Xuanhui Lu

Hollow Gaussian beams and their propagation properties

A new mathematical model, described as hollow Gaussian beams (HGBs), is proposed to describe a dark hollow laser beam (DHB). The area of the dark region across the HGBs can easily be controlled by proper choice of the beam parameters. Based on the Collins integral, an analytical propagation formula for the HGBs through a paraxial optical system is derived. The HGBs also can be expressed as a superposition of a series of Lagurerre-Gaussian modes by use of a polynomial expansion. As a numerical example, the propagation properties of a DHB in free space are illustrated graphically. The HGBs provide a convenient and powerful way to describe and treat the propagation of DHBs and can be used conveniently to analyze atoms manipulated with a DHB.

Radiation force of coherent and partially coherent flat-topped beams on a Rayleigh particle.

Propagations of coherent and partially coherent flat-topped beams through a focusing optical system are formulated. The radiation force on a Rayleigh dielectric sphere induced by focused coherent and partially coherent flat-topped beams is investigated theoretically. It is found that we can increase the transverse trapping range at the planes near the focal plane by increasing the flatness (i.e., beam order) of the flat-topped beam, and we can increase the transverse and longitudinal trapping ranges at the focal plane by decreasing the initial coherence of the flat-topped beam. The trapping stability is also analyzed.

Effect of spatial coherence on radiation forces acting on a Rayleigh dielectric sphere

We show that the radiation forces (RFs) on a Rayleigh dielectric sphere induced by a partially coherent light beam are greatly affected by the spatial coherence. We find that the magnitude of the RFs greatly decreases as the spatial coherence decreases and derive an inequality for the required correlation width sigma(0) (i.e., the spatial coherence of the beam) to stably trap the particles.

Radiation force of highly focused Lorentz-Gauss beams on a Rayleigh particle

The radiation force of highly focused Lorentz-Gauss beams (LG beam) on a dielectric sphere in the Rayleigh scattering regime is theoretically studied. The numerical results show that the Lorentz-Gauss beam can be used to trap particles with the refractive index larger than that of the ambient. The radiation force distribution has been studied under different beam widths of the Lorentz part. The trapping stability under different conditions is also analyzed.

Propagation dynamics of abruptly autofocusing Airy beams with optical vortices

We investigated the propagation dynamics of the Circular Airy Beams (CAB) with optical vortices (OVs) by numerical calculation. Comparing to the common CAB, the maximum intensity of CAB with vortices can be increased greatly at the focal plane and its focal intensity profile is doughnut-shaped when an on-axis vortex is imposed. The case for an off-axis OV and multiple OVs have been investigated as well. We demonstrate that two opposite OVs will annihilate exactly at the focal plane, with the focal intensity is highly increased.

Radiation force of abruptly autofocusing Airy beams on a Rayleigh particle.

The radiation force of circular Airy beams (CAB) on a dielectric Rayleigh particle is investigated in this paper. Our results show that the CAB can be used to trap the particle whose refractive index is larger than the ambient at different positions along the beam axis. Comparing with the Gaussian beam under the same conditions, the longitudinal and the transverse gradient force of CAB on the Rayleigh particle are increased, and the particle can be trapped more stable. Our analyses also demonstrate that the trapping properties of CAB can be modulated by controlling corresponding parameters of CAB.

Abruptly autofocusing property of blocked circular Airy beams

We theoretically and experimentally study the propagation characteristics of the circular Airy beam (CAB) when its first few light rings are blocked. It is shown that the focus position of the blocked CAB will remain the same, and its abruptly autofocusing property will be enhanced. Since the maximum focal intensity almost remains the same when the first ring is blocked, a better result of abruptly autofocusing property can be obtained when only the first ring is eliminated. Compared with the common CAB with a same hollow region, the intensity of the blocked CAB will focus at a different position and the intensity will be increased much larger than the common CAB.

Propagation characteristics of the modified circular Airy beam

We have proposed a kind of modified circular Airy beam (MCAB) based upon a modification of the Fourier spectrum of circular Airy beams (CAB) in this paper. Unlike most abruptly autofocusing beams, the position of peak intensity of MCAB can be moved to any rings behind. Two apodization parameters are introduced to describe the propagation characteristics of MCAB. It is found that the focal position, focal trajectory and the size of focal spot do not change with the apodization parameters; but the abruptly autofocusing property will be greatly enhanced if appropriately apodization parameters are chosen. Comparing with the common CAB and the previous blocked CAB, the MCAB shows better abruptly autofocusing property. It may have more applications in various fields.

Trapping two types of particles by modified circular Airy beams

The radiation force of modified circular Airy beams (MCAB) exerted on both a high-refractive-index particle and a low-refractive-index particle are analyzed in this paper. Our results show that the two kinds of particles can be simultaneously stably trapped by MCAB at different positions. Compared with the common circular Airy beams (CAB) with the same parameters, trapping forces on the two kinds of particles are greatly increased because of the enhanced abruptly autofocusing property and the appearance of hollow region in MCAB. The trapping forces can be modulated by varying parameters of MCAB, and it is important to choose appropriate parameters to trap particles in practice.

Generation of a high-quality partially coherent dark hollow beam with a multimode fiber

We report the experimental generation of a high-quality partially coherent dark hollow beam (DHB) by coupling a partially coherent beam into a multimode fiber (MMF) with a suitable incidence angle. The interference experiment of the generated partially coherent DHB passing through double slits is demonstrated. It is found that the coupling efficiency of the multimode fiber, the quality and the coherence of the generated partially coherent DHB are closely controlled by the coherence of the input beam. A partially coherent elliptical DHB is generated through focusing the generated partially coherent DHB with a cylindrical lens.