Optical trapping and polarization-controlled scattering of dielectric spherical nanoparticles by femtosecond laser pulses
Graphical abstract
Highlights
► Optical trapping behavior of 50-nm-sized polystyrene beads by fs laser beam. ► Trapped nanoparticles in a single trapping site are observed as a bright spot. ► Scattered nanoparticles from the trapping site are observed as a bright locus. ► We evaluated our findings based on radiation and temporal forces of laser pulses. ► Axial electric field induces lateral components of scattering and temporal forces.
Introduction
One of the successful applications of mode-locked lasers is the ultrafast time-resolved spectroscopies, which provide the absorption, vibrational, or emission spectra of atoms or molecules on extremely short time scales after their excitation with ultrashort laser pulses. The research group of Prof. M. Martin is one of pioneers who have utilized transient absorption spectroscopy to decipher the dynamics and mechanisms of fundamental photo-induced processes [1]. Their reports on the insights of driving forces and primary occurring events in the photo-induced dynamics of various chromophores, photoactive proteins, or biomimetics are important advances in our understanding of the photo-processes, particularly the functionality of the biomaterials in relation with their electronic structures [2], [3], [4], [5], [6], [7].
Another important laser application is optical trapping (also called optical tweezers), exploiting the optical gradient force, which can confine micrometer to submicrometer-sized objects in the focal spot [8], [9]. In this phenomenon, a high numerical aperture lens is necessarily required to focus tightly the continuous-wave (cw) laser beams into a diffraction-limited spot size [10], [11]. With its potential ability of non-destructive tool to immobilize, reorient, and transfer the micro-to-submicrometer sized dielectric or metallic particles, this technique has been widely applied in various fields of sciences with target materials ranging from small particles [12], polymers [13], [14], clusters of amino acids [15], [16], [17], [18], [19], to biological substances [20], and has become indispensable in single-molecule measurements [21], [22].
Recently, the optical trapping technique is further developed by utilizing ultrashort laser pulses. By the femtosecond laser pulses, optical trapping of micrometer-sized silica spheres was found to be as effective as cw optical tweezers, and trap stiffness was related to average power of the laser pulses [23]. With the ultrashort laser pulses, however, several phenomena have been revealed, including optical trapping of as small as a few nm-sized CdTe quantum dots or the depositions of CdS nanoparticles with grain size down to 25 nm [24], [25]. For the trapping of gold nanoparticles by laser pulses, the trapping site splits up into two equivalent positions around the focal center, demonstrating that high nonlinear optical susceptibility of the target materials can modify the shapes of gradient force and trapping potential [26]. More recently, the femtosecond laser pulses with the power less than 200 mW has been successfully applied to confine an individual polystyrene bead with a diameter of a few tens of microns (the particle sizes within the framework of geometrical optics regime), but the microparticle was pushed away from the trapping site when the focal position was shifted to its downstream surface due to secondary convergence of the laser pulses that reduces water breakdown threshold [27].
In this article, we report on an experimental study exploring the trapping behavior of the dielectric spherical nanoparticles (50-nm-diameter polystyrene beads), suspended in liquid water medium, by femtosecond laser pulses tightly focused by high numerical aperture lens. We show that as compared with the cw mode, the laser pulses can trap a larger number of nanoparticles. In addition, the laser pulses induce nanoparticle flows out of the focal spot in two opposite directions, in an alternating manner, controlled by the laser polarization. To understand this phenomenon, we evaluate both radiation (gradient and scattering) and temporal forces (the latter is also called pulse radiation force) by adopting Lorentz force of fundamental Gaussian beam exerted on Rayleigh particles [10], [28], and by applying scattering cross section value of the nanoparticles obtained based on Mie theory [29]. We demonstrate that the axial electric field produced by the high numerical aperture objective lens is responsible for the present novel phenomenon.
Section snippets
Optical setup
To experimentally exemplify the trapping behavior of the nanoparticles by femtosecond laser pulses, we developed an experimental setup based on an inverted microscope (Olympus IX71), as shown in Fig. 1. We used a 800-nm fundamental mode of Ti:sapphire (Tsunami; Spectra Physics) laser beam, which can be operated in cw or femtosecond-pulse mode, acting as the trapping beam. When it was operated in the pulse mode, the pulse duration was compressed by a pair of prisms to be about 90 fs, and the
Results
With the laser trapping beam operated in the femtosecond pulse mode at the average power of 350 mW, we observed a brighter scattering light at the focal spot compared with the surrounding area. We should note that such bright scattering light was never observed in a neat solvent. In addition to scattering light at the focal spot, bright locus of scattered polymer beads, just like multiple shooting stars, from the focus spot to the surrounding area was also observed. The bright locus was shaped
Optical trapping and nanoparticle flows
Extracting the intensity of scattering light by line profile is one of useful practical ways to identify the spatial position of nanoparticles in colloidal solution. Typically, a sharp light intensity at the focal spot in the line profiles can be attributed to nanoparticles accumulated by laser trapping, and the number of trapped nanoparticles is associated with the light intensity [31]. Similarly, the existence of such single and sharp peak intensity from the spherical polystyrene beads under
Conclusion
We have presented optical trapping behavior of 50-nm sized dielectric spherical nanoparticles by the tightly focused ultrashort laser pulses. In addition to the single optical trap at the focal spot, the nanoparticles were also scattered from the focus spot to the surrounding area forming a partially opened folding fan-shaped bright locus in two opposite directions, in an alternating manner, perpendicular to the laser polarization. We have shown that as compared with the cw mode, the laser
Acknowledgements
The financial supports from the Ministry of Education of Taiwan (MOE-ATU Project; National Chiao Tung University), the National Science Council of Taiwan (Grant No. NSC 100-2113-M-009-001), and Foundation of the Advancement for Outstanding Scholarship of Taiwan to H.M. are gratefully acknowledged.
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