Focused Laser Light Research Articles

Experimental results using large area picosecond photo-detectors

We present experimental results obtained with the LAPPD Gen-II large-size MCP-PMT. Using a custom-designed PCB capacitively coupled to the anode backplane with different segmentation patterns, the photo-detector is read out using the PETsys TOFPET2 and FastIC ASICs. The results reported in this paper include characterization using focused laser light and temporal response to picosecond illumination at a single photon level.

On the Brownian motion of a colloid trapped in optical tweezers: Experiments and simulations

The trapping potential induced by the interaction of a highly focused laser light with a spherical dielectric particle can be accurately approximated by a parabolic potential. In this work, we revisit experimental and numerical methodologies used to characterize the Brownian motion of a colloidal particle under the influence of a simple harmonic potential produced by optical tweezers. A classic Brownian dynamics simulation is used to model the experimental results, focusing on statistical properties that can be measured by direct visualization of the system using videomicroscopy. This work represents a useful insight into the underlying physics behind the optical tweezers technique, also giving guidelines regarding programming protocols and experimental analysis methodologies, that may be of help for students working with such techniques, as well as for professors teaching undergraduate advanced optics courses.

Spatial extension of the transient gain drop in a microchannel plate for a single-pulse irradiation

It is widely recognized that a microchannel plate gain drops temporarily after a particle initiates an electron avalanche and that a large amount of charge is extracted from the channel. Electron multiplication is expected to deplete the charges from the microchannel and produce depleted charges (wall charges), and this gain drop is known to persist until charges are replenished. In addition, it was reported that a gain drop occurs not only in the activated channels, where the electrons are multiplied, but also in the surrounding channels. A proposed mechanism for this phenomenon is suggested that the wall charges in the activated channels change the electric field in the surrounding channels, which affects the electron multiplications. In this study, the spatial extent of the gain drop of a chevron microchannel plate detector was evaluated by measuring gains as a function of distance from an activated channel. The experiment used two ultraviolet pulses with different beam sizes, which initially activated one channel and the six neighboring channels with carefully focused laser light, and then applied another light pulse to the surrounding 900 μm × 30 μm area. The gain of each channel was measured using a charge-coupled device camera, and the change in gain as a function of the output charge for the laser pulse was analyzed. A gain drop of 20% was observed at a channel located 110 μm far from the initial activated channel when the output charge was 4 pC. The obtained relationship between the radius of gain drop area and the output charge for the laser pulse was in good agreement with that predicted from the transverse electric field due to the wall charges by computational analysis.

Portable contactless caliper.

This work presents a portable optical meter for noncontact thickness measurement. The device shines a focused laser light on a thin and transparent sample, resulting in an interference between light reflecting from the top and from the bottom surface, and the interfering pattern is recorded by a linear sensor array before data analysis with an Arduino microcontroller. The device produced accurate thickness values from glass cover slips and transparent plastic sheets within a fraction of a second per measurement. Additionally, the sample's refractive index is not required a priori. Therefore, it has a high potential to be of use in real-time quality control in transparent thick-film coating and manufacturing.

High-speed feedback based wavefront shaping for spatiotemporal enhancement of incoherent light through dynamic scattering media.

Feedback-based wavefront shaping is a promising and versatile technique for enhancing the contrast of a target signal for both coherent and incoherent light through a highly scattering medium. However, this technique can fail for a dynamic sample with a short correlation time. So far, most proposed methods for high-speed wavefront shaping can only directly enhance the intensity of coherent light but not incoherent light. Here we try to fill this gap to directly enhance incoherent light with high speed, such as fluorescence, which is essential in extending wavefront shaping to biomedical applications. For this purpose, we develop a technique based on a single acousto-optic deflector (AOD) with field-programmable gate array (FPGA) acceleration for spatiotemporal focusing within milliseconds. With the digital time gating of the feedback signal, spatiotemporal focusing of laser light with high contrast can be formed behind dynamic scattering media in milliseconds resulting in fluorescence enhancement. Furthermore, FPGA-based wavefront shaping is shown to effectively enhance fluorescence directly behind dynamic samples with short correlation times.

Quantitative active super-resolution thermal imaging: The melanoma case study.

Super-resolution image acquisition has turned photo-activated far-infrared thermal imaging into a promising tool for the characterization of biological tissues. By the sub-diffraction localization of sparse temperature increments primed by the sample absorption of modulated focused laser light, the distribution of (endogenous or exogenous) photo-thermal biomarkers can be reconstructed at tunable ∼10-50 μm resolution. We focus here on the theoretical modeling of laser-primed temperature variations and provide the guidelines to convert super-resolved temperature-based images into quantitative maps of the absolute molar concentration of photo-thermal probes. We start from camera-based temperature detection via Stefan-Boltzmann's law, and elucidate the interplay of the camera point-spread-function and pixelated sensor size with the excitation beam waist in defining the amplitude of the measured temperature variations. This can be accomplished by the numerical solution of the three-dimensional heat equation in the presence of modulated laser illumination on the sample, which is characterized in terms of thermal diffusivity, conductivity, thickness, and concentration of photo-thermal species. We apply our data-analysis protocol to murine B16 melanoma biopsies, where melanin is mapped and quantified in label-free configuration at sub-diffraction 40 µm resolution. Our results, validated by an unsupervised machine-learning analysis of hematoxylin-and-eosin images of the same sections, suggest potential impact of super-resolved thermography in complementing standard histopathological analyses of melanocytic lesions.

Tunable Spin Wave Propagation in YIG/Fe-Rh Stripe

Here, we present the results of an experimental and numerical study of controllable spin-wave propagation in the yttrium iron garnet stripe with an Fe-Rh slab. The transformation of spin-wave dispersion and transmission are observed by the focusing laser light on top of the Fe-Rh slab. It was shown that the geometry variation and design of the Fe-Rh slab significantly affect spin-wave propagation. The static internal magnetic field profile in the yttrium iron garnet is subject to position and the geometric parameters of the Fe-Rh slab. Additionally, the possibility of width mode filtering was shown for a surface magnetostatic spin wave by the means of micromagnetic simulation. The qualitative correspondence between the behavior of spin waves in the numerical simulation and microwave spectroscopy data was observed. The proposed structure can be used as a functional unit with the simultaneous frequency selective and spin-wave mode filtration regime in planar magnonic networks.

Fusion experiments broke records this year

Scientists this year took two big steps on what has been a decades-long, slow march to fusion power. Researchers in California used lasers to trigger a record-breaking fusion reaction, while a team in Massachusetts demonstrated a powerful magnet that could one day be incorporated into an electricity-generating fusion reactor. Whether these and other recent advances lead to fusion power remains to be seen. Even if they do, viable fusion reactors won’t be built for a decade or more. But people involved in both these projects describe a range of chemical and materials science developments that resulted in these breakthroughs, including nearly flawless lenses that focus laser light and superconducting tape coiled inside a giant magnet. The researchers emphasize that at this point, more than half a century after the first investigations of fusion energy, some of the hardest aspects of their work come not in fundamental physics research but in

Numerical simulation of a plasmonic lens for laser light focusing

In this paper, the design of a plasmonic lens in gold and silver thin films for focusing the light with radial polarization is presented. Using the finite difference time domain method the optimal parameters of the plasmonic lens design are found. It was shown that the silver plasmonic lens produces a tight focal spot with a full width at half maximum of 0.38 of the incident light wavelength.

An automated platform for assembling light-powered hydrogel microrobots and their subsequent chemical binding

This paper presents light powered hydrogel microrobots (100μm), that are directed to specific locations in their environment by an automated platform. The microrobots are actuated by focused laser light and crawl in aqueous environments by periodic volumetric changes of a section of their bodies. The platform consists of a stage, manipulated by stepper drivers and controlled by a Raspberry PI 4. This positions the laser light in the desired locations to move microrobots towards a goal location. The microrobots are localized via a microscope camera and repetitive usage of an algorithm based on Hough Gradient Method. The optimal position for the laser is chosen before every step so that the disk reaches the goal as fast as possible. Multiple disks are moved to form a formation of predefined geometry. An algorithm for finding the optimum sequence of disk movements to suitable positions is introduced. Subsequently, the disks are bound together chemically, using local UV illumination as the binding trigger. The bound formation can perform useful tasks, such as pushing and depositing a cargo at a target location.

Tight focusing of laser light using microlens array combined with radially polarised light converter

ABSTRACT A facile method to prepare a liquid crystal polarisation conversion microlens array (LCPC MLA) is reported. One-step photolithography was used to form a microhole array patterned fluoropolymer poly (butenyl vinyl ether) (CYTOP) layer on the front and back side of a thin glass substrate. On the front side, LC droplets are radially aligned in the microholes, and a twisted radial arrangement texture is formed by the integration of another glass substrate coated with a parallel alignment layer to create the LCPC array. The MLA is located at the rear side of the thin glass substrate and each microlens centre is completely aligned with the centres of the LC droplets in the LCPC array. The LCPC array can convert linearly polarised light to radially polarised light and the MLA can focus the radially polarised light to a spot array. This tightly focused array has a high intensity and at each focused spot, a large component of the electric field is in the longitudinal direction. The LCPC MLA has a compact structure, good stability, and highly uniformity, and it shows potential for applications such as material processing, phase modulation, polarisation compensation, and polarisation imaging.

Implementation of one-qubit quantum gates with individual addressing of two rubidium atoms in two optical dipole traps

We report the results of experiments on implementing individually addressable one-qubit quantum gates on a microwave transition with two 87Rb atoms in two optical dipole traps. Addressing is carried out using additional focused laser light, which results in a differential light shift of the microwave transition frequency. In the absence of addressing in each of the atoms, Rabi oscillations are obtained on the microwave clock transition 5S1/2 (F = 2, mF = 0) → 5S1/2(F = 1, mF = 0) between two working levels of qubits with a frequency of up to 5.1 kHz, a contrast up to 98 %, and a coherence time up to 4 ms. When addressing is turned on, the probability of a microwave transition in the addressed atom is suppressed to an average value of less than 5 %. The Rabi oscillations remaining in the other atom have the same contrast and correspond to the implementation of individually addressable basic one-qubit quantum operations (Hadamard gate and NOT gate) from different initial states of a qubit with an average fidelity of 92% ± 3 %. After renormalising this fidelity to the error in the preparation and measurement of quantum states of qubits, an estimate of 97% ± 3% is obtained for the fidelity of individual qubit rotations.

Simulation of laser light focusing with two-layer dielectric microcyl-inders

Focusing of a linearly polarized laser beam of wavelength 633 nm with two-layer dielectric microcylinders of a circular cross-section and 2-um diameter was simulated using a finite-difference time-domain (FDTD) method, implemented using the FullWAVE software. It was shown that using a cladding whose refractive index (1.8 or 1.9) is higher than that of the core (1.45), it is possible to increase the depth of focus by a factor of 2.57 multiplied by the incident wavelength and shift the focal spot position along the optical axis away from the microcylinder boundary. It was also shown that parameters of the microcylinder could be chosen in such a way that a tighter focal spot was generated, with its full width at half maximum of intensity being 2.27 of the incident wavelength. The intensity at this focus was shown to be 1.4 times higher than that at the focus generated with a homogeneous microcylinder.

Spiral Caustics of Vortex Beams

We discuss the nonparaxial focusing of laser light into a three-dimensional (3D) spiral distribution. For calculating the tangential and normal components of the electromagnetic field on a preset curved surface we propose an asymptotic method, using which we derive equations for calculating stationary points and asymptotic relations for the electromagnetic field components in the form of one-dimensional (1D) integrals over a radial component. The results obtained through the asymptotic approach and the direct calculation of the Kirchhoff integral are identical. For a particular case of focusing into a ring, an analytical relation for stationary points is derived. Based on the electromagnetic theory, we design and numerically model the performance of diffractive optical elements (DOEs) to generate field distributions shaped as two-dimensional (2D) and 3D light spirals with the variable angular momentum. We reveal that under certain conditions, there is an effect of splitting the longitudinal electromagnetic field component. Experimental results obtained with the use of a spatial light modulator are in good agreement with the modeling results.

Viscoelastic Relaxation of the Nuclear Envelope Does Not Cause the Collapse of the Spindle After Ablation in S. pombe.

A large molecular machine called the mitotic spindle is responsible for accurate chromosome segregation in eukaryotic cells. The spindle consists of protein filaments known as microtubules and microtubule-associated proteins such as motors and crosslinkers, which help impart its organization. In the case of the fission yeast S. pombe, these form a single bundle inside the nucleus. During spindle elongation, sliding by motor proteins provides an internal source of extensile forces, which are resisted by the compressive forces of the nuclear envelope. To probe the sources of this force balance, we cut the spindle using focused laser light at various stages of spindle elongation. We find that the spindle pole bodies collapse toward each other post-ablation. While this basic behavior has been previously observed, many questions remain about the timing, mechanics, and molecular requirements of this phenomenon. Here, we quantify the time scale of the relaxation and probe its underlying mechanism. We demonstrate that viscoelastic relaxation of the nuclear envelope cannot explain this phenomenon and provide evidence of active forces as the underlying mechanism.

AO DIVER: Development of a three-dimensional adaptive optics system to advance the depth limits of multiphoton imaging

Multiphoton microscopy (MPM) can non-invasively measure the dynamic biochemical properties deep in scattering biological samples and has the potential to accelerate clinical research with advances in deep tissue imaging. However, in most samples, the imaging depth of MPM is limited to fractions of a millimeter due to blurring caused by refractive index mismatching throughout tissue and background fluorescence, overshadowing the signal in conventional MPM. To overcome these challenges, we developed a novel 3D adaptive optics (AO) system that uses an interpolated network of endogenous guide stars to focus laser light more efficiently into highly scattering samples. The synergistic combination of our AO system with DIVER detection technology enables millimeter-scale imaging with diffraction-limited resolution with optimization times between 15 s and 65 s. We characterized the algorithm and wavefront interpolation performance in a flat 2D sample and in 3D using fluorescent beads embedded in gels of various optical heterogeneity. We also tested the system in biological tissue, improving image brightness by a factor of 5 at depths of ∼0.4 mm in the fresh green fluorescent protein-tagged mouse skin and ∼2 mm in a formalin-fixed yellow fluorescent protein-tagged mouse brain. By collecting forward and back-scattered fluorescence light to optimize the excitation wavefront, AO DIVER allows imaging of the tissue architecture at depths that are inaccessible to conventional multiphoton microscopes.

An analysis of surface breakup induced by laser-generated cavitation bubbles in a turbulent liquid jet

The breakup of turbulent liquid jets by cavitation bubbles was investigated by artificially introducing them by focusing laser light into the jet. The induced surface deformations and ejected liquid structures were characterized using shadowgraphy with a high-speed video camera. The flow velocity of the liquid jets, which were ejected from a 6 mm nozzle, was varied by adjusting the injection pressure from 1 to 5 bar. Deionized water and a dipropylene glycol–water mixture were used to compare the breakup of liquid jets with different surface tension and viscosity. Surface deformation and breakup were found to occur in two stages. One was early breakup of liquid strings into tiny droplets. This was followed by the formation of a larger structure separating into ligaments and larger drops. Averaged time-resolved one-dimensional plots were introduced and implemented to analyze breakup statistically, to address the problem of shot-to-shot variations in the breakup due to the turbulent condition of the jets. Bubble-induced breakup could easily be distinguished from spontaneous breakup with this method. Both the position of bubble formation and the injection pressure had an influence on the scale of the breakup. The deformation of the jet surface was highly affected by shear. The structure of the deformation became less intact when the surface tension was lower. The sizes of the drops produced during the second stage of breakup were analyzed. The bubble-induced breakup produced smaller drops than the spontaneous breakup at lower injection pressure. As expected, lower surface tension favored droplet detachment and smaller sized drops.Graphic abstract

Orbital Energy and Spin Flows in a Strong Focus of Laser Light

In this work, we derive analytical relations for all projections of the orbital energy flow and spin flow in the focal plane of a strongly focused left-hand circularly polarized Gaussian beam, also calculating a torque exerted upon a dielectric ellipsoidal microparticle in the focus of the beam and discussing results of an experiment on rotating the microparticle around its center of mass. The on-axis projection of the torque exerted upon the microparticle is found to be negative, i.e., directed oppositely to the beam propagation axis. Such a torque is supposed to induce a clockwise rotation of a non-absorbing dielectric particle around its center of mass. We have been demonstrated that a reverse energy flow occurs in the light field regions where the longitudinal projection of the spin flow is negative and larger in magnitude than the longitudinal projection of the orbital energy flow, which is always positive.

Spin-orbit and orbit-spin conversion in the sharp focus of laser light: Theory and experiment

Based on the Richards-Wolf theory, it is strictly shown that in the sharp focus of a linearly polarized laser beam, the flux of a spin vector has only transverse components (the effect of photonic wheels or a photonic helicopter). For a linearly polarized optical vortex, the orbit-spin conversion leads to the appearance of both longitudinal and transverse components of the spin angular momentum (SAM) vector in the focus. We show that in the strong focus of a circularly polarized Gaussian beam, the longitudinal component of the SAM is maximal on the optical axis, with the longitudinal component of the orbital angular momentum (OAM) being maximal on a ring. In this way, the effect of SAM and OAM on the motion of a trapped microparticle can be evaluated separately. Spin-orbit conversion is experimentally demonstrated for a circularly polarized Gaussian beam when transverse energy flux (orbital angular momentum) arises in the focus, which is transmitted to a microparticle and causes it to rotate. Switching the handedness of circular polarization (from left to right) switches the direction of microparticle rotation. It is also shown here that an azimuthally polarized vortex beam with an arbitrary integer topological charge generates in the focus a SAM vector with only an axial component (pure magnetization), whereas there is no transverse spin flux.

Correcting anisotropic intensity in light sheet images using dehazing and image morphology.

Light-sheet fluorescence microscopy (LSFM) provides access to multi-dimensional and multi-scale in vivo imaging of animal models with highly coherent volumetric reconstruction of the tissue morphology, via a focused laser light sheet. The orthogonal illumination and detection LSFM pathways account for minimal photobleaching and deep tissue optical sectioning through different perspective views. Although rotation of the sample and deep tissue scanning constitutes major advantages of LSFM, images may suffer from intrinsic problems within the modality, such as light mismatch of refractive indices between the sample and mounting media and varying quantum efficiency across different depths. To overcome these challenges, we hereby introduce an illumination correction technique integrated with depth detail amelioration to achieve symmetric contrast in large field-of-view images acquired using a low power objective lens. Due to an increase in angular dispersion of emitted light flux with the depth, we combined the dehazing algorithm with morphological operations to enhance poorly separated overlapping structures with subdued intensity. The proposed method was tested on different LSFM modalities to illustrate its applicability on correcting anisotropic illumination affecting the volumetric reconstruction of the fluorescently tagged region of interest.