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Computation of the differential signals Á x and Á y given the signals from the quadrant detector. The differential signal Á x represents a particle’s motion along the flow channel ( x -direction). The differential signal Á y represents a particle’s motion transverse to the flow channel ( y -direction).
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We present a detection scheme for nanoscale particles based on the gradient force and torque near a tightly focused laser beam. The focus affects the path of nanoparticles passing by and a quadrant detector records the particle trajectory. A feedback system continuously adjusts the laser power and thereby prevents the particles from being trapped....
Context in source publication
Context 1
... object in a microscope can be monitored by measuring the intensity shifts in the back focal plane of the lens that collimates the outgoing laser field. The detected intensity shift originates from interference between the original laser field and the scattered field from the particle. The signal depends on the relative position of the particle with respect to the focus. Defining the signal from each quadrant by A , B , C and D ( figure 2), motion in the x -direction will induce a change in the signal ð A þ C Þ À ð B þ D Þ . On the other hand, motion in the y -direction can be detected by monitoring the signal ð A þ B Þ À ð C þ D Þ . The two signals can be made independent of the laser intensity by normalization with signal A B C D . In order to understand the process of position tracking, let us calculate the scattered field at the position of the detector. Assume that the focus is located at the origin and the particle is located at the point r 0 1⁄4 f x 0 ; y 0 ; z 0 g . The particle radius is R and the field incident on the particle is E ð r 0 Þ . The incident beam is a paraxial Gaussian TEM 00 mode polarized in the x -direction ( E 1⁄4 E n x ) and propagating along the z -direction. The centre of the detector is located at the point ð 0 ; 0 ; z Þ (c.f. figure 3). Since we consider particles much smaller than the laser wavelength we treat the particles in the dipole (Rayleigh) limit. The polarizability of a particle ...
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... This may result in the transition from laminar to turbulent flow, rendering the device nonfunctional. In optical particle size separation [17]- [19], significant sophistication in the laser/optical system is required to resolve particle sizes smaller than 100 nm. Thermophoresis is relatively insensitive to particles size and, thus, making separation difficult. ...
Airborne particles are separated according to their electrical mobilities using a microfabricated corona ionizer and separator electrodes. Oleic acid particles with sizes ranging from 30 to 300 nm are used to characterize the device. They are generated using a TSI 3075 constant output atomizer. These particles are electrically charged by a microfabricated corona ionizer, and the resultant particle electrical mobility is a function of the size of the particle. A varying DC potential difference of 0-2 kV across the separator electrodes selects charged particles of various electrical mobilities. These separated particles are subsequently counted using a TSI 3025A condensation particle counter. The device demonstrated its ability to separate particles between 50 and 130 nm into five distinct size bins. The operational flow rate is 0.5 L/min, and the micropin-between-planes corona ionizer operates at 1.3 kV with 7 muA. The theoretical and experimental electrical mobilities of the particles are compared.
... Novel applications comprise utilization of metal nanoparticles as non-toxic and non-bleaching labels in biology and biomedicine, e.g. [12][13][14][15] and references therein, as well as highly sensitive biochemical nano-sensors [10,11,[16][17][18][19]. Metallic nanoparticles have been studied extensively in combination with optical microscopy [1,[20][21][22] and considerable effort has been spent on detection and visualization of individual metallic nanoparticles with diameters down to 1.4 nm [23][24][25][26][27]. In this context, confocal interference scattering microscopy (CISM) in combination with tightly focused fundamental as well as higher order laser beams has been used for imaging elastic scattering pattern of individual, spatially isolated metallic nanoparticles [28]. ...
We study spatially isolated, individual gold nanorods placed at a planar interface between two dielectric media using confocal interference scattering microscopy in combination with higher order laser modes. Approaching refractive index matching conditions, we observe that the elastic scattering patterns of individual nanorods exhibit an exponential increase of both the scattering intensity and the signal-to-background ratio. In case refractive index matching conditions are fullfilled, the data acquisition rates are maximized and suitable for in-vivo biological measurements. In all cases, the characteristic two-lobe shape of the scattering patterns of single nanorods remains unchanged while the sign of the image contrast is a direct consequence of the refractive index variation occurring at the interface.
... Therefore, the signal difference between two halves of the photodetector is a direct measure of the particle's coordinates. The latter can be calculated as [39] ...
... For other particle positions, the detector signal is nonzero. For more details, the interested reader is referred to [39]. ...
... The back aperture of the collimating objective is imaged by a lens onto a quadrant detector (SPOT 9DMI, UDT Sensors, Hawthorne, CA). In order to extend the detection capabilities of our sensor to a broader range of particle sizes, we implemented a feedback loop that attenuates the laser power when the backscattered light from a larger particle exceeds a given threshold [39]. In our experiment, however, the feedback is deactivated in order to ease the analysis and discussion. ...
We have developed two different optical techniques for the detection of nanoscale particles. One of the methods is based on measuring the optical gradient force exerted on a nanoparticle as it passes through a confined optical field, and the other method uses a background-free interferometric scheme to detect the scattered field amplitude from a laser-irradiated particle. In both cases, the measured signal depends on the third power of the particle size (R<sup>3</sup>) as opposed to the R<sup>6</sup> dependence inherent to traditional scattering-based detection methods. The weaker size dependence in our schemes leads to a better signal-to-noise ratio (SNR) for small particles. Similar to mass spectrometry, the first detection method influences the trajectory of a particle as it passes through a tightly focused laser beam. On the other hand, the second detection method combines an interferometer with a split detector that yields no signal in the absence of a particle. For both systems, we demonstrate real-time (1 ms) detection of single nanoparticles in a microfluidic system and discuss the limits of each detection approach
... Optical traps have been also used in the process of measuring various parameters. These include particle size by Mie resonances [13], and by their ability to be trapped [59], the medium's viscosity [60], a waveguides' evanescent field [61] and various biological properties discussed below. ...
The trapping of microparticles by optical methods using a focussed laser beam, known as optical tweezers, has evolved rapidly in the last thirty years. However this process is limited to the trapping of a small number of particles. Evanescent wave trapping allows simultaneous trapping of many particles due to the long length over which a strong intensity gradient is present. A channel waveguide used to produce such an evanescent wave can be photolithographically defined on a flat substrate and thus can be integrated with other micron scale processes. This therefore has potential applications in the lab-on-a-chip field that is currently proving so successful, particularly in biochemical areas where evanescent wave manipulation is ideal for its properties of being cheap, robust, contaminant-free and designed for use with aqueous solutions.
This thesis describes both a theoretical and experimental study into the optical trapping and propulsion of gold nanoparticles and latex microparticles above caesium ion-exchanged waveguides. Gold particles of radii varying from 50nm to 250nm and latex particles varying from 1.5μm to 7.5μm were propelled above a waveguide in an aqueous medium. The evanescent field of the channel waveguide was used to both optically trap and propel these particles and speeds of up to 500μm/s were achieved, a full order of magnitude faster than has previously been reported.
The optical forces on the particles were derived and used to predict the trapping ability and the speed of particles and physical, electric, and thermophoretic forces, that also affect the particles were described. In addition, modelling allowed the theoretical optimisation of the waveguides for this process.
Using a counter-propagating wave it is demonstrated that it is possible to both render particles stationary and position them at any point along the waveguide. In addition, devices were fabricated that allow particles to be automatically sorted down either branch of a Y-junction waveguide. These results demonstrate, that evanescent wave based, integrated optical devices for trapping are feasible and it is anticipated that this will lead to devices for real-life applications being realised.
The optical gradient force patterns in focal region of radially polarized beams is investigated. The pure phase plate consists of three concentric zones: center circle zone, inner annular zone and outer annular zone. And the phase variance of annular zone is adjustable. The tunable phase plate can be implemented through liquid crystal spatial light modulator, which can conveniently alter the phase shift and polarization rotation angle of each section of the incident laser beam in the control of computer. Numerical simulations show that the evolution of the focal shape is evident by changing polarization rotation angle of each section portion of the vector beam. Consequently, the optical gradient force patterns will change and some interesting optical trap patterns may occur, which shows that radially polarized beams can be used to construct controllable optical tweezers.
The focusing properties of phase and amplitude modulated radially polarized vortex beams in a 4pi focusing system are theoretically investigated near the focal plane by using Richards-Wolf vectorial diffraction method. The amplitude modulation of vortex beams can be adjusted by changing the start integration value. The phase modulation of vortex beams can be realized by adding liquid crystal variable retarder with the phase delay angle δ. The simulated results show that multiple spherical spots can be obtained near the focus of the 4pi focusing system with the decrease of amplitude. The phase delay angle δ of the input beams can generate extruding effect for the electrical field distribution near the focus of the 4pi focusing system. Some special intensity distributions can be obtained by changing topological charge m and phase delay angle δ. Optical chain can be generated in the case of m = 1. Dark channel can be obtained in the case of m = 2. These special focusing beams can also transform with phase modulation. With the increase of phase δ, the multiple spherical spots at m = 0 change slowly into an optical chain, and finally become a dark channel. In contrast, the optical chain at m = 1 changes slowly into multiple spherical spots; and the dark channel at m = 2 changes into the superposition of optical spherical spots and the optical chain. These special focusing beams have potential applications in optical trapping and micro-manipulation.
The focusing property of concentric three-ring non-uniform mixing polarization vector beams through a high numerical aperture is theoretically investigated near the focal plane by using Richards-Wolf vectorial diffraction method. This kind of vector beams can be obtained by letting a concentric three-ring local linearly polarized cylindrical vector beam passes through a phase retarder with a phase angle δ. The three ring local linearly polarized cylindrical vector beam consists of the outer circle whose polarization is along the radially inward, the inner circle portion whose polarization is along the radially outward and the centray circle whose polarization direction has an angle φ 2 with radial direction. The results of the numerical simulation show that the focusing intensity distribution of the non-uniform mixing polarization vector beams is closely related with parameters φ 2 and δ. It is shown that the 3D dark optical chain along optical axis can be obtained near the focus by modulating the value of φ 2 and δ, which has potential application in the optical micro-manipulation.
As far as we know, this is thefirst time optical communication between signaling and receiving
nanoparticles has been analyzed and simulated for tumor smart-targeting. The original aim was to
achieve smart communication between the receiving nanoparticles far from the tumor and the signaling
nanoparticles placed near the tumor, which can be used to lead to effective detection and treatment of
cancerous tumors. The latter case can contribute to effective targeting by signaling nanoparticles and
can aid in tuning drug delivery by the receiving agents. In this method, the gathering of nanoparticles at
the tumor site is realized by the optical force that contributes to the interaction between the opticalfield
and the nanoparticles around the tumor in such a way that enhances the population of nanoparticles
towards the tumor. Furthermore, the manipulation of the optical force is controlled by the engineering
of plasmon hybridization of core–shell nanoparticles. Unlike recent smart-targeting which is based on a
biological cascade; in this work the communication between the agents is done optically, so the
construction of fake sites in the normal tissues will be avoided, together with any harmful effects from them.
We implement phase-sensitive optical detection and characterization of nano-particles.The elimination of the phase contribution to the detected signal helps improve the measured particle size accuracy and resolution, compared to standard interferometric techniques.
We introduce a new nanoparticle detection scheme which combines phase-sensitive heterodyne interferometry and background-free dark-field detection to enable us characterize in real-time single human viruses and bacteriophage down to ∼24 nm in radius.
