Yaoju Zhang

Longer axial trap distance and larger radial trap stiffness using a double-ring radially polarized beam

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.

Theoretical study on all-optical magnetic recording using a solid immersion lens

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.

All-optical magnetic superresolution with binary pupil filters

A method to produce magnetic superresolution in all-optical magnetic storage is proposed. Two-zone amplitude-only and phase-only filters are designed to improve the quality of the superresolved magnetization pattern, to increase the magnetic spot intensity, to reduce the magnetic spot size, and to control the sidelobe effect. A procedure for designing a rotationally symmetric pupil-plane mask to control the magnetization intensity distribution near focus is presented. As a practical implementation, we have applied our method to obtain superresolving two-zone phase filters that can improve the all-optical magnetic recording density.

Generation of 2-

We propose an experimental technique for efficient 2- μm light generation based on the intracavity noncritical phase matching optical parametric oscillator (OPO). Intra-cavity pumped by a diode pumped actively Q-switched Nd:YVO4 laser operating at 1.3 μm, we have achieved efficient 2093-nm signal beam emission making full use of noncritical phase matching KTA-OPO. At a diode pump power of 9.8 W and a Q-switching repetition rate of 30 kHz, a signal beam with a spectral linewidth of ~ 1 nm and an average power of 1.01 W have been obtained, giving a diode pump power to signal power conversion efficiency of 10.3%. The short pulse width of ~ 3.4 ns is achieved, which is much shorter than other approaches with Q-switched operation to generate 2-μm laser beams.

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The near-field focusing properties of actual hybrid amplitude–phase binary subwavelength Fresnel zone plates (HBSFZPs) are studied theoretically. The analysis based on the exact vector angular spectrum method is done for a radially polarized beam incident on the HBSFZPs. The results show that the near-field subwavelength focusing with a long depth of focus can be obtained using an HBSFZP, which is very useful for near-field subwavelength photolithography and high-resolution microscopy. The position of the actual focus in the near-field focusing HBSFZPs depends on the evanescent wave rather than the propagating wave. The etch depth has an important influence on focusing properties of HBSFZPs.

Light Based on a Noncritical Phase Matching OPO Technique

Laser induced strange blue luminescence in several Raman crystals has been investigated. The blue luminescence at about 473 nm has the characteristic of no orientation and only produced in the crystal where the fundament laser oscillated. The experimental results show that the blue luminescence must result from the fundamental laser around 1.0 µm rather than Stokes-shifting. The spectrum detected is similar for different crystals. This blue luminescence is obviously strange and inconsistent with traditional luminescence theories, which maybe a brand-new luminescence theory.

Analysis of near-field subwavelength focusing of hybrid amplitude-phase Fresnel zone plates under radially polarized illumination

We report on the third Stokes light from a potassium titanyle arsenate (KTA) crystal with a Raman shift of 234 cm−1. A 25 mm long x-cut KTA crystal was intracavity pumped by an acousto-optic Q-switched Nd:Lu0.5Y0.5VO4 polarized laser at 1065 nm. The fundamental, first, second, and third Stokes lights oscillated and shared the same resonator, but only the third Stokes light at 1151 nm was output based on appropriate coated cavity mirrors. At an incident pump power of 8.2 W and a pulse repetition frequency of 15 kHz, an average output power of 430 mW and a pulse width of 4.7 ns were obtained, with the diode to Raman output conversion efficiency of 5.2%.

Laser Induced Blue Luminescence Phenomenon

We reported a compact self-cascaded KTA-OPO source for 2.6 μm coherent light generation. The OPO is driven in a diode end-pumped and Q-switched Nd:YVO4 laser cavity. Two OPO processes occurred in the same KTA crystal with non-critical phase matching. At an incident diode pump power of 8.7 W and a pulse repetition frequency of 60 kHz, the OPO can generate a maximum average output power of 445 mW at 2.59 μm. The slope efficiency was about 12.7%, and the power fluctuation was less than 8%. Therefore, the self-cascade OPO based on KTA offers a promise scheme for the rugged and compact mid-infrared 2.6 μm laser generation.

Third-Stokes light operation in KTA crystal derived by Nd:Lu0.5Y0.5VO4 crystal laser

Nd:YAG/KTA intra-cavity Raman lasers with two different Raman shifts are experimentally compared for eradiating around 1.15 µm. The different Raman processes are attributed to polarization dependent Raman gain of KTA crystal and nearly linear polarization of Nd:YAG fundamental light with Q-switch operation. Under an incident pump power of 10.2 W and a pulse repetition frequency of 15 kHz, 1.24 W output for the third-Stokes at 1150 nm in X(ZZ)X Raman configuration with the Raman shifts of 234 cm-1 and 1.21 W output for the first-Stokes at 1146 nm in X(YY)X Raman configuration with the Raman shifts of 671 cm-1were achieved by rotating the KTA crystal. The conversion efficiency in both Raman configurations were about 12% respect to the same incident pump power. The slight differences of spectra and pulses are investigated.

Compact self-cascaded KTA-OPO for 2.6 μm laser generation

We propose a method to obtain nano-scale 3D super- resolution in STED ∞uorescence microscopy. A double-ring-shaped cylindrical vector vortex beam, with an appropriate vortex angle and a proper truncation parameter of the beam, is used to generate a 3D dark spot as the erase spot. A single-ring-shaped radially polarized beam is used as a pump beam, which can generate a sharper 3D bright spot. The volume of the generated 3D dark spot is small and the uniformity of the light wall surrounding the spot is quite high. Consequently, the 3D super-resolution ability of a STED microscope is improved and nano-scale three-dimensional resolutions are obtained.