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Conventionally, a collimated visible laser beam can only be focused to a transverse region volume with a waist of about 200 nm and an axial waist of about 1200 nm due to the wave nature of light. Several techniques have been used to bypass the diffraction limit, including a recently developed one using the emission from an optically trapped upconverting nanoparticle. This nanoparticle has a diameter smaller than 200 nm, such that it emits like a dipole into a 45° cone. Thus, not only is the emission coming from a particle smaller in size than the waist of the diffraction‐limited spot but also the cone is smaller than that of the volume of a very tightly focused beam. Here, the technique is developed even further where the emission coming from a specific region of the cone is selected by an optical fiber‐based pinhole in the detection path. Using this technique, a confocal depth of about 200 nm is achieved as the overlap between the emission cone and the detection volume. This would correspond to a volume of about 250 attoliters for a 1.5 μm diameter particle which can be reduced to 40 attoliters by using a 500 nm diameter particle.

Controlling the translational and rotational motion of microparticles is a topic of current interest in soft matter and statistical mechanics. Roll rotation of a microparticle in a hydro-thermophoretic trap – which is a device that uses the flow of fluids and thermophoresis to trap and manipulate microparticles – was recently reported (Nalupurackal et al., 2023). Here, we study the coupled translational and rotational motion of a microparticle in a hydro-thermophoretic trap. We demonstrate, using experiments and theory, that the dynamical system describing the motion of the microparticle undergoes bifurcations when the power of lasers used in a hydro-thermophoretic trap is changed. We show that the advective fluid flow from laser heating and thermophoresis depend on the power of the laser through two distinct exponents. We obtain these exponents from experimental data. Using these exponents and a linear stability analysis of the equations of motion, we show that locations at which a microparticle can be trapped (stable fixed points of the dynamical system) can be tuned by changing the power of the lasers at the two hotspots. At higher power values, we find that there are no stable fixed points; hence, the microparticle can no longer be trapped. Our experimental findings support these theoretical results.

In this article, we explore the effect of two different infrared (IR) laser wavelengths on the optical properties of trapped nano-diamonds containing high-density ensembles of nitrogen vacancy (NV) centers. We investigate 975 nm and 1064nm wavelengths for trapping lasers and find that NV photoluminescence quenching is more prominent for 1064nm illumination than for 975 nm illumination when simultaneously excited with a 532 nm laser. In order to understand the underlying mechanism, we develop a rate-equation-based model that takes into account various transition probabilities. The model suggests that the findings cannot be explained only by imposing modification of the NV charge-state ratio under varied illumination wavelengths, and, thus, we speculate that the effective ionization and recombination rates associated with NV charge states for the studied samples are highly wavelength-dependent in the probed regime. Importantly, the results demonstrate that 975 nm laser is desirable for optical trapping of NV-diamonds, especially for NV-based sensing applications.

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)1744-683X10.1039/D2SM00627H], 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.

Upconverting particles like Yb and Er-doped NaYF _4 are known to heat up after illumination with light at pump wavelength due to inefficient upconversion processes. Here we show that NaYF _4 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.

The realization of microscopic heat engines has gained a surge of research interest in statistical physics, soft matter, and biological physics. A typical microscopic heat engine employs a colloidal particle trapped in a confining potential, which is modulated in time to mimic the cycle operations. Here, we use a lanthanide-doped upconverting particle (UCP) suspended in a passive aqueous bath, which is highly absorptive at 975 nm and converts near infra red (NIR) photons to visible, as the working substance of the engine. When a single UCP is optically trapped with a 975 nm laser, it behaves like an active particle by executing motion subjected to an asymmetric temperature profile along the direction of propagation of the laser. The strong absorption of 975 nm light by the particle introduces a temperature gradient and results in significant thermophoretic diffusion along the temperature gradient. However, the activity of the particle vanishes when the trapping wavelength is switched to 1064 nm. We carefully regulate the wavelength-dependent activity of the particle to engineer all four cycles of a Stirling engine by using a combination of 1064 nm and 975 nm wavelengths. Since the motion of the particle is stochastic, the work done on the particle due to the stiffness modulation per cycle is random. We provide statistical estimation for this work averaged over five cycles which can be extended towards several cycles to make a Stirling engine. Our experiment proposes a robust set-up to systematically harness temperature which is a crucial factor behind building microscopic engines.