Axial electrokinetic trapping of single particles at kHz feedback rates

Anti-Brownian Electrokinetic (ABEL) trapping has proven to be a valuable novel tool for analysis at the single-nanoparticle level. In previous work, we explored axial (in the z-direction only) ABEL trapping with planar ITO electrodes based on video image analysis. In this work, we improved the method by using total-internal-reflection (TIR) in combination with a single-photon-counting module. This allows us to axially trap single nanoparticles with a homogeneous field at feedback rates of several kHz such that screening of the electric field becomes negligible

Electrokinetic trapping of non-fluorescent nanoparticles in water

Anti-Brownian electrokinetic trapping enables the confinement of single nanoparticles in solution by applying feedback electric forces that counteract Brownian motion. This technique facilitates the long-term observation of nanoparticles to study their different physio- and bio- chemical properties. However, the method has been greatly restricted to nanoparticles that can be visualized by photoluminescence. In this work, we demonstrate the electrokinetic trapping of fluorescence-free nanoparticles that scatter the evanescent field induced by total internal reflection (a). Using the measured intensity of scattered photons (b) as a feedback, we generate an external electric field (c) that holds the particle at the desired distance from the glass surface. As a result, we are able to trap the nanoparticle and trace its response to the applied voltage (d) at kilohertz rates without any fluorescent labeling. Our approach significantly extends the range of nanoobjects that can be trapped at the single-particle level in an aqueous solution based purely on their light-scattering properties

Polarized photoluminescence from structured nanoparticles

Quantum dots efficiently transform blue photons into green or red photons by photoluminescence in quantum dot LED TVs. For spherical quantum dots, the absorption and emission are independent of the polarization of the photons. For semiconductor nanoparticles with other shapes, the absorption depends on the polarization of the incident blue photons and the emitted light can become polarized. In this presentation it is explained how photoluminescent nanoparticles can be used to generate a backlight that emits linearly polarized light. By giving the semiconductor nanoparticles a particular shape, the anisotropy of absorption and emission can be independently tuned

Patterned photo-alignment and surface topography for chiral liquid crystal superstructures with unique electro-optic properties

The combination of long-pitch and short-pitch chiral liquid crystals (CLCs) with patterned surface anchoring or surface topography is investigated with the aim to develop new electro-optic components. Highly efficient large-angle 1D diffraction gratings and metastable 2D gratings with hysteresis switching are demonstrated as well as electro-optic components with a uniform lying helix-like structure at intermediate voltages

Measurement of the amplitude and phase of the electrophoretic and electroosmotic mobility based on fast single‐particle tracking

The electrophoretic mobility of micron-scale particles is of crucial importance in applications related to pharmacy, electronic ink displays, printing, and food technology as well as in fundamental studies in these fields. Particle mobility measurements are often limited in accuracy because they are based on ensemble averages and because a correction for electroosmosis needs to be made based on a model. Single-particle approaches are better suited for examining polydisperse samples, but existing implementations either require multiple measurements to take the effect of electroosmosis into account or are limited in accuracy by short measurement times. In this work, accurate characterization of monodisperse and polydisperse samples is achieved by measuring the electrophoretic mobility on a particle-to-particle basis while suppressing electroosmosis. Electroosmosis can be suppressed by measuring in the middle of a microchannel while applying an AC voltage with a sufficiently high frequency. An accurate measurement of the electrophoretic mobility is obtained by analyzing the oscillating particle motion for urn:x-wiley:01730835:media:elps7439:elps7439-math-0001 per particle with a high-speed camera measuring at urn:x-wiley:01730835:media:elps7439:elps7439-math-0002, synchronized to the applied electric field. Attention is paid to take into account the effect of the rolling shutter and the non-uniform sampling in order to obtain the accurate amplitude and phase of the electrophoretic mobility. The accuracy of method is experimentally verified and compared with a commercial apparatus for polystyrene microspheres in water. The method is further demonstrated on a range of particle materials and particle sizes and for a mixture of positively and negatively charged particles

Large angle forward diffraction by chiral liquid crystal gratings with inclined helical axis

A layer of chiral liquid crystal (CLC) with a photonic bandgap in the visible range has excellent reflective properties. Recently, two director configurations have been proposed in the literature for CLC between two substrates with periodic photo-alignment: one with the director parallel to the substrates and one with the director in the bulk parallel to the tilted plane. The transmission experiments under large angles of incidence (AOI) presented in this work prove that, in the bulk, the director does not remain parallel with the substrates. Because of the inclined helical axis, the full reflection band can be observed at a smaller AOI than in planar CLC. For sufficiently large AOI, the reflection of diffracted light is prohibited by total internal reflection and efficient diffraction occurs in the forward direction

Tilted chiral liquid crystal gratings for efficient large‐angle diffraction

Deflecting light with high efficiency over large angles with thin optical ele- ments is challenging but offers tremendous potential for applications, such as wearable displays and optical communication systems. Compared to the complex production of metasurfaces, the self-organization of liquid crystal (LC) superstructures provides an elegant and flexible way to produce high- quality thin optical components. The periodically varying dielectric tensor in short-pitch chiral LC gives rise to a photonic bandgap, which can be exploited to realize efficient diffractive mirrors in the visible wavelength range. How- ever, large-angle diffractive devices require a small in-plane period, leading to complex self-assembly behavior in the bulk. This work demonstrates that by patterning photoalignment layers at the surfaces with a period comparable to the chiral pitch, the LC self-assembles into a tilted, defect-free helical struc- ture. The director configuration is calculated by finite element simulations and it is experimentally demonstrated that a single photoaligned substrate is sufficient to template the tilted chiral structure in the bulk. This structure effectively (88%) diffracts light over large angles (≈46°) and enables novel micrometer-thin (≈3 μm) optical components that can be produced with an elegant manufacturing process. Due to flexibility of photoalignment, this process could easily be implemented in emerging photonic applications

Waveguiding of photoluminescence in a layer of semiconductor nanoparticles

Semiconductor nanoparticles (SNPs), such as quantum dots (QDs) and core/shell nanoparticles, have proven to be promising candidates for the development of next-generation technologies, including light-emitting diodes (LEDs), liquid crystal displays (LCDs) and solar concentrators. Typically, these applications use a sub-micrometer-thick film of SNPs to realize photoluminescence. However, our current knowledge on how this thin SNP layer affects the optical efficiency remains incomplete. In this work, we demonstrate how the thickness of the photoluminescent layer governs the direction of the emitted light. Our theoretical and experimental results show that the emission is fully outcoupled for sufficiently thin films (monolayer of SNPs), whereas for larger thicknesses (larger than one tenth of the wavelength) an important contribution propagates along the film that acts as a planar waveguide. These findings serve as a guideline for the smart design of diverse QD-based systems, ranging from LEDs, where thinner layers of SNPs maximize the light outcoupling, to luminescent solar concentrators, where a thicker layer of SNPs will boost the efficiency of light concentration