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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Ultrafast Tm-Doped Fiber Amplifier with 1 kW Average Output Power
摘要： Ultrafast thulium-doped fiber lasers emitting in the 2 μm wavelength region have recently attracted a significant amount of attention regarding applications in material processing, life sciences, as well as in fundamental science, e.g. high harmonic generation, particle acceleration, mid-IR generation. In addition to the application-relevant emission wavelength, Tm-doped silica fibers possess many interesting characteristics, such as a broad gain bandwidth spanning from 1700 nm to 2100 nm and more than 70% slope efficiency by exploiting cross-relaxation processes. Furthermore, the longer operation wavelength reduces the impact of detrimental nonlinear effects in fiber lasers, and allows for a larger mode field area with diffraction-limited beam quality as compared to the well-established ytterbium-doped fiber laser systems. In this contribution, we report on a new average power record from a thulium-doped fiber amplifier. We use state-of-the-art laser diodes to reach a combined pump power of more than 2 kW at 793 nm. The use of a polarization maintaining thulium-doped photonic crystal fiber allows for an optical-to-optical efficiency of more than 60% and a pump-limited average output power of 1150 W. After compression to 265 fs pulse duration, 1060 W of average power and 50 MW of peak power at 80 MHz pulse repetition rate are achieved, which represents a performance level that is currently not achievable with a single ultrafast ytterbium-doped fiber amplifier. Additionally, we confirmed a close to diffraction-limited beam quality (M2<1.1) up to the highest power level. From the simulation of the amplification process, the average heat-load of the fiber reaches 95 W/m at full power level, without the onset of transverse mode instability. This average heat-load is significantly higher than the critical value of about 30 W/m, which was found to be the threshold for transverse mode instability in ytterbium-doped fiber lasers.
关键词： ultrafast,high power,Tm-doped fiber amplifier,photonic crystal fiber
更新于2025-09-11 14:15:04
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-matrix-with-time-dependence theory for ultrafast atomic processes in arbitrary light fields
摘要： We describe an ab initio and nonperturbative R-matrix with time-dependence theory for ultrafast atomic processes in light fields of arbitrary polarization. The theory is applicable to complex, multielectron atoms and atomic ions subject to ultrashort (particularly few-femtosecond and attosecond) laser pulses with any given ellipticity, and it generalizes previous time-dependent R-matrix techniques restricted to linearly polarized fields. We discuss both the fundamental equations, required to propagate the multielectron wave function in time, as well as the computational developments necessary for their efficient numerical solution. To verify the accuracy of our approach, we investigate the two-photon ionization of He, irradiated by a pair of time-delayed, circularly polarized, femtosecond laser pulses, and compare photoelectron momentum distributions, in the polarization plane, with those obtained from recent time-dependent close-coupling calculations. The predictive capabilities of our approach are further demonstrated through a study of single-photon detachment from F? in a circularly polarized, femtosecond laser pulse, where the relative contribution of the co- and counter-rotating 2p electrons is quantified.
关键词： ultrafast atomic processes,R-matrix,circularly polarized laser pulses,single-photon detachment,time-dependence,photoelectron momentum distributions,arbitrary light fields,multielectron atoms
更新于2025-09-11 14:15:04
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High power 4.2-cycle mid-infrared pulses from a self-compression optical parametric oscillator
摘要： We demonstrate the generation of few-cycle mid-infrared (MIR) pulses from a self-compressed optical parametric oscillator (OPO), synchronously pumped by a 105 fs Yb3+ doped fiber laser system. By using a ring cavity design, 4.2-cycle (45.3 fs) MIR pulses at a central wavelength of 3250 nm (with a 10-dB bandwidth of 648 nm) are obtained in a simple, practical and compact configuration. Detailed theoretical analysis and numerical simulation are performed to better understand the self-compression mechanism in such an OPO cavity. In addition, when pumped at 3.8 W, the OPO delivers idler output power up to 330 mW and the corresponding pump depletion is 74.7%.
关键词： Nonlinear optics,Pulse compression,Ultrafast lasers
更新于2025-09-11 14:15:04
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Imaging charge migration in the asymmetric molecule with the holographic interference in strong-field tunneling ionization
摘要： Coherent superposition of several electronic states in molecules induces attosecond charge migration. We theoretically demonstrate a way to observe this attosecond charge migration in the asymmetric molecule HeH2+ using strong-field photoelectron holography, complementary to our previous work (He M et al. 2018 Phys. Rev. Lett. 120 133204) where the charge migration in the symmetric molecule was monitored. We disentangle the holographic interference from other types of interference in the photoelectron momentum distribution with the Fourier frequency filter method. By employing the holographic fringes originating from the ground state of HeH2+ as a reference, we develop a feasible approach to visualize the charge migration in the asymmetric molecule with attosecond temporal resolution.
关键词： strong laser field,above-threshold ionization,ultrafast phenomena
更新于2025-09-10 09:29:36
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Beyond the molecular movie: Dynamics of bands and bonds during a photoinduced phase transition
摘要： Ultrafast nonequilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photoinduced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a nonequilibrium population of excited electronic states. Using femtosecond photoemission, we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photoholes in shaping the potential energy landscape and enables a combined momentum- and real-space description of PIPTs, including the ultrafast formation of chemical bonds.
关键词： Silicon surface,Photoinduced phase transitions,Chemical bonds,Electronic structure,Indium nanowires,Femtosecond photoemission,Ultrafast dynamics
更新于2025-09-10 09:29:36
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Dissociating diatomic molecules in ultrafast and intense light
摘要： An ab initio theory is devised for the quantum dynamics of molecules undergoing multiple ionization in ultrafast and intense light. Specifically, the intertwined problem of photoionization, radiative, and electronic transitions in the course of dissociation is addressed which arises, e.g., when molecules are exposed to xuv light or x rays from free electron lasers or attosecond light sources, but the approach is equally useful in optical strong-field physics. The coherent interaction of the molecule with the light in a specific charge state is also treated. I set out from an abstract formulation in terms of the quantum optical notion of system-reservoir interaction using a master equation in Lindblad form and analyze its short-time approximation. First, I express it in a direct sum rigged Hilbert space for an efficient solution with numerical methods for systems of partial differential equations. Second, I derive a treatment via quantum Monte Carlo wave packet (MCWP) propagation. The formalism is concretized to diatomic molecules in Born-Oppenheimer approximation whereby molecular rotation is disregarded. The numerical integration of the master equation is carried out with a suitably factored density matrix that exploits the locality of the Hamiltonian and the Lindblad superoperator with respect to the internuclear distance. The formulation of the MCWP for molecules requires a thorough analysis of the quantum jump process; namely, the dependence on the continuous distance renders a straight wave packet promotion useless and, instead, a projected outer product needs to be employed involving an integrated quantum jump operator.
关键词： Ultrafast and intense light,Born-Oppenheimer approximation,Dissociation of diatomic molecules,Quantum Monte Carlo algorithm,Master equation in Lindblad form
更新于2025-09-10 09:29:36
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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) - Studying Solvation by Non-linear THz Spectroscopy
摘要： We are using non-linear THz spectroscopy to study the ultrafast solvation dynamics of water. Hydrogen bond rearrangements via librational or diffusional motions constitute the heart of network dynamics in liquid water. Reorientation of water molecules on the ps and sub-ps time scale is a decisive step in the formation and breaking of hydrogen bonds. Here we report the results of ultrafast nonlinear THz experiments.
关键词： solvation dynamics,ultrafast dynamics,non-linear THz spectroscopy,hydrogen bond
更新于2025-09-10 09:29:36
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Structural Dynamics of Dimethyl Sulfoxide Aqueous Solutions Investigated by Ultrafast Infrared Spectroscopy: Using Thiocyanate Anion as a Local Vibrational Probe
摘要： The microscopic structure of dimethyl sulfoxide (DMSO) aqueous solutions was investigated by FTIR spectroscopy and ultrafast IR spectroscopy. The structural dynamics of the binary mixtures were reflected by using thiocyanate anion (SCN-) as a local vibrational probe. FTIR spectra of SCN- anion showed the hydrogen bond networks of water are affected by the presence of DMSO molecules, and the peak position and bandwidth of SCN- anions are red-shifted and narrowed accordingly due to the weak hydration in the binary mixture. The vibrational lifetime of the SCN- anion showed almost linear enhancement with the increase of DMSO, which can be explained by the weak interaction between SCN- and the hydrophobic groups in DMSO molecule. However, the rotational dynamics of SCN- are slowing down significantly and showed a maximum response at XDMSO (mole fraction) of 0.35, which is mainly caused by the confinement of SCN- anions positioned in the vicinity of the complex structure formed between DMSO and water molecules. The concentration dependent rotational dynamics of water molecules and SCN- anions are having similar behavior, indicated that the complex structure can be formed between water and DMSO molecules due to the strong interaction. The result also demonstrates that the structural inhomogeneity in aqueous solution can be unraveled by monitoring the vibrational relaxation dynamics of SCN- anion serving as the local vibrational probe.
关键词： structural dynamics,thiocyanate anion,FTIR spectroscopy,Dimethyl sulfoxide,ultrafast IR spectroscopy,aqueous solutions,vibrational probe
更新于2025-09-10 09:29:36
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Disentangling the Electron and Hole Dynamics in Janus CdSe/PbSe Nanocrystals through Variable Pump Transient Absorption Spectroscopy
摘要： CdSe/PbSe Janus hetero-nanocrystals (HNCs) represent an interesting system where structurally immiscible CdSe and PbSe co-exist in a single structure with intermixed electronic states. Here, we have investigated the carrier dynamics in Janus CdSe/PbSe HNCs through ultrafast transient absorption spectroscopy by selectively exciting either the CdSe or the PbSe domains. Upon excitation of the CdSe domain sub-picosecond hole transfer to the hybrid interfacial PbSe states were identified. On the other hand, the transfer of hot electron from PbSe to CdSe was evident upon creation of hot electrons in PbSe domain without exciting the CdSe domain. The photo-excited hot hole also gets transferred to the hybrid interfacial states in sub-ps time (~1ps) bleaching the corresponding transition. The decay of the localized hole was found to be much slower compared to the electron which can be beneficial for carrier extraction and multiexciton generation. The finding of hot electron transfer in a single structure and slow decay of holes can thus prove to be advantageous for future design of photovoltaic devices.
关键词： carrier dynamics,ultrafast transient absorption spectroscopy,CdSe/PbSe Janus hetero-nanocrystals,photovoltaic devices,hot electron transfer
更新于2025-09-10 09:29:36
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Evidence of depolarization and ellipticity of high harmonics driven by ultrashort bichromatic circularly?polarized fields
摘要： High harmonics generated by counter-rotating laser fields at the fundamental and second harmonic frequencies have raised important interest as a table-top source of circularly polarized ultrashort extreme-ultraviolet light. However, this emission has not yet been fully characterized: in particular it was assumed to be fully polarized, leading to an uncertainty on the effective harmonic ellipticity. Here we show, through simulations, that ultrashort driving fields and ultrafast medium ionization lead to a breaking of the dynamical symmetry of the interaction, and consequently to deviations from perfectly circular and fully polarized harmonics, already at the single-atom level. We perform the complete experimental characterization of the polarization state of high harmonics generated along that scheme, giving direct access to the ellipticity absolute value and sign, as well as the degree of polarization of individual harmonic orders. This study allows defining optimal generation conditions of fully circularly polarized harmonics for advanced studies of ultrafast dichroisms.
关键词： circularly polarized fields,high harmonics,ultrafast dichroisms,ultrashort pulses,polarization state
更新于2025-09-10 09:29:36