Laser Excitation Source Research Articles

Impact of altering phase modulation and geometrical shape of laser beam via a phase-only spatial light modulator on laser-induced fluorescence

This study investigates the impact of modulating and shaping a laser beam via a phase-only spatial light modulator (SLM) on the intensity of the laser-induced fluorescence (LIF) signal. Different phase modulations ranging from 0 to 2π are applied to the SLM window to evaluate the effect of the modulated beam on the emitted LIF signal from extra virgin olive oil EVOO samples. The laser beam is also shaped into circular and square geometries with varying diameters to determine the optimum size for maximizing the LIF signal strength. The results reveal that the highest LIF signal intensity could be obtained after modulating the incident laser beam with phase shifts of 0 and 2π, with an improvement factor of approximately 3 for Avanti olive oil and 1.42 for Al Jouf olive oil. Furthermore, laser beam shaping into a circular aperture with a radius of 2.45 mm or a square aperture with a side length of 2.45 mm yields the highest LIF signal improvement, with enhancement factors of approximately 2.9 for Avanti olive oil and 1.42 for Al Jouf olive oil. This work demonstrates the potential of SLM-based beam shaping techniques to optimize LIF measurements by tailoring the wavefront characteristics of the excitation laser source.

Low blue-hazard white-light emission based on color-tunable triplet-triplet annihilation upconversion

The commonly used artificial light sources, such as fluorescent lamps and white light-emitting diodes, often have a high ratio of blue light emission, which poses potential blue light hazards, especially one of the main culprits leading to eye diseases. Therefore, developing novel white lighting sources with low blue-hazard is highly appreciated. In this work, an air-stable and color-tunable triplet–triplet annihilation upconversion (TTA-UC) mechanism was proposed to realize the low blue-hazard white-light emission. The proposed design was composed of three primary RGB colors from the annihilator (9,10-diphenylanthracene, DPA), the laser excitation source, and the photosensitizer (palladium (II) octaetylporphyrin, PdOEP), respectively. The introduction of oil-in-water (o/w) microemulsion can effectively block the potential oxygen-induced triplet-quenching and benefit high UC efficiency. Moreover, either raising ambient temperatures or adding isobutanol can activate the UC process to yield white-light emission. Notably, the white-light emission with a Commission Internationale de l’Eclairage (CIE) coordinate of (0.33, 0.33) as well as a low ratio of blue emission (14.2 %) was achieved at an ambient temperature of 42 °C. Therefore, the proposed air-stable TTA-UC mechanism can significantly lower the blue-hazard and provide a novel solution for applications in lighting and display.

Shape and location recovery of laser excitation sources in photoacoustic imaging using topological gradient optimization

This study focuses on source term inversion in fractional partial differential equations, specifically applied to photoacoustic imaging. This work contributes to advancing imaging techniques and provides practical insights for medical diagnostics and materials characterization. Our aim in this paper is to develop an accurate method for recovering the location and the shape of a laser excitation source from partial boundary data. To achieve this, we reformulate our inverse problem as an optimization challenge. We utilize topological sensitivity analysis to establish an asymptotic expansion of a relevant shape function. These theoretical findings form the basis for a rapid and precise detection algorithm. Additionally, we present several numerical experiments that demonstrate the effectiveness and accuracy of our proposed approach.

Deep operator networks for bioheat transfer problems with parameterized laser source functions

To numerically characterize optothermal interactions at the laser-tissue interface during tissue imaging or surgery, the bioheat transfer partial differential equation (PDE) needs to be solved with a laser excitation source function. Considering various possible experimental scenarios, the PDEs become parametric and need to be solved for several optical and laser parameters. Classical numerical solvers that are used for performing such simulations are computationally expensive and time consuming. In this work, we use the recent deep operator network (DeepONet) framework for learning the mapping between infinite dimensional function spaces of the parameterized laser source functions and the solutions of the parameterized bioheat transfer PDE. We propose a new neural network architecture, Res-DeepONet, for training a purely physics-informed DeepONet with laser heating functions comprising of Gaussian beam profiles. We also showcase the versatility and robustness of our network architecture within purely data-driven and hybrid training frameworks for applications where labeled ground truth data may be accessible. The proposed neural network architecture exhibits excellent generalization performance on unseen Gaussian source functions with 10× higher accuracy when compared to traditional feedforward network architectures. Furthermore, zero-shot performance studies highlight the exceptional extrapolation capabilities of our network with 50× higher accuracy compared to traditional DeepONet architectures at inference times that are two orders of magnitude faster than classical solvers.

Time-resolved laser-induced fluorescence spectroscopy using CW diode laser for diagnostics of argon-ion velocity distribution near AC-biased electrode.

Controlling the ion velocity in an ion sheath by applying an alternating current (AC) voltage to an electrode and/or a substrate is critical in plasma material processes. To externally control the velocity distribution of incident ions on a substrate, the application of tailored-waveform AC voltages instead of sinusoidal voltages has garnered interest in recent years. In this study, to investigate temporal changes in ion-velocity distributions, we developed a time-resolved laser-induced fluorescence spectroscopy (LIF) system using a continuous-wave diode laser as an excitation-laser source. A time-resolved LIF system entails the capture of temporally continuous and spectrally discrete LIF spectra during an AC voltage cycle. By measuring temporal changes in the LIF signal intensity at various excitation-laser wavelengths, the argon-ion velocity distribution near the electrode following the AC voltage can be characterized. The results of applying sinusoidal, triangular, and rectangular bias waveforms indicate that the LIF measurement scheme proposed herein can be used to investigate the dynamic behavior of ion-velocity distributions controlled by tailored-waveform AC voltages.

Size Dependence of Gold Nanorods for Efficient and Rapid Photothermal Therapy.

In recent years, gold nanomaterials have become a hot topic in photothermal tumor therapy due to their unique surface plasmon resonance characteristics. The effectiveness of photothermal therapy is highly dependent on the shape and size of gold nanoparticles. In this work, we investigate the photothermal therapeutic effects of four different sizes of gold nanorods (GNRs). The results show that the uptake of short GNRs with aspect ratios 3.3-3.5 by cells is higher than that of GNRs with aspect ratios 4-5.5. Using a laser with single pulse energy as low as 28 pJ laser for 20 s can induce the death of liver cancer cells co-cultured with short GNRs. Long GNRs required twice the energy to achieve the same therapeutic effect. The dual-temperature model is used to simulate the photothermal response of intracellular clusters irradiated by a laser. It is found that small GNRs are easier to compact because of their morphological characteristics, and the electromagnetic coupling between GNRs is better, which increases the internal field enhancement, resulting in higher local temperature. Compared with a single GNR, GNR clusters are less dependent on polarization and wavelength, which is more conducive to the flexible selection of excitation laser sources.

Comparison of the penetration depth in mouse brain in vivo through 3PF imaging using AIE nanoparticle labeling and THG imaging within the 1700 nm window.

3-Photon microscopy (3PM) excited at the 1700 nm window features a smaller tissue attenuation and hence a larger penetration depth in brain imaging compared with other excitation wavelengths in vivo. While the comparison of the penetration depth quantified by effective attenuation length le with other excitation wavelengths have been extensively investigated, comparison within the 1700 nm window has never been demonstrated. This is mainly due to the lack of a proper excitation laser source and characterization of the in vivo emission properties of fluorescent labels within this window. Herein, we demonstrate detailed measurements and comparison of le through the 3-photon imaging of the mouse brain in vivo, at different excitation wavelengths (1600 nm, 1700 nm, and 1800 nm). 3PF imaging and in vivo spectrum measurements were performed using AIE nanoparticle labeling. Our results show that le derived from both 3PF imaging and THG imaging is the largest at 1700 nm, indicating that it enables the deepest penetration in brain imaging in vivo.

Optical-domain modulation cancellation method for background-suppression and dual-gas detection in light-induced thermo-elastic spectroscopy

To address the issues of signal saturation and measurement range reduction caused by the background in light-induced thermoelastic spectroscopy (LITES) employing the first harmonic (1 f) modulation, an optical-domain modulation cancellation method (MOCAM) was reported in the 1f-based LITES. A balance laser source with the opposite sine modulation phase as the excitation laser source synchronously excites the quartz tuning fork (QTF) to realize background suppression. By optimizing the modulation amplitude of the balance laser and the modulation phase difference between the two laser sources, the background level was suppressed by two orders of magnitude from ∼ 7 mV to ∼ 18 μV, close to the QTF thermal noise level (∼ 1 μV). Combining the excitation laser with the balance laser enables MOCAM to overcome common mode interferences, e.g. optical path shift, laser power fluctuation, and focus position change. Furthermore, methane and water vapor (CH4/H2O) dual-gas detection based on MOCAM and time-division multiplexing method was demonstrated, and measurement range is enlarged with easy calibration due to zero background. The MOCAM-based 1f-LITES technique delivers effective background suppression and provides robustness to common mode interferences, thus establishes a feasible solution for mobile vehicle or airborne gas monitoring applications with serious environmental disturbances.

Revisiting the Raman Spectra of Carbonate Minerals

This work presents a new discussion about the vibrational properties of the carbonate ion displayed in several different environments. The microparameters introduced by cation substitution and different crystal lattices in addition to the crystal aggregation are present in the discussion. The work comments on how the Raman modes are affected by these changes by using data obtained with four different laser excitation sources. Raman spectra excited at 1064 nm are reported at 1 cm−1 resolution. New observations and approaches based on the Raman modes highlight the differences observed in the relative intensity and width of the bands. The new data contribute to the understanding of these materials and their spectra, bringing new observations based on the Raman modes. This work presents a new approach highlighting the differences observed in the relative intensity and width of the Raman bands. The results indicate some evidence of the influence of the crystal habit and/or the growth of the mineral itself on the Raman spectrum. In addition, the data show the influence of cation substitution upon Raman bandwidth and the interference of the size of the spot of the laser in the measurement.

Structural and morphological characterizations of ex-situ sol-gel derived luminescent Nd3+-Yb3+ion co-doped zinc-silicate dense glass-ceramic

We proposed a novel ex-situ sol-gel synthesis of Nd3+ (0.7 mol%) and Yb3+ (X; X = 0.0, 0.7, 1.5, 3.0 mol%) ion co-doped Zinc-Silicate (SZNYX) dense glass-ceramic(DGC) samples. The structural analyses of the SZNY07-DGC sample were performed at different annealing temperatures using X-ray diffraction (XRD) analysis and Fourier transforms infrared (FTIR) spectroscopy. The tetragonal willemite Zn2SiO4 structure with an R-3(148) space group was confirmed from XRD results. The formation of the pure oxide phases (bridging oxygen) was noticed at FTIR spectra with the complete removal of hydroxyl impurity at higher annealing temperatures. The presence of ZnO crystallites with the inter-planer spacing of ⁓ 0.28 nm and Nd3+/Yb3+ ion pair in an amorphous silica host was confirmed via transmission electron microscopy (TEM) and TEM-energy dispersive spectroscopy (EDS) respectively. The white up-conversion luminescence (UCL) under a 980 nm excitation was attributed to the simultaneous emission of wavelength in the visible spectrum. The intensity of UCL was initially rising with an increase in Yb3+ ion concentration whereas the optimized luminescence was obtained for samples with 1.5 mol% Yb3+ ion concentration. The CIE chromaticity coordinates (0.34, 0.35) of corresponding UCL spectra were invariant with the change in Yb3+ ion concentration. The photoluminescence (PL) confocal mapping of the SZNY15-DGC sample was performed at different locations under a 980 nm laser excitation source which turned out non-uniform in nature throughout the surface. Thus as-synthesized SZNYX-DGC samples have spectacular upconversion luminescence for near-infrared excitation which may be utilized in thermal sensing, photovoltaic, displays, and photonic devices.

Characterisation of the transient mixing behaviour of evaporating near-critical droplets

With technical progress, combustion pressures have been increased over the years, frequently exceeding the critical pressure of the injected fluids. For conditions beyond the critical point of the injected fluids, the fundamental physics of mixing and evaporation processes is not yet fully understood. In particular, quantitative data for validation of numerical simulations and analytical models remain sparse. In previous works, transient speed of sound studies applying laser-induced thermal acoustics (LITA) have been conducted to investigate the mixing behaviour in the wake of an evaporating droplet injected into a supercritical atmosphere. LITA is a seedless, non-intrusive measurement technique capable of direct speed of sound measurements within these mixing processes. The used setup employs a high-repetition-rate excitation laser source and, therefore, allows the acquisition of time-resolved speed of sound data. For the visualisation of the evaporation process, measurements are accompanied by direct, high-speed shadowgraphy. In the present work, the measured speed of sound data are evaluated by applying an advection-controlled mixing assumption to estimate both the local mole fraction and mixing temperature. For this purpose, planar spontaneous Raman scattering results measured under the same operating conditions are evaluated using an advection-controlled mixing assumption with the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. Successively, the resulting concentration–temperature field is used for the estimation of local mixture parameters from the detected speed of sound data. Moreover, models using the PC-SAFT equation of state and the NIST database for the computation of the speed of sound are compared. The investigations indicate a classical two-phase evaporation process with evaporative cooling of the droplet. The subsequent mixing of fluid vapour and ambient gas also remains subcritical in the direct vicinity of the droplet.

Nondestructive determination of carotenoids in kiwifruit leaves infected with Pseudomonas syringae pv. actinidiae by surface‐enhanced Raman spectroscopy combined with chemical imaging

AbstractSurface‐enhanced Raman spectroscopy (SERS), a noninvasive and nondestructive technique, was used to distinguish healthy and cankered kiwifruit leaves. Silver nanoparticles with iodide and calcium ions (Ag@ICNPs) were synthesized as SERS substrates and significantly enhanced the characteristic signal peaks of carotenoids. Due to its simple operation and high sensitivity, the detection process is straightforward and efficient. Using a 532 nm laser excitation source, spectra were collected from the leaf surface over a wavelength range of 400–2000 cm−1, and the characteristic carotenoid peak at 1520 cm−1 was selected for integral imaging. Detection of changes in carotenoid spectral characteristics on the surface of kiwifruit leaves could be used as an early warning of kiwifruit canker. Raman spectroscopy and high‐performance liquid chromatography confirmed the existence of two main carotenoids, lutein and β‐carotene, and their intensity changed significantly following infection, especially in the later stages of infection. The results of principal component analysis and support vector machine (SVM) analysis of preprocessed spectra showed that samples of healthy, early disease and advanced disease leaves clustered separately, and the accuracy of SVM classification was up to 98%. The results of chemical imaging showed that the average carotenoid signal decreased with extended incubation time and there were significant differences among healthy, early disease and advanced disease stages. Therefore, SERS can be used as a general tool for in situ nondestructive determination of surface carotenoid changes in kiwifruit leaves during disease, and thereby effectively monitor the nutritional quality of kiwifruit before harvest.

Bimetallic Ag-Cu Alloy SERS Substrates as Label-Free Biomedical Sensors: Femtomolar Detection of Anticancer Drug Mitoxantrone with Multiplexing.

Surface-enhanced Raman spectroscopy (SERS) has been recognized as a promising label-free technology for clinical monitoring due to its high sensitivity and multiplexing ability, which should accelerate the screening of important drugs in the blood and plasma of cancer patients in a simpler, faster, and less-expensive manner. In this work, bimetallic Ag-Au and Ag-Cu alloy microflowers (MFs) with tunable surface compositions were fabricated on a glass cover slip by simple thermolysis of a metal alkyl ammonium halide precursor and used as SERS substrates for the sensitive detection of anticancer drug mitoxantrone (MTO). Two different laser excitation sources, 532 and 632.8 nm, were used to explore the possibility of surface-enhanced resonance Raman scattering. The Ag-Cu substrate showed superior detection capability over Ag-Au, whereby the sensor recorded a noteworthy "limit of detection" value of 1 fM for MTO. Theoretical electromagnetic field maps were simulated on appropriately chosen plasmonic systems to compare the electromagnetic field enhancements with the experimental SERS efficiencies of the substrates. Further, using a 10% Ag-Cu substrate, efficient multiplexing detection of MTO was demonstrated with another anticancer drug doxorubicin (DOX) in water and mouse blood plasma.

First Experimental Evidence of Anti-Stokes Laser-Induced Fluorescence Emission in Microdroplets and Microfluidic Systems Driven by Low Thermal Conductivity of Fluorocarbon Carrier Oil

With the advent of many optofluidic and droplet microfluidic applications using laser-induced fluorescence (LIF), the need for a better understanding of the heating effect induced by pump laser excitation sources and good monitoring of temperature inside such confined microsystems started to emerge. We developed a broadband highly sensitive optofluidic detection system, which enabled us to show for the first time that Rhodamine-B dye molecules can exhibit standard photoluminescence as well as blue-shifted photoluminescence. We demonstrate that this phenomenon originates from the interaction between the pump laser beam and dye molecules when surrounded by the low thermal conductive fluorocarbon oil, generally used as a carrier medium in droplet microfluidics. We also show that when the temperature is increased, both Stokes and anti-Stokes fluorescence intensities remain practically constant until a temperature transition is reached, above which the fluorescence intensity starts to decrease linearly with a thermal sensitivity of about −0.4%/°C for Stokes emission or −0.2%/°C for anti-Stokes emission. For an excitation power of 3.5 mW, the temperature transition was found to be about 25 °C, whereas for a smaller excitation power (0.5 mW), the transition temperature was found to be about 36 °C.

Multi-resolution non-contact laser-sensing damage detection technique for composite laminates

Numerous ultrasonic (UT) modalities have been developed to perform inspections of engineered structures. However, increased precision often comes at the cost of longer inspection times. As an example, guided waves propagating in plate-like structures allow fast inspection of large areas due to their long propagation distances. Damage characterization accuracy is, however, limited in this case due to relatively large wavelengths. On the other hand, local inspection with high-frequency bulk waves provides much more accurate damage characterization thanks to shorter wavelengths. The drawback of local inspection is the long scanning time necessary for large components. Here, we discuss a two-step laser-interferometer-based non-contact approach for the inspection of carbon-fiber-reinforced structures. First, a full-field screening using guided waves is performed to determine the possible damage locations. The identified hot spots are further evaluated in the second step with a high-frequency bulk wave system. A piezoceramic exciter is used for the guided wave setup, while a pulsed laser excitation source is used in the bulk wave setup. The proposed approach minimizes sample preparation time and is completely non-intrusive. The feasibility of the proposed framework is demonstrated in a complex shape, thin-walled composite structure with multiple defects.

Trace photoacoustic SO2 gas sensor in SF6 utilizing a 266 nm UV laser and an acousto-optic power stabilizer.

A sulfur dioxide (SO2) gas sensor based on the photoacoustic spectroscopy technology in a sulfur hexafluoride (SF6) gas matrix was demonstrated for SF6 decomposition components monitoring in the power system. A passive Q-switching laser diode (LD) pumped all-solid-state 266 nm deep-ultraviolet laser was exploited as the laser excitation source. The photoacoustic signal amplitude is linear related to the incident optical power, whereas, a random laser power jitter is inevitable since the immature laser manufacturing technology in UV spectral region. A compact laser power stabilization system was developed for better sensor performance by adopting a photodetector, a custom-made internal closed-loop feedback controller and a Bragg acousto-optic modulator (AOM). The out-power stability of 0.04% was achieved even though the original power stability was 0.41% for ∼ 2 hours. A differential two-resonator photoacoustic cell (PAC) was designed for weak photoacoustic signal detection. The special physical constants of SF6 buffer gas induced a high-Q factor of 85. A detection limit of 140 ppbv was obtained after the optimization, which corresponds to a normalized noise equivalent absorption coefficient of 3.2 × 10-9 cm-1WHz-1/2.

Tm3+; Yb3+:Zn2TiO4 near infrared to blue upconversion phosphors for anti-counterfeit applications.

Tm3+; Yb3+:Zn2TiO4 samples have been synthesized using a solid state reaction route. The phase, lattice parameters, crystallite size has been examined using X-ray Diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). An intense peak of Yb3+ codoped samples is observed near ∼957 nm due to the 2F7/2 → 2F5/2 transition in diffuse reflectance spectra (DRS), which confirms the presence of Yb3+ ion in the prepared compound. The optical band gap of Yb3+ codoped samples has been calculated using Kubelka-Munk function. The Raman spectra corresponds to incorporation of Tm3+/Yb3+ at the octahedral and tetrahedral site of the spinel host. The emission spectra recorded by using 370 nm excitation wavelength shows intense blue colour band corresponding to the 1G4 → 3H6 transition of Tm3+ ion. The upconversion (UC) emission spectra recorded by using 980 nm laser excitation source shows emission bands due to the 1G4 → 3H6, 1G4 → 3F4 and 3H4 → 3H6 transitions of Tm3+ ion in the host matrix lying in the blue, red and NIR regions respectively. There is effective enhancement of about ∼35 times in the blue UC emission intensity with incorporation of Yb3+ at 3% doping concentration in the prepared sample. The anti-counterfeit application of the optimized upconverting phosphor has been successfully demonstrated.

Automatic classification of Candida species using Raman spectroscopy and machine learning

One of the problems that most affect hospitals is infections by pathogenic microorganisms. Rapid identification and adequate, timely treatment can avoid fatal consequences and the development of antibiotic resistance, so it is crucial to use fast, reliable, and not too laborious techniques to obtain quick results. Raman spectroscopy has proven to be a powerful tool for molecular analysis, meeting these requirements better than traditional techniques. In this work, we have used Raman spectroscopy combined with machine learning algorithms to explore the automatic identification of eleven species of the genus Candida, the most common cause of fungal infections worldwide. The Raman spectra were obtained from more than 220 different measurements of dried drops from pure cultures of each Candida species using a Raman Confocal Microscope with a 532 nm laser excitation source. After developing a spectral preprocessing methodology, a study of the quality and variability of the measured spectra at the isolate and species level, and the spectral features contributing to inter-class variations, showed the potential to discriminate between those pathogenic yeasts. Several machine learning and deep learning algorithms were trained using hyperparameter optimization techniques to find the best possible classifier for this spectral data, in terms of accuracy and lowest possible overfitting. We found that a one-dimensional Convolutional Neural Network (1-D CNN) could achieve above 80 % overall accuracy for the eleven classes spectral dataset, with good generalization capabilities.

Investigation of size-dependent spontaneous and stimulated visible WGM emissions via both ultraviolet and visible excitations for sensing applications

In this work, we investigated both spontaneous and stimulated whispering gallery mode (WGM) emissions of 2 mol. % Li+-doped ZnO (Li-ZnO) microspheres with different sizes under 325 and 488 nm wavelength laser excitations, respectively. It was found that all the microspheres exhibit stimulated emissions under a visible laser excitation source of 488 nm wavelength after the threshold pumping power. Thereafter, we studied the dependence of threshold pumping power on the size of microresonators to achieve stimulated emissions by individual microspheres. Furthermore, two microspheres (MS2 and MS3) are excited via a 325 nm UV laser, and surprisingly, the WGM peaks of higher intensity are observed in the visible rather than in the UV spectral region. We expected that most of the emissions are achieved via defect states transitions instead of inter-band transitions in the microresonators. It was found that WGMs in each microsphere exhibit a linear spectral shift of 3–5 nm with increasing pumping power of 488 nm excitation laser source. We believe that these proposed microspheres can be utilized effectively as WGM-based visible lasers and sensors.

The Wavelength-Dependent SERS Template Based on a Nanopillar Array.

Surface-enhanced Raman spectroscopy (SERS) can be regarded as a powerful tool for probing chemical molecules by effectively enhancing Raman signals. However, the enhancement factors depend on the SERS template, the probed molecular structures, and the excitation laser wavelength. Herein, we proposed a simple and easily fabricated nanostructured template for SERS and analyzed the wavelength-dependent factors. Three types of golden nanopillar arrays on silicon wafers were designed and manufactured. The SERS signals of the Rhodamine 6G (R6G) molecules were extracted. Three laser sources, a blue 17 mW 458 nm diode laser, a green 20 mW 532 nm laser, and a red 6 mW 633 nm laser, were employed as the excitation laser sources. The 458 nm laser was located far from the resonate spectrum of R6G. The optical intensity distributions for the different SERS templates excited by three laser beams were also simulated. The enhancement factors (EFs) of R6G on the three nanostructured templates were measured and compared. The photoluminescence spectrum of the nanostructured templates and SERS signals of R6G were also measured. In addition, the experimental results concerned optical simulations. The analysis tool that was used was a convolution profile of multiple Lorentzian line shapes with a Gaussian profile. It is helpful to understand the SERS signals when the excitation laser wavelength is located out of the resonance region of molecules. It can also provide a new design approach to fabricate an SERS Template with a nanopillar array for different excitation wavelengths.