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A new method to generate and control the amplitude and phase distributions of a optical vortex beam is proposed. By introducing a holographic grating on top of the dielectric waveguide, the free space vortex beam and the in-plane guiding wave can be converted to each other. This microscale holographic grating is very robust against the variation of geometry parameters. The designed vortex beam generator can produce the target beam with a fidelity up to 0.93, and the working bandwidth is about 175 nm with the fidelity larger than 0.80. In addition, a multiple generator composed of two holographic gratings on two parallel waveguides are studied, which can perform an effective and flexible modulation on the vortex beam by controlling the phase of the input light. Our work opens a new avenue towards the integrated OAM devices with multiple degrees of optical freedom, which can be used for optical tweezers, micronano imaging, information processing, and so on.
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We theoretically and experimentally studied a novel class of vortex beams named open vortex beams (OVBs). Such beams are generated using Gaussian beams diffracted by partially blocked fork-shaped gratings (PB-FSGs).The analytical model of OVBs in the near field and far field is given by superpositions of Hypergeometric (HyG) modes. Unlike conventional integer and fractional vortex beams, the OVBs can have both an open ring structure and an integer topological charge (TC). The TC is decided by the circumference covered by the open ring. It is also quantitatively shown that a $pi/2$ rotation of the open ring occurs in the propagation of an OVB due to the Gouy phase shift. Futhermore, we demonstrate experimental generation and detection of OVBs. Our experimental results are in very good agreement with the theory. We believe that the OVB can be the potential candidate for numerous applications, such as particle manipulation, quantum information and optical metrology.
High harmonic generation (HHG) is an extreme nonlinear frequency up-conversion process during which extremely short duration optical pulses at very short wavelengths are emitted. A major concern of HHG is the small conversion efficiency at the single emitter level. Thus ensuring that the emission at different locations are emitted in phase is crucial. At high pump intensities it is impossible to phase match the radiation without reverting to ordered modulations of either the medium or the pump field itself, a technique known as Quasi-Phase-Matching (QPM). To date, demonstrated QPM techniques of HHG were usually complicated and/or lacked tunability. Here we demonstrate experimentally a relatively simple, highly and easily tunable QPM technique by using a structured pump beam made of the interference of different spatial optical modes. With this technique we demonstrate on-the-fly, tunable quasi-phase-matching of harmonic orders 25 to 39 with up to 30 fold enhancement of the emission.
Fractional vortex beams (FVBs) with non-integer topological charges attract much attention due to unique features of propagations, but there still exist different viewpoints on the change of their total vortex strength. Here we have experimentally demonstrated the distribution and number of vortices contained in FVBs at Fraunhofer diffraction region. We have verified that the jumps of total vortex strength for FVBs happens only when non-integer topological charge is before and after (but very close to) any even integer number, which originates from two different mechanisms for generation and movement of vortices on focal plane. Meanwhile, we have also measured the beam propagation factor (BPF) of such FVBs, and have found that their BPF values almost increase linearly in one component and oscillate increasingly in another component. Our experimental results are in good agreement with numerical results.
Surface plasmon polaritons carrying orbital angular momentum are of great fundamental and applied interest. However, common approaches for their generation are restricted to having a weak dependence on the properties of the plasmon-generating illumination, providing a limited degree of control over the amount of delivered orbital angular momentum. Here we experimentally show that by tailoring local and global geometries of vortex generators, a change in circular polarization handedness of light imposes arbitrary large switching in the delivered plasmonic angular momentum. Using time-resolved photoemission electron microscopy we demonstrate pristine control over the generation and rotation direction of high-order plasmonic vortices. We generalize our approach to create complex topological fields and exemplify it by studying and controlling a bright vortex, exhibiting the breakdown of a high-order vortex into a mosaic of unity-order vortices while maintaining the overall angular momentum density. Our results provide tools for plasmonic manipulation and could be utilized in lab-on-a-chip devices.
Vector vortex beams are a class of optical beams with singularities in their space-variant polarization. Vector vortex beam lasers have applications in many areas including imaging and communication, where vertical-cavity lasers emitting Gaussian beams have been most widely used so far. Generation of vector vortex beams from vertical-cavity lasers has required external control or modulation. Here, by utilizing a polarization-selective subwavelength grating as one of the reflectors in a vertical semiconductor microcavity, we design the spin textures of the polariton mode and demonstrate polariton lasing in a single-mode, radially polarized vector vortex beam. Polarization and phase distributions of the emission are characterized by polarization-resolved imaging and interferometry. This method of vector vortex laser beam generation allows low threshold power, stable single-mode operation, scalability, and on-chip integration, all of which are important for applications in imaging and communication.
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