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1. Few-cycle, mJ-level, mid-wave infrared pulses generated via post-compression of a chirped pulse amplifier

   https://doi.org/10.1364/OL.524212  

   Alphonse_Marra, Z; Wu, Yi; Belden, Nicholas; Chang, Zenghu (May 2024, Optics Letters)

   Few-cycle pulses were generated by passing a beam from a cryogenically cooled Fe:ZnSe chirped-pulse amplifier (CPA) at a repetition rate of 400 Hz through a gas-filled hollow core fiber (HCF) followed by dispersion-compensating bulk CaF₂. The krypton-filled fiber at 370 kPa yielded 1.14-mJ, 42-fs pulses centered at 4.07 µm, while the oxygen-filled fiber at 310 kPa delivered 0.78-mJ, 39-fs pulses spanning from 3 to 5.5 µm. This work is a step toward a high repetition rate mid-wave infrared driver of isolated attosecond keV x-ray pulses. 

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2. Spatial Beam Self-Cleaning in Second-Harmonic Generation

   https://doi.org/10.1038/s41598-020-64080-7  

   Krupa, K.; Fona, R.; Tonello, A.; Labruyère, A.; Shalaby, B. M.; Wabnitz, S.; Baronio, F.; Aceves, A. B.; Millot, G.; Couderc, V. (December 2020, Scientific Reports)

   null (Ed.)

   Abstract We experimentally demonstrate the spatial self-cleaning of a highly multimode optical beam, in the process of second-harmonic generation in a quadratic nonlinear potassium titanyl phosphate crystal. As the beam energy grows larger, the output beam from the crystal evolves from a highly speckled intensity pattern into a single, bell-shaped spot, sitting on a low energy background. We demonstrate that quadratic beam cleanup is accompanied by significant self-focusing of the fundamental beam, for both positive and negative signs of the linear phase mismatch close to the phase-matching condition. 

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3. CAVITY-BASED FREE-ELECTRON LASER RESEARCH AND DEVELOPMENT: A JOINT ARGONNE NATIONAL LABORATORY AND SLAC NATIONAL LABORATORY COLLABORATION

   https://doi.org/10.18429/JACoW-FEL2019-TUD04  

   G. Marcus, F.-J. Decker (November 2019, 39th Free Electron Laser Conf./ FEL2019, Hamburg, Germany)

   One solution for producing longitudinally coherent FEL pulses is to store and recirculate the output of an amplifier in an X-ray cavity so that the X-ray pulse can interact with following fresh electron bunches over many passes. The X-ray FEL oscillator (XFELO) and the X-ray regenerative amplifier FEL (XRAFEL) concepts use this technique and rely on the same fundamental ingredients to realize their full capability. Both schemes require a high repetition rate electron beam, an undulator to provide FEL gain, and an X- ray cavity to recirculate and monochromatize the radiation. The shared infrastructure, complementary performance char- acteristics, and potentially transformative FEL properties of the XFELO and XRAFEL have brought together a joint Argonne National Laboratory (ANL) and SLAC National Laboratory (SLAC) collaboration aimed at enabling these schemes at LCLS-II. We present plans to install a rectangu- lar X-ray cavity in the LCLS-II undulator hall and perform experiments employing 2-bunch copper RF linac accelerated electron beams. This includes performing cavity ring-down measurements and 2-pass gain measurements for both the low-gain XFELO and the high-gain XRAFEL schemes. 

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4. Entropic thermodynamics of nonlinear photonic chain networks

   https://doi.org/10.1038/s42005-020-00484-1  

   Wu, Fan O.; Jung, Pawel S.; Parto, Midya; Khajavikhan, Mercedeh; Christodoulides, Demetrios N. (December 2020, Communications Physics)

   null (Ed.)

   Abstract The convoluted nonlinear behaviors of heavily multimode photonic structures have been recently the focus of considerable attention. The sheer complexity associated with such multimode systems, allows them to display a host of phenomena that are otherwise impossible in few-mode settings. At the same time, however, it introduces a set of fundamental challenges in terms of comprehending and harnessing their response. Here, we develop an optical thermodynamic approach capable of describing the thermalization dynamics in large scale nonlinear photonic tight-binding networks. For this specific system, an optical Sackur-Tetrode equation is obtained that explicitly provides the optical temperature and chemical potential of the photon gas. Processes like isentropic expansion/compression, Joule expansion, as well as aspects associated with beam cleaning/cooling and thermal conduction effects in such chain networks are discussed. Our results can be used to describe in an effortless manner the exceedingly complex dynamics of highly multimoded nonlinear bosonic systems. 

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5. Spatially homogeneous few-cycle compression of Yb lasers via all-solid-state free-space soliton management

   https://doi.org/10.1364/OE.443942  

   Zhu, Bingbing; Fu, Zongyuan; Chen, Yudong; Peng, Sainan; Jin, Cheng; Fan, Guangyu; Zhang, Sheng; Wang, Shunjia; Ru, Hao; Tian, Chuanshan; et al (January 2022, Optics Express)

   The high power and variable repetition-rate of Yb femtosecond lasers makes them very attractive for ultrafast science. However, for capturing sub-200 fs dynamics, efficient, high-fidelity and high-stability pulse compression techniques are essential. Spectral broadening using an all-solid-state free-space geometry is particularly attractive, as it is simple, robust and low-cost. However, spatial and temporal losses caused by spatio-spectral inhomogeneities have been a major challenge to date, due to coupled space-time dynamics associated with unguided nonlinear propagation. In this work, we use all-solid-state free-space compressors to demonstrate compression of 170 fs pulses at a wavelength of 1030nm from a Yb:KGW laser to ∼9.2 fs, with a highly spatially homogeneous mode. This is achieved by ensuring that the nonlinear beam propagation in periodic layered Kerr media occurs in spatial soliton modes, and by confining the nonlinear phase through each material layer to less than 1.0 rad. A remarkable spatio-spectral homogeneity of ∼0.87 can be realized, which yields a high efficiency of >50% for few-cycle compression. The universality of the method is demonstrated by implementing high-quality pulse compression under a wide range of laser conditions. The high spatiotemporal quality and the exceptional stability of the compressed pulses are further verified by high-harmonic generation. Our predictive method offers a compact and cost-effective solution for high-quality few-cycle-pulse generation from Yb femtosecond lasers, and will enable broad applications in ultrafast science and extreme nonlinear optics. 

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