecond scale, as revealed by the optical-pump terahertz-probe method, suggests that the MXene layers have minimal impact on the phase transition speed. Moreover, the reduced phase transition threshold can remarkably alleviate the photothermal effect and inhibit temperature rise and diffusion in VO2 triggered by laser. This study offers a blueprint for designing VO2/MXene hybrid films with reduced phase transition thresholds. It holds significant potential for the development of low-power, intelligent optical and electrical devices including, but not limited to, terahertz modulators based on phase transition phenomena.

Controlled Optical Contrast Caused by Reversible Laser-Induced Phase Transitions in GeTe and Ge2Sb2Te5 Thin Films in the Spectral Range from 500 to 20,000 nm

Controlled Optical Contrast Caused by Reversible Laser-Induced Phase Transitions in GeTe and Ge2Sb2Te5 Thin Films in the Spectral Range from 500 to 20,000 nm

Modeling of Short-Pulse Laser Interactions with Monolithic and Porous Silicon Targets with an Atomistic-Continuum Approach.

The acquisition of reliable knowledge about the mechanism of short laser pulse interactions with semiconductor materials is an important step for high-tech technologies towards the development of new electronic devices, the functionalization of material surfaces with predesigned optical properties, and the manufacturing of nanorobots (such as nanoparticles) for bio-medical applications. The laser-induced nanostructuring of semiconductors, however, is a complex phenomenon with several interplaying processes occurring on a wide spatial and temporal scale. In this work, we apply the atomistic-continuum approach for modeling the interaction of an fs-laser pulse with a semiconductor target, using monolithic crystalline silicon (c-Si) and porous silicon (Si). This model addresses the kinetics of non-equilibrium laser-induced phase transitions with atomic resolution via molecular dynamics, whereas the effect of the laser-generated free carriers (electron-hole pairs) is accounted for via the dynamics of their density and temperature. The combined model was applied to study the microscopic mechanism of phase transitions during the laser-induced melting and ablation of monolithic crystalline (c-Si) and porous Si targets in a vacuum. The melting thresholds for the monolithic and porous targets were found to be 0.32 J/cm2 and 0.29 J/cm2, respectively. The limited heat conduction mechanism and the absence of internal stress accumulation were found to be involved in the processes responsible for the lowering of the melting threshold in the porous target. The results of this modeling were validated by comparing the melting thresholds obtained in the simulations to the experimental values. A difference in the mechanisms of ablation of the c-Si and porous Si targets was considered. Based on the simulation results, a prediction regarding the mechanism of the laser-assisted production of Si nanoparticles with the desired properties is drawn.

Ultrafast laser ablation of gold in liquids: Effect of laser pulse overlap-induced surface porosity on size distribution of formed nanoparticles

Pulsed Laser Ablation in Liquids (PLAL) manifested itself as a powerful tool for the synthesis of nanoparticles (NPs) from a variety of materials of high demand in biomedicine. The mechanisms and regimes of nanostructures formation, however, still require clarification in order to better control the final NPs characteristics. Here, we present a numerical study of femtosecond laser-ablative production of metal (gold) NPs in water ambient using an advanced atomistic continuum approach, combining the Molecular Dynamics (MD) and Two Temperature Model into the frames of a single MD-TTM computational method. The model describes non-equilibrium laser-induced phase transitions at atomic level and accounts for the effect of free carriers in continuum. With the MD-TTM model we investigated the effect of slight porosity arising due to a partial overlap of laser craters during the scanning of laser beam over the target surface under a high repetition rate of laser pulses. For that purpose, we perform a simulation of 270 fs laser pulse interaction with solid and porous gold targets at the incident fluence of 2.5 J/cm2. The obtained results revealed the manifestation of different regimes of ablation and different yield of the obtained NPs. The simulation results are compared with the corresponding experimental data. We found that depending on the scanning speed the ablation can follow thermal and spallation mechanisms of the material ejection, which are responsible for the appearance of fine and course populations of NPs correspondingly. In the case of clean target’s surface ablation, when at high scanning speed of 3840 mm/s each pulse hits a fresh area, the material ejection is governed by both mechanisms, which leads to the appearance of bimodal population of NPs. Alternatively, a moderate scanning speed of 840 mm/s results in a partial overlap of the sequential laser spots on the surface and generated slight porosity removes the accumulation of laser-induced stresses. The spallation mechanism of the material ejection is therefore suppressed, which results in generation of a monomodal fine population of NPs with a 20 times larger yield. The obtained data are of importance to predict and control size characteristics of laser-synthesized nanomaterials.

X-FAST: A versatile, high-throughput, and user-friendly XUV femtosecond absorption spectroscopy tabletop instrument.

We present the X-FAST (XUV Femtosecond Absorption Spectroscopy Tabletop) instrument at the University of Wisconsin-Madison. The instrument produces femtosecond extreme ultraviolet photon pulses via high-harmonic generation in the range of 40-72eV, as well as optical pump pulses for transient-absorption experiments. The system implements a gas-cooled sample cell that enables studying the dynamics of thermally sensitive thin-film samples. This paper provides potential users with specifications of the optical, vacuum, data acquisition, and sample cooling systems of the X-FAST instrument, along with performance metrics and data of an ultrafast laser-induced phase transition in a Ni2MnGa Heusler thin film.

Discrete thermokinetic computational model of laser-induced phase transitions in phase-changing materials

This paper presents a thermokinetic computational model of phase transitions in GST225 (germanium–antimony–tellurium) thin films [as well as other phase change materials (PCMs)] induced and initiated by the impact of nano- and femtosecond laser pulses in a wide energy fluence range according to the results of experimental studies using Raman spectra and thin-film samples of TEM cross-sectional image analysis. Applying this phase transition model makes it possible to understand the mechanism of the induced phase transition regarding the usage of PCMs in photonics and optoelectronic devices, which require precise control of the phase state of their PCM-based active elements for their functioning. The proposed model shows the internal structure of the sample, generating both the profile of the crystalline fraction distribution over the sample's depth, providing images of virtual TEM sections, as well as the volume distribution of the crystalline phase.

Impact of the uniaxial strain on terahertz modulation characteristics in flexible epitaxial VO2 film across the phase transition.

Exploring flexible electronics is on the verge of innovative breakthroughs in terahertz (THz) communication technology. Vanadium dioxide (VO2) with insulator-metal transition (IMT) has excellent application potential in various THz smart devices, but the associated THz modulation properties in the flexible state have rarely been reported. Herein, we deposited an epitaxial VO2 film on a flexible mica substrate via pulsed-laser deposition and investigated its THz modulation properties under different uniaxial strains across the phase transition. It was observed that the THz modulation depth increases under compressive strain and decreases under tensile strain. Moreover, the phase-transition threshold depends on the uniaxial strain. Particularly, the rate of the phase transition temperature depends on the uniaxial strain and reaches approximately 6 °C/% in the temperature-induced phase transition. The optical trigger threshold in laser-induced phase transition decreased by 38.9% under compressive strain but increased by 36.7% under tensile strain, compared to the initial state without uniaxial strain. These findings demonstrate the uniaxial strain-induced low-power triggered THz modulation and provide new insights for applying phase transition oxide films in THz flexible electronics.

Laser-Induced Phase Transition and Patterning of hBN-Encapsulated MoTe2.

Transition metal dichalcogenides exhibit phase transitions through atomic migration when triggered by various stimuli, such as strain, doping, and annealing. However, since atomically thin 2D materials are easily damaged and evaporated from these strategies, studies on the crystal structure and composition of transformed 2D phases are limited. Here, the phase and composition change behavior of laser-irradiated molybdenum ditelluride (MoTe2 ) in various stacked geometry are investigated, and the stable laser-induced phase patterning in hexagonal boron nitride (hBN)-encapsulated MoTe2 is demonstrated. When air-exposed or single-side passivated 2H-MoTe2 are irradiated by a laser, MoTe2 is transformed into Te or Mo3 Te4 due to the highly accumulated heat and atomic evaporation. Conversely, hBN-encapsulated 2H-MoTe2 transformed into a 1T' phase without evaporation or structural degradation, enabling stable phase transitions in desired regions. The laser-induced phase transition shows layer number dependence; thinner MoTe2 has a higher phase transition temperature. From the stable phase patterning method, the low contact resistivity of 1.13kΩµm in 2H-MoTe2 field-effect transistors with 1T' contacts from the seamless heterophase junction geometry is achieved. This study paves an effective way to fabricate monolithic 2D electronic devices with laterally stitched phases and provides insights into phase and compositional changes in 2D materials.

Optical Creation of Skyrmions by Spin Reorientation Transition in Ferrimagnetic CoHo Alloys.

Manipulating magnetic skyrmions by means of a femtosecond (fs) laser pulse has attracted great interest due to their promising applications in efficient information-storage devices with ultralow energy consumption. However, the mechanism underlying the creation of skyrmions induced by an fs laser is still lacking. As a result, a key challenge is to reveal the pathway for the massive reorientation of magnetization from trivial to nontrivial topological states. Here, we studied a series of ferrimagnetic CoHo alloys and investigated the effect of a single laser pulse on the magnetic states. Thanks to the time-resolved magneto-optical Kerr effect and imaging techniques, we demonstrate that the laser-induced phase transitions from single domains into a topological skyrmion phase are mediated by the transient in-plane magnetization state, in real time and space domains, respectively. Combining experiments and micromagnetic simulations, we propose a two-step process for creating skyrmions through laser pulse irradiation: (i) the electron temperature enhancement induces a spin reorientation transition on a picosecond (ps) timescale due to the suppression of perpendicular magnetic anisotropy (PMA) and (ii) the PMA slowly restores, accompanied by out-of-plane magnetization recovery, leading to the generation of skyrmions with the help of spin fluctuations. This work provides a route to control skyrmion patterns using an fs laser, thereby establishing the foundation for further exploration of topological magnetism at ultrafast timescales.

Laser-driven first-order spin reorientation and Verwey phase transitions in magnetite Fe3O4 beyond the range of thermodynamic equilibrium

Ultrafast photo-induced phase transitions occurring under the impact of femtosecond laser pulses provide versatile opportunities for switching solids between distinctly-different crystalline, electronic, and magnetic states and thus modify their functional properties in a significant way. In this paper, we report on the laser-induced spin reorientation and Verwey phase transitions in a single crystalline ferrimagnetic magnetite ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. Using femtosecond optical and magneto-optical pump-probe techniques, we define the range of the initial sample temperatures and laser fluences when partial or complete photo-induced phase transitions occur from a monoclinic insulating to a cubic metallic state with concomitant switching of magnetic anisotropy from the uniaxial to the cubic one. We thus reveal a connection between these phase transitions when driven by femtosecond laser pulses. Using transient linear and quadratic magneto-optical effects, we examine magnetization dynamics launched the switching of the magnetic anisotropy axis. We unveil the presence of the domains undergoing the laser-induced phase transitions even below the established threshold fluence for the transitions, as well as when the material is initially in the cubic phase. This is the manifestation of the first order of these both laser-induced phase transitions beyond the range of thermodynamic equilibrium.

Liquid-Modulated Photothermal Phenomena in Porous Silicon Nanostructures Studied by μ-Raman Spectroscopy

In the present study, the effect of liquid filling of the nanopore network on thermal transport in porous Si layers was investigated by μ-Raman spectroscopy. The values of thermal conductivity of porous Si and porous Si-hexadecane composites were estimated by fitting the experimentally measured photoinduced temperature rise with finite element method simulations. As a result, filling the pores with hexadecane led to (i) an increase in the thermal conductivity of the porous Si-hexadecane composite in a wide range of porosity levels (40-80%) and (ii) a suppression of the characteristic laser-induced phase transition of Si from cubic to hexagonal form.

Femtosecond laser-induced phase transition in VO2 films.

VO2 is a very promising material due to its semiconductor-metal phase transition, however, the research on fs laser-induced phase transition is still very controversial, which greatly limits its development in ultrafast optics. In this work, the fs laser-induced changes in the optical properties of VO2 films were studied with a variable-temperature Z-scan. At room temperature, VO2 consistently maintained nonlinear absorption properties at laser repetition frequencies below 10 kHz while laser-induced phase transition properties appeared at higher repetition frequencies. It was found by temperature variation experiments at 100 kHz that the modulation depth of the laser-induced VO2 phase transition was consistent with that of the ambient temperature-induced phase transition, which was increased linearly with thickness, further confirming that the phase transition was caused by the accumulation of thermal effects of a high-repetition-frequency laser. The phase transition process is reversible and causes substantial changes in optical properties of the film, which holds significant promise for all-optical switches and related applications.

Laser-induced Fano asymmetry, electron-phonon coupling, and phase transition in lanthanide sesquioxide (Ln2O3; Ln = Eu, Gd, Dy) nanoparticles: A Raman spectroscopic investigation

Laser power-dependent Raman spectroscopy is deployed to probe Fano interference in asymmetrically broadened Tg modes and the associated line shift in three technologically sound, meticulously characterized rare-earth sesquioxide systems. Group theoretical analysis is accompanied to introspect the Raman-active optic modes in cubic, monoclinic, and trigonal phases and identify the laser heating-induced local phase transitions. With increasing laser intensity, a regular redshift and larger negative asymmetry in the Raman peaks are detected, which is attributed to moderations in Fano scattering by enhanced electron–phonon coupling amid the focussed photoexcited electron plasma and is illustrated using a Feynman diagram. A quantitative study is thereby performed to unveil the intrinsic nature of discrete-continuum Fano resonance in the nanoparticles of interest emphasizing the high sensitivity of Raman spectra to the excitation strength that perturbs the generic vibrational features at the Brillouin zone center by influencing the interference conditions, force constant, and length of the associated bonds compelled by tensile stress. A rising trend of the charge–phonon coupling constant (λ) with laser power validates a stronger particle–quasiparticle coupling, whereas a shorter anharmonic phonon lifetime (τanh) indicates faster interactions. Using Allen's formalism, the charge density of states [N(εF)] at the Fermi level per spin and molecule is calculated, which pertains to a negative regression dependence in the λN(εF)−τanh dynamics.

Size-Dependent Threshold of the Laser-Induced Phase Transition of Colloidally Dispersed Copper Oxide Nanoparticles

Size-Dependent Threshold of the Laser-Induced Phase Transition of Colloidally Dispersed Copper Oxide Nanoparticles

Low Energy Switching of Phase Change Materials Using a 2D Thermal Boundary Layer.

The switchable optical and electrical properties of phase change materials (PCMs) are finding new applications beyond data storage in reconfigurable photonic devices. However, high power heat pulses are needed to melt-quench the material from crystalline to amorphous. This is especially true in silicon photonics, where the high thermal conductivity of the waveguide material makes heating the PCM energy inefficient. Here, we improve the energy efficiency of the laser-induced phase transitions by inserting a layer of two-dimensional (2D) material, either MoS2 or WS2, between the silica or silicon substrate and the PCM. The 2D material reduces the required laser power by at least 40% during the amorphization (RESET) process, depending on the substrate. Thermal simulations confirm that both MoS2 and WS2 2D layers act as a thermal barrier, which efficiently confines energy within the PCM layer. Remarkably, the thermal insulation effect of the 2D layer is equivalent to a ∼100 nm layer of SiO2. The high thermal boundary resistance induced by the van der Waals (vdW)-bonded layers limits the thermal diffusion through the layer interface. Hence, 2D materials with stable vdW interfaces can be used to improve the thermal efficiency of PCM-tuned Si photonic devices. Furthermore, our waveguide simulations show that the 2D layer does not affect the propagating mode in the Si waveguide; thus, this simple additional thin film produces a substantial energy efficiency improvement without degrading the optical performance of the waveguide. Our findings pave the way for energy-efficient laser-induced structural phase transitions in PCM-based reconfigurable photonic devices.

Temperature-dependent electrical and structural characterization of laser-induced graphitic microwires in CVD diamond

An array of 2500 vertical graphitic microwires was fabricated within a single-crystal diamond plate with the purpose of creating distributed electrically conductive structures intended for the development of electronic devices operating at high temperatures. To this end, the structural, morphological, and electrical properties of the diamond/graphite system were investigated as a function of temperature up to 550 °C, by means of optical and secondary electron microscopy (SEM), micro-Raman spectroscopy and current-voltage measurements. The vertical microstructuring of the diamond bulk was obtained by a laser-induced phase transition from diamond to graphite by means of ultra-short laser pulses (100 fs duration, 800 nm wavelength, 1 kHz repetition rate). As inferred from SEM micrographs, the graphitic wires display a high-aspect-ratio with length of approximately 200 μm and diameter of about 10 μm. The electrical resistivity of the single microwire is estimated to be 0.49 ± 0.15 Ω cm at room temperature, then decreasing linearly with temperature with a coefficient of approximately −1 × 10−2 K−1. Raman spectroscopy results point out the absence of structural alterations after high-temperature operations.

Melting of gold by ultrashort laser pulses: advanced two-temperature modeling and comparison with surface damage experiments

The ultrafast laser-induced solid-liquid phase transition in metals is still not clearly understood and its accurate quantitative description remains a challenge. Here we systematically investigated, both experimentally and theoretically, the melting of gold by single femto- and picosecond near-infrared laser pulses. Two laser systems with wavelengths of 800 and 1030 nm and pulse durations ranging from 124 fs to 7 ps were used and the damage and ablation thresholds were determined for each irradiation condition. The theoretical analysis was based on two-temperature modeling. Different expressions for the electron-lattice coupling rate and contribution of ballistic electrons were examined. In addition, the number of free electrons involved in the optical response is suggested to be dependent on the laser intensity and the influence of the fraction of involved electrons on the damage threshold was investigated. Only one combination of modelling parameters was able to describe consistently all the measured damage thresholds. Physical arguments are presented to explain the modeling results.

Dynamics of ultrafast phase transitions in MgF2 triggered by laser-induced THz coherent phonons

The advent of free-electron lasers opens new routes for experimental high-pressure physics, which allows studying dynamics of condensed matter with femtosecond resolution. A rapid compression, that can be caused by laser-induced shock impact, leads to the cascade of high-pressure phase transitions. Despite many decades of study, a complete understanding of the lattice response to such a compression remains elusive. Moreover, in the dynamical case (in contrast to quasi-static loading) the thresholds of phase transitions can change significantly. Using the third harmonic pump–probe technique combined with molecular dynamics to simulate the terahertz (THz) spectrum, we revealed the dynamics of ultrafast laser-induced phase transitions in MgF2 in all-optical experiment. Tight focusing of femtosecond laser pulse into the transparent medium leads to the generation of sub-TPa shock waves and THz coherent phonons. The laser-induced shock wave propagation drastically displaces atoms in the lattice, which leads to phase transitions. We registered a cascade of ultrafast laser-induced phase transitions (P42/mnm ⇒ Pa-3 ⇒ Pnam) in magnesium fluoride as a change in the spectrum of coherent phonons. The phase transition has the characteristic time of 5–10 ps, and the lifetime of each phase is on the order of 40–60 ps. In addition, phonon density of states, simulated by molecular dynamics, together with third-harmonic time-resolved spectra prove that laser-excited phonons in a bulk of dielectrics are generated by displacive excitation (DECP) mechanism in plasma mediated conditions.