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Yaoju Zhang
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ABSTRACT: The theoretical analyses in this paper show that a highly focused double-ring radially polarized Laguerre-Gaussian beam with a topological charge of 1 (R-LG(11)) can generate a small three-dimensional (3D) dark spot surrounded by an almost 100% uniform light shell in all directions. The cleanness and size of the 3D dark spot, the uniformity and strength of the light shell surrounding the dark spot, and the light efficiency all depend on the truncation parameter β of the R-LG(11) beam and the numerical aperture (NA) of the system. When β=1.6 and the NA is close to its utmost, an almost 100% uniform light shell surrounding the 3D dark spot can be achieved and the dark spot is very clean. If the NA is lowered but β is increased to 1.95, we can also achieve an almost 100% uniform light shell and light efficiency can reach 90%, but the disadvantage is that the center of the dark spot is not too clean. A not-too-clean 3D dark spot, if the light shell surrounding it is very uniform, is acceptable for many applications. Therefore, 3D dark spots surrounded by a high uniform light shell, generated by simply adjusting the truncation parameter of the R-LG(11) beam and the NA of the system, are useful for superresolution fluorescence microscopy, dark spot microscopy, and the dark spot trap.
Applied Optics 11/2010; 49(32):6217-23. · 1.41 Impact Factor
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ABSTRACT: The optical trapping forces acting on a metallic Rayleigh particle are calculated for the case where a double-ring-shaped radially polarized beam is applied. The influence of the off-focus distance and the off-axis distance of a trapping particle on the trapping force is investigated. Compared with the use of the conventional single-ring-shaped radially polarized beam, the longer axial trap distance and the larger radial trap stiffness are predicted using a double-ring-shaped radially polarized beam in an optical trap. These features are useful for improving the trapping ability of an optical trap system where a longer axial trap distance is needed.
Optics Letters 04/2010; 35(8):1281-3. · 3.40 Impact Factor
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ABSTRACT: An optical-trap method based on the illumination of a double-ring-shaped radially polarized beam (R-TEM11*) is proposed. The numerical results based on the vector diffraction theory show that a highly focused R-TEM11* beam not only can produce a bright spot but also can form an optical cage in the focal region by changing the truncation parameter β, defined as the ratio of the radius of the aperture to the waist of the beam. The radiation forces acting on Rayleigh particles are calculated by using the Rayleigh scattering theory. The bright spot generated by the R-TEM11* beam with a β value close to 2 can three-dimensionally trap a particle with a refractive index larger than that of the ambient. An optical cage or three-dimensional dark spot generated by the R-TEM11* beam with a β value close to 1.3 can three-dimensionally trap a particle with refractive index smaller than that of the ambient. Because the adjustment of the truncation parameter can be actualized by simply changing the radius of a circular aperture inserted in the front of the lens, only one optical-trap system in the present method can be used to three-dimensionally trap two types of particles with different refractive indices.
Phys. Rev. A. 02/2010; 81(2).
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ABSTRACT: A simple expression for the magnetic filed of a highly focused radially polarized light is derived and the incorrect results for the time averaged Poynting vector and the trapping stability for a gold particle presented in the paper "Trapping metallic Rayleigh particles with radial polarization" by Zhan (Opt. Express 12, 3377-3382 (2004)) are corrected.
Optics Express 11/2009; 17(24):22235-9. · 3.59 Impact Factor
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ABSTRACT: Distributions of the optical field in a solid immersion lens recording system are calculated for higher-order radially polarized modes of the incidence. Results show that two higher-order radially polarized modes of R-TEM(11) (* )and R-TEM(21) * are useful to near-field optical recording, but further higher-order modes such as R-TEM(31) (* ), R-TEM(41) (* ), and R-TEM(51) (* ) are not useful due to the strong side-lobe intensity. Compared with R-TEM(01) (* ) beam focusing, the full width at half-maximum of the recording spot is decreased markedly and the focal depth is increased substantially by using R-TEM(11) (* )beam focusing. The effect of the beam width of the R-TEM(11) (* ) mode is also discussed.
Optics Express 04/2009; 17(5):3698-706. · 3.59 Impact Factor
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ABSTRACT: We present a simple model for performing high-density all-optical magnetic recording using a solid immersion lens. The magnetization distribution in the magneto-optic film placed in the vicinity of the solid immersion lens is studied using the vector diffraction theory and the inverse Faraday effect. Simulation results show that although the transverse components of magnetization are nonzero, the axial component dominates. The magnetization direction of the axial component can be reversed by changing the helicity of the incident circularly polarized laser pulses. For a lower-numerical-aperture (NA) system, a larger and circular magnetization zone is obtained and the deviation angle of magnetization direction departing from the optical axis is smaller in the effective magnetization zone, which is useful to vertical magnetic recording. For a higher-NA system, a smaller magnetization zone is generated, but the three-dimensional magnetization distribution has to be considered.
Journal of the Optical Society of America B 12/2008; 26(1):176-182. · 2.18 Impact Factor
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Yaoju Zhang
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ABSTRACT: A simple and rigorous analytical expression of the propagating field behind an axicon illuminated by an azimuthally polarized beam has been deduced by use of the vector interference theory. This analytical expression can easily be used to calculate accurately the propagation field distribution of azimuthally polarized beams throughout the whole space behind an axicon with any size base angle, not just restricted inside the geometric focal region as does the Fresnel diffraction integral. The numerical results show that the pattern of the beam produced by the azimuthally polarized Gaussian beam that passes through an axicon is a multiring, almost-equal-intensity, and propagation-invariant interference beam in the geometric focal region. The number of bright rings increases with the propagation distance, reaching its maximum at half of the geometric focal length and then decreasing. The intensity of bright rings gradually decreases with the propagation distance in the geometric focal region. However, in the far-field (noninterference) region, only one single-ring pattern is produced and the dark spot size expands rapidly with propagation distance.
Applied Optics 11/2007; 46(29):7252-7. · 1.41 Impact Factor
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Yaoju Zhang
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ABSTRACT: Ignoring the effect of the small aperture, we deduce the optical field distribution of the so-called plano-convex solid immersion mirror with a small aperture on the apex (PC-SIM) by using the vector diffraction theory. The simulation results show that a PC-SIM, like a solid immersion lens (SIL), can achieve high resolution. Unlike the SIL, the PC-SIM can effectively reduce the spreading of the spot size with increasing distance from the interface. The size and intensity of the spot are related not only to the refractive index of the solid immersion medium but also to the structure parameter of the PC-SIM. The size of a spot smaller than a quarter wavelength can be obtained simply by optimizing the structure parameter of a PC-SIM but not by decreasing the size of the small aperture.
Journal of the Optical Society of America A 02/2007; 24(1):211-4. · 1.56 Impact Factor
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Yaoju Zhang
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ABSTRACT: A new solid immersion mirror called the planoellipsoidal (PE) solid immersion mirror (SIM) for the near-field optical storage is proposed and developed. The PE SIM has a small aperture on the apex of the ellipsoidal surface. The intensity distribution of the transmitted field is calculated by using the vector diffraction theory. Compared with a conventional solid immersion lens (SIL), the proposed PE SIM has the following features. A PE SIM replaces three optical elements of the collimator, objective, and SIL in a conventional SIL optical storage system, so that the optical system equipped with the PE SIM is not only simple in its assembly but is also effective in making an optical head unit. The PE SIM obtains light from a point light source and focuses it directly on the recording layer, which may be useful for a compact optical data storage system. The convex ellipsoidal surface of the PE SIM can reduce the risk of the SIM touching the surface of the recording medium. In addition, the spreading of the spot size with the increase of distance is very small in the PE SIM.
Applied Optics 01/2007; 45(34):8653-8. · 1.41 Impact Factor
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Yaoju Zhang
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ABSTRACT: Both the reflection inside a hemisphere solid immersion lens (SIL) and the reflection inside the gap between the SIL and the optical recording medium are considered. The near-field SIL imaging theory for high numerical aperture is developed by using the vector diffraction and thin-film optics. Numerical results show that the spot size, Strehl ratio, and sidelobe intensity have an oscillatory behavior with the change of thickness of the air gap, which results from the interference effect of the transmitted field. We find that for smaller spot size, the Strehl ratio is smaller but the sidelobe intensity is larger. A certain thickness of air gap is useful for optical storage, which is less than 63 nm for the system in the simulated examples.
Journal of the Optical Society of America A 10/2006; 23(9):2132-6. · 1.56 Impact Factor
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Yaoju Zhang
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ABSTRACT: Two types of novel solid immersion lens are designed and investigated theoretically using the vector diffraction theory. The advantages of these so-called high-performance supersphere solid immersion lenses (HPSILs) are that they can improve the Strehl ratio of the focused spot and increase the focal depth of near-field optical systems. Both the spot size and the sidelobe intensity are not increased, however, compared with those of the standard Weierstrass solid immersion lens. These HPSILs will be useful for near-field optical data storage and photolithography.
Applied Optics 08/2006; 45(19):4540-6. · 1.41 Impact Factor
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ABSTRACT: On the basis of the vectorial Rayleigh diffraction integrals and stationary-phase method, the analytic expression describing the vectorial field distribution of radially polarized Gaussian beams diffracted by an axicon is derived. The theoretical analysis and simulation calculation show that the radial component of the diffraction field is the propagation-invariant first-order Bessel beam when the radially polarized Gaussian beam illuminates the axicon. However, the longitudinal component possesses no such behavior because of its intrinsic r dependence, and its central intensity is the maximum. The longitudinal component is related to the open angle and index of the axicon, which has to be considered when the open angle and index are large. For a small open angle and index, the longitudinal component can be neglected, and the scalar approximation is valid.
Journal of the Optical Society of America A 12/2005; 22(11):2542-6. · 1.56 Impact Factor
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ABSTRACT: We propose a method for high-density all-optical magnetic recording. Our analyses, based on the vector diffraction theory, show that owing to the inverse Faraday effect, circularly polarized laser pulses focused by a high numerical aperture (NA) lens can induce a small magnetization domain. For an example, the FWHM of the effective magnetization domain is 0.4646λ when NA=0.85. The magnetization direction is basically perpendicular to the surface of the optic-magneto film within the effective magnetization domain and the switching direction of magnetization can be controlled by the helicity of the incident circularly polarized light. These characteristics are useful to next-generation high-density all-optical magnetic storage.
Physics Letters A.
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ABSTRACT: Based on the vectorial Rayleigh diffraction integral and the hard-edge aperture function expanded as the sum of finite-term complex Gaussian functions, an approximate analytical expression for the propagation equation of vectorial Gaussian beams diffracted at a circular aperture is derived and some special cases are discussed. By using the approximate analytical formula and diffraction integral formula, some numerical simulation comparisons are done, and some special cases are discussed. We find that a circular aperture can produce the focusing effect but the beam becomes the shape of ellipse in the Fresnel region. When the Fresnel number is equal to unity, the beam is circular and the focused spot reaches a minimum.
Optics & Laser Technology.
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ABSTRACT: The optical trapping characteristics of highly focused higher-order radially polarized beams (R-TEMp1*) acting on a Rayleigh particle are studied theoretically. Numerical results show that as the order p of beam increases and the numerical aperture NAo of the objective decreases, the axial trap distance increases but the trap depth and maximum restoring force decreases. In a limit of NAo = 1, three higher-order R-TEMp1* beams of p = 1, 2, 3, like the fundamental lowest-order radially polarized beam of p = 0, can three-dimensionally trap a particle to the focus but the axial trap stiffness decreases with the increase of p. When NAo = 0.95, the focus is still a stable trap point for the two beams of p = 0 and 1 but it becomes an unstable trap point for the two beams of p = 2 and 3. The trap stability is also discussed for higher-order radially polarized beam illumination.Research Highlights► The axial trap distance is increased by high-order radially polarized beams. ► Three-dimensional trap for a particle is related with the order of the beam. ► The trap stability and trap stiffness is also dependent of the order. ► The radially polarized beam of p = 1 order is very valid to trap Rayleigh particles.
Optics Communications 284(7):1734-1739. · 1.49 Impact Factor
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ABSTRACT: We suggest a simple and high efficient method for trapping particles in the evanescent field. In this method, a single plane wave is normally incident on the cylindrical surface of a cylindrical lens and then incident on the plane surface of the lens at an angle larger than the critical angle. Multiple reflections of light within the cylindrical lens create two evanescent waves with different directions in the transmitted field. Interference of two evanescent waves comes into being a standing wave which can stably trap particles close to the top of the cylindrical lens. Based on the Rayleigh approximation, we obtain analytical expressions of optical force acting on a Rayleigh particle placed in the vicinity of the lens. We find that the trap stiffness and trap depth is dependent on the radius of the cylindrical lens, wavelength and polarization of light, and incident angle at the lens–liquid interface.
Optics Communications.
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ABSTRACT: The focal field of the solid immersion lens (SIL) system with an annular filter is calculated by using vector method. Polarization of the transmitted field, size and shape of spot are analyzed. Numerical results show that, as the increase of the obstruction ratio of annular filter for low-NA (numerical aperture) SIL systems, the shape of spot is almost invariant and the size of spot decreases. But both the axial component of the transmitted field and the depth of focus increase with the obstruction ratio. For high-NA SIL systems, an annular-amplitude filter couldn't produce superresolving effects. It is especially noted that, when the obstruction angle is near the critical angle, the shape of spot of the SIL with high-NA becomes fairly bad due to the stronger axial component.
Optik - International Journal for Light and Electron Optics.
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ABSTRACT: An analytical expression describing the vectorial field distribution of Gaussian light beams diffracted by an axicon is obtained. The theoretical analysis and simulation calculation show that for the linearly x-polarized light incident on an axicon, the y-component of the diffraction field is very small and the x-component dominates. The intensity of the z-component along the propagation direction is related with the open angle and index of axicon. The open angle plays the more important role in determining the polarization than does the index. For a small open angle, the z-polarized effect can be neglected and the scalar method is simple and valid to evaluating the diffraction field distribution of axicon. However, the vectorial method has to be used for great open angle.
Optik - International Journal for Light and Electron Optics.
