Macroscopic effects in noncollinear high-order harmonic generation

Enhancing the efficiency of high-order harmonics with two-color non-collinear wave mixing in silica

The emission of high-order harmonics from solids under intense laser-pulse irradiation is revolutionizing our understanding of strong-field solid-light interactions, while simultaneously opening avenues towards novel, all-solid, coherent, short-wavelength table-top sources with tailored emission profiles and nanoscale light-field control. To date, broadband spectra in solids have been generated well into the extreme-ultraviolet (XUV), but the comparatively low conversion efficiency in the XUV range achieved under optimal conditions still lags behind gas-based high-harmonic generation (HHG) sources. Here, we demonstrate that two-color high-order harmonic wave mixing in a fused silica solid is more efficient than solid HHG driven by a single color. This finding has significant implications for compact XUV sources where gas-based HHG is not feasible, as solid XUV wave mixing surpasses solid-HHG in performance. Moreover, our results enable utilizing solid high-order harmonic wave mixing as a probe of structure or material dynamics of the generating solid, which will enable reducing measurement times compared to the less efficient regular solid HHG. The emission intensity scaling that follows perturbative optical wave mixing, combined with the angular separation of the emitted frequencies, makes our approach a decisive step for all-solid coherent XUV sources and for studying light-engineered materials.

Photon Pathways and the Nonperturbative Scaling Law of High Harmonic Generation

High harmonic generation (HHG) has become a core pillar of attosecond science. Traditionally described with field-based models, HHG can also be viewed in a parametric picture, which predicts all properties of the emitted photons, but not the nonperturbative efficiency of the process. Driving HHG with two noncollinear beams and deriving analytically the corresponding yield scaling laws for any intensity ratio, we herein reconcile the two interpretations, introducing a generalized photonic description of HHG. It is in full agreement with field-based simulations and experimental data, opening the route to smart engineering of HHG with multiple driving beams.

Strong chiral response in non-collinear high harmonic generation driven by purely electric-dipole interactions

High harmonic generation (HHG) records the ultrafast electronic response of matter to light, encoding key properties of the interrogated quantum system, such as chirality. The first implementation of chiral HHG [Cireasa et al, Nat. Phys.11, 654 (2015)10.1038/nphys3369] relied on the weak electronic response of a medium of randomly oriented chiral molecules to the magnetic component of an elliptically polarized wave, yielding relatively weak chiro-optical signals. Here we apply state-of-the-art semi-analytical modelling to show that elliptically polarized light can drive a strong chiral response in chiral molecules via purely electric-dipole interactions - the magnetic component of the wave does not participate at all. This strong chiro-optical response, which remains hidden in standard HHG experiments, can be mapped into the macroscopic far-field signal using a non-collinear configuration, creating new opportunities for imaging chiral matter and chiral dynamics on ultrafast time scales.

Noncollinear Enhancement Cavity for Record-High Out-Coupling Efficiency of an Extreme-UV Frequency Comb

We demonstrate a femtosecond enhancement cavity with a crossed-beam geometry for efficient generation and extraction of extreme-ultraviolet (XUV) frequency combs at a 154 MHz repetition rate. We achieve a record-high out-coupled power of 600  μW, directly usable for spectroscopy, at a wavelength of 97 nm. This corresponds to a >60% out-coupling efficiency. The XUV power scaling and generation efficiency are similar to that achieved with a single Gaussian-mode fundamental beam inside a collinear enhancement cavity. The noncollinear geometry also opens the door for the generation of isolated attosecond pulses at >100  MHz repetition rate.

Helicity-Selective Enhancement and Polarization Control of Attosecond High Harmonic Waveforms Driven by Bichromatic Circularly Polarized Laser Fields

High harmonics driven by two-color counterrotating circularly polarized laser fields are a unique source of bright, circularly polarized, extreme ultraviolet, and soft x-ray beams, where the individual harmonics themselves are completely circularly polarized. Here, we demonstrate the ability to preferentially select either the right or left circularly polarized harmonics simply by adjusting the relative intensity ratio of the bichromatic circularly polarized driving laser field. In the frequency domain, this significantly enhances the harmonic orders that rotate in the same direction as the higher-intensity driving laser. In the time domain, this helicity-dependent enhancement corresponds to control over the polarization of the resulting attosecond waveforms. This helicity control enables the generation of circularly polarized high harmonics with a user-defined polarization of the underlying attosecond bursts. In the future, this technique should allow for the production of bright highly elliptical harmonic supercontinua as well as the generation of isolated elliptically polarized attosecond pulses.

Phase matching of noncollinear sum and difference frequency high harmonic generation above and below the critical ionization level

We investigate the macroscopic physics of noncollinear high harmonic generation (HHG) at high pressures. We make the first experimental demonstration of phase matching of noncollinear high-order-difference-frequency generation at ionization fractions above the critical ionization level, which normally sets an upper limit on the achievable cutoff photon energies. Additionally, we show that noncollinear high-order-sum-frequency generation requires much higher pressures for phase matching than single-beam HHG does, which mitigates the short interaction region in this geometry. We also dramatically increase the experimentally realized cutoff energy of noncollinear circularly polarized HHG, reaching photon energies of 90 eV. Finally, we achieve complete angular separation of high harmonic orders without the use of a spectrometer.

Tunable orbital angular momentum in high-harmonic generation

Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly nonlinear light-matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photon's angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules and violation of dipolar selection rules in atoms.

Perturbative High Harmonic Wave Front Control

We pattern the wave front of a high harmonic beam by intersecting the intense driving laser pulse that generates the high harmonic with a weak control pulse. To illustrate the potential of wave-front control, we imprint a Fresnel zone plate pattern on a harmonic beam, causing the harmonics to focus and defocus. The quality of the focus that we achieve is measured using the spectral wave-front optical reconstruction by diffraction method. We will show that it is possible to enhance the peak intensity by orders of magnitude without a physical optical element in the path of the extreme ultraviolet (XUV) beam. Through perturbative wave-front control, XUV beams can be created with a flexibility approaching what technology allows for visible and infrared light.

In situ frequency gating and beam splitting of vacuum- and extreme-ultraviolet pulses

Monochromatization of high-harmonic sources has opened fascinating perspectives regarding time-resolved photoemission from all phases of matter. Such studies have invariably involved the use of spectral filters or spectrally dispersive optical components that are inherently lossy and technically complex. Here we present a new technique for the spectral selection of near-threshold harmonics and their spatial separation from the driving beams without any optical elements. We discover the existence of a narrow phase-matching gate resulting from the combination of the non-collinear generation geometry in an extended medium, atomic resonances and absorption. Our technique offers a filter contrast of up to 10⁴ for the selected harmonics against the adjacent ones and offers multiple temporally synchronized beamlets in a single unified scheme. We demonstrate the selective generation of 133, 80 or 56 nm femtosecond pulses from a 400-nm driver, which is specific to the target gas. These results open new pathways towards phase-sensitive multi-pulse spectroscopy in the vacuum- and extreme-ultraviolet, and frequency-selective output coupling from enhancement cavities.

Non-collinear high-order harmonic generation by three interfering laser beams

High order harmonic generation (HHG) has shown its impact on several applications in Attosecond Science and Atomic and Molecular Physics. Owing to the complexity of the experimental setup for the generation and characterization of harmonics, as well as to the large computational costs of numerical modelling, HHG is generally performed and modelled in collinear geometry. Recently, several experiments have been performed exploiting non-collinear geometry, such as HHG in a grating of excited molecules created by crossing beams. In such studies, harmonics were observed at propagation directions different from those of the driving pulses; moreover the scattered harmonics were angularly dispersed.In this work we report on a new regime of HHG driven by multiple beams, where the harmonics are generated by three synchronized, intense laser pulses organized in a non-planar geometry. Although the configuration we explore is well within the strong-field regime, the scattered harmonics we observe are not angularly dispersed.