Optical Trapping-Formed Colloidal Assembly with Horns Extended to

Letter pubs.acs.org/NanoLett

Optical Trapping-Formed Colloidal Assembly with Horns Extended to the Outside of a Focus through Light Propagation Tetsuhiro Kudo,* Shun-Fa Wang, Ken-ichi Yuyama, and Hiroshi Masuhara* Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan S Supporting Information *

ABSTRACT: We report optical trapping and assembling of colloidal particles at a glass/solution interface with a tightly focused laser beam of high intensity. It is generally believed that the particles are gathered only in an irradiated area where optical force is exerted on the particles by laser beam. Here we demonstrate that, the propagation of trapping laser from the focus to the outside of the formed assembly leads to expansion of the assembly much larger than the irradiated area with sticking out rows of linearly aligned particles like horns. The shape of the assembly, its structure, and the number of horns can be controlled by laser polarization. Optical trapping study utilizing the light propagation will open a new avenue for assembling and crystallizing quantum dots, metal nanoparticles, molecular clusters, proteins, and DNA. KEYWORDS: Optical trapping, colloidal assembly, light propagation, optical binding, glass/solution interface

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shkin and coauthors1 demonstrated that a laser beam tightly focused into solution can be used for an optical trapping of colloidal particles. Burns et al.2 conducted the optical trapping and assembling of the particles at a glass/ solution interface with a loosely focused laser beam for constructing crystalline structures that arise as a consequence of light-induced interactions between the particles (so-called optical binding). The optical binding force depends on the properties of the laser beams such as wavelength and laser polarization.2−5 Recently, artificially created fluctuating light fields were used for the studies of the optical binding force.6 The optical trapping phenomenon has been expected to provide a new direction for self-organization of assemblies and crystals. Fournier group7 showed that the assembly structure, formed in the manner similar to that of Burns et al.,2 can be used as an optical mirror. Evanescent waves created by a total reflection with a prism,8−14 plasmonic mental nanostructures,15−17 and photonic-crystal slabs,18,19 as well as shaped light field generated by a spatial light modulator,20−22 were used for self-assembling of particles at the interface. In these previous studies, the laser beam was widely illuminated to collect the particles to form the assembly, and obviously all these assemblies were trapped only in the irradiated area. Here, we report novel optical trapping and assembling mechanism in which the propagation of trapping laser through the assembly leads to extension of the assembly much larger than the focal spot. We demonstrate the optical trapping of polystyrene particles at the glass/solution interface. A purchased sample of the polystyrene particles (Polyscience, mean diameter 500 nm) dispersed in water is diluted 1000 times with water, yielding particle concentration of about 3.6 × 108 particles/ml in order © XXXX American Chemical Society

to observe the assembling dynamics at slow speed. The sample solution is sandwiched by cover glasses (Matsunami) with a 120 μm thickness spacer (Electron Microscopy Sciences). As shown in Figure 1, an optical trapping system is constructed on

Figure 1. Setup for optical trapping and assembling at the glass/ solution interface. The dashed-line rectangular inset shows the sketch of the sample chamber sandwiched by glass substrates. The trapping laser is focused at the upper-glass/solution interface of the chamber. Abbreviations used: NA = numerical aperture; CCD = charge-coupled device. Received: January 10, 2016 Revised: April 21, 2016

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DOI: 10.1021/acs.nanolett.6b00123 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters

Figure 2. Transmission images of the colloidal assembly formed during optical trapping with the linearly polarized light. (a) The image before turning on the laser. (b−f) Time evolution of assembly with vertical polarization. (g,h) The assembly constructed with horizontal and 45°-titled polarizations, respectively. Arrows denote the direction of light polarization. The length of the black bars is 10 μm.

Figure 3. Transmission images of the colloidal assembly formed during optical trapping with the circularly polarized light. (a) The image before turning on the laser. (b−g) Time evolution of the assembly. (h) The assembly formed with laser power of 1.8 W. The circular arrows denote the direction of light polarization. The length of the black bars is 10 μm.

Figure 4. Possible formation mechanism for the assembly structures with four and six horns. (a,b) Spatial profiles of calculated laser intensity at the focal plane for linearly polarized light and circularly polarized light. The laser power, numerical aperture, and beam diameter before the objective lens for theoretical calculation are 1.4 W, 0.95, and 9 mm, which are parameters of the present experiments. (c,d) The sketches of the growth process of tetragonal structure with four horns and close-packed structure with six horns assembled by the linearly polarized light and the circularly polarized light, respectively. The gradually expanding effective irradiation area created by the multiple scattering is represented by a light-red cloud; dark-red lines denote the propagation of trapping laser toward the horn directions from the center of the assemblies. (e,f) Backscattering images of 1064 nm laser observed from assembles with four and six horns. The length of the white bars corresponds to 10 μm.

The trapping laser power of 1.4 W in the present experiment is about 100 times higher than a minimum laser power (15 mW) that is necessary to form their aggregate (see Supporting Information 1). The direction of a linearly polarized light is controlled by turning a λ/2 wave plate and a circularly polarized light is generated with a λ/4 wave plate. Transmission images are recorded by a charge-coupled device (CCD) camera (30 fps) under illumination of a halogen lamp with condenser lens (0.52 numerical aperture). A spectrometer (Andor SR303i)

an inverted microscope (Nikon Eclipse Ti), and a 1064 nm continuous wave laser (Spectra-Physics, Nd3+:YVO4 laser, J20IBL-106C) is used as the trapping laser. This optical setup is conventional and similar to that reported previously by our group.23,24 The laser located at the bottom side of the sample chamber is focused at the interface between an upper glass substrate and water through objective lens (Olympus, 0.95 numerical aperture, 40× magnification). A beam diameter of 9 mm is adjusted before the objective lens by collimation lenses. B

DOI: 10.1021/acs.nanolett.6b00123 Nano Lett. XXXX, XXX, XXX−XXX

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effective wavelength (described below), which is a characteristic of the optical binding (see Supporting Information 3). For the circularly polarized light, we found that the assembly sticks out short six horns after its gradual expansion (Figure 3g and Movie S2) and the angles between the neighboring horns are about 60°. The growth process is dynamic and stochastic and we sometimes observed that the assembly suddenly develops four horns instead of six horns. After a while, these four horns disappear and the assembly returns back to the gradual expansion to six horns. We conducted the optical trapping experiment with circularly polarized light of 1.8 W ten times. The probability that we only observed six horns within 9 min irradiation was 50%. It should be emphasized that we have never observed six horns with linearly polarized light and six horns are only achieved with circularly polarized light. The six horns obtained with the 1.8 W laser are longer than those obtained with the 1.4 W laser (see Figure 3h and Movie S3). We believe that the laser power required to form a single horn is the same and the total laser power is divided into four or six horns. Therefore, four horns formed with linearly polarized light are much longer compared to six horns formed with circularly polarized light. The assemblies were also monitored by 1064 nm backscattering imaging. It is noticeable in Figure 4e,f that the horns grew from the focus to the outside. This indicates directly that the trapping laser is propagated along the horn. We consider that the light propagation elongates the horns by binding optically subsequent particles along the direction of light propagation. Additionally, the backscattering light of the 1064 nm laser is emitted also from the assembly, which may be due to the multiple scattering in the assembly. We propose that the assembly structure with four horns is tetragonal and that with six horns is close-packed, which is depicted in Figure 4c-(v),d(v), together with schematic picture of light propagation through the rows of linearly adjoined particles. It was previously reported that, by observing fluorescence emissions from the dye-doped particles in the colloidal layer, the close-packed structure leads to analogous six-fold light propagation along the directions of linearly adjoined particles.27 The assembly structures with four and six horns can be directly differentiated by measuring transmittance spectra as shown in Figure 5. The transmittance for both linearly and

combined with a CCD camera (Aundor iDus) is equipped to obtain the transmittance spectra with a pinhole (50 μm in diameter). The transmittance is estimated as a ratio of the transmission intensity obtained during the assembly formation and to that before the laser irradiation. A short-pass filter (Semrock: FF01-1010/SP-25) is inserted to cut the backscattering light of the trapping laser from the transmission image. For the measurement of 1064 nm backscattering imaging, this filter is taken out and the halogen lamp is turned off. Figures 2 and 3 represent time evolution of the assembly formed by the optical trapping with the linearly and the circularly polarized lights, respectively. For the linearly polarized light, the assembly is elongated along the polarization direction as shown in Figure 2c. The particles forming the assembly move dynamically and collide randomly with each other. The long and short axes of the ellipse-like assembly are about 14 and 12 μm, respectively. These dimensions are much larger than the focal spot (about 1 μm). This expansion of the assembly may be caused by multiple scattering in and near-field diffraction from constituent particles (see Supporting Information 2). The elliptical assembly was rotated by changing the polarization direction, as shown in Figure 2g. For the circularly polarized light, the assembly has a circular shape, as shown in Figure 3e−g with a diameter of about 20 μm. We calculated laser beam profiles at a focal plane similarly as done in previous theoretical studies.25,26 As shown in Figure 4a,b, elliptical and circular laser beam profiles are obtained for the linearly and the circularly polarized lights, respectively. The anisotropy of the elliptic distribution is due to the tightly focusing optics. When the linearly polarized light (say xpolarized) is introduced into a high NA objective lens, the xpolarized light is tilted and other y- and z-components are generated through focusing. Because the calculated profiles correlate well with the assembled shapes, we think that the shapes of assembly are determined by trapping laser beam profiles. After the growth of the elliptical assembly is almost completed, the assembly sticks out four horns consisting of a single row of linearly aligned particles. The horns appear within several seconds as shown in Figure 2d (Movie S1 shows the video in Figure 2c−e). The horns become longer with time as shown in Figure 2d−f. When the growth of the assembly is saturated, more and more particles are observed around the assembly especially at the edge-side of the horns, as shown in Figure 2d−f. Those particles are responsible for the elongations, and indeed the horn in Figure 2f reaches about 10 μm. Note that such length would be difficult to achieve with a direct trapping laser. The horns are tilted at about 45° from the light polarization direction. Although the horns swing randomly, they always maintain an X-like shape. Figure 2h displays the assembly formed using laser irradiation with 45° tilted polarization. The horns appear in the direction rotated by 45° in comparison to that in Figure 2f. It should be noted that the direction of the horns is not rotated when the polarization is rotated within a few seconds after the horn formation. After a while, the horns start to appear in the direction of 45° tilted from the linear polarization. We believe that it might be possible to rotate the whole assembly with horns by rotating the linear polarization at the rate slower than the time scale of the assembling dynamics. Although the particles move randomly in the horns, their interparticle distance is sometimes about 1.2 μm. This distance is about twice the size of an

Figure 5. Typical transmittance spectra observed for assemblies with four horns (blue line) and six horns (red line). The vertical line indicates the position of the dips for both spectra. The exposure time for both spectra is 1 s. C

DOI: 10.1021/acs.nanolett.6b00123 Nano Lett. XXXX, XXX, XXX−XXX

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results in gathering of the particles around the edge of the Lphenylalanine crystal even if the laser was not illuminating directly the edge. We believe that the present finding will hold also for quantum dots, metal nanoparticles, molecular clusters, proteins, and DNA, enabling us to design their assemblies and crystals by optical force through light propagation.

circularly polarized lights is diminished, while its degree in the shorter wavelength region is larger than that in the longer one due to scattering of transmission light. We believe the reason for lower transmittance for the assembly with six horns compared to that for four horns is because the close-packed structure is denser than the tetragonal one. Dips appearing around 720 nm in both spectra are interpreted as the second mode of Bragg reflection wavelength (see Supporting Information 4). The possible formation mechanism for structures with four and six horns are summarized in Figure 4c,d. After turning on the trapping laser the particles start to gather in the optical potential. The effective irradiated area gradually extends due to the multiple scattering (Figure 4c-(ii),d-(ii)) resulting in the growth of the assembly. The shapes of the assemblies, elliptical for linearly polarized light and circular for circularly polarized light (Figure 4c-(ii),d-(ii)), are determined by the laser beam profiles (Figure 4a,b). The structure is formed from the gathered and randomly moving particles in the assembly. We believe that the tetragonal and the close-packed structures are prepared stochastically by filling up the particles in each optical potential field (Figure 4c-(iii),d-(iii)). The propagation of the trapping laser through the adjoining particles becomes apparent outside the focus in the form of horns, where the number of horns is determined by packing order, tetragonal or closepacked, of the assembly. The effective wavelength along the horn direction is estimated to be about 570 nm, taking into account water refractive index and the incident angle, which is similar to the particle diameter. Under such conditions, the horn may serve as a one-dimensional photonic crystal or a waveguide for propagating the trapping laser efficiently. This light propagation stabilizes the horns and optically binds subsequent particles as many as possible until the laser intensity at the horn edge becomes weaker than the intensity required for overcoming thermal fluctuations. Thus, the tetragonal structure with four horns and the close-packed structure with six horns are assembled with the linearly and the circularly polarized lights, respectively (Figure 4c-(v),and d-(v)). In summary, we have experimentally demonstrated that a single assembly of polystyrene particles is prepared at the focal point and expands much larger than the irradiated area with sticking out horns through the propagation of trapping laser. Despite a considerable research of the optical trapping experiments conducted in the past three decades, our work seems to be the first observation because the majority of optical trapping experiments have been carried out inside solution. The formation of the horns is enabled with strong laser intensity, with an appropriate relation between the particle size and the trapping wavelength, and only at the glass/solution interface. Indeed, these three conditions could not have been simultaneously set without deep insights of integrating light propagation and optical trapping. The shape of the assembly, its structures, and the number of horns can be controlled by laser polarization. The horns are formed as a result of light propagation along the adjoined particles, which suggests that waveguide-like structure could be self-organized by optical trapping. We believe that the light propagation mechanism reported here for assemblies formed by optical trapping at the glass/solution and air/solution interfaces applies generally to nanoparticles, molecules, and their clusters. Optical trapping induced crystallization of L-phenylalanine at the air/solution interface,28,29 reported by us previously, can be a good example here. We observed that adding colloidal particles to the solution

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00123. (1) Minimum laser power for assembling, (2) large assembly due to multiple scattering, (3) interparticle distance due to optical binding force, (4) Bragg reflection wavelength, and additional figures (PDF) The video of the colloidal assembly with four horns formed during optical trapping with the linearly polarized light. The laser power is 1.4 W and the direction of the polarization is vertical. The video recording time is from 4 min (Figure 2c in main text) to 4 min 30 s (Figure 2e). (AVI) The video of the colloidal assembly with six horns formed during optical trapping with the circularly polarized light. The laser power is 1.4 W. The video recording time is from 16 min to 16 min 15 s and the transmission image of Figure 3g in main text is included in this video recording time. (AVI) The video of the colloidal assembly with six horns formed during optical trapping with the circularly polarized light. The laser power is 1.8 W. The video recording time is from 5 min 50 s to 6 min 10 s and the transmission image of Figure 3h in main text is included in this video recording time. (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (T.K.) [email protected]. *E-mail: (H.M.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is partly supported by the MOE-ATU Project (National Chiao Tung University) of the Ministry of Education, Taiwan, to H.M., the National Science Council of Taiwan to K.Y. (MOST 103-2113-M-009-022-MY2) and to H.M. (MOST 103-2113-M-009-003). T.K. deeply acknowledges support from the Japan Society for the Promotion of Science Postdoctoral Fellowships for Research Abroad and also thanks Professor Hajime Ishihara for his kind understandings and strong support.

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REFERENCES

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