Article pubs.acs.org/JPCC
Picosecond Motional Relaxation of Nanoparticles in Femtosecond Laser Trapping Masayasu Muramatsu,*,†,§ Tse-Fu Shen,† Wei-Yi Chiang,† Anwar Usman,‡ and Hiroshi Masuhara*,† †
Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan Department of Chemistry, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Negara Brunei Darussalam
‡
ABSTRACT: Repetitive drag and release dynamics by impulsive force is characteristic of optical trapping by femtosecond laser pulses. We studied the dynamics utilizing double pulse train and found that trapped polystyrene particles are ejected repetitively from the focal spot and its frequencies become less for longer interval of the pulse trains. The ejection changes drastically in a few-ps interval region, although particles cannot move appreciable distance in such a short time. It means that displacement of particles by a conventional diffusive motion is not dominant and another fast process has an important role in femtosecond pulse trapping. We also revealed that the silica nanoparticles shows a decay at few-ps, indicating that the picosecond decay is not due to a material property but considered to be a general dynamics. We propose that a picosecond relaxation process of inertia force of particles is important for understanding laser trapping dynamics by femtosecond laser pulses.
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around the focal spot in pulse trapping of gold nanoparticles.21 We have also studied interesting phenomena in femtosecond laser trapping of polystyrene nanoparticles, such as pulse-width and pulse repetition rate dependences of trapping behaviors22,23 and considered their mechanisms. For instance, we revealed that two-photon absorption process of the target nanoparticles enhances trapping ability of the particles in femtosecond laser trapping.24 Those results provide a hint of highly nonlinear optical character of optical trapping with femtosecond laser pulses especially for target materials in the Rayleigh regime. We have recently shown higher trapping efficiency of 50 nm sized particles by femtosecond pulsed laser than CW laser.22,25,26 Furthermore, it shows unconventional directional ejection of particles from the trapping site, which is perpendicular to the polarization of the trapping beam. We consider that the trapping efficiency and characteristic phenomenon by femtosecond laser pulses is closely related to the fast gathering and escaping dynamics of nanoparticles by the repetitions of “on” and “off” times of the optical forces. Thus, to understand the mechanism of highly efficient pulse trapping, temporal evolution of motion of particles under trapping force need to be revealed. In this paper, we study motional relaxation of nanoparticles in water solution in femtosecond pulse trapping. For this purpose, we have newly developed a powerful technique to investigate picosecond to nanosecond dynamics in femtosecond laser trapping utilizing a double pulse train. By using this technique, the trapping efficiency of nanoparticles is observed as a function of time
INTRODUCTION Interaction between light and material is an important topic in the physical chemistry, which has been studied for a long time. In recent years, nanomaterial attracts more and more interest particularly from the viewpoint of energy conversion, bio medical application, and so on. Their interaction with light is being studied extensively.1,2 Spectroscopy, reaction, fabrication, and patterning of nanomaterial become a main stream of research in physical chemistry. Optical trapping is also an important subject that dealing with the interaction between light and nanometer size objects.3−7 With its ability of noncontact and nondestructive immobilization and transportation of objects, recently the laser trapping is applied to not only simple manipulation of small objects8−11 but also important chemical phenomena such as aggregations of particles and molecules,12,13 polymerization,14 and crystallizations.15−17 Although the optical manipulation technique is widely applied, the light source of trapping has been mostly limited to continuous wave (CW) lasers. The trapping force is kept constant for CW laser, while very high photon pressure is induced during the impulsive pulses with high peak power and no force is applied on the target for pulse intervals in the femtosecond laser trapping. Thus, impulsive drag and release will be repeated in the pulsed laser trapping, which is quite different dynamics compared to CW laser trapping. Recently, several results of femtosecond laser trapping are reported. Dholakia et al. found that a submicrometer or micrometer-sized silica sphere is trapped as effective as CW laser by femtosecond laser.18,19 On the other hand, few-nm sized quantum dots are stably trapped by picosecond pulse laser at much lower power than by CW laser.20 Moreover, it was found that the trapping site splits into two equivalent positions © XXXX American Chemical Society
Received: February 4, 2016
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DOI: 10.1021/acs.jpcc.6b01217 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
threshold (twice of amplitude of the background signal, shown in Figure 4) on the observation circle, we assume the ejection is occurring in the video frame. We analyzed all of 1800 image frames in 1 min video file as above, and estimated the frequencies of ejection. The interferometric two-photon fluorescence autocorrelation trace was measured using same setup with trapping experiment and detected by an avalanche photo diode (Newport, PCD100). Surface modified silica nanoparticle was synthesized as following procedures. 500 μL of 50 nm sized silica nanoparticles solution (3.8 × 1014 particles/mL) was mixed with 5 mL of ethanol (Sigma-Aldrich; ≥99.8%), 250 μL of an ammonium hydroxide solution (Sigma-Aldrich; 28% NH3 in H2O), and 106 μL of triethoxy(octyl)silane (Sigma-Aldrich; ≥ 97.5%). The solution was placed inside an ultrasonic washer for 1 h at room temperature, and stirred for 1 h at 60 °C. The particles ware washed by water three times after 15 min of centrifuge with 13,000 rpm.
interval between the pulse trains, and the dynamics of particles after the irradiation of pulses is revealed in several time regions that is related to the mechanism of efficient femtosecond laser trapping.
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EXPERIMENTAL SECTION Experimental setup is shown in Figure 1a. We used an 800 nm fundamental mode of Ti:sapphire laser (Spectra Physics,
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RESULTS AND DISCUSSION At the beginning, we measured interferometric two-photon fluorescence autocorrelation trace of rhodamine B in ethanol solution at the sample position to evaluate our system (Figure 2). We adjusted to get the best overlap between the first and
Figure 1. Schematic diagram of the experimental set up of double pulse train trapping; λ/2 is a half-wave plate (a). A double pulse train prepared from two optical paths (b). Time interval between the pulse trains can be controlled with keeping total laser power and pulse repetition rate. Figure 2. Interferometric two-photon fluorescence autocorrelation trace of Rhodamine B in ethanol solution and envelope curves calculated assuming a Gaussian pulse with 100 fs fwhm.
Tsunami) pumped by the second harmonic generation (SHG) of a CW Nd:YVO4 laser (Spectra Physics, Millennia Pro) as a trapping beam. The beam is divided into 1:1 by a polarizing beamsplitter, and the polarization of each beam is controlled by a wave plate. After a beam goes through an optical delay line, two beams are made collinear again by a half mirror. An example of a linearly polarized double pulse train prepared from two optical paths is shown in Figure 1b. The repetition rate of the total pulses is fixed at 160 (80 + 80) MHz, while the interval between the pulse trains is adjustable by a delay line. The optical delay line is controlled by a stepping motor (Sigma Koki, SGSP26−200(X) and SHOT-302GS) with the minimum step size as 500 nm. It is then focused into a sample cell by an objective lens (60×; NA 0.90) mounted on the inverted microscope (Olympus, IX71). At the sample, the laser is focused on the diffraction-limited size. The sample cell containing 50 nm sized spherical nanoparticles (3.6 × 1014 particles/mL) is illuminated by the white-light from a halogen lamp (λ = 400−750 nm) through a dark-field condenser lens (NA 1.2−1.4). The scattering light from the nanoparticles is collected by the objective lens, and detected by using a chargecoupled device (CCD) camera (JAI, CV-A55 IR EIA) running at 30 interlaced frames per second for 1 min. Saved images are analyzed as following. We make an observation circle with 5μm radius from the focal spot (red circle in Figure 4). If the intensity of light scattered by the ejecting particles exceeds the
second pulse trains by seeing an image of the focal spot on a cover glass with CCD camera. We confirmed that the focusing positions do not change with the movement of delay stage. As a result of interferometric autocorrelation measurement, we obtained a symmetrical curve with its ratio of the maximum intensity to the background as 8:1. The solid line for the envelope is the curve calculated assuming Gaussian pulse with 100 fs fwhm. It indicates that the dispersion of pulse is small and a nearly ideal condition of pulse is preserved even after the objective lens and our alignment procedure works properly. We applied a double pulse train technique to trapping dynamics of nanoparticles in femtosecond time region. Dark-field images of optical trapping of polystyrene nanoparticles by linearly polarized pulse trains in perpendicular with each other with femtoseconds of time intervals are shown in Figure 3. It shows a bright spot at the focal position and bright areas outside of the beam center that are ascribed to trapping and ejecting nanoparticles, respectively. The directions of ejection of particles are changed with intervals of the pulse trains. As is revealed in Figure 2, two pulse trains are overlapped and interfere with each other with time interval in ≤100 fs. When a phase of intervals corresponds to odd-number of π (2.7 fs), the direction of the composite vector of two beams coincides with B
DOI: 10.1021/acs.jpcc.6b01217 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
ps delay without interference of two pulses), the present ejection by double pulse trains becomes much weaker. It indicates that the higher peak power of the pulses is more important to trap nanoparticles compared to higher repetition rate of the pulses in the tens of MHz region. This result is consistent with the efficient trapping in fs pulses compared to CW laser which can be regarded as infinite repetition rate pulses. The ejection of nanoparticles became weaker for longer intervals. We already reported that the ejection of nanoparticles is ascribed to the formation of transient assembly of the optically confined nanoparticles in the focus spot by the pulsed laser.22 We also confirmed that the ejection occurs more frequently in highly concentrated samples. The ejection occurs only when the number of particles at the focal spot becomes larger than a certain threshold. After the ejection process, surrounding particles are gathered to the focal spot again until they reach a certain number of particles for the next ejection. In high concentration solution, particles have high possibility to diffuse into trapping site and will be soon trapped by laser. It can rapidly gather large number of particles and the ejection occurs frequently. Hence, we can evaluate the number of trapped particles by femtosecond laser from the ejection frequency,25,27 and we analyzed our data as described in the experimental section. The ejection data are shown in Figure 5a as a function of time interval between the pulse trains. The total laser power of the double pulse train was 200 mW, and it consisted of linearly polarized beams in parallel with each other. The frequency of ejection shows an exponential decay curve. Interestingly, it changes drastically in a few-ps interval region. In such a short time, particles cannot move appreciable distance by the diffusion. We estimate the diffusion coefficient of 50 nm sized particles in water at the room temperature as ca. 11.0 pm2 ps−1 according to the Stokes−Einstein equation
Figure 3. Dark-field images detected by a CCD camera revealing trapping and ejections of polystyrene nanoparticles by two of linearly polarized pulse trains in perpendicular with each other. Time intervals between the pulse trains and phases of 800 nm laser are shown on the figures. Red and blue arrows describe directions of each beam. The scale bar of 10 μm is shown in an image.
the direction of the vertical axis. On the other hand, when a phase is equal to multiples of 2π (5.3 fs), the direction of the composite vector becomes horizontal. Ejection occurs to a direction that is perpendicular to the composite vector. It is consistent with results of the ejection by single pulse train with linearly polarized beam. It indicates we succeeded in controlling trapping and ejecting behaviors of particles by double pulse train. We also controlled direction of ejection using circularly polarized double pulse train with different rotational direction (data are not shown). These results show that a double pulse train technique is useful to control temporal evolution of pulses precisely even in the femtosecond time region. Because pulses are interfered with each other in time region of a few hundred fs, it changes the trapping condition drastically and makes trapping dynamics so complicated. Thus, in the following sessions, we treat processes in longer than one picosecond where pulses are independent. We show trapping and ejection images obtained by irradiating linearly polarized double pulse trains in parallel with each other in Figure 4. In these picosecond time intervals, the pulses of two trains do not overlap temporally with each other. Compared to the 80 MHz pulse train with the laser power of the sum of two trains (this is virtually equal to the 0
D=
kBT 6πηr
(1)
where T is the absolute temperature, kB is the Boltzmann’s constant, η is the dynamic viscosity, and r is the radius of the spherical particle. It means that the displacement of particles is not dominant for the relaxation dynamics in femtosecond laser trapping. As one of the possible origins of a few ps relaxation, first we considered that electron excited state dynamics of polystyrene is related to trapping dynamics. Namely, polystyrene has the
Figure 4. Dark-field images of trapping and ejections of polystyrene nanoparticles by two of linearly polarized pulse trains in parallel with each other (upper panels). Time intervals between the pulse trains are in picoseconds. The scale bar of 10 μm is shown in an image. The bottoms are binarized images. The threshold is set to twice of background. Red circles describes 5 μm away from the focal spot. C
DOI: 10.1021/acs.jpcc.6b01217 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 5. Ejection frequencies of polystyrene (a) and surface modified silica (b) nanoparticles from the focal spot as a function of time interval between the pulse trains. It represents how many image frames show the ejection for 1 min (1800 frames) video. The power of trapping laser is 200 mW for (a) and 280 mW for (b).
Figure 6. A. Schemes of optical trapping of nanoparticles by repetitive femtosecond laser pulses. (1) In
