Javier E. Garay

Photonic materials with sub-wavelength features have enabled unique light-matter interactions; however, fabrication methods face challenges in achieving high-resolution features and tunable structures at scale. In this work, laser-processing of photonic materials is studied. First, multilayered gratings are fabricated using two-photon absorption lithography for polarization-sensitive structural colors. By varying the grating design and light polarization conditions, the transmitted structural color response is analyzed. Next, we investigate the laser focusing beyond Abbe’s diffraction limit using ultrasmooth plasmonic pyramidal arrays. The electric field enhancement due to nanoscale curvature at the apex is characterized, and pyramidal structures of different metals are integrated into a high-precision substrate patterning setup. Overall, this work showcases high-resolution feature size and structural control in laser-based techniques, offering insights into novel micro- and nanofabrication techniques.

This paper presents a study of several levels of simplification of the geometrical features of the locomotive unit, in good agreement with these proposed in the European standards, in order to study the sensitivity of the compression wave profile and the pressure gradient measured at the tunnel entrance. The European standards permits in computational fluid dynamic studies the simplification of the bogies while other aerodynamically significant features shall be modeled in detail. Here the snowplow, the coupler elements and the bogies are studied to determine the influence of each element on the compression wave and the maximum pressure gradient of a intercity train entering into a tunnel. It is observed that the coupler introduces a delay in the pressure rise measured at the tunnel walls when the train nose is entering. The snowplow introduces a flow detachment that increases the effective cross-sectional area of the train, and so increases the pressure rise. The removal of the bogies clearly modifies the train head cross-sectional area, so its substitution by a dummy box provokes an increase in the pressure rise that approximates the maximum pressure gradient to that obtained from the full-detailed case.

Tamper materials allow for pressure maximization in a shock experiment up until their saturation limit. The Fabbro model provides the basis for the prediction for pressure enhancement from tamper materials. In this paper, we report the performance of two novel, to the best of our knowledge, tamper materials (yttrium aluminum garnet (YAG) and gadolinium gallium garnet (GGG)) using the Fabbro model and experimental data. The pressure enhancement using a laser-driven shock in the 2 GPa range and the saturation curve as these novel tampers compare to traditional tampers is explored. The unexpectedly low saturation points of YAG and GGG are discussed because of variable surface quality of commercially available samples. Overall, this study’s findings suggest that YAG and GGG show promise as alternative tampers, provided that anticipated enhancements in surface quality are achieved. At fluences of 5 J/cm 2 , a strong dependence of maximum pressure on the total acoustic impedance of the sample is not observed, and tamper selection becomes negligible.

Transparent alumina ceramics are known for high toughness and the ability to withstand high temperatures, making them ideal materials for use in extreme environments and high-power optical devices. However, polycrystalline alumina, which has a hexagonal crystal structure, is difficult to make highly transparent due to birefringent scattering loss. Research has shown that birefringent scattering can be reduced by having finer grains and/or aligned grains. Here, we present an analytical birefringence scattering model that can model realistic microstructures, by including chord length distributions and grain orientations, and predict the birefringence scattering loss quantitatively. Our modeled results match well with existing experimental data for transparent fine grained and aligned alumina. This model is derived from first-principles and is applicable to other transparent ceramics.

The effect of laser pulse duration on energy coupling into a planar silicon target is investigated in experiments at the OMEGA-EP facility by varying the laser pulse length <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>τ</a:mi></a:math>—spanning 3 orders of magnitude from 100 ps to 10 ns—while maintaining a constant peak laser intensity, <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:msub><b:mi>I</b:mi><b:mn>0</b:mn></b:msub><b:mo>=</b:mo><b:mn>5</b:mn><b:mo>×</b:mo><b:msup><b:mn>10</b:mn><b:mn>14</b:mn></b:msup></b:mrow></b:math> W/<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msup><c:mi mathvariant="normal">cm</c:mi><c:mn>2</c:mn></c:msup></c:math>. In theoretical models, the ablation pressure primarily scales for a given material with laser intensity and wavelength, which are all fixed variables here, allowing us to explore the specific role of laser pulse duration. Two-dimensional radiation-hydrodynamics simulations benchmarked with optical probing of the expanding plasma show that the pulse duration is critical for the ablation pressure to reach a steady state. Moreover, the pulse duration impacts shock decay and multiple wave effects, which strongly dictate the evolving shock profile that propagates within the laser-shocked target as ultimately measured by rear-surface diagnostics. The shock velocities inferred from the theoretical model, after considering shock decay, impedance matching, and shock Hugoniot, are found to be in good agreement with velocimetry measurements. However, discrepancies are observed with simulations for the shorter (0.1 ns) and longer (10 ns) pulse durations, which are respectively attributed to unaccounted contributions of kinetic absorption mechanisms and instabilities in simulations. Published by the American Physical Society 2024

Abstract Therapeutic ultrasound for brain stimulation has increased in the last years. This energy has shown promising results for treating Alzheimer’s disease, Parkinson’s disease and traumatic brain injury, among other conditions. However, the application of ultrasound in the brain should trespass a natural but highly attenuating and distorting barrier, the cranium. Implantable ceramic materials can be used to replace part of the cranium as an alternate method to enhance ultrasound transmission. In this work, it is presented the acoustic characterization of alumina ceramic disks that can be employed as cranial implants for acoustic windows-to-the-brain. Alumina samples were prepared using current-activated pressure-assisted densification and were acoustically characterized. Acoustic impedance and attenuation of the samples were determined for different porosities. Additionally, measured and modeled acoustic fields are presented and analyzed in terms of the total ultrasound transmitted through the ceramics. Results indicate a resonant behavior in the alumina disks when the thickness corresponds to a half-wavelength of ultrasound; this resonance permits a total of 95.4% of ultrasound transmission; for thicknesses out of the resonant zone, transmission is 53.0%. Alumina proves to be an excellent medium for ultrasound transmission that, in conjunction with its mechanical and optical properties, can be useful for cranium replacement in mixed opto-acoustic applications.

Therapeutic ultrasound for brain stimulation has increased in the last years. This energy has shown promising results for treating Alzheimers disease, Parkinsons disease, and traumatic brain injury, among other conditions. However, the application of ultrasound in the brain should trespass a natural but highly attenuating and distorting barrier, the cranium. Implantable ceramic materials can be used to replace part of the cranium as an alternate method to enhance ultrasound transmission. In this work, it is presented the acoustic characterization of alumina ceramic disks that can be employed as cranial implants for acoustic windows-to-the-brain. Alumina samples were prepared using current-activated pressure-assisted densification and were acoustically characterized. Acoustic impedance and attenuation of the samples were determined for different porosities. Additionally, measured and modeled acoustic fields are presented and analyzed in terms of the total ultrasound transmitted through the ceramics. Results indicate a resonant behavior in the alumina disks when the thickness corresponds to a half-wavelength of ultrasound; this resonance permits a total of 95.4% of ultrasound transmission; for thicknesses out of the resonant zone, transmission is 53.0%. Alumina proves to be an excellent medium for ultrasound transmission that, in conjunction with its mechanical and optical properties, can be useful for cranium replacement in mixed opto-acoustic applications.

We report compression tests on micropillars manufactured from bulk specimens of partially devitrified SAM2×5 (Fe 49.7 Cr 17.7 Mn 1.9 Mo 7.4 W 1.6 B 15.2 C 3.8 Si 2.4 ). Yield strength values of ≈6 GPa are obtained. Such a high strength can be attributed to the higher glass transition temperature (883 K) of this material, which impedes the multiplication of shear bands under loading, and to the presence of hard crystalline domains that result from devitrification of the amorphous powders during powder consolidation. The Vickers hardness of the specimens is found to be strongly correlated to the processing temperature and, hence to the volume of crystalline phases present in the specimens. As the processing temperature is increased, there is a reduction in free volume from the structural relaxation process in the amorphous alloy, leading to the eventual nucleation of crystalline phases of BCC Fe, Cr 2 B, Cr 21.30 Fe 1.7 C 6 , or Fe 23 B 2 C 4 , during the densification process. These results shed light on the relationship between nanocrystalline domains and the mechanical behavior of Fe‐based amorphous/crystalline composites.

Despite Neodymium laser systems being well-established and ever popular, there is motivation to improve gain and scale in inexpensive host materials such as Yttrium Aluminum Garnet (YAG) and Fine-Grain Al₂O₃. Thermal management through host materials with improved thermal properties is a promising pathway to stronger pumping and subsequently higher gain. Benefits of polycrystalline ceramic gain media, as well as various ceramic fabrication methods will be discussed. While polycrystalline Nd:YAG can be fabricated using traditional densification techniques of sintering and Hot Isostatic Pressing (HIP), in order to create polycrystalline Nd:Al₂O₃, one must turn to Current-Activated Pressure-Assisted Densification (CAPAD), a method of ceramic fabrication that utilizes high heating rates and pressure to reduce hold temperatures and times, reducing diffusion and subsequent grain growth.

This research looks to enhance our understanding of the laser-material interaction within silicon, considering variations in free carrier density. Silicon exhibits distinct optical behaviors, ranging from transparency to non-transparency, contingent on its doping concentration, particularly at a 1064 nm wavelength. Our experimental investigation delves into the quantitative assessment of damage size and the qualitative characterization of damage morphology induced by singlepulse 1064 nm laser irradiation. In this experiment, we vary laser intensities and focal depths to show their influence on the damage features of single crystal silicon with varying doping concentrations. The damage size and qualitative characteristics can be used to better understand the mechanisms responsible for the laser damage. Additionally, we can see when the damaged silicon is exhibiting pure melting or a form of ordered damage at higher intensities. The findings of this study give insight into the optimization of laser processing techniques that require precise control over material ablation, and phase change as cutting and material joining. Furthermore, the insights garnered from this work contribute to a broader understanding of the interplay between laser parameters and material properties. This study represents a move towards unlocking the potential of laser-matter interactions in shaping the future of silicon advanced manufacturing technologies.

Nonlinear optical properties play a key role in technologies such as broadband laser light sources and ultrafast laser machining. With the emergence of transparent nanocrystalline Al2O3 ceramics as an alternative to single crystal alumina (sapphire), it is critical to understand their nonlinear optical behavior. Here, we report the demonstration of supercontinuum generation in polycrystalline alumina ceramics. Substantial broadening was observed when a focused 515 nm pulsed (260 fs) laser propagated through the ceramic sample. The broadening increased with increasing laser power and displayed stokes/anti-stokes asymmetries. At higher incident power, permanent damage was observed. Our results show that transparent nanocrystalline Al2O3 ceramics have a higher material removal rate than single crystal alumina. These results have interesting implications for laser machining as well as integrated photonics.

We investigate the ablation of silicon as a function of laser pulse length at a constant fluence using time-resolved x-ray spectroscopy data obtained from OMEGA EP experiments at the University of Rochester's Laboratory for Laser Energetics. Our targets consisted of three-layer planar structures composed of Si (50 μm), Cu (25 μm), and SiO2 (500 μm) layers. The Si layer was irradiated by a 351-nm laser with varying pulse widths of 250 ps, 500 ps, 1 ns, and 10 ns while maintaining a constant fluence of ∼27.9 kJ/cm2. Electron temperatures and densities of the ablated plasma were determined by analyzing the time-resolved x-ray spectroscopy data through a comparison of experimental measurements with synthetic results obtained from Si atomic calculations in a steady state and non-local thermodynamic equilibrium. These calculations were computed using PrismSPECT [MacFarlane et al., High Energy Density Phys. 3, 181 (2007)]. Additionally, radiation-hydrodynamics simulations with FLASH are used to generate simulated plasma-density and plasma-temperature profiles, which are then compared with the experimental measurements. Our analyses reveal that increasing the laser pulse length at a constant fluence results in a decrease in electron temperatures and densities. Furthermore, the longer pulses with lower intensities lead to deeper ablation regions before reaching the peak ablation but lower ionization balances in the silicon layer. These findings emphasize the critical role of laser pulse length in plasma ablation and shock generation for laser-impulse studies.

A 100 ps laser is used to probe the pressure generation, depth of the non-solid ablator, and the non-linear optical effects through tamper materials. Samples consisted of an aluminum ablator with tampers of sapphire and coverslip glass. In general, the sapphire tamped sample achieves higher pressures at lower laser intensities as compared to the coverslip glass tamped sample. Attempts to model the details of this set of experimental data with standard available radiation coupled hydrodynamic codes make clear that more physics is needed in these simulations to accurately predict the impact of the tamper material on the pressure generation and the depth of non-solid aluminum.

Abstract Polycrystalline materials have shown promise in advanced optical applications because of their unique optical and mechanical properties. Most polycrystalline materials have non‐uniform optical properties due to residual porosity, secondary phases, and/or crystalline anisotropies (e.g., birefringence). These optical inhomogeneities manifest as scattering that reduces the transparency of the polycrystal. Even in the case of a single‐phase polycrystal with negligible porosity, birefringence scattering will always be present whenever the crystal is anisotropic (non‐cubic). Multiple models for predicting birefringence scattering have been suggested in the literature that are successful in describing scattering loss in specific material systems. Here a first‐principles model of birefringent scattering that is applicable to any single‐phase, unaligned transparent polycrystal is derived. The model can treat grain size distributions and is not limited to a specific material system. The derivation culminates in an equation that describes birefringence scattering coefficient spectra using the single‐crystal refractive index tensor and chord length distribution (measured from a representative polished surface micrograph). The model should be useful for designing materials and characterization. For example, it can be used to predict transmissions of transparent polycrystals for a range of grain sizes or to characterize the average grain size using a transmission measurement.

Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity. The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts ($NaNO_3$ and $KNO_3$), solar salt ($NaNO_3-KNO_3$ mixture), and chloride salt ($NaCl-KCl-MgCl_2$). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems.

Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity. The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts ($NaNO_3$ and $KNO_3$), solar salt ($NaNO_3-KNO_3$ mixture), and chloride salt ($NaCl-KCl-MgCl_2$). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems.

The generation of laser ablation depth data in the ultrafast (100ps) time regime is important for the validation of radiation hydrodynamic codes in that time regime.We present a technique using data from a velocimetry diagnostic to determine the hot electron penetration depth into a metal sample.