Light Intensification Research Articles

Harnessing a silicon carbide nanowire photoelectric synaptic device for novel visual adaptation spiking neural networks.

Visual adaptation is essential for optimizing the image quality and sensitivity of artificial vision systems in real-world lighting conditions. However, additional modules, leading to time delays and potentially increasing power consumption, are needed for traditional artificial vision systems to implement visual adaptation. Here, an ITO/PMMA/SiC-NWs/ITO photoelectric synaptic device is developed for compact artificial vision systems with the visual adaption function. The theoretical calculation and experimental results demonstrated that the heating effect, induced by the increment light intensity, leads to the photoelectric synaptic device enabling the visual adaption function. Additionally, a visual adaptation artificial neuron (VAAN) circuit was implemented by incorporating the photoelectric synaptic device into a LIF neuron circuit. The output frequency of this VAAN circuit initially increases and then decreases with gradual light intensification, reflecting the dynamic process of visual adaptation. Furthermore, a visual adaptation spiking neural network (VASNN) was constructed to evaluate the photoelectric synaptic device based visual system for perception tasks. The results indicate that, in the task of traffic sign detection under extreme weather conditions, an accuracy of 97% was achieved (which is approximately 12% higher than that without a visual adaptation function). Our research provides a biologically plausible hardware solution for visual adaptation in neuromorphic computing.

Temporal and spatial laser intensification within nodular defects overcoated with multilayer dielectric mirrors over a wide range of defect geometries.

The nodular defect shape and the laser incidence angle have a dramatic impact on the spatial distribution of light intensification within the nodule as well as how the laser light is drained from the defect. Nodular defect geometries unique to ion beam sputtering, ion-assisted deposition, and electron-beam (e-beam) deposition, respectively, are modeled in this parametric study over a wide range of nodular inclusion diameters and layer count for optical interference mirror coatings constructed with quarter-wave thicknesses and capped with a half wave of the low index material. It was found for hafnia (n=1.9) and silica (n=1.45) multilayer mirrors that the light intensification in nodular defects with a C factor of 8, typical of e-beam deposited coatings deposited with a wide range of deposition angles, was maximized for a 24-layer design. For intermediate size inclusion diameters, increasing the layer count for normal incidence multilayer mirrors reduced the light intensification within the nodular defect. A second parametric study explored the impact of the nodule shape on the light intensification for a fixed number of layers. In this case, there is a strong temporal trend for the different nodule shapes. Narrow nodules tend to drain more laser energy through the bottom of the nodule into the substrate while wide nodules tend to drain more laser energy through the top of the nodule when irradiated at normal incidence. At a 45° incidence angle, waveguiding is an additional method to drain laser energy from the nodular defect. Finally, laser light resonates within nodular defects longer than within the adjacent nondefective multilayer structure.

Generation of chemical deposits on fused silica optics and their modulation on the light intensity distribution.

Deposited substances generated from hydrofluoric acid based (HF-based) etching are found to be the precursors that deteriorate the resistance of fused silica optics to laser damage. In this paper, the surface of polished fused silica was treated with a buffer oxide etchant (BOE, 5-10%wt. HF + 10%wt. N H 4 F+80-85% w t.H 2 O). The optic surface area affected by the etching-induced deposits (N H 4)S i F 6 was found to increase significantly with the amount of material removal and sensitivity to the post-cleaning procedure. Three shapes of deposited particles are simultaneously identified on the treated samples. The 3D finite-difference time-domain (FDTD) was used to simulate the light field distribution near the deposited particles, which demonstrates that fused silica is subject to more light modulation when a particle is exposed on the front surface laser than on the rear surface laser. In addition, dense particles barely increase the light intensification factor (LIF) while markedly increase the light focused probability inside fused silica. Moreover, a multiple linear regression (MLR) analysis for the LIF builds a fitting plane model of the LIF and the particle height as well as the horizontal size, revealing that the LIF increases with the size of the deposited particle, especially its height. The laser damage testing results indicate that a deposited layer of ∼76n m in height had little influence on the laser-induced damage threshold (LIDT) of the optics and larger deposited particles with up to 9µm thickness may deteriorate the LIDT to 41.62% that of the reference sample. Both the simulation and experimental results demonstrate that deposited particles with a height of more than 0.1µm should be inhibited to achieve fused silica with superior laser damage performance.

Revisiting Defect-Induced Light Field Enhancement in Optical Thin Films.

Based on a finite-difference time-domain method, we revisited the light field intensification in optical films due to defects with different geometries. It was found that defect can induce the local light intensification in optical films and the spherical defects resulted in the highest light intensification among the defect types investigated. Light intensification can increase with defect diameter and the relative refractive index between the defect and the film layer. The shallow defects tended to have the highest light intensification. Finally, the extinction coefficient of the defect had a significant effect on light intensification. Our investigations revealed that the light field intensification induced by a nano-defect is mainly attributed to the interference enhancement of incident light and diffracted or reflected light by defects when the size of the defect is in the subwavelength range.

Light Field Intensification in Optical Films Induced by Intercoupling of Defects and Organic Contamination.

Based on the finite-difference time-domain method, light field intensification in optical films due to the intercoupling of defects and organic contamination was analyzed. The results show that the intercoupling between the defect and the organic contamination droplet leads to an increase in the local electric field and the coupling effect enhances with the decreasing distance between the defect and the organic contamination droplet and the increasing diameter of the organic contamination droplet. The coupling effect of the defect and the organic contamination layer depends on not only the thickness of the organic contamination layer but also the refractive index of the organic contamination layer. With the thickness and the refractive index of the organic contamination layer increasing, the peak value of the electric field decreases. This work deepens the physical understanding of the degradation mechanism of laser-induced damage in optical thin films used in vacuum.

Etching behavior of ground fused silica and light enhancement modulated by surface/subsurface cracks

AbstractThe mechanical defects (scratches/cracks) originated from grinding processing need to be dislodged so that improving the optical performance of fused silica. In this paper, HF‐based (HF represents hydrofluoride acid) etchant was used as a subsequent fabrication for loose‐abrasive ground fused silica, and the surface morphology and characteristics of samples were evaluated, meanwhile, the electric/light field distribution around treated cracks was simulated using the finite difference time domain (FDTD) method. The results show that sufficient material removal amount blunts the cracks to be a series of parabolic‐shaped and smooth craters fused with each other, whose diameter and depth grow as a power function with etched depth. Triple subsurface defect depth should be removed so as to mitigate surface roughness to be ∼2.46 µm and promote transmission from initial 22.55%–54.02%. FDTD simulation results reveal that deep etching alleviates the light intensification factor (LIF) of craters within 1.8 as the diameter‐to‐depth ratio of craters is increased over 10. Moreover, a thickness removal model of ground fused silica involving the cracks distribution characteristics, cracks etching behavior as well as the LIF variation of craters is established, in which the supposed etched depth is determined to promote the manufacturing efficiency and laser damage resistance of the material.

Characterization of surface defects using a phase retrieval technique in a high-power laser system.

In the framework of high-power lasers, surface defects on optics can generate strong light intensification and induce damage sites on downstream optics. To evaluate this intensification during high-energy laser shots, a three-step method is proposed. First, a dedicated measurement bench is designed to measure the intensification induced by defects on a wide variety of optics, including amplifier slabs, KDP crystals, mirrors, gratings, and vacuum windows, for propagation distances up to 2000 mm. A multi-resolution single-beam multiple-intensity reconstruction phase retrieval algorithm is then used to reconstruct a model of the defect, in both amplitude and phase, from a set of intensification measurements. Finally, the impact of the modeled defect on downstream optics is evaluated with a simulation of the high-power laser system. This method is experimentally validated through a case study of damage identified on one of the Laser Mégajoule (LMJ) beams, characterized with the method presented in this paper. The long-distance impact on the LMJ beam is estimated by simulation and compared to a direct near-field measurement.

Relativistic flying laser focus by a laser-produced parabolic plasma mirror

The question of electromagnetic field intensification towards the values typical for strong field Quantum Electrodynamics is of fundamental importance. One of the most promising intensification schemes is based on the relativistic-flying mirror concept, which shows that the electromagnetic radiation reflected by the mirror will be frequency up-shifted by a factor of 4 gamma^2 (gamma: the Lorentz factor of the mirror). In laser-plasma interactions, such a mirror travels with relativistic velocities and typically has a parabolic form, which is advantageous for light intensification. Thus, a relativistic-flying parabolic mirror reflects the counter-propagating radiation in a form of focused and flying electromagnetic wave with a high frequency. The relativistic-flying motion of the laser focus makes the electric and magnetic field distributions of the focus complicated, and the mathematical expressions describing the field distributions of the focus is important. We present analytical expressions describing the field distribution formed by an ideal flying mirror having a perfect reflectance over the entire surface and wavelength range. The peak field strength of an incident laser pulse with a center wavelength of lambda_0 and an effective beam radius of w_e is enhanced by a factor proportional to gamma^3 (w_e/lambda_0) in the relativistic limit. Electron-positron pair production is investigated in the context of invariant fields based on the enhanced electromagnetic field. The pair production rate under the relativistic-flying laser focus is modified by the Lorentz gamma-factor and the beam radius-wavelength ratio (w_e/lambda_0). We show that the electron-positron pairs can be created by colliding two counter-propagating relativistic-flying laser focuses in vacuum, each of which is formed when a 180 TW laser pulse is reflected by a relativistic-flying parabolic mirror with a gamma = 12.2.

Application of light diffraction theory to qualify the downstream light field modulation property of mitigated KDP crystals

Micro-milling can effectively remove laser damage sites on a KDP (potassium dihydrogen phosphate) surface and then improve the laser damage resistance of the components. However, the repaired KDP interfaces could cause light propagating turbulence and downstream light intensification with the potential risk to damage downstream optics. In order to analyze the downstream light field modulation caused by Gaussian mitigation pits on KDP crystals, a computational model of the downstream light diffraction based on the angular spectrum theory and the Gaussian repair contour is established. The results show that the phase offset caused by the repaired surface produces a large light field modulation near the rear KDP surface. The modulation generated in the whole downstream light field is greater than that caused by the amplitude change. Therefore, the phase characteristics of the outgoing light could be suggested as a vital research topic for future research on the downstream light field modulation caused by mitigation contours. Significantly, the experimental results on the downstream light intensity distribution have good agreement with the simulation ones, which proves the validity of the established downstream light diffraction model. The phase characterization of the outgoing light is proposed as an evaluation tool in the repair of KDP crystals. The developed analytical method and numerical discrete algorithm could be also applicable in qualifying the repair quality of other optical components applied in high-power laser systems.

Intensification and location of relativistic laser pulses against pointing fluctuations with micro-tube plasma lenses

We analyze, via an off-axis incident model and three-dimensional particle-in-cell simulations, the influence of beam pointing fluctuations (BPFs) on the propagation properties of relativistic laser pulses in micro-tubes. It is observed that the in-tube laser intensity can be further amplified in the BPFs-induced off-axis incident case. But the intensification factor exhibits strong polarization-dependence. When the laser pulse is linearly polarized in the off-axis incident plane (p-polarization), more electrons may be dragged out from about a half of the tube inner surface in each across section than in the on-axis incident case, enhancing the effects of relativistic nonlinearity and channel focusing. The area for generating more electrons is reduced by one half in the s-polarization case, resulting in less efficient light intensification. The BPFs-induced off-axis incident also leads to pulse shortening. Moreover, light confinement in the tube core is evident and the laser pulse tends to be coaxial with the tube.

Model Development for Nanosecond Laser-Induced Damage Caused by Manufacturing-Induced Defects on Potassium Dihydrogen Phosphate Crystals.

Nanosecond laser-induced damage on (potassium dihydrogen phosphate) KDP crystals is a complex process, which involves coupled actions of multi-physics fields. However, the mechanisms governing the laser damage behaviors have not been fully understood and there have been no available models to accurately describe this complex process. In this work, based on the theories of electromagnetic, thermodynamic, and hydrodynamic fields, a coupled multi-physics model is developed to describe the transient behavior of laser-supported energy deposition and diffusion accompanied by the surface defect (e.g., surface cracks)-initiated laser damage process. It is found that the light intensification caused by the defects near the crystal surface plays a significant role in triggering the laser-induced damage, and a large amount of energy is quickly deposited via the light intensity-activated nonlinear excitation. Using the developed model, the maximum temperature of the crystal material irradiated by a 3 ns pulse laser is calculated, which agrees well with previously reported experimental results. Furthermore, the modeling results suggest that physical processes such as material melting, boiling, and flowing have effects on the evolution of the laser damage process. In addition, the experimentally measured morphology of laser damage sites exhibits damage features of boiling cores, molten regions, and fracture zones, which are direct evidence of bowl-shaped high-temperature expansion predicted by the model. These results well validate that the proposed coupled multi-physics model is competent to describe the dynamic behaviors of laser damage, which can serve as a powerful tool to understand the general mechanisms of laser interactions with KDP optical crystals in the presence of different defects.

Potential damage threats to downstream optics caused by Gaussian mitigation pits on rear KDP surface

Abstract To determine whether a potassium dihydrogen phosphate (KDP) surface mitigated by micro-milling would potentially threaten downstream optics, we calculated the light-field modulation based on angular spectrum diffraction theory, and performed a laser damage test on downstream fused silica. The results showed that the downstream light intensification caused by a Gaussian mitigation pit of 800 μm width and 10 μm depth reached a peak value near the KDP rear surface, decreased sharply afterward, and eventually kept stable with the increase in downstream distance. The solved peak value of light intensification exceeded 6 in a range 8–19 mm downstream from the KDP rear surface, which is the most dangerous for downstream optics. Laser damage sites were then induced on the fused silica surface in subsequent laser damage tests. When the distance downstream was greater than 44 mm with a downstream light intensification of less than 3, there were no potential damage threats to downstream optics. The study proves that a mitigated KDP surface can cause laser damage to downstream optical components, to which attention should be paid in an actual application. Through this work, we find that the current manufacturing process and the mitigation index still need to be improved. The research methods and calculation models are also of great reference significance for related studies like optics mitigation and laser damage.

Characterization of manufacturing-induced surface scratches and their effect on laser damage resistance performance of diamond fly-cut KDP crystal

Manufacturing-induced defects have drawn more and more attentions to improve the laser damage resistance performance of KDP crystal applied in high-power laser systems. Here, the morphology of surface scratches on diamond fly-cut KDP crystal is characterized and their effect on the laser damage resistance is theoretically and experimentally investigated. The results indicate that surface scratches could lower laser-induced damage threshold (LIDT) by modulating incident lasers and producing resultant local light intensifications. The induced maximum light intensity enhancement factors (LIEFs) are dependent on scratch shapes and dimensions. The diffraction effects originating from scratch edges are responsible for the strongest light intensification. Even for ultra-precision finished KDP surface with scratches that well satisfy the currently applied scratch/dig specification, the induced LIEFs are quite high, indicating that the actual defect dimension allowance should be amended and specified according to the defect-induced LIEFs. The effect of scratches on laser damage resistance is experimentally verified by the tested LIDT, which is approximately consistent with the simulation one. The morphologies of laser damage sites further confirm the role of scratches in lowering LIDT. This work could offer new perspective and guidance for fully evaluating the performance of ultra-precision manufactured optical materials applied in high-power laser facilities.

Modeling of residual tool mark formation and its influence on the optical performance of KH2PO4 optics repaired by micro-milling

In the micro-milling repair process of surface defects on KH2PO4 (KDP) optics, residual tool marks are inevitably introduced on repaired surfaces. These tool marks could present great potential risks in lowering laser-induced damage thresholds of repaired KDP optics, which has not been reported in the previous works. In this work, the formation process of micro-milled tool marks was initially modeled and then the effect of these residual marks on the optical performance of KDP optics was investigated theoretically and experimentally. A 3D surface generation model for repair contours is proposed to predict the profiles of residual tool marks on micro ball-end milled surfaces, and then a numerical FEM model adopting the calculated profiles of residual tool marks is established to investigate the light intensification inside KDP optics based on electromagnetic theory. The relationship between repair machining processes, tool marks profiles and light intensifications as well as laser damage resistance of repaired KDP optics, has been discussed comprehensively and systematically. Practical repair experiments and laser damage tests verified the theoretical results very well. It revealed that the residual height of tool marks can induce apparent diffraction effect, consequently causing severe light intensification inside KDP optics. The micro milling strategies (e.g. a combination of layer-milling and spiral milling paths) can also exert a positive role in improving the laser damage resistance of KDP optics. An optimized repair process flow, adopting layer-milling path as rough milling and employing spiral-milling with path intervals of 10∼15 µm as fine milling, is recommended for the future practical engineering repair of full-aperture KDP optics in ICF facilities.

Resistance of Scratched Fused Silica Surface to UV Laser Induced Damage

Scratches in fused silica are notorious laser damage precursors to UV laser damage initiation. Ductile and brittle scratches were intentionally generated using various polishing slurries. The distribution, profile and the dimension of scratches were characterized. The damage resistance of polished surfaces was evaluated using raster scanning damage testing protocol. The results show that both ductile and brittle scratches greatly increase area proportion of laser damage about one to two orders of magnitude relative to unscratched surface and brittle scratches are more deleterious. Moreover, finite difference time domain (FDTD) simulation was used to numerically calculate the light field distribution around scratches on rear surface (i.e. exit surface for light) which indicates that modulated light intensity is susceptible to the profile and size of scratches. FDTD simulation results also indicate that the light field intensification is elevated with the dimension of scratches and light modulation effects in triangular scratches are usually not as notable as serrated and parabolic scratches.

Incident laser modulation by tool marks on micro-milled KDP crystal surface: Numerical simulation and experimental verification

Micro-milling has been accepted as the most promising method to repair the micro-defects on the surface of KH2PO4 (KDP) optics. However, surface tool marks are inevitably introduced during the micro-milling repairing process, and could possess great potential risks in lowering the laser-induced damage threshold of KDP optics. The primary cause of laser damage growth of nonlinear crystals has been considered as its internal light intensification. In this work, how the tool marks impact the incident laser modulation as well as the laser-induced damage resistance of micro-milled KDP optics was theoretically and experimentally investigated. The results indicate that periodic tool marks can cause diffraction effect and result in significant relative light intensity modulation (IRmax), up to 5.6 times higher than that inside smooth crystal surfaces. Although the change trends of IRmax with respect to tool marks on both surfaces of KDP optics are similar, the IRmax induced by the rear-surface tool marks is nearly twice higher than that induced by the front-surface tool marks, which means the rear surface with tool marks are more vulnerable to be damaged. The period of tool marks determines the modulation degree and distribution patterns of light intensity inside KDP crystal while the residual height of tool marks can only slightly regulate the modulation degree of light intensity. The tool marks with a period of 1 μm normally give rise to serious light intensification and should be strictly excluded, while the period of tool marks from 10 μm to 20 μm is conducive to the laser damage resistance of micro-milled KDP optics, which were verified by the tests of transmittance capacity and laser damage resistance, and is supposed to be preferred in the actual repairing process of full-aperture KDP optics.

Extending Nighttime Combustion Source Detection Limits with Short Wavelength VIIRS Data

The Visible Infrared Imaging Radiometer Suite (VIIRS) collects low light imaging data at night in five spectral bands. The best known of these is the day/night band (DNB) which uses light intensification for imaging of moonlit clouds in the visible and near-infrared (VNIR). The other four low light imaging bands are in the NIR and short-wave infrared (SWIR), designed for daytime imaging, which continue to collect data at night. VIIRS nightfire (VNF) tests each nighttime pixel for the presence of sub-pixel IR emitters across six spectral bands with two bands each in three spectral ranges: NIR, SWIR, and MWIR. In pixels with detection in two or more bands, Planck curve fitting leads to the calculation of temperature, source area, and radiant heat using physical laws. An analysis of January 2018 global VNF found that inclusion of the NIR and SWIR channels results in a doubling of the VNF pixels with temperature fits over the detection numbers involving the MWIR. The addition of the short wavelength channels extends detection limits to smaller source areas across a broad range of temperatures. The VIIRS DNB has even lower detection limits for combustion sources, reaching 0.001 m2 at 1800 K, a typical temperature for a natural gas flare. Comparison of VNF tallies and DNB fire detections in a 2015 study area in India found the DNB had 15 times more detections than VNF. The primary VNF error sources are false detections from high energy particle detections (HEPD) in space and radiance saturation on some of the most intense events. The HEPD false detections are largely eliminated in the VNF output by requiring multiband detections for the calculation of temperature and source size. Radiance saturation occurs in about 1% of the VNF detections and occurs primarily in the M12 spectral band. Inclusion of the radiances affected by saturation results in temperature and source area calculation errors. Saturation is addressed by identifying the presence of saturation and excluding those radiances from the Planck curve fitting. The extremely low detection limits for the DNB indicates that a DNB fire detection algorithm could reveal vast numbers of combustion sources that are undetectable in longer wavelength VIIRS data. The caveats with the DNB combustion source detection capability is that it should be restricted to pixels that are outside the zone of known VIIRS detected electric lighting.

Effect of surface scallop tool marks generated in micro-milling repairing process on the optical performance of potassium dihydrogen phosphate crystal

Micro-milling is the prevailing method to repair laser-induced surface damage on potassium dihydrogen phosphate crystal components applied in high-power laser systems. Scallop tool marks are inevitably generated on repaired surfaces due to the interaction of neighboring tool paths involved in the micro-milling process. In this work, the effect of scallop tool marks on the optical performance of micro-milled crystal surfaces is theoretically and experimentally investigated. The results indicate that surface tool marks could be categorized into four bands having different period lengths (L) according to the levels of induced light intensification: high-frequency (0.1 μm ≤ L ≤ 0.7 μm), high-risk (0.7 μm ≤ L ≤ 1.2 μm), intermediate-frequency (1.2 μm ≤ L ≤ 100 μm), and low-frequency (100 μm ≤ L ≤ 1000 μm) bands. Tool marks in high-risk band play dominant role in degrading the repairing effectiveness that they should be strictly excluded. While for intermediate-frequency band marks, the induced light intensification is low, and the laser damage resistance and transmittance capacity are comparable to those of tool mark-free surface. The experimental results agree well with simulation results, which suggest that machining parameters corresponding to tool marks in intermediate-frequency band should be preferred in the actual micro-milling repairing process.

Effect of brittle scratches on transmission of optical glass and its induced light intensification during the chemical etching

Chemical etching experiments are carried out to expose and change the morphology of multiple scratches and single scratches in a set of K9 glasses, during which the transmission of some glasses is measured. The results show that brittle scratches have an influence on the transmission, and the transmission decreases at first and then increases obviously with the increase of etching time for the glass with a high density of cracks. The scattered field distribution around trailing indent crack(s) and the transmission of the glass with a scratch etched deeply (adjacent cracks have coalesced with each other) are simulated under different parameters. Using finite-difference time-domain (FDTD) algorithm, the light intensification effect around two single-evolving scratches and two adjacent cracks with different parameters are simulated, respectively. The results indicate that the position with the maximum light intensity changes for the glass with a scratch and its light intensity enhancement factors (LIEFs) will generally decrease as the etching process progresses. Compared with the morphology evolution of a scratch during the etching process, the changing mechanisms of transmission and LIEFs are revealed. This work will contribute to the improvement of service performance of optical glass by chemical etching process.

Optimization of morphological parameters for mitigation pits on rear KDP surface: experiments and numerical modeling.

In high power laser systems, precision micro-machining is an effective method to mitigate the laser-induced surface damage growth on potassium dihydrogen phosphate (KDP) crystal. Repaired surfaces with smooth spherical and Gaussian contours can alleviate the light field modulation caused by damage site. To obtain the optimal repairing structure parameters, finite element method (FEM) models for simulating the light intensification caused by the mitigation pits on rear KDP surface were established. The light intensity modulation of these repairing profiles was compared by changing the structure parameters. The results indicate the modulation is mainly caused by the mutual interference between the reflected and incident lights on the rear surface. Owing to the total reflection, the light intensity enhancement factors (LIEFs) of the spherical and Gaussian mitigation pits sharply increase when the width-depth ratios are near 5.28 and 3.88, respectively. To achieve the optimal mitigation effect, the width-depth ratios greater than 5.3 and 4.3 should be applied to the spherical and Gaussian repaired contours. Particularly, for the cases of width-depth ratios greater than 5.3, the spherical repaired contour is preferred to achieve lower light intensification. The laser damage test shows that when the width-depth ratios are larger than 5.3, the spherical repaired contour presents higher laser damage resistance than that of Gaussian repaired contour, which agrees well with the simulation results.