John T. Bahns, Subramanian K. R. S. Sankaranarayanan, Noel C. Giebink, Hui Xiong, Stephen K. Gray
Directed colloidal synthesis of conductive organic–inorganic hybrid mesoscale structuresis reported. The technique is simple but allows hierarchical assembly and 2D patterning of materials. A focused laser spot is used to direct the colloidal assembly of nanoparticles into electrically conductive organic–inorganic hybrid mesoscale filaments with arbitrary permanent patterns on a glass surface.
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Olav Gaute Hellesø , Pål Løvhaugen , Ananth Z Subramanian , James S. Wilkinson and Balpreet Singh Ahluwalia
Waveguide trapping has emerged as a useful technique for parallel and planar transport of particles and biological cells and can be integrated with lab-on-a-chip applications. However, particles trapped on waveguides are continuously propelled forward along the surface of the waveguide. This limits the practical usability of waveguide trapping technique with other functions (e.g analysis, imaging) that require particles to be held stationary during diagnosis. In this paper, an optical waveguide loop with an intentional gap at the centre is proposed to hold propelled particles and cells. The waveguide acts as a conveyor belt to transport and deliver the particles/cells towards the gap. At the gap, the diverging light fields hold the particles at a fixed position. The proposed waveguide design is numerically studied and experimentally implemented. The optical forces on the particle at the gap are calculated using the finite element method. Experimentally, the method is used to transport and trap micro-particles and red blood cells at the gap with varying separations. The waveguides are only 180 nm thick and thus could be integrated with other functions on the chip, e.g. microfluidics or optical detection, to make an on-chip system for single cell analysis and to study the interaction between cells.
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Anita Jannasch, Ahmet F. Demirörs, Peter D. J. van Oostrum, Alfons van Blaaderen & Erik Schäffer
Optical tweezers are exquisite position and force transducers and are widely used for high-resolution measurements in fields as varied as physics, biology and materials science. Typically, small dielectric particles are trapped in a tightly focused laser and are often used as handles for sensitive force measurements. Improvement to the technique has largely focused on improving the instrument and shaping the light beam, and there has been little work exploring the benefit of customizing the trapped object. Here, we describe how anti-reflection coated, high-refractive-index core–shell particles composed of titania enable single-beam optical trapping with an optical force greater than a nanonewton. The increased force range broadens the scope of feasible optical trapping experiments and will pave the way towards more efficient light-powered miniature machines, tools and applications.
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Hidenori Suzuki, Yoshiki Matsuzaki, Azusa Muraoka, and Maki Tachikawa
By use of an optical trap, we can levitate micrometer-sized drops of purified water and cool them below the melting point free from contact freezing. Raman spectra of the OH stretching band were obtained from those supercooled water droplets at temperatures down to −35 °C. According to the two-state model, an enthalpy change due to hydrogen-bond breaking is derived from temperature dependence of the spectral profile. The isobaric heat capacity calculated from the enthalpy data shows a sharp increase as the temperature is lowered below −20 °C in good agreement with conventional thermodynamic measurements.
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Raktim Dasgupta, Sunita Ahlawat, and Pradeep Kumar Gupta
Optical sorting was demonstrated by selective trapping of a set of microspheres (having specific size or composition) from a flowing mixture and guiding these in the desired direction by a moving array of optical traps. The approach exploits the fact that whereas the fluid drag force varies linearly with particle size, the optical gradient force has a more complex dependence on the particle size and also on its optical properties. Therefore, the ratio of these two forces is unique for different types of flowing particles. Selective trapping of a particular type of particles can thus be achieved by ensuring that the ratio between fluid drag and optical gradient force on these is below unity whereas for others it exceeds unity. Thereafter, the trapped particles can be sorted using a motion of the trapping sites towards the output. Because in this method the trapping force seen by the selected fraction of particles can be suitably higher than the fluid drag force, the particles can be captured and sorted from a fast fluid flow (about 150 μm/s). Therefore, even when using a dilute particle suspension, where the colloidal trafficking issues are naturally minimized, due to high flow rate a good throughput (about 30 particles/s) can be obtained. Experiments were performed to demonstrate sorting between silica spheres of different sizes (2, 3, and 5 μm) and between 3 μm size silica and polystyrene spheres.
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A Yu Maklygin, A V Priezzhev, A Karmenian, Sergei Yu Nikitin, I S Obolenskii, Andrei E Lugovtsov and Kisun Li
We have fabricated double-beam optical tweezers and demonstrated the possibility of their use for measuring the interaction forces between red blood cells (erythrocytes). It has been established experimentally that prolonged trapping of red blood cells in a tightly focused laser beam does not cause any visible changes in their shape or size. We have measured the interaction between red blood cells in the aggregate, deformed by optical tweezers.
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Koichiro Hori , David P. Penaloza , Atsuomi Shundo and Keiji Tanaka
Surfactant molecules often form micelles with a large aspect ratio, “worm-like” micelles, leading to network structures based on their entanglements. By using optical tweezers, we could detect a heterogeneity in the viscoelastic properties of the worm-like micelle solution, which is observed when the measurement timescale is shorter than the relaxation time − the time at which dissolution of the micelle entanglements occurs.
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Mariko Toshimitsu, Yuriko Matsumura, Tatsuya Shoji, Noboru Kitamura, Mai Takase, Kei Murakoshi, Hiroaki Yamauchi, Syoji Ito, Hiroshi Miyasaka, Atsushi Nobuhiro, Yoshihiko Mizumoto, Hajime Ishihara, and Yasuyuki Tsuboi
Optical trapping of flexible polymer chains to a metallic nano-structured surface was explored by microscopic imaging and confocal fluorescence spectroscopy. A fluorescence-labeled poly(N-isopropylacrylamide) was targeted, being a representative thermo-responsive polymer. Upon resonant plasmonic excitation, it was clearly observed that polymers were assembled into the excitation area to form molecular assemblies. Simultaneously, fluorescence from the area was obviously intensified, indicating an increase in the number of polymer chains at the area. The excitation threshold of light intensity that was required for obvious trapping was 1 kW/cm2, which was much lower by a factor of 104 than that for conventional trapping using a focused laser beam. The morphology of the assemblies was sensitive to excitation intensity. We precisely evaluated temperature rise (
T) around the metallic nanostructure upon plasmonic excitation: T ~ 10 K at 1 kW/cm2 excitation. This temperature rise was an origin of a repulsive force that blocked stable trapping. Based on experimental observations and theoretical calculations, we quantitatively evaluated the plasmon-enhanced trapping force and the thermal repulsive force (Soret effect). The overall mechanisms that were involved in such plasmon-enhanced optical trapping are discussed in detail. The smooth catch-and-release trapping (manipulation) of polymer chains was successfully demonstrated.
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Miguel A. Santiago-Cordoba, Murat Cetinkaya, Svetlana V. Boriskina, Frank Vollmer, Melik C. Demirel
Microcavity and whispering gallery mode (WGM) biosensors derive their sensitivity from monitoring frequency shifts induced by protein binding at sites of highly confined field intensities, where field strengths can be further amplified by excitation of plasmon resonances in nanoparticle layers. Here, we propose a mechanism based on optical trapping of a protein at the site of plasmonic field enhancements for achieving ultra sensitive detection in only microliter-scale sample volumes, and in real-time. We demonstrate femto-Molar sensitivity corresponding to a few 1000 s of macromolecules. Simulations based on Mie theory agree well with the optical trapping concept at plasmonic ‘hotspots’ locations.
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S.E. Skelton, M. Sergides, R. Patel, E. Karczewska, O.M. Maragó, P.H. Jones
We investigate the manipulation of microscopic and nanoscopic particles using the evanescent optical field surrounding an optical fiber that is tapered to a micron-scale diameter, and propose that this scheme could be used to discriminate between, and thereby sort, metallic nanoparticles. First we show experimentally the concept of the transport of micron-sized spheres along a tapered fiber and measure the particle velocity. Having demonstrated the principle we then consider theoretically the application to the optical trapping and guiding of metallic nanoparticles, where the presence of a plasmon resonance is used to enhance optical forces. We show that the dynamics of the nanoparticles trapped by the evanescent field can be controlled by the state of polarization of the fiber mode, and by using more than one wavelength differently detuned from the nanoparticle plasmon resonance. Such a scheme could potentially be used for selectively trapping and transporting nano- or microscopic material from a polydisperse suspension.
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