We propose a method for generating axial multifocal spots (AMS) with a high numerical aperture (NA) objective. The AMS is generated by using phase-only modulation at the back aperture of the objective. Without using any iteration algorithm, the modulated phase distribution is directly calculated by an additional phase analytical formula with different focal distances. By dividing the back aperture of the objective into multi sectorial zones and applying the corresponding additional phase with different focal distances, the AMS can be created. Numerical simulation shows that the numbers of the axial focus depends solely on the different phase distribution calculated by different focal distances. By engineering the phase pattern with different focal distances, axial multifocal spots with different spacing can be realized. Furthermore, combined with vortex phase, the AMS with specific shape spots also can be created. In addition, the AMS focused by incident beams of circular polarization, radial polarization and angular polarization are also studied. This kind of AMS may be found applications in optical imaging, especially in three-dimensional (3D) biological imaging, and also be attractive in mult-plane optical trapping.
Thermal lens (TL) and thermal mirror (TM) effects have been widely used for measuring the thermo-optical properties in materials. However, most previous research is not a direct two-dimensional measurement of the phase difference induced by photothermal effects, and the TL and TM effects cannot be measured simultaneously. We present an integrated digital holography (IDH) for measuring photothermal effects induced by femtosecond laser pulses with the laser excitation fluence below the ablation threshold. The photothermal effects of a metal sample induced by femtosecond laser pulses are studied. Our theoretical analysis reveals that when the energy of the femtosecond laser is below the ablation threshold, the theory of heat conduction and thermoelasticity can be used to explain the TL and TM effects caused by the laser-induced nonuniform temperature distribution. The experimental results show that both the nanoscale surface deformation of the TM effect and the refraction index change of the TL effect can be measured simultaneously by using the IDH. This IDH setup could be suitable for measuring the optical and thermal properties of materials.
A new aperture-synthesis approach in femtosecond-pulse digital holography for obtaining a high-resolution and a whole field of view of the reconstructed image is proposed. The subholograms are recorded only by delay scanning holograms that have different delay times between the object and reference beams. In addition, by using image processing techniques, the synthesis aperture digital hologram can be superposed accurately. Analysis and experimental results show that the walk-off in femtosecond off-axis digital holography caused by low coherent can be well eliminated. The resolution and the field of view of the reconstructed image can be improved effectively.
Because of their spiral wave front, phase singularity, zero-intensity center and orbital angular momentum, dark hollow vortex beams have been found many applications in the field of atom optics such as atom cooling, atom transport and atom guiding. In this paper, a method for generating confluent hypergeometric beam by computer-generated hologram displayed on the spatial light modulator is presented. The hologram is formed by interference between a single ring Laguerre-Gaussian beam and a plane wave. The far-field Fraunhofer diffraction of this optical field transmitted from the hologram is the confluent hypergeometric beam. This beam is a circular symmetric beam which has a phase singularity, spiral wave front, zero-intensity center, and intrinsic orbital angular momentum. It is a new dark hollow vortex beam
A primary spherical aberration phase factor is introduced for the spherical refracting system and the diffraction integral of the system is obtained. By omitting this primary spherical aberration phase factor, a specific crude imaging system is designed by cascading single spherical refracting fractional Fourier transform units. The spherical aberration of the system are numerically investigated. The relation between fractional Fourier transform and source of spherical aberration of imaging system is discussed.
Nanopore arrays were fabricated by self-organized anodization on aluminum. A two step anodization process was used to oxidize aluminum in H<sub>2</sub>SO<sub>4</sub> solution. Hexagonally ordered pore array films were obtained by dissolving the remained aluminum base of anodic alumina in saturated HgCl<sub>2</sub> solution. Scanning electron microscopy and X-ray Diffraction were used to investigate the morphology and crystal structures of the porous anodic alumina films. An investigation was made on the optical transmission, optical absorption and photoluminescence of the porous anodic alumina films. The results show that the highly ordered anodic porous alumina films are amorphous. The transmission spectra and absorption spectra of porous anodic alumina films abruptly curve at wavelength of 360nm, the optical transmission increases at wavelength more than 360nm, the optical absorption increases at wavelength less than 360nm. The photoluminescence intensity and peak position of the porous anodic alumina films depend strongly on the excitation wavelength, there is a wide blue photoluminescence band in the wavelength range of 340- 600nm.