Coherent soft x-ray (SXR) sources enable fundamental studies in the important water window spectral region. Until now, such sources have been limited to repetition rates of 1 kHz or less, which limits count rates and signal-to-noise ratio for a variety of experiments. SXR generation at high repetition rate has remained challenging because of the missing high-power mid-infrared (mid-IR) laser sources to drive the high-harmonic generation (HHG) process. Here we present a mid-IR optical parametric chirped pulse amplifier (OPCPA) centered at a wavelength of 2.2 µm and generating 16.5-fs pulses (2.2 oscillation cycles of the carrier wave) with 25 W of average power and a peak power exceeding 14 GW at 100-kHz pulse repetition rate. This corresponds to the highest reported peak power for high-repetition-rate mid-IR laser systems. The output of this 2.2-µm OPCPA system was used to generate a SXR continuum extending beyond 0.6 keV through HHG in a high-pressure gas cell. MainProgress in laser technology has enabled rapid development in attosecond science which led to many scientific discoveries [1,2]. Further advances in attosecond science are closely linked to high-harmonic generation (HHG) sources [3,4], and therefore to state-of-the-art laser systems to drive the HHG process into new performance frontiers. Specifically, there is currently great interest in scaling HHG sources to parameters beyond those available in conventional Ti:sapphire amplifier driven beamlines, in particular to higher photon energies and higher repetition rates. Photon energies extending up to 1.6 keV were generated at 20 Hz repetition rate [5]. Recently, multiple research groups have developed 1-kHz laser sources capable of producing coherent soft x-ray (SXR) radiation spanning up to the oxygen K-edge at 543 eV [6][7][8]. Such high-photon-energy sources are interesting for a variety of spectroscopic studies since core electrons can be accessed directly. For example, this enables direct probing of biological molecules in aqueous solutions [9], tracking of electronic, vibrational and rotational [10] as well as magnetization dynamics [8]. Furthermore, the high photon energies allow for the shortest probe pulses ever produced [11]. On the other hand, high repetition rates are especially important for applications limited by space-charge effects, such as the investigation of photoemission delays from surfaces [12,13]. The coherent SXR radiation in the above examples is generated via HHG. At a given intensity I and carrier wavelength λ, the maximum energy of the generated photons scales with ~I·λ 2 of the driving laser field [14]. Thus, to obtain a high-energy cut-off without excessive ionization of the target, which would prevent phasematching, mid-IR driving lasers are required. Longer driving wavelengths also give rise to higher phasematching pressures, which increases the number of potential emitters [15]. On the other hand, the singleatom yield drops rapidly with wavelength, with a scaling of around ~λ -5.5 for a fixed energy interval [16]. This ...

We study the coupling interactions between a progressively elongated silver nanoparticle and a silver film on a glass substrate. Specifically, we investigate how the coupling between localized surface plasmons (LSPs) and propagating surface plasmon polaritons (SPPs) is influenced by nanoparticle length. Although the multiple resonances supported by the nanoparticle are effectively standing wave surface plasmons, their interaction with the SPP continuum of the underlying Ag film indicates that their spectral response is still localized in nature. It is found that these LSP-SPP interactions are not limited to small particles, but that they are present as well for extremely long particles, with a transition to the SPP coupling interactions of a bilayer metallic film system beginning at a particle length of approximately 5 μm. Coupling of metallic nanostructures in plasmonic systems has been a topic of major research interest, as it leads to effects such as strong near-field confinement, useful for trapping, sensing, nonlinear interactions, and surface-enhanced Raman scattering [1][2][3][4][5][6]. Coupling between finite-size plasmonic nanostructures and a conductive film is of particular interest as it involves the interaction of discrete localized resonances with a continuum of delocalized surface plasmon polaritons (SPPs) [7][8][9][10][11]. Systems exhibiting coupling of just a few localized resonances with a delocalized continuum have already been researched [7,8]. However, the interaction between an ever-growing number of higher order localized plasmon resonances and a SPP resonance continuum has yet to be discussed. Specifically, it is well known that coupling between a localized surface plasmon (LSP) resonance and a SPP continuum leads to an observable anticrossing at the LSP resonance frequency [7,8,[12][13][14][15][16][17][18][19][20][21]. What is not clear, however, is if this still holds true for a large number of localized resonances, as will be investigated in this Letter. The system studied here consists of a 2D silver nanoparticle of 40 nm height and variable length L spaced 100 nm above a 50 nm thick Ag film on a SiO 2 substrate of refractive index n 1.46 [ Fig. 1(a)]. A 2D geometry was chosen since effects along the third dimension are irrelevant. We use the dielectric data for silver measured by Johnson and Christy [22]. The simulation method carried out in this study is based on Green's tensor formalism in 2D [23]. As shown in Fig. 1(a), a horizontally oriented electric dipole source placed 100 nm to the left of the particle is used for excitation. A study of the system response for a finite length L is made via the same Fourier analysis method as in a previous work [24]. The plots obtained display the magnitude of the parallel k-vector component k x at each given frequency. Note that use of this Fourier analysis method leads to an unavoidable spectral broadening in k-space, which is inversely proportional to the particle length L, but does not pose any limitation on the current study.The sampling...

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In optical parametric amplification (OPA) of broadband pulses, a non-collinear angle between the interacting waves is typically introduced in order to achieve broadband phase-matching. Consequently, bandwidth and beam geometry are closely linked. This coupling restricts the geometrical layout of an OPA system. Here, we demonstrate a quasi-phase-matching (QPM) geometry for broadband OPA in which a transverse component is introduced to the QPM grating to impose an additional momentum on the generated wave. This momentum shift detunes the wavelength where the signal and the idler are group-velocity matched, thereby allowing for broadband phase-matching without having to add a non-collinear angle between the interacting waves. We present two experimental configurations making use of this principle, and propose a third configuration with the potential to further simplify ultra-broadband OPA system architectures.

We experimentally demonstrate a novel use of a spatial light modulator (SLM) for shaping ultrashort pulses in time-gated amplification systems. We show that spectral aberrations because of the device's pixelated nature can be avoided by introducing a group delay offset to the pulse via the SLM, followed by a time-gated amplification. Because of phase wrapping, a large delay offset yields a nearly-periodic grating-like phase function (or a phase grating). We show that, in this regime, the phase grating periocidity defines the group delay spectrum applied to the pulse, while the grating's amplitude defines the fraction of light that is delayed. We therefore demonstrate that a one-dimensional (1D) SLM pixel array is sufficient to control both the spectral amplitude and the phase of the amplified pulses.