Pump Fluence Research Articles

A versatile laser-based apparatus for time-resolved ARPES with micro-scale spatial resolution.

We present the development of a versatile apparatus for 6.2eV laser-based time and angle-resolved photoemission spectroscopy with micrometer spatial resolution (time-resolved μ-ARPES). With a combination of tunable spatial resolution down to ∼11 μm, high energy resolution (∼11meV), near-transform-limited temporal resolution (∼280fs), and tunable 1.55eV pump fluence up to 3 mJ/cm2, this time-resolved μ-ARPES system enables the measurement of ultrafast electron dynamics in exfoliated and inhomogeneous materials. We demonstrate the performance of our system by correlating the spectral broadening of the topological surface state of Bi2Se3 with the spatial dimension of the probe pulse, as well as resolving the spatial inhomogeneity contribution to the observed spectral broadening. Finally, after in situ exfoliation, we performed time-resolved μ-ARPES on a ∼30 μm flake of transition metal dichalcogenide WTe2, thus demonstrating the ability to access ultrafast electron dynamics with momentum resolution on micro-exfoliated materials.

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.

Microstructured Organic Cavities with High‐Reflective Flat Reflectors Fabricated by Using a Nanoimprint‐Bonding Process

AbstractThe integration of photonic microstructure into organic microcavities represents an effective strategy for manipulating eigenstates of cavity or polariton modes. However, well‐established fabrication processes for microstructured organic microcavities are still lacking. This study proposes a nanoimprint‐bonding process as a novel fabrication method for microstructured organic microcavities. This process relies on a UV nanoimprint technique utilizing two different photopolymer resins, enabling the independent fabrication of highly reflective reflectors and photonic microstructures without compromising the accuracy of each. The resulting organic microcavities demonstrate spatially localized photonic modes within dot structures and their nonlinear responses on the pumping fluence. Furthermore, a highly precise photonic band is confirmed within a honeycomb lattice structure, which is owing to the high quality factor of the cavity achievable with the nanoimprint‐bonding process. Additionally, a topological edge state is also observable within a zigzag lattice structure. These results highlight the significant potential of the fabrication method for advancing organic‐based photonic devices, including lasers and polariton devices.

Chirality manipulation of ultrafast phase switches in a correlated CDW-Weyl semimetal

Light engineering of correlated states in topological materials provides a new avenue of achieving exotic topological phases inaccessible by conventional tuning methods. Here we demonstrate a light control of correlation gaps in a model charge-density-wave (CDW) and polaron insulator (TaSe4)2I recently predicted to be an axion insulator. Our ultrafast terahertz photocurrent spectroscopy reveals a two-step, non-thermal melting of polarons and electronic CDW gap via the fluence dependence of a longitudinal circular photogalvanic current. This helicity-dependent photocurrent reveals continuous ultrafast phase switches from the polaronic state to the CDW (axion) phase, and finally to a hidden Weyl phase as the pump fluence increases. Additional distinctive attributes aligning with the light-induced switches include: the mode-selective coupling of coherent phonons to the polaron and CDW modulation, and the emergence of a non-thermal chiral photocurrent above the pump threshold of CDW-related phonons. The demonstrated ultrafast chirality control of correlated topological states here holds large potentials for realizing axion electrodynamics and advancing quantum-computing applications.

Giant Ultrafast All-Optical Modulation Based on Exceptional Points in Exciton-Polariton Perovskite Metasurfaces.

Ultrafast all-optical modulation with optically resonant nanostructures is an essential technology for high-speed signal processing on a compact optical chip. Key challenges that exist in this field are relatively low and slow modulations in the visible range as well as the use of expensive materials. Here we develop an ultrafast all-optical modulator based on MAPbBr3 perovskite metasurface supporting exciton-polariton states with exceptional points. The additional angular and spectral filtering of the modulated light transmitted through the designed metasurface allows us to achieve 2500% optical signal modulation with the shortest modulation time of 440 fs at the pump fluence of ∼40 μJ/cm2. Such a value of the modulation depth is record-high among the existing modulators in the visible range, while the main physical effect behind it is polariton condensation. Scalable and cheap metasurface fabrication via nanoimprint lithography along with the simplicity of perovskite synthesis and deposition make the developed approach promising for real-life applications.

Coherent and Incoherent Ultrafast Dynamics in Colloidal Gold Nanorods.

The study of the mechanisms that control the ultrafast dynamics in gold nanoparticles is gaining more attention, as these nanomaterials can be used to create nanoarchitectures with outstanding optical properties. Here pump-probe and two-dimensional electronic spectroscopy have been synergistically employed to investigate the early ultrafast femtosecond processes following photoexcitation in colloidal gold nanorods with low aspect ratio. Complementary insights into the coherent plasmonic dynamics at the femtosecond time scale and incoherent hot electron dynamics over picosecond time scales have been obtained, including important information on the different sensitivity to the pump fluence of the longitudinal and transverse plasmons and their different contributions to the photoinduced broadening and shift.

Terahertz emission characterization of silicon based ferromagnetic heterostructures

Terahertz spectroscopy and imaging have many applications, so the generation of broadband terahertz radiation is very important, but now it faces some challenges. Opto-spintronic terahertz emitters, composed of nanometer-thin magnetic multilayer, can produce high-quality broad-band terahertz pulses. Integration of opto-spintronic terahertz emitters onto the silicon wafers is the first step towards their usage in modern photonic devices. In this work, Ta/CoFeB/Ir heterostructures are deposited on thermally oxidized silicon wafers by dc magnetron sputtering. Under the illumination of a femtosecond laser pulse on the Ta/CoFeB/Ir trilayer heterostructure grown on silicon substrate, a spin current can be generated in the ferromagnetic layer due to the ultrafast demagnetization. The spin current is transported and injected into the neighboring non-magnetic metal layers of Ta and Ir. Consequently, the spin current can be converted into the charge current due to the strong spin-orbit coupling. The sub-picosecond transient charge current gives rise to the terahertz radiation that enters into the free space. The terahertz electric field is fully inverted when the magnetization is reversed, which indicates a strong connection between THz radiation and spin order of the heterostructure. The THz radiation from Ta/CoFeB/Ir heterostructure covers the 0.1–2.5 THz frequency range with a maximum value of about 0.64 THz. We also investigate the dependence of THz peak-to-peak value on the pump fluence. The THz emission is found to be saturated at a pump fluence of ~0.73 mJ/cm<sup>2</sup>. Our results demonstrate the existence of the strong spin-orbit coupling in the heavy metal Ir. Furthermore, we optimize the THz emission from the Ta/CoFeB/Ir heterostructure by changing the thickness of Ir layer. According to the thickness dependence of THz emission from the heterostructure, the propagation length of the spin current at THz frequencies is extracted to be about 0.59±0.12 nm, which is shorter than the GHz experimental measurement (~1.34 nm). Our experimental observation is consistent with that in the antiferromagnet IrMn layer, which may be attributed to different transport regimes. Theoretically, the optimized thickness values for CoFeB and Ir layers are 2.4 nm and 1.1 nm, respectively.

Strong transient magnetic fields induced by THz-driven plasmons in graphene disks

Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong ( ~ 1°) ultrafast Faraday rotation ( ~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm−2.

Modeling of Charge Injection, Recombination, and Diffusion in Complete Perovskite Solar Cells on Short Time Scales.

A model of charge population decay upon ultrafast optical pulse excitation in complete, working perovskite solar cells is proposed. The equation, including charge injections (extractions) from perovskite to contact materials, charge diffusion, and charge recombination via first-, second-, and third-order processes, is solved using numerical simulations. Results of simulations are positively verified by broadband transient absorption results of mixed halide, triple-cation perovskite (FA0.76MA0.19Cs0.05Pb(I0.81Br0.19)3). The combined analytical and experimental findings reveal the best approaches for the proper determination of the crucial parameters that govern charge transfer dynamics in perovskite solar cells on picosecond and single nanosecond time scales. Measurements from both electron and hole transporting layer sides under different applied bias potentials (zero and close to open circuit potential) and different pump fluence (especially below 5 μJ/cm2), followed by fitting of parameters using numerical modeling, are proposed as the optimal methodology for describing the processes taking place in efficient devices.

Emergence of liquid following laser melting of gold thin films.

X-ray structural science is undergoing a revolution driven by the emergence of X-ray Free-electron Laser (XFEL) facilities. The structures of crystalline solids can now be studied on the picosecond time scale relevant to phonons, atomic vibrations which travel at acoustic velocities. In the work presented here, X-ray diffuse scattering is employed to characterize the time dependence of the liquid phase emerging from femtosecond laser-induced melting of polycrystalline gold thin films using an XFEL. In a previous analysis of Bragg peak profiles, we showed the supersonic disappearance of the solid phase and presented a model of pumped hot electrons carrying energy from the gold surface to scatter at internal grain boundaries. This generates melt fronts propagating relatively slowly into the crystal grains. By conversion of diffuse scattering to a partial X-ray pair distribution function, we demonstrate that it has the characteristic shape obtained by Fourier transformation of the measured F(Q). The diffuse signal fraction increases with a characteristic rise-time of 13 ps, roughly independent of the incident pump fluence and consequent final liquid fraction. This suggests the role of further melt-front nucleation processes beyond grain boundaries.

Photophysical properties of nitrogen-doped carbon quantum dots synthesized by graphite

Here, nitrogen-doped carbon quantum dots (N-CQDs) were successfully synthesized by the solvothermal method using graphite as the carbon source and N,N-dimethylformamide as the nitrogen source. We characterized the structure and chemical constitution of N-CQDs using X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. We investigated the pump- and temperature-dependent photoluminescence (PL) properties and the dynamic exciton recombination processes of N-CQDs, using both steady-state and time-resolved PL techniques. The spectral results show that the PL emission peak located at 518 nm at room temperature, mainly originates from the n-π∗ transition on the surface of N-CQDs. The pump fluence and PL integral intensity were analyzed to demonstrate the existence of single-photon excitation under the 405 nm laser excitation. As the temperature increases, the non-radiative transition gradually increases, which decreases the PL intensity, the full width at half maxima first narrows and then widens and the PL lifetime gradually decreases. Furthermore, we combined the N-CQDs with chip to prepare light-emitting diode (LED). The resulting chromaticity coordinate was obtained to be (0.29, 0.40). This study offers a comprehensive understanding of the luminescence mechanism in N-doped CQDs and introduces a novel approach for the quickly fabrication of full-color display LEDs.

Micrometer-Scale Carrier Transport in the Solid Film of Giant CdSe/CdS Nanocrystals Imaged by Transient Absorption Microscopy.

For the practical applications in solar cells and photodetectors, semiconductor colloidal nanocrystals (NCs) are assembled into a high-concentration film with carrier transport characteristics, the full understanding and effective control of which are critical for the achievement of high light-to-electricity conversion efficiencies. Here we have applied transient absorption microscopy to the solid film of giant CdSe/CdS NCs and discovered that at high pump fluences the carrier transport could reach a long distance of ∼2 μm within ∼30 ps after laser pulse excitation. This intriguing behavior is attributed to the metal-insulator transition and the associated bandlike transport, which are promoted by the enhanced electronic coupling among neighboring NCs with extended wave functions overlap of the excited-state charge carriers. Besides providing fundamental transport information in the regime of high laser pump fluences, the above findings shed light on the rational design of high-power light detecting schemes based on colloidal NCs.

Temporal‐Spatial‐Resolved Lasing Dynamics in Customized Solution Grown Perovskite Single‐Crystal Microcavities

AbstractSingle‐crystal halide perovskites are among the most promising semiconductor candidates for highly efficient optoelectronic devices due to their higher stability against moisture and light radiation and lower trap density than their polycrystalline counterparts. In this work, single‐crystal MAPbBr3 microcavities with controllable geometries and sizes are fabricated using a template‐limited method combined with an anti‐solvent diffusion growth method. The single‐crystal microcavities emit lasers around 550 nm under femtosecond laser excitation, with a lasing threshold of ≈35 µJ cm−2 and a quality factor of 1135, as well as excellent long‐term operation stability. The temporal‐spatial evolution of the lasing process in an individual microcavity is achieved by the microscopic optical Kerr‐gating technique. The temporal behaviors including pulse duration and build‐up time, emission spectra shift, and lasing mode evolution as a function of pump fluence are unveiled, and the complex relationship between the light‐induced carrier dynamics and lasing properties is clarified. Spatial heterogeneous lasing dynamics are also observed in an individual microdisk, which is attributed to the non‐uniform optical losses and carrier densities driven by structural imperfections. This study can provide a fabrication strategy for perovskite on‐chip optoelectronic devices, as well as a deeper understanding of the carrier‐density‐driven lasing dynamics in microcavities.

Time-resolved photo-magnetic switching of bubble domains in ferrimagnetic garnets

We investigate the single-domain switching of magnetization with a femtosecond laser pulse in Co-doped iron garnet films using time-resolved magneto-optical imaging. This switching regime is achieved with a pump fluence that is two times lower than the fluence required for multi-domain switching. We observed the picoseconds magnetization dynamics of switching in a single bubble domain with a diameter of a few micrometers by tuning the pump laser fluence. This may provide opportunities for cold photo-magnetic recording in magnetic bits approaching the nano-size scale.

Modulation of the Transient Magnetization of an EuO/Co Bilayer Tuned by Optical Excitation

AbstractThe magnetic proximity effect provides a promising way to increase the low Curie temperature (TC) of europium monoxide (EuO) toward or even above room temperature, while keeping its stoichiometry and insulating properties. This work studies EuO/Co bilayers using static and time‐resolved magneto‐optical Kerr effect measurements, and explores the influence of magnetic proximity on TC and on the spin dynamics in EuO. Excitation above the EuO bandgap results in an ultrafast enhancement of the EuO magnetization followed by a demagnetization within nanoseconds. This behaviour is also visible upon selectively photoexciting Co in the EuO/Co bilayer placed in an out‐of‐plane magnetic field, which is attributed to propagation of a superdiffusive spin current from Co into EuO. As the spin dynamics of Co shows a transient thermal demagnetization, the bilayer provides a system where the transient magneto‐optical signal can be tuned in amplitude and sign by varying external parameters such as the sample temperature or pump fluence. Moreover, in a strong excitation regime it is possible to measure the magnetic hysteresis of the underlying EuO, which is present up to room temperature – giving experimental evidence for the presence of a tuneable magnetic proximity coupling between Co and EuO.

Generation and characterization of intense terahertz pulses from DSTMS crystal.

Matter manipulation in terahertz range calls for a strong-field broadband light source. Here, we present a scheme for intense terahertz generation from DSTMS crystal driven by a high power optical parametric chirped pulse amplifier. The generated terahertz energy is up to 175 µJ with a peak electric field of 17 MV/cm. The relationship between terahertz energy, conversion efficiency, and pump fluence is demonstrated. This study provides a powerful driving light source for strong-field terahertz pump-probe experimentation.

Ultrafast Charge Dynamics in Bulk Zinc Oxide under Intense Photoexcitation

The photo-induced charge dynamics of textbook wide-bandgap semiconductor ZnO have been investigated on the picosecond time-scale. We performed optical Pump-THz Probe experiments in order to measure the dielectric constant of the material after high-fluence photo-excitation of charge carriers. The technique allows access to both carrier lifetime and scattering rates, and it provides direct access to the intrinsic dielectric function changes upon excitation. A complex dynamic is unveiled in the high-fluence pumping regime, where the relaxation time is in the hundreds of picoseconds range and increases with increasing Pump fluence, while the onset of photoconductivity takes place in a few picoseconds. The plasma frequency and the relaxation time dependence on the Pump fluence are discussed.

Light-driven transitions in quantum paraelectrics

Motivated by recent experiments on pump-induced polar ordering in the quantum paraelectric SrTiO$_3$, we study a driven phonon system close to a second order phase transition. Analyzing its classical dynamics, we find that sufficiently strong driving leads to transitions into polar phases whose structures, determined by the light polarization, are not all accessible in equilibrium. In addition, for certain intensity profiles we demonstrate the possibility of two-step transitions as a function of fluence. For even stronger field intensities, the possibility of period-doubling and chaotic behavior is demonstrated. Finally we develop a generalized formalism that allows us to consider quantum corrections to the classical dynamics in a systematic fashion. We predict a shift in the critical pump fluence due to quantum fluctuations with a characteristic dependence on the fluence increase rate, which can be observed in experiments.

Ultrafast Switching from the Charge Density Wave Phase to a Metastable Metallic State in 1T-TiSe_{2}.

The ultrafast electronic structures of the charge density wave material 1T-TiSe_{2} were investigated by high-resolution time- and angle-resolved photoemission spectroscopy. We found that the quasiparticle populations drove ultrafast electronic phase transitions in 1T-TiSe_{2} within 100fs after photoexcitation, and a metastable metallic state, which was significantly different from the equilibrium normal phase, was evidenced far below the charge density wave transition temperature. Detailed time- and pump-fluence-dependent experiments revealed that the photoinduced metastable metallic state was a result of the halted motion of the atoms through the coherent electron-phonon coupling process, and the lifetime of this state was prolonged to picoseconds with the highest pump fluence used in this study. Ultrafast electronic dynamics were well captured by the time-dependent Ginzburg-Landau model. Our work demonstrates a mechanism for realizing novel electronic states by photoinducing coherent motion of atoms in the lattice.

Single-Color and Two-Color Femtosecond Pump–Probe Experiments on Graphitic Carbon Nitrides Revealing Their Charge Carrier Kinetics

Graphitic carbon nitride related materials have been one type of the most promising photocatalysts. Comprehensive understanding of their genuine charge carrier kinetic aspects will strengthen their photocatalytic applications. Herein, we synthesized two samples with different termination/functionalization groups, CN and NCN, and performed both single-color and two-color femtosecond pump–probe experiments on them. We found that both the recovery kinetics obtained from single-color femtosecond pump–probe experiments (SCFPPE) and the decay kinetics obtained from two-color femtosecond pump–probe experiments (TCFPPE) for CN and NCN display stronger pump fluence dependences, indicating that their genuine charge carrier kinetics should be measured under very low pump fluence. The charge carrier kinetics obtained under higher pump fluence would introduce a dramatically short picosecond component due to carrier–carrier interactions. With the obtained pump fluence-dependent charge carrier kinetics, for the first time, we derived the absorption cross section of CN and NCN at pump wavelength. The derived absorption cross section of CN at ∼398 nm is ∼5.2 × 10–15 cm2, and the derived absorption cross section of NCN at ∼398 nm is ∼8.3 × 10–15 cm2. Under very low pump fluence, we also found that the recovery kinetics of CN and NCN obtained from SCFPPE is obviously faster than their decay kinetics obtained from TCFPPE, suggesting that an additional fast decay photophysical process of CN and NCN might not be detected in TCFPPE. On the base of those femtosecond studies, we proposed a model for CN and NCN to understand how the pump fluence affect their charge carrier kinetics. We also discussed how the introduced termination/functionalization group in CN and NCN affects their genuine charge carrier kinetics so as to modify their photocatalytic hydrogen production activities. FTIR, XPS, XRD, and SEM EDX of CN and NCN were also measured for characterizing their chemical compositions and structures.