We demonstrate the realization of a coherent random fiber laser (RFL) in the extremely weakly scattering regime, which contains a dispersive solution of polyhedral oligomeric silsesquioxanes nanoparticles (NPs) and laser dye pyrromethene 597 in carbon disulfide that was injected into a hollow optical fiber. Multiple scattering of polyhedral oligomeric silsesquioxanes NPs greatly enhanced by the waveguide confinement effect was experimentally verified to account for coherent lasing observed in our RFL system. This Letter extends the NPs-based RFLs from the incoherent regime to the coherent regime.

We demonstrate laser-plasma acceleration of high charge electron beams to the ~10 MeV scale using ultrashort laser pulses with as little energy as 10 mJ. This result is made possible by an extremely dense and thin hydrogen gas jet. Total charge up to ~0.5 nC is measured for energies >1 MeV. Acceleration is correlated to the presence of a relativistically self-focused laser filament accompanied by an intense coherent broadband light flash, associated with wavebreaking, which can radiate more than ~3% of the laser energy in a sub-femtosecond bandwidth consistent with half-cycle optical emission. Our results enable truly portable applications of laser-driven acceleration, such as low dose radiography, ultrafast probing of matter, and isotope production.Laser-driven electron acceleration in plasmas has achieved many successes in recent years, including record acceleration up to 4 GeV in a low emittance quasi-monoenergetic bunch [1] and generation of high energy photons [2][3][4][5]. In these experiments, the driver laser pulse typically propagates in the 'bubble' or 'blow-out' regime [6,7] for a normalized peak vector potential 2 0 0 / 1 a eA mc = > > . Plasma densities are deliberately kept low for resonant laser excitation and to avoid dephasing [7]. Essentially all of these experiments use 10 TW −1 PW laser drivers, with repetition rates ranging from 10 Hz to an hour between shots [8].For many modest lab scale and portable applications, however, a compact, relatively inexpensive, high average current source of laser-accelerated relativistic electrons is sufficient and desirable. In this paper we describe experiments using a very dense, thin hydrogen gas jet, where the relativistic self-focusing threshold is exceeded even with ~10 mJ laser pulses and MeV-scale energy electron bunches are generated. This enables applications, such as ultrafast low dose medical radiography, which would benefit from a truly portable source of relativistic particle beams. We note that prior work has shown electron bunch generation of modest charge and acceleration (~10 fC/pulse, <150 keV) from a 1 kHz, ~10 mJ laser driving a thin (~100 μm), low density continuous flow argon or helium jet [9].Central to our experiment is a thin, high density pulsed hydrogen sonic gas jet, which reaches a maximum peak molecular density of 9×10 at our laser wavelength of λ 0 =800nm. The density profile is near-Gaussian, with a full width at half maximum (FWHM) in the range 150-250 μm, depending on the height of the optical axis above the jet orifice. Earlier versions of this jet were run in both pulsed [10] and continuous flow [11] for nitrogen and argon. High densities are achieved using a combination of high valve backing pressure and cryogenic cooling of the valve feed gas, which is forced through a 100μm diameter needle orifice. Cooling to −160C enables a significant density increase for a given valve backing pressure. Figure 1 shows the experimental

We demonstrate laser-driven acceleration of electrons to MeV-scale energies at 1 kHz repetition rate using <10  mJ pulses focused on near-critical density He and H₂ gas jets. Using the H₂ gas jet, electron acceleration to ∼0.5  MeV in ∼10  fC bunches was observed with laser pulse energy as low as 1.3 mJ. Increasing the pulse energy to 10 mJ, we measure ∼1  pC charge bunches with >1  MeV energy for both He and H₂ gas jets.

We report on, to the best of our knowledge, the first results of laser plasma wakefield acceleration driven by ultrashort mid-infrared (IR) laser pulses (λ=3.9  μm, 100 fs, 0.25 TW), which enable near- and above-critical density interactions with moderate-density gas jets. Relativistic electron acceleration up to ∼12  MeV occurs when the jet width exceeds the threshold scale length for relativistic self-focusing. We present scaling trends in the accelerated beam profiles, charge, and spectra, which are supported by particle-in-cell simulations and time-resolved images of the interaction. For similarly scaled conditions, we observe significant increases in the accelerated charge, compared to previous experiments with near-infrared (λ=800  nm) pulses.

We demonstrate a new highly tunable technique for generating meter-scale low density plasma waveguides. Such guides can enable electron acceleration to tens of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by optical field ionization induced by two timeseparated Bessel beam pulses: The first pulse, a 𝐽 beam, generates the core of the waveguide, while the delayed second pulse, here a 𝐽 or 𝐽 beam, generates the waveguide cladding. We demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths with on axis plasma densities as low as 𝑁 ~5 × 10 cm .

We have demonstrated the realization of a random polymer fiber laser (RPFL) based on laser dye Pyrromethene 597-doped one-dimensional disordered polymer optical fiber (POF). The stabilized coherent laser action for the disordered POF has been obtained by the weak optical multiple scattering of the polyhedral oligomeric silsesquioxanes nanoparticles in the core of the POF in situ formed during polymerization, which was enhanced by the waveguide confinement effect. Meanwhile, the threshold of our RPFL system is almost one order of magnitude lower than that of the liquid core random fiber laser reported previously, which promotes the development of random lasers.

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