Generation of the stable propagation Bessel beam and the axial multifoci beam with pure phase elements.

A recently proposed method is upgraded to convert two amplitude phase modulation systems (APMSs) to pure phase elements (PPEs), for generating the stable propagation Bessel beam and the axial multifoci beam, respectively. Phase functions of the PPEs are presented analytically. Numerical simulations by the complete Rayleigh-Sommerfeld method demonstrate that the converted PPE has implemented the same optical functionalities as the corresponding APMS, in either the longitudinal or the transverse direction. Compared with the traditional APMS, the converted PPE possesses many advantages such as fabrication process simplification, system complexity reduction, production cost conservation, alignment error avoidance, and experimental precision enhancement. These inherent advantages position the PPE as an ideal choice and driving force behind further advancements in optical system technology.

Ideal suppression of Bessel beam intensity oscillations with a vortex phase plate.

We propose what we believe to be a new kind of diffractive phase element, i.e., vortex phase plate (VPP) with phase singularities along the azimuth direction. Phase function of the proposed VPP is given analytically. Axial intensity oscillations of propagating Bessel beams are ideally suppressed by using the proposed VPP. Compared with the traditional amplitude mask, the proposed VPP takes such advantages as a simpler fabrication procedure and a lower cost.

Generation of stable propagation Bessel beams and axial multifoci beams with binary amplitude filters.

The binary amplitude filter (BAF) is employed to generate stable propagation Bessel beams and axial multifoci beams, rather than the traditional continuous amplitude filter (CAF). We introduce a parameter along the azimuth direction, i.e., angular order of the BAF, to weaken transverse intensity asymmetry. Numerical simulations reveal that the BAF implements the same optical functionalities as the CAF. The BAF holds advantages over the traditional CAF: a simpler fabrication process, a lower cost, and a higher experimental accuracy. It is believed that the BAF should have many practical applications in future optical systems.

Flattening axial intensity oscillations of a diffracted Bessel beam through a cardioid-like hole.

We present a new feasible way to flatten the axial intensity oscillations for diffraction of a finite-sized Bessel beam, through designing a cardioid-like hole. The boundary formula of the cardioid-like hole is given analytically. Numerical results by the complete Rayleigh-Sommerfeld method reveal that the Bessel beam propagates stably in a considerably long axial range, after passing through the cardioid-like hole. Compared with the gradually absorbing apodization technique in previous papers, in this paper a hard truncation of the incident Bessel beam is employed at the cardioid-like hole edges. The proposed hard apodization technique takes two advantages in suppressing the axial intensity oscillations, i.e., easier implementation and higher accuracy. It is expected to have practical applications in laser machining, light sectioning, or optical trapping.

New design model for high efficiency cylindrical diffractive microlenses.

A new model, i.e., the decreasing thickness model (DTM) is proposed and employed for designing the cylindrical diffractive microlenses (CDMs). Focal performances of the designed CDMs are theoretically investigated by solving Maxwell's equations with the boundary element method. For comparison, the CDMs designed by the traditional equal thickness model (ETM) are also studied. Theoretical simulations demonstrate that focal performances of the designed CDMs are improved a lot via replacing the traditional ETM with the proposed DTM. Concretely, the focal efficiency is heightened and the focal spot size is shrunk. Experimental measurements verify the theoretical simulations well. Especially, the above-mentioned improvements become more prominent for the CDM with a higher numerical aperture.

An ultrathin terahertz lens with axial long focal depth based on metasurfaces.

The plasmonic resonance effect on metasurfaces generates an abrupt phase change. We employ this phase modulation mechanism to design the longitudinal field distribution of an ultrathin terahertz (THz) lens for achieving the axial long-focal-depth (LFD) property. Phase distributions of the designed lens are obtained by the Yang-Gu iterative amplitude-phase retrieval algorithm. By depositing a 100 nm gold film on a 500 µm silicon substrate and etching arrayed V-shaped air holes through the gold film, the designed ultrathin THz lens is fabricated by the micro photolithography technology. Experimental measurements have demonstrated its LFD property, which basically agree with the theoretical simulations. In addition, the designed THz lens possesses a good LFD property with a bandwidth of 200 GHz. It is expected that the designed ultrathin LFD THz lens should have wide potential applications in broadband THz imaging and THz communication systems.