Spiral Phase Plate Research Articles

Design and implementation of broadband optical vortices in photonic crystal slabs

We propose and discuss a simple method to generate broadband optical vortices, utilizing the inherent topological vortex structures of polarization around bound states in the continuum supported by a photonic crystal slab in the momentum space to induce the generation of vortex beams. Since the proposed structure is composed of silicon pillars arranged periodically, it lacks a true optical geometric center. It is insensitive to the position of the incident light and does not require a specific optical alignment process compared to traditional spiral phase plates. Furthermore, because it is composed of dielectric pillars, it can achieve vortex beam generation at any desired working wavelength. We also discuss the robustness of its structure, showing that it can be immune to certain manufacturing defects. Therefore, the proposed structure not only provides a new method for manipulating the angular momentum of photons but also has potential new applications in integrated optical information processing and optical tweezers.

Compact multi-ring perfect vortex beam generator fabricated by laser direct writing

Perfect vortex beams (PVBs) have received much attention in recent years since the annular intensity distributions are independent of the topological charge (TC). However, the cost-effective preparation of micrometer-scale monolithic devices capable of generating multiple PVBs through a simple approach remains a significant challenge. In this work, a design of double-ring perfect spiral phase plates (DPSPPs) is presented for the generation of PVBs at two distinct locations along the radial direction. The respective radii and spacing of the double-ring PVBs can be tuned by changing the control parameters. The proposed DPSPPs are fabricated by the flexible femtosecond laser direct writing (FsDLW) technique. The experimentally generated double-ring vortex beams with different TCs possess an almost constant radius, which is consistent with the characteristics of PVBs. Furthermore, the double-ring PVBs with different fractional and trigonometric function TCs and the multi-ring PVB are also demonstrated. The design and fabrication methods are expected to facilitate the miniaturization of the applications of PVBs in optical manipulation, optical communication, and high-capacity information storage.

Finite-difference time-domain simulation of THz light pulse with orbital angular momentum produced using a spiral phase plate

A silicon spiral phase plate (SPP) was proposed for a pulsed terahertz signal to produce orbital angular momentum with a topological charge of $l=1$. The resulting field distribution after the signal passes through the SPP was simulated using finite-difference time-domain method and observed at different propagation distances. It shows that the helical spatial structure and optical vortex were best observed at the farthest distance of 7.5 cm. Additionally, the cross-section field distribution at different times as the signal passes through the SPP indicates a helical wavefront structure. Moreover, the output of a source with different central frequencies was observed when using the same SPP and showed a hollow distribution indicating a vortex despite not being located in the center.

Scanless Spectral Imaging of Terahertz Vortex Beams Generated by High‐Resolution 3D‐Printed Spiral Phase Plates

Terahertz technology has experienced significant advances in the past years, leading to new applications in the fields of spectroscopy, imaging, and communications. This progress requires the development of dedicated optics to effectively direct, control and manipulate terahertz radiation. In this regard, 3D printing technologies have shown great potential, offering fast prototyping, high design flexibility, and good reproducibility. While traditional 3D printing techniques allow for the preparation of terahertz optical components operating at relatively low frequencies (<0.4 THz) due to their limited resolution, two‐photon polymerization lithography (TPL) exhibits high detail resolution and low surface roughness and can thus potentially enable the fabrication of high‐frequency terahertz devices. Here, as a proof of principle, spiral phase plates operating at 1 THz are designed and fabricated by means of TPL. Moreover, these samples are characterized via a rapid and scanless terahertz imaging technique customized to obtain a coherent hyperspectral analysis of the generated vortex beams at varying distances along propagation. Numerical simulations are also conducted for comparison with experiments, revealing a good agreement. Current limitations of the technique are found to be mainly related with terahertz loss in TPL polymers, and possible solutions are discussed.

Quantum enhanced mechanical rotation sensing using wavefront photonic gears

Quantum metrology leverages quantum correlations for enhanced parameter estimation. Recently, structured light enabled increased resolution and sensitivity in quantum metrology systems. However, lossy and complex setups impacting photon flux hinder true quantum advantage while using high dimensional structured light. We introduce a straightforward mechanical rotation quantum sensing mechanism, employing high-dimensional structured light and use it with a high-flux (45 000 coincidence counts per second) N00N state source with N = 2. The system utilizes two opposite spiral phase plates with topological charge of up to ℓ = 16 that converts mechanical rotation into wavefront phase shifts and exhibit a 16-fold enhanced super-resolution and 25-fold enhanced sensitivity between different topological charges, while retaining the acquisition times, and with negligible change in coincidence count. Furthermore, the high efficiency together with the high photon flux enables detection of mechanical angular acceleration in real-time. Our approach paves the way for highly sensitive quantum measurements, applicable to various interferometric schemes.

Fourier-transform spectroscopy based on the rotational Doppler effect

We propose a new Fourier-transform spectroscopy technique based on the rotational Doppler effect. The technique offers an application for optical vortex frequency combs, where each frequency component carries a unique amount of orbital angular momentum (OAM). Here, we emulate a vortex comb using a tunable single frequency laser and a collection of spiral phase plates, generating up to 11 distinct OAM modes. Unlike in traditional Fourier-transform spectroscopy based on the Michelson interferometer (linear Doppler effect), the spectral resolution of vortex-comb spectroscopy is not limited by the mechanical scan distance of the instrument, but the instrument can be operated continuously without interruptions, leading to fast mode-resolved measurements.

A Method for Fingerprint Edge Enhancement Based on Radial Hilbert Transform

Fingerprints play a significant role in various fields due to their uniqueness. In order to effectively utilize fingerprint information, it is necessary to enhance image quality. This paper introduces a method based on Radial Hilbert transform (RHLT), which simulates the vortex filter using the point spread function (PSF) of spiral phase plate (SPP) with a topological charge l=1, for fingerprint edge enhancement. The experimental results show that the processed fingerprint image has more distinct edges, with an increase in information entropy and average gradient. Unlike classical edge detection operators, the fingerprint edge image obtained by the RHLT method exhibits a lower mean square error (MSE) and a higher peak signal-to-noise ratio (PSNR). This indicates that the RHLT method provides more accurate edge detection and demonstrates higher noise-resistance capabilities. Due to its ability to highlight edge information while preserving more original features, this method has great application potential in fingerprint image processing.

Tight focusing of terahertz vortex beams formed by laser dielectric resonator

Wave characteristics of vortex laser beams during their tight focusing have been theoretically studied. The Rayleigh–Sommerfeld theory was used to describe propagation in free space of laser beams excited by the modes of a waveguide dielectric resonator. It is shown that at the topological charge of the spiral phase plate n = 0, the studied EH11 mode has a maximum of radiation intensity on the axis. Introduction of a topological charge leads to the appearance of a minimum of radiation intensity on the axis as well as to the increase in the size of the focal spot. However, for the TE01 mode with the topological charges n = 0 and n = 2, the intensity distribution retains a ring shape, while at n = 1 the beam profile turns into the Gaussian-like one. The wave front in the focal region of the lens for the components of the EH11 and TE01 modes transforms from spherical to spiral one with increasing the topological charge.

Perfect vortex beams generation based on reflective geometric phase metasurfaces

Perfect vortex beam (PVB) is a propagating light field with radial intensity distribution independent of topological charge and carrying orbital angular momentum (OAM). PVBs can be generated through the Fourier transformation of a Bessel-Gaussian (BG) beam, which traditionally requires a precisely aligned optical setup consisting of a spiral phase plate, a conical lens, and a Fourier lens. However, such traditional schemes are usually bulky and unstable. Here, we have utilized a method that differs from conventional approaches, employing metasurfaces designed as planar Pancharatnam-Berry (PB) phase elements to substitute for all the required components. Unlike traditional methods based on reflective or refractive elements, metasurface elements can significantly reduce the system's footprint. Because the use of metallic metasurfaces allows for a more flattened design plane, in this paper, we have demonstrated the generation of PVB within the 8 GHz microwave frequency band based on a single-layer metallic metasurface. The metasurface is composed of a square ring metal structure, which can serve as a half wave plate to provide the required geometric phase distribution for incident circularly polarized light to generate PVB. By rigorously optimizing the parameters of the square-ring structures, we have generated vortex beams carrying OAM with different topological charges. These vortex beams exhibit a constant radial intensity distribution, which validates their "perfect" characteristics. In addition, we also extracted the mode purity of different vortex beams, so that the mode states of different vortex beams can be distinguished. These results provide broader value for the application of perfect vortex beams in communication.

Computerized simulation of 2-dimensional phase contrast images using spiral phase plates in neutron interferometry

Abstract We present calculations of interferograms (interference patterns) of one or multiple spiral phase plates that would be observed with a perfect crystal neutron interferometer of Mach–Zehnder type. A spiral phase plate (SPP) in one of the two coherent beam paths produces a twist in the phase front and thus a vortex beam with intrinsic angular momentum, which in the case of neutrons should be observed as a characteristic interference pattern that appears complementary to each other in both detectors behind the interferometer. Adding additional SPPs in one beam path of the interferometer yield interference patterns similar to that of a single SPP but only due to the cumulative step height. All simulated interferograms have been calculated on the basis of dynamical neutron diffraction without any assumption of a neutron orbital angular momentum and show very convincing agreement with experimental results from the literature, see e.g. (C. W. Clark, R. Barankov, M. G. Huber, M. Arif, D. G. Cory, and D. A. Pushin, “Controlling neutron orbital angular momentum,” Nature, vol. 525, pp. 504–506, 2015). In particular, this clarifies, that the cited experiments do not give evidence of the quantization of interactions caused by a twist of the phase front of a neutron wave in the interferometer and thus no evidence for the effect of a neutron orbital angular momentum.

Observation of ferroelectric domain walls using nonlinear spiral interferometry

Nonlinear optical methods based on second harmonic generation have been widely used to observe ferroelectric domain structures. However, most previous methods have some flaws, such as limitations in structure patterns and time-consuming scanning processes. We have developed a technique called nonlinear spiral interferometry to observe domain walls, which avoids these problems. By placing a spiral phase plate on the rear focal plane of the imaging system, the intensity of the second harmonic wave can be concentrated at 180° domain walls, while regions with homogeneous polarization appear as a dark field. This phenomenon originates from the interference of point spread functions with spiral phase, and the principle is applicable to samples with any polarized pattern. Using this method, disturbing miscellaneous peaks can be suppressed, and imaging contrast is improved due to the redistribution of energy. This technique is verified through theoretical calculations and experiments, providing an effective and convenient way to observe domain structures.

Wavelength-tolerant generation of Bessel-Gaussian beams using vortex phase plates

With their nearly non-diffracting and self-healing nature, Bessel-Gaussian beams (BGBs) are attractive for many applications ranging from free-space communications to nonlinear optics. BGBs can successfully be generated on background laser beams by imprinting and subsequently annihilating multiply charged optical vortices followed by focusing the resulting ring-shaped beam with a thin lens. For high-power applications optical vortices are preferentially created by spiral phase plates because of their high damage threshold. These are fabricated to realize an azimuthal change of the accumulated phase of a multiple of 2π at a predetermined wavelength. This raises the expectation that the use of spiral phase plates for the generation of BGBs is limited to the design wavelength and therefore not applicable to broadband applications involving short-pulse lasers. In this paper we present experimental data showing that this limitation can be overcome in a broad spectral range around the design wavelength. Experimental cross-sections of the BGBs for several off-design wavelengths are found in a good quantitative agreement with the theoretical Bessel functions at distances up to 540 cm after the focus of the lens.

Generation of wavelength- and orbital angular momentum-tunable extreme-ultraviolet vortex beams using a spiral phase mirror

To generate or manipulate the orbital angular momentum (OAM) of light, various optical elements, such as spiral phase plates, spatial light modulators, diffractive optical elements, and spiral phase mirrors (SPMs) have been used. In contrast to other optical components, the SPM depends only on the angle of incidence, which allows for the manipulation of the OAM values by simply rotating a single SPM or changing the angle of incidence of light. This study theoretically demonstrated the generation of high-quality vortex beams in the X-ray region (within a range of several tens of nanometers) beyond the existing visible and infrared regions using a SPM. Based on the geometry of grazing incidence, by rotating a single SPM, we obtained wavelength-tunable vortex beams with an identical OAM value of l = 1 in the 20–40 nm wavelength range and OAM-tunable vortex beams with various OAM values ranging from l = 3 to l = 5 at a 10 nm wavelength. Based on these characteristics, the tuning range of an SPM was discussed and the variation in the major radius of the elliptical beam formed on the SPM by grazing incidence was analyzed to realize SPM manufacturing. This novel optical technique holds great promise for enabling light-matter interactions at the nanometer scale, such as in the molecular or atomic regions, by implementing easily controllable wavelength- and OAM-tunable vortex beams based on a single SPM.

Compact Q-switched vortex waveguide laser modulated by buried Ag nanoparticles in SiO2

Vortex beam, particularly pulsed vortex laser, has expanded the potential applications in optics due to their unique wave-front phase structure and the determined photon orbital angular momentum. In this study, we present a novel approach including the integration of fused silica embedded with silver nanoparticles (Ag:SiO2) into the Nd:YAG waveguide platform for achieving nanosecond vortex pulses. Using a spiral phase plate for in-cavity phase modulation, we successfully demonstrate a Q-switched vortex laser with a high repetition rate in the megahertz range and short pulse width in the nanosecond regime. Additionally, we conduct a comprehensive analysis of the near-field distributions of Ag nanoparticles with varying sizes and distributions using COMSOL simulation. This study exemplifies the integration of silica-based photonic elements for realizing a high-stability and cost-effective nanosecond vortex laser system.

Scanning Electrochemical Probe Lithography for Ultra-Precision Machining of Micro-Optical Elements with Freeform Curved Surface.

Two challenges should be overcome for the ultra-precision machining of micro-optical element with freeform curved surface: one is the intricate geometry, the other is the hard-to-machining optical materials due to their hardness, brittleness or flexibility. Here scanning electrochemical probe lithography (SECPL) is developed, not only to meet the machining need of intricate geometry by 3D direct writing, but also to overcome the above mentioned mechanical properties by an electrochemical material removal mode. Through the electrochemical probe a localized anodic voltage is applied to drive the localized corrosion of GaAs. The material removal rate is obtained as a function of applied voltage, motion rate, scan segment, etc. Based on the material removal function, an arbitrary geometry can be converted to a spatially distributed voltage. Thus, a series of micro-optical element are fabricated with a machining accuracy in the scale of 100 s of nanometers. Notably, the spiral phase plate shows an excellent performance to transfer parallel light to vortex beam. SECPL demonstrates its excellent controllability and accuracy for the ultra-precision machining of micro-optical devices with freeform curved surface, providing an alternative chemical approach besides the physical and mechanical techniques.

Nanometer-scale electron beam shaping with thickness controlled and stacked nanostructured graphite

The generation of small electron probes is the basis for various techniques in which such a probe is scanned across a sample, and special probe shapes like vortices can be desirable, e.g., to gain insight into magnetic properties. Micron-scale phase plates or holographic masks, in combination with demagnifying optics, are usually used for creating such special probe wave functions. Here, we present the fabrication of nanometer-sized phase plates based on thickness-selected and stacked graphite layers as well as an analysis of their performance. First, a spiral phase plate is demonstrated that creates a vortex beam with an orbital angular momentum of 1 and an outer radius of 2.5 nm. Second, a three-level Fresnel lens built from two nanopatterned graphite membranes is presented, which achieves a focal spot with a full width at half maximum of 5.5 nm. Third, an array of electron sieves is demonstrated, each of which creates a focal spot with a radius of 2 nm, and the array is applied as a Shack–Hartmann wavefront detector. These elements allow the generation of few-nanometer sized focused probes or vortices without the need for additional optical elements.

Short-wavelength-infrared upconversion edge enhancement imaging based on a Laguerre-Gaussian composite vortex filter

Edge-enhanced imaging by spiral phase contrast has proven instrumental in revealing phase or amplitude gradients of an object, with notable applications spanning feature extraction, target recognition, and biomedical fields. However, systems deploying spiral phase plates encounter limitations in phase mask modulation, hindering the characterization of the modulation function during image reconstruction. To address this need, we propose and demonstrate an innovative nonlinear reconstruction method using a Laguerre-Gaussian composite vortex filter, which modulates the spectrum of the target. The involved nonlinear process spectrally transforms the incident short-wavelength-infrared (SWIR) signal from 1550 to 864 nm, subsequently captured by a silicon charge-coupled device. Compared with conventional schemes, our novel filtering method effectively suppresses the diffraction noise, significantly enhancing image contrast and resolution. By loading specific phase holograms on the spatial light modulator, bright-field imaging, isotropic, amplitude-controlled anisotropic, and directional second-order edge-enhanced imaging are realized. Anticipated applications for the proposed SWIR edge-enhanced imaging system encompass domains such as artificial intelligence recognition, deep tissue medical diagnostics, and non-destructive defect inspection. These applications underscore the valuable potential of our cutting-edge methodology in furthering both scientific exploration and practical implementations.

Spatial shaping of low- and high-order harmonics generated using vortex beams

We demonstrate the generation of the low- and high-order harmonic vortex beams from a single spiral phase plate illuminated by different laser wavelengths. The second harmonic (532 nm) originates from the application of the wavefront-structured 1064 nm femtosecond pulses with fractional orbital angular momentum (OAM) during propagation through a lithium triborate crystal, while the third harmonic (500 nm) originates from the application of the wavefront-structured near-IR (1500 nm) femtosecond pulses with integer OAM during propagation through a 150 μm thick fused silica plate. The topological charges (TCs) of the second and third harmonics are measured and compared. The increase in TC and the peculiarities in OAM variations during modification of the polarisation of the incident radiation are analysed and discussed. The two-colour-pump-driven second-harmonic vortex radiation interacted with an Ar gas jet to generate vortex harmonics up to the 14th order with double-lobe complex spatial profiles in the extreme ultraviolet region.

Improving millimeter-wave imaging quality using the vortex phase method

This paper investigates a new vortex wave imaging approach to improve the imaging quality of small metal targets of size less than 1.5 mm. Antennas with different spiral phase plates are designed to efficiently transmit vortex beams with orbital angular momentums (OAMs). By analyzing the OAM spectrum of the target, it was discovered that the predominant reflection contains a particular OAM mode that carries abundant azimuthal information. This can be explained by the OAM selectivity of the target and the guidance of the vortex transmitting beam. A simple reflection vortex imaging system was designed to capture the phase information. Measurement results show that the high image contrast reaches 14.9%, which is twice as high as that of the imaging without OAM. Both of simulations and experiments demonstrate that the vortex phase imaging approach proposed in this paper can effectively improve the imaging quality at 80 GHz. This approach is suitable for other millimeter wave imaging systems and is helpful to improve the resolution in anti-terrorism security checks.

Optical vortices by an adaptive spiral phase plate

An Adaptive Spiral Phase Plate (ASPP) based on liquid crystal (LC) and the transmission electrode technique is theoretically and experimentally demonstrated. This ASPP design enables the generation of high-quality optical vortices with topological charges ranging from ±1 to ±4 using a single device, with higher charges being directly achievable by employing higher-birefringence LC or thicker cells. The continuous reconfigurability of the optical phase shift, achieved through a simple control mechanism involving only two low voltages and 100 electrodes that distribute the voltage, sets this device as an accurate approximation to a continuous ASPP. The measured conversion efficiency ranges between almost 100% and 95% for the first and fourth topological charges, respectively. This device offers remarkable advantages, such as complete reconfigurability, allowing adjustment of operating wavelengths and topological charges. The fabrication process mirrors that of a standard LCD cell, ensuring a cost-effective and reliable solution. In summary, the proposed ASPP is a key advancement, providing superior light conversion efficiency, simplicity, and the capability for on-the-fly reconfiguration in various optical applications.