This PDF file contains the front matter associated with SPIE Proceedings Volume 12198, including the Title Page, Copyright information, Table of Contents, and Conference Committee Page.

Flow cytometry (FC) is a pivotal tool for studying the physical and chemical properties of particles. State-of-the-art FC systems are highly advanced, yet they are expensive, bulky, and require high sample volume, qualified operators, and periodic maintenance. The manipulation of particles suspended in viscoelastic fluids has received increasing attention, especially for miniaturized flow cytometry technologies. This study presents a miniaturized optical capillary FC device using the viscoelastic focusing technique. A straight, one inlet/outlet microcapillary device is precisely aligned to a fiber-coupled laser source and detectors. Forward scattered, side scattered, and fluorescently emitted light signals are collected and analyzed in a real-time environment. The developed platform fits onto an inverted microscope stage enabling real-time microscopy imaging of the particles of interest together with the flow cytometry analysis. We achieved stable viscoelastic focusing and performed FC measurements for rigid polystyrene beads (diameters: 2 – 15 μm), non-spherical human erythrocytes, and canonical shape metaphase human chromosomes. We performed cytometry measurements with a throughput of 100 events/s yielding a coefficient of variation of 2%. This newly developed FC device is a versatile tool and can be operated with any inverted microscope to get the mutual benefits of optical and imaging FC measurements. Furthermore, it is possible to extend these benefits by adding more back-end tools, such as optical trapping and Raman spectroscopy.

Nanophotonic structures optimise the strength of optical forces, enabling trapping at the nanoscale. To improve the impact of nanotweezers in biological studies, it is necessary to move from individual traps to large multiplexed arrays. Here, we discuss the state-of-the-art of nanotweezers for multiplexed trapping, describing advantages and drawbacks of the configurations that have demonstrated the strongest impact in this field. Finally, we focus on our latest results with a dielectric metasurface that supports strong resonances with thousands of trapping sites. We demonstrate near-field enhancement and simulate trapping performance for 100 nm particles, verifying the possibility to trap > 1000 particles with a low total power of P < 30 mW. The multiplexed trapping with dielectric metasurfaces can open up new biological studies on viruses and vesicles.

Common atomic physics courses jump from the square and harmonic well potentials straight to the hydrogen atom. However, there is a missing link in between, the spherical well potential. Although it is included in some textbooks, the lack of an experimental backing means the problem quickly becomes mathematically complex. Here we have built an optical toy atom using the scattering of an optically levitated, evaporating water droplet. We find a greatly simplified Mie scattering spectrum composed of a series of evolving Fano resonances organized in a set of combs. The whole spectrum can be intuitively explained through an analogy to a quantum spherical well potential. This produces a model of an atom including ground and excited states, quantized angular momentum, and tunneling.

Photoacoustic spectroscopy and photothermal spectroscopy are two common methods to probe aerosol particle absorption coefficients and can be performed both on aerosol ensembles and on the single particle level. With photothermal spectroscopy typically changes in the particle’s light scattering pattern upon heating or cooling are observed with photo-diodes or cameras. In photoacoustic spectroscopy, the acoustic response to periodic light absorption is recorded e.g. with a microphone. Although both methods are closely related through their excitation process, the detection pathways are quintessentially different. In our single particle optical trapping setup, however, we observe a previously unnoticeable, unidirectional coupling between modulated Mie scattering (result of the photothermal effect) and photoacoustic spectroscopy. The coupling manifests itself via differently shaped, sudden features in the acoustic signal. Our analysis suggests a non-trivial interaction between light scattering of single, optically trapped particles and the photoacoustic signal generation based on interactions of light with the acoustic resonator’s walls. Measurements over several trapping powers and photoacoustic excitation powers support this conclusion. How the coupling manifests itself, such as shape and strength, can be conclusively explained by the structure of the particle’s momentary phase function (scattering intensity) calculated by classical Mie theory. This allows us to formulate conditions to either utilise or minimise the coupling effects in future experiments.

There is an unsettled problem in choosing the correct expressions for the local momentum density and angular momentum density of electromagnetic fields (or indeed, of any non-scalar field). If one only examines plane waves, the problem is moot, as the known possible expressions all give the same result. The momentum and angular momentum density expressions are generally obtained from the energy-momentum tensor, in turn obtained from a Lagrangian. The electrodynamic expressions obtained by the canonical procedure are not the same as the symmetric Belinfante reworking. For the interaction of matter with structured light, for example, twisted photons, this is important; there are drastically different predictions for forces and angular momenta induced on small test objects. We show situations where the two predictions can be checked, with numerical estimates of the size of the effects.

In optical experiments involving a single photon that takes alternative paths through an optical system and ultimately interferes with itself (e.g., Young’s double-slit experiment, Mach- Zehnder interferometer, Sagnac interferometer), there exist fundamental connections between the linear and angular momenta of the photon on the one hand, and the ability of an observer to determine the photon’s path through the system on the other hand. This paper examines the arguments that relate the photon momenta (through the Heisenberg uncertainty principle) to the “which path” (German: welcher Weg) question at the heart of quantum mechanics. We show that the linear momenta imparted to apertures or mirrors, or the angular momenta picked up by strategically placed wave-plates in a system, could lead to an identification of the photon’s path only at the expense of destroying the corresponding interference effects. We also describe a thought experiment involving the scattering of a circularly-polarized photon from a pair of small particles kept at a fixed distance from one another. The exchange of angular momentum between the photon and the scattering particle in this instance appears to provide the “which path” information that must, of necessity, wipe out the corresponding interference fringes, although the fringe-wipe-out mechanism does not seem to involve the uncertainty principle in any obvious way.

Self-propelled particles are found in many biological systems as well as in numerous synthetic systems where self-motile colloids and artificial swimmers have recently been realized. These active systems exhibit a variety of process not found in equilibrium systems. Most studies of active matter have been performed with smooth landscapes; however, there is an increasing amount of work on active matter coupled to ordered or disordered substrates. Due to the size scale of the active particles, suitable random, periodic, or quasiperiodic substrates could be made optically. Here we present recent results for active matter on periodic substrates and discuss some future directions. We also enumerate many of the active matter versions of nonactive systems that could be realized with periodic substrates, including an active matter glass, commensurate-incommensurate transitions, solitons, and sliding states. We show that the driven dynamics of active matter can produce directional locking on periodic substrates. Finally, we discuss the possibility of introducing a dynamical substrate in order to create active matter versions of classical time crystals.

Negatively charged nitrogen-vacancy (NV^-) centers in diamond have a plethora of potential applications in quantum systems, including sensing and computing^1-3. Photothermal heating can limit the utility of NV^- center nanodiamonds, especially under high laser irradiances^4-6. A composite of nanodiamonds with NV^- defects and ytterbium-doped cubic sodium yttrium fluoride (Yb:α-NaYF₄ or NaYF) could offset the photothermal heating of nanodiamonds by the anti-Stokes fluorescence cooling of Yb³⁺ ions⁷. We present a novel preparation method for generating a NV^- diamond NaYF composite material based on a hydrothermal synthesis approach. Particle size was determined to be 230 ± 90 nm by SEM, and DLS data show a permanent connection between nanodiamonds and NaYF. Nanodiamonds are observed on the surfaces of NaYF materials. Nanodiamonds may also be incorporated within the body of individual NaYF grains, however the question of whether nanodiamonds are fully incorporated into the host NaYF material remains to be answered. The temperatures of host material and NV^- defects are accessed using mean fluorescence wavelength shifts and Debye-Waller factor thermometry respectively. The obtained temperature changes with increasing 1020 nm irradiance show good agreement. Two data sets showed photothermal heating of around 10 and 13 K at 6.3 MW/cm². Increased particle smoothness and sizes could lead to coolable composite materials.

Microlaser designs based on the coupling of whispering gallery modes (WGMs) with the upconversion processes which take place within lanthanide-doped nanoparticles (UCNPs) have been demonstrated and shown to have many valuable qualities, such as high Q factors and low lasing thresholds. One obstacle that these microlaser designs still face is the challenges caused by photothermal heating of the gain medium, which could be solved through the design of a radiation balanced microlaser. In this work, WGM microresonators composed of 5 μm diameter polystyrene spheres are fabricated with a layer of Yb³⁺-doped NaYF₄ UCNPs in order to test if the anti-Stokes cooling properties of the UCNPs can cool the microresonator and its environment under laser irradiation. We find via calibrated mean fluorescence spectroscopy that the UCNPs can cool their local environment by as much as 23 °C and significantly reduce the heating of the aqueous environment surrounding the microresonator, showing promise for inclusion in a design for a radiation balanced microlaser.

The hopping of a nanoparticle between two adjacent potential wells is a fundamental process in various physical, chemical, and biological phenomena. However, it is tricky to implement an experimental measure to study this process because handling a single nanoparticle is not a simple problem. We propose a 3D tapered metallic nanoantenna with a bow-tie-shaped hole illuminated by two lasers: a continuous-wave (CW) laser and a femtosecond laser. The CW laser produced a double-well potential inside the hole that trapped a single nanoparticle. The femtosecond laser generated a second harmonic signal by enhancing the nonlinear optical effect on the metal surface, which could be easily filtered and monitored. This two-laser platform provides the freedom to choose between the means for capturing a nanoparticle and the means for observing them. We controlled the landscape of the double-well potential by combining the gap size of a nanoantenna and optical pump power. The hopping of trapped nanoparticles over the central potential barrier was monitored and showed a maximum at the specific input laser power. This phenomenon agreed well with the theoretical prediction considering the thermal energy of a nanoparticle.

Up-converting particles (UCP) absorb wavelengths in IR region and emit light in visible region by multiphoton absorption process. When optically trapped with 975 nm laser, these particles show active Hot Brownian Motion (HBM) due to the temperature difference created across the particle by the trapping laser. This is akin to an active particle optically confined in a tweezers with properly oriented motion. However, the activity vanishes when trapped with 1064 nm laser. We carefully maneuver the activity dependence of UCPs on laser wavelength to build a Stirling engine. A Stirling cycle consists of an isothermal expansion followed by isochoric cooling, isothermal compression and isochoric heating. Here, activity of the UCP in an optical trap is analogous to effective temperature which is controlled by the 975 nm laser. Whereas, the confinement of the trapped particle is similar to volume which can be altered by changing the trap stiffness of the 1064 nm laser trap. In this work, We first trap a UCP simultaneously with 1064 nm laser and 975 nm laser. Gradually decreasing 1064 nm laser power keeping 975 nm laser power constant decreases the trap stiffness resulting in less confinement of the UCP while keeping the activity constant. This process is considered as isothermal expansion. There can also be another process where 975 nm is increased and 1064 nm laser power is reduced leaving the total intensity constant. That would amount to isochoric process. We explore all these processes towards the Stirling cycle.

Single-beam optical tweezers that use continuous wave (CW) lasers for trapping microscopic particles can be understood in terms of force-balancing light pressure from a tightly focused laser beam. High-repetition-rate femtosecond lasers for single-beam optical trapping research have matured as a technique and have garnered increasing interest. There are important differences between the theoretical models for femtosecond laser tweezers and the CW tweezers, e.g., in the sensitive detection of background-free two-photon fluorescence. The instantaneous trapping potential is due to the high peak power of each laser pulse, while the sustained stable trapping regime is a consequence of the high repetition rate of successive pulses. Simulating real-time scenarios for predicting optical trapping behavior continues to be a challenging problem. However, the capability and usefulness of optical tweezers setups with both CW and pulsed lasers are well established. For a tightly focused beam as used in an optical tweezer, cumulative heating can occur despite the minimal absorption cross-section of the trapping medium or the trapped particle, which reaches its maximum value near the focus. A temperature gradient from the laser focal spot is thus generated outwards from the laser focus in the medium, creating a refractive index gradient across the focusing region. The refractive index attains its minimum value at the focus, gradually increasing as a function of increasing distance from it. Since the trapping force and potential depend on the refractive index of the medium, the thermal effect impacts the force and potential of the trapped particle significantly. With CW lasers, computational evidence of temperature rise at the focus of optical tweezers has been posited, which, unfortunately, is not a feasible approach for ultrafast lasers, given their inherent computational complexities. A better understanding of high photon-flux induced processes and a working model of the single-beam optical tweezers that could address both CW and pulsed lasers would be ideal for elucidating the effects of this inherent thermal gradient of the optical tweezers. We demonstrate a framework that includes all possible nonlinear effects arising from high photon flux interactions and validate this with experimental results. Our approach allows a coherent and consistent treatment for both CW and ultrafast cases. We have the purely thermal nonlinear effects for the CW laser case, while for the ultrafast laser case, we include both the thermal and the Kerr type nonlinearities. Such a source-sensitive model is amenable to high throughput computations when coupled with a suitable paradigm for modeling experimental conditions as well.

Astrocytes in the brain migrate to sites of injury where they can take up damaging molecules extruded from injured cells to protect neurons. The astrocytic response to cell death is critical to our understanding of ways to mitigate secondary injury from a traumatic brain injury(TBI). We previously showed that a laser could be used to induce a single cell death (photolysis) in order to monitor the surrounding astrocytic response. We found that photolysis leads to a calcium transient in surrounding astrocytes. Here we show that cells treated with the internal calcium chelator BAPTA-AM do not exhibit a transient. Similarly, cells whose endoplasmic reticulum (ER) has been depleted through blocking of the SERCA pump do not show a calcium increase. Cells treated with EGTA to chelate external calcium showed no statistical significance when compared to cells in regular medium with calcium. Therefore, it is concluded that the ER stores are largely responsible for the cytosolic calcium transient.

ABO blood typing is the determination of four different blood groups: type A, B, AB, or O. Clinically approved ABO blood typing methods are suffering from expensive reagents and multiple time-consuming cross-referencing steps, creating the need for fast, sustainable, sensitive, and label-free technologies. Raman spectroscopy techniques have shown potential to distinguish biomolecules and blood components such as purified serum proteins, albumin, and globulin. In combination with machine learning tools, the accuracy and specificity of Raman spectroscopic measurements can be improved and adapted to clinical applications. This study presents a multivariate analysis of human-blood samples for ABO blood typing using Raman spectroscopy and support vector machine (SVM) classification. A custom-built NIR Raman spectroscopy setup with a 785 nm wavelength laser is coupled into an inverted microscope to collect Raman spectra from each blood sample. Donor samples are drawn from EDTA tubes into a fused silica microcapillary without dilution and sample preparation steps. Raman measurements from more than 270 donor samples are analyzed to get accurate blood typing predictions. The blood types are distinguished pairwise by an average AUC score of 0.94, showing great potential of the developed system for future blood typing applications.

The stochastic differential equations (SDEs) representing a bead’s motion in an optical tweezer are stiff, meaning that the ratio of bead’s inertia to the viscous force from the surrounding fluid is extremely large. A scaling technique can be used to improve the computational time required to solve these SDEs numerically using adaptive SDE solvers. However, this scaling changes the SDEs as well as the power spectral density (PSD) of the bead’s position that the SDEs predict. This work shows that the scaling technique can be used effectively without too much loss of information but with significant reduction in computational time. The model uses Mie Scattering theory to compute the laser beam force, while Stokes’ law is used to calculate the drag force. Adaptive Stiff solvers, available in Julia programming language, are used to solve the SDEs. The PSD analysis is done in MATLAB. Experimental datasets for 2000nm, 1950nm, 990nm and 500nm diameter polystyrene beads are compared with the numerical results. We focus on the 2000nm bead because it is the only case where we can obtain PSD directly from the experimental data; in the other cases the PSD is indirectly obtained from experimental data. Interestingly, the 990nm and 500nm beads overshoot the focal point of the optical trap. This work presents the PSD analysis for these cases in addition to the reduction in computational time using the proposed scaling approach.

Here, we present experimental results of optical trapping of dielectric microparticles with structured laser beam created by computer-generated holograms using a spatial light modulator. We compared the trapping efficiencies with converging obstructed Gaussian and Gaussian beams.

We show the 3D quasi-steady-state trapping of ethanol vapor microbubbles and their real-time manipulation in liquids driven by the Marangoni force that is triggered by using a low-power continuous-wave laser. The light absorption phenomenon, caused by the silver nanoparticles photodeposited on the core of optical fibers, is employed for both the generation of vapor microbubbles and triggering the Marangoni effect. The thermal effects, activated by either the silver nanoparticles on the optical fiber´s core or the light bulk absorption, modulate the surface tension of the bubble wall, generating longitudinal and transversal forces, similar to the optical ones. The balance of the optothermal forces drives the quasi-steady-stated 3D manipulation of microbubbles. By numerical simulations, we acquire expressions for the temperature profiles and present analytical expressions for the Marangoni force. Moreover, using an array of three fibers with photodeposited nanoparticles is used to demonstrate the transfer of bubbles from one fiber to another by sequentially switching on and off the lasers.

The cell membrane has fluctuations due to thermal and athermal sources. That causes the membrane to flicker. Conventionally, only the normal (perpendicular to the membrane) fluctuations are studied and then used to ascertain the membrane properties like the bending rigidity. It is here that we introduce a different concept, namely the slope fluctuations of the cell membrane which can be modelled as a gradient of the normal fluctuations. This can be studied using a new technique where a birefringent particle placed on the membrane turns in the out of plane sense, called the pitch sense. We introduce the pitch detection technique in optical tweezers relying upon asymmetric scattering from a birefringent particle under crossed polarizers. We then go on to use this pitch detection technique to ascertain the power spectral density of membrane slope fluctuations and find it to be (frequency)⁻¹ while the normal fluctuations yields (frequency)^−5/3. We also explore a different regime where the cell is applied with the drug Latrunculin-B which inhibits actin polymerization and find the effect on membrane fluctuations. We find that even as the normal fluctuations now become (frequency)^−4/3, the slope fluctuations spectrum still remains (frequency)⁻¹, with exactly the same coefficient as the case when the drug was not applied. Thus, this presents a convenient opportunity to study the membrane parameters like bending rigidity as a function of time after applying the drug. This would be the first time the membrane bending rigidity could be studied as a function of time upon the application of Lat-B without reverting to AFM.

Atomic Force Microscopes (AFM) with 10 nm tip is employed to estimate work of adhesion at nano-scale. The AFM tip is pressed against the surface with forces around a few nano-Newtons and retracted back until it breaks from the surface. Thus estimating the work of adhesion due to this technique can be termed as “hard probing” of the surface. Whereas, we propose another configuration in which a spherical particle is trapped near the surface using a linearly polarized light and the particle attaches to the surface by work of adhesion. Here, by moving the surface in tangential direction, the particle is forced into a rolling motion. This motion can be used to estimate work of adhesion and this technique can be called “soft probing”. We used the soft probing configuration to estimate rolling work of adhesion of a birefringent 3 μm particle on a glass surface. Further, we have studied the effects of PolydimethylSiloxane (PDMS) which is a hydrophobic surface. This technique is used to probe the rolling work of adhesion of 500 nm nanodiamond bearing Nitrogen-vacancy centers which are birefringent due to the stress in the crystal. These nanodiamonds have a contact diameter as small as 50 nm because of their relatively high Young’s modulus. The rolling work of adhesion estimated using our soft probing configuration is about 1 mJ/m², while using the AFM tips to estimate work of adhesion at nanoscale yields about 50 mJ/m².