Phase Zone Plate Research Articles

Chiral metasurface zone plate for transmission-reflection focusing of circularly polarized terahertz waves.

The properties of traditional Fresnel zone plates have been greatly enhanced by metasurfaces, which allow the control of polarization, orbital angular momentum, or other parameters on the basis of focusing. In this Letter, a new, to the best of our knowledge, method for circularly polarized wave manipulation based on a zone plate is proposed. Chiral meta-atoms and binary geometric phase are used for the simultaneous focusing of reflected and transmitted terahertz waves. The silicon-based dielectric chiral units, which show great performance of spin-selective transmission near 0.54 THz, separate the orthogonal circularly polarized components. A binary Pancharatnam-Berry (P-B) phase gradient is obtained by rotating the unit 90 degrees, then the phase zone plate can be easily designed. The simulation results show that the proposed chiral metasurface zone plate has the function of reflection-transmission separation and focusing for the circularly polarized terahertz waves. In addition, we also demonstrate the possibility of using a 1064-nm continuous infrared laser to adjust the intensity of our devices, based on photo-generated carriers in silicon. The design principle of the chiral metasurface zone plates can be extended to other wavelengths, providing new ideas for the regulation of circularly polarized light.

Self-seeded free-electron lasers with orbital angular momentum

X-ray beams carrying orbital angular momentum (OAM) are an emerging tool for probing matter. Optical elements, such as spiral phase plates and zone plates, have been widely used to generate OAM light. However, due to the high impinging intensities, these optics are challenging to use at X-ray free-electron lasers (XFELs). Here, we propose a self-seeded free-electron laser (FEL) method to produce intense X-ray vortices. Unlike passive filtering after amplification, an optical element will be used to introduce the helical phase to the radiation pulse in the linear regime, significantly reducing thermal load on the optical element. The generated OAM pulse is then used as a seed and significantly amplified. Theoretical analysis and numerical simulations demonstrate that the power of the OAM seed pulse can be amplified by more than two orders of magnitude, reaching peak powers of several tens of gigawatts. The proposed method paves the way for high-power and high-repetition-rate OAM pulses of XFEL light.

Ultrashort Laser Pulse Focusing by Amplitude and Phase Zone Plates

In this paper, using the frequency-dependent finite-difference time-domain method, a femtosecond cylindrical vector beam of second-order focusing binary zone plates (BZP) is investigated. It is shown that the relief material has a significant effect on the electromagnetic field formed in the focal plane. It is also shown that, in the case of tight focusing of a second-order cylindrically polarized laser pulse, a reverse energy flux is formed in the focus near the optical axis. For the quartz BZP, the energy backflow is maximum. For aluminum and chromium BZPs, the reverse energy flux is approximately two times less, and there is no energy backflow in the focus formed by the gold BZP. This study will be useful for surface nanostructuring applications where a focused short pulse is applied.

Creating axial bifocal spots by solving the binary phase zone plate in inverse in 4Pi focusing system

We propose an approach to obtain bifocal spots with variable axial distance in 4Pi focusing system. The approach is to inversely solve the radii of a binary phase zone plate (BPZP) according to the conceived 4Pi focal field distribution. The ring radius of the BPZP solved in reverse has an analytic expression by the Richards-Wolf vector diffraction theory, which is directly related to the axial offset distance of the bifocal spots. With the BPZP designed by the analytic expression to modulate the incident radially polarized beam, bifocal spots with variable axial distance can be achieved in the 4Pi focusing region, and two focal spots have the same size. Moreover, some special focal fields, such as optical chain and optical cage, can also be created by selecting suitable axial offset distance to design the BPZP for modulating radially polarized beam. In addition, axial multi-spherical spots can also be generated by superposition of a BPZP and an annular obstruction.

Comparison of the focused optical vortices produced by high-aperture phase conventional and spiral zone plates

Using the FDTD simulation, sharp focusing of a linearly polarized Gaussian beam with an embedded topological charge m = 3 by a phase zone plate and focusing of a Gaussian beam by a phase spiral zone plate with topological charge m = 3 were studied. The obtained results showed that proposed elements formed different patterns of intensity at a focal plane. The spiral zone plate forms a focal spot with three petals. At a distance of 13.5 μm from the focus, the lobe structure of the intensity (and energy flux) is replaced by an annular distribution.

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.

Composite spiral multi-value zone plates.

We present composite spiral multi-value phase zone plates that are achieved by sectioning a spiral multi-value phase zone plate into several radial regions. Each region is composed of specially structured Fresnel zones with optimized phase values and an embedded basic topological charge. In numerical studies, it is shown that the proposed element is capable of producing equal intensity arrays of petal-like modes as well as dark optical ring lattice structures along the optical axis in multiple focal planes of the diffractive element. Additionally, it is demonstrated that the generated petal-like modes can be rotated in a controllable manner by implementing an angular frequency shift between the two composited spiral multi-value phase zone plates. We also illustrate that the rotation angle is independent of the diffraction order. Experimental results are included to verify the theoretical outcomes, where the phase pattern of the composite spiral multi-value zone plate is encoded onto a spatial light modulator.

Compact radial shearing interferometer with a randomly encoded cosinusoidal zone plate for wavefront measurements.

A radial shearing interferometer (RSI) using a randomly encoded cosinusoidal zone plate (RECZP) to measure the wavefront is proposed. The RECZP has two foci, i.e., a virtual focus and a real focus, so its Fresnel diffractions contain only two beams. These two beams can be regarded as the extended beam and the contracted beam in the RSI, respectively. This RSI is composed of a RECZP and a charge-coupled device (CCD). The radial shearing rate is continuously adjustable by changing the distance between the CCD and RECZP, which is good for measurement sensitivity and dynamic range for different situation requirements. In the simulation experiment, we analyzed the influence of beam tilt error, distance error of zone plate and CCD, CCD camera nonlinearity, and noise on wavefront reconstruction results. We also analyzed the effects of different fabrication errors (randomly encoded principle error, sidewall angle error, depth error, and alignment error of amplitude zone plate and phase zone plate) on the diffraction intensity distributions, which determine the fabrication tolerance of the RECZP. Experimentally compared with a ZYGO interferometer, the RECZP-RSI optical system can get good results.

Multifocal multi-value phase zone plate for 3D focusing.

We demonstrate a method for creating a three-dimensional (3D) array of focal spots by combination of a multi-focal diffractive lens and a two-dimensional multi-value phase grating. The multi-focal Fresnel-based lens is created by means of encoding special nonlinearities into the phase structure of a Fresnel zone plate and is represented as a mathematical superposition of this phase function with a refractive lens. The imposed nonlinearity type enables the creation of multiple focal spots with uniform intensity along the optical axis. We demonstrate the example of a 3D multi-value phase grating, which creates five focal planes with a $5 \times 5$5×5 transverse array of focal spots with equal energy distribution in each plane. Experimental results are included to verify the theoretical outcomes, where the phase pattern of a 3D multi-value phase grating is encoded onto a spatial light modulator.

Comparative simulation of spiral zone plates with a relief from silica glass and silver

Comparative simulation of optical vortices generation by spiral zone plates with a relief from silver and silica glass using a frequency depended finite difference time domain method. The right circularly polarized Gaussian pulse was used as incident light. An analysis of the Umov-Poynting vector, amplitude and phase distributions in the focal plane shows was the presence of complex optical vortices with an inverse energy flow in its’ centre. Influence of the amplitude spiral zone plate relief height on energy backflow is studied. Comparison of simulation results for two types of spiral zone plate confirms the possibility of using the amplitude analogue instead of the phase zone plate since it is simpler to manufacture.

Formation and Characterization of Hole Nanopattern on Photoresist Layer by Scanning Near-Field Optical Microscope.

Patterning of lines of holes on a layer of positive photoresist SX AR-P 3500/6 (Allresist GmbH, Strausberg, Germany) spin-coated on a quartz substrate is carried out by using scanning near-field optical lithography. A green 532 nm-wavelength laser, focused on a backside of a nanoprobe of 90 nm diameter, is used as a light source. As a result, after optimization of parameters like laser power, exposure time, or sleep time, it is confirmed that it is possible to obtain a uniform nanopattern structure in the photoresist layer. In addition, the lines of holes are characterized by a uniform depth (71–87 nm) and relatively high aspect ratio ranging from 0.22 to 0.26. Numerical modelling performed with a rigorous method shows that such a structure can be potentially used as a phase zone plate.

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.

Fibonacci terahertz imaging by silicon diffractive optics.

Fibonacci or bifocal terahertz (THz) imaging is demonstrated experimentally employing a silicon diffractive zone plate in continuous wave mode. Images simultaneously recorded in two different planes are exhibited at 0.6THz frequency with the spatial resolution of wavelength. Multifocus imaging operation of the Fibonacci lens is compared with a performance of the conventional silicon phase zone plate. Spatial profiles and focal depth features are discussed varying the frequency from 0.3 to 0.6THz. Good agreement between experimental results and simulation data is revealed.

Modeling of the optical vortex generation using a silver spiral zone plate

Using a FDTD-method implemented in the FullWAVE (RSoft) software, a process of generating optical vortices during the propagation of a left- or right-hand circularly polarized Gaussian pulse through a phase and amplitude spiral zone plate with topological charge 2, a diameter of 8 μm, and a focal length of 532 nm is numerically modeled. An investigation of the amplitude and phase distributions in the focal plane shows the presence of optical vortices with topological charge 2. Analysis of the longitudinal component of the Umov-Poynting vector shows the presence of an inverse energy flow in the focal plane. In the case of using an amplitude spiral zone plate, the magnitude of the reverse flow decreases sharply until it disappears completely for the left circularly polarized incident beams.

Zone Plate Fabrication Using a Low Power Femtosecond Laser

With an advantage in photonic integration and device miniaturization, fabrication of micron sized diffractive optical elements has gained attention due to their low volume, light weight and flexible design. We discuss the fabrication of phase zone plates in soda-lime and borosilicate glass using a femtosecond laser, and its characterization. Our technique of fabrication involves a one-step laser direct writing process in comparison to the popular photolithography based micro-structuring. In this work, the femtosecond laser oscillator system is used to successfully fabricate phase zone plate structures at low energies and high repetition rate. In addition to this, a laser induced breakdown spectroscopy (LIBS) study was done to confirm the consistency of glass obtained from different manufacturers through its composition

High efficiency positive and negative phase zone plates

We design a new kind of phase zone plates (PZPs) to improve the diffraction efficiency of soft x ray zone plates (ZPs). The design replaces blank parts of PZPs with metals of negative phase shift at the working energy, which is called as the positive and negative PZPs (PNPZPs). According to the calculation, PNPZPs have a higher maximum efficiency than conventional ZPs with the same zone width. With the help of a negative phase coefficient, it is much easier to achieve a π phase shift in one period, resulting in a smaller zone height. This design can help fabricate finer PZPs to achieve a better image resolution.

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.

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.

Comparative modeling of amplitude and phase zone plates

In this paper using FDTD-method, we conduct a comparative modeling of a sharp focus of the laser light by using binary-phase and binary-amplitude Fresnel zone plates, with the latter being fabricated as thin metal films of 20-30-nm thickness. Subwavelength local maximums are found near the rings of the silver zone plates, with their intensity being comparable with the intensity in the focus. They can be explained by the excitation of surface plasmons.

Novel Temporal Zone Plate Designs With Improved Energy Efficiency and Noise Performance

We propose and demonstrate novel temporal zone plate designs offering a higher energy efficiency and an improved background noise performance than previously proposed approaches using similar experimental configurations. Two kinds of novel temporal zone plates, namely, temporal amplitude zone plates and temporal phase Fresnel lenses, are introduced and studied. The proposed designs are demonstrated for optical linear pulse compression. In the reported experiments, temporal amplitude zone plates are shown to provide a 96% improvement in energy efficiency (or light-collecting efficiency) and significantly reduce noise background as compared with equivalent temporal intensity zone plates. On the other hand, a temporal phase Fresnel lens is shown to offer an ideal light-collecting efficiency of 100%, greatly exceeding the efficiency of an equivalent temporal phase zone plate. Performance in a practical setup is however limited by the achievable modulation bandwidth and a 37% efficiency improvement over previous designs is experimentally demonstrated.