Subwavelength Diffractive Optical Elements Research Articles

Diffractive optical element design based on vector diffraction theory and improved PSO-SA algorithm

The existence of the beam vector effect requires rigorous electromagnetic computational methods for the optimization of subwavelength diffractive optical elements (DOEs). To design DOEs with high performance, we proposed an optimization strategy combining a global optimization algorithm with the finite difference time domain (FDTD) method. First, scalar initial solutions are obtained by the Gerchberg–Saxton algorithm. Then, regarding the diffraction efficiency, the uniformity error and figure of merit are calculated by FDTD as evaluation functions, and the optimization of the initial solutions is implemented by improved particle swarm optimization and continues with the simulated annealing algorithm. After several numerical simulations, the analysis results confirm the robustness of the algorithm and the reduction of the computational cost. The experimental data demonstrate that the correlation of diffraction energy orders between the simulated design and fabricated sample are above 85%. This method efficiently improves the DOEs design.

Subwavelength Diffractive Optical Elements for Generation of Terahertz Coherent Beams with Pre-Given Polarization State.

Coherent terahertz beams with radial polarization of the 1st, 2nd, and 3rd orders have been generated with the use of silicon subwavelength diffractive optical elements (DOEs). Silicon elements were fabricated by a technology similar to the technology used before for the fabrication of DOEs forming laser terahertz beams with pre-given mode content. The beam of the terahertz Novosibirsk Free Electron Laser was used as the illuminating beam. The experimental results are in good agreement with the results of the computer simulation.

Development of subwavelength diffractive optical elements manufacturing process for photonic devices

Subwavelength diffractive optical elements with characteristic dimensions close to the wavelength can implement interesting optical functions, replacing, advantageously, other structures with dimensional features much smaller than the wavelength. The fabrication process becomes easier and the performance is better controlled. A study of a subwavelenght DOE manufacturing process with use of available technologies will be presented.

Diffractive optical elements for the generating cylindrical beams of different orders

The paper proposes a method for calculating the phase function of subwavelength diffractive optical elements. The method is based on diffraction gratings with a varying period for generating vector beams with arbitrary-order cylindrical polarization. Formulas for the phase function of the grating are obtained with due regard for the period variation for increasing the efficiency of the polarization conversion of the incident beam. The obtained phase functions are supposed to be used for creating polarization-conversion diffractive optical elements for noise-resistant optical communication systems.

Vector fuzzy control iterative algorithm for the design of sub-wavelength diffractive optical elements for beam shaping

The vector fuzzy control iterative algorithm (VFCIA) is proposed for the design of phase-only sub-wavelength diffractive optical elements (SWDOEs) for beam shaping. The vector diffraction model put forward by Mansuripur is applied to relate the field distributions between the SWDOE plane and the output plane. Fuzzy control theory is used to decide the constraint method for each iterative process of the algorithm. We have designed a SWDOE that transforms a circular flat-top beam to a square irradiance pattern. Computer design results show that the SWDOE designed by the VFCIA can produce better results than the vector iterative algorithm (VIA). And the finite difference time-domain method (FDTD), a rigorous electromagnetic analysis technique, is used to analyze the designed SWDOE for further confirming the validity of the proposed method.

An improved vector model for analyzing the diffraction field of sub-wavelength diffractive optical elements

An improved vector model to analyze the diffraction field of sub-wavelength diffractive optical elements is proposed. Based on the pioneer model put forward by Prof. M. Mansuripur, the Fresnel formula is further applied in our work to calculate the transmitted light field. A revised model is built, the updated vector diffraction formula is deduced and a computing example is given. Compared with Mansuripur's method, our method leads to results closer to those obtained by finite-difference time domain (FDTD), which is always considered as a more rigorous algorithm. Therefore, this paper provides the practical and fast algorithm for computing the diffraction light field of sub-wavelength diffractive optical elements.

Design of Subwavelength Diffractive Optical Elements using Genetic Algorithm and FDTD Method

We present efficient design of subwavelength diffractive optical elements (DOEs) based on finite-difference time-domain (FDTD) method and optimization by genetic algorithm (GA). Using the GA-FDTD method, we successfully optimize the structure of polarizing beam splitter (PBS) and focusing lens. We also show design of hybrid functional DOEs that double as PBS and lens.

Subwavelength surface-relief gratings for stellar coronagraphy

We present a new design of a phase mask coronagraph implemented with subwavelength diffractive optical elements consisting of optimized surface-relief gratings. Phase mask coronagraphy is a recent technique that seeks to accommodate both high dynamic and high angular resolution imaging of faint sources around bright astrophysical objects such as exoplanets orbiting their host stars. The original design we propose is a new, integrated, and flexible solution to the pi phase-shift chromaticity of the phase mask coronagraphs. It will allow broadband observations, i.e., shorter integration times and object characterizations, by means of spectroscopic analysis. The feasibility of the component manufacturing is also considered through a tolerance study.

Subwavelength gratings for phase mask coronagraphy: the 4QZOG and AGPM coronagraphs

We present two new phase mask coronagraphs implemented with subwavelength diffractive optical elements. The first one is an evolution of the four quadrant phase mask coronagraph (FQPM), which resolves the over the whole K band. These coronagraphs could be used alone on single-pupil telescopes either in space or on the ground (with an adaptive optics system) to detect exoplanets.

Use of subwavelength gratings in TIR incidence as achromatic phase shifters

Nulling interferometry constitutes a very promising technique in observational astrophysics. This method consists in attenuating the signal of a bright astrophysical object in order to detect much fainter nearby features, e.g. exoplanets around their host star. An on-axis destructive interference is created by adjusting the phases of the beams coming from various telescopes. The huge flux ratio between the parent star and the planet (106 in the thermal infrared) requires unprecedented high performance broadband phase shifters. We present a new design for these key components called Achromatic Phase Shifters (APS). We propose to use subwavelength diffractive optical elements under total internal reflection (TIR) incidence. Our component can be seen as an evolution of the Fresnel Rhomb technology.

Perfectly matched layer absorbing boundary conditions in rigorous vector analysis of axially symmetric diffractive optical elements

We extended Berenger’s split-field perfectly matched layer (PML) absorbing boundary conditions (ABCs) to cylindrical coordinates for the use in body-of-revolution finite-difference time-domain (BOR FDTD) method that is suitable for the analysis of axially symmetric subwavelength diffractive optical elements (SWDOE). As application, we use our algorithm in the analysis of a multilevel microlens and its counterpart of binary SWDOE. Numerical results illustrate the efficiency of our PML ABCs.

Subwavelength surface-relief gratings fabricated by microcontact printing of self-assembled monolayers

We have designed and tested subwavelength diffractive optical elements consisting of surface-relief gratings made by microcontact printing of self-assembled monolayers. The first device is a beam deflector for 1.55-mum operation consisting of a surface-relief grating made up of eight pillars over one period (9.3 mum) of the grating. The widths of the pillars vary to approximate a linear phase profile within each grating period. The second device is a quarter-wave plate for 632.8-nm operation consisting of a subwavelength surface-relief grating with a 300-nm period and 58% duty cycle.

Analysis of diffractive optical elements using a nonuniform finite-difference time-domain method

We present the analysis of diffractive optical elements (DOEs) using a two-dimensional nonuniform finite-difference time- domain (FDTD) method. Because the feature sizes in a DOE profile are in general irregular, their analysis using a conventional formulation of the FDTD, i.e., a uniform orthogonal grid, typically requires a high spatial sampling. This in turn raises the computational time and memory requirements for analysis. However, by using a nonuniform grid configuration one can more accurately represent the computation boundary of the DOE, and consequently reduce computational costs. To this end we apply our method to the analysis of both multilevel and subwavelength DOEs to illustrate its utility.

Three-dimensional analysis of subwavelength diffractive optical elements with the finite-difference time-domain method

We present a three-dimensional (3D) analysis of subwavelength diffractive optical elements (DOE's), using the finite-difference time-domain (FDTD) method. To this end we develop and apply efficient 3D FDTD methods that exploit DOE properties, such as symmetry. An axisymmetric method is validated experimentally and is used to validate the more general 3D method. Analyses of subwavelength gratings and lenses, both with and without rotational symmetry, are presented in addition to a 2 x 2 subwavelength focusing array generator.

Vector-based synthesis of finite aperiodic subwavelength diffractive optical elements

We present an optimization-based synthesis algorithm for the design of diffractive optical elements (DOE’s) that are finite in extent, have subwavelength features, and are aperiodic. The subwavelength nature of the DOE’s precludes the use of scalar diffraction theory, and their finite extent and aperiodic nature prevents the use of coupled-wave analysis. To overcome these limitations, we apply the boundary element method (BEM) as the propagation model in the synthesis algorithm. However, the computational costs associated with the conventional implementation of the BEM prevent the design of realistic DOE’s in reasonable time frames. Consequently, an alternative formulation of the BEM that exploits DOE symmetry is developed and implemented on a parallel computer. Designs of finite extent, subwavelength, and aperiodic DOE’s, such as a lens and a focusing beam splitter, are presented.

High-efficiency subwavelength diffractive optical element in GaAs for 975 nm

We have fabricated subwavelength diffractive optical elements with binary phase profiles for operation at 975 nm. The individual surface-relief features of the elements are smaller than the wavelength of light in the material. By modulating the size and spacing of the features we form artificial, gradient, effective index-of-refraction surfaces. The blazed transmission gratings were designed with rigorous coupled-wave analysis and fabricated by direct-write electron-beam lithography and reactive ion-beam etching in GaAs. The gratings have minimum features 63 nm wide. Transmission measurements show 85% diffraction efficiency into the first order.