Dynamic switching of optical vortices with dynamic gamma-correction liquid crystal spiral phase plate

Generation and Detection of Optical Vortices with Multiple Cascaded Spiral Phase Plates

Spiral phase plate (SPP) is the widely used method in the generation of vortex beam (VB) with fixed topological charges (TCs) for specific wavelength. Although VB with large TCs can be directly generated by using the SPP with high vortex order. The fabrication of high-quality SPPs with high vortex orders usually requires complex manufacturing process and high machining accuracy. An alternative method to generate VBs with large TCs is cascaded multiple SPPs with low order. In this study, we numerically calculate the transmitted light field of cascaded multiple SPPs according to the Huygens–Fresnel diffraction integral, and perform the experimental verifications. Based on cascading 6 SPPs (3 SPPs with TCs of 2, and 3 SPPs with TCs 4, respectively), an VB with TCs as high as 18 is generated. Furthermore, The TCs of the generated VB are detected by coaxial and off-axis interfering with fundamental Gaussian beam or its conjugate beam, respectively. The generated fork and spiral patterns allow us to distinguish the value and sign of TCs carried by the VB. The experimental results coincide well with the theoretical simulations. The fork pattern shows better resolution than the spiral one, and the petal pattern with small spiral allows us to distinguish large TCs with a higher resolution.

Generation of optical vortices by using binary vortex producing lenses

Experimental high-quality optical vortices of different topological charges are generated by using a vortex producing lens with two phase levels. In our setup, the lens is displayed on a liquid-crystal spatial light modulator that only attains phase modulation of around 1.2π. This achievement opens the real possibility of creating high-quality optical vortices with devices of very low phase modulation capacity. The experimental setup is fully described, and the considerations to set the optimal parameters to obtain high-quality optical vortices are discussed and experimentally established. The phase and intensity of the optical vortices are recovered. The phase is obtained through a phase-shifting method that is directly programmed onto the modulator avoiding any class of mechanical displacement.

Focused laser lithographic system with sub-wavelength resolution based on vortex laser induced opacity of photochromic material

A focused laser lithographic system combines with vortex laser induced opacity of photochromic layer to write patterns with linewidth below wavelength. A photochromic layer is formed by coating the mixture of metanil yellow and aqueous PVA solution on the photoresist layer. In our system, the center of a lithographic laser with a 405 nm wavelength coincides with the center of vortex laser with a 532 nm wavelength. When a photochromic layer is illuminated by both lasers simultaneously, the absorbance for the lithographic laser decreases at the hollow region of the vortex laser but increases at its annular region, so that a transparent aperture for the lithographic laser is created and its size could be tuned by changing the power of vortex laser; therefore, the linewidth of written patterns is variable. Experimentally, using a 20× lens (NA = 0.4), this system condenses the linewidth of written patterns from 6614 to 350 nm.

Fraunhofer diffraction of a partially blocked spiral phase plate

The Fraunhofer diffraction pattern from a partially blocked spiral phase plate (SPP) produces a partial vortex output pattern that is rotated by 90 degrees compared with the input. The rotation direction depends on whether the angular phase pattern increases in the clockwise or counterclockwise direction. In this work, we present an explanation of this effect based on careful examination of classical diffraction theory and show new experimental results. This approach is very convenient for easily determining the sign of the vortex charge.

Focusing properties of Fresnel zone plates with spiral phase

Focusing properties of Fresnel zone plates with spiral phase with integer and fractional topological charges illuminated by plane wave are studied. Numerical results show that hollow beams can be generated and can also be controlled by the number of the zones and the topological charge, which implies the potential applications of such kind of zone plate in trapping and manipulating particles.

Square Fresnel zone plate with spiral phase for generating zero axial irradiance

A type of square Fresnel zone plate with spiral phase for generating zero axial irradiance is proposed. Diffraction patterns of the zone plates with different generalized topological charges are given by numerical simulation, which also suggests their potential applications in generating hollow beams.

Fabrication and testing of l = 2 optical vortex phase masks for coronography

In this paper we present the fabrication process and tests of two different types of l = 2 spiral phase plates (SPPs), designed for an Optical Vortex Coronagraph (OVC) in the visible wavelength regime. Each phase mask is realized dividing the spirals area in sectors respectively of 8 and 512 of levels using lithographic nanofabrication approach. The SPPs produces different optical vortices (OVs) with topological charge l that depends on the number of steps and on the wavelength. We found that the residual light in the central dark region of the OV tends to zero as the number of steps increases.

Fabrication of liquid crystal polymer axial waveplates for UV-IR wavelengths

We show the opportunity of fabricating axially symmetric waveplates fine tuned to a desired wavelength. High quality waveplates are obtained using liquid crystal polymer layers on photoaligning substrates extending their functional range from UV to IR wavelengths. We characterize the effect of the waveplate on laser beams showing formation of a doughnut beam with over 240 times attenuation of intensity on the axis. We pay attention that the power density is strongly reduced on the doughnut ring as well and use this opportunity for taking charge coupled devices (CCDs) out of a deep saturation regime. Strong deformation of the beam profile is observed when the vortex axis is shifted towards the periferies of the beam. We demonstrate feasibility of using this phenomenon for shaping the profile of light beams with a set of waveplates.

Vortex characteristics of Fraunhofer diffractions of a plane wave by a spiral phase plate limited by pseudoring polygonal apertures

We introduce a multilevel spiral phase plate (SPP) limited by a pseudoring polygonal aperture (PRPA). Such an SPP has the advantages of easier fabrication and greater suppression of the sidelobes of the diffraction field over that generated with a polygonal aperture (PA). The Fraunhofer diffraction fields generated by an SPP with a PRPA or with a PA have the same topological charge features and a similar diffraction pattern. Numerical evaluations show that the maximum bright annular-intensity difference between the diffraction patterns for the SPP with a PRPA and that of a PA does not exceed 3% under optimal design parameters.

Diffraction of a finite-radius plane wave and a Gaussian beam by a helical axicon and a spiral phase plate

We derive what we believe to be new analytical relations to describe the Fraunhofer diffraction of the finite-radius plane wave by a helical axicon (HA) and a spiral phase plate (SPP). The solutions are deduced in the form of a series of the Bessel functions for the HA and a finite sum of the Bessel functions for the SPP. The solution for the HA changes to that for the SPP if the axicon parameter is set equal to zero. We also derive what we believe to be new analytical relations to describe the Fresnel and Fraunhofer diffraction of the Gaussian beam by a HA are derived. The solutions are deduced in the form of a series of the hypergeometric functions. We have fabricated by photolithography a binary diffractive optical element (a HA with number n=10) able to produce in the focal plane of a spherical lens an optical vortex, which was then used to perform rotation of several polystyrene beads of diameter 5 microm.