Second-harmonic generation in optically trapped nonlinear particles with pulsed lasers

Nonlinearity-Induced Optical Torque

Optically induced mechanical torque driving rotation of small objects requires the presence of absorption or breaking cylindrical symmetry of a scatterer. A spherical nonabsorbing particle cannot rotate due to the conservation of the angular momentum of light upon scattering. Here, we suggest a novel physical mechanism for the angular momentum transfer to nonabsorbing particles via nonlinear light scattering. The breaking of symmetry occurs at the microscopic level manifested in nonlinear negative optical torque due to the excitation of resonant states at the harmonic frequency with higher projection of angular momentum. The proposed physical mechanism can be verified with resonant dielectric nanostructures, and we suggest some specific realizations.

Plasmonic tweezers: for nanoscale optical trapping and beyond

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science.

Nonlinear accelerated orbiting motions of optical trapped particles through two-photon absorption

Vortex beams carrying optical angular momentum (AM) could drive the orbital motion of a small particle around the optical axis. In general, the orbital rotation speed of trapped particles increases linearly with the increasing laser power. Beyond the linear optics regime, in this work, we investigate both the optical force and torque on a two-photon absorbing Rayleigh particle produced by the tightly focused femtosecond-pulsed circularly polarized vortex beam. Different from the trapping dynamics of particles without two-photon absorption (TPA), it is shown that the orbital motion of trapped particles with TPA accelerates nonlinearly as the laser power increases. Moreover, the orbital motion acceleration of trapped particles is proportional to the TPA coefficient. The corresponding underlying mechanism is discussed in detail. Our results may find interesting applications in the characterization of the optical nonlinearity of a single nanoparticle, and AM manipulation and particle transportation in the nonlinear optics regime.

Longitudinal position dependence of the second-harmonic generation of optically trapped silica microspheres

We report the design of a setup combining the simultaneous and independent optical trapping and second-harmonic generation (SHG) of 1 µm diameter silica microspheres with two independent laser beams. Optical trapping is achieved with a tightly focused continuous wave infrared laser beam whereas the SHG intensity from the trapped microparticles is obtained with a 810 nm femtosecond pulsed laser. The silica microparticles are dispersed in an aqueous solution, and a microfluidic channel flow is used to remove untrapped microparticles. We show by the perpendicular displacement of the optical trap from the microfluidic channel wall that it is possible to control the contribution of the channel wall/solution interface to the overall SHG intensity. Stable trapping and SHG detection of two microparticles is also demonstrated. Combining the independent trapping of centrosymmetrical silica microparticles with SHG offers new avenues for analytical studies like surface sensing or all-optical devices where the SHG intensity is controlled by the trapping beam.

Bio-Molecular Applications of Recent Developments in Optical Tweezers

In the past three decades, the ability to optically manipulate biomolecules has spurred a new era of medical and biophysical research. Optical tweezers (OT) have enabled experimenters to trap, sort, and probe cells, as well as discern the structural dynamics of proteins and nucleic acids at single molecule level. The steady improvement in OT's resolving power has progressively pushed the envelope of their applications; there are, however, some inherent limitations that are prompting researchers to look for alternatives to the conventional techniques. To begin with, OT are restricted by their one-dimensional approach, which makes it difficult to conjure an exhaustive three-dimensional picture of biological systems. The high-intensity trapping laser can damage biological samples, a fact that restricts the feasibility of in vivo applications. Finally, direct manipulation of biological matter at nanometer scale remains a significant challenge for conventional OT. A significant amount of literature has been dedicated in the last 10 years to address the aforementioned shortcomings. Innovations in laser technology and advances in various other spheres of applied physics have been capitalized upon to evolve the next generation OT systems. In this review, we elucidate a few of these developments, with particular focus on their biological applications. The manipulation of nanoscopic objects has been achieved by means of plasmonic optical tweezers (POT), which utilize localized surface plasmons to generate optical traps with enhanced trapping potential, and photonic crystal optical tweezers (PhC OT), which attain the same goal by employing different photonic crystal geometries. Femtosecond optical tweezers (fs OT), constructed by replacing the continuous wave (cw) laser source with a femtosecond laser, promise to greatly reduce the damage to living samples. Finally, one way to transcend the one-dimensional nature of the data gained by OT is to couple them to the other large family of single molecule tools, i.e., fluorescence-based imaging techniques. We discuss the distinct advantages of the aforementioned techniques as well as the alternative experimental perspective they provide in comparison to conventional OT.

Second-harmonic generation of single BaTiO3 nanoparticles down to 22 nm diameter

We investigate the second-harmonic generation (SHG) signal from single BaTiO3 nanoparticles of diameters varying from 70 nm down to 22 nm with a far-field optical microscope coupled to an infrared femtosecond laser. An atomic force microscope is first used to localize the individual particles and to accurately determine their sizes. Power and polarization-dependent measurements on the individual nanoparticles reveal a diameter range between 30 and 20 nm, where deviations from bulk nonlinear optical properties occur. For 22 nm diameter particles, the tetragonal crystal structure is not applicable anymore and competing effects due to the surface to volume ratio or crystallographic modifications are taking place. The demonstration of SHG from such small nanoparticles opens up the possibilities of using them as bright coherent biomarkers. Moreover, our work shows that measuring the SHG of individual nanoparticles reveals critical material properties, opening up new possibilities to investigate ferroelectricity at the nanoscale.

Gold nanorods as probes in two-photon fluorescence correlation spectroscopy: emerging applications and potential artifacts

Owing to the highly efficient two-photon fluorescence of gold nanorods and very short fluorescence lifetime compared with the rotational correlation time, the rotation and diffusion of a single gold nanorod can be easily observed by two-photon fluorescence correlation spectroscopy (TP-FCS). This property, along with the previous successful use as a contrast agent in two-photon fluorescence imaging, suggests a potential application in TP-FCS as well. Although the FCS measurement becomes highly efficient with gold nanorods as probes, the amplitude and temporal decay of the measured correlation functions depend critically on excitation power. Here, we investigate various photophysical processes of gold nanorods to determine the cause of such a sensitive power dependency. This understanding provides a basis for choosing appropriate FCS models to recover reasonable physical parameters. Although the correlation function amplitude G(0) is 32 times lower when the excitation power increases from 20 µW to 1.12 mW, the application of a saturation-modified FCS model yields very good fit to each data set and the fitted concentration of 0.64 nM is comparable to the 0.7 nM given by the inductively coupled plasma mass spectrometry measurement. The FCS assay appears to be an efficient method for the quantification of gold nanorods when correctly interpreted. However, even with the saturation considered in the fitting model, the fitted rotational and translational diffusion rates are getting faster as the power increases. This indicates that other effects such as photothermal effects may raise the local temperature, and thus increasing the rotational and translational diffusion rate.

Effect of pulse temporal shape on optical trapping and impulse transfer using ultrashort pulsed lasers

We investigate the effects of pulse duration on optical trapping with high repetition rate ultrashort pulsed lasers, through Lorentz-Mie theory, numerical simulation, and experiment. Optical trapping experiments use a 12 femtosecond duration infrared pulsed laser, with the trapping microscope's temporal dispersive effects measured and corrected using the Multiphoton Intrapulse Interference Phase Scan method. We apply pulse shaping to reproducibly stretch pulse duration by 1.5 orders of magnitude and find no material-independent effects of pulse temporal profile on optical trapping of 780nm silica particles, in agreement with our theory and simulation. Using pulse shaping, we control two-photon fluorescence in trapped fluorescent particles, opening the door to other coherent control applications with trapped particles.

Three-dimensional harmonic holographic microcopy using nanoparticles as probes for cell imaging

Luminescent markers play a key role in imaging techniques for life science since they provide a contrast mechanism between signal and background. We describe a new type of marker using second harmonic generation (SHG) from noncentrosymmetric BaTiO(3) nanocrystals. These nanoparticles are attractive due to their stable, non-saturating and coherent signal with a femtosecond-scale response time and broad flexibility in the choice of excitation wavelength. We obtained monodispersed BaTiO(3) nanoparticles in colloidal suspensions by coating the particle surface with amine groups. We characterized the SHG efficiency of 90-nm BaTiO(3) particles experimentally and theoretically. Moreover, we use the coherent SHG signal from BaTiO(3) nanoparticles for three-dimensional (3D) imaging without scanning. We built a harmonic holographic (H(2)) microscope which records digital holograms at the second harmonic frequency. For the first time, high-resolution 3D distributions of these SHG markers in mammalian cells are successfully captured and interpreted by the H(2) microscope.

Nonlinear optical response from single spheres coated by a nonlinear monolayer

We detected the second-order nonlinear response from single isolated spheres comprised from a centrosymmetric material but covered by a layer of a material with strong second-order nonlinear properties and isolated from an ensemble by the optical trapping technique. We show that when large size parameter spheres are used, the measured second-harmonic efficiency deviates strongly from the prediction of the nonlinear Rayleigh scattering theory. Our results are in very good agreement with the predictions from the exact nonlinear Mie scattering theory.

Femtosecond optical tweezers for in-situ control of two-photon fluorescence

We perform a comparison of optical tweezing using continuous wave (cw) and femtosecond lasers. Measurement of the relative Q-values in the femtosecond and cw regimes shows that femtosecond optical tweezers are just as effective as cw optical tweezers. We also demonstrate simultaneous optical tweezing and in-situ control of two-photon fluorescence (at 400nm) from dye-doped polymer microspheres. By switching the 800 nm tweezing laser source between femtosecond and cw regimes, we turned the fluorescent signal from the tweezed particle on and off while maintaining an equivalent tweezing action. Femtosecond lasers can thus be used for optical tweezing and simultaneously utilized to induce nonlinear multi-photon processes such as two-photon excitation or even photoporation.

Radiation force on a nonlinear microsphere by a tightly focused Gaussian beam

We determine the characteristics of the radiation force that is exerted on a nonresonant nonlinear (Kerr-effect) rigid microsphere by a strongly focused Gaussian beam when diffraction and interference effects are significant (sphere radius a < or = illumination wavelength lambda). The average force is calculated from the surface integral of the energy-momentum tensor consisting of incident, scattered, and internal electromagnetic field vectors, which are expressed as multipole spherical-wave expansions. The refractive index of a Kerr microsphere is proportional to the internal field intensity, which is computed iteratively by the Rytov approximation (residual error of solution, 10(-30). The expansion coefficients for the field vectors are calculated from the approximated index value. Compared with that obtained in a dielectric (linear) microsphere in the same illumination conditions, we find that the force magnitude on the Kerr microsphere is larger and increases more rapidly with both a and the numerical aperture of the focusing objective. It also increases nonlinearly with the beam power unlike that of a linear sphere. The Kerr nonlinearity also leads to possible reversals of the force direction. The proposed technique is applicable to other types of weak optical nonlinearity.

Simulated dynamic behavior of single and multiple spheres in the trap region of focused laser beams

An enhanced photon propagation method is used to calculate the forces and torque present on each sphere of a system of particles located in the vicinity of focused laser-trapping beams. Infinitesimal trajectory displacements are computed through classical mechanics and the new particle position used to define the next trapping system geometry considered. Repeated applications of the process, implemented as a computer program, enables full trajectory plotting and the dynamic behavior of the systems to be explored as a function of time.