Efficient Optical Trapping of CdTe Quantum Dots by Femtosecond

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Efficient Optical Trapping of CdTe Quantum Dots by Femtosecond Laser Pulses Wei-Yi Chiang, Tomoki Okuhata, Anwar Usman, Naoto Tamai, and Hiroshi Masuhara J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2014 Downloaded from http://pubs.acs.org on June 17, 2014

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The Journal of Physical Chemistry

Efficient Optical Trapping of CdTe Quantum Dots by Femtosecond Laser Pulses

Wei-Yi Chiang,† Tomoki Okuhata,§ Anwar Usman,‡,* Naoto Tamai,§,* Hiroshi Masuhara†,* †

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, 1001 Ta Hsueh Rd., Hsinchu 30010, Taiwan, § Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda 669-1337, Japan ‡ Solar and Photovoltaic Engineering Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia,

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Abstract

The development in optical trapping and manipulation has been showing rapid progress, most of it is in the small particle sizes in nanometer scales, substituting the conventional continuous-wave lasers with high-repetition-rate ultrashort laser pulse train, and nonlinear optical effects. Here, we evaluate two-photon absorption in optical trapping of 2.7 nm-sized CdTe quantum dots (QDs) with high-repetition-rate femtosecond pulse train by probing laser intensity dependence of both Rayleigh scattering image and the two-photon-induced luminescence spectrum of the optically trapped QDs. The Rayleigh scattering imaging indicates that the two-photon absorption (TPA) process enhances trapping ability of the QDs. Similarly, a nonlinear increase of the two-photon-induced luminescence with the incident laser intensity fairly indicates the existence of TPA process.

Keywords: CdTe quantum dots, optical trapping, two-photon absorption, femtosecond laser pulses, Rayleigh scattering, dark-field microscopy, two-photon-induced luminescence.


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INTRODUCTION Optical gradient force originating from the transfer of momentum from a tightly focused continuous-wave (cw) laser beam to a micron- or submicron-sized object located around the focal spot enables contactless and accurate optical confinement, deposition, and manipulation.1-3 This so-called optical trapping technique has been showing tremendous and increasing impacts on optical micromanipulation in many research areas in physics, chemistry, biology, and material science.4-8 Optical manipulation of small objects on the nanometre scales has been of particular interest. Because polarizability of the particles reduces with their volume, while diffusion coefficient increases inversely with their diameter, the optical trapping potential generated in the diffraction-limited focal spot cannot suppress their thermal Brownian motion. Therefore, the smallest size of metallic or dielectric polystyrene particles that can be stably trapped by sub-W level cw beam into the limiting spot diameter of 1 µm is mostly about 10–20 nm.2,9-11 Several efforts have been devoted to reduce the size limit as well as to increase the trapping efficiency of the nanoparticles. First effort to answer these two challenges is the use of an additional cw laser beam with wavelength falling at the absorption band of the target particle. This so-called single-photon resonant optical trapping has been successfully demonstrated to elongate biased diffusion of 40 nm dielectric particles by 7-fold when 532-nm laser excitation beam (2.4 µW) is applied in addition to near-infrared (1064-nm) laser trapping beam (600 mW), as compared to that without resonance effect.12 Recently, the resonance effect has also been demonstrated in optical trapping of myoglobin (a small protein) which absorbs the near-infrared laser trapping beam.13 The fundamental mechanism of the resonant optical trapping has been documented by Ishihara’s group.14-16 Second effort is to generate intense light field at the focal spot either by creating localized plasmon in the trapping region17 or by substituting cw- with pulsed-mode lasers.18 The former case has been demonstrated in optical trapping of nanocrystal CdSe/ZnS quantum dots (QDs) in the presence of gold nanodimer arrays,19 and in the latter case the impulsive peak powers of high-repetition-rate ultrashort laser pulses has been showing important advantages for future optical nanomanipulation, including stable trapping of a large number of semiconductor nanocrystal CdTe QDs by picosecond (ps) laser pulses at average power as low as 100 mW.18 The other important optical trapping of nanoparticles is two-photon excitation effect induced by cw lasers to trap individual semiconductor QDs20,21 or nanorods.22 This finding has started new development of efficient optical manipulation of QDs or other small particles in nanometer scales. In parallel, the nonlinear resonance laser manipulation has been theoretically proposed by considering the nonlinear optical effect beyond the perturbative regime.23 More recently, femtosecond (fs) laser pulses have been revealed new phenomena in optical 3|Page ACS Paragon Plus Environment


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trapping of Rayleigh particles, including the split of potential trapping minimum of gold nanoparticles into two off-axis trapping sites parallel to the laser polarization,24 reversible trapping and release of λ-DNA without any permanent fixing on the plasmonic substrate,25 and ejections of optically trapped polystyrene nanoparticles along the directions perpendicular to the laser polarization.26 In these cases, the focused fs pulse train not only generates attractive forces exerted on the nanoparticles without severe temperature increase, but it also stimulates nonlinear optical (NLO) processes in the trapped particles and its transverse fields induces repulsive forces perpendicular to the polarization vector. So far, the important characteristic of the utilization of ultrashort laser pulses in optical trapping is their impulsive peak power that can generate strong transient gradient, transient scattering, and temporal forces.27 By controlling the laser pulse width and the numerical aperture (NA) objective lens in the trapping experiment, one can achieve third-order NLO process to strengthen optical force or to modify the optical trapping potential24 as well as to generate three-dimensional optical forces that can control directional ejections of the trapped nanoparticles.28 Because single photon resonance trapping by cw lasers can improve optical trapping, we expect that two photon resonance could be easily induced by impulsive peak power of fs laser pulses, leading to a new window in optical nanomanipulation. In this work we specifically demonstrate that high-repetition-rate fs laser pulses can enhance optical trapping ability of CdTe QDs. In contrast to our report on optical trapping of CdTe QDs by ps laser pulses in our previous work, in which we focus on trapping stability and effect of medium,18 here we evaluate the impact of two-photon absorption (TPA) of CdTe QDs.29,30 By analyzing the incident laser power dependence of both Rayleigh scattering and twophoton-induced luminescence (TPL) intensities, as described later, we report a new insight that TPA enhances the trapping ability of the QDs. Considering that the optical force exerted on such Rayleigh particles is transiently generated by the ultrashort laser pulses18,27 and that the meansquare displacement of the QDs within interval time (12.5 ns) between consecutive pulses is much smaller than the trapping area, the high-repetition-rate pulse train can optically trap or confine a large number of QDs in three dimensions within a few µm3-sized trapping site.28 Such effective optical trapping and second-order NLO susceptibility (χ(2)) of CdTe QDs may allow TPA process to occur in optically trapped QDs, similarly to the process observed for a single micron-sized dye-doped polystyrene bead trapped by fs laser pulses.31

EXPERIMENTAL SECTION Materials. The thioglycolic acid-capped CdTe QDs were synthesized from water-soluble thiol compounds by using an adaptation of the method reported.32-35 This type of QDs have been 4|Page ACS Paragon Plus Environment

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reported to have χ(2) in the range of 43–85 pm/V 36 and third-order NLO susceptibility (χ(3)) in the order of 1.8×10-22 m2/V2 (equivalent to 1.2×10-14 esu)37 at 800 nm excitation. This CdTe QDs can also be considered to be a convenient model system for TPA study owing to their high TPA cross section ( σ TPA = 104 GM),29,38 high absorption coefficient in visible region, bright photoluminescence,35 highly monodispersed-colloid, and high photostability.32 In comparison, σ TPA of CdTe QDs is equivalent to that of CdSe QDs,39 but it is one and two orders of magnitude higher than that for the same size CdS and ZnS QDs (~102 GM).40,41 However, the χ(3) of CdTe QDs is much smaller than those of silicon (5.1×10-11 esu),42 silver (1.7×10-10 esu),42 or gold nanoparticles (5×10-8 esu).43 Optical Trapping. The optical trapping setup in this work is generally the same as we used in optical trapping of dielectric nanoparticles.26 Briefly, the setup is based on an inverted microscope (Olympus IX70) equipped with a Ti:sapphire laser (Tsunami; 800 nm) operating in cw- or pulse-mode (90 fs, 80 MHz) as the trapping beam. By using an objective lens (UPLanApo; 60×, NA 0.90), the beam was tightly focused at 100 µm above the bottom substrate of a sample chamber containing 10-6 M colloidal solution of 2.7-nm-sized thioglycolic acidcapped CdTe QDs suspended in water. At the focal plane, beam waist of focusing 800-nm beam is estimated to 0.46 µm, equivalent to focal area of 6.64×10-9 cm2. Due to the transparency of water at 800 nm, the laser-induced temperature elevation at the focal spot was estimated to be less than 2 °C/W. Thus, optical trapping-induced temperature elevation can be excluded. We have systematically evaluated the optical trapping behavior of the CdTe QDs by two complementary detection systems, side-by-side, either Rayleigh scattering imaging or TPL microspectroscopy, not both detections at the same time. As schematically shown in Figure 1A, for the scattering imaging, we employed a narrowband white light illumination (λ=695–705 nm) through a dark-field condenser lens (NA 1.2–1.4), a long-wave pass filter (Semrock; RazorEdge 633 nm), and a CCD camera (JAI; CV-A55IR E) running at 30 frames per second. For TPL spectroscopy (Figure 1B), the backward luminescence induced by two-photon absorption of the trapping beam has been acquired with a CCD-coupled polychromator (Princeton Instrument; SpectraPro2300i) running at 200 ms integrating time. In both detection systems, hyper Rayleigh scattering was naturally eliminated by the dichroic mirror which is completely reflective for λ> β , the polarizability can be approximately expressed as α ≈ α 0 + α1 I ( r , t ) . For isotropic target particles with negligibly low NLO susceptibilities, the gradient force and potential trapping can be written as9,27 Fgrad = nm2 ε 0α∇ I ( r , t ) / 2 and U trap = − nm2 ε 0αI ( r , t ) , where ε 0 in vacuum permittivity, and in the two-photon resonance regime they become Fgradient = nm2 ε 0 (α 0 + α 1I ( r , t ))∇ I ( r , t ) / 2

(2a)

U trap = − nm2 ε 0 (α 0 + α1 I ( r , t )) I ( r , t )

(2b)

This Eq. (2) suggests that the incident laser intensity-dependent polarizability creates an additional driving force exerted on the nanoparticle due to the TPA which comprises a quadratic term of I (r ) . Therefore, given that the trapping ability is directly related to the depth of the potential trapping well at the focal spot, it is reasonable to consider that C has both linear and TPA-induced nonlinear dependences on I (r ) . As shown in Figure 3, the data suggests the presence of such a nonlinear term. If we consider that the nonlinear term reflects the laser intensity dependence of trapping ability due to the effect of TPA process, this effect enhances the trapping ability for I 0 = 24−48 MW/cm2 and at laser intensities higher than 48 MW/cm2 the enhancement factor decreases with laser intensity, resulting in a quasi-linear dependence. The result indicates that the TPA effect in the optical trapping can be divided into three laser-intensity zones. At very low laser intensities, the pulse train is even unable to generate stable trap or to overcome thermal diffusion of QDs out from the trapping site. With our optical trapping and dark-field microscopic systems, the scattering light is inappreciable before the laser power exceeded I 0 = 12 MW/cm2. Above this laser intensity, the number of optically trapped QDs increases gradually, indicating that despite having TPA in concert with the optical trapping its impact is not detected for laser power below I 0 = 24 MW/cm2. At moderate laser intensities, 2 I 0 = 24−48 MW/cm , at which the pulse train can effectively and stably confine a large number 7|Page ACS Paragon Plus Environment


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of QDs, the TPA effect works in concert, enhancing considerably and non-linearly the trapping ability of the QDs. For the incident laser pulses with higher intensities, the trapping ability shows a quasi-linear dependence though the generated transient optical forces and potential well in the trapping site should be higher with laser intensity. This suggests that, in this regime, the enhanced trapping ability due to TPA might be limited by thermal diffusion, trapping size, and photophysical processes other than TPA. For instance, we should take strong repulsive forces into account for the ultrashort pulse duration in this experiment.27,28 Thus, the optically trapped QDs are easily ejected from the highly occupied trapping site.27 Because the distance between QDs in the trapping site may reduce to within nm order, absorbing two photons and emitting single photon can create additional optical momentum or evanescent light field in the trapping site that also generate inter-particle radiation forces to destabilize optical trap of the QDs.23 In light of the TPA process occurs in the optically trapped QDs, we evaluated the dependence of TPL spectra on the incident laser power. It is noteworthy that the TPL spectral feature is comparable with the single-photon emission. Irrespective of the incident laser intensity, the TPL intensity increases rapidly within 0.4 s of trapping time of before saturation throughout the laser trapping without any spectral change (Figure 4). This indicates that the trapping site is rapidly filled by the QDs within 0.4 s of trapping time, and then the number of optically trapped QDs is unchanged. Figure 5A shows TPL spectra of the trapped CdTe QDs at different incident laser intensities. The consistent TPL spectral feature during the optical trapping and at different intensities indicates that there is no modification either in particle size or in electronic structure of the optically trapped CdTe QDs. Because TPL is a second-order nonlinear absorption process, theoretically, ITPL is approximately given by

ITPL = ATPL ∫ I 2 (r )CdV

(3)

where a constant ATPL accounts for σ TPA , quantum yield, and light collection efficiency. This means that ITPL is simply proportional to the square of I 0 , if C is constant, as it is observed for an optically trapped micron-sized particle by fs laser pulses.31 The above-mentioned Rayleigh scattering measurement indicates that C has both linear and TPA-induced nonlinear dependences on I 0 , therefore ITPL should show sum of linear, quadratic, cubic, and quartic terms of I 0 . However, a logarithmic plot of ITPL against I 0 (shown in Figure 5B) indicates that the data points deviate even from a straight line with slope of 2.0. The data suggest as if the measured ITPL do not support TPA similarly to those observed in individual QDs aggregates optically trapped by cw laser21 or ps pulse train.18 The deviation of the ITPL from the quadratic or higher terms of I 0 can be attributed to several possible accompanying phenomena as follows. 8|Page ACS Paragon Plus Environment

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First, an individual QD absorbs two photons simultaneously from a single pulse generating one exciton, and the charge carrier in the conduction band then relaxes to the valence band via radiative recombination process.45 An excess energy of the photogenerated QDs (Eg=522 nm) upon absorbing two photons of 800 nm and generating an exciton is 0.23 Eg. Such an excess energy, however, is not enough to create another exciton through carrier multiplication process, and it is dissipated via exciton-phonon interaction and induces local heating46-50 that may destabilize optical trap of the QDs.23 Second, unusual decrease with increasing excitation intensity at high laser intensities can be attributed to rapid Auger photoionization or excitonexciton annihilation as usually observed in the thiol-capped CdTe QDs.51,52 Third, the effect of photobleaching from a fluorescent emitting state to a dark state and blue-shift can probably induce intermittent emission of the optically trapped QDs, as it is observed under cw laser illumination.53 However, within a few hundred seconds of observation window the TPL spectra of the QDs remain intact without bleaching or shifting. This may indicate that high dynamics and mobility of QDs under the high-repetition-rate pulse train stabilize the emission color or minimize the emission intermittency of the QDs. Fourth, temporal force and NLO properties of the QDs in fs laser trapping can destabilize and eject QDs from the trapping site. Overall, all these photo-processes obscure the actual relationship between ITPL and I (r ) in the optical trapping QDs with fs laser pulses. This means that we already integrate the electronic relaxation dynamics of densely excited QDs and nanomechanics of their trapping and manipulation, which provides us a new insight of developing nanooptical devices and systems. In addition to the TPL spectra, it is also fundamentally important to evaluate whether the TPA or other NLO processes in the optical trapping of CdTe QDs are induced. For instance, at an average laser power of 150 mW, with β of the QDs 1×10-9 cm2/W,54 and refractive index 55 2 n 0 = 2 .4 , I 0 is 45 MW/cm and β I 0 / n0 is about 0.019, which is negligibly small. Moreover, χ(3) of CdTe QDs (1.2×10-14 esu)37 is more than five orders of magnitude lower than that of gold nanoparticles (5×10-8 esu).43 These values indicate that NLO effects such as Kerr effect or split trapping well as in case of gold nanoparticles24 are unsurprisingly not resolved for CdTe QDs in the current work. However, due to high σ TPA ,29,38 the efficient TPA in CdTe QDs enhances trapping ability of the nanoparticles. In contrast to the enhanced trapping of particles with cwlaser through single-photon resonance, which requires at least a-few-tens-nm-sized particles and an additional cw laser beam (in µW level of laser power) with wavelength locating at the absorption band of incorporated dyes, the capability of ultrashort laser pulses to increase trapping ability of a few nm-sized particles through two-photon resonant optical trapping is of fundamental interest in future optical trapping and nanomanipulation.



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CONCLUSIONS Using two complementary Rayleigh scattering imaging and TPL microspectroscopic systems, we have evaluated the impact of TPA induced by fs laser pulse-trapping beam on optical trapping of CdTe QDs. The Rayleigh scattering imaging demonstrates that, in a certain laser intensity region, TPA enhances trapping ability of the QDs. The impact of TPA in higher intensity region is apparently affected by thermal diffusion, trapping volume, and photophysical processes other than TPA. A nonlinear increase of the ITPL with the incident laser intensity fairly indicates the existence of TPA process, though the actual incident-laser-power dependence of ITPL is obscured due to several photo-processes other than the TPA. With the capability of ultrashort laser pulses to increase trapping ability of a few nm-sized particles through TPA process, one can further explore two-photon resonant optical trapping as well as optical manipulation of nm- or sub-nm-sized particles for the single nanoparticle or macromolecular trapping. With the TPA process and optical trapping behavior of the nanoparticles or macromolecules that can be controlled by optical properties of target nanoparticles and/or parameters of the laser pulses, such optical nanomanipulation can be a promising technique for fabrication of lab-on-a-chip devices.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] [email protected] (H.M.)

(A.U.);

[email protected]

(N.T.);

Notes The authors declare no competing financial interest. 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 (Grant No. NSC 100-2113M-009-001). H.M. also thanks to Foundation of the Advancement for Outstanding Scholarship of Taiwan. The reviewers are also acknowledged for their valuable comments and advices.

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(39) Dakovski, G. L.; Shan, J. Size Dependence of Two-Photon Absorption in Semiconductor Quantum Dots. J. Appl. Phys. 2013, 114, 014301. (40) Szeremeta, J.; Nyk, M.; Wawrzynczyk, D.; Samoc, M. Wavelength Dependence of Nonlinear Optical Properties of Colloidal CdS QDs. Nanoscale 2013, 5, 2388-2393. (41) Vivas, M. G.; Cury, J. F.; Schiavon, M. A.; Mendonca, C. R. Two-Photon Absorption of ZnS Quantum Dots: Interpreting the Nonlinear Spectrum. J. Phys. Chem. C 2013, 117, 8530-8535. (42) López-Suárez, A.; Rangel-Rojo, R.; Torres-Torres, C.; Benami, A.; Tamayo-Rivera, L.; Reyes-Esqueda, J. A.; Cheang-Wong, J. C.; Rodríguez-Fernández, L.; Crespo-Sosa, A.; Oliver, A. Enhancement of the Optical Kerr Effect Exhibited by an Integrated Configuration of Silicon Quantum Dots and Silver Nanoparticles. J. Phys.: Conf. Ser. 2011, 274, 012145. (43) Bloemer, M. J.; Haus, J. W.; Ashley, P. R. Degenerate of Four-Wave Mixing in Colloidal Gold as a Function of Particle Size. J. Opt. Soc. Am. B 1990, 7, 790-795. (44) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles, Wiley, New York, 1983. (45) Klimov, V. I. Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Nanocrystals. J. Phys. Chem. B 2000, 104, 6112-6123. (46) Schaller, R. D.; Klimov, V. I. High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion. Phys. Rev. Lett. 2004, 92, 186601. (47) Nozik, A. J. Exciton Multiplication and Relaxation Dynamics in Quantum Dots: Applications to Ultrahigh-Efficiency Solar Photon Conversion. Inorg. Chem. 2005, 44, 6893-6899. (48) Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.; Ellingson, R. J.; Nozik, A. J. Multiple Exciton Generation in Colloidal Silican Nanocrystals. Nano Lett. 2007, 7, 2506-2512. (49) Sukhovatkin, V.; Hinds, S.; Brzozowski, L.; Sargent, E. H. Colloidal Quantum-Dot Photodetectors Exploiting Multiexciton Generation. Science 2009, 324, 1542-1544. (50) Kobayashi, Y.; Pan, L.; Tamai, N. Effects of Size and Capping Reagents on Biexciton Auger Recombination Dynamics of CdTe Quantum Dots. J. Phys. Chem. C 2009, 113, 11783-11789. (51) Wu, F.; Lewis, J. W.; Kliger, D. S.; Zhang, J. Z. Unusual Excitation Intensity Dependence of Fluorescence of CdTe Nanoparticles. J. Chem. Phys. 2003, 118, 12-16. (52) Padilha, L. A.; Neves, A. A. R.; Cesar, C. L.; Barbosa, L. C.; Cruz, C. H. B. Recombination Processes in CdTe Quantum-Dot-Doped Glasses. Appl. Phys. Lett. 2004, 85, 3256-3258. (53) Christensen, E. A.; Kulatunga, P.; Lagerholm, B. C. A Single Molecule Investigation of the Photostability of Quantum Dots. Plos One 2012, 7, e44355. (54) Dancus, I.; Vlad, V. I.; Petris, A.; Gaponik, N.; Shavel, A.; Eychmuller, A. Nonlinear Optical Properties of CdTe QDs Near the Resonance Regime. J. Optoelectron. Adv. Mater. 2008, 10, 149-151. (55) Chen, J.; Chen, X.; Xu, R.; Zhu, Y.; Shi, Y.; Zhu, X. Refractive Index of Aqueous Solution of CdTe Quantum Dots. Opt. Commun. 2008, 281, 3578-3580.



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Figure Captions FIGURE 1. Schematic diagrams of the experimental setups; (A) Rayleigh scattering imaging, (B) two-photon-induced luminescence microspectroscopy (DM is dichroic mirror, LPF is long-wave pass filter, and SPF is short-wave pass filter), and (C) absorption and emission spectrum of 2.7 nm-sized thioglycolic acid-capped CdTe QDs dispersed in water.

FIGURE 2. Rayleigh scattering images recorded by CCD camera at the incident laser power 150 mW ( I 0 = 45 MW/cm2) for a water suspension containing CdTe QDs. Inset: (left) the same image for water solvent containing no CdTe QDs and (right) line profile crossing the center of the image.

FIGURE 3. Rayleigh scattered light intensity, I s , as a function of the incident laser power in term of photon intensity I 0 in the range of 3−105 MW/cm2. Insert: schematic illustrations indicating optical trapping of QDs at three laser intensity regions: (A) without TPA at low laser intensities, (B) the impact of TPA at moderate laser intensities, and (C) saturation of TPA at high laser intensities.

FIGURE 4. TPL spectrum at different times 0–100 s after switching on the incident trapping beam.

FIGURE 5. (A) TPL spectra at different incident laser intensities, I 0 , in the range of 6−102 MW/cm2. (B) logarithmic plot of ITPL as function of I 0 . The straight line is the guide line with a slope 2.0.


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FIGURE 1

B

A

Oil 800 nm DM 800 nm DM

SPF

LPF SPF Polychromator

CCD

Single-photon PL Intensity (a.u.)

C

551

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


522

400

500

600

700

Wavelength (nm)



FIGURE 2

3 2 1

µm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

-1 -2 -3 0

100 I s [a.u.]


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FIGURE 3

(A)

(B)

(C)

z

z

z

(A)

x

x

x

50

y

y

y

(B)

(C)

40

Is [a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30 20 10 0 0

20

40

60

80

100

I0 [mW/cm2]



FIGURE 4

3 2 1 0 2

0 500

550

600

650

700

3

4

TPL [10 counts]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5

100 20

1 im 0.5 g T 0.2 pin

e

[ s]

ap r T

Wavelength [nm]


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FIGURE 5

A

6

102 MW/cm 90 75 60 45 30 15 6

ITPL (103 counts)

5 4 3

2

2 1 0 500

550

600

650

700

750

Wavelength (nm) B 4

10

3

10

ITPL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

10

1

10

10

100 2

I0 (MW/cm

)

FIGURE 4



Table of Content

Efficient Optical Trapping of CdTe Quantum Dots by Femtosecond Laser Pulses

Anwar Usman, Wei-Yi Chiang, Tomoki Okuhata, Naoto Tamai, Hiroshi Masuhara

50 40

Is [a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30

( A

20 10 0 0

20

40

60

80

100

I0 [mW/cm2]


Efficient Optical Trapping of CdTe Quantum Dots by Femtosecond

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