Laser Imprint Research Articles

Reversible Laser Imprinting of Phase Change Photonic Structures in Integrated Waveguides.

Formation of laser-induced periodic surface structures (LIPSS) is known as a fast and robust method of functionalization of material surfaces. Of particular interest are LIPSS that manifest as periodic modulation of phase state of the material, as it implies reversibility of phase modification that constitute rewritable LIPSS, and recently was demonstrated for chalcogenide phase change materials (PCMs). Due to remarkable properties of chalcogenide PCMs─nonvolatality, prominent optical contrast and ns switching speed─such novel phase change LIPSS hold potential for exciting applications in all-optical tunable photonics. In this work we explore phase change LIPSS formation in thin films of Ge2Sb2Te5 (GST) integrated with planar and rib waveguides. We demonstrate that by fine-tuning laser radiation, the morphology of phase change LIPSS can be controlled, including their period and fill factor, and investigate the limitations of multicycle rewriting of the structures. We also demonstrate the formation of phase change LIPSS on a 1D waveguide, which has potential for use as tunable Bragg filters or structures for on-demand light decoupling into the far-field. The presented concept of applying phase change LIPSS offers a promising approach to enable fast and simple tuning in integrated photonic devices.

Solid-to-plasma transition of polystyrene induced by a nanosecond laser pulse within the context of inertial confinement fusion.

Laser direct drive (LDD) inertial confinement fusion (ICF) involves irradiating a spherical target of thermonuclear fuel coated with an ablator, usually made of polystyrene. Laser energy absorption near the target surface leads to matter ablation, hydrodynamic shocks, and ultimately capsule implosion. The conservation of spherical symmetry is crucial for implosion efficiency, yet spatial modulations in laser intensity can induce nonuniformities, causing the laser imprint phenomenon. Understanding laser imprint, especially considering the initial solid state, is essential for advancing LDD ICF. A first microscopic model of solid-to-plasma transition was built in 2019, accounting for laser absorption in the solid state with a band-structure-based ionization model. This model has been improved to include chemical fragmentation and a more accurate description of electron collision frequency in various matter states. The latest development involves assessing the model's reliability by comparing theoretical predictions with experimental observations. Despite the success of this approach, questions remain, leading to further investigations and observations under different irradiation conditions. This work presents an experiment with a nanosecond pulse, taking into account hydrodynamic effects, and measures transmission dynamics over the entire laser beam area to observe two-dimensional effects. The objective is to adapt the theoretical model, couple it with a hydrodynamic code, and observe additional effects related to the initial solid state.

Effects of Laser Bandwidth in Direct-Drive High-Performance DT-Layered Implosions on the OMEGA Laser.

In direct-drive inertial confinement fusion, the laser bandwidth reduces the laser imprinting seed of hydrodynamic instabilities. The impact of varying bandwidth on the performance of direct-drive DT-layered implosions was studied in targets with different hydrodynamic stability properties. The stability was controlled by changing the shell adiabat from (α_{F}≃5) (more stable) to (α_{F}≃3.5) (less stable). These experiments show that the performance of lower adiabat implosions improves considerably as the bandwidth is raised indicating that further bandwidth increases, beyond the current capabilities of OMEGA, would be greatly beneficial. These results suggest that the future generation of ultra-broadband lasers could enable achieving high convergence and possibly high gains in direct drive ICF.

Joint process of laser shock polishing and imprinting for metallic nanostructure fabrication

Most fabrication methods of metallic nanostructures are time-consuming with multi-steps, which are difficult to achieve low-cost batch processing. In this study, joint laser process of laser shock polishing & imprinting (LSPI) is proposed to efficiently fabricate large-area nanostructures on the surface of cheap industrial aluminum foil without any pre-processing step. LSPI tailors the laser parameters of single laser manufacturing equipment to apply laser shock polishing (LSP) to polish the aluminum foil, and then laser shock imprinting (LSI) to obtain nanostructures with high accuracy, integrity and uniformity. This unique characteristic of proposed LSPI is enabled by pre-taking LSP process to improve the yield of following LSI process through significantly decreasing the surface roughness of aluminum foil. LSPI also reduces the aluminum residue inside the silicon mold, which improves the integrity of resultant nanostructure and facilitates the reuse of the mold. The working mechanism of LSPI are also investigated to reveal its fundamental fabrication principle, which may inspire other low-cost and efficient batch fabrication method for metallic nanostructures manufacturing in the future.

Laser pulse shape designer for direct-drive inertial confinement fusion implosions

Abstract Pulse shaping is a powerful tool for mitigating implosion instabilities in direct-drive inertial confinement fusion (ICF). However, the high-dimensional and nonlinear nature of implosions makes the pulse optimization quite challenging. In this research, we develop a machine-learning pulse shape designer to achieve high compression density and stable implosion. The facility-specific laser imprint pattern is considered in the optimization, which makes the pulse design more relevant. The designer is applied to the novel double-cone ignition scheme, and simulation shows that the optimized pulse increases the areal density expectation by 16% in one dimension, and the clean-fuel thickness by a factor of four in two dimensions. This pulse shape designer could be a useful tool for direct-drive ICF instability control.

The importance of laser wavelength for driving inertial confinement fusion targets. I. Basic physics

We reinvestigate the role that laser wavelength plays in driving inertial confinement fusion (ICF) targets. Different assumptions underlie previous analytic frameworks that provide predictions for wavelength scaling of many important target parameters. These are explored and compared to radiation-hydrodynamics simulations of laser-driven targets. We are particularly interested here in lasers with wavelengths between 0.193 μm [wavelength of the Argon Flouride (ArF) excimer laser] and 0.527 μm (the frequency-doubled glass Nd:glass laser). Short-wavelength drivers have significant advantages for directly driven ICF targets, which are summarized here. We show that constraints such as providing a certain pressure or avoiding laser-plasma instability thresholds allow shorter laser wavelengths to provide energy savings, pressure enhancements, and/or higher hydrodynamic efficiencies. We also consider potential disadvantages, such as increased laser imprint or exposure to the Landau–Darrieus instability. These are shown to be either minor and/or can be easily remediated.

Benchmarking solid-to-plasma transition modeling for inertial confinement fusion laser-imprint with a pump-probe experiment

For laser direct-drive (LDD) fusion implosions, intense laser beams are used to directly illuminate the inertial confinement fusion (ICF) capsule. The laser beams' intensity nonuniformity (due to speckles) on the target can impose perturbation seeds which are subsequently amplified by Rayleigh-Taylor instability growth, thereby leading to degradation of ICF implosion performance. To devise methods to mitigate this issue, adequate understanding of the underlying so-called laser-imprinting process is required. Here, we report measurements and modeling of the initial plasma formation process which has been shown to affect the laser imprint. Specifically, we measured the transient transmission of a femtosecond probe pulse through a polystyrene target for different 100-picosecond pump pulse intensities pertaining to ICF conditions. The experimental data are used to benchmark a microphysics model of initial plasma formation that overall describes the observed dynamics, thus providing a validated solid-to-plasma modeling for laser-imprinting purposes in radiation-hydrodynamic codes to accurately simulate and design LDD targets.

Mitigation of the ablative Rayleigh–Taylor instability by nonlocal electron heat transport

The effects of electron nonlocal heat transport (NLHT) on the two-dimensional single-mode ablative Rayleigh–Taylor instability (ARTI) up to the highly nonlinear phase are reported for the first time through numerical simulations with a multigroup diffusion model. It is found that as well as its role in the linear stabilization of ARTI growth, NLHT can also mitigate ARTI bubble nonlinear growth after the first saturation to the classical terminal velocity, compared with what is predicted by the local Spitzer–Härm model. The key factor affecting the reduction in the linear growth rate is the enhancement of the ablation velocity Va by preheating. It is found that NLHT mitigates nonlinear bubble growth through a mechanism involving reduction of vorticity generation. NLHT enhances ablation near the spike tip and slows down the spike, leading to weaker vortex generation as the pump of bubble reacceleration in the nonlinear stage. NLHT more effectively reduces the nonlinear growth of shorter-wavelength ARTI modes seeded by the laser imprinting phase in direct-drive laser fusion.

Mitigating laser imprint with a foam overcoating

In direct-drive inertial confinement fusion, laser imprint can cause areal density perturbations on the target shell that seed the Rayleigh–Taylor instability and further degrade the implosion. To mitigate the effect of laser imprint, a foam overcoating layer outside the target shell has been suggested to increase the thermal smoothing of the conduction region (between the ablation front and the critical density surface) and mass ablation of the ablation front. In this paper, we use a two-dimensional radiation hydrodynamic code FLASH to investigate the laser imprint mitigation performance and find other physical mechanisms of foam overcoatings. First, radiation ablation dynamically modulates density distribution not only to increase the frequency of the perturbed ablation front oscillation but also to decrease the amplitude of the oscillation. Second, a larger length of the shocked compression region reduces the amplitude of the perturbed shock front oscillation. The areal density perturbations decrease with the decrease in the perturbations of the ablation front and shock front. Based on the abovementioned physical mechanisms, we propose the optimal ranges of foam parameters to mitigate laser imprint with the aid of dimensional analysis: the foam thickness is about two to three times that of the perturbation wavelength, and the foam density is about 1/2–3/2 times that of the critical density.

Causes of fuel–ablator mix inferred from modeling of monochromatic time-gated radiography of OMEGA cryogenic implosions

Here, we present evidence, in the context of OMEGA cryogenic target implosions, that laser imprint, known to be capable of degrading laser-direct-drive target performance, plays a major role in generating fuel–ablator mix. OMEGA cryogenic target implosions show a performance boundary correlated with acceleration-phase shell stability; for sufficiently low adiabats (where the adiabat is the ratio of the pressure to the Fermi pressure) and high in-flight aspect ratios (IFAR's), the neutron-weighted shell areal density and neutron yield relative to the clean simulated values sharply decline. Direct evidence of Rayleigh–Taylor fuel–ablator mixing was previously obtained using a Si Heα backlighter driven by an ∼20-ps short pulse generated by OMEGA EP. The shadow cast by the shell shortly prior to stagnation, as diagnosed using backlit radiographs, shows a softening near the limb, which is evidence of an ablator–fuel mix region for a low-adiabat implosion (α ∼ 1.9, IFAR = 14) but not for a moderate adiabat implosion (α ∼ 2.5, IFAR = 10). We find good agreement between experimental and synthetic radiographs in simulations that model laser imprint and account for uncertainty in the initial ablator thickness. We further explore the role of other mechanisms such as classical instability growth at the fuel–ablator interface, species concentration diffusion, and long-wavelength drive and target asymmetries.

Direct-drive implosion experiment of diamond capsules fabricated with hot filament chemical vapor deposition technique

Diamond is a promising alternative capsule material for direct-drive inertial confinement fusion. Previous investigations suggest that the stiffness and higher density of diamond can mitigate laser imprinting, which is an initial perturbation on the ablation surface caused by non-uniform laser irradiation. We conduct diamond capsule fabrication using the hot filament chemical vapor deposition (HF-CVD) technique. This technique is the most suitable diamond deposition method in terms of mass production, since the deposition area can be easily extended just by increasing the number of hot filaments. This paper presents the first direct-drive implosion experiment concerning diamond capsules fabricated using the HF-CVD technique. The compression of thin diamond capsules, with an inner capsule diameter of 482 μm and a shell thickness of 1.6 μm, is successfully achieved, and the implosion trajectory of diamond capsules, with the same diameter and thickness, is consistent with 1D radiation hydrodynamic simulation calculations. However, for diamond capsules with an inner diameter of 482 μm and a shell thickness of 2.4 μm, asymmetry of the implosion trajectory and unexpected x-ray emission are observed. This is attributed to the remaining silicon (Si) mold inside the capsule left over from the fabrication process, revealing that the etching method of Si in diamond capsules should be improved.

Improved modeling of the solid-to-plasma transition of polystyrene ablator for laser direct-drive inertial confinement fusion hydrocodes.

The target performance of laser direct-drive inertial confinement fusion (ICF) can be limited by the development of hydrodynamic instabilities resulting from the nonhomegeneous laser absorption at the target surface, i.e., the laser imprint on the ablator. To understand and describe the formation of these instabilities, the early ablator evolution during the laser irradiation should be considered. In this work, an improved modeling of the solid-to-plasma transition of a polystyrene ablator for laser direct-drive ICF is proposed. This model is devoted to be implemented in hydrocodes dedicated to ICF which generally assume an initial plasma state. The present approach consists of the two-temperature model coupled to the electron, ion and neutral dynamics including the chemical fragmentation of polystyrene. The solid-to-plasma transition is shown to significantly influence the temporal evolution of both free electron density and temperatures, which can lead to different shock formation and propagation compared with an initial plasma state. The influence of the solid-to-plasma transition on the shock dynamics is evidenced by considering the scaling law of the pressure with respect to the laser intensity. The ablator transition is shown to modify the scaling law exponent compared with an initial plasma state.

Measurements of laser-imprint-induced shock velocity nonuniformities in plastic targets with the Nike KrF laser

We report results of direct-drive laser imprint experiments measuring velocity perturbation profiles of shock waves produced by the Nike krypton fluoride laser. A new high-resolution two-dimensional velocimeter system was successfully implemented on the Nike laser facility and used for sensitive optical measurements of the velocity perturbations. Planar polystyrene targets with and without a thin high-Z overcoat (400 Å Au or 600 Å Pd) were irradiated by four, eight, and sixteen Nike laser beams to examine laser imprint and its mitigation. The results from the uncoated targets showed that the shock velocity perturbations decreased with an increasing number of laser beams overlapped on target, precisely as anticipated by the beam averaging effect on laser imprint. In the experiment on the shocks driven in the high-Z coated targets, the shock velocity perturbations were further reduced by a factor of 2–6 compared to their counterparts in the uncoated experiment, with the amplitude of the velocity fluctuations measured as small as 20 m/s rms for shock velocities of 20 km/s. These experiments allowed more direct measurements of laser imprint effects without relying on the Rayleigh–Taylor hydrodynamic amplification, providing valuable quantitative data for calibrating radiation-hydrodynamic simulations of laser imprint.

Order-of-magnitude laser imprint reduction using pre-expanded high-Z coatings on targets driven by a third harmonic Nd:glass laser

A hybrid early time x-ray drive followed by conventional laser direct drive obtained by utilizing a thin high-Z overcoat on plastic ablator targets has proven to be very effective at reducing laser imprint in experiments driven by the Nike krypton-fluoride laser. An inherent low level laser prepulse from that laser system heated and expanded the coating prior to the main pulse. Here, we report on results using a frequency tripled Nd:glass laser system (Omega EP) where the inherent prepulse was several orders of magnitude smaller. By applying a separate prepulse, these experiments allowed us to test if the prepulse is important for mitigating imprint with the high-Z layer. The results show that the high-Z coating is much more effective at reducing laser imprint when it is pre-expanded, in this case, by an externally generated low level soft x-ray prepulse of ∼10 J/cm2. With the x-ray prepulse, laser imprint at spatial wavelengths below 100 μm was reduced by an order of magnitude, similar to that observed with the laser prepulse on Nike. Furthermore, the experiments here establish that high-Z coating is effective in reducing imprint even in the case of a fixed speckle pattern. Rayleigh–Taylor-amplified laser imprint and high-Z layer dynamics were measured using through-foil and side-on x-ray radiography. Pre-expansion times from 30 down to 6 ns were effective, potentially compatible with laser prepulse generation using existing NIF and OMEGA front ends; however, temporal beam smoothing appears to be necessary for the laser prepulse that directly illuminates the high-Z coating. The highest imprint reduction is observed for Pd and Au coatings of at least 400 Å thickness; thicknesses down to 200 Å show a reduction in imprint with an adjusted prepulse.

Plasma hydrodynamic experiments on NRL Nike KrF laser

The krypton-fluoride Nike laser delivers up to 2 KJ in 248 nm ultraviolet radiation to planar targets with the most uniform illumination of all existing high-energy lasers for inertial confinement fusion (ICF) research. That, combined with high-resolution monochromatic x-ray imaging pioneered at NRL in the 1990s and the recently added two-dimensional (2D) VISAR diagnostics, provides a unique platform for experimental studies of hydrodynamic phenomena important to the inertial confinement fusion (ICF). These include absolute experimental determination of the primary Hugoniots of low-density plastic foams in the Mbar pressure range. Although foam materials have many uses for the manufacturing of ICF targets ranging from laser fusion to Z-pinch-driven dynamic hohlraums, no experimental data on their shock compressibility in the ICF-relevant pressure range was available until our experiments. We describe the first systematic experimental study of the nonlinear, multi-mode perturbation evolution triggered by localized target non-uniformities, such as straight grooves and dots, which emulate fill tubes, tents, and other target-mounting structures in laser capsules. The experimental data turned out to be counter-intuitive and challenging for simulations. Finally, the 2D VISAR snap-shot images of the shock velocity field made it possible to directly measure the roughness of a uniformly driven shock front propagating into the target, which is caused by ISI-smoothed laser imprint. Initial measurements for planar CH targets with and without thin high-Z layers added for the imprint mitigation have been successfully performed.

Novel Hot-Spot Ignition Designs for Inertial Confinement Fusion with Liquid-Deuterium-Tritium Spheres.

A new class of ignition designs is proposed for inertial confinement fusion experiments. These designs are based on the hot-spot ignition approach, but instead of a conventional target that is comprised of a spherical shell with a thin frozen deuterium-tritium (DT) layer, a liquid DT sphere inside a wetted-foam shell is used, and the lower-density central region and higher-density shell are created dynamically by appropriately shaping the laser pulse. These offer several advantages, including simplicity in target production (suitable for mass production for inertial fusion energy), absence of the fill tube (leading to a more-symmetric implosion), and lower sensitivity to both laser imprint and physics uncertainty in shock interaction with the ice-vapor interface. The design evolution starts by launching an ∼1-Mbar shock into a DT sphere. After bouncing from the center, the reflected shock reaches the outer surface of the sphere and the shocked material starts to expand outward. Supporting ablation pressure ultimately stops such expansion and subsequently launches a shock toward the target center, compressing the ablator and fuel, and forming a shell. The shell is then accelerated and fuel is compressed by appropriately shaping the drive laser pulse, forming a hot spot using the conventional or shock ignition approaches. This Letter demonstrates the feasibility of the new concept using hydrodynamic simulations and discusses the advantages and disadvantages of the concept compared with more-traditional inertial confinement fusion designs.

Dependences of morphology and surface roughness on growth conditions of diamond capsules for the direct-drive inertial confinement fusion

In order to achieve the ignition by direct-drive inertial confinement fusion (ICF), smooth and symmetric fuel capsule is required. Our previous research found that stiffness of the capsule material contributes to reduce the laser imprinting. Therefore, diamond is considered to be the most promising candidate as the stiff ablator material for direct-drive ICF capsule. In this study, aiming at improving the surface roughness on the diamond capsule, we obtained dependence of methane concentration and gas pressure on surface roughness and morphology by hot filament chemical vapor deposition technique.

Direct-drive double-shell implosion: A platform for burning-plasma physics studies.

Double-shell ignition designs have been studied with the indirect-drive inertial confinement fusion (ICF) scheme in both simulations and experiments in which the inner-shell kinetic energy was limited to ∼10-15kJ, even driven by megajoule-class lasers such as the National Ignition Facility. Since direct-drive ICF can couple more energy to the imploding shells, we have performed a detailed study on direct-drive double-shell (D^{3}S) implosions with state-of-the-art physics models implemented in radiation-hydrodynamic codes (lilac and draco), including nonlocal thermal transport, cross-beam energy transfer (CBET), and first-principles-based material properties. To mitigate classical unstable interfaces, we have proposed the use of a tungsten-beryllium-mixed inner shell with gradient-density layers that can be made by magnetron sputtering. In our D^{3}S designs, a 70-μm-thick beryllium outer shell is driven symmetrically by a high-adiabat (α≥10), 1.9-MJ laser pulse to a peak velocity of ∼240 km/s. Upon spherical impact, the outer shell transfers ∼30-40 kJ of kinetic energy to the inner shell filled with deuterium-tritium gas or liquid, giving neutron-yield energies of ∼6 MJ in one-dimensional simulations. Two-dimensional high-mode draco simulations indicated that such high-adiabat D^{3}S implosions are not susceptible to laser imprint, but the long-wavelength perturbations from the laser port configuration along with CBET can be detrimental to the target performance. Nevertheless, neutron yields of ∼0.3-1.0-MJ energies can still be obtained from our high-mode draco simulations. The robust α-particle bootstrap is readily reached, which could provide a viable platform for burning-plasma physics studies. Once CBET mitigation and/or more laser energy becomes available, we anticipate that break-even or moderate energy gain might be feasible with the proposed D^{3}S scheme.

Functional Micro–Nano Structure with Variable Colour: Applications for Anti-Counterfeiting

Colour patterns based on micro-nano structure have attracted enormous research interests due to unique optical switches and smart surface applications in photonic crystal, superhydrophobic surface modification, controlled adhesion, inkjet printing, biological detection, supramolecular self-assembly, anti-counterfeiting, optical device and other fields. In traditional methods, many patterns of micro-nano structure are derived from changes of refractive index or lattice parameters. Generally, the refractive index and lattice parameters of photonic crystals are processed by common solvents, salts or reactive monomers under specific electric, magnetic and stress conditions. This review focuses on the recent developments in the fabrication of micro-nano structures for patterns including styles, materials, methods and characteristics. It summarized the advantages and disadvantages of inkjet printing, angle-independent photonic crystal, self-assembled photonic crystals by magnetic field force, gravity, electric field, inverse opal photonic crystal, electron beam etching, ion beam etching, laser holographic lithography, imprinting technology and surface wrinkle technology, etc. This review will provide a summary on designing micro-nano patterns and details on patterns composed of photonic crystals by surface wrinkles technology and plasmonic micro-nano technology. In addition, colour patterns as switches are fabricated with good stability and reproducibility in anti-counterfeiting application. Finally, there will be a conclusion and an outlook on future perspectives.

Modeling the solid-to-plasma transition for laser imprinting in direct-drive inertial confinement fusion.

Laser imprinting possesses a potential danger for low-adiabat and high-convergence implosions in direct-drive inertial confinement fusion (ICF). Within certain direct-drive ICF schemes, a laser picket (prepulse) is used to condition the target to increase the interaction efficiency with the main pulse. Whereas initially the target is in a solid state (of ablators such as polystyrene) with specific electronic and optical properties, the current state-of-the-art hydrocodes assume an initial plasma state, which ignores the detailed plasma formation process. To overcome this strong assumption, a model describing the solid-to-plasma transition, eventually aiming at being implemented in hydrocodes, is developed. It describes the evolution of main physical quantities of interest, including the free electron density, collision frequency, absorbed laser energy, temperatures, and pressure, during the first stage of the laser-matter interaction. The results show that a time about 100 ps is required for the matter to undergo the phase transition, the initial solid state thus having a notable impact on the subsequent plasma dynamics. The nonlinear absorption processes (associated to the solid state) are also shown to have an influence on the thermodynamic quantities after the phase transition, leading to target deformations depending on the initial solid state. The negative consequences for the ICF schemes consist in shearing of the ablator and possibly preliminary heating of the deuterium-tritium fuel.