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Quasi-parametric amplification (QPA) is a nonlinear interaction in which the idler wave is depleted through some loss mechanism. QPA plays an important role in signal amplification in ultrafast photonics and quantum light generation. The QPA process has a number of features characterized by the non-Hermitian parity-time ($\mathcal{PT}$) symmetry. In this report, we explore new interaction regimes and uncover multiple $\mathcal{PT}$-symmetry phase transitions in such QPA process where transitions are particularly sensitive to external parameters. In particular, we demonstrate the feasibility of detection of $10^{-11}$ inhomogeneities of the doped absorber, which is order of magnitude more sensitive than similar measurements performed in a linear absorption regime. In doing so, we reveal a family of $\mathcal{PT}$-symmetry phase transitions appearing in the QPA process and provide a novel nonlinear optical sensing mechanism for precise optical measurements., Comment: 17 pages, 6 figures

The continuous advancements in ultrafast lasers, characterized by high pulse energy, great average power, and ultrashort pulse duration, have opened up new frontiers and applications in various fields such as high-energy-density science. In this study, we investigated the implementation of non-Hermitian nonlinear parametric amplification by introducing anti-parity-time (anti-PT) symmetry to three-wave interaction processes. By exploring the parameter space defined by the coupling coefficient, phase mismatch, and absorption, we categorized the behavior of the non-Hermitian optical parametric system into four distinct quadrants, representing unbroken/broken anti-PT symmetry and amplification/attenuation, and amplification-attenuation boundaries and exceptional lines can be observed in such parametric space. Through simulations of the dynamical behavior of the interacting waves, we demonstrated the rich evolutions of the signal and idler waves in systems belonging to the respective quadrants and near exceptional points, revealed by the unique performance of eigenmodes. Our findings provide insights into the evaluation of energy flow direction in optical parametric amplification engineering by the directly linked parameter space, which contribute to a deeper understanding of photonics and laser science, potentially leading to new applications in these fields., Comment: 12 pages; 4 figures

Optical quadratic nonlinearity is ultrafast in nature, while parametric interaction usually manifests only the broadband characteristic. Enormous progress has been made toward broadband phase-matching for parametric amplification and wide applications. In existing devices of broadband parametric amplification, the power efficiency of conversion is restricted to approximately the energy efficiency, and the desire for the signal power enhancement necessitates additional pulse compression after amplification. Here, we demonstrate ultrafast parametric amplification having an extraordinary power efficiency of 1155%; this allows the generation of intense femtosecond pulses without the need for a pulse stretcher and compressor. Direct femtosecond signal amplification by picosecond pumping is enabled by an ultrafast parametric environment in which the pre-delayed signal of faster speed gradually overtakes and effectively depletes the pump of slower speed as they propagate in a nonlinear crystal. The demonstrated technique should lead to breakthroughs in ultrafast lasers as well as applications., Comment: 15 pages, 4 figures

Limited by the difficulty in acceleration synchronization, it has been a long-term challenge for on-chip dielectric laser-based accelerators (DLA) to bridge the gap between non-relativistic and relativistic regimes. Here, we propose a DLA based on a spatio-temporal coupling (STC) controlled laser pulse, which enables the acceleration of a non-relativistic electron to a sub-MeV level in a single acceleration structure (chirped spatial grating). It provides high precision temporal and spatial tuning of the driving laser via the dispersion manipulation, leading to a synchronous acceleration of the velocity increasing electrons over a large energy range. Additionally, the STC scheme is a general method and can be extended to driving fields of other wavelengths such as terahertz pulses. Our results bring new possibilities to MeV-scale portable electron sources and table-top acceleration experiments.

In strong-field physics experiments with intense lasers, it is of paramount importance to single-shot diagnose the temporal contrast between laser pulse peak and its noise pedestal. This allows fast optimization of pulse contrast and meaningful comparison with theory for each pulse shot, and it can help new outcomes from clean laser-plasma interactions. Thus far, high contrast ratios up to ~10^10, required by present petawatt (PW) class lasers, have been accessible in both generation and single-shot characterization. However, ultrahigh contrast ~10^13, required by the planned 200-PW lasers, challenges intense laser technology and remains an open question. This paper reports on the first demonstration of such an ultrahigh-contrast measurement by adapting single-shot cross-correlator (SSCC). We introduce an ultrafast method that enables to determine the SSCC detection limit. Our strategy mimics the test laser having known ultrahigh contrast in the measurement frame of time-to-space mapping. The ultimate contrast-measurement limit of 10^13 is achieved, which corresponds to the highest pulse intensity set by SSCC damage threshold and the lowest noise pedestal set by single-photon detection. As a consequence, photon noise in the detection is observed and increases as the noise pedestal reduces. The demonstrated measurement ability at the photon noise limit is applied to a high-contrast laser system based on second-harmonic generation and optical parametric chirped-pulse amplification, suggesting accessible of ultrahigh contrast pulses., Comment: 12 pages, 4 figures

With the proposal of dual-wavelength pumping (DWP) scheme, DWP Er:ZBLAN fiber lasers at 3.5 um have become a fascinating area of research. However, limited by the absence of suitable saturable absorber, passively Q-switched and mode-locked fiber lasers have not been realized in this spectral region. Based on the layer-dependent bandgap and excellent photoelectric characteristics of black phosphorus (BP), BP is a promising candidate for saturable absorber near 3.5 um. Here, we fabricated a 3.5-um saturable absorber mirror (SAM) by transferring liquid-phase exfoliated BP flakes onto a gold-coated mirror. With the as-prepared BP SAM, we realized stable Q-switching and continuous-wave mode-locking operations in the DWP Er:ZBLAN fiber lasers at 3.5 um. To the best of our knowledge, it is the first time to achieve passively Q-switched and mode-locked pulses in 3.5 um spectral region. The research results will not only promote the development of 3.5-um pulsed fiber lasers but also open the photonic application of two-dimensional materials in this spectral region, Comment: 7 pages, 7 figures

Lasers that generate ultra-intense light pulses are under development for experiments in high-field and high-energy-density physics, as well as for applications such as particle acceleration. Extensions to even higher powers are being considered for future investigations that can only be imagined today, such as the quantum electrodynamics of plasmas and isolated attosecond-pulse generation with solid targets. For all of these areas, it is vital to produce high-contrast pulses, so that no pre-plasma is created in the target before the arrival of the main pulse. However, noise is unavoidable in high-gain amplification, and is manifested in the form of background light that accompanies pulses generated by chirped-pulse amplification (CPA). Here, we introduce a linear filtering technique based on spatio-spectral coupling, which allows in-band filtering of amplified pulses for the first time. Experiments demonstrate approximately 40 times contrast enhancement in optical parametric chirped-pulse amplification (OPCPA) and provide a foundation for scaling to much higher performance. The simplicity, efficiency, and direct compatibility with existing techniques for short-pulse generation will make spatio-spectral filtering attractive to a wide range of applications in ultrafast optics and time-resolved spectroscopy, and may open new directions in noise reduction., Comment: 13 pages, 4 figures

We experimentally study a new kind of parametric noise that is initiated from signal scattering and enhanced through optical parametric amplification. Such scattering noise behaves similarly to the parametric super-fluorescence in the spatial domain, yet is typically much stronger. In the time domain, it inherits the chirp of signal pulses and can be well compressed. We demonstrate that this scattering-initiated parametric noise has little influence on the amplified pulse contrast but can degrade the conversion efficiency substantially., Comment: 8 pages, 5 figures