Optical Trapping Dynamics of a Single Polystyrene Sphere - American

Jan 17, 2016 - Department of Chemistry, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Negara Brunei. Darussalam...
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Optical Trapping Dynamics of a Single Polystyrene Sphere: Continuous Wave versus Femtosecond Lasers Tsung-Han Liu,† Wei-Yi Chiang,† Anwar Usman,*,‡ and Hiroshi Masuhara*,† †

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Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, 1001 Ta Hsueh Rd., Hsinchu 30010, Taiwan, ROC ‡ Department of Chemistry, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Negara Brunei Darussalam ABSTRACT: Understanding of optical trapping dynamics of a single particle in the trapping site is important to develop its optical manipulation for molecular assembly and chemical application. For micrometer-sized Mie particles, similar trapping efficiency of the conventional continuous wave (cw) laser or high-repetition-rate femtosecond (fs) laser pulse train has been established [Dholakia et al., Opt. Express 2010, 18, 7554−7568], in contrast to higher efficiency of the laser pulses to trap dielectric Rayleigh particles. To further explore and clarify the switching phenomena of optical trapping efficiency with cw laser and fs laser pulse and to elucidate its nature, we study the immobilization dynamics of a single polystyrene sphere with 500 nm in diameter (which is comparable to focal beam size) in shallow potential well. By observing trapping events and immobilization time of the particle with a size in Lorenz−Mie regime, distinct from well-known Rayleigh particle and ray optics approximations, we found that immobilization time is only linearly related to the incident laser power ≤40 mW, and at higher laser powers cw laser is more efficient than fs laser pulses to immobilize the particle. This finding means that the dynamics of the particle in this size region is still affected by the strong transient force fields induced by high-repetition-rate ultrashort pulse train as usually observed for Rayleigh particles. This may provide an understanding that the dynamics of the target particle in the trapping site is size- and laser mode-dependent.

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discussed in detail in numerous reviews.17,21,27−29 In recent years, the most important fundamental topic in this particular field is to study trapping dynamics of individual particles in their colloidal solutions. In this sense, stable trapping of individual micrometer-sized dielectric particles by a focused cw laser beam at sub-Watt level have been reported,30,31 and it is well-known that such a laser beam cannot overcome Brownian motion of particles in nanometer scales.19,32 In these experiments using cw lasers to trap particles, the smallest particle size is 5−10 nm for gold particles33,34 or 10−20 nm for dielectric particles,19,32 nearly two orders smaller than the wavelength of a near-infrared laser. There have been several effective approaches to reduce the size limit of trapped particles, including the advances of singleand two-photon resonance effects of the target particles,35−39 localized surface plasmon in the trapping region,40,41 or highrepetition-rate ultrashort laser pulses as trapping beam.42 In the latter case, in particular, a large number of semiconductor quantum dots can be stably trapped by a pulse train of picosecond laser.42 Besides, the high trapping ability of femtosecond (fs) laser pulses has also been demonstrated in

INTRODUCTION Size, position, and geometric control of micrometer- or nanometer-sized materials are important for fundamental physical principles and phenomena1 as well as their applications in bottom-up approaches for fabrications of functional nanodevices.2,3 One of the important strategies for such a dynamic control is optical manipulation which includes, for instance, optically controlled particle depositions,4−9 particle assembly formations,10−12 in situ polymerization,13 crystallization,14−17 and crystal growth.14−17 An essential tool in such optical manipulation is optical trapping which is a noninvasive and nondestructive technique utilizing a tightly focused continuous wave (cw) near-infrared laser beam to locally induce a high gradient light intensity at the focal spot.10,18−21 With the incident light of sub-Watt level, light−matter interactions generate optical forces in the range of 10−10 to 10−13 N in the focal spot toward the beam center; thus, this technique is also effective to confine, gather, and precisely transfer target objects to a desired position in three-dimensional space. Therefore, optical trapping is also suitable to optically manipulate individual biomolecules, such as viruses and bacteria ranging from a few tens to a few hundred of nanometers in size22−24 or biomolecules inside a living organism where the biomolecules are in their native environment.25,26 Such various aspects of optical trapping and optical manipulation have been © 2016 American Chemical Society

Received: September 19, 2015 Revised: January 17, 2016 Published: January 17, 2016 2392

DOI: 10.1021/acs.jpcc.5b09146 J. Phys. Chem. C 2016, 120, 2392−2399

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The Journal of Physical Chemistry C optical trapping of 50 nm-sized polystyrene beads.43 These findings have therefore indicated that a tightly focused beam of ultrashort laser pulses has a higher trapping ability than the conventional cw lasers to trap nanoparticles in the Rayleigh regime where the particles are regarded as point electric dipoles.42−44 This is in sharp contrast to the same trapping efficiency of fs laser pulses and cw lasers when the target particle is in micrometer scales.30 Therefore, understanding the trapping efficiency variation of laser pulse train versus cw lasers in a certain range of particle size is, in fact, not only practically important but also of fundamental interests. For the incident light of a few tens milliwatt level, by which the potential well is shallow and the Brownian motion of the particles is not completely overcome, the particles are not stably trapped and thus they are axially ejected.45 We should therefore consider that the particles only transit or they are immobilized shortly in the trapping site. The trapping stability, particularly for the particle immobilized with shallow potential well has been of interest. In this sense, Dholakia group reported that trapping efficiency of 780 nm-sized silica bead (a particle in Lorenz-Mie regime) under 800 nm excitation is pulse-duration independent,45 supporting the similar trapping efficiency of cwand fs-laser for 1.28 μm-sized particle.30,46 A fascinating finding in such optical trapping with fs pulse train was axial ejection of 2.1-μm blue-fluorescent-dyed polystyrene bead from the trapping site almost immediately when it was immobilized by the incident laser above 15 mW.45 Interestingly, the particle is trapped for up to a few minutes before an axial ejection only under laser power between 3.7 and 7 mW. This emphasizes that the trapping stability depends on peak power and pulse shape of the laser pulses.45 On the other hand, the role of low average power fs pulses at high repetition rate in stable optical trapping of dielectric nanoparticles less than 100 nm in diameter has been explained based on the inertial response time of the nanoparticles to the impulsive momentum transfer by laser pulses.47,48 Though the axial ejection of the fluorescent bead at higher laser power may be a straightforward self-focusing effect under fs excitation which is opposite to an enhanced trapping efficiency under cw excitation, but the detailed explanation on such remarkable different phenomena of fs and cw mode in optical trapping of nanoparticle and microsized particles was still an open question. Regarding the above-mentioned issue, quantitative and systematic evaluation of optical manipulation is indispensable. Accordingly, there have been several concrete approaches proposed and then demonstrated, such as estimating stiffness of trapping wells through positioning of a single micrometer-sized particle trapped three-dimensionally,49 monitoring amount of trapped nanoparticles reflected by dynamic scattering or fluorescence fluctuations at the focal spot,10,11,42,44 and evaluating immobilization time (also called transit time) of nanoparticles in potentials of localized plasmon.50,51 As a direct and effective method for measurements of a single particle with a few hundreds of nm in size, in our case, the latest approach is preferred. Thus, we designed the following experiment that can clearly monitor the dynamics of a single particle optically immobilized by either laser pulse train or the conventional cw lasers. In this article, we report optical immobilization of 500 nmsized polystyrene beads suspended in water by introducing low power trapping beams. This size of particle should obey Mie theory, distinct from Rayleigh particle and ray optics

approximations. This particle size is of interest because it is approximately comparable to the beam diameter in the focal spot, thus one can expect that only a single particle occupy the trapping potential when it interacts with the incident beam. In addition, as the dielectric particle of this size can be tracked by bright field microscopic images, temporal immobilization of the particle in the shallow trapping potentials was continuously monitored by recording bright field images and detecting transmittance of the trapping beam using an avalanche photodiode (APD). Our approach offers clear observation of trapping events and immobilization time of the particle. Thus, this approach is distinct from the earlier measurements of trapping stiffness which require deep potential and micrometersized target particles.49 By comparing immobilization time of the particles trapped under cw- and fs-mode laser, we show how immobilization time of a single particle in threedimensional space tends to be longer when the particle is optically trapped by cw-mode laser at high laser power, whereas there is no remarkable difference at lower laser powers. These experiments would be significant steps toward future studies of optical trapping with ultrashort laser pulses, where there are several characteristic features occurring in concert; including periodical attractive and repulsive forces by the laser pulses, and repetitive relaxation between consecutive pulses.52

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EXPERIMENTAL SECTION Sample. We used a colloidal solution of 500 nm-sized polystyrene beads with particle density 7.28 × 108 particles/ mL, which was obtained by diluting a commercially available suspension (Polyscience; particle density 3.64 × 1011 particles/ mL) with distilled water. With the diluted colloidal solution, we expect that number of particles around the focal spot is very low. The solution was sonicated and was immediately used for trapping experiments to avoid aggregation. The sample cell consisted of a Teflon chamber (EMS; 120-μm thickness) sandwiched between two coverglass plates (Matsunami; 100 μm thickness). The inner well of the chamber (20 mm in diameter) was filled with 20 μL of the diluted suspension. Optical Setup. The optical trapping setup in this work is generally the same as we reported earlier.43,53 Briefly, we employed a laser beam from a Ti:sapphire laser (Tsunami, Spectra Physics) which can be operated at 800 nm in either cwor fs-pulse (120 fs; 80 MHz)-mode as the trapping light source (Figure 1). A pair of prisms was used to further compress the pulse width into 90 fs at fwhm. Laser power of the beam was adjusted by a half-wave plate and a polarizing beam splitter, and was measured by a digital optical power meter (Spectra Physics; 842-PE). The laser power was directly measured after the objective lens. The linearly polarized beam was then collimated, introduced into an inverted microscope (Olympus; IX-71), and focused by an objective lens (Olympus; UPlanFLN, 60 × ; NA 0.90) into a sample cell mounted on the sample stage. The radius of a finite beam waist, Rayleigh length, and a focal volume of the objective lens was estimated to be 0.46 μm, 1.8 μm, and 1.2 μm3, respectively. By controlling the position of the objective lens along the z-axis, the focal position was set at 60 μm above the bottom glass surface inside the solution in the sample cell. Detection Systems. As shown in Figure 1, we monitored the dynamics of optical trapping by detecting bright field images of the particles around the focal spot area. For this purpose, we illuminated the sample by a white-light probe of halogen lamp (λ = 380−750 nm) focused by using a condenser 2393

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can precisely record the immobilization time and confirm the number of immobilized particles simultaneously.

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RESULTS AND DISCUSSION The optically immobilized or trapped 500 nm-sized polystyrene particle is observed by detecting the transmittance of the trapping beam as well as by the bright field images of probe light at the focal area. The immobilization or trapping events were qualitatively confirmed from bright field images, as shown in Figure 2. For quantitative analysis, the immobilization events were also monitored by the transmittance of the trapping beam. Figure 3 shows typical time domain response of APD for the

Figure 1. Schematic diagrams of the experimental setup; DM is dichroic mirror, SPF is short wave pass filter, ND is neutral density filter, APD is avalanche photodiode, and PCB is photon counting board. The linear-polarized light from Ti:sapphire laser in either cw- or fs-mode is introduced to an inverted microscope and then focused into a sample chamber by objective lens. Intensity change of the incident light is monitored by APD, and simultaneously images under halogen lamp illumination are recorded by CCD.

lens (Olympus; 1 × 2-LWUCD). The probe light was collected by the same objective lens and was passed through into a CCD camera (JAI; CV-A55 IR) running at 30 interlaced frames per second. Simultaneously, optical trapping of the particles was monitored by detecting transmitted light of the trapping beam. The transmitted light was collected by the same condenser lens, reflected by a dichroic mirror, collimated by a pair of lenses, and finally focused by an objective lens (Newport; 10 × ; NA 0.25, MV-10) into APD (Newport; PCD100) running at a temporal resolution of 100 ms. When a single particle is trapped and occupies the trapping site, the white light is scattered, and synchronously the incident laser beam is also transmitted and refracted. Consequently, the transmittance signal detected by APD increases when a particle occupies the focal spot. With these two detection systems, side by side, we

Figure 3. Optical trapping of a single particle in fs-mode (blue line) and cw-mode (black line) seen as sudden discrete steps of signal detected by APD at the incident laser power of (A) 10, (B) 20, (C) 30, (D) 40, and (E) 50 mW.

Figure 2. Representative series of bright field images observed by a CCD camera at −0.5−3.1 s, and a typical stepwise signal recorded by APD showing a complete trapping event of a single particle including both trapping and releasing. The contrast of the bright field images have been increased for the sake of clarity. Before the laser was switched on, there was no particle at the beam center. Single particles were randomly trapped after the laser was switched on. The images recorded from 0.03−1.68 s indicate that a particle was immobilized in the focal spot, and then at 1.7 s the particle was released from the trapping site as the particle images gradually became enlarged and blurred. Eventually the particle disappeared from our observation around 3 s after the trapping event started. Such alternative trapping and releasing of a single particle were synchronously confirmed by the signal detected by the APD. 2394

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photoluminescence of polystyrene induced by UV pulsed laser irradiation for several minutes has been reported.55 This phenomenon has been ascribed to the formation of carbonyl groups on the surface of polystyrene particles due to fs laserinduced photochemical oxidation.55 As the photoluminescence is successive processes of photoexcitation, photoreaction, and generation of photoproducts,55 detectable photoluminescence spectrum of accumulated photoproducts during the laser trapping was observed after long irradiation time. Interestingly, immobilization time under the incident fs laser pulse train at high laser power such as 40 mW reversely becomes shorter than that of 30 mW (Figure 3D). Furthermore, out of our expectation, at laser power higher than 30 mW photoluminescence related to multiphoton absorption from an optically excited polystyrene particle has never been observed as immobilization time becomes shorter. The particle is confined by cw lasers with immobilization time up to few seconds for laser power at 40 mW (Figure 3D and 4D) or even up to few tens of seconds for laser power at 50 mW (Figure 3E). On the other hand, we cannot immobilize any single particle for longer than 100 ms at 50 mW under fsmode, and thus no stepwise signal can be observed by APD (Figure 3E). It is noteworthy that time resolution of APD was set to 100 ms in our case, thus if the trapping events taking place under fs-mode at 50 mW are shorter than 100 ms they should be still detectable by the CCD camera (time resolution of 33 ms). In general, we observed that immobilization time of a single particle under the conventional cw laser increases with laser power, whereas fs laser trapping results in a decreased immobilization time at the incident laser power higher than 30 mW. From Figure 4 it is clearly seen that there are more trapping events with relatively shorter immobilization time, irrespective of the incident laser power. Because trapping events give random immobilization time, we fitted the data points with the probability density function as given by f(t) = A exp(−t2/2τ2), where f(t) is frequency of immobilization events, A is a constant, t is immobilization time, and τ is immobilization time constant which should be determined by particle size, optical properties of the particle, and laser power. In Figure 5, we show plots of extracted τ as a function of P, demonstrating a tendency of higher τ of the conventional cw laser trapping as compared with that of fs laser trapping at laser power ≥40 mW. This indicates that particles are more likely to be immobilized for longer duration in the trapping site by cw laser compared

particle trapped by cw- and fs pulse-mode laser at 10−50 mW. The transmittance increases substantially when a single particle is trapped, and it decreases to the initial level when the particle escapes from the trapping site. Thus, the trapping events appear as discrete steps.50,51 Transmittance fluctuation can be attributed to thermally driven motion of the particle in the trapping site and can be used to study the trap dynamics.54 We should note that two particles are also infrequently observed by both APD and bright field images. In this case one of the particles is immediately released from the beam center typically within 200 ms. Such two-particle events are excluded from our analysis of immobilization of a single particle. Practically, we monitored and statistically analyzed more than 200 trapping events of a single particle under laser irradiation at power from 5 to 50 mW under the two different laser modes. We summarized the frequency of trapping events of single particles, and it is plotted versus their immobilization time under cw- and fs pulse-mode laser irradiations at different laser powers as shown in Figure 4.

Figure 4. Frequency of trapping events versus immobilization time counted from the signal detected by APD at the incident laser power from 10 to 50 mW. Blue and black data points represent the frequency of trapping events under fs- and cw- mode laser, respectively. The solid lines are the fitting by the probability density function (see text). We note that there was no stable trapping events observed at 50 mW under fs-mode laser trapping, so only black points are shown in 4E.

As shown in Figure 3A, at relatively low laser power such as 10 mW or lower, distribution of immobilization time in cwmode is almost similar to that in fs-mode (Figure 4A). When laser power is increased to 20−30 mW, immobilization time becomes longer in both laser modes (Figure 3B,C and 4B,C). In this range of laser power, the immobilization time of fs pulse mode is slightly longer than that of cw laser mode. We should note that at a laser power of 30 mW, a few trapping events under fs-mode accompanied by white photoluminescence were clearly observed by the CCD camera after the particle is immobilized in the trapping site for more than 10 s. The luminescence could be assigned to the photoluminescence of polystyrene induced by the fs laser pulse train, and such

Figure 5. Plot of τ as a function of P, showing that the immobilization is linearly related to the incident laser power at the low laser powers and it deviates from the linearity at high laser powers for both cw- and fs-mode lasers. Blue and black data points and respective linear lines are those for fs- and cw-mode lasers. 2395

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s. As shown in Figure 4, these long-immobilization-time events which may induce photoluminescence easily were extremely rare because under higher power fs-mode immobilization time becomes much shorter. If we quantitatively compared the three-photon-absorption efficiency of a single particle immobilized at the focal spot by fs-mode laser at different powers, where it is proportional to cubic of laser power, when the particle was immobilized under fs laser irradiation at 40 mW the photoluminescence should occur at least 4.25 s of immobilization. However, our experimental results suggest that photoluminescence cannot be observable at 40 mW nor higher laser powers, indicating that only long immobilization time leads to photoluminescence. Accordingly, it is considered that polystyrene has to be first triggered by three-photon absorption and a photochemical reaction and then photoluminescence becomes observable due to the photoproduct following the multiphoton absorption. Thus, unobservable photoluminescence fs-mode under laser powers ≥40 mW, where multiphoton absorption is more efficient, is because of shortened immobilization time. The two-step process is also applicable to explain the reason why photoluminescence did not occur immediately after a particle was trapped. In addition, as it has been demonstrated that dye-doped particles excited through two-photon processes are axially ejected from the trapping site,45 multiphoton absorption might affect the immobilization in the form of absorption forces. However, unlike single- and two-photon resonance effects that enhance trapping efficiency of Rayleigh particles,35−39 the effect of the multiphoton absorption on optical trapping of the Mie particle is still an open question. Here, if we consider all immobilization events at low laser powers (or at shallow trapping potential) where the multiphoton effect can be excluded, temperature gradient induced by such a photochemical reaction is negligible. On the other hand, temperature elevation of water as medium induced by a tightly focused 1064 nm cw laser beam through one-photon absorption for overtone transitions has been quantitatively estimated, where local temperature is only elevated less than 2 K when a laser beam at few tens of mW is introduced.58 In our case, in which the 800 nm laser is used and only multiphoton absorption but no one-photon absorption occurs, local temperature elevation is considered not remarkable either. Therefore, at lower laser powers, because of small multiphoton absorption cross section of polystyrene particles and that of water, the particle and medium are basically transparent at 800 nm, temperature around the particle is constant and the effect of local heating can be excluded in all experiments. It is worth mentioning that the linear relation between τ and P, as shown in Figure 5 for low laser powers ≤40 mW, suggests that the approximation in eq 3 is reasonably achieved and the above consideration was fairly valid for the incident beam at low laser powers. Interestingly, τ tends to increase abruptly at high laser powers ≥40 mW for cw lasers, whereas it tends to decrease for fs pulse-mode. A nonlinear increase of τ for the incident cw laser ≥40 mW should indicate exponential relation between τ and P as given by eq 2, rather than the limitation of the low Utrap approximation in eq 3. On the other hand, a clear downward curvature of τ for the incident fs laser ≥40 mW suggests that 500 nm-sized particle has higher trapping stability under the cw beam than fs pulse train. This may indicate that repulsive temporal radiation forces, as discussed below, can contribute in the less efficient immobilization due to possible macroscopic movement of the particle in the focal spot.

with that of fs laser pulse train. On the other hand, it is also suggested that some other factors, as it will be discussed below, become dominant in fs-mode case when laser power is increased. We first consider the classical theory of optical tweezers originated from a tightly focused laser beam to confine and manipulate particles three-dimensionally. For such sub micrometer-sized particles in the Mie-scattering limit, optical confinement occurs when the light is refracted through the particle, generating an exchange of momentum and an optical force that draws the particle to the highest light intensity at the beam center. With a high NA objective lens, typically an incident laser beam of a few hundreds of mW level is required to trap a submicrometer particle27−29 or molecular clusters of amino acids.17 Considering the diffusion coefficient (D) of the submicrometer particle is within 1 μm2/s, the covered area of the particle in time interval between pulses (12.5 ns) due to the thermally induced mobility should be 12.5 × 10−3μm2 which is much smaller than the focal trapping area (1.5 μm2).52 Thus, diffusion of a submicrometer- or micrometer-sized particle from the trapping site within the time interval can be excluded, and its trapping efficiency is quantified by a dimensionless value, Q, as given by56,57 F = (n m P / c )Q

(1)

where F is generated optical trapping force, nmP/c is the momentum transfer per second, nm is refractive index of medium, P is the average incident laser power, and c is the vacuum speed of light.30 When the potential well which is defined as the energy to transfer a particle with a distance r from the infinite to a certain position in the focal spot, Utrap = ∫ F dr, is larger than the kinetic energy of the thermal diffusion,19,32 a necessary condition for stable trapping is achieved. It should be emphasized that the potential well is spatially (and temporally) generated by the interaction between the particle and the focused beam. Thus, temporal fluctuations of transmittance (Figure 3) can be interpreted as fluctuations in the position of the center of mass of the particle in the focal spot or the optical trapping potential. We recall that τ corresponds to the transit time of a particle in the trapping site and it is enhanced by the Boltzmann factor as given by35 τ = τD exp(Utrap/kBT )

(2)

Here τD is diffusion time constant in the focal area when the incident laser beam is switched off, kB is the Boltzmann constant, and T is the temperature of the medium surrounding a particle. Because Utrap is linearly related to P, in the case of shallow trapping potential,Utrap/kBT ≪ 1, eq 2 can be expressed by τ = τD(1 + Utrap/kBT ) = τD(1 + aP)

(3)

Here a constant a accounts for light collection efficiency, dynamics of the optically trapped particle in the trapping site, trapping efficiency, and temperature around the particle. As shown in eq 3, this approximation gives rise a linear relation between τ and P. It is noteworthy that noticeable photoluminescence related to multiphoton absorption from an optically trapped polystyrene particle was observed only under fs laser irradiation at 30 mW after the particle was immobilized for longer than 10 2396

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The Journal of Physical Chemistry C We may recall that trapping efficiency of cw- and fs-pulsemode is exactly the same when the polystyrene particle size is 1.28 μm,30 whereas ultrafast laser pulses can stably trap a large number of 100 nm-sized polystyrene beads,44 3.3 nm-sized CdTe QDs,47,48 or fluorescent latex nanoparticles,42−44 where cw lasers at similar power level cannot lead to a stable potential well. This means that cw- and fs-pulse-modes have a similar trapping efficiency for large particle sizes in Mie and ray optic regimes, whereas the high impulsive peak power of a femtosecond laser pulse at high repetition rate can hold tiny Rayleigh particles. Thus, this current work provides a new particle size region where the cw beam immobilizes a particle in the trapping site longer than fs pulse train for laser powers ≥40 mW. To evaluate this finding, we should take dynamics of momentum transfer under fs pulse irradiation into account. The response of a particle to the pulse train is related to inertial time of the particle,48 as given by t=

2 2 r (ρ / η ) 9

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CONCLUSIONS

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AUTHOR INFORMATION

The development of optical trapping and manipulation has shown rapid progress, much of it owing to improvements in the optical trapping system as well as size and optical properties of the target particles; however, not much progress has been made in understanding the substitution from cw laser to highrepetition-rate ultrashort laser pulses. Using the conventional cw- and fs pulse-mode laser as the trapping beam, we have studied immobilization dynamics of a single 500 nm-sized polystyrene sphere in shallow potential well. Our finding offers trapping events and immobilization time of the particle with a size distinct from well-known Rayleigh particle and ray optics approximations. Immobilization time is only linearly related to the incident laser power ≤40 mW, and it deviates from the linearity at higher laser powers. The clear upward and downward curvature for the cw- and fs pulse-mode laser, respectively, suggests that the particle has higher trapping stability under the cw beam than fs pulse train. This finding further suggests that the dynamics of the particle in this size region is still affected by the strong transient force fields induced by high-repetition-rate ultrashort pulse train. In particular, impulsive peak of fs laser pulses strongly induces several features occurring in concert; including periodical attractive and repulsive forces, repetitive dragging and releasing the particle, and convection or microfluidic flow in the medium. Thus, this finding provides an understanding for optical manipulation that the dynamics of the target particle is size-, laser mode-, and laser power-dependent. In a certain size of Lorenz−Mie particle region, the conventional cw laser in particular at high laser power can be better to immobilize the particle in the potential well as compared with fs laser pulse train.

(4)

where r and ρ is the radius and density of the polystyrene particle, respectively, and η is the viscosity of the medium. By taking into account the values of r = 250 nm, ρ = 1.05 gr/mL, and η = 8.90 × 10−4 Pa s, we estimated t = 16.4 ns which is much longer than laser pulse width (90 fs). Thus, the 500 nmsized particle responds to every single laser pulse less than 16 ns as an instantaneous momentum transfer. As the particle size is increased, the inertial response time also increases. Thus, it is noteworthy to mention that t increases to 42 ns for 1.28 μmsized polystyrene particle.30 If all the optical system is comparable, the similar order of the inertial response time indicates that both Mie particles should response equally to instantaneous momentum transfer of fs laser pulse. However, we should consider that potential well disturbs the Brownian motion of a single Mie particle, leading to temporal immobilization of the particle in the trapping site. Releasing of the particle from such a trapping well is due to incomplete suppressing of Brownian motion, microfluidic flow around the focal spot, or repulsive optical forces that can escape the particle out from the trapping site. Following this sense, the Brownian motion of Mie particles is much more sluggish than that of Rayleigh particles, thus they are less sensitive to the transient force field of laser pulses. As compared with the micrometer-sized particle which shows no difference in trapping efficiency of ultrashort laser pulses and cw lasers,30 we found that dynamics of a few hundreds of nmsized particles is still affected by the strong transient force fields induced by ultrashort laser pulses as usually observed for Rayleigh particles.59 In this regard, impulsive peak power of fs laser pulse train at high repetition rate should generate strong transient optical forces at high frequency, including gradient and scattering forces. Additional repulsive temporal radiation force also increases the kinetic energy, resulting in higher destabilization of the optical trap. Thus, additional repulsive forces generated by fs pulses and particle dynamics in the focal site play an important role in immobilization of a few hundreds of nm-sized Lorenz-Mie particle. Overall, these explain why fs laser pulse train reversely becomes less efficient to immobilize particles in the trapping site as compared with the conventional cw lasers at high laser powers.

Corresponding Authors

* E-mail: [email protected] (A.U.). * E-mail: [email protected] (H.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported the Ministry of Education of Taiwan (MOE-ATU Project; National Chiao Tung University) and the National Science Council of Taiwan (MOST 103-2113-M-009003). H.M. also thanks to Foundation of the Advancement for Outstanding Scholarship of Taiwan.

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REFERENCES

(1) Lehmuskero, A.; Johansson, P.; Rubinsztein-Dunlop, H.; Tong, L.; Kall, M. Laser Trapping of Colloidal Metal Nanoparticles. ACS Nano 2015, 9, 3453−3469. (2) Grier, D. G. A Revolution in Optical Manipulation. Nature 2003, 424, 810−816. (3) Yin, Y.; Alivisatos, A. P. Colloidal Nanocrystal Synthesis and the Organic−Inorganic Interface. Nature 2005, 437, 664−670. (4) Hotta, J.; Sasaki, K.; Masuhara, H. A Single Droplet Formation from Swelled Micelles by Radiation Pressure of a Focused Infrared Laser Beam. J. Am. Chem. Soc. 1996, 118, 11968−11969. (5) Hofkens, J.; Hotta, J.; Sasaki, K.; Masuhara, H.; Iwai, K. Molecular Assembling by the Radiation Pressure of a Focused Laser Beam: Poly(N-isopropylacrylamide) in Aqueous Solution. Langmuir 1997, 13, 414−419. 2397

DOI: 10.1021/acs.jpcc.5b09146 J. Phys. Chem. C 2016, 120, 2392−2399

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The Journal of Physical Chemistry C (6) Ito, S.; Yoshikawa, H.; Masuhara, H. Laser Manipulation and Fixation of Single Gold Nanoparticles in Solution at Room Temperature. Appl. Phys. Lett. 2002, 80, 482−484. (7) Svoboda, K.; Block, S. M. Optical Trapping of Metallic Rayleigh Particles. Opt. Lett. 1994, 19, 930−932. (8) Sanz, M.; Nalda, R. d.; Marco, J. F.; Izquierdo, J. G.; Banares, L.; Castillejo, M. Femtosecond Pulsed Laser Deposition of Nanostructured CdS Films. J. Phys. Chem. C 2010, 114, 4864−4868. (9) Urban, A. S.; Lutich, A. A.; Stefani, F. D.; Feldmann, J. Laser Printing Single Gold Nanoparticles. Nano Lett. 2010, 10, 4794−4798. (10) Neuman, K. C.; Block, S. M. Optical Trapping. Rev. Sci. Instrum. 2004, 75, 2787−2809. (11) Hosokawa, C.; Yoshikawa, H.; Masuhara, H. Optical Assembling Dynamics of Individual Polymer Nanospheres Investigated by SingleParticle Fluorescence Detection. Phys. Rev. E 2004, 70, 061410. (12) Hosokawa, C.; Yoshikawa, H.; Masuhara, H. Cluster Formation of Nanoparticles in an Optical Trap Studied by Fluorescence Correlation Spectroscopy. Phys. Rev. E 2005, 72, 021408. (13) Ito, S.; Tanaka, Y.; Yoshikawa, H.; Ishibashi, Y.; Miyasaka, H.; Masuhara, H. Confinement of Photopolymerization and Solidification with Radiation Pressure. J. Am. Chem. Soc. 2011, 133, 14472−14475. (14) Sugiyama, T.; Adachi, T.; Masuhara, H. Crystallization of Glycine by Photon Pressure of a Focused CW laser Beam. Chem. Lett. 2007, 36, 1480−1481. (15) Rungsimanon, T.; Yuyama, K.; Sugiyama, T.; Masuhara, H.; Tohnai, N.; Miyata, M. Control of Crystal Polymorph of Glycine by Photon Pressure of a Focused Continuous Wave Near-Infrared Laser Beam. J. Phys. Chem. Lett. 2010, 1, 599−603. (16) Masuhara, H.; Sugiyama, T.; Rungsimanon, T.; Yuyama, K.; Miura, A.; Tu, J.-R. Laser-Trapping Assembling Dynamics of Molecules and Proteins at Surface and Interface. Pure Appl. Chem. 2011, 83, 869−883. (17) Sugiyama, T.; Yuyama, K.; Masuhara, H. Laser Trapping Chemistry: From Polymer Assembly to Amino Acid Crystallization. Acc. Chem. Res. 2012, 45, 1946−1954. (18) Ashkin, A. Acceleration and Trapping of Particles by Radiation Pressure. Phys. Rev. Lett. 1970, 24, 156−159. (19) Ashkin, A.; Dziedzic, J. M.; Bjorkholm, J. E.; Chu, S. Observation of a Single-Beam Gradient Force Optical Trap for Dielectric Particles. Opt. Lett. 1986, 11, 288−290. (20) Dholakia, K.; Reece, P.; Gu, M. Optical Micromanipulation. Chem. Soc. Rev. 2008, 37, 42−55. (21) Dholakia, K.; Zemanek, P. Gripped by Light: Optical Binding. Rev. Mod. Phys. 2010, 82, 1767−1791. (22) Ashkin, A.; Dziedzic, J. M. Optical Trapping and Manipulation of Viruses and Bacteria. Science 1987, 235, 1517−1520. (23) Bustamante, C.; Macosko, J. C.; Wuite, G. J. L. Grabbing the Cat by the Tail: Manipulating Molecules One by One. Nat. Rev. Mol. Cell Biol. 2000, 1, 130−136. (24) Bustamante, C.; Chemla, Y. R.; Forde, N. R.; Izhaky, D. Mechanical Processes in Biochemistry. Annu. Rev. Biochem. 2004, 73, 705−748. (25) Dufrêne, Y. F.; Evans, E.; Engel, A.; Helenius, J.; Gaub, H. E.; Müller, D. J. Five Challenges to Bringing Single-Molecule Force Spectroscopy into Living Cells. Nat. Methods 2011, 8, 123−127. (26) Oddershede, L. B. Force Probing of Individual Molecules inside the Living Cell is now a Reality. Nat. Chem. Biol. 2012, 8, 879−886. (27) Ashkin, A. History of Optical Trapping and Manipulation of Small-Neutral Particle, Atoms, and Molecules. IEEE J. Sel. Top. Quantum Electron. 2000, 6, 841−856. (28) Molloy, J. E.; Padgett, M. J. Lights, Actions: Optical Tweezers. Contemp. Phys. 2002, 43, 241−258. (29) Ashkin, A. Optical Trapping and Manipulation of Neutral Particles Using Lasers. Proc. Natl. Acad. Sci. U. S. A. 1997, 94, 4853− 4860. (30) Agate, B.; Brown, C. T. A.; Sibbett, W.; Dholakia, K. Femtosecond Optical Tweezers for In-Situ Control of Two-Photon Fluorescence. Opt. Express 2004, 12, 3011−3017.

(31) Xin, H.; Xu, R.; Li, B. Optical Trapping, Driving, and Arrangement of Particles using a Tapered Fibre Probe. Sci. Rep. 2012, 2, 1−8. (32) Harada, Y.; Asakura, T. Radiation Forces on a Dielectric Sphere in the Rayleigh Scattering Regime. Opt. Commun. 1996, 124, 529−541. (33) Hansen, P. M.; Bhatia, V. K.; Harrit, N.; Oddershede, L. Expanding the Optical Trapping Range of Gold Nanoparticles. Nano Lett. 2005, 5, 1937−1942. (34) Hajizadeh, F.; Reihani, S. N. S. Optimized Optical Trapping of Gold Nanoparticles. Opt. Express 2010, 18, 551−559. (35) Hosokawa, C.; Yoshikawa, H.; Masuhara, H. Enhancement of Biased Diffusion of Dye-Doped Nanoparticles by Simultaneous Irradiation with Resonance and Nonresonance Laser Beams. Jpn. J. Appl. Phys. 2006, 45, L453−L456. (36) Shoji, T.; Kitamura, N.; Tsuboi, Y. Resonant Excitation Effect on Optical Trapping of Myoglobin: The Important Role of a Heme Cofactor. J. Phys. Chem. C 2013, 117, 10691−10697. (37) Jauffred, L.; Oddershede, L. B. Two-Photon Quantum Dot Excitation during Optical Trapping. Nano Lett. 2010, 10, 1927−1930. (38) Jauffred, L.; Richardson, A. C.; Oddershede, L. B. ThreeDimensional Optical Control of Individual Quantum Dots. Nano Lett. 2008, 8, 3376−3380. (39) Head, C. R.; Kammann, E.; Zanella, M.; Manna, L.; Lagoudakis, P. G. Spinning Nanorods − Active Optical Manipulation of Semiconductor Nanorods using Polarised Light. Nanoscale 2012, 4, 3693−3697. (40) Tsuboi, Y.; Shoji, T.; Kitamura, N.; Takase, M.; Murakoshi, K.; Mizumoto, Y.; Ishihara, H. Optical Trapping of Quantum Dots Based on Gap-Mode-Excitation of Localized Surface Plasmon. J. Phys. Chem. Lett. 2010, 1, 2327−2333. (41) Juan, M. L.; Righini, M.; Quidant, R. Plasmon Nano-Optical Tweezers. Nat. Photonics 2011, 5, 349−356. (42) Pan, L.; Ishikawa, A.; Tamai, N. Detection of Optical Trapping of CdTe Quantum Dots by Two-Photon-Induced Luminescence. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 161305. (43) Usman, A.; Chiang, W.-Y.; Masuhara, H. Optical Trapping and Polarization-Controlled Scattering of Dielectric Spherical Nanoparticles by Femtosecond Laser Pulses. J. Photochem. Photobiol., A 2012, 234, 83−90. (44) De, A. K.; Roy, D.; Dutta, A.; Goswami, D. Stable Optical Trapping of Latex Nanoparticles with Ultrashort Pulsed Illumination. Appl. Opt. 2009, 48, G33−G37. (45) Shane, J. C.; Mazillu, M.; Lee, W. M.; Dholakia, K. Effect of Pulse Temporal Shape on Optical Trapping and Impulse Transfer using Ultrashort Pulsed Lasers. Opt. Express 2010, 18, 7554−7568. (46) De, A. K.; Roy, D.; Goswami, D. Towards Stable Trapping of Single Macromolecules in Solution. Proc. SPIE 2010, 7762, 776203. (47) De, A. K.; Goswami, D. Towards Controlling Molecular Motions in Fluorescence Microscopy and Optical Trapping: A Spatiotemporal Approach. Int. Rev. Phys. Chem. 2011, 30, 275−299. (48) Roy, D.; Goswami, D.; De, A. K. Exploring the Physics of Efficient Optical Trapping of Dielectric Nanoparticles with Ultrafast Pulsed Excitation. Appl. Opt. 2015, 54, 7002−7006. (49) Sasaki, K.; Tsukima, M.; Masuhara, H. Three-Dimensional Potential Analysis of Radiation Pressure Exerted on a Single Microparticle. Appl. Phys. Lett. 1997, 71, 37−39. (50) Pang, Y.; Gordon, R. Optical Trapping of a Single Protein. Nano Lett. 2012, 12, 402−406. (51) Zehtabi-Oskuie, A.; Jiang, H.; Cyr, B. R.; Rennehan, D. W.; AlBalushi, A. A.; Gordon, R. Double Nanohole Optical Trapping: Dynamics and Protein-Antibody Co-Trapping. Lab Chip 2013, 13, 2563−2568. (52) Usman, A.; Chiang, W.-Y.; Masuhara, H. Optical Trapping of Nanoparticles by Ultrashort Laser Pulses. Sci. Prog. 2013, 96, 1−18. (53) Chiang, W.-Y.; Usman, A.; Masuhara, H. Femtosecond PulseWidth Dependent Trapping and Directional Ejection Dynamics of Dielectric Nanoparticles. J. Phys. Chem. C 2013, 117, 19182−19188. 2398

DOI: 10.1021/acs.jpcc.5b09146 J. Phys. Chem. C 2016, 120, 2392−2399

Article

The Journal of Physical Chemistry C (54) Chen, C.; Juan, M. L.; Li, Y.; Maes, G.; Borghs, G.; Dorpe, P. V.; Quidant, R. Enhanced Optical Trapping and Arrangement of NanoObjects in a Plasmonic Nanocavity. Nano Lett. 2012, 12, 125−132. (55) Kim, E.; Kyhm, J.; Kim, J. H.; Lee, G. Y.; Ko, D.-H.; Han, I. K.; Ko, H. White Light Emission from Polystyrene under Pulsed Ultra Violet Laser Irradiation. Sci. Rep. 2013, 3, 3253. (56) Ashkin, A. Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime. Biophys. J. 1992, 61, 569− 582. (57) Felgner, H.; Muller, O.; Schliwa, M. Calibration of light forces in optical tweezers. Appl. Opt. 1995, 34, 977−982. (58) Ito, S.; Sugiyama, T.; Toitani, N.; Katayama, G.; Miyasaka, H. Application of Fluorescence Correlation Spectroscopy to the Measurement of Local Temperature in Solutions under Optical Trapping Condition. J. Phys. Chem. B 2007, 111, 2365−2371. (59) Ambardekar, A. A.; Li, Y.-Q. Optical levitation and manipulation of stuck particles with pulsed optical tweezers. Opt. Lett. 2005, 30, 1797−1799.

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DOI: 10.1021/acs.jpcc.5b09146 J. Phys. Chem. C 2016, 120, 2392−2399