| 1. |
- Alemán Hérnandez, Felipe Ademir, et al.
(author)
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Frequency comb enhanced Brillouin microscopy
- 2020
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In: Optics Express. - : Optica Publishing Group. - 1094-4087. ; 28:20, s. 29540-29552
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Journal article (peer-reviewed)abstract
- Brillouin light scattering (BLS) microscopy is a well established and powerful technique to study acoustic and magnetic excitations in the frequency domain with sub-micron spatial resolution. Many other spectroscopic techniques have benefited from the introduction of femtosecond laser sources to optically pump and stimulate the sample under investigation. In BLS microscopy, the use of femtosecond lasers as the excitation source introduces several challenges, primarily since the measured frequency shift is small and the signal levels are weak due to the low duty cycle of typical femtosecond lasers. Here we present a method to evade these challenges. A strong enhancement of the weak scattering amplitude on selected modes is observed by pumping the sample with a high repetition rate frequency comb laser source. The laser beam can be focused to the diffraction limit, providing a micron pumping area. We can thus preserve the innate high frequency and spatial resolution of BLS microscopy. Furthermore, we are able to induce a point-like source of mode-selected elementary excitations which propagate away from the pumping spot. We conclude that we have demonstrated frequency comb pumped BLS microscopy as an attractive tool for studies of ultrafast induced laser dynamics directly in the frequency domain. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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| 2. |
- Aquila, Andrew, et al.
(author)
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Time-resolved protein nanocrystallography using an X-ray free-electron laser
- 2012
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In: Optics Express. - 1094-4087. ; 20:3, s. 2706-2716
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Journal article (peer-reviewed)abstract
- We demonstrate the use of an X-ray free electron laser synchronized with an optical pump laser to obtain X-ray diffraction snapshots from the photoactivated states of large membrane protein complexes in the form of nanocrystals flowing in a liquid jet. Light-induced changes of Photosystem I-Ferredoxin co-crystals were observed at time delays of 5 to 10 µs after excitation. The result correlates with the microsecond kinetics of electron transfer from Photosystem I to ferredoxin. The undocking process that follows the electron transfer leads to large rearrangements in the crystals that will terminally lead to the disintegration of the crystals. We describe the experimental setup and obtain the first time-resolved femtosecond serial X-ray crystallography results from an irreversible photo-chemical reaction at the Linac Coherent Light Source. This technique opens the door to time-resolved structural studies of reaction dynamics in biological systems.
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| 3. |
- Borejdo, J., et al.
(author)
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Fluorescence correlation spectroscopy in surface plasmon coupled emission microscope
- 2006
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In: Optics Express. - 1094-4087. ; 14:17, s. 7878-7888
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Journal article (peer-reviewed)abstract
- Study of dynamics of single molecules by Fluorescence Correlation Spectroscopy (PCS) requires that the rate of photon detection per molecule be high, that the background be low, and that there be a large change in fluorescent signal associated with change in a position of a molecule. PCS applied to microscopic Surface Plasmon Coupled Emission (SPCE) suggests a powerful method to meet those requirements. In this method, the observational volume is made shallow by placing a sample on a thin metal film and illuminating it with the laser beam at Surface Plasmon Resonance (SPR) angle through high numerical aperture objective. The illuminating light excites surface plasmons in the metal film that produce an evanescent wave on the aqueous side of the interface. The thickness of the detection volume is a product of evanescent wave penetration depth and distance-dependent fluorescence coupling to surface plasmons. It is further reduced by a metal quenching of excited fluorophores at a close proximity (below 10 nm) to a surface. The fluorescent light is emitted through the metal film only at an SPCE angle. Objective collects emitted light, and a confocal aperture inserted in its conjugate image plane reduces lateral dimensions of the detection volume to a fraction of a micrometer. By using diffusion of fluorescent microspheres, we show that SPCE-FCS is an efficient method to measure molecular diffusion and that on gold surface the height of the detection volume is ∼35 nm.
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| 4. |
- Deschout, Hendrik, et al.
(author)
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Straightforward FRAP for quantitative diffusion measurements with a laser scanning microscope
- 2010
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In: Optics Express. - 1094-4087. ; 18:22, s. 22886-22905
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Journal article (peer-reviewed)abstract
- Confocal or multi-photon laser scanning microscopes are convenient tools to perform FRAP diffusion measurements. Despite its popularity, accurate FRAP remains often challenging since current methods are either limited to relatively large bleach regions or can be complicated for non-specialists. In order to bring reliable quantitative FRAP measurements to the broad community of laser scanning microscopy users, here we have revised FRAP theory and present a new pixel based FRAP method relying on the photo bleaching of rectangular regions of any size and aspect ratio. The method allows for fast and straightforward quantitative diffusion measurements due to a closed–form expression for the recovery process utilizing all available spatial and temporal data. After a detailed validation, its versatility is demonstrated by diffusion studies in heterogeneous biopolymer mixtures.
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| 5. |
- Engström, David, 1976, et al.
(author)
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Calibration of spatial light modulators suffering from spatially varying phase response
- 2013
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In: Optics Express. - 1094-4087 .- 1094-4087. ; 21:13, s. 16086-16103
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Journal article (peer-reviewed)abstract
- We present a method for converting the desired phase values of a hologram to the correct pixel addressing values of a spatial light modulator (SLM), taking into account detailed spatial variations in the phase response of the SLM. In addition to thickness variations in the liquid crystal layerof the SLM, we also show that these variations in phase response can be caused by a non-uniform electric drive scheme in the SLM or by local heating caused by the incident laser beam. We demonstrate that the use of a global look-up table (LUT), even in combination with a spatially varyingscale factor, generally does not yield sufficiently accurate conversion forapplications requiring highly controllable output fields, such as holographicoptical trapping (HOT). We therefore propose a method where the pixeladdressing values are given by a three-dimensional polynomial, with twoof the variables being the (x;y)-positions of the pixels, and the third theirdesired phase values. The coefficients of the polynomial are determined bymeasuring the phase response in 8×8 sub-sections of the SLM surface; thedegree of the polynomial is optimized so that the polynomial expressionnearly replicates the measurement in the measurement points, while stillshowing a good interpolation behavior in between. The polynomial evaluationincreases the total computation time for hologram generation by onlya few percent. Compared to conventional phase conversion methods, for an SLM with varying phase response, we found that the proposed methodincreases the control of the trap intensities in HOT, and efficiently preventsthe appearance of strong unwanted 0th order diffraction that commonlyoccurs in SLM systems.
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| 6. |
- Engström, David, 1976, et al.
(author)
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Grid-free 3D multiple spot generation with an efficient single-plane FFT-based algorithm
- 2009
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In: Optics Express. - 1094-4087. ; 17:12, s. 9989-10000
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Journal article (peer-reviewed)abstract
- Algorithms based on the fast Fourier transform (FFT) for the design of spot-generating computer generated holograms (CGHs) typically only make use of a few sample positions in the propagated field. We have developed a new design method that much better utilizes the information-carrying capacity of the sampled propagated field. In this way design tasks which are difficult to accomplish with conventional FFT-based design methods, such as spot positioning at non-sample positions and/or spot positioning in 3D, are solved as easily as any standard design task using a conventional method. The new design method is based on a projection optimization, similar to that in the commonly used Gerchberg-Saxton algorithm, and the vastly improved design freedom comes at virtually no extra computational cost compared to the conventional design. Several different design tasks were demonstrated experimentally with a liquid crystal spatial light modulator, showing highly accurate creation of the desired field distributions.
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| 7. |
- Engström, David, 1976, et al.
(author)
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Unconventional structure-assisted optical manipulation of high-index nanowires in liquid crystals
- 2012
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In: Optics Express. - 1094-4087. ; 20:7, s. 7741-7748
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Journal article (peer-reviewed)abstract
- Stable optical trapping and manipulation of high-index particles in low-index host media is often impossible due to the dominance of scattering forces over gradient forces. Here we explore optical manipulation in liquid crystalline structured hosts and show that robust optical manipulation of high-index particles, such as GaN nanowires, is enabled by laser-induced distortions in long-range molecular alignment, via coupling of translational and rotational motions due to helicoidal molecular arrangement, or due to elastic repulsive interactions with confining substrates. Anisotropy of the viscoelastic liquid crystal medium and particle shape give rise to a number of robust unconventional trapping capabilities, which we use to characterize defect structures and study rheological properties of various thermotropic liquid crystals.
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| 8. |
- Eriksson, Emma, 1980, et al.
(author)
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The effect of external forces on discrete motion within holographic optical tweezers
- 2007
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In: Optics Express. - 1094-4087. ; 15:26, s. 18268-18274
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Journal article (peer-reviewed)abstract
- Holographic optical tweezers is a widely used technique to manipulate the individual positions of optically trapped micron-sized particles in a sample. The trap positions are changed by updating the holographic image displayed on a spatial light modulator. The updating process takes a finite time, resulting in a temporary decrease of the intensity, and thus the stiffness, of the optical trap. We have investigated this change in trap stiffness during the updating process by studying the motion of an optically trapped particle in a fluid flow. We found a highly nonlinear behavior of the change in trap stiffness vs. changes in step size. For step sizes up to approximately 300 nm the trap stiffness is decreasing. Above 300 nm the change in trap stiffness remains constant for all step sizes up to one particle radius. This information is crucial for optical force measurements using holographic optical tweezers.
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| 9. |
- Galinskiy, Ivan, et al.
(author)
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Measurement of particle motion in optical tweezers embedded in a Sagnac interferometer
- 2015
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In: Optics Express. - 1094-4087. ; 23, s. 27071-27084
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Journal article (peer-reviewed)abstract
- We have constructed a counterpropagating optical tweezers setup embedded in a Sagnac interferometer in order to increase the sensitivity of position tracking for particles in the geometrical optics regime. Enhanced position determination using a Sagnac interferometer has previously been described theoretically by Taylor et al. [Journal of Optics 13, 044014 (2011)] for Rayleigh-regime particles trapped in an antinode of a standing wave. We have extended their theory to a case of arbitrarily-sized particles trapped with orthogonally-polarized counter-propagating beams. The working distance of the setup was sufficiently long to optically induce particle oscillations orthogonally to the axis of the tweezers with an auxiliary laser beam. Using these oscillations as a reference, we have experimentally shown that Sagnac-enhanced back focal plane interferometry is capable of providing an improvement of more than 5 times in the signal-to-background ratio, corresponding to a more than 30-fold improvement of the signal-to-noise ratio. The experimental results obtained are consistent with our theoretical predictions. In the experimental setup, we used a method of optical levitator-assisted liquid droplet delivery in air based on commercial inkjet technology, with a novel method to precisely control the size of droplets.
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| 10. |
- Guldbrand, Stina, 1970, et al.
(author)
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Two-photon fluorescence correlation microscopy combined with measurements of point spread function; investigations made in human skin
- 2010
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In: Optics Express. - 1094-4087. ; 18:15, s. 15289-15302
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Journal article (peer-reviewed)abstract
- Two-photon excitation fluorescence correlation spectroscopy (TPFCS) has been applied in connection to measurements of the point spread function (PSF) for quantitative analysis of sulphorhodamine B (SRB) in excised human skin. The PSF was measured using subresolution fluorescent beads embedded in the skin specimen. The PSF, measured as full width at half maximum (FWHM) was found to be 0.41 ± 0.05 μm in the lateral direction, and 1.2 ± 0.4 μm in the axial direction. The molecular diffusion of SRB inside the skin ranged between 0.5 and 15.0 × 10 −8 cm2/s. The diffusion coefficient is not dependent on depths down to 40 μm. The fluorophores were found to accumulate on the upper layers of the skin. This work is the first TPFCS study in human skin. The results show that TPFCS can be used for quantitative analyses of fluorescent compounds in human skin.
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