arXiv subjects

Yuxi Fu

Publications and source records attributed to Yuxi Fu.

At least 19 recordsLinked to original sources

arXiv2026

Complex-gate all-optical frequency-resolved optical gating for ultrabroadband isolated attosecond pulse characterization

We experimentally demonstrate all-optical frequency-resolved optical gating (AO-FROG) for the characterization of ultrabroadband isolated attosecond pulses (IAPs) generated by a mid-infrared sub-cycle laser field. By extending the AO-FROG framework beyond the conventional phase-only modulation approximation, we develop a strong-field approximation (SFA)-based theoretical framework and show that the weak perturbing field induces both phase and amplitude modulations during high-order harmonic generation. A complex-valued gate function is therefore required for accurate pulse reconstruction. Using a 2.26-$\mu$m sub-cycle driving laser, we characterize IAPs spanning 100-180 eV in argon. The measured AO-FROG traces exhibit delay-dependent spectral modulations arising from perturbation-induced modifications of the electron trajectories and ionization probability. SFA simulations reproduce the experimental observations and confirm the importance of including ionization-induced amplitude modulation. The reconstructed temporal and spectral properties reveal an IAP duration of approximately 300 as and its spectral phase, providing access to the attosecond chirp of the generated pulses. Our results establish AO-FROG as a promising approach for temporal characterization of ultrabroadband attosecond sources driven by long-wavelength infrared fields.

physics.optics↗

arXiv2026

3-VASS Reachability is in EXPSPACE

A VASS can be viewed as a finite-state automaton manipulating a fixed number (called its dimension) of counters holding non-negative values. The reachability problem, asking whether there is a run from one configuration, defined by a state and values of the counters, to another configuration, has been a long-standing algorithmic challenge in theoretical computer science. When the dimension is part of the input, the problem has been shown to be ACKERMANN-complete in 2021. For fixed dimension greater than 2, and in particular for dimension 3, the exact complexity of the reachability problem remains unclear. For a long time the known algorithms for the 3-dimensional VASS reachability problem had been non-elementary, while the best known lower bound is merely PSPACE hardness inherited from dimension 2. A recent breakthrough in (Czerwi\'nski, Jecker, Lasota, Orlikowski, ICALP 2025) gave the first elementary upper bound for the problem, namely 2-EXPSPACE. In this paper it is shown that the reachability problem in 3-VASS belongs to EXPSPACE. The proof is based on a hierarchical pumpability analysis, yielding a doubly-exponential length bound on the shortest runs between two configurations.

cs.FL↗

arXiv2026

Improving Reachability in Vector Addition Systems through Pumpability

Vector addition systems (VAS) constitute an important model of computation and concurrency that is equally expressive as the Petri net model. Recently, a lot of research has been conducted on vector addition systems with states (VASS), which are VASes equipped with a finite state control. Results on VASS naturally carry over to VAS, but no straightforward improvement is available. In this paper, we investigate the reachability problem in VAS in fixed dimensions. Based on a pumpability analysis of VAS that refines Rackoff's extraction for VASS, we obtain an F_{d-2} upper bound for the d-dimensional VAS reachability problem, improving the F_d upper bound inherited from the d-dimensional VASS reachability problem. Low-dimensional VASes are also considered. In particular, we establish a PSPACE upper bound for reachability in 4-dimensional VAS and an ELEMENTARY upper bound for 5-dimensional VAS, while the same upper bounds were known only for 2-VASS and 3-VASS, respectively. The result for 4-VAS particularly hinges on a simplified projection technique developed for geometrically 2-dimensional VASSes, whose reachability problem is shown to be equivalent to 2-VASS.

cs.FL↗

arXiv2026

On the Reachability Problem for One-Dimensional Thin Grammar Vector Addition Systems

Vector addition systems with states (VASS) are a classic model in concurrency theory. Grammar vector addition systems (GVAS), equivalently, pushdown VASS, extend VASS by using a context-free grammar to control addition. In this paper, our main focus is on the reachability problem for one-dimensional thin GVAS (thin 1-GVAS), a structurally restricted yet expressive subclass. By adopting the index measure for complexity, and by generalizing the decomposition technique developed in the study of VASS reachability to grammar-generated derivation trees of GVAS, an effective integer programming system is established for a thin 1-GVAS. In this way, a nondeterministic algorithm with $\mathbf{F}_{2k}$ complexity is obtained for the reachability of thin 1-GVAS with index $k$, yielding a tighter upper bound than the previous one.

cs.LO↗

arXiv2025

A Unifying Approach to Probabilistic Testing Equivalences

Probabilistic concurrent systems are foundational models for modern mobile computing. In this paper, a unifying approach to probabilistic testing equivalences is proposed. With the help of a new distribution-based semantics for probabilistic models and a probabilistic testing framework with respect to process predicates, the internal characterization and the external characterization for testing equivalences are studied. The latter characterization can be viewed as the generalization of the classical fair/should equivalence and may equivalence. These equivalences are shown to be congruences. A thorough comparison between these equivalences and probabilistic bisimilarities is carried out. The techniques introduced in this paper can be easily extended to other probabilistic concurrent models. To showcase this flexibility, a case study is carried out on the pCSP model.

cs.LO↗

arXiv2025

A Programming Language for Feasible Solutions

Runtime efficiency and termination are crucial properties in the studies of program verification. Instead of dealing with these issues in an ad hoc manner, it would be useful to develop a robust framework in which such properties are guaranteed by design. This paper introduces a new imperative programming language whose design is grounded in a static type system that ensures the following equivalence property: All definable programs are guaranteed to run in polynomial time; Conversely, all problems solvable in polynomial time can be solved by some programs of the language. The contribution of this work is twofold. On the theoretical side, the foundational equivalence property is established, and the proof of the equivalence theorem is non-trivial. On the practical side, a programming approach is proposed that can streamline program analysis and verification for feasible computations. An interpreter for the language has been implemented, demonstrating the feasibility of the approach in practice.

cs.PL↗

arXiv2025

Nonadiabatic effect in high order harmonic generation revealed by a fully analytical method

We propose a fully analytical method for describing high-order harmonic generation (HHG). This method is based on the strong-field approximation (SFA) and electron-trajectory theory, but utilizes the perturbation expansion on the Keldysh parameter $\gamma$. This expansion allows us to clearly differentiate the nonadiabatic and adiabatic effects on HHG. We show that the nonadiabatic effect relating to high-order expansion depends on the laser wavelength and remarkably enhances the HHG yields for cases of short wavelengths, providing deeper insights into wavelength-dependent HHG yields which are important in producing attosecond pulses. Especially, our method provides the analytical and accurate descriptions of nonadiabatic exit velocity and position of the tunneling electron at the tunnel exit. These descriptions are meaningful for constructing a fully analytical and quantitative Coulomb-included HHG model, which is crucial in HHG-based attosecond measurement.

physics.optics↗

arXiv2025

Pulse duration dependence of material response in ultrafast laser-induced surface-penetrating nanovoids in fused silica

The focused ultrafast laser, with its ability to initiate nonlinear absorption in transparent materials, has emerged as one of the most effective approaches for micro-nano processing. In this study, we carried out research on the processing of high-aspect-ratio nanovoids on fused silica by using the single-pulse ultrafast Bessel beam. The thermodynamic response behaviors of the materials on surface and deep inside are found to exhibit pronounced disparities with the variation in laser pulse duration. As the pulse duration increases from 0.2 ps to 9.0 ps, the intensity of material ablation on silica surface exhibits a gradually decreasing trend, while for the void formation deep inside silica, the void diameter exhibits a trend of initial increase followed by decrease. In particular, no nanovoids are even induced deep inside when the pulse duration is 0.2 ps. The mechanism causing such differences is discussed and considered to be related to the peak intensity, group velocity dispersion, and plasma defocusing. By covering a polymer film on silica surface to influence the energy deposition, the thermomechanical response behaviors of the materials to laser pulse duration are modulated, and the material sputtering on nanovoid opening is suppressed. On this basis, surface-penetrating nanovoid arrays are fabricated on a 2-mm-thick silica sample using 2 ps Bessel beam. Given the nanovoid diameter of approximately 150 nm, the aspect ratio of the nanovoids on fused silica sample exceeds 13000:1. This outcome creates significant possibilities for the stealth dicing and processing of 3D photonic crystals, optical integrated devices, and nanofluidics.

physics.optics↗

arXiv2025

Reachability in Geometrically $d$-Dimensional VASS

Reachability of vector addition systems with states (VASS) is Ackermann complete~\cite{leroux2021reachability,czerwinski2021reachability}. For $d$-dimensional VASS reachability it is known that the problem is NP-complete~\cite{HaaseKreutzerOuaknineWorrell2009} when $d=1$, PSPACE-complete~\cite{BlondinFinkelGoellerHaaseMcKenzie2015} when $d=2$, and in $\mathbf{F}_d$~\cite{FuYangZheng2024} when $d>2$. A geometrically $d$-dimensional VASS is a $D$-dimensional VASS for some $D\ge d$ such that the space spanned by the displacements of the circular paths admitted in the $D$-dimensional VASS is $d$-dimensional. It is proved that the $\mathbf{F}_d$ upper bounds remain valid for the reachability problem in the geometrically $d$-dimensional VASSes with $d>2$.

cs.CC↗

arXiv2024

Perturbed three-channel waveform synthesizer for efficient isolated attosecond pulse generation and characterization

The generation of gigawatt-class isolated attosecond pulses (IAPs) is vital for attosecond pump-probe experiments. In such experiments, the temporal duration of IAPs must be determined quickly and accurately. In this study, we developed a perturbed three-channel waveform synthesizer for efficient IAPs generation and characterization at low repetition rates ( 10 Hz). Intense IAPs centered at photon energies of 60 eV (227 as duration) in Ar and 107 eV (128 as duration) in Ne were generated by the driving field from a three-channel waveform synthesizer and characterized using all-optical frequencyresolved optical gating (AO-FROG), which accelerated the measurement time to several minutes, providing fast feedback for the tunability of the IAP source. The peak power of the IAPs is higher than that reported in the literature.

physics.optics↗

arXiv2024

Improved Algorithm for Reachability in $d$-VASS

An $\mathsf{F}_{d}$ upper bound for the reachability problem in vector addition systems with states (VASS) in fixed dimension is given, where $\mathsf{F}_d$ is the $d$-th level of the Grzegorczyk hierarchy of complexity classes. The new algorithm combines the idea of the linear path scheme characterization of the reachability in the $2$-dimension VASSes with the general decomposition algorithm by Mayr, Kosaraju and Lambert. The result improves the $\mathsf{F}_{d + 4}$ upper bound due to Leroux and Schmitz (LICS 2019).

cs.LO↗

arXiv2024

Analyzing Divergence for Nondeterministic Probabilistic Models

Branching and weak probabilistic bisimilarities are two well-known notions capturing behavioral equivalence between nondeterministic probabilistic systems. For probabilistic systems, divergence is of major concern. Recently several divergence-sensitive refinements of branching and weak probabilistic bisimilarities have been proposed in the literature. Both the definitions of these equivalences and the techniques to investigate them differ significantly. This paper presents a comprehensive comparative study on divergence-sensitive behavioral equivalence relations that refine the branching and weak probabilistic bisimilarities. Additionally, these equivalence relations are shown to have efficient checking algorithms. The techniques of this paper might be of independent interest in a more general setting.

cs.LO↗

arXiv2023

Reachability in 3-VASS is in Tower

The reachability problem for vector addition systems with states (VASS) has been shown to be \textsc{Ackermann}-complete. For every $k\geq 3$, a completeness result for the $k$-dimensional VASS reachability problem is not yet available. It is shown in this paper that the $3$-dimensional VASS reachability problem is in \textsc{Tower}, improving upon the current best upper bound $\mathbf{F}_7$ established by Leroux and Schmidt in 2019.

cs.FL↗

arXiv2022

Generation of subcycle isolated attosecond pulses by pumping ionizing gating

We present a novel approach named as pumping ionizing gating (PIG) for the generation of isolated attosecond pulses (IAPs). In this regime, a short laser is used to ionize a pre-existing gas grating, creating a fast-extending plasma grating(FEPG) having an ionization front propagating with the velocity of light. A low-intensity long counterpropagating pump pulse is then reflected by a very narrow region of the ionization front, only where the Bragg conditions for resonant reflection is satisfied. Consequently, the pump reflection is confined within a sub-cycle region called PIG, and forms a wide-band coherent IAP in combination with the frequency up-conversion effect due to the plasma gradient. This approach results in a new scheme to generate IAPs fromlong picosecond pump pulses. Three-dimensional (3D) simulations show that a 1.6-ps, 1-{\mu}m pump pulse can be used to generate a 330 as laser pulse with a peak intensity approximately 33 times that of the pump and a conversion efficiency of around 0.1%.These results highlight the potential of the PIG method for generating IAPs with high conversion efficiency and peak intensity.

physics.optics↗

arXiv2021

A custom-tailored multi-TW optical electric field for gigawatt soft-x-ray isolated attosecond pulses

The bottleneck for an attosecond science experiment is concluded to be the lack of a high-peak-power isolated attosecond pulse source. Therefore, currently, generating an intense attosecond pulse would be one of the highest priority goals. In this paper, we review a TW-class parallel three-channel waveform synthesizer for generating a gigawatt-scale soft-x-ray isolated attosecond pulse (IAP) using high-order harmonics generation (HHG). Simultaneously, using several stabilization methods, namely, the low-repetition-rate laser carrier-envelope phase stabilization, Mach-Zehnder interferometer, balanced optical cross-correlator, and beam-pointing stabilizer, we demonstrate a stable 50-mJ three-channel optical-waveform synthesizer with a peak power at the multi-TW level. This optical-waveform synthesizer is capable of creating a stable intense optical field for generating an intense continuum harmonic beam thanks to the successful stabilization of all the parameters. Furthermore, the precision control of shot-to-shot reproducible synthesized waveforms is achieved. Through the HHG process employing a loose-focusing geometry, an intense shot-to-shot stable supercontinuum (50-70 eV) is generated in an argon gas cell. This continuum spectrum supports an IAP with a transform-limited duration of 170 as and a submicrojoule pulse energy, which allows the generation of a GW-scale IAP. Another supercontinuum in the soft-x-ray region with higher photon energy of approximately 100-130 eV is also generated in neon gas from the synthesizer. The transform-limited pulse duration is 106 as. According to this work, the enhancement of HHG output through optimized waveform synthesis is experimentally proved. The high-energy multicycle pulse with 10-Hz repetition rate is proved to have the same controllability for optimized waveform synthesis for HHG as few- or subcycle pulses from a 1-kHz laser.

physics.optics↗

arXiv2021

Nonlinear terahertz-wave radiation generated by mid-infrared dual-chirped optical parametric amplification femtosecond pulse laser excitation

We demonstrate an efficient procedure for terahertz-wave radiation generation by applying a nonlinear wavelength conversion approach using a mid-infrared pump source. We used a 3.3 {\mu}m dual-chirped optical parametric amplification source and a 1.5 {\mu}m fiber laser for a comparison of energy conversion efficiencies. Nonlinear inorganic and organic crystals were used, and for the periodically-poled lithium niobate crystal, the efficiencies were 1.3*10^-4 and 5.6*10^-12 for the 3.3 {\mu}m source and the 1.5 {\mu}m source, respectively. We confirmed that nonlinear crystals could be pumped with tera-watt / cm^2 class by using an mid-infrared source, which reduces several undesirable nonlinear optical effects.

physics.optics↗

arXiv2020

Apparatus for generation of nanojoule-class water-window high-order harmonics

In our recent study [Commun. Phys. 3, 92 (2020)], we have developed an approach for energy-scaling of high-order harmonic generation in water-window region under neutral-medium condition. More specifically, we obtained nanojoule-class water-window soft x-ray harmonic beam under phase match condition. It has been achieved by combining a newly developed terawatt-class mid-infrared femtosecond laser and a loose focusing geometry for high-order harmonic generation. The generated beam is more than 100 times intense compared to previously reported results. The experimental setup included two key parts: terawatt mid-infrared femtosecond driving laser [ Sci. Rep. 8, 7692 (2018)] and specially designed gas cell. Despite the dramatic drop in the optimal gas pressure due to loose focusing geometry, it still reached 1 bar level for helium. Moreover, faster leaking speed caused by larger pinhole size of the gas cell made the use of a normal gas cell impossible. Thus, we have designed a double-structured pulsed-gas cell with a differential pumping system, which enabled providing sufficiently high gas pressure. Moreover, it allowed reducing gas consumption significantly. Robust energy-scalable apparatus for high-order harmonic generation developed in in this study will enable the generation of over tens nanojoule water-window attosecond pulses in the nearest future.

physics.optics↗

arXiv2020

Reachability is Tower Complete

A complete characterization of the complexity of the reachability problem for vector addition system has been open for a long time. The problem is shown to be Tower complete.

cs.LO↗