Laser Excitation Research Articles

Enhanced Quantum Efficiency via Co-substitution in Sr2[MgAl5N7]:Eu2+ for Spectroscopic Applications including Laser Displays with Ultra-high Luminescence Saturation Threshold.

A series of Sr2[Mg1-xLixAl5-xSixN7]:0.01Eu2+ (SMAN-xLS, 0≤x≤0.5) red phosphors were devised and synthesized via the high temperature solid state reaction and the effects of the co-substitution of [Mg-Al]5+ by [Li-Si]5+ on structural and luminescence properties is investigated. With the entry of [Li-Si]5+ into theSr2[MgAl5N7]:0.01Eu2+ (SMAN) lattice, the substitution of Al3+ by Si4+ shifts the emission peak from 657 nm to 647 nm and reduces the excitation in green region. When the amount of [Li-Si]5+ co-substitution is x=0.1, the luminescence intensity and thermal stability of the sample are enhanced, in which the external quantum efficiency is elevated by 49.6%. The increase in lattice rigidity gives rise to higher luminescence intensity, and the introduced trap levels for the compensation of luminescence enhances the thermal stability. Under blue laser excitation, the SMAN-0.1LS can achieve an ultra-high luminescence saturation threshold of 52.22W/mm2, which is a breakthrough performance that has enormous potential for application in high-power laser display light sources. By measuring the pressure-dependent luminescence of SMAN-0.1LS, the emission peak can be shifted from 650 nm to 702 nm with the increase of pressure, and the sensitivity dλ/dP is 5.07 nm/GPa, which is indicative of the potential application of this system as an optical pressure sensor.

Enhanced Quantum Efficiency via Co‐substitution in Sr2[MgAl5N7]:Eu2+ for Spectroscopic Applications including Laser Displays with Ultra‐high Luminescence Saturation Threshold

A series of Sr2[Mg1‐xLixAl5‐xSixN7]:0.01Eu2+ (SMAN‐xLS, 0≤x≤0.5) red phosphors were devised and synthesized via the high temperature solid state reaction and the effects of the co‐substitution of [Mg‐Al]5+ by [Li‐Si]5+ on structural and luminescence properties is investigated. With the entry of [Li‐Si]5+ into theSr2[MgAl5N7]:0.01Eu2+ (SMAN) lattice, the substitution of Al3+ by Si4+ shifts the emission peak from 657 nm to 647 nm and reduces the excitation in green region. When the amount of [Li‐Si]5+ co‐substitution is x=0.1, the luminescence intensity and thermal stability of the sample are enhanced, in which the external quantum efficiency is elevated by 49.6%. The increase in lattice rigidity gives rise to higher luminescence intensity, and the introduced trap levels for the compensation of luminescence enhances the thermal stability. Under blue laser excitation, the SMAN‐0.1LS can achieve an ultra‐high luminescence saturation threshold of 52.22W/mm2, which is a breakthrough performance that has enormous potential for application in high‐power laser display light sources. By measuring the pressure‐dependent luminescence of SMAN‐0.1LS, the emission peak can be shifted from 650 nm to 702 nm with the increase of pressure, and the sensitivity dλ/dP is 5.07 nm/GPa, which is indicative of the potential application of this system as an optical pressure sensor.

Modeling of fluence-dependent hole-burned spectra and hole-growth kinetics using multiple two-level system model.

Numerical formalism is presented that perfectly describes resonant low-temperature hole-burned spectra (including zero-phonon holes, ZPHs) and spectral hole-growth dynamics of Al-phthalocyanine tetrasulphonate embedded in hyperquenched glassy water films over more than seven orders of fluence magnitude (0.4 µJ/cm2-5.9 J/cm2). Frequency changes during spectral hole-burning (HB) are traditionally explained with the help of a single extrinsic two-level-system (TLSext) associated with impurity molecules. The new multiple two-level system (n-TLSext) models and data analysis presented in this work show that each chromophore in an amorphous medium can couple with multiple independent TLSext, which maintain perfect photo-memory, allowing a full return of the photoproduct to the initial ("preburn") state. Modeling reveals that the experimentally observed narrow photoproduct peak at higher energies, in close vicinity of the zero-phonon hole (ZPH), reflects a dynamical feature of the HB process populating so-called "terminal" states (states that do not interact with laser excitation). Within the n-TLSext model, each chromophore possesses multiple possibilities to create a photoproduct when in interaction with the burning laser, i.e., chromophores can interact with burning laser-light multiple times until reaching the terminal states. Due to phonon-assisted absorption, terminal states are typically at higher energies than the ZPH, in agreement with the hole burned spectra reported for many molecules embedded in various amorphous solids. However, many HB systems reveal both blue- (high-energy) and red-shifted (low-energy) antiholes (i.e., photoproducts). We suggest that future modeling of resonant holes in various proteins using our n-TLSext model will provide more insight on the complexity of the protein energy landscape.

High Luminescence Saturation Y3Al5O12: Ce3+‐Diamond Composite Color Converter for High‐Brightness Laser‐Driven White Lighting

AbstractPhosphor‐converted laser lighting confronts enormous challenges in the development of color converters with high luminescence saturation owing to the thermal issues from laser irradiation and phosphor conversion. Herein, a high luminescence saturation Y3Al5O12: Ce3+‐diamond (YAG‐diamond) composite color converter by introducing diamond particles in phosphor‐in‐glass film (PiGF) coated on transparent sapphire substrate is proposed for high‐brightness laser‐driven white lighting. By optimizing the diamond content to 6 wt.%, the YAG‐diamond converter shows a thermal conductivity (TC) of 14.7 W·m−1·K−1 and displays higher luminescence intensity at 175 °C (1.15 times that at 25 °C). Under laser excitation, the YAG‐diamond converter achieves a luminous flux (LF) of 1990 lm at the laser power density (LPD) saturation threshold (ST) of 24.82 W·mm−2, which is far better than the PiGF@sapphire converter with a LF of 1364 lm@17.64 W·mm−2. The laser‐driven converter yields stable white laser light with a correlated color temperature (CCT) of 5714 K and a chromaticity coordinate of (0.3278, 0.3372) at the ST of 24.82 W·mm−2. Furthermore, the working temperature of the converter is decreased by 59 °C (26.6%) under LPD of 10.89 W·mm−2. These results indicate that the proposed YAG‐diamond composite converter displays excellent opto‐thermal performances, which may be a promising luminescence material in high‐brightness white laser lighting.

Plasmon-enhanced two photon excited emission from edges of one-dimensional plasmonic hotspots with continuous-wave laser excitation.

One-dimensional junctions between parallelly and closely arranged multiple silver nanowires (NWs) exhibit a large electromagnetic (EM) enhancement factor (FR) owing to both localized and surface plasmon resonances. Such junctions are referred to as one-dimensional (1D) hotspots (HSs). This study found that two-photon excited emissions, such as hyper-Rayleigh, hyper-Raman, and two-photon fluorescence of dye molecules, are generated at the edge of 1D HSs of NW dimers with continuous-wave near-infrared (NIR) laser excitation and propagated through 1D HSs; however, they were not generated from the centers of 1D HSs. Numerical EM calculations showed that FR of the NIR region for the edges of 1D HSs was larger than that for the centers by ∼102 times, resulting in the observation of two-photon excited emissions only from the edge of 1D HSs. The analysis of the NW dimer gap distance dependence of FR revealed that the lowest surface plasmon (SP) mode, compressed and localized at the edges of 1D HSs, was the origin of the large FR in the NIR region. The propagation of two-photon-excited emissions was supported by the higher-order coupled SP mode.

On-The-Flight trapping, LIBS analysis and discrimination of single meteorite particles generated by laser ablation

BackgroundThousands of micrometeorites fall to the Earth on a daily basis. Most of these meteorites have a rocky composition, but others are mainly composed of iron and nickel. Due to their small size, often ca. 100 μm in diameter, the process of searching for, collecting, and identifying these samples is remarkably tedious. In this work, we introduce a minimally invasive methodology for evaluating the full elemental composition of micrometeorites using optical emission spectroscopy of single particles produced by laser ablation of bulk targets. ResultsBulk meteorite samples were directly ablated within an ablation cell. From few micrograms of ablated matrix, we originated dry aerosols consisting of multielemental particles which were representative of the sample chemistry. SEM images confirmed that the generated particles exhibited spherical geometry. Particles were first optically trapped in air and, then, analyzed by laser-induced breakdown spectroscopy (LIBS). LIBS spectra evidenced compositional differences among samples. For example, Campo de Cielo meteorite featured a high iron content due to its metallic nature whereas LIBS results for Jbilet Winselwan and NWA 869 suggested that pyroxene components dominated the composition of the samples. In contrast, NWA 13739 and Vaca Muerta contained high aluminum intensity, thus indicating that the feldspathic component was dominant, as then verified by XRD. The intra-sample compositional variability were quite satisfactory, as revealed by the RSD data, below 45 %. For quantitative analysis, the percentages of FeO, SiO2, Al2O3, Na2O, MgO, TiO2, CaO, K2O, MnO, SrO, Cr2O3, and Li2O were calculated using CF-LIBS. SignificanceThis work demonstrates the applicability of laser excitation of individual particles in an optical trap for the multielemental analysis of meteorites. The methodology provides a complete overview of the samples, is capable of classificating them according to their main phases and yield preliminary quantitative information about those phases. Therefore, we present a minimally destructive pathway to be used as the first step in the inspection of these delicate samples. If the particles represent nanostructures inherent to the meteorites, it would constitute a major step in the analysis of extraterrestrial material that may provide fundamental insights into the structure and composition of the original materials at the microscale, a topic that remains an active area of research worldwide.

Stable Alkyne‐Bridged Conjugated Polymer Nanoparticles With a High NIR‐II Photothermal Conversion Efficiency of 71% for Effective Photothermal Tumor Therapy

AbstractOrganic photothermal materials (PTMs) in the near‐infrared (NIR) range (1000–1700 nm) are more favorable for photothermal therapy (PTT) therapeutic applications because of their greater depth of tissue penetration and better biosafety. However, the photothermal conversion efficiency (PCE) of the few NIR‐II‐responsive organic PTMs that have been investigated is still slightly low. Here, a stable alkyne‐bridged conjugated polymer, BBT‐BTE, are explored with a high PCE for tumor ablation within the NIR‐II window. The BBT‐BTE synthesized by the Stille coupling reaction has strong NIR‐II absorption, and their nanoparticles (NPs) achieved a 71% PCE under 1064 nm laser excitation, with good photothermal stability. BBT‐BTE NPs are shown to be effective in completely ablating tumors without any recurrence in both in vitro and in vivo experiments. Additionally, the biosafety of these NPs is further proven by biochemical analysis and tissue biopsy. This study developed a high‐performance NIR‐II PTM and offered practical and feasible insights into the design of efficient photothermal materials specifically for the NIR‐II window.

Room-temperature non-volatile optical manipulation of polar order in a charge density wave

Utilizing ultrafast light-matter interaction to manipulate electronic states of quantum materials is an emerging area of research in condensed matter physics. It has significant implications for the development of future ultrafast electronic devices. However, the ability to induce long-lasting metastable electronic states in a fully reversible manner is a long-standing challenge. Here, by using ultrafast laser excitations, we demonstrate the capability to manipulate the electronic polar states in the charge-density-wave material EuTe4 in a non-volatile manner. The process is completely reversible and is achieved at room temperature with an all-optical approach. Each induced non-volatile state brings about modifications to the electrical resistance and second harmonic generation intensity. The results point to layer-specific phase inversion dynamics by which photoexcitation mediates the stacking polar order of the system. Our findings extend the scope of non-volatile all-optical control of electronic states to ambient conditions, and highlight a distinct role of layer-dependent phase manipulation in quasi-two-dimensional systems with inherent sublayer stacking orders.

Understanding AFM-IR Signal Dependence on Sample Thickness and Laser Excitation: Experimental and Theoretical Insights.

Photothermal induced resonance (PTIR), also known as atomic force microscopy-infrared (AFM-IR), enables nanoscale IR absorption spectroscopy by transducing the local photothermal expansion and contraction of a sample with the tip of an atomic force microscope. PTIR spectra enable material identification at the nanoscale and can measure sample composition at depths >1 μm. However, implementation of quantitative, multivariate, nanoscale IR analysis requires an improved understanding of PTIR signal transduction and of the intensity dependence on sample characteristics and measurement parameters. Here, PTIR spectra measured on three-dimensional printed conical structures up to 2.5 μm tall elucidate the signal dependence on sample thickness for different IR laser repetition rates and pulse lengths. Additionally, we develop a model linking sample thermal expansion dynamics to cantilever excitation amplitudes that includes samples that do not fully thermalize between consecutive pulses. Remarkable qualitative agreement between experiments and theory demonstrates a monotonic increase in the PTIR signal intensity with thickness, with decreasing sensitivities at higher repetition rates, while signal intensity is nearly unaffected by laser pulse length. Although we observe slight deviations from linearity over the entire 2.5 μm thickness range, the signal's approximate linearity for bands of sample thicknesses up to ≈500 nm suggests that samples with comparably low topographic variations are most amenable to quantitative analysis. Importantly, we measure absorptive undistorted profiles in PTIR spectra for strongly absorbing modes, up to ≈1650 nm, and >2500 nm for other modes. These insights are foundational toward quantitative nanoscale PTIR analyses and material identification, furthering their impact across many applications.

Concentration-Dependent Photoluminescence of Carbon Quantum Dots Useable in LED.

We synthesized carbon quantum dots (CQDs) using a solvothermal method with o-phenylenediamine as the carbon and nitrogen source. The sample was characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. When we continued the optical characterization of the CQDs, we were surprised to discover that the colors of the synthesized CQDs changed with the dilution of the original solution. In addition, the photoluminescence (PL) of CQDs under 405 nm continuous wave laser excitation was also investigated. It was found that CQDs with different concentrations exhibited different PL spectra. In order to explain the mechanism of different PL spectra, chemical characterization of the CQDs at different concentrations was performed again, revealing that the color change is independent of particle size and surface functional groups. Systematic optical characterization and theoretical analysis indicate that this color change results from the interparticle distance. Furthermore, we investigated the PL lifetimes of CQDs using time-resolved PL measurements and found that the PL lifetime values change with the concentration of CQDs, which is attributed to nonradiative transitions. Finally, we fabricated warm white-light-emitting diodes with CQDs that are proportionally adjusted in concentrations. The investigation developed a simple and effective method to tune the color of CQDs by adjusting the concentration through dilution of the original solution, which provides a new approach for the preparation and regulation of multicolor CQDs.

Roles of shallow energy level defect states on amplified spontaneous emission enhancement of CH3NH3PbBr3 perovskite crystals

Abstract In this work, we demonstrated the role of shallow energy level defect states on the emission of CH3NH3PbBr3 perovskite polycrystals under laser excitation. The perovskite polycrystals were synthesized by a simple, one-step, low-cost solution self-assembled method. By adjusting the sample preparation temperature from 303 to 373 K, we could manipulate the number of shallow energy level defect states, which were evaluated through low-temperature photoluminescence measurement. This led to an evolution of the CH3NH3PbBr3 perovskite polycrystals’ emission from amplified spontaneous emission to random lasing emission. As a result, the most efficient lasing threshold of 4 μJ mm−2 was achieved with the CH3NH3PbBr3 perovskite polycrystals synthesized at the optimum temperature of 333 K. Furthermore, the surface morphologies and the crystal structure of the CH3NH3PbBr3 perovskite polycrystals were also taken into consideration to unravel the role of defects in the CH3NH3PbBr3 perovskite polycrystals.

Rod-shaped LuAG:Ce transparent ceramic enabling ultrahigh forward efficiency for laser lighting in a transmissive mode.

Ceramic phosphors have high thermal conductivity and high thermal stability, showing great potential for use in laser lighting. However, it is difficult to further improve the forward efficiency in transmissive mode because of the arbitrarily emitting ceramic phosphors and light loss by secondary optical components. Here, an effective design of rod-shaped LuAG:Ce transparent ceramics was proposed, and the silicone encapsulated ceramic-based devices could operate stably under 3.5 W laser excitation, possessing a luminous efficiency of 150-180 lm/W, far exceeding the level of existing commercial transmissive mode. Besides, because of the gradual absorption of blue light and the gradient distribution of heat, the rod-shaped LuAG:Ce transparent ceramics could bear a power density of 46 W/mm2 without luminous saturation, and the thermal-induced luminous degradation only accounted for 7% under a 15 min operation. The ceramic-based laser lighting sources with low divergence angle (∼4°) and uniform spatial distribution were obtained. Our optimized transparent ceramic rod and encapsulation scheme provided a solution to improve the efficiency of a transmissive mode for laser lighting.

Charge Transfer Plasmons Enabled by Supramolecular Plug: From Optoelectronic Switching to Enhanced Chiral Sensing.

Miniaturization and integration of plasmonic nanodevices are fundamentally limited by quantum tunneling, which leads to quantum plasmonics with reduced local E-field intensity. Despite significant efforts devoted to modeling and deterring the detrimental effect of quantum plasmonics, the modulation and application of electron transport through the subnanometer gaps seems rarely exploited due to the limited tunability of conventional quantum materials. Here, we establish a supramolecular plasmonic system made of pillar[5]arene complexes and plasmonic resonators (nanoparticle-on-mirror, NPoM). The supramolecular assemblies significantly enhance the gap conductance of NPoM, which results in a blue-shift of the coupled plasmons. Plasmonic hot-electron transport with laser excitation further modulates the gap plasmons, which are fully reversible and beneficial for enhanced chiroptic sensing. Such a conductive supramolecular plasmonic system not only suggests an optoelectronic switching strategy for charge transfer plasmons but also provides a superior sensing platform for single molecules.

Improved upconversion luminescence of NaBiF4: Tm3+/Yb3+/Al3+ as a ratio thermometer

NaBiF4: 0.5 %Tm3+/20 %Yb3+/x%Al3+ upconversion luminescence materials were synthesized by co-precipitation method. The crystal structure, upconversion luminescence properties and temperature measurement properties systematically studied. Under 980 nm laser excitation, the characteristic transitions of Tm3+ were observed, corresponding to 1G4 → 3H6 (475 nm), 1G4 → 3F4 (650 nm), 3F2, 3 → 3H6 (700 nm) and 3H4 → 3H6 (800 nm), respectively. Al3+ substitution significantly increases the upconversion luminescence intensity, and the fluorescence lifetime of 1G4 level shortens from 277.8 to 179.1 μs. An anomalous thermal enhancement behavior is observed. The optical thermometry properties of Tm3+ based on the non-thermally coupled energy levels 3F2, 3 and 3H4 have been studied using the fluorescence intensity ratio technique. Relative sensitivity and absolute sensitivity show maximum values at 316 K, which are 1.1 % K−1 and 0.21 % K−1, respectively. The above results demonstrate that this material is a promising optical ratio thermometer.

Single- and dual-tube He-Ne lasers for high-power excitation of LG01-mode vortex beam

Abstract In this study, aiming to extend the power of LG01-mode vortex light at 632.8nm, the commercial He-Ne laser was upgraded to dual-tube gain geometry with the assistance of an intracavity spot defect for spatial filtering. We investigated the impact of using single and double He-Ne gas gain tubes on the powers of vortex laser output , and achieved a maximum 4.7-mW power of LG01-mode vortex light, correspionding to an optical-optical conversion efficiency of 39.2% and making it the highest power of vortex light obtained in He-Ne laser.

High-spectral-resolution quantum Fourier-transform infrared spectroscopy with pulsed laser excitation

High-spectral-resolution quantum Fourier-transform infrared spectroscopy with pulsed laser excitation

Ultrasensitive and Adjustable Nanothermometers Based on Er3+-Sensitized Core@Shell Nanoparticles for Use in the First Biological Window.

In recent years, intensive research has focused on lanthanide-doped nanoparticles (NPs) used as noncontact temperature sensors, particularly in nanomedicine. These NPs must be capable of excitation and emission within biological windows, where biological materials usually show better transparency for radiation. In this article, we propose that NPs sensitized with Er3+ ions can be applied as temperature sensors in biological materials. We synthesized the NPs through a reaction in high-boiling solvents and confirmed their crystal structure and the formation of core@shell NPs by using X-ray diffraction, high-resolution transmission electron microscopy, and element distribution mapping within the NPs. NaErF4@NaYF4, NaYF4:12.5% Er3+, 2.5% Tm3+@NaYF4, NaYF4:7.5% Er3+@NaYF4, and NaYF4:12.5% Er3+, 2.5% Ho3+@NaYF4 exhibited intense upconversion (UC) emission under 1532 nm laser excitation detectable also in the whole human blood. We propose that this UC results from energy transfer between Er3+ ions and from Er3+ to Tm3+ or Ho3+ codopants. To determine the mechanism of UC, we measured the dependence of the emission band intensities on the laser power densities. Importantly, we also analyzed the temperature-dependent emission of the NPs within the 295-360 K range. Based on the collected emission spectra, we calculated the luminescence intensity ratios (LIRs) of the emission bands to assess their potential for optical temperature sensing. The temperature-sensing properties varied with the concentration of Er3+ ions and the presence of additional Tm3+ or Ho3+ codopants. Depending on the NP composition and the emission bands used for luminescence ratio calculations, the maximum relative temperature sensitivity ranged from 4.55%·K-1 to 1.12%·K-1, with temperature resolution between 0.05 and 2.53 K at room temperature. Finally, as proof of using NPs as temperature sensors in biomedicine, we successfully measured the temperature-dependent emission of NaYF4:7.5% Er3+@NaYF4 NPs dispersed in whole blood under 1532 nm excitation. We demonstrated that the ratio of Er3+ ion emission bands changes with temperature, indicating that these NPs have potential applications in temperature sensing within biological environments. We also confirmed the properties of NPs as temperature sensors by measuring the temperature reading uncertainty and the repeatability of the LIR readings during heating-cooling cycles, thereby confirming the excellent properties of the studied systems as temperature sensors.

Reactive oxygen species-mediated organic long-persistent luminophores light up biomedicine: from two-component separated nano-systems to integrated uni-luminophores.

Organic luminophores have been widely utilized in cells and in vivo fluorescence imaging but face extreme challenges, including a low signal-to-noise ratio (SNR) and even false signals, due to non-negligible background signals derived from real-time excitation lasers. To overcome these challenges, in the last decade, functionalized organic long-persistent luminophores have gained much attention. Such luminophores could not only overcome the biological toxicity of inorganic long-persistent luminescent materials (metabolic toxicity and leakage risk of inorganic heavy metals), but also continue to emit long-persistent luminescence after removing the excitation source, thus effectively improving imaging quality. More importantly, organic long-persistent luminophores have good structure tailorability for the construction of activable probes, which is favorable for biosensing. Recently, the development of reactive oxygen species (ROS)-mediated long-persistent (ROSLP) luminophores (especially organic small-molecule ROSLP luminophores) is still in the rising stage. Notably, ROSLP luminophores for in vivo imaging have experienced from two-component separated nano-systems to integrated uni-luminophores, which obtained gradually better designability and biocompatibility. In this review, we summarize the progress and challenges of organic long-persistent luminophores, focusing on their development history, long-persistent luminescence working mechanisms, and biomedical applications. We hope that these insights will help scientists further develop functionalized organic long-persistent luminophores for the biomedical field.

Fourier Ptychographic Coherent Anti-Stokes Raman Scattering Microscopy with Point-Scanning for Super-Resolution Imaging.

Fourier ptychography (FP) is a high resolution wide-field imaging method based on the extended aperture in the Fourier space, which is synthesized from raw images with varying illumination angles. If FP is extended to coherent nonlinear optical imaging, the resolution could be further improved due to the increase of the cutoff frequency of the synthesized coherent optical transfer function (C-OTF) with respect to the order of nonlinear optical processes. However, there is a fundamental conflict between wide-field FP and nonlinear optical imaging, whereby the nonlinear optical imaging typically requires a focused excitation laser beam with high power density. To tackle the problem, in this work, a unique point-scanning FP (PS-FP) method is presented for super-resolution nonlinear optical imaging, in which the nonlinear optical signal is obtained by using focused laser beam, while the conventional FP algorithm can still be used to retrieve the super-resolution image. PS-FP coherent anti-Stokes Raman scattering (PS-FP-CARS) imaging on a variety of samples, where a 1.8-fold expansion of the OTF is achieved experimentally for enhancing vibrational imaging. Further theoretical calculation shows that the C-OTF of PS-FP higher-order CARS (PS-FP-HO-CARS) can be expanded up to ≈4.9-fold, thereby improving the spatial resolution by ≈3-fold in comparison with conventional point-scanning CARS with under tightly focused beams. The generality of PS-FP method developed in this work can be adapted to other coherent nonlinear optical imaging modalities for super-resolution imaging in tissue and cells.

Multi-mode high resolution TFM imaging of microdefects based on laser ultrasonic full matrix capture

The identification and visualization of submillimeter microdefects have long been a critical focus within the realm of ultrasonic testing. The utilization of high-frequency signals is imperative due to the small acoustic interaction structure. An additional benefit of using lasers is their non-contact nature and ability to utilize multiple imaging modes, effectively reducing false or missed detections and enhancing credibility. This study proposes an advanced optimization algorithm for imaging and quantitative evaluation of submillimeter microdefects using laser ultrasonic full matrix capture (FMC) and total focusing method (TFM). The analysis of acoustic directionality under laser excitation and detection provides a foundation for mode selection. In preprocessing, the multi-scale principal component analysis (MSPCA) method is introduced to suppress coherent noise and blind spots. Moreover, phase coherence methods are implemented to further reduce mode artifacts, background noise, and residual surface acoustic wave (SAW). Additionally, multi-mode imaging and mode fusion are achieved using reflected longitudinal waves, reflected shear waves, converted longitudinal waves, and converted shear waves. Experiments were carried out on two different types of microdefects, namely side drilled holes (SDHs) and blind holes (BHs), resulting in high resolution TFM imaging with a minimum BH size of 0.3 mm. The outcomes demonstrate the efficacy of MSPCA in suppressing coherent noise, with further enhancement in imaging quality after phase coherence weighting. In addition, the −6 dB threshold of multi-mode images also provided the size information. This research is poised to contribute to the resolution of quality assessment challenges in powder metallurgy, welding, and additive manufacturing processes.