| 1. |
- Andersson, Magnus, et al.
(author)
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Optimizing active and passive calibration of optical tweezers
- 2011
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In: Journal of optics. - : IOP Publishing. - 0150-536X .- 2040-8978 .- 2040-8986. ; 13:4
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Journal article (peer-reviewed)abstract
- To obtain quantitative information from optical trapping experiments it is essential to perform a precise force calibration. Therefore, sources of noise should be pinpointed and eliminated. Fourier analysis is routinely used to calibrate optical trapping assays because it is excellent for pinpointing high frequency noise. In addition, Allan variance analysis is particularly useful for quantifying low frequency noise and for predicting the optimal measurement time. We show how to use Allan variance in combination with Fourier analysis for optimal calibration and noise reduction in optical trapping assays. The methods are applied to passive assays, utilizing the thermal motion of a trapped particle, and to active assays where the bead is harmonically driven. The active method must be applied in assays where, for example, the viscoelastic properties of the medium or the size or shape of the trapped object are unknown. For measurement times shorter than the optimal calibration time the noise is larger in active than in the passive assays. For times equal to or longer than the optimal measurement time, though, the noise on passive and active assays is identical. As an example, we show how to quantify the influence on measurement noise of bead size and chamber geometry in active and passive assays.
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| 2. |
- Lindgren, Georg
(author)
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A detailed statistical representation of the local structure of optical vortices in random wavefields
- 2012
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In: Journal of Optics. - : IOP Publishing. - 2040-8986 .- 2040-8978. ; 14:3
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Journal article (peer-reviewed)abstract
- The statistical properties near phase singularities in a complex wave field are here studied by means of the conditional distributions of the real and imaginary Gaussian components, given a common zero crossing point. The exact distribution is expressed as a Slepian model, where a regression term provides the main structure, with parameters given by the gradients of the Gaussian components at the singularity, and Gaussian non-stationary residuals that provide local variability. This technique differs from the linearisation (Taylor expansion) technique commonly used. The empirically and theoretically verified elliptic eccentricity of the intensity contours in the vortex core is a property of the regression term, but with different normalization compared to the classical theory. The residual term models the statistical variability around these ellipses. The radii of the circular contours of the current magnitude are similarly modified by the new regression expansion, and also here the random deviations are modelled by the residual field.
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| 3. |
- Shomali, Ramin, et al.
(author)
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Measurement of the atmospheric primary aberrations by a 4-aperture differential image motion monitor
- 2011
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In: Journal of Optics. - : IOP Publishing. - 2040-8986 .- 2040-8978. ; 13:5
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Journal article (peer-reviewed)abstract
- The present paper investigates and discusses the ability of the Hartmann test with a 4-aperture differential image motion monitor (DIMM) to measure the atmospheric primary aberrations which, in turn, can be used for the calculation of the atmospheric coherence time. Through performing numerical simulations, we show that the 4-aperture DIMM is able to measure the defocus and astigmatism terms correctly whereas its results are not reliable for the coma. The most important limitations in the measurement of the primary aberrations by the 4-aperture DIMM are the centroid displacements of the spots which are caused by the higher order aberrations. This effect is negligible in the calculation of the defocus and astigmatisms, whereas it cannot be ignored in the calculation of the coma.
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| 4. |
- Dalarsson, Mariana, et al.
(author)
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Lossy gradient index transmission optics with arbitrary periodic permittivity and permeability and constant impedance throughout the structure
- 2012
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In: Journal of Optics. - : IOP Publishing. - 2040-8978 .- 2040-8986. ; 14:6, s. 065102-
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Journal article (peer-reviewed)abstract
- A remarkably simple exact analytical solution is obtained for the electromagnetic field distribution across infinite metamaterial-containing composites with any arbitrary periodic variation of complex effective permittivity and permeability for the case of constant impedance across the structure. Arbitrary temporal dispersion and losses are allowed and the model is generally applicable to different inhomogeneous and anisotropic media simultaneously containing positive and negative refractive index constituents, as long as the effective medium approximation remains valid. The analytical solution is validated by a dispersive numerical model of lossy metamaterials that uses a transmission line matrix method based on Z-transforms, where a close agreement between the analytic and numerical results is obtained.
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| 5. |
- Song, Yi, et al.
(author)
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Efficient coupling between dielectric and hybrid plasmonic waveguides by multimode interference power splitter
- 2011
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In: Journal of optics. - : IOP Publishing. - 0150-536X .- 2040-8978 .- 2040-8986. ; 13:7, s. 075002-
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Journal article (peer-reviewed)abstract
- Multimode interference power splitters based on hybrid plasmonic waveguides are investigated theoretically. Balanced power splitting is achieved in designed 1 × 3 and 1 × 2 power splitters between a silicon-on-insulator waveguide and several hybrid plasmonic waveguides, with total transmission efficiencies at 76.1% and 78.3% at the wavelength of 1550 nm, respectively.
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| 6. |
- Song, Yi, et al.
(author)
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Subwavelength hybrid plasmonic nanodisk with high Q factor and Purcell factor
- 2011
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In: Journal of optics. - : IOP Publishing. - 0150-536X .- 2040-8978 .- 2040-8986. ; 13:7, s. 075001-
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Journal article (peer-reviewed)abstract
- Optical nanodisk resonators based on a hybrid plasmonic waveguiding geometry areinvestigated theoretically in the wavelength range of 1200–2000 nm with their radii varyingfrom 320 to 1000 nm. Due to the fact that a hybrid plasmonic structure can efficiently confinelight in a deep-subwavelength mode volume with a relatively low propagation loss, thenanodisks designed exhibit simultaneously a high Q factor and a high Purcell factor. For ahybrid plasmonic nanodisk with the radius of 1000 nm, a Q factor of 819 and a Purcell factor of1827 are achieved at the telecommunication wavelength of 1558 nm.
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| 7. |
- Song, Yi, et al.
(author)
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Subwavelength hybrid plasmonic nanodisk with highQfactor and Purcell factor
- 2011
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In: Journal of Optics. - : Institute of Physics Publishing (IOPP). - 2040-8978 .- 2040-8986. ; 13:7
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Journal article (peer-reviewed)abstract
- Optical nanodisk resonators based on a hybrid plasmonic waveguiding geometry are investigated theoretically in the wavelength range of 1200–2000 nm with their radii varying from 320 to 1000 nm. Due to the fact that a hybrid plasmonic structure can efficiently confine light in a deep-subwavelength mode volume with a relatively low propagation loss, the nanodisks designed exhibit simultaneously a high Q factor and a high Purcell factor. For a hybrid plasmonic nanodisk with the radius of 1000 nm, a Q factor of 819 and a Purcell factor of 1827 are achieved at the telecommunication wavelength of 1558 nm.
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| 8. |
- Tidström, Jonas, et al.
(author)
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Photonic crystal cavity embedded in electromagnetically induced transparency media
- 2010
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In: Journal of Optics. - : IOP Publishing. - 2040-8978 .- 2040-8986. ; 12:3, s. 035105-
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Journal article (peer-reviewed)abstract
- Photonic crystal cavities are known for their high quality factor and small modal volume. Electromagnetically induced transparency (EIT) is known for its dramatic dispersion. We study the fundamental cavity mode of a two-dimensional photonic crystal embedded in an EIT medium. Comparison of finite-difference time-domain simulations with an analytical intracavity-EIT model yields similar results: the modal density of the photonic crystal cavity increases and the cavity resonance is pulled from the empty cavity resonance to the two-photon resonance of the EIT medium. As a consequence, the quality factor may be enhanced by more than two orders of magnitude. For a miniature sized 2.5 lambda x 2.5 lambda photonic crystal cavity based on a 5 x 5 high index rod structure with a missing centre rod, the quality factor can be increased from 200 to 40 000.
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| 9. |
- Trivedi, Rahul P., et al.
(author)
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Optical manipulation of colloids and defect structures in anisotropic liquid crystal fluids
- 2010
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In: Journal of Optics. - : IOP Publishing. - 2040-8978 .- 2040-8986. ; 13:4
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Journal article (peer-reviewed)abstract
- Optical trapping in anisotropic fluids such as liquid crystals shows inherently different behavior compared to that in isotropic media. Anisotropic optical and visco-elastic properties of these materials result in direction-sensitive and polarization-dependent interaction of the focused laser beam with colloidal inclusions, defects and structures of long-range molecular order, providing new means of non-contact optical control. Optical trapping properties are further enriched by laser-induced realignment of the optical axis that can be observed in these liquid crystalline materials at relatively low trapping laser powers. Optical manipulation of particles and defects in these anisotropic fluids is of immense importance for their fundamental study and from the standpoint of technological applications such as light-directed colloidal self-assembly and generation of tunable photonic architectures in liquid crystals. We review the basic physical mechanisms related to optical trapping in anisotropic liquid crystalline fluids and demonstrate how it can be employed in quantitative studies of colloidal interactions and both topological and mechanical properties of defects.
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| 10. |
- Yan, Min, et al.
(author)
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Lithography-free broadband visible light absorber based on a mono-layer of gold nanoparticles
- 2014
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In: Journal of Optics. - : IOP Publishing. - 2040-8978 .- 2040-8986. ; 16:2, s. 025002-
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Journal article (peer-reviewed)abstract
- We experimentally demonstrate a large area, optically opaque plasmonic absorber which can absorb 95% of visible light with an effective thickness of less than 150 nm. The absorber comprises, from top to bottom, a mono-layer of random gold nanoparticles, a dielectric spacer, and a bottom gold reflector. Reflectometry analyses show that its absorption is insensitive to the incidence's polarization or angle when the incident angle is less than 50 degrees. At a larger incident angle, reflection increases and absorption spectra differ for two polarizations. Numerical simulations based on a 3D finite-element method suggest that the high absorbance is due to collective efforts of dipolar particle resonances, most often strongly coupled and forming chain resonances, as well as coupling of light to the surface plasmon polariton, irrespective of the incidence's polarization, through the top-layer particles. Similar high absorptivity is also demonstrated with silver or aluminum as the bottom reflector. These highly efficient visible light absorbers can be potential candidates for a range of passive and active photonic applications, including solar-energy harvesting as well as producing artificial colors on a large scale.
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