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Noise and spectral stability of deep-UV gas-filled fiber-based supercontinuum sources driven by ultrafast mid-IR pulses
摘要： Deep-UV (DUV) supercontinuum (SC) sources based on gas-filled hollow-core fibers constitute perhaps the most viable solution towards ultrafast, compact, and tunable lasers in the UV spectral region, which can even also extend into the mid-infrared (iR). noise and spectral stability of such broadband sources are key parameters that define their true potential and suitability towards real-world applications. In order to investigate the spectral stability and noise levels in these fiber-based DUV sources, we generate an SC spectrum that extends from 180 nm (through phase-matched dispersive waves - DWs) to 4 μm by pumping an argon-filled hollow-core anti-resonant fiber at a mid-IR wavelength of 2.45 μm. We characterize the long-term stability of the source over several days and the pulse-to-pulse relative intensity noise (RIN) of the DW at 275 nm. The results indicate no sign of spectral degradation over 110 hours, but the RIN of the DW pulses at 275 nm is found to be as high as 33.3%. Numerical simulations were carried out to investigate the spectral distribution of the RIN and the results confirm the experimental measurements and that the poor noise performance is due to the high Rin of the mid-IR pump laser, which was hitherto not considered in numerical modelling of these sources. The results presented herein provide an important step towards an understanding of the noise mechanism underlying such complex light-gas nonlinear interactions and demonstrate the need for pump laser stabilization.
关键词： Deep-UV,supercontinuum sources,spectral stability,ultrafast mid-IR pulses,gas-filled fiber,noise
更新于2025-09-23 15:19:57
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Hidden isolated OH at the charged hydrophobic interface revealed by two-dimensional heterodyne-detected VSFG spectroscopy
摘要： Water around the hydrophobic groups mediates hydrophobic interaction that plays key roles in many chemical and biological processes. Thus, molecular-level elucidation of the properties of the water in the vicinity of the hydrophobic group is important. We report on the structure and dynamics of water at two oppositely charged hydrophobic interfaces, i.e., the tetraphenylborate ion (TPB-)/water and tetraphenylarsonium ion (TPA+)/water interfaces, which are clarified by two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy. The obtained 2D HD-VSFG spectra of anionic TPB- interface reveal the existence of distinct π-hydrogen bonded OH groups in addition to the usual hydrogen bonded OH groups, which is hidden in the steady-state spectrum. In contrast, 2D HD-VSFG spectra of cationic TPA+ interface only show the presence of usual hydrogen bonded OH groups. The present study demonstrates that the sign of the interfacial charge governs the structure and dynamics of water molecules that face the hydrophobic region.
关键词： sum-frequency generation,ultrafast spectroscopy,hydrophobic interfaces,interfacial water,vibrational dynamics
更新于2025-09-23 15:19:57
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Near-direct bandgap WSe <sub/>2</sub> /ReS <sub/>2</sub> type-II pn heterojunction for enhanced ultrafast photodetection and high-performance photovoltaics
摘要： PN heterojunctions comprising layered van der Waals (vdW) semiconductors have been used to demonstrate current-rectifiers, photodetectors, and photovoltaic devices. However, a direct or near-direct heterointerface bandgap for enhanced photogeneration in high light-absorbing few-layer vdW materials remains unexplored. In this work, for the first time, density functional theory calculations show that the heterointerface of few-layer group-6 transition metal dichalcogenide (TMD) WSe2 with group-7 ReS2 results in a sizeable (0.7 eV) near-direct type-II bandgap. The interlayer IR bandgap is confirmed through IR photodetection and micro-photoluminescence measurements demonstrate type-II alignment. Few-layer flakes exhibit ultrafast response time (5 μs) and high responsivity (3 A/W) and large photocurrent-generation and responsivity-enhancement at the hetero-overlap region (10-100×). Large open-circuit voltage of 0.64 V and short-circuit current of 2.6 μA enable high output electrical power. Finally, long term air-stability and facile single contact metal fabrication process make the multi-functional few-layer WSe2/ReS2 heterostructure diode technologically promising for next-generation optoelectronics.
关键词： van der Waals heterostructure,pn heterojunction,ultrafast photodetection,near-direct bandgap,infrared photodetection,interlayer bandgap
更新于2025-09-23 15:19:57
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Defect mediated transport in self-powered, broadband and ultrafast photoresponse of MoS <sub/>2</sub> /AlN/Si based photodetector
摘要： By combining unique properties of ultrathin 2D materials with conventional 3D semiconductors, devices with enhanced functionalities can be realized. Here we report a self-powered and ultrafast photodetector based on hybrid MoS2/AlN/Si heterostructure. The heterojunction is formed by depositing MoS2 thin film by pulsed laser deposition on AlN/Si(111) template. The vertical transport properties of the device under dark and light illumination conditions, exhibit an excellent photoresponse in a broad range of wavelengths (300–1100 nm) at 0 V. The maximum responsivity of this photodetector is found to be 9.93 A/W at wavelength of 900 nm. The device shows an ultrafast temporal response with response/recovery times of 12.5/14.9 μs. X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy reveal presence of native oxygen impurities in AlN, throughout the bulk of the film. These oxygen defects form a deep donor level in AlN and play a crucial role in the transport of the photogenerated carriers, resulting in enhanced device performance.
关键词： broadband and ultrafast photoresponse,deep defects,MoS2,AlN,pulsed laser deposition
更新于2025-09-23 15:19:57
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Tilted Snowplow Ponderomotive Electron Acceleration With Spatio-Temporally Shaped Ultrafast Laser Pulses
摘要： We propose a novel scheme for using the ponderomotive force of a tilted ultrafast laser pulse to accelerate electrons in free space. The tilt of the intensity envelope results from the angular dispersion of the pulse’s spectrum and slows down the interaction of the pulse with free electrons. The slower effective pulse velocity allows time for the electrons to accelerate from rest while remaining on the wave. We present both non-relativistic and relativistic analytic single-particle models in the adiabatic ponderomotive approximation, describing the process for an ideal infinite tilted pulse as well as a finite width beam. The analysis predicts the threshold intensity as a function of the pulse front tilt angle and shows that in the ideal case the output energy of the electrons is four times that of the ponderomotive potential at the capture threshold. Full-field simulations using the 2D OSIRIS 4.0 particle-in-cell code confirm the basic scheme. This tilted pulse acceleration scheme shows promise as a lab-scale method of accelerating electrons to the MeV level with good energy and angular resolution, to be used for ultrafast electron diffraction or injection into a second stage accelerator.
关键词： ponderomotive force,spatio-temporal pulse shaping,ultrafast lasers,laser electron accelerators,relativistic kinematics
更新于2025-09-23 15:19:57
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Expansion dynamics and chemistry evolution in ultrafast laser filament produced plasmas
摘要： Laser ablation in conjunction with optical emission spectroscopy is a potential non-contact, standoff detection method for all elements in the periodic table and certain isotopes such as radionuclides. Currently, significant development efforts are on-going to use ultrafast laser filaments for remote detection of materials. The application of filaments is of particular interest in extending the range of stand-off capability associated with elemental and isotopic detection via laser-induced breakdown spectroscopy. In this study, we characterize the expansion dynamics and chemical evolution of filament-produced uranium (U) plasmas. Laser filaments are generated in the laboratory by loosely focusing 35 femtosecond (fs), 6 milli Joule (mJ) pulses in air. Two-dimensional spectrally-integrated and time-resolved imaging was performed to study hydrodynamics and evolution of U atomic and UO molecular emission in filament-produced U plasmas. Our results highlight that filament ablation of U plasmas gives a cylindrical plasma plume morphology with an appearance of plume splitting into slow and fast moving components at later times of its evolution. Emission from the slow-moving region shows no distinct spectral features (i.e. broadband-like) and is contributed in part by nanoparticles generated during ultrafast laser ablation. Additionally, we find U atoms and U oxide molecules (i.e. UO, UxOy) co-exist in the filament produced plasma, which can be attributed to the generation of low-temperature plasma conditions during filament ablation.
关键词： optical emission spectroscopy,Laser ablation,plasma chemistry,ultrafast laser filaments,uranium plasmas
更新于2025-09-23 15:19:57
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Ultrafast spectroscopic investigation of the effect of solvent additives on charge photogeneration and recombination dynamics in non-fullerene organic photovoltaic blends
摘要： The PBDB-TF:IT-4F blend is a kind of state-of-the-art non-fullerene photovoltaic blend. Herein, the effects of 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) additives on the neat and blend film morphologies and the related ultrafast photophysical processes are studied. It is found that both DIO and CN can lead to an enhanced structural order in the in-plane direction and increased average lifetime of excitons in neat PBDB-TF films. The face-on orientation of PBDB-TF still exists in the DIO-processed PBDB-TF:IT-4F blend film, while in the case of the CN-processed blend film, molecular packing orientation is similar to that of the pristine blend film. The blend samples prepared with the two additives show increased initial exciton yields. Interestingly, in the blend samples prepared with the DIO additive, the recombination loss via the formation of polymer triplet excitons can be effectively suppressed, in comparison to the pristine and CN-processed samples. Both the DIO- and CN-processed devices show increased short-circuit current densities. The DIO-processed device is also found to have a superior fill factor due to suppressed recombination loss. The work provides a comprehensive insight into the ultrafast photophysical processes in varied blend morphologies induced by additives and their effect on the photovoltaic parameters of the devices.
关键词： charge photogeneration,ultrafast spectroscopy,recombination dynamics,solvent additives,non-fullerene photovoltaic blend
更新于2025-09-23 15:19:57
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Photocarrier relaxation pathway in selenium quantum dots and its application for UV-Vis photodetecting
摘要： Chain-like materials have recently attracted significant attention due to their unique structure and outstanding electro-optical properties. However, the photocarrier dynamics and pathways in these materials that determine the electro-optical performances of the prepared devices have barely been touched. Herein, one typical chain-like materials e quantum dots (Se QDs) were prepared via a facile liquid phase exfoliation approach. The photocarrier dynamics in selenium quantum dots were systematically investigated via ultrafast transient absorption spectroscopy in the ultraviolet-visible regime. Four photocarrier decay pathways with different lifetime times were firstly detected, endues the elucidation and reconstruction of the energy schematic diagram of Se QDs. Thanks to the broadband photo-response and fast recovery time of Se QDs, a photoelectrochemical (PEC)-typed photodetector was proposed for the first time to our knowledge, demonstrating excellent photodetection properties. Considering the feasible fabrication method and clear photocarrier pathways, the excellent photocurrent density and stability of this photodetector undoubtedly guarantee the promising of selenium for advanced photonics devices.
关键词： broadband photo-response,photocarrier dynamics,ultrafast transient absorption spectroscopy,selenium quantum dots,photoelectrochemical photodetector
更新于2025-09-23 15:19:57
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Sub-100 nm 2D nanopatterning on large scale by ultrafast laser energy regulation
摘要： Coupling ultrafast light to surface nanoreliefs leads to periodic patterns achieving record processing scales down to tens of nanometers. Driven by near-field interactions, the promising potential of the spontaneous pattern formation relies on the scale-up manufacturing one-step process. Here, we report the self-assembly of unconventional arrays of nanocavities of 20 nm diameter with a periodicity down to 60 nm upon ultrafast laser irradiation of a nickel surface. In stark contrast to laser-induced surface ripples, which are stochastic and suffer from a lack of regularity, the 2D patterns present an unprecedented uniformity on extreme scales. The onset of nanocavity arrays ordered in a honeycomb lattice is achieved by overcoming the anisotropic polarization response of the surface by a delayed action of cross-polarized laser pulses. The origin of this self-arrangement is identified as a manifestation of Marangoni convection instability in a nanoscale melt layer, destabilized by the laser-induced rarefaction wave.
关键词： nanopatterning,self-assembly,ultrafast laser,Marangoni convection,nanocavities
更新于2025-09-23 15:19:57
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Dynamic quantum state holography
摘要： We present a pulse-shaper-based holographic technique for the time-resolved and phase-sensitive observation of ultrafast quantum dynamics. The technique combines bichromatic white light polarization pulse shaping with the tomographic reconstruction of photoelectron wave packets. The physical scheme is based on the interference of a probe wave packet from N + 1 resonance-enhanced multiphoton ionization via the target states and a reference wave packet from M + 1 multiphoton ionization of the ground state. To create the wave packets, we employ carrier-envelope phase stable bichromatic (Mω:Nω) pump-probe pulse sequences. The scheme is demonstrated on femtosecond Rydberg wave-packet dynamics in potassium atoms using corotating circularly polarized (2ω:3ω) pulse sequences. The interference of continuum states with different angular momenta yields a crescent-shaped photoelectron wave packet rotating in the laser polarization plane due to the interplay of the optical phase and the accumulated quantum phase. Carrier-envelope phase control of the rotation provides access to the photoelectron asymmetry, enabling background-free detection of the crescent’s angular motion which maps the bound-electron dynamics.
关键词： photoelectron wave packets,ultrafast quantum dynamics,Rydberg wave-packet dynamics,dynamic quantum state holography,bichromatic white light polarization pulse shaping
更新于2025-09-23 15:19:57