Intense Laser Illumination Research Articles

Photovoltaic high-throughput microfluidic platform for the mass-production of hybrid viscous microdroplets towards diverse applications

Scientists have been pursuing a compact, electrodeless microfluidic platform with a simple structure to realize the efficient cascading manipulations of serial aqueous microdroplets. In this study, we present a photovoltaic microfluidic platform to handle viscous Sodium-Alginate-doped aqueous microdroplets dispersed in a continuous surfactant-doped oil phase. Three kinds of optical response (Non-coalescence, Successful coalescence, and Bouncing back) are observed between adjacent microdroplets. Both the surfactant and SA concentrations are employed to optimize the laser-illumination intensity range of successful coalescence. We realized, that by simply applying focused-laser illumination, the sequential cascading of microfluidic manipulations including trapping, coalescence, mixing, and release of viscous aqueous microdroplets could be controlled in a reconfigurable, high-throughput manner. By properly adjusting the laser-illumination intensity, we can control the number of the microdroplets participating in the coalescence as well as the degree of the liquid mixing inside coalescent microdroplets, thus realizing the mass-production of hybrid viscous aqueous microdroplets. With this platform, we prepare Janus microparticles possessing magnetic and phosphorescent properties for assembling a magnetically driven displayer. Moreover, bioreactors are prepared with both immobilized yeast cells for effective fermentation and embedded magnetic nanoparticles for convenient recovery. Additionally, we generate, on this platform, chains of fluorescent double-emulsion microdroplets for optically encoding the information of the target microdroplet. The photovoltaic MHTE platform, with a simple chip design and good integrability with other LN-based photonic components, allows for an electrodeless, biocompatible manipulation of serial aqueous microdroplets with minimal temperature fluctuation, showing great potential in diverse fields including optics, MEMS, biotechnology and chemical sciences.

Time-resolved study of laser-induced phase separation in CsPb(IxBr1−x)3 perovskite under high pressure

Mixed-halide perovskites have attracted extensive interest with their broadly tunable bandgap and optoelectronic properties, which makes them ideal candidates for various solar cells and light-emitting diodes. However, the mixed-halide perovskites often encounter phase separation and degradation under light illumination, which prevents them from many optoelectronic applications. The study on the underlying mechanism and the controlling of the phase separation is crucial to improve the tailored properties for practical applications. Here, we report our systematical investigations of the time-resolved photoluminescence shift and crystal structure evolution on the nanocrystalline CsPb(IxBr1−x)3 perovskite under intense laser illumination and found the increasing contribution of bromine-rich phase over time. Furthermore, we subject the nanocrystalline CsPb(IxBr1−x)3 perovskite to a quasi-hydrostatic pressure environment and observed that the phase separation slows down quickly with increasing pressure and can be totally suppressed at a rather mild pressure below 0.2 GPa. These findings suggest the light-induced separation of the crystalline structure, and their optoelectronic properties can be largely suppressed, which provides a useful approach to overcome the problem caused by the intense light applications.

Efficient Surfactant-Mediated Photovoltaic Manipulation of Fl-Scale Aqueous Microdroplets for Diverse Optofluidic Applications on LiNbO3 Platform.

The electrodeless biocompatible manipulation of femtoliter-scale aqueous microdroplets remains challenging to date. The appropriate isolation of electrostatic charges from fL-scale aqueous microdroplets is crucial for electrodeless optoelectronic manipulation based on space-charge-density modulation. Here, we propose surfactant-mediated photovoltaic manipulation, in which the surfactant layers self-assembled at the water-oil and oil-LN interfaces are employed to isolate photovoltaic charges. The reduced electrostatic attenuation, remarkable hydrophobicity, and strong electrical breakdown suppression of the surfactant layers enable the stable and swift manipulation of fL-scale aqueous microdroplets using μW-level laser in an oil medium. By virtue of the surfactant-mediated photovoltaic manipulation, we realized a controllable merging/touching/detaching switch of aqueous microdroplets by adjusting the laser illumination intensity and position and demonstrated the cascading biochemical operations and microreactions of aqueous microdroplets and microdroplet strings. To demonstrate its potential in photonic MEMS assemblies, we demonstrated the end coupling of a tightly focused-laser-beam into a ZnO microrod leveraging the refraction effect occurring at the water/oil interface. Moreover, because of the selective permeability of the droplet-interface-bilayer developed between the touching microdroplets, we achieved in situ adjustment of the size of the microdroplets and the fluorescent solute contained in the microdroplets, aiming at constructing multicomponent fluorescent microdroplets with tunable whispering-gallery-mode (WGM) characteristics. This article is protected by copyright. All rights reserved.

Thermal management of tungsten by spectrally selective thin film optical filters under laser illumination

Surface temperature management of refractory metals, such as Tungsten, is crucial for their use in high-temperature applications like aerospace, defense, and laser instruments. In this study, distributed Bragg reflectors (DBR) were utilized to reduce the temperature caused by intense laser illumination. To address this problem, we developed a coupled optical and thermal model and investigated multiple factors, including material composition, morphology, architecture, and source-related parameters, such as wavelength. Our results indicate that our DBR coating designs can significantly decrease the temperature of the Tungsten.

Giant optomechanical spring effect in plasmonic nano- and picocavities probed by surface-enhanced Raman scattering

Molecular vibrations couple to visible light only weakly, have small mutual interactions, and hence are often ignored for non-linear optics. Here we show the extreme confinement provided by plasmonic nano- and pico-cavities can sufficiently enhance optomechanical coupling so that intense laser illumination drastically softens the molecular bonds. This optomechanical pumping regime produces strong distortions of the Raman vibrational spectrum related to giant vibrational frequency shifts from an optical spring effect which is hundred-fold larger than in traditional cavities. The theoretical simulations accounting for the multimodal nanocavity response and near-field-induced collective phonon interactions are consistent with the experimentally-observed non-linear behavior exhibited in the Raman spectra of nanoparticle-on-mirror constructs illuminated by ultrafast laser pulses. Further, we show indications that plasmonic picocavities allow us to access the optical spring effect in single molecules with continuous illumination. Driving the collective phonon in the nanocavity paves the way to control reversible bond softening, as well as irreversible chemistry.

Novel controlling pathway for metallic nanoparticles by laser assisted ion-reduction process

In this work, the morphological and plasmonic features of the AgNPs which formed by ion-reduction process was carried out extensively. The application of the laser beam during the ion- reduction process has significant effect in the reconstruction of the formed AgNPs with small dimensions and non-frequent morphologies, according to the laser illumination intensity. For non-illumination process the deposited form of the AgNPs appear aggregated into cluster of layer AgNPs size due to the chemical reaction at Si interface, the AgNPs sizes varied from 0.85 to1.2 µm; while at lower laser intensity of about 250 mW/cm2 the AgNPs sizes varied from 0.1 to 1.0 µm, while at high intensity upto 400 mW/cm2 the AgNPs sizes varied from 0.05 to 0.4 µm. The hot spot dimension for non-illumination process varied from 1 to 11 nm while at low intensity of 250 mW/cm2 the hot spot dimension varied from 1to 8 nm. At high intensity upto 400 mW/cm2 , the hot spot varied from 0.1 to 14 nm. The XRD for the generated Ag nanoparticles / Si nanocrystallites, for non- illumination the grain size about 6.171 nm and SSD about 92.687 m2 /g while at low intensity of 250 mW/cm2 the grain size about 4.759nm and SSD about 120.191 m2 /g. At high intensity of 350 mW/cm2 , the grain size about 2.037nm and SSD about 280.847m2 /g uniform distributed AgNPs with minimum hot spot regions can be realized with 350mW/cm2 laser illumination intensity. This process is considerable as a novel work which can be adopted modification at the plasmonic features of metallic nanoparticles for SERs application.

Laser Cooling Assisted Thermal Management of Lightsails

A lightsail can be accelerated to ultra-high speed by the radiation pressure of a laser having an intensity of the order of GW/m$^2$, which though presents a critical challenge in the thermal management of lightsails. In this letter, we explore the applicable regimes of solid-state laser cooling in dissipating heat in additional to the previously explored radiative cooling approach. We begin by examining the cooling capacity of laser cooling, and show that the cooling rate from a micron-thick layer doped with ytterbium ions can exceed that of blackbody thermal emission. This allows more intense laser illumination upon material damage, and consequently shortened acceleration distance. Next, we explore the impact of the limited operating bandwidth of laser cooling to account for the Doppler shift of the pumping laser, and conclude that laser cooling is helpful for target velocities $\lesssim5\%$ for room-temperature operations.

Towards biochemical microreactor: Nonlocal photovoltaic actuation of aqueous microdroplets in oil-infused PDMS channels based on LiNbO3: Fe crystal

Biocompatible, pump-free microfluidic actuation is a challenging task for optically integrated lab-on-chips. Herein, photovoltaic actuation of aqueous microdroplets is demonstrated inside the oil-infused PDMS channels fabricated on a c-cut LiNbO3:Fe (LN:Fe) substrate. This photovoltaic actuation is based on the repulsive response of aqueous microdroplets to laser illumination, and its “nonlocal” feature can avoid the direct laser illumination on aqueous microdroplets containing fragile biochemical targets. Both the direct electrostatic force acting on the aqueous microdroplet and the repulsive force induced by the compressed convection flow of infused oil are found contributing to this “nonlocal” feature. Due to the transition of electrostatic action from the dielectrophoretic (DEP) to electrophoretic (EP) mechanism, the maximal velocity of photovoltaic actuation in the straight-line channel exhibits a pronounced threshold effect on the laser illumination intensity. Thanking to the strong EP action, the aqueous microdroplet can be actuated to squeeze through a narrow pass in the right-angle channel. Precipitation and chromogenic chemical microreactions as well as mortal reactions of a single paramecium using the photovoltaic actuation are demonstrated for the future on-chip biochemical applications.

Hydrophobic-substrate based water-microdroplet manipulation through the long-range photovoltaic interaction from a distant LiNbO3:Fe crystal.

Development of photovoltaic water-microdroplet manipulation using LN:Fe crystals has to meet the requirement of the hybrid and heating-avoided design of biological lab-on-chips. To fulfill this, we demonstrate a successful manipulation of a water microdroplet on a hydrophobic substrate by utilizing the long-range photovoltaic interaction from a distant LN:Fe crystal (see Visualization 1). The maximal manipulation distance (MMD) is found to be dependent on the laser-illumination intensity at the LN:Fe crystal and it can be tuned up to a sub-centimeter level (∼4 mm). Basing on the two-center model of light-induced charge transport in the LN:Fe crystal, we establish an analytic model to describe the force balance during the microdroplet manipulation under a long-range photovoltaic interaction. Either shortening the manipulation distance or increasing the illumination intensity can enhance the photovoltaic interaction and increase the velocity of the microdroplet being manipulated. An abrupt shape change followed by a fast repelling movement of the water microdroplet is observed under a strong photovoltaic interaction (see Visualization 2).

Optimizing the performance of extended-gate field-effect transistor (EGFET) pH sensor by regulating the structural properties of the nanostructured porous silicon layer

We report the performance of extended-gate field-effect transistor (EGFET) pH sensors based on the properties of nanostructured porous silicon layer (nano PSi). Four different forms of nano PSi layers with different characteristic features are used as a sensor for determining the pH level. The synthesized nano PSi layer was prepared by laser-assisted etching of n-type (100) by using different illumination intensities 15, 30, 45 and 60 mW/cm2 of 530 nm laser wavelength. The structural and morphological properties of nano PSi were investigated through exploring scanning electron microscopy, X-ray diffraction (XRD) and gravimetrical measurements. Nano PSi with pore-like structures of various pore shapes and sizes are as follows: fine circular pores, star full pore, circular pores and rectangular pores; star full pore shape was prepared through varying the laser illumination intensity. The Fourier transform infrared spectroscopy, current–voltage measurements and specific surface area of were employed to describe the density of the hydrogenated dangling bonds (Si–Hx) of the nano PSi layer. The synthesized nano PSi pH sensors were tested and dry out in the pH range from 3 to 11 at room temperature. The performance of nano PSi pH sensors was comparatively analyzed based on the density of (Si–Hx), specific surface area, porosity and layer thickness of nano PSi. Concerning EGFET pH sensor showed noticeably enhanced pH sensitivity and linearity with increasing the density of (Si–Hx) and specific surface area. Higher sensitivity with excellent linearity was obtained for pore-like structures of circular pore shapes when the laser illumination intensity was about 45 mW/cm2.

Controlling the symmetry of inorganic ionic nanofilms with optical chirality

Manipulating symmetry environments of metal ions to control functional properties is a fundamental concept of chemistry. For example, lattice strain enables control of symmetry in solids through a change in the nuclear positions surrounding a metal centre. Light–matter interactions can also induce strain but providing dynamic symmetry control is restricted to specific materials under intense laser illumination. Here, we show how effective chemical symmetry can be tuned by creating a symmetry-breaking rotational bulk polarisation in the electronic charge distribution surrounding a metal centre, which we term a meta-crystal field. The effect arises from an interface-mediated transfer of optical spin from a chiral light beam to produce an electronic torque that replicates the effect of strain created by high pressures. Since the phenomenon does not rely on a physical rearrangement of nuclear positions, material constraints are lifted, thus providing a generic and fully reversible method of manipulating effective symmetry in solids.

Laser-processing of grinded and mechanically abraded Nb-surfaces

The effect of pulsed laser polishing on rough niobium surfaces was investigated. We created different well-defined roughness profiles with standard emery papers and subsequently remelted random surface areas with a size of about 2 × 2 mm2 with nanosecond laser pulses (wavelength of 1064 nm, pulse length of 10 ns). Pristine as well as laser-treated surfaces were investigated using optical profilometry and atomic force microscopy, and the surface topography was described by means of correlation functions. Uniformly rough and highly smooth surface geometries were achieved for fractals above and below 7 μm, respectively. Moreover, the behavior of foreign particles during the laser processing was investigated in detail. The polishing procedure was also monitored point by point by detecting electrical signals, i.e., sample charging, which resulted from the intense laser illumination. The measured electrical charges were found to be correlated with the local surface texture. Thus, regions with initially high roughness profiles and regions with extensive laser-induced defects could be directly identified from the detected electrical signals.

The effects of hydrostatic pressure and temperature on the nonlinear optical properties of shallow-donor impurities in semiconductors in a magnetic field

We present a theoretical study of the nonlinear optical properties of shallow-donor impurities in semiconductors subjected to magnetic fields, hydrostatic pressures, and intense laser illumination within the Voigt configuration. The donor energy levels and their wave functions are obtained using a combination of nonperturbative and variational methods where intense laser field effects are exactly taken into account through a laser-dressed Coulomb potential (LdCP). The combined effects of radiation and magnetic fields, hydrostatic pressures, and temperatures on the linear, third-order nonlinear, and total optical absorption coefficients (OACs) for the 1s→2p± and 2pz transitions are investigated using a compact density-matrix approach. We find that the transition energies and geometric factors can be increased or decreased by changing external fields via the LdCP or by changing hydrostatic pressures and temperatures. In this way, saturable absorption depends not only on the incident optical intensity but also on the laser field, which is more easily realized in the z-polarization direction. The peak positions and magnitudes of the linear, third-order nonlinear, and total OACs can be effectively adjusted with an appropriate choice of these external perturbations. Moreover, hydrostatic pressures and temperatures affect these OACs in an opposite way. This opens a promising route to design new and efficient impurity-based devices manipulated by external perturbations.

Efficient SnO2/CuO/porous silicon nanocomposites structure for NH3 gas sensing by incorporating CuO nanoparticles

In this paper, a porous silicon (Psi) surface modification process, including the incorporating of pure and CuO mixed SnO2 within a Psi layer, was investigated. A Psi layer with a pore-like structure prepared via laser-assisted etching process with a laser illumination intensity of (40) mW/cm2 was employed as a substrate for materialization SnO2/CuO/Psi nanocomposites for gas sensing application. Pure SnO2 and SnO2:CuO nanoparticles were synthesized via a chemical spray pyrolysis process on the Psi surface at 350 °C. The materialization of SnO2:CuO/Psi nanocomposites were recognized through Field-Emission Scanning Electron Microscopy, X-ray Diffraction, Atomic Force Microscopy, and Energy-dispersive X-ray Analysis. The results displayed that the morphological properties of SnO2:CuO nanoparticles were definitely controlled by varying the CuO contents. Different shapes (nearly cubic shape and spherical shape) with various sizes of SnO2 and SnO2:CuO within the Psi surface were achieved by increasing the CuO content. SnO2:CuO/Psi nanocomposites, plans, grain size, average surface roughness, homogeneity and specific surface area of SnO2:CuO nanoparticles were considerably influenced by CuO content. The lower grain size and higher average surface roughness and specific surface area were attained when the percentage of CuO was about 30% within the structure of 70% SnO2:30% CuO, and the performance of the NH3 gas sensor revealed a short response time with a high sensitivity of (88%).

Studies of BiCuSeO thin films for potential infrared detector application

In this work, we prepared BiCuSeO polycrystalline films by evaporating precursors following with selenization treatment. Their optical and photoelectrical properties were investigated from visible to infrared light. Optical absorption spectrum indicated that the absorption coefficient was 104–105 cm−1 from 0.4 μm to 2 μm. Photo induced voltages were clearly detected when the film surfaces were irradiated by the lasers with various wavelengths and powers. The voltage had a linear dependence with the laser illumination intensity, which indicated the main response mechanism of photo-thermal electric effect in the films. All the results suggested that the BiCuSeO film was a potential material for near infrared detector application.

Optimizing of Gold Nanoparticles on Porous Silicon Morphologies for a Sensitive Carbon Monoxide Gas Sensor Device

A set of carbon monoxide (CO) gas sensors based on porous silicon (PSi)/gold nanoparticle (AuNP) hetro structures were fabricated. Different forms of PSi surface morphologies were studied as a substrate for growth of AuNPs. Simple dipping process of PSi in hydrogen tetrachloroaurate (III) solution (HAuCl4) at fixed concentrations of 10−2 M/3.5 HF was used to synthesize AuNPs. The n-type PSi was equipped through photo-electrochemical etching process at current density value of 10 mA/cm2 under illumination condition of 530-nm wavelength and laser illumination intensity of 20 to 80 mW/cm2. Three different forms of PSi morphology, meso, macro, and double layers with pore shapes and sizes, were prepared. The structural and surface morphology properties of PSi-based substrate before and after deposition of AuNPs were investigated through studying of scanning electron microscopy (SEM), photoluminescence (PL), and X-ray diffraction (XRD). The electrical property (J-V) was carried out in primary vacuum and CO at low pressure. The results show that PSi surface morphologies strongly influenced the AuNP sizes and hence the sensor performance. It was found that decrease the AuNP sizes could be occasioned in high and fast current response.

Thermal Effect on Plasmon-induced Electron Transfer System under Intense Pulsed Laser Illumination

The effect of plasmon excitation on metal nanostructures was investigated under electrochemical potential control with intense pulsed laser illumination. The well-defined Au nano-rod structures showed a distinct thermal effect in the form of shape transformation, in the absence of the plasmon-induced electron transfer reaction. It was found that the shape transformation only occurred under the resonance condition of the localized surface plasmon mode excitation.

Nonlinear Focal Modulation Microscopy.

We demonstrate nonlinear focal modulation microscopy (NFOMM) to achieve superresolution imaging. Traditional approaches to superresolution that utilize point scanning often rely on spatially reducing the size of the emission pattern by directly narrowing (e.g., through minimizing the detection pinhole in Airyscan, Zeiss) or indirectly peeling its outer profiles [e.g., through depleting the outer emission region in stimulated emission depletion (STED) microscopy]. We show that an alternative conceptualization that focuses on maximizing the optical system's frequency shifting ability offers advantages in further improving resolution while reducing system complexity. In NFOMM, a spatial light modulator and a suitably intense laser illumination are used to implement nonlinear focal-field modulation to achieve a transverse spatial resolution of ∼60 nm (∼λ/10). We show that NFOMM is comparable with STED microscopy and suitable for fundamental biology studies, as evidenced in imaging nuclear pore complexes, tubulin and vimentin in Vero cells. Since NFOMM is readily implemented as an add-on module to a laser-scanning microscope, we anticipate wide utility of this new imaging technique.

Laser-flash-photolysis-spectroscopy: a nondestructive method?

Herein, we report the effect of the laser illumination during the diffuse-reflectance laser-flash-photolysis measurements on the morphological and optical properties of TiO2 powders. A grey-blue coloration of the TiO2 nanoparticles has been observed after intense laser illumination. This is explained by the formation of nonreactive trapped electrons accompanied by the release of oxygen atoms from the TiO2 matrix as detected by means of UV-vis and EPR spectroscopy. Moreover, in the case of the pure anatase sample a phase transition of some TiO2 nanoparticles located in the inner region from anatase to rutile occurred. It is suggested that these structural changes in TiO2 are caused by an energy and charge transfer to the TiO2 lattice.

Ultraviolet photodetectors based on MgZnO thin film grown by RF magnetron sputtering

We demonstrate the growth of high quality, single phase, wurtzite MgxZn1-xO thin films on p-type Si (111) substrate by magnetron sputtering using Mg0·3Zn0.7O as target and no buffer layer is used for the growth. The films are highly oriented along the c-axis and have nanorod-like morphology. The Mg content in MgxZn1-xO films varies in a large range (40.7–51at. %) by changing the substrate temperature from room temperature to 250°C. The MgxZn1-xO films maintain the hexagonal phase up to 150°C substrate temperature and the Mg content at this temperature is 43.7at.%. The heterostructures of MgxZn1-xO/Si are fabricated into metal-semiconductor-metal photodetectors. The signal to noise ratio (S/N) is as high as 4.3×104% at 2V bias under 325nm laser at relatively low laser illumination intensity (2.77mW), and the output photocurrent increases with the increase in the UV illumination intensity at both −10V and +10V biased voltage. The photocurrents increase with the rise in the illumination intensity. The peak responsivity is 4.6A/W at 292nm with a cutoff wavelength of 305nm and at 9V bias voltage.