Optical trapping, where microscopic particles are trapped and manipulated by light is a powerful and widespread technique, with the single-beam gradient trap (also known as optical tweezers) in use for a large number of biological and other applications.
The forces and torques acting on a trapped particle result from the transfer of momentum and angular momentum from the trapping beam to the particle.
Despite the apparent simplicity of a laser trap, with a single particle in a single beam, exact calculation of the optical forces and torques acting on particles is difficult. Calculations can be performed using approximate methods, but are only applicable within their ranges of validity, such as for particles much larger than, or much smaller than, the trapping wavelength, and for spherical isotropic particles.
This leaves unfortunate gaps, since wavelength-scale particles are of great practical interest because they are readily and strongly trapped and are used to probe interesting microscopic and macroscopic phenomena, and non-spherical or anisotropic particles, biological, crystalline, or other, due to their frequent occurance in nature, and the possibility of rotating such objects or controlling or sensing their orientation.
The systematic application of electromagnetic scattering theory can provide a general theory of laser trapping, and render results missing from existing theory. We present here calculations of force and torque on a trapped particle obtained from this theory and discuss the possible applications, including the optical measurement of the force and torque.
Laser trapping near the surface of a nanostructured substrate is demonstrated. Stable microbubbles with radii of 1-20micrometers have been created and manipulated with sub-micron precision by a focused laser beam in an immersion oil covering arrays of pairs of gold nanopillars deposited on a glass substrate. The threshold for bubble creation and trapping characteristics depended on near-field coupling of nanopillars. The nanometric laser tweezers showed giant trapping efficiency of Q~50 for the trapped microbubbles.
We present experimental evidence of plasmonic-enhanced optical tweezers, of polystyrene beads in deionized water in the vicinity of metal-coated nanostructures. The optical tweezers operate with a continuous wave (CW) near-infrared laser. We employ a Cu/Au bilayer that significantly improves dissipation of heat generated by the trapping laser beam and avoid de-trapping from heat convection currents. We investigate the improvement of the optical trapping force, the effective trapping quality factor, and observe an exponential distance dependence of the trapping force from the nanostructures, ex…
We report on the experimental observation of incoherently coupled dark vector soliton trapping in a fiber laser. Dark vector solitons along the two orthogonal polarization directions though with large difference in central frequency, energy and darkness could be still synchronously trapped as a group velocity locked dark vector soliton in virtue of the incoherent interactions. Our numerical simulation could well rebirth the experimental observations and confirm the existence of incoherently coupled dark vector soliton under certain conditions.
We investigate the phenomenon of the transverse breaking of symmetry of Gaussian lasers transmitted through dielectrics, showing for which incidence conditions and beam parameters it can be observed in optical experiments. The numerical analysis, done by using the integral form of the transmitted beam at the lower interface of a dielectric prism, shows, for incidence approaching the critical region, a laser planar trapping. By using an analytic approximation for the intensity of lower transmitted beam, we model the wave front curvature by a closed expression which depends on the refractive ind…
To identify the optimum conditions for the optical trapping of a droplet under microgravity, we theoretically analyzed the efficiency of trapping with counter laser beams. We found that the distance between the two foci is an important parameter for obtaining stable trapping conditions. We also performed an optical trapping experiment with counter laser beams under microgravity. The experimental results correspond well to the theoretical prediction.
Optical manipulation has attracted remarkable interest owing to its versatile and non-invasive nature. However, conventional optical trapping remains inefficient for the nanoscopic world. The emergence of plasmonics in recent years has brought a revolutionary change in overcoming limitations due to diffraction and the requirements for high trapping laser powers. Among the near-field optical trapping cavity-based systems, Fano resonant optical tweezers have a robust trapping capability. In this work, we experimentally demonstrate sequential trapping of 20 nm particles through the use of metamat…
Standard optical tweezers rely on optical forces that arise when a focused laser beam interacts with a microscopic particle: scattering forces, which push the particle along the beam direction, and gradient forces, which attract it towards the high-intensity focal spot. Importantly, the incoming laser beam is not affected by the particle position because the particle is \emph{outside} the laser cavity. Here, we demonstrate that \emph{intracavity nonlinear feedback forces} emerge when the particle is placed \emph{inside} the optical cavity, resulting in orders-of-magnitude higher confinement al…
It has recently been shown that optical reflection gratings fabricated directly into an atom chip provide a simple and effective way to trap and cool substantial clouds of atoms [1,2]. In this article we describe how the gratings are designed and micro-fabricated and we characterise their optical properties, which determine their effectiveness as a cold atom source. We use simple scalar diffraction theory to understand how the morphology of the gratings determines the power in the diffracted beams.
Analytical solution for optical trapping force on a spherical dielectric particle for an arbitrary positioned focused beam is presented in a generalized Lorenz-Mie and vectorial diffraction theory. In this case the exact electromagnetic field is considered in the focal region. A double tweezers setup was employed to perform ultra sensitive force spectroscopy and observe the forces, demonstrating the selectively couple of the transverse electric (TE), transverse magnetic (TM) modes by means of the beam polarization and positioning, and to observe correspondent morphology-dependent resonances (M…
We present a laser beam shaping method using acousto-optic deflection of light and discuss its application to dipole trapping of ultracold atoms. By driving the acousto-optic deflector with multiple frequencies, we generate an array of overlapping diffraction-limited beams that combine to form an arbitrary-shaped smooth and continuous trapping potential. Confinement of atoms in a flat-bottomed potential formed by a laser beam with uniform intensity over its central region confers numerous advantages over the harmonic confinement intrinsic to Gaussian beam dipole traps and many other trapping s…
Simulating real-time scenarios to predict optical trapping behavior for continuous wave (CW) as well as femtosecond pulsed lasers is a challenging problem. This is especially so, because, for a tightly focused laser, one must also include optical Kerr effect as well as thermal nonlinearity. We show here the distinct differences between CW and femtosecond pulsed laser mediated optical trapping via their effect in scattering and gradient forces in the Rayleigh regime. In our newly developed model, it will also be easier to predict the stability of the trap in real optical trapping scenario as we…
An on-chip multi-grating device is proposed to interface single-atoms and integrated photonic circuits, by guiding and focusing lasers to the area with ~10um above the chip for trapping, state manipulation, and readout of single Rubidium atoms. For the optical dipole trap, two 850 nm laser beams are diffracted and overlapped to form a lattice of single-atom dipole trap, with the diameter of optical dipole trap around 2.7um. Similar gratings are designed for guiding 780 nm probe laser to excite and also collect the fluorescence of 87Rb atoms. Such a device provides a compact solution for future…
We reduce the intensity noise of laser light by using an electro-optic modulator and a cousto-optic modulator in series. The electro-optic modulator reduces noise at high frequency(10 kHz to 1 MHz), while the acousto-optic modulator sets the average power of the light and reduces noise at low frequency (up to 10 kHz). The light is then used to trap single sodium atoms in an optical tweezer, where the lifetime of the atoms is limited by parametric heating due to laser noise at twice the trapping frequency. With our noise eater, the noise is reduced by up to 15 dB at these frequencies, and the l…
We present a high-power tunable deep-ultraviolet (DUV) laser that uses two consecutive cavity enhanced doubling stages with LBO and CLBO crystals to produce the fourth harmonic of an amplified homebuilt external cavity diode laser. The system generates up to 2.75 W of 261.5 nm laser light with a ~2 W stable steady-state output power and performs second harmonic generation in a largely unexplored high intensity regime in CLBO for continuous wave DUV light. We use this laser to perform fluorescence spectroscopy on the $X^1Σ\leftarrow A^1Π$ transition in a cold, slow beam of AlCl molecules and pr…
When light is confined in all three directions and in dimensions of the order of the light wavelength, full discretization of the photon spectra and distinctive phenomena occur, the Purcell effect and the inhibition of emission of atoms being two paradigmatic examples. Diverse solid-state devices that confine light in all three dimensions have been developed and applied. Typically the confinement volume, operating wavelength, and quality factor of these resonators are set by construction, and small variations of these characteristics with external perturbations are targeted for applications in…
In this work, we demonstrate trapping of microparticles using a plasmonic tweezers based on arrays of annular apertures. The transmission spectra and the E- field distribution are simulated to calibrate the arrays. Theoretically, we observe sharp peaks in the transmission spectra for dipole resonance modes and these are redshifted as the size of the annular aperture is reduced. We also expect an absorption peak at approximately 1,115 um for the localised plasmon resonance. Using a laser frequency between the two resonances, multiple plasmonic hotspots are created and used to trap and transport…
Plasmonic nanoapertures generate strong field gradients enabling efficient optical trapping of nano-objects. However, because the infrared laser used for trapping is also partly absorbed into the metal leading to Joule heating, plasmonic nano-optical tweezers face the issue of local temperature increase. Here, we develop three independent methods based on molecular fluorescence to quantify the temperature increase induced by a 1064 nm trapping beam focused on single and double nanoholes milled in gold films. We show that the temperature in the nanohole can be increased by 10°C even at the mode…
We fabricate a miniature spherical mirror for tightly focusing an optical dipole trap for neutral atoms. The mirror formation process is modelled to predict the dimensions for particular fabrication parameters. We integrate the spherical mirror with a neutral atom experiment to trap and detect a single atom with high efficiency. The mirror serves the dual purpose of focusing the dipole trap as well as collection of the atomic fluorescence into an optical fibre.
We analyze evanescent fields of laser written waveguides and show that they can be used to trap atoms close to the surface of an integrated optical atom chip. In contrast to subwavelength nanofibres it is generally not possible to create a stable trapping potential using only the fundamental modes. This is why we create a stable trapping potential by using two different laser colors, such that the waveguide supports two modes for the blue detuned laser, while for the red detuned light the waveguide has only a single mode. In particular, we study such a two-color trap for Cesium atoms, and calc…
We have demonstrated Zeeman slowing and capture of neutral Ra-225 and Ra-226 atoms in a magneto-optical trap (MOT). The intercombination transition 1S0-3P1 is the only quasi-cycling transition in radium and was used for laser-cooling and trapping. Repumping along the 3D1-1P1 transition extended the lifetime of the trap from milliseconds to seconds. Room-temperature blackbody radiation was demonstrated to provide repumping from the metastable 3P0 level. We measured the isotope shift and hyperfine splittings on the 3D1-1P1 transition with the laser-cooled atoms, and set a limit on the lifetime o…
Photophoretic force due to the optically-induced thermal effect provides an effective way to manipulate the light-absorbing particles suspended in ambient gases. However, how this force temporally responds to the intensity modulation of the illumination light is unclear. Here, by vertically trapping a micron-sized absorbing particle with a negative photophoretic force generated by a focused Gaussian beam, we demonstrate that the temporal change in the photophoretic force in response to the intensity modulation is remarkably slow (with a time constant up to ~1 s) due to the slow change in the p…
Optical trapping can be used to manipulate the three-dimensional (3-D) motion of spherical particles based on the simple prediction of optical forces and the responding motion of samples. However, controlling the 3-D behaviour of non-spherical particles with arbitrary orientations is extremely challenging, due to experimental difficulties and the extensive computations. Here, we achieved the real-time optical control of arbitrarily shaped particles by combining the wavefront shaping of a trapping beam and measurements of the 3-D refractive index (RI) distribution of samples. Engineering the 3-…
Plasmonic nanostructures can overcome Abbe's diffraction limit to generate strong gradient fields, enabling efficient optical trapping of nano-sized particles. However, it remains challenging to achieve stable trapping with low incident laser intensity. Here, we demonstrate a Fano resonance-assisted plasmonic optical tweezers (FAPOT), for single nanoparticle trapping in an array of asymmetrical split nano-apertures, milled on a 50 nm gold thin film. Stable trapping is achieved by tuning the trapping wavelength and varying the incident trapping laser intensity. A very large normalized trap stif…
We report the first efficient trapping of rare-earth Yb atoms with a high-power violet laser diode (LD). An injection-locked violet LD with a 25 mW frequency-stabilized output was used for the magneto-optical trapping (MOT) of fermionic as well as bosonic Yb isotopes. A typical number of $4\times 10^6$ atoms for $^{174}$Yb with a trap density of $\sim 1\times10^8/$cm$^3$ was obtained. A 10 mW violet external-cavity LD (ECLD) was used for the one-dimensional (1D) slowing of an effusive Yb atomic beam without a Zeeman slower resulting in a 35-fold increase in the number of trapped atoms. The ove…
Developing angular trapping methods, which will enable optical tweezers to rotate a micronized bead, is of great importance for the studies of biomacromolecules during a wide range of torque-generation processes. Here we report a novel controlled angular trapping method based on composite Janus particles. We used a chemically synthesized Janus particle, which consists of two hemispheres made of polystyrene (PS) and poly(methyl methacrylate) (PMMA) respectively, as a model system to demonstrate this method. Through computational and experimental studies, we demonstrated the feasibility to contr…
A frequency-stabilized light source emitting at 556 nm is realized by frequency-doubling a 1112-nm laser, which is phase-locked to a fiber-based optical frequency comb. The 1112-nm laser is either an ytterbium (Yb)-doped distributed feedback fiber laser or a master-slave laser system that uses an external cavity diode laser as a master laser. We have achieved the continuous frequency stabilization of the light source over a five-day period. With the light source, we have completed the second-stage magneto-optical trapping (MOT) of Yb atoms using the 1S0 - 3P1 intercombination transition. The t…
High-power and narrow-linewidth laser light is a vital tool for atomic physics, being used for example in laser cooling and trapping and precision spectroscopy. Here we produce Watt-level laser radiation at 457.49 nm and 460.86 nm of respective relevance for the cooling transitions of cadmium and strontium atoms. This is achieved via the frequency doubling of a kHz-linewidth vertical-external-cavity surface-emitting laser (VECSEL), which is based on a novel gain chip design enabling lasing at > 2 W in the 915-928 nm region. Following an additional doubling stage, spectroscopy of the $^1S_0\to{…
We describe a high-power, frequency-tunable, external cavity diode laser (ECDL) system near 626 nm useful for laser cooling of trapped $^9$Be$^+$ ions. A commercial single-mode laser diode with rated power output of 170 mW at 635 nm is cooled to $\approx - 31$ C, and a single longitudinal mode is selected via the Littrow configuration. In our setup, involving multiple stages of thermoelectric cooling, we are able to obtain $\approx$130 mW near 626 nm, sufficient for efficient frequency doubling to the required Doppler cooling wavelengths near 313 nm in ionized Beryllium. In order to improve no…
The optical trapping techniques have been extensively used in physics, biophysics, micro-chemistry, and micro-mechanics to allow trapping and manipulation of materials ranging from particles, cells, biological substances, and polymers to DNA and RNA molecules. In this Letter, we present a convenient and effective way to generate a novel phenomenon of trapping, named trap split, in a conventional four-level double-Λatomic system driven by four femtosecond Laguerre-Gaussian laser pulses. We find that trap split can be always achieved when atoms are trapped by such laser pulses, as compared to Ga…
Radiation pressure forces in a focussed laser beam can be used to trap microscopic absorbing particles against a substrate. Calculations based on momentum transfer considerations show that stable trapping occurs before the beam waist, and that trapping is more effective with doughnut beams. Such doughnut beams can transfer angular momentum leading to rotation of the trapped particles. Energy is also transferred, which can result in heating of the particles to temperatures above the boiling point of the surrounding medium.
We present theoretical and experimental results of coherent population trapping spectra on a multi-level $^{40}$Ca$^+$-type configuration, adding a third beam to the standard two-laser $Λ$ system to avoid undesired optical pumping. We show that the extra laser can preserve the nature of the dark resonances or introduce decoherence depending on its power. Experiments are carried out using a single trapped $^{40}$Ca$^+$ ion in the $S_{1/2}-P_{1/2}-D_{3/2}$ manifold. Theoretically, the problem is solved with a Floquet-like expansion of the Liouvillian that correctly predicts all of the measured s…
The principle of optical trapping is conventionally based on the interaction of optical fields with linear induced polarizations. However, the optical force originating from the nonlinear polarization becomes significant when nonlinear optical nanoparticles are trapped by ultrafast laser pulses. Herein we establish the time-averaged optical forces on a nonlinear optical nanoparticle using high-repetition-rate ultrafast laser pulses, based on the linear and nonlinear polarization effects. We investigate the dependence of the optical forces on the magnitudes and signs of the refractive nonlinear…
We present a joint theoretical and experimental characterization of the coherent population trapping (CPT) resonance excited on the D1 line of 87Rb atoms by bichromatic linearly polarized laser light. We observe high-contrast transmission resonances (up to 25%), which makes this excitation scheme promising for miniature all-optical atomic clock applications. We also demonstrate cancellation of the first-order light shift by proper choice of the frequencies and relative intensities of the two laser field components. Our theoretical predictions are in good agreement with the experimental results…
Optical trapping and manipulation using laser beams play a key role in many areas including biology, atomic science, and nanofabrication. Here, we propose and experimentally demonstrate the first use of a vortex-pair beam in optical trapping and manipulation. We successfully trap two spherical microparticles simultaneously by a single vortex-pair beam. Precisely position-controllable manipulation of the trapped spherical microparticles is realized by adjusting the off-axis distance of the vortices on the initial phase plane of the vortex-pair beam. Based on the feature of the vortex-pair beam,…
After pumped from $5s_{1/2}$ ground state to $6p_{1/2}$ state, the population inversion between $6s_{1/2}$ and $5p_{1/2,3/2}$ will be established for Rubidium four-level active optical clock. In this paper, we calculate AC Stark shift due to lattice trapping laser which dominates the frequency shift of clock transition in lattice trapped Rubidium four-level active optical clock. Several magic wavelengths are found that can form desired optical lattice trapping potential. By choosing a proper intensity and linewidth of trapping laser, the fractional frequency uncertainty of clock transition due…
Transmission of high power laser beams through partially absorbing materials modifies the light propagation via a thermally-induced effect known as thermal lensing. This may cause changes in the beam waist position and degrade the beam quality. Here we characterize the effect of thermal lensing associated with the different elements typically employed in an optical trapping setup for cold atoms experiments. We find that the only relevant thermal lens is represented by the $TeO_2$ crystal of the acousto-optic modulator exploited to adjust the laser power on the atomic sample. We then devise a s…
Optical tweezers, the three-dimensional confinement of a nanoparticle by a strongly focused beam of light, have been widely employed in investigating biomaterial nanomechanics, nanoscopic fluid properties, and ultrasensitive detections in various environments such as inside living cells, at gigapascal pressure, and under high vacuum. However, the cryogenic operation of solid-state-particle optical tweezers is poorly understood. In this study, we demonstrate the optical trapping of metallic and dielectric nanoparticles in superfluid helium below 2 K, which is two orders of magnitude lower than …
We develop an optical tweezers system using a single dual-mode optical fiber where mesoscopic absorbing particles can be trapped in three dimensions and manipulated employing photophoretic forces. We generate a superposition of fundamental and first order Hermite-Gaussian beam modes by the simple innovation of coupling a laser into a commercial optical fiber designed to be single mode for a wavelength higher than that of the laser. We achieve robust trapping of the absorbing particles for hours using both the pure fundamental and superposition mode beams and attain large manipulation velocitie…
Acoustic tweezers can manipulate microscopic objects and cells independently of the optical, magnetic and electrical properties of the objects or their medium. However, because ultrasonic waves are attenuated within few millimeters, existing devices must synthesize intricate and powerful acoustic fields in a very narrow footprint immediately close to the manipulated object. Here we show that the design of microscale acoustic tweezers can be considerably simplified by taking advantage of the nonlinear nature of the acoustic trapping force. In our experiment, a featureless piezoelectric crystal …
We carry out a systematic study of the static and dynamic polarizabilities of the potassium atom using a first-principles high-precision relativistic all-order method in which all single, double, and partial triple excitations of the Dirac-Fock wave functions are included to all orders of perturbation theory. Recommended values are provided for a large number of electric-dipole matrix elements. Static polarizabilities of the 4s, 4p_j, 5s, 5p_j, and 3d_j states are compared with other theory and experiment where available. We use the results of the polarizability calculations to identify magic …
We present experimental results on optical trapping of Yb-doped $β-$NaYF sub-wavelength-thickness high-aspect-ratio hexagonal prisms with a micron-scale radius. The prisms are trapped in vacuum using an optical standing wave, oriented with the normal vector to their face along the beam propagation direction, and exhibit characteristic modes of three translational and two torsional degrees of freedom. The measured motional spectra are compared with numerical simulations. This plate-like geometry simultaneously enables trapping with low photon-recoil-heating, high mass, and high trap frequency, …
We demonstrate an optical bottle beam trap created by interfering two fundamental Gaussian beams with different waists. The beams are derived from a single laser source using a Mach-Zehnder interferometer whose arms have unequal magnifications. Destructive interference of the two beams from the Mach-Zehnder leads to a three dimensional intensity null at the mutual focus of the beams. We demonstrate trapping of cold Cesium atoms in a blue detuned bottle beam trap.
Superpositions of paraxial Laguerre-Gauss laser beam modes to generate optical potentials based on the optical dipole force are investigated theoretically. Parabolic and other monomial potentials with even powers, in cylindrical and spherical symmetry, with large diameters, can be synthesized. This superposition approach promises to help with high quality atom cloud manipulation and imaging.
In this article we explore the requirements for enabling high quality optically detected magnetic resonance (ODMR) spectroscopy in a conventional gradient force optical tweezers using nanodiamonds containing nitrogen-vacancy (NV$^{-}$) centres. We find that modulation of the infrared (1064 nm) trapping laser during spectroscopic measurements substantially improves the ODMR contrast compared with continuous wave trapping. The work is significant as it allows trapping and quantum sensing protocols to be performed in conditions where signal contrast is substantially reduced. We demonstrate the ut…
