High-Brightness and Attosecond Electron Beams

The generation of relativistic electron bunches with durations in the attosecond range can lead to pump/probe beams which can be fruitfully employed to unveil ultrafast dynamics. The two-color ionization injection and the resonant multi-pulse ionization injection (ReMPI) for LWFA, or their equivalent form for the PWFA, a.k.a. the trojan-horse scheme, resulting in being extremely flexible yet capable of generating high-brightness electron beams. All these schemes use a driver to excite a large-amplitude plasma wave and a short-wavelength ionization pulse to extract electrons from a dopant. The driver can be a single long-wavelength laser pulse (two-color), a train of resonantly delayed pulses (ReMPI) or a charged beam (trojan horse). In any of these schemes, the electrons extracted by the very low normalized amplitude ionization slip back out the pulse with a very small residual transverse momentum. As the electrons are accelerated and focused by the wakefield, they are eventually trapped in the bucket and further accelerated. Moreover, during the slippage in the back of the bucket, the electron beam is compressed in both the longitudinal and radial directions and can reach longitudinal sizes of tens of nm, thus generating electron bunches that can reach attosecond-scale duration.

ReMPI with two driver pulses. q-3D simulation with FB-PIC

Highly Efficient Laser Wakefield Acceleration

Recently, an advanced laser technology called Thin Film Compression has been developed, which is capable of shortening and intensifying tens of femtosecond long, Joule scale energy pulses. These laser pulses now operate at a cutting-edge level of 100 terawatts and can fire 10 times per second, offering significant potential for enhancing the particle acceleration techniques. In this study, we utilized a supercomputer and quasi 3D Particle In Cell codes for high-fidelity numerical modeling to explore how these laser pulses interact with plasma. Our findings reveal that more than 50% of the energy delivered by such a laser pulse can be transferred to electrons, accelerating them to energies exceeding 100 megaelectronvolts (MeV). The resulting electron beam carries a substantial charge, measured in several nanocoulombs.

Nanowire Implosion Under Laser Prepulse Irradiation

Nanowire array targets exhibit high optical absorption when interacting with short, intense laser pulses, leading to enhanced particle acceleration. However, these interactions are highly sensitive to the laser prepulse and can be significantly affected.

Radiation hydrodynamics simulations have shown that an array of aligned aluminum nanowires undergoes implosion when irradiated by the Amplified Spontaneous Emission (ASE) pedestal of a 1 PW laser with an intensity on the order of 10¹¹ W/cm². In particular, the electron density profile is radially compressed at the nanowire tips by the rocket-like propulsion of the ablated plasma.

The mass density compression increases up to 2.9 times the initial density when a denser nanowire array is used, due to the additional ablation pressure from neighboring nanowires. This pronounced compression occurs for nanowires located at the center of the laser focal spot when irradiated with a 500 ps ASE pedestal.

These findings provide valuable insights for selecting appropriate target designs in experiments aimed at optimizing accelerated particle production using high-power lasers on nanostructured targets. Understanding and mitigating the effects of the ASE prepulse is crucial for maximizing the performance of these advanced particle acceleration schemes.

Laser-induced implosion of nanowire arrays leads to density compression, with the effect modulated by nanowire spacing and laser parameters-critical factors for optimizing particle acceleration performance.

Double Layer Targets for Proton Acceleration

In laser-driven proton acceleration, double-layer targets provide a sophisticated means of enhancing proton generation by combining a near-critical-density (NCD) layer with a solid layer. The NCD layer, positioned on the laser-irradiated side, acts as a lens that further focuses and intensifies the incoming laser pulse up to several times of its already extreme value.

In addition to its role in focusing the laser, the NCD layer offers another key advantage: it can shield the solid target from unintended laser prepulses. These prepulses, often present in high-power laser systems, can prematurely heat or disrupt the solid layer, reducing the efficiency of the proton acceleration. By absorbing or deflecting these prepulses, the NCD layer protects the solid layer, ensuring that the main laser pulse arrives in optimal conditions for proton generation. This combination of laser focusing and prepulse shielding makes the double-layer target an important advancement in the pursuit of efficient, high-energy proton beams, with promising applications in fields like medical therapies and fusion research.

Nonlinear Phenomena at the Focal Spot

As high energy photons propagate inside a laser beam they can absorb photons from the background field and produce electron-positron pairs. The produced pairs can also absorb laser photons and create high energy photons. These pair creation (nonlinear Breit-Wheeler - NBW) and photon emission (nonlinear Compton scattering - NCS) events can trigger one other inside a long enough pulse, ultimately leading to a cascade formation. In the NBW process, an energetic photon (ħωɣ ≫ mc2) plays the role of a seed photon, which by absorbing N laser photons ensures the energy-momentum balance of the reaction: ħωɣ + Nħω → e⁺ + e^-. Remarkably enough, QED predicts that pair creation can occur without a seed photon, solely through a focussed, ultra-high intensity laser. Although quanta in the laser field typically have low energy (ħω ≪ mc²), the reaction can in principle become energetically possible because collective energy of the merging photons scales with their number. The probability of creating pairs is suppressed for sub-critical field strengths, reminiscent of a quantum tunneling amplitude.

Left: QED cascade process initiated by a seed photon, decaying into electron-positron pairs, which further radiate high energy photons.
Right: Ultra intense, tightly focused laser beam can create pairs from vacuum via Schwinger mechanism.

Low intensity behavior of NCS AND NBW was probed in the famous SLAC E-144 experiment. Anticipated experiments at ELI-NP will open up the possibility to probe these phenomena at much higher intensities, for which the theory predicts drastically different scaling behavior.

Vacuum Birefringence

Vacuum birefringence is a prediction of QED that implies a different propagation speed of probe photons in an external field, depending on their relative polarization. It can be deduced from the Heisenberg-Euler (HE) theory, which is the effective field of QED when the Fermionic degrees of freedom are integrated out. While indirect hints of this phenomenon exist, to this day, no laboratory experiment was able to measure this prediction. Moreover, deviations from the HE prediction would indicate possible new particles/fields. At ELI-NP, two experimental proposals are considered as future experiments: one is considering GeV gamma photons as a probe, while the other one is all-optical.

Nuclear Physics under High Intensity Frontier

Despite numerous achievements and recent progress, nuclear physics is often considered an old field of research nowadays. On the other hand, developments in few - and many-body methods, field-theoretical frameworks, and reliable experimental techniques have made the field mature enough to explore many new frontiers. In this regard, extending existing knowledge to an emerging field of physics - where particles interact with a relatively low-energy (< 100 MeV) but high intensity field (intense enough so that multi-particle processes become comparable or more important than one-to-one processes) - can lead to exciting discoveries. Investigations can be realized under a spatially-dense environment (e.g., astrophysical events such as the core of a neutron star), or a highly time-compressed beam source (e.g., particle sources generated by laser-matter interaction using high-power laser systems). With unlimited potentials such as nuclear photonics, astrophysics, medical applications, and more, our research is focused on expediting further developments toward this paradigm.

Construction of nuclear forces through effective field theories

To describe physical phenomena, one needs an interaction to start with. Effective field theory (EFT) in principle provides such a venue and has been adopted in essentially all ab-initio calculations (from nuclear structure to reaction) today. However, the current implementation of EFT following Weinberg power counting generates non-renormalizable results. This indicates a serious drawback, as the internal consistency of the theory is lost. Our focus is on developing a more consistent theory, and therefore provide a truly model-independent framework for many-body calculations.

On the nuclear structure side, we perform ab-initio calculations through no-core shell model and coupled cluster methods. For heavier nuclei, we adopt the energy-density-functional (EDF) method, with its effective interactions constructed via EFT principles beyond the mean field. For excited states and/or exotic nuclei, an improved shell-model is adopted to provide key observables.

A probe beam (green wavy line) interacting with a virtual e+e- loop in a background field

Nuclear Photonics with High Power Lasers

The ability to manipulate matter on a near-atomic scale to produce new structures, materials and devices has enabled modern nanotechnology. Similarly, our aim is to provide the first efficient manipulation of matter on the nuclear scale, i.e., nuclear photonics. Intense beams (electron, gamma-photon, proton, neutron, ions) generated from high-power lasers via laser-matter interactions play a key role as game-changers. One example is reflected in the population and controlled depletion of nuclear isomers. Our theoretical studies lead to practical applications, including (but not limited to) medical imaging and therapy, energy storage and production, radioactive material treatment, precision measurement and probe of fundamental physics within and beyond the standard model.

A new scheme for isomer pumping and depletion (see arXiv:2404.07909 [nucl-th]).

Quantum Metrology

Quantum metrology is one of the four pillars of the emerging quantum technologies. Within this broad subject, we are interested in a number topics:

• Theoretical phase sensitivity investigations via the quantum Fisher information
• Interferometric phase sensitivity with realistic detection schemes
• Interferometric phase sensitivity enhancements via quantum metrology

The four stages of quantum parameter estimation. The parameter to be estimated (𝜑) interacts with our system. A POVM (positive operator value measurement) yields the measurements results. Finally, an estimator is constructed based on the available data.

Phase measurement in quantum mechanics is problematic, since a phase operator cannot be properly defined. A convenient workaround is found via the theory of quantum parameter estimation. A central object in the optimality estimation is given by the quantum Fisher information (QFI), since this object determines the optimum estimator variance via the quantum Cramér-Rao bound (QCRB).

The classically available phase sensitivity is limited by the so-called shot-noise limit (blue line). However, quantum metrology allows for a much more convenient sub-shotnoise regime (light-red area), limited by the Heisenberg limit (red line).

By employing non-classical states of light such as squeezed vacuum or NOON states, the classical shot-noise regime (bounded by the so-call standard quantum limit (SQL) or shot-noise limit (SNL, 𝚫²𝜑∼1/N) can be overcome and the more favorable Heisenberg limit (𝚫²𝜑∼1/N²). Thus, bounded by the same resource in terms of average number of input photons, N, one can benefit from an enhanced phase sensitivity.
While the ideal MZI can be optimized in closed form, more complicated problems arise in the case of a lossy MZI. This topic is actively addressed in our group and work to extend the technique from reference to the lossy case is under way.

Optimizing an unbalanced Mach-Zehnder interferometer in terms of phase sensitivity is a two-step process. One first optimizes the first BS via the QFI. With this value fixed, one finally proceeds to the optimization of the second BS, taking also in consideration the employed detection scheme.