Advances in Design and Fabrication of Micro-Structured Solid Targets for High-Power Laser-Matter Interaction

Abstract

Accelerated particles have multiple applications in materials research, medicine, and the space industry. In contrast to classical particle accelerators, laser-driven acceleration at intensities greater than 10¹⁸ W/cm², currently achieved at TW and PW laser facilities, allow for much larger electric field gradients at the laser focus point, several orders of magnitude higher than those found in conventional kilometer-sized accelerators. It has been demonstrated that target design becomes an important factor to consider in ultra-intense laser experiments. The energetic and spatial distribution of the accelerated particles strongly depends on the target configuration. Therefore, target engineering is one of the key approaches to optimizing energy transfer from the laser to the accelerated particles. This paper provides an overview of recent progress in 2D and 3D micro-structured solid targets, with an emphasis on fabrication procedures based on laser material processing. Recently, 3D laser lithography, which involves Two-Photon Absorption (TPA) effects in photopolymers, has been proposed as a technique for the high-resolution fabrication of 3D micro-structured targets. Additionally, laser surface nano-patterning followed by the replication of the patterns through molding, has been proposed and could become a cost-effective and reliable solution for intense laser experiments at high repetition rates. Recent works on numerical simulations have also been presented. Using particle-in-cell (PIC) simulation software, the importance of structured micro-target design in the energy absorption process of intense laser pulses—producing localized extreme temperatures and pressures—was demonstrated. Besides PIC simulations, the Finite-Difference Time-Domain (FDTD) numerical method offers the possibility to generate the specific data necessary for defining solid target material properties and designing their optical geometries with high accuracy. The prospects for the design and technological fabrication of 3D targets for ultra-intense laser facilities are also highlighted.

1. Introduction

Laser-driven particle accelerators have been proposed as “table-top” installations suitable, not only for research, but also for medical applications, such as proton therapy for tumor treatments, the generation of radionuclides for radiopharmaceutics, and radiation hardening for space industry [1,2,3]. Due to their very high local electric field in a narrow volume compared to the synchrotron accelerators, ultra-intense pulsed lasers focused on various types of targets offer the possibility to accelerate particles in the range of hundreds of MeV up to a few GeV. The energetic and spatial distribution of the particles strongly depends on the target configuration. Target engineering is one of the key approaches to optimizing the energy transfer from lasers to accelerated particles. The micro-structured targets having different shapes and sizes leads to the emission of accelerated particles after interaction with an intense laser pulse through a mechanism known as Target Normal Sheath Acceleration (TNSA) or Radiation Pressure Acceleration (RPA), depending on the experimental conditions and target configuration [4,5]. In a practical application, diverse requirements are imposed on the secondary sources. For example, in some applications, mono-energetic particles at higher energies with a better collimated beam are required, or large energetic spectra and tunability could be desired for other applications. Moreover, the need for higher irradiation doses triggers the development of high-power lasers with repetition rates of 1 to 10 Hz. The use of solid targets at high repetition rates of laser pulses raises significant challenges related to target site refreshing rate, debris mitigation, target insertion accuracy at the laser focus, electro-magnetic pulse (EMP) generation, etc. Theoretical and experimental studies demonstrated that the cone targets can be properly projected and used in predefined interaction conditions. Then, the absorption of the ultra-intense electromagnetic field can increase. This may lead to the production of particles with a much higher energy and better directionality for the same laser intensity [6]. Also, via an appropriate design, the EMP’s field strength and frequency distribution can be controlled [7].

Many solid target configurations have been studied, such as nano-gratings created on target surfaces for increasing the energy coupling between laser beam and target [8] and cones-shaped targets acting as beam concentrators [9,10,11]. Foam targets or low-density targets have also been demonstrated to increase the energy of accelerated ions as compared to flat solid targets [12]. The divergence of accelerated particles can be improved for particles carrying orbital angular momentum (OAM). This was demonstrated in Laser Wake-Field Acceleration (LWFA) and TNSA by using circularly polarized laser pulses or by helical laser beams produced by spiral phase plates [13,14,15]. However, for future development of the target designs, it will be interesting to see if it is possible to find the right mechanism and target geometry to induce OAM for the accelerated particle beams at the level of the target. The target engineering approach requires proper efforts toward the development of microprocessing technologies for target fabrication, as well as in the direction of the development of computational resources in order to numerically simulate and optimize the various target designs. Based on PIC (particle-in-cell) simulation software, it demonstrated the importance of structured micro-target design in the energy absorption process of intense laser pulses, producing localized extreme temperatures and pressures. Also, the FDTD (Finite-Difference Time-Domain) numerical method offers the possibility to generate the specific data necessary to define solid target material properties and to design their optical geometries with high accuracy.

This paper provides a review of the recent progress of 2D and 3D micro-structured solid target design and future perspectives, highlighting their major contribution in the laser field intensity enhancement process. A series of micro-structured targets, such as thin films, foils, micro-spheres, foams materials, or 3D cone targets, made of different materials with different geometrical shapes, will be presented and discussed. The theoretical studies and the fabrication methods suitable for the generation of micro-structured targets required in ultra-intense laser experiments will be reviewed. Recently, the 3D laser lithography that involves the Two-Photon Absorption (TPA) effects in photopolymers has been proposed as a technique for the high-resolution fabrication of 3D micro-structured targets. Target fabrication metrology has been developed based on advanced X-ray microtomography. The theoretical and experimental analysis of the micrometric- and nanometric-sized features evidenced the optical field enhancement in the proximity of these structures [16], which can lead to near-critical plasmas at relativistic intensities for the efficient generation of laser-driven secondary sources. The prospects in the design and technology fabrication of 3D targets for ultra-intense laser facilities were also evidenced.

2. 2D Targets

2.1. Foils Targets

Ultra-thin foils were used for the first time as targets more than 40 years ago in the laser-driven particle and fast ignition of laser fusion applications. Exploited initially in laser fusion processes to simulate the shells of spherical targets [17], these foils were later employed for particle acceleration by interactions with ultra-intense laser pulses. The plasma expansion dynamics in the early stages of interaction with photons, as well as the laser absorption mechanisms related to material and laser intensity, have been investigated and presented in numerous publications, establishing the fundamental mechanisms behind particle acceleration. In the first studies reported in the literature, polystyrene (CH) and aluminum planar thin-foil targets with thicknesses of 5–6 μm were used as targets to simulate the walls of the spherical deuterium–tritium fuel targets commonly employed in the fusion process [17,18]. For these applications, a moderate irradiance level of 10¹²–10¹⁴ W/cm² was required to attain fusion densities and ignition temperatures upon implosion, avoiding in this way the plasma instabilities created at higher intensities. The effect of target thickness on the properties of the emitted X-ray beams was analyzed based on various laser parameters such as laser light intensity, laser pulse contrast, and the spatial distribution of the laser pulse intensity. Later, the application potential triggered interest in optimizing the entire emission process, by varying the target geometry. For instance, X-ray beam emission was reported following the interaction between laser beams (600 ps, 527 nm wavelength) having an irradiance of 10¹³ W/cm² and double foil nickel target designs with nickel film thicknesses ranging from 30 to 80 nm. This configuration ensured an increase in the plasma density and a higher gain for the optimized distance of 100–200 µm between the foils [19]. By simply modifying the target geometry, a much higher plasma density with a concave profile was created, leading to the collimation of the emitted X-ray beam, demonstrating the importance of target geometry in the distribution of accelerated particles. Experimental data, along with numerical models, have shown that using lasers with short wavelengths to interact with materials that have high atomic numbers (Zs) at moderate intensities (10¹³ W/cm²) leads to high X-ray conversion rates [20]. Additionally, the acceleration mechanisms have been formulated to explain the effects occurring in the target, such as TNSA [21,22] or RPA [23], collisionless shock wave acceleration (CSA) [24,25], Coulomb explosion acceleration (CE) [26], breakout afterburner acceleration (BOA) [27], and magnetic vortex acceleration (MVA) [28], depending on the target thickness and irradiation conditions, available for a wide range of targets, from simple foils to structured targets with 3D geometry.

TNSA, considered the main acceleration mechanism in the case of targets with thicknesses in the micrometer range or greater, was tested using 1 PW laser power (500 fs with a peak intensity of 3 × 10²⁰ W/cm²) in interaction with Au target, generating proton beams with particle energy up to 58 MeV [21]. In this case, a charge separation between the two faces of the target took place due to relativistic electrons generated by the laser pulse on the target front, initiating laser-driven ion acceleration on the rear target surface. In contrast, the RPA regime is used for thicknesses of the order of tens of nanometers, along with the circular polarization of the laser pulse and high laser contrast (>10¹⁰), facilitating the emission of ions with high energy into narrow energetic distribution [29]. A combination of RPA and TNSA mechanisms was also investigated, combining the advantages of both methods for the emission of quasi-monoenergetic ion beams with GeV energy from 80 nm ultrathin carbon foils [30]. At high intensities (more than 10²⁰ W/cm²), a relativistic induced transparency (RIT) mechanism was initiated, changing the opacity of the target by the electrons with relativistic energies. The expansion plasma became transparent for the remaining laser pulse, leading to the spatial and spectral modulation of the laser pulse with a significant impact on the particle acceleration and radiation generation of protons to energies exceeding 94 MeV [31].

2.2. Multilayer Target

Although pure target foils have initiated and demonstrated the ability to produce accelerated particles at high energies, a new concept of multilayer target seems to offer a much wider range of options in controlling the properties of emitted particles [32,33], especially of heavy ions, although they cannot be accelerated as easily as protons due to the high charge-to-mass ratio [34]. Due to properties such as the Bragg peak, similar to protons, very small divergence and the fact that it induces an amplified effect on double-strand breaks in DNA through a sharp localization of the peak, heavy ions are seen as an efficient solution for new methods of radiation therapy [35,36]. Having a high impact in many cutting-edge research fields, the heavy ion emission is numerically confirmed to be efficiently generated by the RPA mechanism applied to multilayer target design [37]; although, it is possible to use the TNSA regime at moderate intensities (10¹⁸ W/cm²) by a thermal pressure of the expanding hot plasma effect [38].

Simulation studies of laser absorption and high-energy particle acceleration demonstrate the significant role of multilayer target design in the production of monoenergetic ions, especially heavy ions, by allowing a strong interaction between the main laser pulse with the high Z layer after the laser pre-pulse is consumed in the first layer of target [39,40]. The high cut-off energy of protons emitted from thick titanium and aluminum targets using the pre-pulse method and collisional shock effect was recently investigated [41], showing a high value of proton energy emitted in the pre-plasma’s presence. It was observed that pre-plasma facilitates the propagation of the main pulse through the target, accelerating the electrons from the generated pre-plasma and creating a sheath field behind the target, enhancing the back hot electron cloud, which generates protons with higher energy.

Having different thicknesses up to several hundreds of nanometers, multilayer targets design can be found in a vast combination of materials with different Z values obtained by ultraprecise deposition methods such as pulsed laser deposition (PLD) or chemical vapor deposition (CVD) setups. Each layer can be controlled with nm precision, correlating in this way to the theoretical model used to identify the optimized target design with the target used in experiment. Variable configurations composed of alternating layers of plastic and gold [39], a thin compound ion layer between two light–ion layers [37] or even a triple-layer design [32].

In some cases, multilayer target design is a combination of randomly distributed nanoparticles with a thick carbon matrix, prepared using the co-sputtering technique. A Au nanoparticle of 3–8 nm trapped in a 120 nm thick carbon layer can provide quasi-monoenergetic Au ion bunches with energies of 500 keV range at 10¹⁸ W/cm² intensity, having energy spread less than 10% [38], and can be increased up to a 1.2 GeV (6.1 MeV = nucleon) emission if an optimal double layer target made of 60 μm carbon nanotube foams with a 150 nm Au layer is used for interaction with a 4-petawatt Ti:Sapphire laser beam [42]. As illustrated in Figure 1, the charge–state distribution of Au ions has been imaged by a CCD camera of a Thomson parabola spectrometer (TPS) equipped with a microchannel plate (MCP). Parabola-like traces indicate the distribution of ions with different charge-to-mass ratios on the surface of the MCP. The two solid lines are Au⁺³⁷ and Au⁺⁶¹ and dashed lines represent the constant energy levels. The magnified ion signals are shown in the inset.

The multitude of parameters that give particularity to the experiment, both the laser and the target, amplifies the difficulty of choosing a preferential acceleration mechanism for the emission of heavy ions in the case of targets with a multilayer design. Regardless of the regime used, the efficiency of these types of targets in the emission of mono-energetic heavy ions is observed.

2.3. Double-Foil Target Design

A double-foil target design offers a new approach to generating high-energy proton beams in the presence of a laser pre-pulse. Depending on the focal position of the laser relative to the first foil, the two-layer design, spatially separated, can either enhance the laser contrast through a plasma mirror effect [43] or create a target–back electric field that induces a re-acceleration effect [44], Figure 2.

This setup has been shown to produce proton beams with energies up to 70 MeV using a double-membrane target structure. In the plasma mirror case, the first layer reduces the contribution of the pre-pulse, thereby improving the laser pulse contrast. This results in more effective interaction with the second layer, which generates a highly intense electric field responsible for accelerating protons to high energies. Alternatively, when the configuration is altered such that the first layer becomes the main target, both layers contribute to the acceleration process. In this case, a re-acceleration effect is triggered by the combined influence of the electric fields from the first and second layers. This concept was tested experimentally by delivering a 1 ps laser pulse with 300 J of energy onto a double-layer target composed of two Au foils separated by 830 μm. The laser, focused to a 25 μm diameter spot, first interacting with a thicker 20 μm foil, followed by a thinner 0.3 μm foil. This interaction modified the total sheath electric field, resulting in proton beams with cutoff energies of up to 70 MeV [44]. Similar configurations were investigated numerically for laser pulses with a high temporal contrast and 10²⁰ W/cm² intensity leading to the emission of MeV electron peak energy from two ultrathin polymer foils with a 500 μm distance between them [45]. Also, more complex geometries were proposed for a better conversion of laser energy in hot electrons responsible for ion acceleration: installing two symmetrical solid slices at different angles in front of a double-layer target [46] or a vacuum-sandwiched target [47].

3. 3D Targets

3.1. Foams Targets

The efficiency of converting laser energy into high-energy particle emission has been correlated with target density [48,49]. Low-density targets, such as foams, allow the laser pulse to penetrate deeper into the material, modifying the pulse profile and creating a low-density plasma that accelerates ions more efficiently than solid targets [50,51,52,53]. Foam targets provide a broad range of densities and geometrical configurations, challenging the capabilities of simulation software in analyzing their interactions with laser pulses. Depending on the fabrication method, foam targets may have a disordered structure with variable pore networks and densities, or they can be produced using precise, controlled processes, resulting in well-defined structural dimensions [54,55]. Photolithographic techniques, such as UV lithography or two-photon polymerization, enable precise control over the target design, achieving resolutions down to tens of nanometers [56]. This allows for a highly accurate correlation between the simulated design and the fabricated target. Photolithographic techniques, including UV lithography and two-photon polymerization, provide precise control over target designs, achieving resolutions down to tens of nanometers [56]. This level of accuracy ensures a strong correlation between the simulated design and the fabricated target, particularly for 3D geometries. In addition to these photolithographic methods, techniques like pulsed laser deposition (PLD) and chemical vapor deposition are also employed. These methods offer the possibility to fabricate thin film layers from various materials with sub-micrometric dimensions suitable to be used as targets. For example, these techniques have successfully grown carbon foam layers with densities around 10 mg/cm³ [57], as well as nanotube foams with adjustable densities in double-layer target designs [58]. Recently, more advanced methods combining different materials have produced low-density copper foam shells, with densities of around 0.9 g/cm³, using a multistep templating process [59], as seen in Figure 3. Recently, a polystyrene foam target with a density of 30 mg/cm³ and a thickness of 50 μm generated protons with 23 MeV of energy after interacting with the LFEX laser at Osaka University at a laser intensity of 10¹⁹ W/cm² [60].

These techniques demonstrate the versatility of foam targets in being fabricated and tested for interactions with high-power laser installations. Experimentally, energetic ions in the MeV range with energies up to three times higher than those from solid targets have been achieved.

3.2. Cone-Shaped Targets Fabrication

Over a decade ago, inspired by the fast ignition HiPER target concept [61,62], a cone-shaped target design was proposed as a light concentrator in order to efficiently couple laser radiation to a target and to produce particle beams with higher energy, higher particle density, and better particle beam divergence compared to foil targets [6]. Two-dimensional (2D) PIC code PICLS was used to compute the electro-magnetic field structures, then proton beam characteristics were assessed for cone targets in comparison to flat targets at laser intensity 3 × 10²⁰ W/cm² (Figure 4). It has been numerically demonstrated that the cone geometry dramatically increases the electron density in the tip and allows for the higher conversion efficiency of laser light into very energetic electrons. At these times, the fabrication of such geometries was challenging from a technological point of view. However, the recent development of additive or subtractive manufacturing based on the ultra-short laser processing of material enabled practically unlimited possibilities for target shapes and geometries fabricated with sub-micrometer resolution.

Lately, the need for a large number of identical targets compatible with the new generation of PW laser sources with relatively high repetition rates (over 1 Hz) and with specific shapes and reduced dimensions, has imposed the use of advanced micro-fabrication and nano-fabrication technologies. In this direction, Direct Laser Writing (DLW), Two-Photon Photopolymerization (TPP), or the surface patterning of silicon wafer by laser ablation and mold replication with polydimethylsiloxane (PDMS) [63] are of the greatest interest. The additive manufacturing (AM) based on 3D laser lithography is one of the candidate techniques for the fabrication of 3D targets, including cones, micropillars, microtubes, woodpile structures, etc. The fundamental effect behind this DLW method is the Two-Photon Absorption (TPA) in photoresist materials, followed by the TPP effect. TPP consists of focusing the laser radiation (femtosecond pulses) in the volume of photoresist materials that are transparent to the laser wavelength of the processing laser beam. When near-infrared lasers with ultra-short pulses are used, multi-photon absorption processes occur in the center of the focused laser spot in a small volume of several tens of nanometers due to the extremely high laser peak intensity corresponding to the nonlinear absorption threshold of the focused Gaussian beam. In the focal plane the irradiated photoresist materials are polymerized, resulting in an insoluble structure in certain solvents, characteristic to the used photoresists. The polymerized volume is commonly known as voxel (a 3D pixel), with dimensions below the diffraction limit. By moving the sample in XYZ direction using high precision piezo stages any 3D structure with resolution down to hundreds of nm will be created according to a required design.

Figure 5 presents an example of cone targets produced by 3D laser lithography [9]. In this reference the fabrication procedure of the structures was optimized for different shapes, such as symmetric targets with straight walls and symmetric targets with parabolic walls, as predicted by some numerical simulations [10,64], to couple the laser radiation more efficiently, and inclined targets with parabolic walls, as required in solid target experiments in order to avoid laser back reflections and debris that could damage the focusing optics in the interaction chamber. The typical dimensions of the cones were 100 μm high, 100 μm as the input diameter of the cone, 10 μm as the apex diameter of the cone, and the thickness of the cone walls at about 4 μm, as confirmed by the X-ray micro-computed tomograph (μ-CT) measurements presented in Figure 6.

The perspectives for developing ultra-intense lasers at higher repetition rate, as well as the higher doses of accelerated particles, required, for example, in medical applications, stimulating the invention of new fabrication methods, taking the advantages of the ultra-fast laser processing of surfaces that use the effect of the self-organization of materials and the formation of micro- and nano-patterns on irradiated surfaces, such as laser-induced periodic surface structures (LIPSS), cone-shaped microstructures, conical needles, etc. The patterns can be easily generated on large surfaces at scanning speed of tens to hundreds of mm/s and the surface morphology can be controlled by laser scanning parameters (energy of the laser pulses, diameter of the focused laser spot, repetition rate, scanning speed) and by a processing environment that involves the photochemical etching of the surface during laser irradiation in order to obtain deeper laser ablation and sharper conical needles. Recently, such laser surface patterning has been proposed as the fabrication method for a new type of target. The manufacturing process includes the replication of the patterns via a molding step with PDMS and the realization of a negative structure with a hollow cone-like geometry (Figure 7). This approach is very promising, as it can be adapted to a large class of polymers and is even compatible to the roll-to-roll technology that can be used in order to ensure the mass production of future targets for the laser generation of secondary sources.

4. Numerical Simulation of Targets in Ultra-Intense Laser Irradiation Regime

4.1. Particle-in-Cell (PIC)

Recent studies in the field of ultra-short and ultra-intense pulse laser–matter interactions have encountered notable progresses in the lay out and development of 3D micro- and nano-structured solid targets. Numerical investigations of ultra-short laser beam interaction with solid targets expose the complex mechanisms influenced by laser beam parameters and material characteristics. The development of numerical studies points out the essentiality of advanced modeling methods in precisely describing the dynamics of ultra-short pulse laser interaction with solid targets, leading to enriched applications in engineering and material laser processing. For instance, recent theoretical studies involved a hybrid atomistic–continuum model which represents a combination of classical molecular dynamic (MD) simulations and a two-temperature model (TTM) to accurately capture the dynamics of laser interactions with solid targets [65,66,67]. Additionally, numerous numerical studies on ultra-short and ultra-intense pulse laser interaction with solid targets focusing on radiation shielding, bremsstrahlung dose yields, gamma ray flash or laser-driven particle acceleration are currently based on FLUKA Monte Carlo numerical simulations [68,69,70,71]. Furthermore, considerable research on the dynamics of laser–solid target interaction and radiation generation efficiently employs PIC codes, which are currently the most widely used numerical simulations for ultra-intense laser–plasma interaction studies. Particle-in-cell simulations were efficiently used to optimize laser plasma interactions for ion acceleration. Evolutionary PIC algorithms could investigate the collisional effects on ion dynamics in high-Z solid targets, highlighting ion energy conversion maximization [72,73]. Also, particle-in-cell calculations play an essential role in understanding laser interactions with thin foil solid targets. In this direction, dedicated PIC algorithms were developed to model the interaction of an ultra-high intensity laser pulse with thin foils in a target-normal sheath acceleration regime, revealing a strong collisional absorption process and ion and proton acceleration mechanisms [74,75,76]. Lately, a large number of PIC simulations were focused on modeling the interaction between ultra-short and ultra-intense laser pulses with cone targets, offering insights into particle behavior and magnetic reconnection processes, paving the way for progress in high-energy physics applications [77,78,79,80]. Moreover, noticeable laser pulse intensity enhancement was pointed out by using PIC numerical simulations by one order of magnitude from 8 × 10²⁰ W/cm² by using a micro-cone target [81]. Such approaches may be easily implemented by ultra-intense laser facilities to enable experiments limited by laser intensity. In the context of proton acceleration efficiency increasement, lots of PIC high-intense laser-matter interaction models were developed by implementing nanowire targets with high aspect ratios. The interaction of high-intense lasers with aligned nanowire targets may effectively heat the matter to high-energy-density regimes, enabling experiments in ultra-high field physics [82,83,84,85].

Also, important numerical results were obtained in regard to high-power laser beam conical target interaction performed using a 2D FDTD numerical model [86]. The aim of the study is to determine the electromagnetic field distribution from a spatio-temporal point of view in the proximity of the laser–matter interaction point at different moments in time, considering four different micro-structured PMMA cone targets in the presence of pulse duration variation. The internal wall of the conical target had four different profiles: sine, blaze, one-layer spheres, and step (Figure 8). According to these numerical computations, laser field intensity enhancement in a ultra-intense regime can be achieved by using a single micro-structured conical target in an optimal geometrical outline.

4.2. Finite-Difference Time-Domain (FDTD)

Advanced research indicates that ultra-short-pulsed laser interaction with micro-structured targets leads to higher ion acceleration efficiency, significantly impacting its actual scientific and technological applications. Particularly, the interaction of ultra-short lasers with micro-sphere-layered targets provides a considerable efficiency enhancement in laser-driven proton acceleration, increasing proton cut-off energy in case of small incident angles due to the structures’ geometry-induced effects [87,88]. Lately, 2D-FDTD numerical studies have been performed in order to determine the optimum conditions for extreme electric field generation [89,90]. Also, the FDTD method has been implemented in numerous studies to determine essential computation and to design efficient optical schemes for employing micro-structured solid targets for laser–matter interaction configurations. In this direction, a representative study regarding the influence of target architecture in the energy enhancement process during ultra-short pulse laser–matter interaction is related in [75].

An alternative numerical method employed in laser–matter interaction scheme design and characterization is the FDTD technique, which can provide the instant magnitude of the electromagnetic field of the laser pulse at each sequence of time for each point along the beam propagation direction. Also, the FDTD method offers the possibility to provide necessary data to define material properties and to project optimal setups with high accuracy. Using such an approach, new results in terms of electromagnetic field structures will be determined in accordance with the actual spatial and temporal limits for ultra-short laser pulses in an ultra-intense regime [91,92].

The FDTD numerical study presented here implies a Gaussian laser source that propagates to the fs pulses–target interaction point [75]. The laser source had a diameter of 5.6 μm, a wavelength of λ =800 nm, and a pulse duration of 25 fs being positioned at a distance of 10 μm from the target (Figure 9). Numerical computations were performed for two types of materials for foil targets and nanospheres: aluminum and PMMA. The evolutions of the EMF in the fs pulses–target interaction points for eight different thickness values of the PMMA and aluminum targets coated by nanospheres are presented in Figure 9. The thickness of the one-layer nanospheres target formed by the foil and the nanospheres was maintained as equal to 80 nm in the presence of the foil thickness variation in the range of 10 and 80 nm with a 10 nm step, which automatically induced the modification of the nanospheres diameter with complementary sizes in order to maintain constant target thickness. The numerical investigations were focused on the on-axis EMF intensity evolutions in the Rayleigh range, considering both linear and circular beam polarization, at a specific moment in time, when the maximum field was registered.

A recent study has been performed to provide a purposive solution to controlling the evolution of the compound laser field intensity in the proximity of the laser–matter interaction point by employing micro-patterned solid targets in a coherent laser beams overlapping (CLBO) scheme [93]. Thus, the developed FDTD numerical model could provide a full description of the laser field intensity dynamics in the vicinity of the coherent combined beams–micro-patterned targets interaction point under the phase and displacement error effect. The schematic representation of the optical setup for coherently overlapped laser beams–micro-patterned target interaction is illustrated in Figure 10. The numerical study considered two identical Gaussian laser sources (S1 and S2), both having a pulse duration of 25 fs, a beam diameter of 20 μm, a central wavelength of 0.8 μm, equal output energy values, and a symmetrical position relative to the x axis. The beams were linear s-polarized and focused by a parabolic mirror which had the focal distance of 120 μm and a diameter of 150 μm. The piston error effect was generated by temporal displacement (D) induced between the two coherent sources in the range of 0–24 μm with variable step. The design of the coherently superimposed laser beam–micro-structured target interaction system depicted in Figure 10, included, beside the two identical laser sources, a cone target with a small base diameter ϕ_Tip of 10 μm, a large base diameter ϕ_Base of 70 μm, a height (H) of 80 μm, and a wall thickness of (T) of 4 μm with a background refractive index equal to 1. This numerical study took into consideration a conical target having a one-layer micro-sphere profile of the internal wall, implying four different materials: copper, PMMA, aluminum, and silicon both for the conical target and the micro-spheres. The target internal wall profile was chosen in accordance with previous studies that aimed to obtain laser field enrichment during the propagation through the target. Thus, by considering a coherent beam combination scenario in a conical target with a one-layer micro-sphere internal wall profile, a considerable laser intensity enhancement in the focal region was foreseen due to the micro-spheres’ behavior as an array of near-field lenses that focused the incident beam in multiple spots. The intensity investigations made for the specific case of the coherent overlapping of the two laser sources in the presence of piston error took into consideration the variation of the internal wall micro-sphere dimension in the range of 1–4 μm with 1 μm step. In the presented conditions, a study concerning the laser field structure in the vicinity of a coherently overlapped laser beams–micro-patterned solid target interaction point was elaborated both from spatial and temporal points of view at specific moments in time when the temporal monitors registered the highest electromagnetic fields. The numerical results showed that, in the case of a coherently overlapped beams outline, the material and the geometry of the solid target considerably influenced the evolution of the composed laser field intensity in the vicinity of the laser–solid target interaction point, in the presence of the piston error effect. It was shown that the highest intensity values and the minimum temporal duration of the composed pulse had been obtained in the case of aluminum for all piston error values. This numerical approach based on the FDTD method had proven that laser field intensity may be accurately controlled with high resolution in a coherently overlapped laser beam configuration by using the piston error effect induced by the temporal displacement between two laser sources. This technique can be implemented in a wide variety of ultra-high power laser applications to fine-tune the laser field intensity.

4.3. Raytracing Numerical Modeling

In the context of laser field intensity enhancement in ultra-short-pulsed laser–conical target interaction schemes, a specific raytracing numerical study concerning the effect of beam incident angle variation on pulse duration and laser intensity was reported in [11]. By using a raytracing model performed with Optica-Mathematica software, several schemes based on the CPA method were designed and a comprehensive study for spatio-temporal aberration compensation was performed to optimize the parameters of the laser beam used for interaction with micro-structured conical targets. The Optica optical design program represents a raytracing tool that can perform ray traces through optical systems with detailed labels of the rays exceeding the average machine precision. It is based on an outlined raytracing language for the 2D and 3D optical systems’ characterization with a broad database of materials and optical components. In the present work [11], the ultra-short, pulsed laser–conical target interaction system included a conical target with a base diameter ϕ₁ of 100 μm, a tip diameter ϕ₂ of 15 μm, and a height H of 120 μm, being placed at the output of a Chirped Pulse Amplification (CPA) scheme. The numerical computations were conducted by using a raytracing model generated with Mathematica software considering the optical path provided by the designed CPA laser system. The fluctuations in the beam’s pointing were investigated in the vicinity of the laser–cone target interaction point by considering the lateral displacement (d) of the laser beam spot relative to the center of the target in the range of 5–30 μm in the geometrical scheme depicted in Figure 11. The incident angle of the output laser beam on the conical target varied in the range of 0.1°–1° with 0.1° step. Considering these geometrical aspects, a beam pointing stability study was performed in order to point out the influence of beam spot displacement on laser intensity and pulse duration in the ultra-short-pulsed laser–conical target interaction point. The numerical ray tracing model investigated the evolution of the laser intensity and pulse duration for each beam spot displacement considered. As depicted in Figure 11b, the laser intensity registered in the vicinity of the laser–conical target interaction point exhibited a decrease with the laser spot shift’s increase. For a spot displacement equal to 30 µm, the laser intensity registered a decrease by a factor of almost three, while the pulse duration presented an increase of about 25 fs. As a general overview, these numerical computations point out that the CPA beam pointing fluctuations significantly influenced the laser beam intensity and pulse duration in the well-defined conditions. These numerical results can be useful to investigate, control, or minimize the beam spot shift that usually induces diverse inconveniences during ultra-intense laser experiments employing CPA-based laser systems.

5. Challenges and Future Perspective

Research centers equipped with ultra-intense laser facilities are at the forefront of scientific innovation, exploring unprecedented transformations of matter. These facilities play a crucial role in cutting-edge research, contributing to breakthroughs across various fields. A significant portion of the work conducted in these centers focuses on the medical sector, where new methods of utilizing radiation are being developed to combat different forms of cancer. These approaches involve both the direct application of particles emitted during interactions between laser pulses and target materials, as well as the indirect use of these findings to propose new treatment protocols. Despite these advancements, significant challenges remain in transitioning these technologies from experimental research to clinical practice. Key challenges include the repetition rate of the emitted particles and the energy distribution of the resulting fields—both are essential for ensuring safe and effective treatments. Additionally, concerns exist regarding the stability and reproducibility of results when scaling these techniques for routine use in medical protocols.

Several publications have addressed these limitations by proposing solutions rooted in experimental innovation and theoretical analysis. These studies have identified promising strategies, including the development of more efficient laser–matter interaction methods and new target designs, many of which are relatively low-cost and potentially scalable.

5.1. Repetition Rate Barrier

One of the most pressing technical challenges is the repetition rate of high-power lasers capable of reaching over 1 petawatt (PW), a threshold necessary for generating accelerated particles with energies suitable for medical use, such as in proton therapy. Currently, only a limited number of facilities worldwide can achieve such power levels, and even these are constrained by repetition rates below 1 Hz. Increasing the repetition rate without compromising the peak power remains a key technological bottleneck that researchers are actively working to overcome. In addition to the limitations imposed by laser installation parameters, another critical challenge lies in target assembly, which must facilitate the precise and rapid manipulation of targets to sustain interactions with laser pulses at high repetition rates. The ability to reliably handle these targets is essential for advancing applications in high-energy physics and medical therapies, where repeatability is crucial. Innovative configurations have been proposed in the literature, including systems that manipulate solid targets fabricated by micro-electromechanical system (MEMS) technology on silicon wafers. These cutting-edge designs enable closed-loop operation, allowing the automated realignment of hundreds of targets with minimal human intervention, at repetition rates approaching 1 Hz [94,95]. Such technology is decisive in maintaining the precision required for experiments and clinical applications that depend on stable, high-frequency laser–target interactions. To further enhance system efficiency, researchers have integrated optical alignment components with multi-shot target assemblies based on rotating wheels [96]. This advancement has demonstrated the potential to significantly increase repetition rates, with experimental tests successfully achieving rates up to 10 Hz. These tests, conducted over 1000 consecutive targets, mark a breakthrough in laser technology, opening new possibilities for continuous, high-repetition operation in both research and medical fields.

5.2. Energy Distribution Selection

Another significant challenge involves the selection and manipulation of accelerated particles based on their energy. The energy spectrum produced by the interaction between intense laser pulses and the target is typically broad and varied. However, in fields like radiobiological studies with cell cultures, it is crucial to use particles with narrow and well-defined energy distributions. This necessitates the development of methods that can precisely direct accelerated particles to the target area while ensuring controlled and reproducible energy selection. To achieve this, various techniques have been explored, typically involving the use of magnetic field systems. The selection of specific energy ranges is often accomplished through combinations of permanent quadrupole magnets [97], quadrupole and dipole magnets [98], or pulsed solenoids [99]. By adjusting these components, researchers can create a sophisticated system of magnetic fields capable of both selecting particles with the desired energy and controlling their propagation direction. Recently, a new design was proposed to select the proper energy particles based on two anti-parallel magnetic dipole fields [100]. A series of permanent magnets with modular configuration demonstrated the system’s capability to guide energetic proton beams produced by the VEGA-3 laser (Spanish Pulsed Laser Center—CLPU, Salamanca), according to a simulated patch. This approach enabled more refined control over particle delivery, ensuring that only particles with the required energies were used for experiments or treatments. Such advancements are critical for the precision and reliability needed in applications like targeted cancer therapy or radiobiological research, where the prediction of particle propagation is essential for reproducible results.

6. Conclusions

The last decade of research in the field of ultra-intense lasers demonstrated that laser targets are critical components in the generation of secondary sources. Increasing the laser intensity to higher levels in order to obtain the desired particle energy is an intuitive approach, but not always the most accessible from a technological or economical point of view. Sometimes, the investment in the development of laser targets can offer a shortcut to the expected application of ultra-intense lasers. The recent progress in target manufacturing demonstrates that many interdisciplinary domains such as material sciences, chemistry, solid-state physics, plasma physics, nanosciences, micro- and nano-fabrication, software engineering, etc., can directly contribute to the invention of new technological solutions for the optimization of target design required by a certain application. As presented in this review, there is already a large pool of ideas and efforts involved in proposing various target designs. By combining RPA and TNSA mechanisms, it was demonstrated that quasi-monoenergetic ion beams with GeV energy can be extracted from ultrathin carbon foils, measuring just 80 nm, or from a double-layer target designed with a mixture of carbon nanotube foams and a 150 nm gold layer. Recent studies have shown that these low-density targets, such as carbon nanotube foams, facilitate a significantly more efficient conversion of laser energy into high-energy particle emission compared to traditional solid targets. This efficiency is primarily due to the ability of low-density materials to modify laser pulse profiles during interaction, allowing for better energy absorption and particle acceleration.

The fabrication methods have become more and more precise as they benefit from the latest developments in nanotechnologies. Also, the teams involved in the development of computational methods have access to the most powerful computing infrastructures and their numerical models can precisely predict in detail the interaction of the laser field with complex target geometries. In this direction, it was shown that through advanced simulation techniques, including particle-in-cell (PIC), Finite-Difference Time-Domain (FDTD), and raytracing analyses, we can propose more complex target designs to effectively control the divergence of emitted particles.

Among the possible fabrication methods, we highlighted the advantages of Two-Photon Photopolymerization (TPP), which allows the precise micro-fabrication of target design. Notably, we demonstrated, for the first time, the use of X-ray μ-CT for target characterization, providing high-resolution insights into the internal structures and material distributions of the targets. This technique enhances our ability to evaluate target integrity and performance, paving the way for more efficient particle beam generation.

It is expected that in the near future the scientific achievements in the field of laser-driven secondary sources will be closer to industrial and medical applications. However, before the foreseen transfer of knowledge to the public or private sector, a lot of efforts have to still to be invested into solving several bottlenecks related to targets, such as the fabrication of targets compatible with higher laser repetition rates, the establishment of protocols that are able to produce a large number of targets at a reasonable price, and the standardization and metrology of target fabrication in order to fulfill reproducibility and safety requirements.