Friday, January 18, 2019

Atoms in complex twisted light

Mohamed Babiker, David L Andrews and Vassilis E Lembessis

The physics of optical vortices, also known as twisted light, is now a well-established and a growing branch of optical physics with a number of important applications and significant inter-disciplinary connections. Optical vortex fields of widely varying forms and degrees of complexity can be realised in the laboratory by a host of different means. The interference between such beams with designated orbital angular momenta and optical spins (the latter is associated with wave polarisations) can be structured to conform to various geometrical arrangements. The focus of this review is on how such tailored forms of light can exert a controllable influence on atoms with which they interact. The main physical effects involve atoms in motion due to application of optical forces. The now mature area of atom optics has had notable successes both of fundamental nature and in applications such as atom lasers, atom guides and Bose–Einstein condensates. The concepts in atom optics encompass not only atomic beams interacting with light, but atomic motion in general as influenced by optical and other fields. Our primary concern in this review is on atoms in structured light where, in particular, the twisted nature of the light is made highly complex with additional features due to wave polarisation. These features bring to the fore a variety of physical phenomena not realisable in the context of atomic motion in more conventional forms of laser light. Atoms near resonance with such structured light fields become subject to electromagnetic fields with complex polarisation and phase distributions, as well as intricately structured intensity gradients and radiative forces. From the combined effect of optical spin and orbital angular momenta, atoms may also experience forces and torques involving an interplay between the internal and centre of mass degrees of freedom. Such interactions lead to new forms of processes including scattering, trapping and rotation and, as a result, they exhibit characteristic new features at the micro-scale and below. A number of distinctive properties involving angular momentum exchange between the light and the atoms are highlighted, and prospective applications are discussed. Comparison is made between the theoretical predictions in this area and the corresponding experiments that have been reported to date.


TEFM Enhances Transcription Elongation by Modifying mtRNAP Pausing Dynamics

Hongwu Yu, Cheng Xue, Mengping Long, Huiqiang Jia, Guosheng Xue, Shengwang Du, Yves Coello, Toyotaka Ishibashi

Regulation of transcription elongation is one of the key mechanisms employed to control gene expression. The single-subunit mitochondrial RNA polymerase (mtRNAP) transcribes mitochondrial genes, such as those related to ATP synthesis. We investigated how mitochondrial transcription elongation factor (TEFM) enhances mtRNAP transcription elongation using a single-molecule optical-tweezers transcription assay, which follows transcription dynamics in real time and allows the separation of pause-free elongation from transcriptional pauses. We found that TEFM enhances the stall force of mtRNAP. Although TEFM does not change the pause-free elongation rate, it enhances mtRNAP transcription elongation by reducing the frequency of long-lived pauses and shortening their durations. Furthermore, we demonstrate how mtRNAP passes through the conserved sequence block II, which is the key sequence for the switch between DNA replication and transcription in mitochondria. Our findings elucidate how both TEFM and mitochondrial genomic DNA sequences directly control the transcription elongation dynamics of mtRNAP.


Combined Force Ramp and Equilibrium High-Resolution Investigations Reveal Multipath Heterogeneous Unfolding of Protein G

Dena Izadi, Yujie Chen, Miles L. Whitmore, Joseph D. Slivka, Kevin Ching, Lisa J. Lapidus, and Matthew J. Comstock

Over the past two decades, one of the standard models of protein folding has been the “two-state” model, in which a protein only resides in the folded or fully unfolded states with a single pathway between them. Recent advances in spatial and temporal resolution of biophysical measurements have revealed “beyond-two-state” complexity in protein folding, even for small, single-domain proteins. In this work, we used high-resolution optical tweezers to investigate the folding/unfolding kinetics of the B1 domain of immunoglobulin-binding protein G (GB1), a well-studied model system. Experiments were performed for GB1 both in and out of equilibrium using force spectroscopy. When the force was gradually ramped, simple single-peak folding force distributions were observed, while multiple rupture peaks were seen in the unfolding force distributions, consistent with multiple force-dependent parallel unfolding pathways. Force-dependent folding and unfolding rate constants were directly determined by both force-jump and fixed-trap measurements. Monte Carlo modeling using these rate constants was in good agreement with the force ramp data. The unfolding rate constants exhibited two different behaviors at low vs high force. At high force, the unfolding rate constant increased with increasing force, as previously reported by high force, high pulling speed force ramp measurements. However, at low force, the situation reversed and the unfolding rate constant decreased with increasing force. Taken together, these data indicate that this small protein has multiple distinct pathways to the native state on the free energy landscape.


Direct Measurement of π Coupling at the Single-Molecule Level using a Carbon Nanotube Force Sensor

Tu Hong, Tianjiao Wang, and Ya-Qiong Xu

We report a carbon nanotube (CNT) force sensor that combines a suspended CNT transistor with dual-trap optical tweezers to explore the interactions between two individual molecules in the near-equilibrium regime with sub-piconewton resolution. The directly measured equilibrium force (1.2 ± 0.5 pN) is likely related to the binding force between a CNT and a single DNA base, where two aromatic rings spontaneously attract to each other due to the noncovalent forces between them. On the basis of our force measurements, the binding free energy per base is calculated (∼0.34 eV), which is in good agreement with theoretical simulations. Moreover, three-dimensional scanning photocurrent microscopy enables us to simultaneously monitor the morphology changes of the CNT, leading to a comprehensive reconstruction of the CNT-DNA binding dynamics. These experimental results shed light on the fundamental understanding of the mechanical coupling between CNTs and DNA molecules and, more importantly, provide a new platform for direct observation of intermolecular interfaces at the single-molecule level.


Nucleotide-dependent DNA gripping and an end-clamp mechanism regulate the bacteriophage T4 viral packaging motor

Mariam Ordyan, Istiaq Alam, Marthandan Mahalingam, Venigalla B. Rao, and Douglas E. Smith

ATP-powered viral packaging motors are among the most powerful biomotors known. Motor subunits arranged in a ring repeatedly grip and translocate the DNA to package viral genomes into capsids. Here, we use single DNA manipulation and rapid solution exchange to quantify how nucleotide binding regulates interactions between the bacteriophage T4 motor and DNA substrate. With no nucleotides, there is virtually no gripping and rapid slipping occurs with only minimal friction resisting. In contrast, binding of an ATP analog engages nearly continuous gripping. Occasional slips occur due to dissociation of the analog from a gripping motor subunit, or force-induced rupture of grip, but multiple other analog-bound subunits exert high friction that limits slipping. ADP induces comparably infrequent gripping and variable friction. Independent of nucleotides, slipping arrests when the end of the DNA is about to exit the capsid. This end-clamp mechanism increases the efficiency of packaging by making it essentially irreversible.


Thursday, January 17, 2019

Enhanced Plasmonic Particle Trapping Using a Hybrid Structure of Nanoparticles and Nanorods

So Yun Lee, Hyung Min Kim, Jinho Park, Seong Keun Kim , Jae Ryoun Youn, and Young Seok Song

Plasmon-enhanced particle trapping was demonstrated using a hybrid structure of nanoparticles and nanorods. In order to intensify localized surface plasmon resonance (LSPR), gold nanoparticles (AuNPs) were deposited on zinc oxide nanorods (ZnONRs). The synergistic effect caused by the hybrid structure was identified experimentally. Numerical analysis revealed that the LSPR-induced photophysical processes such as plasmonic heating and near-field enhancement were improved by the existence of ZnONRs. The role of the ZnONR in enhancing the particle-trapping velocity was explored by examining the scattered electric field, Poynting vector, and temperature gradient over the nanostructures calculated from the simulation. It was found that polystyrene microparticles and Escherichia coli cells were successfully trapped by using the ZnONR/AuNP plasmonic structure. A relatively high dielectric constant and nanorod geometry of ZnO enabled the hybrid substrate to enhance trapping performance, compared with a control case fabricated using only gold nanoislands.


Optimal Nanoparticle Forces, Torques, and Illumination Fields

Yuxiang Liu, Lingling Fan, Yoonkyung E. Lee, Nicholas X. Fang, Steven G. Johnson, and Owen D. Miller

A universal property of resonant subwavelength scatterers is that their optical cross-sections are proportional to a square wavelength, λ2, regardless of whether they are plasmonic nanoparticles, two-level quantum systems, or RF antennas. The maximum cross-section is an intrinsic property of the incident field: plane waves, with infinite power, can be decomposed into multipolar orders with finite powers proportional to λ2. In this article, we identify λ2/c and λ3/c as analogous force and torque constants, derived within a more general quadratic scattering-channel framework for upper bounds to optical force and torque for any illumination field. This framework also solves the reverse problem: computing globally optimal “holographic” incident beams, for a fixed collection of scatterers. We analyze structures and incident fields that approach the bounds, which for wavelength-scale bodies show a rich interplay between scattering channels, and we show that spherically symmetric structures are forbidden from reaching the plane-wave force/torque bounds. This framework should enable optimal mechanical control of nanoparticles with light.


Absorbing particle 3D trap based on annular core fiber tweezers

Zhihai Liu, Zhenyu Zhang, Yu Zhang, Yaxun Zhang, Xiaoyun Tang, Keqiang Liu, Xinghua Yang, Jianzhong Zhang, Jun Yang, Libo Yuan

We proposed and demonstrated a new method to trap and manipulate a strongly absorbing particle by harnessing strong -type photophoretic forces in pure liquid glycerol with a focused annular beam, which was introduced by an annular core fiber optical tweezers. We performed the focused annular light field by integrating a high refractive index silica microsphere on the annular core fiber end facet. The silica microsphere, which acted as a lens, focused laser beam for trapping. We tested and calculated -type photophoretic forces tendency by using the fluorescence dye method, and the simulated results were in agreement with the experimental results. The proposed optical fiber tweezers were small in size, large in capture range, simple to manipulate, and inexpensive in cost. It provided a new approach for absorbing particles trapping. Tailoring the trapping and manipulation of absorbing particles in liquid environments will open avenues for studying the special, yet-unexplored characteristics of absorbing particles.


Transverse spin forces and non-equilibrium particle dynamics in a circularly polarized vacuum optical trap

V. Svak, O. Brzobohatý, M. Šiler, P. Jákl, J. Kaňka, P. Zemánek & S. H. Simpson

We provide a vivid demonstration of the mechanical effect of transverse spin momentum in an optical beam in free space. This component of the Poynting momentum was previously thought to be virtual, and unmeasurable. Here, its effect is revealed in the inertial motion of a probe particle in a circularly polarized Gaussian trap, in vacuum. Transverse spin forces combine with thermal fluctuations to induce a striking range of non-equilibrium phenomena. With increasing beam power we observe (i) growing departures from energy equipartition, (ii) the formation of coherent, thermally excited orbits and, ultimately, (iii) the ejection of the particle from the trap. As well as corroborating existing measurements of spin momentum, our results reveal its dynamic effect. We show how the under-damped motion of probe particles in structured light fields can expose the nature and morphology of optical momentum flows, and provide a testbed for elementary non-equilibrium statistical mechanics.


Optical lateral forces and torques induced by chiral surface-plasmon-polaritons and their potential applications in recognition and separation of chiral enantiomers

Qiang Zhang, Junqing Li and Xingguang Liu

Surface plasmon polaritons carry an intrinsic transverse spin angular momentum which is locked to their propagation direction due to the quantum spin Hall effect of light. We study the chirality-sorting lateral optical forces arising from this phenomenon in a Kretschmann configuration. We show that the characteristics of surface plasmon polaritons are affected by small changes of the environment's chirality. This property can be utilized to detect the medium's chirality in the macro-world. Furthermore, we explain how the the lateral forces and optical torques interact with the chirality of nano-particles located or adsorbed in the vicinity of the surface in the micro-world. Finally, we demonstrate that introducing one handedness of chirality to the dielectric medium in the Kretschmann configuration will benefit the discrimination of chiral enantiomers compared with the nonchiral case. Our work presents physical insights into chirality-selective lateral forces using surface plasmon polaritons and provides a new approach to discriminating and separating chiral enantiomers in the chemical and pharmaceutical industries.