Polymorphic beams and Nature inspired circuits for optical current

Tailoring nonuniform local orbital angular momentum density

As is well known, a light beam with a helical phase carries an optical orbital angular momentum (OAM), which can cause the orbital motion of trapped microparticles around the beam axis. Usually, the speed of the orbital motion is uniform along the azimuthal direction and depends on the amount of OAM and the light intensity. Here, we present the reverse customized method to tailor the nonuniform local OAM density along the azimuthal direction of the focal field, which has a hybrid polarization distribution and maintains a doughnut-shaped intensity profile. Theoretical analysis and experimental results about the orbital motion of the trapped polystyrene sphere show that the nonuniform local OAM density can be tailored by manipulating the polarization states of the focal field. Our results provide an ingenious way to control the local tangential optical force and the speed of the orbital motion of particles driven by the local OAM density and will promote exciting possibilities for exploring ways to control the mechanical dynamics of microparticles in optical trapping and microfluidics.

Composite Diffraction-Free Beam Formation Based on Iteratively Calculated Primitives

To form a diffraction-free beam with a complex structure, we propose to use a set of primitives calculated iteratively for the ring spatial spectrum. We also optimized the complex transmission function of the diffractive optical elements (DOEs), which form some primitive diffraction-free distributions (for example, a square or/and a triangle). The superposition of such DOEs supplemented with deflecting phases (a multi-order optical element) provides to generate a diffraction-free beam with a more complex transverse intensity distribution corresponding to the composition of these primitives. The proposed approach has two advantages. The first is the rapid (for the first few iterations) achievements of an acceptable error in the calculation of an optical element that forms a primitive distribution compared to a complex one. The second advantage is the convenience of reconfiguration. Since a complex distribution is assembled from primitive parts, it can be reconfigured quickly or dynamically by using a spatial light modulator (SLM) by moving and rotating these components. Numerical results were confirmed experimentally.

Cycloid-structured optical tweezers

We designed novel cycloid-structured optical tweezers based on a modified cycloid and holographic shaping techniques. The optical tweezers realize all the dynamic characteristics of the trapped particles, including start, stop, and variable-velocity motions along versatile trajectories. The superiority of the tweezers is experimentally verified using polystyrene micro-sphere manipulation. This work provides a novel platform for more complex manipulations of particles.

18F-fluorodeoxyglucose (18F-FDG) Functionalized Gold Nanoparticles (GNPs) for Plasmonic Photothermal Ablation of Cancer: A Review

The meeting and merging between innovative nanotechnological systems, such as nanoparticles, and the persistent need to outperform diagnostic-therapeutic approaches to fighting cancer are revolutionizing the medical research scenario, leading us into the world of nanomedicine. Photothermal therapy (PTT) is a non-invasive thermo-ablative treatment in which cellular hyperthermia is generated through the interaction of near-infrared light with light-to-heat converter entities, such as gold nanoparticles (GNPs). GNPs have great potential to improve recovery time, cure complexity, and time spent on the treatment of specific types of cancer. The development of gold nanostructures for photothermal efficacy and target selectivity ensures effective and deep tissue-penetrating PTT with fewer worries about adverse effects from nonspecific distributions. Regardless of the thriving research recorded in the last decade regarding the multiple biomedical applications of nanoparticles and, in particular, their conjugation with drugs, few works have been completed regarding the possibility of combining GNPs with the cancer-targeted pharmaceutical fluorodeoxyglucose (FDG). This review aims to provide an actual scenario on the application of functionalized GNP-mediated PTT for cancer ablation purposes, regarding the opportunity given by the 18F-fluorodeoxyglucose (18F-FDG) functionalization.

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.

Tailored optical propulsion forces for controlled transport of resonant gold nanoparticles and associated thermal convective fluid flows

Noble metal nanoparticles illuminated at their plasmonic resonance wavelength turn into heat nanosources. This phenomenon has prompted the development of numerous applications in science and technology. Simultaneous optical manipulation of such resonant nanoparticles could certainly extend the functionality and potential applications of optothermal tools. In this article, we experimentally demonstrate optical transport of single and multiple resonant nanoparticles (colloidal gold spheres of radius 200 nm) directed by tailored transverse phase-gradient forces propelling them around a 2D optical trap. We show how the phase-gradient force can be designed to efficiently change the speed of the nanoparticles. We have found that multiple hot nanoparticles assemble in the form of a quasi-stable group whose motion around the laser trap is also controlled by such optical propulsion forces. This assembly experiences a significant increase in the local temperature, which creates an optothermal convective fluid flow dragging tracer particles into the assembly. Thus, the created assembly is a moving heat source controlled by the propulsion force, enabling indirect control of fluid flows as a micro-optofluidic tool. The existence of these flows, probably caused by the temperature-induced Marangoni effect at the liquid water/superheated water interface, is confirmed by tracking free tracer particles migrating towards the assembly. We propose a straightforward method to control the assembly size, and therefore its temperature, by using a nonuniform optical propelling force that induces the splitting or merging of the group of nanoparticles. We envision further development of microscale optofluidic tools based on these achievements.

Optical Sorting at the Single-Particle Level with Single-Nanometer Precision Using Coordinated Intensity and Phase Gradient Forces

Position-controlled sorting of colloidal nanoparticles (NPs) at the single-particle level is a challenge in nanoscience. Optofluidic potential wells can partially address this challenge, but they have limited flexibility, reconfigurability, and precision. Here we introduce a strategy by feedback-controlled manipulation of NPs using reconfigurable optical traps with designed intensity and phase gradient. Spatiotemporal patterns of these optical traps coordinatively manipulate the NPs based on machine vision of their positions and differentiated scattering intensities. The NPs are always kept inside the optical field during the manipulation and stably trapped once the sorting is accomplished. To substantiate the key advantages of our approach, we present position-controlled optical sorting of single Ag and Au NPs of the same size (150 nm diameter) and ordering of monodisperse Au NPs (80 ± 9 nm diameter) according to their sub-10 nm radius variation, which can hardly be done via other approaches.

Rapidly and accurately shaping the intensity and phase of light for optical nano-manipulation

Holographic optical tweezers can be applied to manipulate microscopic particles in various optical patterns, which classical optical tweezers cannot do. This ability relies on accurate computer-generated holography (CGH), yet most CGH techniques can only shape the intensity profiles while the phase distributions remain poor. Here, we introduce a new method for fast generation of holograms that allows for accurately shaping both the intensity and phase distributions of light. The method uses a discrete inverse Fourier transform formula to directly calculate a hologram in one step, in which a random phase factor is introduced into the formula to enable complete control of intensity and phase. Various optical patterns can be created, as demonstrated by the experimentally measured intensity and phase profiles projected from the holograms. The high-quality shaping of intensity and phase of light provides new opportunities for optical trapping and manipulation, such as controllable transportation of nanoparticles in optical trap networks with variable phase profiles.

Measuring laser beam quality, wavefronts, and lens aberrations using ptychography

We report on an approach for quantitative characterization of laser beam quality, wavefronts, and lens aberrations using ptychography with a near-infrared supercontinuum laser. Ptychography is shown to offer a powerful alternative for both beam propagation ratio M² and wavefront measurements compared with existing techniques. In addition, ptychography is used to recover the transmission function of a microlens array for aberration analysis. The results demonstrate ptychography's flexibility in wavefront metrology and optical shop testing.

Programmable optical transport of particles in knot circuits and networks

A freestyle single-beam laser trap allows for multi-particle optical transport along arbitrary open or closed trajectories with independent control of the all-optical confinement and propulsion forces exerted over the particles. Here, exploiting this manipulation tool, we propose and experimentally demonstrate an optical dynamic routing technique to assist multi-particle transport in knot circuits and networks exhibiting multiple crossing paths. This new functionality for optical transport enables the particle circulation in such complex systems handling traffic jams and making possible particle separation/mixing in them. It is important for the development of programmable particle delivery and other automated optical transport operations of interest in colloidal physics, optofluidics, biophysics, etc.

Characterization and generation of high-power multi-axis vortex beams by using off-axis pumped degenerate cavities with external astigmatic mode converter

The generalized geometric mode with several high-order Hermite-Gaussian (HG) beams localized on ray periodic orbits in the degenerate resonator is generated by an off-axis pumped Nd:YVO₄ laser, by performing beam transformation via an astigmatic mode converter, the generalized geometric modes are found that can be converted into the multi-axis vortex beams with the bundled-rings structures. Experimental results reveal that the generated multi-axis vortex beams can preserve quite stable beam structures even under high-power operation. Moreover, the radius of the bundled rings for the multi-axis vortex beams can be flexibly adjusted by the off-axis pumping to lead to vortex structures with easily controlled orbital angular momentum distribution. The good agreement between the experimental and theoretical results of propagation evolution for the astigmatic transformation of generalized geometric modes further verify the feasibility of using the proposed system to realize various high-powered, multi-center vortex beams with good reliability and predictability.

Dynamic morphing of 3D curved laser traps for all-optical manipulation of particles

The development of optical manipulation techniques focused on the confinement and transport of micro/nano-particles has attracted increased interest in the last decades. In particular the combination of all-optical confinement and propelling forces, respectively arising from high intensity and phase gradients of a strongly focused laser beam, is promising for optical transport. The recently developed freestyle laser trap exploits this manipulation mechanism to achieve optical transport along arbitrary 3D curves. In practice, reconfigurable 3D optical transport of numerous particles is a challenging problem because it requires the ability to easily adapt the trajectory in real time. In this work, we introduce and experimentally demonstrate a strategy for on-task adaptive design of freestyle laser traps based on a dynamic morphing technique. This provides programmable smooth transformation of the 3D shape of the curved laser trap with independent control of the propelling forces along it, that can be configured according to the considered application. Dynamic morphing, proven here on the example of colloidal dielectric micro-particles, significantly simplifies the important problem of real-time reconfigurable 3D optical transport and opens up routes for other sophisticated optical manipulation tasks.

Vector polymorphic beam

A scalar polymorphic beam is designed with independent control of its intensity and phase along a strongly focused laser curve of arbitrary shape. This kind of beam has been found crucial in the creation of freestyle laser traps able to confine and drive the motion of micro/nano-particles along reconfigurable 3D trajectories in real time. Here, we present and experimentally prove the concept of vector polymorphic beam adding the benefit of independent design of the light polarization along arbitrary curves. In particular, we consider polarization shaped tangential and orthogonal to the curve that are of high interest in optical manipulation and laser micromachining. The vector polymorphic beam is described by a surprisingly simple closed-form expression and can be easily generated by using a computer generated hologram.

Fast label-free microscopy technique for 3D dynamic quantitative imaging of living cells

The refractive index (RI) is an important optical characteristic that is often exploited in label-free microscopy for analysis of biological objects. A technique for 3D RI reconstruction of living cells has to be fast enough to capture the cell dynamics and preferably needs to be compatible with standard wide-field microscopes. To solve this challenging problem, we present a technique that provides fast measurement and processing of data required for real-time 3D visualization of the object RI. Specifically, the 3D RI is reconstructed from the measurement of bright-field intensity images, axially scanned by a high-speed focus tunable lens mounted in front of a sCMOS camera, by using a direct deconvolution approach designed for partially coherent light microscopy in the non-paraxial regime. Both the measurement system and the partially coherent illumination, that provides optical sectioning and speckle-noise suppression, enable compatibility with wide-field microscopes resulting in a competitive and affordable alternative to the current holographic laser microscopes. Our experimental demonstrations show video-rate 3D RI visualization of living bacteria both freely swimming and optically manipulated by using freestyle laser traps allowing for their trapping and transport along 3D trajectories. These results prove that is possible to conduct simultaneous 4D label-free quantitative imaging and optical manipulation of living cells, which is promising for the study of the cell biophysics and biology.

Fast optoelectric printing of plasmonic nanoparticles into tailored circuits

Plasmonic nanoparticles are able to control light at nanometre-scale by coupling electromagnetic fields to the oscillations of free electrons in metals. Deposition of such nanoparticles onto substrates with tailored patterns is essential, for example, in fabricating plasmonic structures for enhanced sensing. This work presents an innovative micro-patterning technique, based on optoelectic printing, for fast and straightforward fabrication of curve-shaped circuits of plasmonic nanoparticles deposited onto a transparent electrode often used in optoelectronics, liquid crystal displays, touch screens, etc. We experimentally demonstrate that this kind of plasmonic structure, printed by using silver nanoparticles of 40 nm, works as a plasmonic enhanced optical device allowing for polarized-color-tunable light scattering in the visible. These findings have potential applications in biosensing and fabrication of future optoelectronic devices combining the benefits of plasmonic sensing and the functionality of transparent electrodes.