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Coherent optical and acoustic phonons generated at lattice-matched GaP/Si(001) heterointerfaces
摘要： Thin GaP films can be grown on an exact Si(001) substrate with nearly perfect lattice match. We perform all-optical pump-probe measurements to investigate the ultrafast electron-phonon coupling at the buried interface of GaP/Si. Above-bandgap excitation with a femtosecond laser pulse can induce coherent longitudinal optical (LO) phonons both in the GaP overlayer and in the Si substrate. The coupling of the GaP LO phonons with photoexcited plasma is reduced significantly with decreasing the GaP layer thickness from 56 to 16 nm due to the quasi-two-dimensional confinement of the plasma. The same laser pulse can also generate coherent longitudinal acoustic (LA) phonons in the form of a strain pulse. The strain pulses induce not only a periodic modulation in the optical reflectivity as they propagate in the semiconductors, but also a sharp spike when it arrives at the GaP layer boundaries. The acoustic pulse induced at the GaP/Si interface is remarkably stronger than that at the Si surface, suggesting a possible application of the GaP/Si heterostructure as an opto-acoustic transducer. The amplitude and the phase of the reflectivity modulation varies with the GaP layer thickness, which can be understood in terms of the interference caused by the multiple acoustic pulses generated at the top surface and at the buried interface.
关键词： opto-acoustic transducer,GaP/Si heterointerfaces,coherent acoustic phonons,ultrafast electron-phonon coupling,coherent optical phonons
更新于2025-09-09 09:28:46
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Thermally-induced nonlinear spatial shaping of infrared femtosecond pulses in nematic liquid crystals
摘要： An optically-induced thermal non-linear effect in a nematic liquid-crystal (E7) cell is evidenced through weak light-absorption by the ITO coating of an infrared pulsed femtosecond laser (λ0 = 1.5 μm). Strong spatial self-phase modulation is generated, thus a multiple-ring pattern is observed in the far-field. The sign of the nonlinearity is changed depending on the laser polarization. The refractive index and thermal gradients are measured as a function of the laser intensity and we observe that the temperature can increase close to the nematic phase transition. The fidelity and stability of the process open new prospects for spatial shaping devices and delimits the operating wavelength range for ultrafast liquid-crystal based electro-optic application.
关键词： liquid crystal,spatial light modulators,ultrafast laser
更新于2025-09-09 09:28:46
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[IEEE 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz2018) - Nagoya, Japan (2018.9.9-2018.9.14)] 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Optical Rectification of a 100W Average Power Ultrafast Thin-Disk Oscillator
摘要： We demonstrate THz generation by optical rectification in GaP crystals using excitation average power levels exceeding 100 W. The laser source is a state-of-the-art diode-pumped Yb:YAG modelocked thin-disk laser, capable of generating 120 W at 13.4 MHz directly from a one-box oscillator, without the need for any amplification stages. In this first demonstration, we measured a maximum average power of 78 μW at a central frequency of 0.8 THz. This first result shows that optical rectification of high average power (100 W class) ultrafast sources is within reach, and paves the way towards Watt-level, ultrafast laser pumped THz sources.
关键词： optical rectification,ultrafast lasers,GaP crystals,high average power,THz generation
更新于2025-09-09 09:28:46
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Ultrafast Nonlinear Optical Excitation Behaviors of Mono- and Few-Layer Two Dimensional MoS2
摘要： The layered MoS2 has recently attracted significant attention for its excellent nonlinear optical properties. Here, the ultrafast nonlinear optical (NLO) absorption and excited carrier dynamics of layered MoS2 (monolayer, 3?4 layers, and 6?8 layers) are investigated via Z-scan and transient absorption spectra. Our experimental results reveal that NLO absorption coefficients of these MoS2 increase from ?27 × 103 cm/GW with more layers at 400-nm laser excitation, while the values decrease from 2.0 × 103 cm/GW at 800 nm. In addition, at high pump fluence, when the NLO response occurs, the results show that not only the reformation of the excitonic bands, but also the recovery time of NLO response decreases from 150 ps to 100 ps with an increasing number of layers, while the reductive energy of A excitonic band decreases from 191.7 meV to 51.1 meV. The intriguing NLO response of MoS2 provides excellent potentials for the next-generation optoelectronic and photonic devices.
关键词： ultrafast photonic devices,Ultrafast optics,two-dimensional materials
更新于2025-09-04 15:30:14
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Pump-Push-Probe for Ultrafast All-Optical Switching: The Case of a Nanographene Molecule
摘要： In the last two decades, the three-beam pump–push–probe (PPP) technique has become a well-established tool for investigating the multidimensional configurational space of a molecule, as it permits disclosure of precious information about the multiple and often complex deactivation pathways of the excited molecule. From the spectroscopic point of view, such a tool has revealed details about the efficiency of charge pair generation and conformational relaxation in π-conjugated molecules and macromolecules. In addition, PPP is effectively utilized for modulating the gain signal in conjugated materials by taking advantage of the spectral overlap between stimulated emission and charge absorption in those systems. However, the relatively low stability of conjugated polymers under intense photoexcitation is a crucial limitation for their real employment in plastic optical fibers (POFs) and for signal control applications. Herein, the role of PPP for achieving ultrafast all-optical switching in π-conjugated systems is highlighted. Furthermore, new experimental data on optical switching of a newly synthesized nanographene molecule, namely dibenzo[hi,st]ovalene (DBOV), is reported. The superior environmental and photostability of DBOV and, in general, of graphene nanostructures can represent a great advantage for their effective applications in POFs and information and communications technology.
关键词： ultrafast optical switching,pump–push–probe,conjugated polymers,ultrafast spectroscopy,nanographene
更新于2025-09-04 15:30:14
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Ultrafast Excited State Dynamics of a Stilbene–Viologen Charge Transfer Complex and Its Interaction with Alkanediammonium Salts
摘要： The spectral and thermodynamic properties of charge transfer complexes D?A and D?A?D between (E)-bis(18-crown-6)stilbene (D) and 1,1'-bis(2-ammonioethyl)-4,4'-bipyridinium tetraperchlorate (A) in MeCN were studied. The complex D?A is highly stable, while the complex D?A?D is weakly stable. D?A does not fluoresce due to fast intramolecular processes of direct and reverse electron transfer. The efficiency of D?A fluorescence ignition upon adding alkanediammonium salts depends on the length of a carbon chain. The spectral and kinetic characteristics of D?A and D?A?D CT states were obtained by femtosecond transient absorption spectroscopy. It was established that the characteristic time of back electron transfer in the CT state of D?A?D (770 fs) is significantly higher than that in the CT state of D?A (400 fs). In a number of charge transfer complexes, formed by derivatives of 4,4'-bipyridinium and di-(4-pyridinium)-ethylene with ammonioethyl and ammoniopropyl N-substituents, the rate of back electron transfer depends weakly on the acceptor nature, but is determined by the length of the carbon chain of terminal groups.
关键词： fluorescence,charge transfer complex,ultrafast electron transfer,transient absorption spectroscopy,bis-crown ether
更新于2025-09-04 15:30:14
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Coulomb Explosion Imaging for the Visualization of a Conical Intersection
摘要： Coulomb explosion imaging is proposed as a method to directly map the presence of conical intersections encountered by a propagating wave packet in a molecular system. The case of choice is the non-adiabatic coupling between two dissociative surfaces in the methyl iodide molecule, probed by Coulomb explosion with short, intense near-infrared pulses causing multiple ionization. On-the-fly multidimensional trajectory calculations with surface hopping using perturbation theory and including spin-orbit coupling are performed to visualize the dynamics through the conical intersection and compare with experimental results. The possibilities and limitations of the technique are examined and discussed.
关键词： ultrafast dynamics,strong laser fields,methyl halides,Nonadiabatic processes,surface hopping
更新于2025-09-04 15:30:14
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Subcycle interference in high-order harmonic generation from solids
摘要： Different from the high-order harmonic generation (HHG) from gases, we ?nd that the yield of HHG from solids exhibits unexpected modulations as a function of the driving laser intensity and wavelength. Its mechanism can be unraveled by the interference between currents inside the solids induced by two adjacent Zener tunneling events. Our simulations agree well with the experimental measurements of HHG from ZnSe and solid Ar. We also ?nd that the dephasing time plays a key role in this subcycle interference and can turn it on or off by controlling the overlap between the channels. It provides an avenue to optimize the ultrafast electron dynamics and HHG emission processes in solids, which will be useful for the compact ultrafast EUV light sources. We also propose an experimental scheme by using ultrashort lasers to explore this interference in other solid materials.
关键词： high-order harmonic generation,solids,ultrafast electron dynamics,Zener tunneling,dephasing time
更新于2025-09-04 15:30:14
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Particle-in-cell simulation of ultrafast hot-carrier transport in Fe/Au heterostructures
摘要： We present a theoretical approach for spin-polarized hot-electron transport, as it occurs after excitation by ultrafast optical pulses in heterostructures formed by ferromagnetic and normal metals. We formulate a spin-dependent particle-in-cell model that solves the Boltzmann equation for excited electrons. It includes lifetimes and transmission coefficients as parameters, which can be taken from ab initio calculations or experiment, and can be easily extended to multilayer systems. This approach is capable of describing electron transport in the ballistic, superdiffusive, and diffusive regime including secondary-carrier generation. We apply the model to optically excited carriers in Fe/Au bilayers and Fe/Au/Fe spin-valve structures and analyze theoretically the hot-electron transport dynamics probed in recent experiments on spin valves. We find contributions to the demagnetization dynamics induced in Fe/Au/Fe trilayers regardless of the parallel or antiparallel magnetic alignment of the Fe layers.
关键词： ferromagnetic and normal metals,ultrafast optical pulses,Fe/Au/Fe spin-valve structures,Fe/Au bilayers,Boltzmann equation,spin-polarized hot-electron transport
更新于2025-09-04 15:30:14
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Ultrafast defect dynamics: A new approach to all optical broadband switching employing amorphous selenium thin films
摘要： Optical switches offer higher switching speeds than electronics, however, in most cases utilizing the interband transitions of the active medium for switching. As a result, the signal suffers heavy losses. In this article, we demonstrate a simple and yet efficient ultrafast broadband all-optical switching on ps timescale in the sub-bandgap region of the a-Se thin film, where the intrinsic absorption is very weak. The optical switching is attributed to short-lived transient defects that form localized states in the bandgap and possess a large electron-phonon coupling. We model these processes through first principles simulation that are in agreement with the experiments.
关键词： electron-phonon coupling,optical switches,interband transitions,all optical broadband switching,first principles simulation,amorphous selenium thin films,ultrafast defect dynamics,sub-bandgap region,transient defects
更新于2025-09-04 15:30:14