Electric field-induced collective reorientation of nematic molecules is of importance for fundamental science and practical applications. This reorientation is either homogeneous over the area of electrodes, as in displays, or periodically modulated, as in electroconvection. The question is whether spatially localized three-dimensional solitary waves of molecular reorientation could be created. Here we demonstrate that the electric field can produce particle-like propagating solitary waves representing self-trapped “bullets” of oscillating molecular director. These director bullets lack fore-aft symmetry and move with very high speed perpendicularly to the electric field and to the initial alignment direction. The bullets are true solitons that preserve spatially confined shapes and survive collisions. The solitons are topologically equivalent to the uniform state and have no static analogs, thus exhibiting a particle–wave duality. Their shape, speed, and interactions depend strongly on the material parameters, which opens the door for a broad range of future studies.

Production of stable multidimensional solitary waves is a grand challenge in modern science. Steering their propagation is an even harder problem. Here we demonstrate three-dimensional solitary waves in a nematic, trajectories of which can be steered by the electric field in a plane perpendicular to the field. The steering does not modify the properties of the background that remains uniform. These localized waves, called director bullets, are topologically unprotected multidimensional solitons of (3 + 2)D type that show fore-aft and right-left asymmetry with respect to the background molecular director; the symmetry is controlled by the field. Besides adding a whole dimension to the propagation direction and enabling controlled steering, the solitons can lead to applications such as targeted delivery of information and micro-cargo.

Manipulation of particles by a uniform electric field, known as electrophoresis, is used in a wide array of applications. Of especial interest is electrophoresis driven by an alternating current (AC) as it eliminates electrode blocking and produces a steady motion. The known mechanisms of AC electrophoresis require that either the particle or the surrounding medium are asymmetric. This asymmetry is usually assured before the field is applied, as in the case of Janus spheres. We report on a new mechanism of AC electrophoresis, in which the symmetry is broken only when the field exceeds some threshold. The new mechanism is rooted in the nature of electrophoretic medium, which is an orientationally ordered nematic liquid crystal. Below the threshold, the director field of molecular orientation around a spherical particle is of a quadrupolar symmetry. Above the threshold, the director forms a polar self-confined perturbation around the inclusion that oscillates with the frequency of the applied field and propels the sphere. The director perturbations are topologically trivial and represent particlelike solitary waves, called "director bullets" or "directrons". The direction of electrophoretic transport can be controlled by the frequency of the field. The AC directron-induced liquid crystal enabled electrophoresis can be used to transport microscopic cargo when other modes of electrophoresis such as induced charge electrophoresis are forbidden.

Colloidal semiconductor CdSe-based quantum dots (QDs) show undesirable photoluminescence (PL) intermittency with frequent and long-lasting dark states due to positively charged states, significantly limiting QD optoelectronic and photonics applications. Here, we show that p-phenylenediamine (PPD) can completely suppress the long-lasting dark states in the PL intensity trajectories for single CdSe-based QDs in the full-color emission range from 459 to 800 nm, while hardly influencing any other PL properties of the QDs, such as the PL intensity, lifetime, and emission spectra. The suppression mechanism is investigated by comparing PPD to another amine compound, N,N-dimethylaniline. With a reasonable highest occupied molecular orbital energy, PPD facilitates electron transfer from PPD to the positively charged QDs, thus, neutralizing the extra hot holes via an Auger-assisted process. Therefore, the positively charged states of QDs and the PL intermittency could be eliminated. Moreover, we demonstrate that PPD can effectively suppress the photobleaching of CdSe-based QDs, and the average survival time of single QDs can be extended from a few minutes to more than 1 h. Finally, we demonstrate the application of PPD-stabilized QDs to single-particle tracking and HeLa cell imaging under relevant biological conditions and show their promising potential in various biomedical applications.

We present polarization-independent optical shutters with a sub-millisecond switching time. The approach utilizes dual-frequency nematics doped with a dichroic dye. Two nematic cells with orthogonal alignment are driven simultaneously by a low-frequency or highfrequency electric field to switch the shutter either into a transparent or a light-absorbing state. The switching speed is accelerated via special short pulses of high amplitude voltage. The approach can be used in a variety of electro-optical devices.