The use of femtosecond laser pulses to impulsively excite thermal and mechanical transients in matter has led, in the last years, to the development of picosecond acoustics. Recently, the pump-probe approach has been applied to nano-engineered materials to optically generate and detect acoustic waves in the GHz-THz frequency range. In this paper, we review the latest advances on ultrafast generation and detection of thermal gradients and pseudo-surface acoustic waves in two-dimensional lattices of metallic nanostructures. Comparing the experimental findings to the numeric analysis of the full thermo-mechanical problem, these materials emerge as model systems to investigate both the mechanical and thermal energy transfer at the nanoscale. The sensitivity of the technique to the nanostructures mass and shape variations, coupled to the phononic crystal properties of the lattices opens the way to a variety of applications ranging from hypersonic waveguiding to mass sensors with femtosecond time-resolution.ens the way to a variety of applications ranging from hypersonic waveguiding to mass sensors with femtosecond time-resolution.
The transient evolution of carriers in an intrinsic graphene under ultrafast excitation, which is caused by the collisionless interband transitions, is studied theoretically. The energy relaxation due to the quasielastic acoustic phonon scattering and the interband generation-recombination transitions due to thermal radiation are analyzed. The distributions of carriers are obtained for the limiting cases when carrier-carrier scattering is negligible and when the intercarrier scattering imposes the quasiequilibrium distribution. The transient optical response (differential reflectivity and tran…
Ultrafast optical pump-probe spectroscopy measurement on monolayer graphene observes significant optical nonlinearities. We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200fs, where stimulated emission completely compensates absorption loss in the graphene layer. To quantitatively investigate this transient, extremely dense photoexcited Dirac-fermion state, we construct a …
We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L$_3$ resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.
We investigate theoretically the dynamic multiferroic response of coupled ferroelectric/ferromagnetic composites upon excitation by a photo-induced acoustic strain pulse. Two magnetoelectric mechanisms are considered: interface strain- and charge-mediated magnetoelectric couplings. The former results in demagnetization, depolarization and repolarization within tens of picoseconds via respectively magnetostriction and piezoelectricity. Charge magnetoelectric interaction affects the ferroelectric/ferromagnetic feedback response leading to magnetization recovery. Experimental realization based on…
The ultrafast dynamics of surface electromagnetic waves photogenerated on aluminum film perforated with subwavelength holes array was studied in the visible spectral range by the technique of transient photomodulation with 100 fs time resolution. We observed a pronounced blueshift of the resonant transmission band that reveals the important role of plasma attenuation in the optical response of nanohole arrays. The blueshift is inconsistent with plasmonic mechanism of extraordinary transmission and points to the crucial role of interference in the formation of transmission bands. The transient …
Time-resolved pump-probe measurements were made at variable heat accumulation in Co/Pd superlattices. Heat accumulation increases the baseline temperature and decreases the equilibrium magnetization. Transient ultrafast demagnetization first develops with higher fluence in parallel with strong equilibrium thermal spin fluctuations. The ultrafast demagnetization is then gradually removed as the equilibrium temperature approaches the Curie temperature. The transient magnetization time-dependence is fit well with the spin-flip scattering model.
Transition metal dichalcogenides (TMDs) are emerging as promising two-dimensional (2d) semiconductors for optoelectronic and flexible devices. However, a microscopic explanation of their photophysics -- of pivotal importance for the understanding and optimization of device operation -- is still lacking. Here we use femtosecond transient absorption spectroscopy, with pump pulse tunability and broadband probing, to monitor the relaxation dynamics of single-layer MoS2 over the entire visible range, upon photoexcitation of different excitonic transitions. We find that, irrespective of excitation p…
It is known that ultrafast laser pulses can be used to deterministically switch magnetisation and create skyrmions, however the deterministic creation of a single Néel skyrmion after ultrafast demagnetisation remains an open question. Here we show domain wall skyrmions also emerge in systems with broken inversion symmetry after exposure to an ultrafast laser pulse, carrying an integer topological charge. Whilst domain wall skyrmions do not appear in the relaxed state due to quick thermal decay following an Arrhenius law, they play a key role in controlling the final skyrmion population through…
We demonstrate an ultrafast reversible modulation of resonant terahertz (THz) response in strongly photoexcited metamaterials. The transient spectral-temporal response of the dipole transition ~1.6 THz exhibits a distinct non-monotonic variation as a function of pump fluence. The transition energy shift, strength, spectral width and density-dependent ultrafast relaxation manifest a remarkable re-emergence of the resonances after initial quenching. Our simulation, incorporating the first-order diffraction from the photoinduced transient grating, reproduces the salient features, providing a new …
We present ultrafast x-ray diffraction experiments on thin films of metallic SrRuO$_3$ (SRO) after their excitation with ultrashort intense laser pulses. Depending on the layer thickness, the data exhibit a transient splitting of the (002) SRO Bragg peak evidencing the generation and propagation of sharp acoustic strain waves. These distinct structural dynamics are due to the exceptionally fast electron-phonon relaxation that gives rise to a quasi-instantaneous thermal stress in SRO. The interpretation is corroborated by numerical simulations which show excellent agreement with the experimenta…
In this article, we study non-radiative and radiative relaxation processes in a hybrid formed by combining Ag nanoparticle (NP) and CdTe quantum dots (QD) using transient transmission spectroscopy. The ultrafast transient transmission of hybrid, when excited at 400 nm, shows a faster recovery of hot electrons at a shorter time scale (few picoseconds) while it shows a slower recovery at a longer time scale (few tens of picoseconds). Further, it is found that the contribution of CdTe QD to the transient transmission is increased in the presence of Ag NP. However, the radiative relaxation in CdTe…
Ultrafast carrier dynamics in the topological insulator Bi2Se3 have recently been intensively studied using a variety of techniques. However, we are not aware of any successful experiments exploiting transient absorption (TA) spectroscopy for these purposes. Here we demonstrate that if the ~730 nm wavelength pumping (~1.7 eV photon energy) is applied to ultrathin Bi2Se3 films, TA spectra cover the entire visible region, thus unambiguously pointing to two-photon excitation (~3.4 eV). The carrier relaxation dynamics is found to be governed by the polar optical phonon cascade emission occurring i…
Black phosphorus has recently emerged as a promising material for high performance electronic and optoelectronic device for its high mobility, tunable mid-infrared bandgap and anisotropic electronic properties. Dynamical evolution of photo excited carriers and its induced change of transient electronic properties are critical for materials' high field performance, but remains to be explored for black phosphorus. In this work, we perform angle resolved transient reflection spectroscopy to study the dynamical evolution of anisotropic properties of black phosphorus under photo excitation. We find…
The injection of spin currents in semiconductors is one of the big challenges of spintronics. Motivated by the ultrafast demagnetisation and spin injection into metals, we propose an alternative femtosecond route based on the laser excitation of superdiffusive spin currents in a ferromagnet such as Ni. Our calculations show that even though only a fraction of the current crosses the Ni-Si interface, the laser-induced creation of strong transient electrical fields at a ferromagnet-semiconductor interface allows for the injection of chargeless spin currents with a record spin polarisations of 80…
Ultrafast carrier relaxation in the 2D topological insulator (TI) Bi2Se3 [gapped Dirac surface states (SS)] and how it inherits ultrafast relaxation in the 3D TI Bi2Se3 (gapless Dirac SS) remains a challenge for developing new optoelectronic devices based on these materials. Here ultrashort (100 fs) pumping pulses of ~340 nm wavelength (~3.65 eV photon energy) were applied to study ultrafast electron relaxation in the 2D TI Bi2Se3 films with a thickness of 2 and 5 quintuple layers (~2 and ~5 nm, respectively) using transient absorption (TA) spectroscopy in the ultraviolet-visible spectral regi…
An ultrafast spin current can be induced by femtosecond laser excitation in a ferromagnetic (FM) thin film in contact with a nonmagnetic (NM) metal. The propagation of an ultrafast spin current into NM metal has recently been found in experiments to generate transient spin accumulation. Unlike spin accumulation in equilibrium NM metals that occurs due to spin transport at the Fermi energy, transient spin accumulation involves highly nonequilibrium hot electrons well above the Fermi level. To date, the diffusion and dissipation of this transient spin accumulation has not been well studied. Usin…
In spin-based electronics, information is encoded by the spin state of electron bunches. Processing this information requires the controlled transport of spin angular momentum through a solid, preferably at frequencies reaching the so far unexplored terahertz (THz) regime. Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin-current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is employed to drive spins from a ferromagnetic Fe thin film into a nonmagnetic cap layer that has either low (Ru) or high (Au) …
Bulk layered MX2 transition metal chalcogenides (M = Mo, W and X = S, Se) are known to exhibit an indirect to direct band gap transition as the number of layers is reduced. Previous time-resolved work has principally focused on the investigation of the transient evolution of the band structure after photo-excitation, but additional information on the dynamics of the concomitant lattice rearrangement is needed to fully understand this phenomenon. Here, ultrafast electron diffraction is used to probe the atomic motion and bond dilation in monolayer WSe2 with femtosecond temporal resolution. The …
Experimental investigations of ultrafast magnetization dynamics increasingly employ resonant magnetic spectroscopy in the ultraviolet spectral range. Besides allowing to disentangle the element-specific transient response of functional magnetic systems, these techniques also promise to access attosecond to few-femtosecond dynamics of spin excitations. Here, we report on a systematic study of transient magnetic circular dichroism (MCD) on the transition metals Fe, Co and Ni as well as on a FeNi and GdFe alloy and reveal a delayed onset between the electronic and magnetic response. Supported by …
The transient acoustic response of a free-standing, polycrystalline thin Au film upon femtosecond optical excitation has been studied by time-resolved Debye-Scherrer X-ray diffraction using ultrashort Cu K$_α$ X-ray pulses from a laser-driven plasma X-ray source. The temporal strain evolution has been determined from the transient shifts of multiple Bragg diffraction peaks. The experimental data are in good agreement with the results of calculations based on the two-temperature model and an acoustic model assuming uni-axial strain propagation in the laser-excited thin film.
Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light induced insulator-to-metal phase transition in vanadium …
Two dimensional electronic spectroscopy and transient grating measurements were performed, for the first time, on nitrogen-vacancy centers in diamond. These measurements reveal energy transfer and vibrational pathways with consequences for spin coherence.
Ultrafast Laser-Induced Magnetic Relaxation in Artificial Spin Ice Driven by Dipolar Interactions
ETH Zurich（苏黎世联邦理工学院）；Instituto de Ciencia de Materiales de Madrid, CSIC（马德里材料科学研究所，西班牙国家研究委员会）；Physik funktioneller Materialien, Universität Wien（维也纳大学功能物理研究所）；NEST Research Group, Department of Computer Science, The University of Manchester（曼彻斯特大学计算机学院NEST研究组）
We examine the multiple exciton population dynamics in PbS quantum dots by ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We simultaneously probe the first three excitonic transitions over a broad spectral range. Transient spectra show the presence of first order bleach of absorption for the 1S_h-1S_e transition and second order bleach along with photoinduced absorption band for 1P_h-1P_e transition. We also report evidence of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures of carrier multiplication (multiexcitons for the single absorbed…
Ultrafast Band-Gap Renormalization in Bilayer Graphene
Christian Albrecht University of Kiel（基尔大学）；Institute of Experimental and Applied Physics, Christian Albrecht University of Kiel（基尔大学实验与应用物理研究所）；Ruprecht Haensel Laboratory, German Electron Synchrotron DESY（德国电子同步辐射中心鲁普雷希特·亨塞尔实验室）；Kiel Nano, Surface and Interface Science KiNSIS, Christian Albrecht University of Kiel（基尔纳米、表面与界面科学研究所 (KiNSIS), 基尔大学）；The Pennsylvania State University（宾夕法尼亚州立大学）；Department of Materials Science and Engineering, The Pennsylvania State University（宾夕法尼亚州立大学材料科学与工程系）；Dimensional Crystal Consortium, Materials Research Institute, The Pennsylvania State University（宾夕法尼亚州立大学材料研究所二维晶体联盟）；Center for 2-Dimensional and Layered Materials, The Pennsylvania State University（宾夕法尼亚州立大学二维及层状材料中心）；Center for Atomically Thin Multifunctional Coatings, The Pennsylvania State University（宾夕法尼亚州立大学原子级厚度多功能涂层中心）
Strain engineering is a powerful technology which exploits stationary external or internal stress of specific spatial distribution for controlling the fundamental properties of condensed materials and nanostructures. This advanced technique modulates in space the carrier density and mobility, the optical absorption and, in strongly correlated systems, the phase, e.g. insulator/metal or ferromagnetic/paramagnetic. However, while successfully accessing nanometer length scale, strain engineering is yet to be brought down to ultrafast time scales allowing strain-assisted control of state of matter…
The impact of semiconductor surface roughness on the electrical and thermal contact resistances and thermoEMF of thermoelectric material (TEM)-metal transient contact layer is studied theoretically. The distribution of hollows and humps on the rough surface is simulated by the truncated Gaussian distribution. The impact of distribution parameters on the electrical contact resistance and thermoEMF of thermoelectric material-metal contact is studied.
Efficient operation of electronic nanodevices at ultrafast speeds requires understanding and control of the currents generated by femtosecond bursts of light. Ultrafast laser-induced currents in metallic nanojunctions can originate from photo-assisted hot electron tunneling or lightwave-induced tunneling. Both processes can drive localized photocurrents inside a scanning tunneling microscope (STM) on femto- to attosecond time scales, enabling ultrafast STM with atomic spatial resolution. Femtosecond laser excitation of a metallic nanojunction, however, also leads to the formation of a transien…
In this work we present a theoretical description of the transient response of the Fermi Edge Singularity (FES). We study the linear and the nonlinear response of an n-doped QW to laser pulses in the Coherent Control (CC) and Four Wave Mixing (FWM) Configurations. By means of a bosonization formalism we calculate the FWM signal emitted by the sample when it is excited by pulses spectrally peaked around the FES and we show that the long time behavior of the nonlinear signal is very similar to the linear case.
We investigate the excited state electronic structure of the model phase transition system In/Si(111) using femtosecond time- and angle-resolved photoemission spectroscopy (trARPES) with an XUV laser source at 500 kHz . Excited state band mapping is used to characterize the normally unoccupied electronic structure above the Fermi level in both structural phases of indium nanowires on Si(111): the metallic (4x1) and the gapped (8x2) phases. The extracted band positions are compared with the band structure calculated using density functional theory (DFT) within both the LDA and GW approximations…
We have used a femtosecond pump-probe impulsive Raman technique to explore the ultrafast dynamics of micelle suspended single walled carbon nanotubes (SWNTs) in various pH environments. The structures of coherent phonon spectra of the radial breathing modes (RBMs) exhibit significant pH dependence, to which we attribute the effect of the protonation at the surface of SWNTs, resulting in the modification of electronic properties of semiconductor SWNTs. Analysis of the time-domain data using a time-frequency transformation uncovers also a second transient longitudinal breathing mode, which vanis…
