Study of laser induced dynamics on macroscopic mirror

Yukun Yuan, Chunyang Gu, Siyu Huang, Le Song, Guangpeng Yan, Zexiao Li and FengZhou Fang

Light carries energy and momentum, which can be transferred to the irradiated target during the process of reflection, refraction or transmission. The energy transfer can be seen from heat generation and thermal expansion of the target, while the momentum can exert a microscopic force, called an optical force, that is extremely difficult to detect because it is so small. A theoretical model for describing the force distribution on a macroscopic spherical mirror irradiated by a single laser pulse is established in this study. Finite element analysis has been performed to predict the dynamics of the mirror and it shows good consistency with the experimental results. It indicates the differences in the variation of mechanical response due to the optical force and thermal deformation due to photon absorption. A parametric study was also performed to analyze the relationship between mirror geometry and laser dynamics. These studies verify the thermal-mechanical coupling in light-material interactions for a pulsed-laser irradiation model, thereby being applicable in precision measurement of optical force at a microscopic level.

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Newtonian orbits of nanoparticles interacting with structured light beams

Manuel F Ferrer-Garcia and Dorilian Lopez-Mago

We perform numerical analysis to study the orbits described by subwavelength-size particles interacting with complex structured light beams. Our solution to the particle dynamics considers: (i) the gradient force, (ii) the transverse radiation pressure, and (iii) polarisation-dependent curl force. The last two terms, (ii) and (iii), constitute the scattering forces. Multiples examples are provided to show the polarisation effects in the trajectories. For the single optical vortex case, the particle is always expelled due to the polarisation-dependent terms. Optical forces due to vector beams, such as cylindrical vector beams and full-Poincaré beams have been analysed finding closed and open orbits, respectively. Trapping control has been achieved by varying the separation distance in the off-axis superposition of optical vortices.

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Interference of axially-shifted Laguerre–Gaussian beams and their interaction with atoms

K Koksal, Vasileios E Lembessis, J Yuan and M Babiker
Counter-propagating co-axial Laguerre–Gaussian (LG) beams are considered, not in the familiar scenario where the focal planes coincide at z = 0, but when they are separated by a finite axial distance d. The simplest case is where both beams are doughnut beams which have the same linear polarisation. The total fields of this system are shown to display novel amplitude and phase distributions and are shown to give rise to a ring or a finite ring lattice composed of double rings and single central ring. When the beams have slightly different frequencies the ring lattice pattern becomes a finite set of rotating Ferris wheels and the whole pattern also moves axially between the focal planes. We show that the fields of such an axially shifted pair of counter-propagating LG beams generate trapping potentials due to the dipole force which can trap two-level atoms in the components of the ring lattice. We also highlight a unique feature of this system which involves the creation of a new longitudinal optical atom trapping potential due to the scattering force which arises solely when $d\ne 0$. The results are illustrated using realistic parameters which also confirm the importance of the Gouy and curvature effects in determining the ring separation both radially and axially and gives rise to the possibility of atom tunnelling between components of the double rings.

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Theoretical simulation of Gaussian beam interferometric optical tweezers with symmetrical construction

Mohammadbagher Mohammadnezhad, Salah Raza Saeed and Abdollah Hassanzadeh

Multi-beam optical tweezers can be very effective and suitable for creating optical lattices and dexterous manipulation of the microscopic world. In an earlier work, a symmetrical multi-beam optical tweezers setup was proposed and theoretically investigated. In that study, however, we assumed that the incident beams were plane waves, and a plane wave cannot exist in real situations. The main aim of this paper is to investigate multi-beam optical tweezers under the realistic condition of using the Gaussian beams. The effects of changing the polarization state and the number of incident beams on potential landscapes are investigated. A comparison between the Gaussian beam and plane wave lattices is also performed. The results show that in the case of p-polarization, a very small spot (a sub-λ spot) can be achieved by increasing the number of incident beams. However, for s-polarized incident beams instead of a bright center, a dark center intensity distribution is obtained. The lattices constructed by the interference of the Gaussian beams have deeper traps in the central region compared to the plane wave lattices.

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Extended linear detection range for optical tweezers using a stop at the back focal plane of the condenser

S Masoumeh Mousavi, Akbar Samadi, Faegheh Hajizadeh and S Nader S Reihani

Optical tweezers are indispensable micro-manipulation tools. It is known that optical tweezers are force rather than position sensors due to the shorter linear range of their position detection system. In this paper, we have shown for the first time, that positioning an optical stop at the BFP of the condenser can overcome this problem by extending the linear detection range. This method would be valuable for the force spectroscopy applications of optical tweezers.

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Dissipation effect on optical force and torque near interfaces

Daigo Oue

The Fresnel–Snell law, which is one of the fundamental laws in optics and gives insights on the behaviour of light at interfaces, is violated if there exists dissipation in the transmitting media. In order to overcome this problem, we extend the angle of refraction from a real number to a complex number. We use this complex-angle approach to analyse the behaviour of light at interfaces between lossy media and lossless media. We reveal that dissipation makes the wavenumber of the light exceed the maximum allowed at lossless interfaces. This is surprising because, in general, dielectric loss only changes the intensity profiles of the light, so this excess wavenumber cannot be produced in the bulk even if there exists dielectric loss. Additionally, anomalous circular polarisation emerges with dissipation. The direction of the anomalous circular polarisation is transverse, whereas without dissipation the direction of circular polarisation has to be longitudinal. We also discuss how the excess wavenumber can increase optical force and how the anomalous circular polarisation can generate optical transverse torque. This novel state of light produced by dissipation will pave the way for a new generation of optical trapping and manipulation.

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Harnessing optical nonlinearity to control reversal of trapping force under pulsed excitation: a theoretical investigation

Anita Devi and Arijit K De

The dramatic influence of optical Kerr effect on the nature of trapping force/potential under pulsed excitation has recently been explored, particularly in the context of trapping of dielectric nanoparticles (Devi and De 2016 Opt. Express 24 21485–96, Devi and De 2017 Phys. Rev. A 96 023856). However, the utility of such effect has yet to be fully understood, which we discuss here. For a variety of nanoparticles (core, core/shell, and hollow-core), we theoretically show how optical force/potential depend on the nature of the material under pulsed excitation and, most importantly, how the force/potential reverses from repulsive to attractive for certain hollow-core nanoparticles made of high nonlinear refractive index material.

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Optical gradient force for tuning, actuation, and manipulation of nonlinearity in graphene nanomechanical resonator

Aneesh Dash, Chandan Samanta, Praveen Ranganath, S K Selvaraja and A K Naik

Graphene nano-mechanical resonators integrated over waveguides provide a powerful sensing platform based on the interaction of graphene with the evanescent wave. An integrated actuation scheme that does not compromise this interaction is required for optimal usage of the ultra-sensitive platform. Conventional electrical and optical actuation techniques are not favorable towards efficient utilization of the near-field interaction. We propose tuning and actuation of these resonators using on-chip optical gradient force due to the guided wave as an alternative to these conventional techniques. We have used the fundamental quasi-TM optical mode in a silicon waveguide in a finite-element model. We obtain a force–distribution that is spatially correlated with the fundamental mechanical mode of the graphene nano-mechanical resonator. We demonstrate that for an evanescent continuous-wave (CW) optical power of 8 μW, the resonant frequency of the device can be tuned by about 24.5%. With an intensity-modulated optical power ≤0.1 μW, the mechanical mode can be driven to nonlinearity. We also demonstrate cancellation of the Duffing nonlinearity at a CW power of 5.4 μW, which can be used to improve the linear dynamic range of vibration. The distributed optical gradient force can produce linear resonant amplitudes that are 50% higher than those obtained using conventional actuation schemes. This actuation scheme is robust against fluctuations in the evanescent optical power and in the refractive index of the side-cladding of the waveguide. This ensures minimal cross-talk from the optical mode to the mechanical mode in nano-mechanical sensing applications.

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Optical forces and torques exerted on coupled silica nanospheres: novel contributions due to multiple scattering

R M Abraham Ekeroth

When illuminated by light, optically coupled nanoparticles suffer the action of multiple electromagnetic forces. In general, two kinds of forces are assumed: binding forces that make the particles attract/repel each other and scattering forces that push the system forwards. Tangential forces and orbital torques can also be induced to align the interacting particles with the electric field. In this work, new degrees of freedom were found for two coupled silica nanospheres under illumination with linearly-polarized plane waves. The results have a general validity for arbitrary mesoscale systems: multiple scattering of light induces unusual torques and deviation forces. These torques include spin contributions to the movement of the whole system. The results are supported by previous works and pave the way for the engineering of nanoscale devices and nanorotators. Any application based on photonics at mesoscales should take into account the new movements predicted here.

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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.

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A review of complex vector light fields and their applications

Carmelo Rosales-Guzmán, Bienvenu Ndagano and Andrew Forbes

Vector beams, and in particular vector vortex beams, have found many applications in recent times, both as classical fields and as quantum states. While much attention has focused on the creation and detection of scalar optical fields, it is only recently that vector beams have found their place in the modern laboratory. In this review, we outline the fundamental concepts of vector beams, summarise the various approaches to control them in the laboratory, and give a concise overview of the many applications they have spurned.

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Optical transportation of micro-particles by non-diffracting Weber beams

Weiwei Liu, Jie Gao and Xiaodong Yang

The optical transportation of solid polystyrene particles by the non-diffracting Weber beams is demonstrated with both forward and backward motion along the parabolic main lobes of Weber beams in three dimensions. The Weber beams are generated from the complex field modulation based on the spatial light modulator for realizing the optical transportation of micrometer polystyrene particles in an optical tweezers setup. The particle motion and velocity distribution along the main lobes of Weber beams in two different parabolic shapes are characterized and compared.

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Optical manipulation of chiral nanoparticles in vector Airy beam

Wanli Lu, Xu Sun, Huajin Chen, Shiyang Liu and Zhifang Lin

The optical manipulation of chiral nanoparticles in a vector Airy beam with linear polarization is theoretically investigated, and to calculate the optical forces acting on a spherical chiral particle of an arbitrary size beyond the paraxial approximation, a rigorous numerical method based on the generalized Lorenz–Mie theory and Maxwell stress tensor method is presented. It is found that the chiral nanoparticle not only can be stably trapped within the main lobe due to the transverse optical gradient force but also can be transported faster than a conventional particle without chirality along curved trajectories because of the longitudinal scattering optical force. In addition, the particle chirality significantly enhances the longitudinal optical force while slightly affecting the transverse optical force. Our results may provide an additional handle for the optical manipulation of chiral nanoparticles.

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Spatial multiplexing for tailored fully-structured light

E Otte, K Tekce and C Denz

Fully-structured light is an emerging approach to sculpt light in all its degrees of freedom, i.e. amplitude, phase and polarization with a high transverse resolution and complex modulation patterns. Such an attractive and versatile approach for advanced optical trapping or fabricating novel materials still poses fundamental as well as technical challenges. Though the implementation of spatial light modulators (SLMs) has opened up a promising path for this task, up to now, fully-structured light can only be achieved using interferometric methods, multiple SLMs, or split-screen techniques reducing spatial resolution. We present a sophisticated single-beam approach based on spatial multiplexing, which does not only allow joint customization of phase and polarization combined with natural and prospectively even on-demand amplitude modulation, but also ensures high spatial resolution by the use of a single standard SLM in the full-screen mode. We demonstrate the capabilities of our approach realizing double phase modulated light fields, as well as first- and higher-order vector modes with additional global phase modulation and natural amplitude shaping. These findings open new perspectives to optically trap polarization-sensitive, i.e. magnetic particles and advance laser material machining in anisotropic, birefingent matter.

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Optical properties of coupled silicon nanowires and unusual mechanical inductions

R M Abraham Ekeroth

A recent study of the photonic coupling between metallic nanowires has revealed new degrees of freedom in the system. Unexpected spin torques were induced on dimers when illuminated with linearly polarized plane waves. As near-field observables, the spectra of torques showed more resolved resonances than the peaks in typical far-field spectra. Here, the study is extended to silicon dimers. The optical properties of high-dielectric systems are governed by volume resonances, not by surface resonances as is the case in plasmonic arrangements. Differently from plasmonic systems, which show strong mechanical inductions only for p-polarized light, high-dielectric systems experience the action of strong forces and torques for both polarizations s and p. The asymmetry in strong near-fields is responsible for the unusual mechanics of the system. Some consequences of this may include the breaking of the action−reaction principle or the appearance of pulling forces. This numerical study is based on an exact method. The work provides ideas for the design of nanorotators and nanodetectors. It suggests a new viewpoint about optical forces: the resultant dynamics of topological variations of electromagnetic fields.

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Photonic crystal resonances for sensing and imaging

Giampaolo Pitruzzello and Thomas F Krauss

This review provides an insight into the recent developments of photonic crystal (PhC)-based devices for sensing and imaging, with a particular emphasis on biosensors. We focus on two main classes of devices, namely sensors based on PhC cavities and those on guided mode resonances (GMRs). This distinction is able to capture the richness of possibilities that PhCs are able to offer in this space. We present recent examples highlighting applications where PhCs can offer new capabilities, open up new applications or enable improved performance, with a clear emphasis on the different types of structures and photonic functions. We provide a critical comparison between cavity-based devices and GMR devices by highlighting strengths and weaknesses. We also compare PhC technologies and their sensing mechanism to surface plasmon resonance, microring resonators and integrated interferometric sensors.

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Nanofiber quantum photonics

Kali P Nayak, Mark Sadgrove, Ramachandrarao Yalla, Fam Le Kien and Kohzo Hakuta

Recent advances in the coherent control of single quanta of light, photons, is a topic of prime interest, and is discussed under the banner of quantum photonics. In the last decade, the subwavelength diameter waist of a tapered optical fiber, referred to as an optical nanofiber, has opened promising new avenues in the field of quantum optics, paving the way toward a versatile platform for quantum photonics applications. The key feature of the technique is that the optical field can be tightly confined in the transverse direction while propagating over long distances as a guided mode and enabling strong interaction with the surrounding medium in the evanescent region. This feature has led to surprising possibilities to manipulate single atoms and fiber-guided photons, e.g. the efficient channeling of emission from single atoms and solid-state quantum emitters into the fiber-guided modes, high optical depth with a few atoms around the nanofiber, trapping atoms around a nanofiber, and atomic memories for fiber-guided photons. Furthermore, implementing a moderate longitudinal confinement in nanofiber cavities has enabled the strong coupling regime of cavity quantum electrodynamics to be reached, and the long-range dipole–dipole interaction between quantum emitters mediated by the nanofiber offers a platform for quantum nonlinear optics with an ensemble of atoms. In addition, the presence of a longitudinal component of the guided field has led to unique capabilities for chiral light–matter interactions on nanofibers. In this article, we review the key developments of the nanofiber technology toward a vision for quantum photonics on an all-fiber interface.

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Three-dimensional vectorial multifocal arrays created by pseudo-period encoding

Tingting Zeng, Chenliang Chang, Zhaozhong Chen, Hui-Tian Wang and Jianping Ding

Multifocal arrays have been attracting considerable attention recently owing to their potential applications in parallel optical tweezers, parallel single-molecule orientation determination, parallel recording and multifocal multiphoton microscopy. However, the generation of vectorial multifocal arrays with a tailorable structure and polarization state remains a great challenge, and reports on multifocal arrays have hitherto been restricted either to scalar focal spots without polarization versatility or to regular arrays with fixed spacing. In this work, we propose a specific pseudo-period encoding technique to create three-dimensional (3D) vectorial multifocal arrays with the ability to manipulate the position, polarization state and intensity of each focal spot. We experimentally validated the flexibility of our approach in the generation of 3D vectorial multiple spots with polarization multiplicity and position tunability.

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Metalens optical 3D-trapping and manipulating of nanoparticles

Yurii E Geints and Alexander A. Zemlyanov

A principal design of a single-beam optical trap is proposed based on the planar metalens assembled from an ordered array of dielectric microspheres arranged in a closely packed micro-assembly with a hollow center. By means of FDTD numerical simulation, we study in the detail the spatial structure of the optical field within the trap active zone and present an example of metalens-trap operation demonstrating the capturing and propulsion of a glass nanoparticle by the optical field.

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Spinning of particles in optical double-vortex beams

Manman Li, Shaohui Yan, Yansheng Liang, Peng Zhang and Baoli Yao

Optical spin angular momentum, an intrinsic part of optical angular momemtum, can induce a spinning motion of a trapped particle around its own axis in optical manipulation. Focusing of a type of double-vortex (DV) input field obtained by linearly superposing two optical vortex beams with equal but opposite topological charges, yields a multi-lobe focal field, each of which has non-vanishing optical spin angular momentum, and is capable of trapping particle while spinning the particle around a certain axis. Significantly, both the focusing properties and the spinning dynamics are strongly polarization dependent. For instance, the focused field of a circularly polarized double-vortex (CPDV) beam carries transverse and longitudinal spin angular momenta, inducing axial spinning of the trapped particles, whereas the focused field of a radially polarized double-vortex (RPDV) beam possesses purely transverse spin angular momentum and can drive the particles to spin transversely to the optical axis. These results may find potential applications in light beam shaping and optical manipulations.

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