Fresnel Phase Zone Plate Research Articles

Silicon Fresnel Zone Plate Metalens with Subwavelength Gratings.

Metalenses are planar optical components that have demonstrated immense potential for integrated optics. In particular, they are capable of high-efficiency subwavelength focusing without the bulkiness of traditional lenses. Dielectric metalenses operating in the C-band typically employ relatively tall, amorphous silicon structures arranged in a periodic array. Phase control spanning from 0 to 2π is accessed by varying the geometry of these scattering structures. The full 2π phase range is necessary to impose a hyperbolic focusing phase profile, but this is difficult to achieve without custom fabrication practices. In this work, we propose a binary phase Fresnel zone plate metalens designed for the standard 500 nm silicon-on-insulator platform. Our design uses subwavelength gratings with trapezoidal segmentation to form concentric rings. The effective index of the grating is set with the duty cycle using a single full-etch step to form the binary phase profile of the zone plate. The metalens design can be easily tuned to achieve longer focal lengths at different wavelengths. It offers a simple platform for high-throughput wavelength-scale focusing elements in free-space optics, including for microscopy and medical imaging.

Chromatic confocal measurement method using a phase Fresnel zone plate.

A chromatic confocal measurement method based on a phase Fresnel zone plate (FZP) is described. Strong dispersion of FZP results in significant axial focal shift. The axial dispersion curve is close to linear within a certain wavelength range determined by the quantitative calculation using the vectorial angular spectrum theory. A 11.27 mm diameter phase FZP with a primary focal length of 50 mm was processed using standard photolithography technology and used as the dispersive objective in a homemade chromatic confocal measurement system. The calibrated axial measurement range exceeds 16 mm, the axial resolution reaches 0.8 µm, and the measurement accuracy of displacement is better than 0.4%. This chromatic confocal sensor has been practically used in the measurement of step height, glass thickness, and 3D surface profile. The proposed method has the obvious characteristics of simplicity, greatly reduced cost and superior performance. It is believed that this sensing method has broad application prospects in glass, coating, machinery, electronics, optics and other industries.

Achromatic focusing effect of metasurface-based binary phase Fresnel zone plate

Traditional binary phase Fresnel zone plate (BPFZP) fails to provide a single-layered solution for multi-wavelength achromatic focusing. In this article, by combining metasurfaces and BPFZP model, we propose an ultra-thin single-layered transmissive achromatic phase Fresnel zone plate (PFZP) with large numerical aperture (NA) working in the visible range that is hardly possible using traditional Fresnel zone plate. We present designs of both monochromatic and achromatic BPFZP displaying almost equal simplicity despite lens size or NA while possessing moderate focusing efficiency attributed to the BPFZP model. The achromatic BPFZP designed consists of periodic cylindrical TiO2 nanopillars with thickness of 375 nm, holds NA of 0.97 and experiences average focal distance deviation of 8%. The circularly-symmetric composing antennas also provide the achromatic BPFZP with polarization-independence performance. This design method with certain error robustness makes it feasible for large-scale fabrication and compact integration with CCD/CMOS or fiber devices for future multi-wavelength applications with high resolution.

Polarization-Independent Metasurface Lens Based on Binary Phase Fresnel Zone Plate.

Based on the binary phase Fresnel zone plate (FZP), a polarization-independent metasurface lens that is able to focus incident light with any polarization state, including circular, linear, and elliptical polarizations, has been proposed and investigated. We demonstrate that the metasurface lens consisting of metal subwavelength slits can operate in a wide bandwidth in the visible range, and has a higher focusing efficiency than that of an amplitude FZP lens without phase modulation. A multi-focus FZP metasurface lens has also been designed and investigated. The proposed lens can provide potential applications in integrated nanophotonic devices without polarization limitations.

Flash X-ray backlight technique using a Fresnel phase zone plate for measuring interfacial instability

Interfacial instabilities in high-energy-density plasmas are important topics in various research fields such as inertial confinement fusion, planetary sciences, and astrophysics. The growth of the instabilities can be examined in the laboratory by using high-power lasers coupled with high-resolution imaging technique. The resolutions both in space and time are essential for the observation of tiny and transient phenomena in high-energy-density plasmas. Here, the growth of a sinusoidal corrugation on the surface of a polystyrene foil during the laser-driven Richtmyer-Meshkov instability was measured by high-resolution X-ray shadowgraphy. In our experiment, a high-intensity short-pulse laser produced the X-ray flash that ensures a better temporal resolution. A Fresnel Phase Zone Plate was used for the improvement of spatial resolution, which realized an accuracy of 5.0 ± 1.0 µm in our system.

Two-channel high-resolution quasi-monochromatic X-ray imager for Al and Ti plasma.

High-resolution, high-sensitivity X-ray imaging is a real challenge in high-energy density plasma experiments. We present an improved design of the Fresnel ultra high-resolution imager instrument. Using an Ultra-High-Intensity (UHI) laser to generate hot and dense plasma in a small volume of an Al-Ti mixed target provides simultaneous imaging of both Al and Ti X-ray emission. Specifically, the Al Heβ (or Lyβ) and the Ti Heα lines are imaged with a resolution of (2.7 ± 0.3) μm and (5.5 ± 0.3) μm, respectively. It features two transmission Fresnel phase zone plates fabricated on the same substrate, each associated with a multilayer mirror for spectral selection. Their spatial resolution has been measured on the PTB synchrotron radiation facility laboratory at BESSY II and on the EQUINOX laser facility. Results obtained on an UHI experiment highlight the difference of emission zone sizes between Al and Ti lines and the versatility of this instrument.

Electrically tunable binary phase Fresnel lens based on a dielectric elastomer actuator.

We propose and demonstrate an all-solid-state tunable binary phase Fresnel lens with electrically controllable focal length. The lens is composed of a binary phase Fresnel zone plate, a circular acrylic frame, and a dielectric elastomer (DE) actuator which is made of a thin DE layer and two compliant electrodes using silver nanowires. Under electric potential, the actuator produces in-plane deformation in a radial direction that can compress the Fresnel zones. The electrically-induced deformation compresses the Fresnel zones to be contracted as high as 9.1% and changes the focal length, getting shorter from 20.0 cm to 14.5 cm. The measured change in the focal length of the fabricated lens is consistent with the result estimated from numerical simulation.

High-resolution quasi-monochromatic X-ray imaging using a Fresnel phase zone plate and a multilayer mirror.

High-resolution, high-sensitivity X-ray imaging is a real challenge in laser plasma diagnostic to attain reliable data in high-energy density plasma experiments. In this context, ultra-high-intensity lasers generate hot and dense plasma but only in a small volume. An experiment has been performed at the LULI2000 laser facility to diagnose such plasma conditions from thermal spectroscopic data. To image the emission zone plasma's Al Heβ, a Fresnel-lens-based X-ray imager has been developed. It features a 846 μm-diameter Fresnel Phase Zone Plate (FPZP) and a Pd/B4C multilayer mirror (thickness d = 5.1 nm). This association can be used between 1500 eV and 2100 eV. The FPZP's efficiency was measured on a synchrotron facility (SOLEIL) and its spatial resolution in a laser facility (EQUINOX). The mirror reflectivity was measured on the synchrotron facility BESSY II. With experimental conditions, the system resolution reaches 3.8 ± 0.6 μm with an adequate efficiency in the 1800 eV-1900 eV energy range with a solid angle of 9 × 10-6 sr. Consequently, a FPZP is an excellent optics setup for high-resolution quasi-monochromatic X-ray imaging and provides a good collection angle. Bragg-Fresnel lenses, based on the principle of FPZP and mirrors, are currently designed for an X-ray imager at the Laser MégaJoule facility.

Image restoration in diffractive optical systems using deep learning and deconvolution

In recent years, several pioneering works were dedicated to imaging systems based on simple diffractive structures like Fresnel lenses or phase zone plates. Such systems are much lighter and cheaper than classical refractive optical systems. However, the quality of images obtained by diffractive optics suffers from stronger distortions of various types. In this paper, we show that a combination of the high-precision lens design with post-capture computational reconstruction allows one to attain a much higher image quality. The proposed reconstruction procedure uses a sequence of color correction, deconvolution, and a feedforward deep learning neural network. An improvement both in lens manufacturing and in image processing may contribute to the emergence of ultra-lightweight imaging systems varying from cameras for nano- and picosatellites to surveillance systems.

Two-dimensional X-ray focusing by off-axis grazing incidence phase Fresnel zone plate on the laboratory X-ray source

The results of studying a two-dimensional X-ray focusing by an off-axis grazing incidence phase Fresnel zone plate on the laboratory X-ray source are presented. This optics enables obtaining a focal spot of ~2µm on the laboratory X-ray source with a focusing efficiency of ~30% at a high signal/noise ratio.

Remarkable optical coupling enhancement with laser selective focusing devices

In this paper, Fresnel zone plates (FZPs) have been employed as optical coupling devices to enhance the response of versatile laser detection systems. Two kinds of FZPs, amplitude-Fresnel zone plate (AFZP) and phase-Fresnel zone plate (PFZP), were fabricated and studied for comparison. The simulation and experiment results indicate that the FZPs have selective focusing abilities for wavelength and coherence of the incident lights, which contributes to high collection efficiency to the laser signals as well as suppression to the ambient lights. A 34 and 26 times gain in response were obtained when illuminated by coherent laser light by applying an AFZP and PFZP to an InGaAs/InP p-i-n photodetector, respectively. And a 30 % reduction in response was achieved by using an AFZP when illuminated by incoherent Tungsten-halogen lamp. In addition, 9.6 times of response enhancement has been observed after utilizing the AFZP for the laser ranging system.

Diffraction theory of high numerical aperture subwavelength circular binary phase Fresnel zone plate.

An analytical model of vector formalism is proposed to investigate the diffraction of high numerical aperture subwavelength circular binary phase Fresnel zone plate (FZP). In the proposed model, the scattering on the FZP's surface, reflection and refraction within groove zones are considered and diffraction fields are calculated using the vector Rayleigh-Sommerfeld integral. The numerical results obtained by the proposed phase thick FZP (TFZP) model show a good agreement with those obtained by the finite-difference time-domain (FDTD) method within the effective extent of etch depth. The optimal etch depths predicted by both methods are approximately equal. The analytical TFZP model is very useful for designing a phase and hybrid amplitude-phase FZP with high-NA and short focal length.

Effect of the shadowing in high-numerical-aperture binary phase Fresnel zone plates

Shadowing occurs for the groove zone plate when radiation origin from one zone of the zone plate must pass through another before reaching the focus point. Using the vector diffraction theory, we study the influence of the shadowing on the focusing properties of binary phase Fresnel zone plates (FZPs). The results show that a low-numerical-aperture (NA) FZP can be treated as an ideal in-plane FZP and the shadowing effect can be neglected. For a high-NA FZP, however, the shadowing effect from the etch depth has to be considered. Owing to the shadowing, the intensity of focusing spot decreases markedly and the size of focusing spot increases slightly in a high-NA FZP. The optimal etching depth of an actual FZP is smaller than that of an ideal FZP.

Analysis of near-field subwavelength focusing of hybrid amplitude–phase Fresnel zone plates under radially polarized illumination

The near-field focusing properties of actual hybrid amplitude–phase binary subwavelength Fresnel zone plates (HBSFZPs) are studied theoretically. The analysis based on the exact vector angular spectrum method is done for a radially polarized beam incident on the HBSFZPs. The results show that the near-field subwavelength focusing with a long depth of focus can be obtained using an HBSFZP, which is very useful for near-field subwavelength photolithography and high-resolution microscopy. The position of the actual focus in the near-field focusing HBSFZPs depends on the evanescent wave rather than the propagating wave. The etch depth has an important influence on focusing properties of HBSFZPs.

Analysis of imaging an extended X-ray source by using a Fresnel phase zone plate

To image a large-size object with a high spatial resolution in a kiloelectronvolt (keV) X-ray range, a method is proposed to analyze and simulate the imaging of an extended X-ray source by a Fresnel phase zone plate (FPZP), based on the translational invariance of the point spread function in a 1 mm square area on the objective plane. Using this method, the imaging of an extended source of a different size is simulated under a typical experimental condition of image-to-source magnification of 10 for an FPZP of an outmost zone width of 0.35 μm. The results show that the image contrast decreases with the increase of the source size, and the zeroth-order and the minus first-order diffractions of the FPZP contribute mainly to the image background enhancement and the contrast decrease. The spatial resolution to the objective plane is also found to be reduced. For a 1-mm-square-shape source with a sinusoidal-distribution intensity modulation of contrast 1, the image modulation contrast is below 0.4, and the spatial resolution is 0.75 μm.

Multiple beam splitter using volumetric multiplexed Fresnel zone plates fabricated by ultrafast laser-writing

A simple approach to producing a 1×N beam splitter is demonstrated by fabricating a volumetric multiplex phase Fresnel zone plate in bulk transparent material. This comprised four layers of zone plates created in borosilicate glass by femtosecond laser direct-writing, each shifted laterally relative to the illumination axis, creating four separate beamlets. Since the power transmitted in each beamlet is proportional to the diffraction efficiency of the corresponding zone plate, the power ratios can be customized by adjusting the fabrication parameters of each layer. This approach demonstrates the potential of femtosecond laser direct-writing for the fabrication of complex optical elements in transparent media as components of integrated monolithic photonic devices.

Tight focusing of partially polarized vortex beams by binary phase Fresnel zone plates

Based on vectorial Debye theory, the focusing properties of partially polarized vortex beam by high numerical aperture Fresnel zone plate are investigated. The effects of the numerical apertures of and the phase difference of binary phase Fresnel zone plates, the topological charge of vortex beam and the degree of polarization of incident beam on the intensity distribution and degree of coherence in the focal plane are investigated in detail. It is shown that elliptical light spots and the flat top beam can be obtained by selecting certain parameters. Studies of degree of coherence reveal that the degree of coherence between x and y components of the electric field, which is zero in the source plane, is improved in the focal plane for vortex beam, but it is hardly changed for the nonvortex beam. It is also proved that any two of the three electric field components Ex, Ey and Ez are completely coherent everywhere in the focal region if the incident light beam is linearly polarized.

Angular momentum conversion of elliptically polarized beams focused by high numerical-aperture phase Fresnel zone plates

Based on the vectorial Debye theory, we investigate the tight focusing of elliptically polarized light beams by high numerical-aperture (NA) phase Fresnel zone plates (FZP). The conversion of the spin angular momentum (SAM) and the orbital angular momentum (OAM) in the tight focusing of the elliptically polarized beams is investigated in great detail. It is shown that the direction and magnitude of the OAM can be directly modulated by the input polarization, providing effective evidence that the SAM carried by an elliptically polarized light beam is converted into OAM in the tight focusing process. The properties of phase retardation between x and y components of the focused field are also investigated. It is found that focused field of x and y components still keeps its original elliptical polarization state, indicating that the focused field composed of x and y components still carries SAM, whereas the focused field of z component is changed into carrying OAM. The influences of the total zone number and the phase difference of binary phase FZPs with high NA on the intensity distribution in the focal plane are also investigated.

Subwavelength focusing behavior of high numerical-aperture phase Fresnel zone plates under various polarization states

An analytical model is developed to study the subwavelength focusing characteristics of a binary phase Fresnel zone plate (FZP). The model shows that high numerical-aperture phase FZP under the illumination of linear polarized light produces rotationally asymmetric focal spot with beamwidth varying from 0.36λ to 0.79λ, where λ is the wavelength. On the other hand, rotationally symmetric focal spot with minimum beamwidth of 0.39λ can be obtained from the illumination of radial polarized light.

Generation of Soft X-ray Submicron Beam Using Fresnel Phase Zone Plate

A submicron soft X-ray beam is generated by a highly coherent soft X-ray laser at 13.9 nm with a Fresnel phase zone plate. The focused X-ray beam profile is measured using a knife-edge scan method, and the spot diameter and peak intensity are 0.5 µm at full width at half maximum and 1 ×1011 W/cm2, respectively.