Zi-Yu Chen

High-power lasers offer ultrahigh intensities for plasma interactions, but they lack advanced techniques to control the properties of the fields, because no optical elements could withstand their high intensities. The vibrant field of metasurfaces has transformed modern optics by enabling unprecedented control over light at subwavelength through deliberate design. However, metasurfaces have traditionally been limited to solid-state materials and low light intensities. Extending the sophisticated capabilities of metasurfaces from solids into the plasma realm would open new horizons for high-field science. Here, we present a proof-of-concept experimental demonstration of plasma-state metasurfaces (PSMs) via the photonic spin Hall effect and the generation of stable-propagating vortex beams under intense laser irradiation. Time-resolved pump-probe measurements reveal that the functionality of PSMs can persist for several picoseconds, making them suitable for controlling ultra-intense femtosecond lasers, even in state-of-the-art multi-petawatt systems. Harnessing the powerful toolkit of metasurfaces, this approach holds the promise to revolutionize our ability to manipulate the amplitude, phase, polarization, and wavefront of high-power lasers during their pulse duration. It also opens new possibilities for innovative applications in laser-plasma interactions such as compact particle acceleration and novel radiation sources.

Bilayer graphene exhibits a rich and fascinating electronic structure highly sensitive to the interlayer stacking pattern. In particular, in-plane sliding can trigger Lifshitz transitions, dramatic changes in Fermi surface topology that crucially influence the material's quantum properties. However, experimental detection of these transitions remains difficult. Here, we investigate high harmonic generation (HHG) in sliding bilayer graphene undergoing Lifshitz transitions using first-principles simulations. We show that the resulting HHG spectra exhibit marked and phase-specific fingerprints, including the emergence of perpendicular harmonic components, pronounced anisotropic responses, and the angular-dependent harmonic yield. Our numerical study shows that the HHG spectra drastically change across the phase transition, which might connect to the Lifshitz transition, but is more strongly associated with the symmetry group of the solid being changed in tandem, providing insights into the symmetry and anisotropy of the different phases.

Relativistically intense Bessel-Gaussian beams hold significant promise for high-intensity laser-plasma interactions, yet an effective method for their generation remains unexplored. Here we numerically demonstrate the direct generation of such beams by reflecting Gaussian beams off conical plasma mirrors in the relativistic and extreme nonlinear regime. Three-dimensional particle-in-cell simulations reveal that high-quality Bessel-Gaussian beams can be produced with above fivefold intensity enhancement and over 85% energy conversion efficiency. Furthermore, this scheme enables the generation of high-harmonic Bessel-Gaussian beams with short wavelengths and attosecond pulse durations. The proposed method is experimentally straightforward, offering a practical pathway for applications requiring intense Bessel-Gaussian beams.

Relativistic electron beams produced by intense lasers over short distances have important applications in high energy density physics and medical technologies. Vacuum laser acceleration with plasma mirrors asinjectors has garnered substantial research interest recently. However, a persistent challenge remains unresolved that electrons inevitably detach from the laser acceleration phase due to velocity mismatch. Here, we employ flying focus lasers to address this limitation. Through three-dimensional particle-in-cell simulations, we demonstrate that flying focus lasers can achieve a substantial enhancement in relativistic electron charge yield compared to conventional Gaussian lasers. This improvement stems from two key attributes: (1) the subluminal propagation velocity of the peak intensity keeps a larger electron population synchronized within the longitudinal ponderomotive acceleration region, and (2) flying focus lasers sustain higher magnitudes of the longitudinal ponderomotive force over longer distances in comparison to Gaussian lasers. This approach offers high-charge relativistic electron sources ideal for demanding applications such as high-flux Thomson scattering and radiography.

Laser-driven proton acceleration holds great promise for compact particle sources. One of the critical challenges is controlling the divergence and improving the collimation quality of the generated proton beams. Here, we propose high-order hollow laser beams to enhance proton collimation in magnetic vortex acceleration from near-critical-density plasmas. Three-dimensional particle-in-cell simulations demonstrate that highly collimated and energetic proton beams with a peak divergence angle of about 1.6° and energies reaching 120 MeV can be generated at a realistic laser intensity of 2×1021 W/cm2. It is identified that the distinct multi-ring intensity profile of the high-order hollow pulses promotes the formation of uniform plasma channels, essential for establishing directional accelerating and focusing fields. This scheme offers a path toward high-repetition-rate and highly collimated energetic proton beam generation, desirable for diverse applications, including high-resolution radiography and advanced medical treatments.

Relativistic electron beams produced by intense lasers over short distances have important applications in high energy density physics and medical technologies. Vacuum laser acceleration with plasma mirrors injectors has garnered substantial research interest recently. However, a persistent challenge remains unresolved that electrons inevitably detach from the laser acceleration phase due to velocity mismatch. Here, we employ flying focus lasers to address this limitation. Through three-dimensional particle-in-cell simulations, we demonstrate that flying focus lasers can achieve a substantial enhancement in relativistic electron charge yield compared to conventional Gaussian lasers. This improvement stems from two key attributes: (1) The subluminal propagation velocity of the peak intensity keeps a larger electron population synchronized within the longitudinal ponderomotive acceleration region, and (2) Flying focus lasers sustain higher magnitudes of the longitudinal ponderomotive force over longer distances in comparison to Gaussian lasers. This approach offers high-charge relativistic electron sources ideal for demanding applications such as high-flux Thomson scattering and radiography.

High-power lasers offer ultrahigh intensities for plasma interactions, but they lack advanced techniques to control the properties of the fields, because no optical elements could withstand their high intensities. The vibrant field of metasurfaces has transformed modern optics by enabling unprecedented control over light at subwavelength through deliberate design. However, metasurfaces have traditionally been limited to solid-state materials and low light intensities. Extending the sophisticated capabilities of metasurfaces from solids into the plasma realm would open new horizons for high-field science. Here, we experimentally demonstrate plasma-state metasurfaces (PSMs) through the photonic spin Hall effect and stable-propagating vortex beam generation irradiated by intense light. Time-resolved pump-probe measurements reveal that the functionality of PSMs can persist for several picoseconds, making them suitable for controlling ultra-intense femtosecond lasers, even in state-of-the-art multi-petawatt systems. Harnessing the powerful toolkit of metasurfaces, this approach holds the promise to revolutionize our ability to manipulate the amplitude, phase, polarization, and wavefront of high-power lasers during their pulse duration. It also opens new possibilities for innovative applications in laser-plasma interactions such as compact particle acceleration and novel radiation sources.

Coherent focusing of high-order harmonic generation (HHG) from optically curved relativistic plasma mirrors provides a promising route to achieve unprecedented high intensities, opening up prospects to study phenomena occurring in extremely strong fields, the region of which is however limited within extremely small spatial and temporal scales. Here through three-dimensional particle-in-cell simulations we demonstrate HHG driven by Bessel-Gaussian beams exhibiting the intriguing feature of self-healing up to the relativistic and extreme nonlinear regime. This allows a secondary intensification of reflected fields after the focus of curved plasma mirrors and thus extends the high-intensity region in both space and time, beneficial for extreme field applications.

Identifying all-optical high-order harmonic observables as reliable signatures to probe the topology of matter has triggered intense interest. Recently, extensive ab initio simulations challenged the previously explored approaches and proposed a new potential feature, i.e., angular dependence of harmonic yield, as a possible fingerprint of material topology [O. Neufeld et al., Phys. Rev. X 13, 031011 (2023)]. However, nontopological contributions from the band structure, spin-orbit coupling (SOC), and Berry curvature have not been ruled out. Here, we clarify this point by studying the elaborately chosen sample system of monolayer black phosphorus, which exhibits similar band structures and are free from SOC as well as Berry curvature effects for both the trivial and topological phases induced by strain. We find the previously proposed angle-dependent harmonic difference to vanish, suggesting it to be an unreliable observable to probe material topology.