Metasurface-based perfect vortex beam for optical eraser

Optical vortices carry orbital angular momentum (OAM) with a helical wavefront characterized by topological charge (TC), producing annular intensity profiles and enabling diverse applications. A key limitation of conventional vortex beams is that the annular diameter scales with TC, complicating simultaneous coupling or superposition of multiple OAM modes and affecting particle manipulation dynamics. Perfect vortex beams (PVBs) address this by maintaining constant ring diameter independent of TC, but conventional implementations rely on bulky components (spiral phase plates, axicons, Fourier lenses), hindering compact integration and CMOS-compatible fabrication. Metasurfaces, particularly high-index dielectric platforms, offer compact, transmissive, and efficient phase control suitable for integrated PVB generation. Prior metasurface work has demonstrated vortex beams and PVBs, but achieving large TCs with broadband performance remains desirable for communication bandwidth expansion and multimode coupling. Additionally, linking metasurface-generated beams with optical (quantum) eraser experiments can elucidate quantum behaviors. This study demonstrates GaN-based metasurface PVBs (MPVBs) with large TCs (−32 and +16) at visible wavelengths, investigates their pre-convergence behavior via interferometry, and integrates them into optical eraser experiments.

The paper reviews the OAM/vortex beam field and its applications in communications, micromachining, optical trapping, nonlinear and quantum optics. Conventional PVBs are typically realized via Fourier transforms of Bessel–Gaussian beams using macroscopic components, limiting integration. Metasurfaces provide planar, subwavelength phase control: plasmonic implementations suffer from transmission inefficiency, while dielectric metasurfaces enable high-efficiency transmissive operation via resonant meta-atoms. Prior metasurface advances include visible metalenses (including GaN platforms) and vortex/PVB generation; however, metasurface-based PVBs with larger TCs and broadband behavior are still sought to enhance OAM multiplexing and coupling. Separately, optical eraser (quantum eraser) concepts—from foundational proposals and delayed-choice demonstrations to variants involving entanglement and microscopy—motivate integrating metasurface-generated structured light to probe quantum-like behaviors. This work builds on those foundations to realize high-TC, broadband MPVBs and demonstrate an optical eraser using them.

Design and simulation: MPVBs are designed using the Pancharatnam–Berry (PB) geometric phase, implementing phase control solely via in-plane rotation of identical dielectric meta-structures. CST simulations optimize a rectangular meta-atom array for operation at 450 nm. Lattice period is 220 nm and height is 800 nm. Simulated polarization conversion efficiency (PCE) reaches ~98% for meta-atom dimensions of width 90 nm and length 160 nm (PCE defined as cross-polarized transmission divided by the sum of cross- and co-polarized transmission). The target phase profile for perfect vortex generation combines a focusing term and azimuthal phase term proportional to TC l. The parameter controlling annular diameter is set to d = 5. Device aperture diameter is 100 µm with a design focal length f = 150 µm. Two TCs are implemented: l = −32 and l = +16.

Fabrication: An undoped 800-nm-thick GaN layer is grown on double-polished c-plane sapphire via MOCVD. A 400-nm SiO2 hard mask is deposited, followed by spin-coating electron-beam resist and e-beam lithography to define meta-structure patterns. A 33-nm Cr layer is deposited by e-gun evaporation. Patterns are transferred using RIE and ICP-RIE to etch GaN and realize high-aspect-ratio meta-structures with smooth, near-vertical sidewalls. SEM imaging confirms pattern fidelity for both TC devices (top, tilt, center, and edge views).

Optical characterization: Broadband response is assessed at visible wavelengths (450, 500, 550, 580, 600, 650 nm). Intensity distributions are recorded in x–z and x–y planes to observe convergence and annular ring profiles. A Mach–Zehnder interferometer is employed to verify helical phase via interference with spherical or planar reference beams, tracking spiral/dislocation patterns across z positions (50, 75, 100, 125, 150, 200 µm). The optical eraser configuration is based on a Mach–Zehnder interferometer augmented with linear polarizers: two polarizers (co- or cross-polarized) placed before the second beam splitter, and an optional third polarizer between the second beam splitter and CCD set at 45° to the first polarizer to erase which-path information. Optical eraser tests are performed at 450, 530, and 610 nm and at z-positions corresponding to the emergence of the flower-like interference during helicity switching.

• High-TC MPVBs: Demonstrated transmissive GaN metasurface PVBs with TCs of −32 and +16 at visible wavelengths.
• Broadband operation: Clear intensity convergence across 450–650 nm; annular intensity distributions measured before and beyond the focal plane, confirming broadband capability.
• Convergence distances (z) to minimum annulus: For −32/+16 TCs at wavelengths 450, 500, 550, 580, 600, 650 nm, the converging distances are 116/116, 88/91, 71/76, 62/69, 53/64, and 43/55 µm, respectively.
• Ring diameter scaling: Observed increase of ring diameter with wavelength consistent with R ∝ (NA·λ)/f; beyond the design focal length (150 µm), devices with different TCs exhibit similar annular diameters.
• Interferometric verification: Mach–Zehnder measurements show spiral interference patterns whose branch number matches designed TCs and reveal a helicity switch prior to convergence, with a transitional flower-like interference pattern.
• Optical eraser demonstration: • With two co-polarized arms, interference is observed; with cross-polarized arms, interference is suppressed (which-path marked). • Introducing a third polarizer at 45° restores interference by erasing which-path information. • Results are consistent for reference beams with spherical and planar wavefronts and reproduced at 450, 530, and 610 nm.
• Device performance metrics: Simulated PCE up to ~98% for the optimized meta-atom (period 220 nm; height 800 nm; width 90 nm; length 160 nm). Device aperture 100 µm; design focal length 150 µm. High-aspect-ratio GaN meta-structures achieved with smooth sidewalls.

The work addresses the challenge of generating perfect vortex beams with large TCs in a compact, transmissive, and potentially CMOS-compatible platform. By employing PB-phase GaN metasurfaces, the authors demonstrate high-TC MPVBs that maintain uniform annular diameters across different TCs beyond the focal plane while operating over a broad visible band. Interferometric measurements elucidate pre-convergence dynamics, notably a helicity switch accompanied by flower-like interference, which facilitates uniformization of ring diameters past the designed focal length. The optical eraser experiments confirm control over interference via polarization-based which-path marking and erasure, validating the suitability of MPVBs for experiments probing quantum-like behaviors of structured light. Collectively, these results demonstrate metasurface-enabled scalability to large TCs, robust broadband performance, and functional integration into interferometric/quantum-optical setups, supporting applications in OAM communications, beam combining, and quantum optics.

The study demonstrates GaN metasurface-based perfect vortex beams with TCs up to −32 and +16, achieving high simulated polarization conversion efficiency and broadband operation from 450 to 650 nm. Mach–Zehnder interferometry reveals pre-convergence helicity switching and flower-like interference that enable uniform annular diameters beyond the focal length. Integration into optical eraser configurations confirms polarization-dependent suppression and restoration of interference, reproduced at 450, 530, and 610 nm with both spherical and planar reference beams. These results highlight the potential of MPVBs for advancing quantum optics experiments and compact optical device engineering, paving the way for probing quantum behaviors with structured light.