Showing papers by "Min Gu published in 2000"
TL;DR: In this article, the authors show that for turbid tissue medium where Mie scattering is dominant, multiple scattering not only reduces the illumination power in the forward direction but also exhibits an anisotropic distribution of scattered photons.
Abstract: We show, both theoretically and experimentally, that for a turbid tissue medium where Mie scattering is dominant, multiple scattering not only reduces the illumination power in the forward direction but also exhibits an anisotropic distribution of scattered photons Thus, a signal level under two-photon excitation drops much faster than that under single-photon excitation although image resolution is much higher in the former case As a result, the penetration depth under two-photon excitation is limited by the strength of two-photon fluorescence and is not necessarily larger than that under single-photon excitation
60 citations
TL;DR: In this paper, the authors reported that the transverse trapping efficiency for a metallic particle can be enhanced by use of a laser beam obstructed by a circular opaque disk, and that the enhancement factor for a p- or s-polarized trapping beam is at least 1.7 or 2.5, respectively.
Abstract: We report that the transverse trapping efficiency for a metallic particle can be enhanced by use of a laser beam obstructed by a circular opaque disk. In the case of gold particles, the enhancement factor for a p- or s-polarized trapping beam is at least 1.7 or 2.5, respectively. The dependence of the transverse trapping efficiency for gold particles (diameter=2 μm) on the size of the obstruction is measured and agrees with the theoretical prediction based on the ray-optics model.
35 citations
TL;DR: In this article, effective point spread functions for fluorescence microscopic imaging are introduced to investigate the effects of scattering particle size and optical gating on image resolution under single-photon (1p) and two photon (2p) excitation.
Abstract: In this article, effective point spread functions for fluorescence microscopic imaging are introduced to investigate the effects of scattering particle size and optical gating on image resolution under single-photon (1p) and two-photon (2p) excitation. The dependence of image resolution on these effects shows a deeper penetration depth under 2p excitation due to the use of a longer illumination wavelength and the nonlinear dependence of the fluorescence on excitation intensity. The fundamental difference between 1p and 2p fluorescence imaging is that 1p fluorescence imaging mainly depends on the fluorescence light excited by scattered photons, in which case the penetration depth is limited by the degradation in image resolution. However, 2p fluorescence imaging is determined by the fluorescence light excited by ballistic photons, in which case the penetration depth is limited by the loss in signal strength. The results also reveal that the pinhole gating method is efficient in 1p fluorescence imaging, but exhibits a limited influence on 2p fluorescence imaging. It is also demonstrated that in 2p fluorescence imaging, a high numerical aperture objective gives a strong signal while retains an image of high resolution if the turbid medium is not so thick.
28 citations
TL;DR: It is demonstrated that two-photon excitation can provide significant improvement in penetration depth through turbid media, due to reduced scattering experienced by the excitation beam.
Abstract: The effect of multiple scattering in a turbid medium on single-photon and two-photon fluorescence microscopy is experimentally investigated for different scattering characteristics including scattering anisotrophy and optical thickness of a turbid medium. It is demonstrated that two-photon excitation can provide significant improvement in penetration depth through turbid media, due to reduced scattering experienced by the excitation beam. It is also shown that the limiting factor in obtaining high-quality images under single-photon excitation is the fast degradation of image resolution caused by multiple scattering, while for two-photon excitation it is limited by the degradation of image contrast due to the reduction in fluorescence strength.
23 citations
TL;DR: Three-dimensional fluorescence spatial distributions under single-photon and two- photon excitation within a turbid medium are studied with Monte Carlo simulation and show that the optical sectioning property of two-Photon fluorescence microscopy is degraded in thick turbid media or when the numerical aperture of an objective becomes low.
Abstract: Three-dimensional fluorescence spatial distributions under single-photon and two-photon excitation within a turbid medium are studied with Monte Carlo simulation It is demonstrated that two-photon excitation has an advantage of producing much less fluorescence light outside the focal region compared with single-photon excitation With the increase of the concentration of scattering particles in a turbid medium, the position of the maximum fluorescence intensity point shifts from the geometric focal region toward the medium surface Further studies show that the optical sectioning property of two-photon fluorescence microscopy is degraded in thick turbid media or when the numerical aperture of an objective becomes low
19 citations
TL;DR: In this article, the effects of handling and fixation processes on the two-photon fluorescence spectroscopy of endogenous fluorophors in mouse skeletal muscle were investigated and the results indicated that formalin was the least disruptive fixative, having only a weak effect on the autofluorescence spectra of muscle tissue.
Abstract: We investigated the effects of handling and fixation processes on the two-photon fluorescence spectroscopy of endogenous fluorophors in mouse skeletal muscle. The skeletal muscle was handled in one of two ways: either sectioned without storage or sectioned following storage in a freezer. The two-photon fluorescence spectra measured for different storage or fixation periods show a differential among those samples that were stored in water or were fixed either in formalin or methanol. The spectroscopic results indicate that formalin was the least disruptive fixative, having only a weak effect on the two-photon fluorescence spectroscopy of muscle tissue, whereas methanol had a significant influence on one of the autofluorescence peaks. The two handling processes yielded similar spectral information, indicating no different effects between them.
17 citations
01 Jan 2000
TL;DR: The spectroscopic results indicate that formalin was the least disruptive fixative, having only a weak effect on the two-photon fluorescence spectroscopy of muscle tissue, whereas methanol had a significant influence on one of the autofluorescence peaks.
Abstract: We investigated the effects of handling and fixation processes on the two-photon fluorescence spectroscopy of endogenous fluorophors in mouse skeletal muscle. The skeletal muscle was handled in one of two ways: either sectioned without storage or sectioned following storage in a freezer. The two-photon fluorescence spectra measured for different storage or fixation periods show a differential among those samples that were stored in water or were fixed either in formalin or methanol. The spectroscopic results indicate that formalin was the least disruptive fixative, having only a weak effect on the two-photon fluorescence spectroscopy of muscle tissue, whereas methanol had a significant influence on one of the autofluorescence peaks. The two handling processes yielded similar spectral information, indicating no different effects between them.
16 citations
TL;DR: It is shown both theoretically and experimentally that two-photon fluorescence imaging experiences less spherical aberration and can thus propagate to a deeper depth within a thick medium.
Abstract: We compare the effects of spherical aberration on the penetration depth of single-photon and two-photon excitation for instances in which the aberration is caused by the refractive-index mismatch when a beam is focused through an interface. It is shown both theoretically and experimentally that two-photon fluorescence imaging experiences less spherical aberration and can thus propagate to a deeper depth within a thick medium.
16 citations
Patent•
16 Feb 2000TL;DR: In this article, a method of erasable/rewritable optical data storage is provided in which a laser beam from a laser operated in either a pulsed or continuous wave mode is focussed by an optical system onto a photorefractive polymeric material (32) to cause two photon excitation of the material to record data which may subsequently be erased by illuminating the material with radiation having a wavelength in the ultraviolet or visible spectrum to erase the recorded data.
Abstract: A method of erasable/rewritable optical data storage is provided in which a laser beam from a laser (10) operated in either a pulsed or continuous wave mode is focussed by an optical system (11-18) onto a photorefractive polymeric material (32) to cause two photon excitation of the material to record data which may subsequently be erased by illuminating the material with radiation having a wavelength in the ultraviolet or visible spectrum to erase the recorded data. New photorefractive polymeric materials having a relatively narrow absorption band for use in the method are also disclosed.
14 citations
TL;DR: In this paper, it was found that the degree of polarization of scattered evanescent waves decreases with the size of gold particles, which is contrary to that previously observed for dielectric particles.
Abstract: Depolarization of evanescent waves scattered by laser-trapped gold particles of 0.1, 0.5 and 2 μm in diameter is experimentally characterized in order to reveal its dependence on the size of particles. It is found that the degree of polarization of scattered evanescent waves decreases with the size of gold particles, which is contrary to that previously observed for dielectric particles. This feature becomes advantageous in particle-trapped near-field microscopy since less depolarized photons carry more information of a sample. With the help of polarization gating, this property is demonstrated in images of the evanescent wave interference pattern as well as the surface of a glass prism.
12 citations
TL;DR: In this paper, a comparison of two-photon and singlephoton fluorescence images showed that multiple scattering leads to a dominant limiting factor of the signal-to-noise ratio in the former case, while it results in a dominant limit of resolution in the latter case.
Abstract: Fluorescent microspheres of diameter 10μm embedded in a turbid medium consisting of polystyrene beads suspended in water are imaged under two-photon and single-photon excitation. A comparison of two-photon and single-photon fluorescence images shows that multiple scattering leads to a dominant limiting factor of the signal-to-noise ratio in the former case, while it results in a dominant limiting factor of resolution in the latter case. These results are qualitatively consistent with the predication by the Monte-Carlo simulation based on Mie scattering theory.
TL;DR: This feature issue comprises a collection of papers from groups in the forefront of the research of high-numerical-aperture optical systems and their applications, and contributions include surface-plasmon microscopy, interference microscope, optical coherence tomography, polarized-light microscope, and the optical theories of these techniques.
Abstract: This feature issue comprises a collection of papers from groups in the forefront of the research of high-numerical-aperture optical systems and their applications. We have assembled 13 papers from a wide subject area within optical microscopy. Contributions include surface-plasmon microscopy, interference microscopy, optical coherence tomography, polarized-light microscopy, and the optical theories of these techniques.
01 Jan 2000
07 May 2000
TL;DR: In this paper, the authors reported on the 3D optical bit data storage in photorefractive polymers using two-photon excitation, which allows one to develop a low-cost, erasable, and rewritable 3D storage method.
Abstract: Summary form only given. Recently, we have reported on the three-dimensional (3D) optical bit data storage in photorefractive polymers using two-photon excitation, which allows one to develop a low-cost, erasable, and rewritable 3D optical storage method. The photorefractive polymer we used consists of 2,5-dimethyl-4-(p-nitrophenylazo)anisole, 2,4,7-trinitro-9-fluorenone, 9-ethylcarbazole and poly(N-vinylcarbazole).
27 Apr 2000
TL;DR: The result imply that two-photon fluorescence spectroscopy can be a potential technique for identification of healthy and malignant biological tissues.
Abstract: Two-photon fluorescence spectroscopy has been performed on rat skeletal muscles to investigate the effect of fixation processes on the micro-environments of the endogenous fluorophors in rat skeletal muscles. The two-photon fluorescence spectra measured for different fixation periods show a differential among those samples that were fixed in water, formalin and methanol, respectively. The results imply that two-photon fluorescence spectroscopy can be a potential technique for identification of healthy and malignant biological tissues.