Estimation of the proximal temperature rise of an excited upconversion particle by detecting the wavefront of emission.

Monitoring the temperature distribution within a local environment at the micro and nanoscale is vital as many processes are solely thermal. Various thermometric techniques have been explored in the community, and out of these, fluorescent nano/micro particle-based mechanisms are accepted widely (fluorescence intensity ratio (FIR) techniques, where the ratio of populations in two consecutive energy levels is compared with Boltzmann distribution). We describe a new technique to account for the temperature rise near an illuminated upconverting particle (UCP) using wavefront imaging, which is more sensitive than the conventional thermometric techniques on the microscale. We rely on a thermo-optical phase microscopic technique by reconstructing the wavefront of emission from an upconverting particle using a Shack-Hartmann wavefront sensor. The wavefront maps the local phase distribution, which is an indicator of the surroundings' optical parameters, particularly the suspended medium's temperature-induced refractive index in the presence of convection currents. We describe how these extracted phase values can provide information about the optical heating due to the particle and hence its local environment along the direction of the emission. Our findings demonstrate the detection of a minimum temperature rise of 0.23 K, while the FIR methods indicate a minimum of 0.3 K rise. This technique is used to study the temperature increase in the backscattered direction for an upconverting particle illuminated on pump resonance. We also estimate the Soret coefficient for an upconverting particle optically trapped on pump resonance and experiencing anisotropic heating across the body.

High-resolution detection of pitch rotation in an optically confined hexagonal-shaped upconverting particle.

A rigid body can have six degrees of freedom, of which three are with rotational origin. In the nomenclature of the airlines, the in-plane degree of rotational freedom can be called yaw while the first out-of-plane degree of freedom can be called pitch with the second one being called roll. Among these, only the yaw sense has been studied extensively in the optical tweezers literature, while the pitch rotation is starting to be explored. In this paper, we show a way to detect the pitch rotation in a hexagonal-shaped particle using photonic force microscopy using the forward scattered light under crossed polarizers and making it incident on a split photodiode. In this way, the pitch angle can be detected at high resolution and bandwidth. We apply this technique to detect continuous pitch rotation and also exhibit a power spectral density for an anisotropic particle optically trapped in a linearly polarized light and exhibiting Brownian motion.

Studying fluctuating trajectories of optically confined passive tracers inside cells provides familiar active forces.

In recent years, there has been a growing interest in studying the trajectories of microparticles inside living cells. Among other things, such studies are useful in understanding the spatio-temporal properties of a cell. In this work, we study the stochastic trajectories of a passive microparticle inside a cell using experiments and theory. Our theory is based on modeling the microparticle inside a cell as an active particle in a viscoelastic medium. The activity is included in our model from an additional stochastic term with non-zero persistence in the Langevin equation describing the dynamics of the microparticle. Using this model, we are able to predict the power spectral density (PSD) measured in the experiment and compute active forces. This caters to the situation where a tracer particle is optically confined and then yields a PSD for positional fluctuations. The low frequency part of the PSD yields information about the active forces that the particle feels. The fit to the model extracts such active force. Thus, we can conclude that trapping the particle does not affect the values of the forces extracted from the active fits if accounted for appropriately by proper theoretical models. In addition, the fit also provides system properties and optical tweezers trap stiffness.

Controlled roll rotation of a microparticle in a hydro-thermophoretic trap.

In recent years, there has been a growing interest in controlling the motion of microparticles inside and outside a focused laser beam. A hydro-thermophoretic trap was recently reported [Nalupurackal et al., Soft Matter 18, 6825 (2022)], which can trap and manipulate microparticles and living cells outside a laser beam. Briefly, a hydro-thermophoretic trap works by the competition between thermoplasmonic flows due to laser heating of a substrate and thermophoresis away from the hotspot of the laser. Here, we extend that work to demonstrate the controlled roll rotation of a microparticle in a hydro-thermophoretic trap using experiments and theory. We experimentally measure the roll angular velocity of the trapped particle. We predict this roll rotation from theoretical computation of the fluid flow. The expression for the angular velocity fits the experimental data. Our method has potential applications in microrheology by employing a different mode of rotation.

Formation of Two-Dimensional Magnetically Responsive Clusters Using Hematite Particles Self-Assembled via Particle-Induced Heating at an Interface.

Hematite particles, which exhibit a high magnetic moment, are used to apply large forces on physical and biological systems under magnetic fields to investigate various phenomena, such as those of rheology and micromanipulation. However, the magnetic confinement of these particles requires complicated field configurations. On the other hand, laser-assisted optical confinement of single hematite particles results in thermophoresis and subsequent ejection of the particle from the laser spot. Herein, we explore an alternative strategy to induce the self-assembly of hematite. In this strategy, with indirect influence from an optically confined and heated upconverting particle (UCP) at an air-water interface, there is the generation of convection currents that facilitate assembly. We also show that the assembly remains at the interface even after removal of the laser light. The hematite particle assemblies can then be moved using magnetic fields and employed to perform interfacial rheology.

Accelerated self assembly of particles at the air-water interface with optically assisted heating due to an upconverting particle.

Particles can be assembled at the air-water interface due to optically induced local heating. This induces convection currents in the water which brings particles to the surface. We improve the technique by employing an upconverting particle (UCP), which, when illuminated with 975 nm light, not only emits visible emission but also generates heat owing to the poor efficiency of the upconversion process. This induces strong convection currents which makes particles dispersed in the suspension assemble at the interface and immediately under the UCP. We show assembly of polystyrene particles of 1 µm diameter and diamonds of 500 nm diameter bearing Nitrogen-Vacancy (NV) centers around the UCP. We also show, for the first time, that the microdiamonds are assembled within about 30 nm at the bottom of the UCP by utilizing non-radiative energy transfer that reduces the lifetime of the 550 nm emission from about 90 µs to about 50 µs.

Generation of partial roll rotation in a hexagonal NaYF4 particle by switching between different optical trapping configurations.

Typically a rigid body can have three degrees of rotational freedom. Among these, there can be two types of out-of-plane rotational modes, called the pitch and the roll. The pitch motion is typically to turn the particle along an axis orthogonal to the axis of symmetry. However, rotation about the axis of symmetry (called the roll motion) has so far not been shown in optical tweezers. It is here that we use a hexagonal shaped particle (NaYF4) which prefers to align side on with the optical tweezers [Rodriguez-Sevilla et al., Nano Letters 16, 8005 (2016)]. In this work, we find that the stable configuration of the hexagonal particle changes while using one beam and two beams, so that when one of the tweezers beams is switched on and off, the particle tends to switch between the different configurations. Thus we get a controlled roll motion. This is the first time that controlled partial roll motions have been generated in optical tweezers.

Facets of optically and magnetically induced heating in ferromagnetically doped-NaYF4 particles.

Upconverting particles like Yb and Er-doped NaYF4 are known to heat up after illumination with light at pump wavelength due to inefficient upconversion processes. Here we show that NaYF4 particles which have been co-doped not only with Yb and Er but also Fe improves the photothermal conversion efficiency. In addition, we show for the first time that alternating magnetic fields also heat up the ferromagnetic particles. Thereafter we show that a combination of optical and magnetic stimuli significantly increases the heat generated by the particles.