Diffractive Filters Research Articles

Asymmetric Diffraction in Plasmonic Meta-Gratings Using an IT-Shaped Nanoslit Array.

Diffraction is a fundamental phenomenon that reveals the wave nature of light. When a plane wave is transmitted or reflected from a grating or other periodic structures, diffracted light waves propagate at several angles that are specified by the period of the given structure. When the optical period is shorter than the wavelength, constructive interference of diffracted light rays from the subwavelength-scale grating forms a uniform plane wave. Many studies have shown that through the appropriate design of meta-atom geometry, metasurfaces can be used to control light properties. However, most semitransparent metasurfaces are designed to perform symmetric operation with regard to diffraction, meaning that light diffraction occurs identically for front- and back-side illumination. We propose a simple single-layer plasmonic metasurface that achieves asymmetric diffraction by optimizing the transmission phase from two types of nanoslits with I- and T-shaped structures. As the proposed structure is designed to have a different effective period for each observation side, it is either diffractive or nondiffractive depending on the direction of observation. The designed structure exhibits a diffraction angle of 54°, which can be further tuned by applying different period conditions. We expect the proposed asymmetric diffraction meta-grating to have great potential for the miniaturized optical diffraction control systems in the infrared band and compact optical diffraction filters for integrated optics.

Spectral Fresnel filter for pulsed broadband terahertz radiation

We proposed a simple and cost-effective method to manipulate the temporal and spectral properties of pulsed terahertz waves. A deep modulation of a pulse spectrum was both numerically and experimentally verified using Fresnel apertures with a radius ranging from several to several tens of the central wavelength of the broadband terahertz radiation. N-fold frequency minima were formed in the spectrum at a specific axial position behind the filter. Non-paraxial properties of this filter were also analyzed. A significant value (35%) of the ratio of the longitudinal to the transverse field component at the filter frequency was obtained. The measured results agree well with the simulation and theoretical predictions. The property of such a diffractive Fresnel notch filter can benefit the generation of longitudinal terahertz fields and relevant applications.

Wavefront Aberration Sensor Based on a Multichannel Diffractive Optical Element.

We propose a new type of a wavefront aberration sensor, that is, a Zernike matched multichannel diffractive optical filter, which performs consistent filtering of phase distributions corresponding to Zernike polynomials. The sensitivity of the new sensor is theoretically estimated. Based on the theory, we develop recommendations for its application. Test wavefronts formed using a spatial light modulator are experimentally investigated. The applicability of the new sensor for the fine-tuning of a laser collimator is assessed.

Multifunctional volumetric meta-optics for color and polarization image sensors

Three-dimensional elements, with refractive index distribution structured at subwavelength scale, provide an expansive optical design space that can be harnessed for demonstrating multifunctional free-space optical devices. Here we present 3D dielectric elements, designed to be placed on top of the pixels of image sensors, that sort and focus light based on its color and polarization with efficiency significantly surpassing 2D absorptive and diffractive filters. The devices are designed via iterative gradient-based optimization to account for multiple target functions while ensuring compatibility with existing nanofabrication processes, and they are experimentally validated using a scaled device that operates at microwave frequencies. This approach combines arbitrary functions into a single compact element, even where there is no known equivalent in bulk optics, enabling novel integrated photonic applications.

Computational multispectral video imaging [Invited

Multispectral imagers reveal information unperceivable to humans and conventional cameras. Here, we demonstrate a compact single-shot multispectral video-imaging camera by placing a micro-structured diffractive filter in close proximity to the image sensor. The diffractive filter converts spectral information to a spatial code on the sensor pixels. Following a calibration step, this code can be inverted via regularization-based linear algebra to compute the multispectral image. We experimentally demonstrated spectral resolution of 9.6nm within the visible band (430-718nm). We further show that the spatial resolution is enhanced by over 30% compared with the case without the diffractive filter. We also demonstrate Vis-IR imaging with the same sensor. Because no absorptive color filters are utilized, sensitivity is preserved as well. Finally, the diffractive filters can be easily manufactured using optical lithography and replication techniques.

Novel optical filters based on curved grating structure

A novel modified Rowland grating structure is proposed in this paper. Optical filters with the proposed structure are designed and fabricated with both high input and output angles. The passband width, coupling loss of the filters are investigated as a function of the output waveguide width. Nearly aberration free diffraction filters with an ultracompact footprint less than 0.5mm2 were obtained with the proposed structure.

Technique to separate lidar signal and sunlight

Sunlight contamination dominates the backscatter noise in space-based lidar measurements during daytime. The background scattered sunlight is highly variable and dependent upon the surface and atmospheric albedo. The scattered sunlight contribution to noise increases over land and snow surfaces where surface albedos are high and thus overwhelm lidar backscatter from optically thin atmospheric constituents like aerosols and thin clouds. In this work, we developed a novel lidar remote sensing concept that potentially can eliminate sunlight induced noise. The new lidar concept requires: (1) a transmitted laser light that carries orbital angular momentum (OAM); and (2) a photon sieve (PS) diffractive filter that separates scattered sunlight from laser light backscattered from the atmosphere, ocean and solid surfaces. The method is based on numerical modeling of the focusing of Laguerre-Gaussian (LG) laser beam and plane-wave light by a PS. The model results show that after passing through a PS, laser light that carries the OAM is focused on a ring (called "focal ring" here) on the focal plane of the PS filter, very little energy arrives at the center of the focal plane. However, scattered sunlight, as a plane wave without the OAM, focuses at the center of the focal plane and thus can be effectively blocked or ducted out. We also find that the radius of the "focal ring" increases with the increase of azimuthal mode (L) of LG laser light, thus increasing L can more effectively separate the lidar signal away from the sunlight noise.

Microscope system with on axis programmable Fourier transform filtering

We propose an on-axis microscope optical system to implement programmable optical Fourier transform image processing operations, taking advantage of phase and polarization modulation of a liquid crystal on silicon (LCOS) display. We use a Hamamatsu spatial light modulator (SLM), free of flickering, which therefore can be tuned to fully eliminate the zero order component of the encoded diffractive filter. This allows the realization of filtering operation on axis (as opposed to other systems in the literature that require operating off axis), therefore making use of the full space bandwidth provided by the SLM. The system is first demonstrated by implementing different optical processing operations based on phase-only blazed gratings such as phase contrast, band-pass filtering, or additive and substractive imaging. Then, a simple Differential interference contrast (DIC) imaging is obtained changing to a polarization modulation scheme, achieved simply by selecting a different incident state of polarization on the incident beam.

Broadband Gerchberg-Saxton algorithm for freeform diffractive spectral filter design.

A multi-wavelength expansion of the Gerchberg-Saxton (GS) algorithm is developed to design and optimize a surface relief Diffractive Optical Element (DOE). The DOE simultaneously diffracts distinct wavelength bands into separate target regions. A description of the algorithm is provided, and parameters that affect filter performance are examined. Performance is based on the spectral power collected within specified regions on a receiver plane. The modified GS algorithm is used to design spectrum splitting optics for CdSe and Si photovoltaic (PV) cells. The DOE has average optical efficiency of 87.5% over the spectral bands of interest (400-710 nm and 710-1100 nm). Simulated PV conversion efficiency is 37.7%, which is 29.3% higher than the efficiency of the better performing PV cell without spectrum splitting optics.

Tuneable broadband optical filter based on soft-composite materials

We report on the realization and characterization of a ‘free space’ diffractive optical filter based on a periodic structure realized in soft composite materials. By combining a high stability optical holographic setup with a light sensitive composite mixture, we have realized a structure made of polymeric slices alternated to pure, well aligned, nematic liquid crystals. Its polarization dependent diffractive properties have been exploited for realizing a diffractive, band-gap based, broadband optical filter. The sample is characterized in terms of its morphological, optical and electro-optical properties. We show that both electric fields and temperature variations can be exploited to tune the position of the diffractive band-gap of about 83 nm and 116 nm respectively.

Tunable diffractive filters for robust NIR and IR spectroscopy

Three different types of specialized spectrometers have been implemented. These sensors are well suited for gas detection or other industrial applications where long-term stability is a key requirement. These sensors are based on tunable diffractive filters that were developed in the past and that we have now adapted to fit into the presented spectrometers, each of them developed to perform a dedicated measurement. The diffractive filters share the common feature that a single surface hologram integrates the functions of lenses, beam splitters and spectral filters. In order to perform a compensated measurement, it is possible to switch between several filter functions, which can be done without the need for complicated or precise mechanical control. These features make it possible for the spectrometers to be made robust against long-term drift, which eliminates the need for frequent recalibration and maintenance. In addition, the simplicity of the sensor assembly makes the sensors well suited for a wide range of applications; the requirement is either a low production cost or robustness to harsh industrial environment. The sensors are or have been industrialized, with application mainly for gas spectroscopy. Results in terms of sensitivity and long-term drift were obtained in the field and are presented.

Specular properties of chemically etched polyethylene terephthalate foils in the soft X-ray region

Results from angular measurements of reflections of radiation at a wavelength of 13.5 nm of poly-ethyleneterephthalate (PETP) foils before and after chemical etching are described. The observed reduction in reflections at grazing incidence in contrast to reflections from ideal smooth sample is due to the characteristics of the surface microrelief of the foils and its changes due to etching. The results of this work can be used to study the spectral transmission of X-ray and UV diffractive filters made on the basis of track membranes.

On the role of scattering and absorption edges in waveguide collimation of X-rays

The role of incoherent scattering in damping of transmitted radiation is studied on the basis of the statistical theory of X-ray scattering in a narrow rough collimator. Approximate expressions for the damping coefficients of waveguide modes are obtained, and damping associated with scattering is found to be proportional to the product of a root-mean-square amplitude of roughnesses by the correlation length. The effect of a considerable suppression of scattering near the absorption edges due to dispersion is predicted; this can also be observed from a decrease in the back-scattered beam intensity. The roughnesses not only reduce the radiation transmission by diffractive filters, but also adversely affect the selectivity of radiation filtering. The analytic expressions for the damping coefficients of waveguide modes make it possible to generalize the results to collimators with various properties of the surfaces.

Finite element method as applied to the study of gratings embedded in complementary metal-oxide semiconductor image sensors

We present a new formulation of the finite element method (FEM) dedicated to the rigorous solution of Maxwell's equations and adapted to the calculation of the scalar diffracted field in optoelectronic subwavelength periodic structures [for both transverse electric (TE) and transverse magnetic (TM) polarization cases]. The advantage of this method is that its implementation remains independent of the number of layers in the structure, the number of diffractive patterns, the geometry of the diffractive object, and the properties of materials. The spectral response of large test photodiodes that can legitimately be represented in 2-D has been measured on a dedicated optical bench and compared to the theory. The validity of the model as well as the possibility of conceiving in this way simple processible diffractive spectral filters are discussed.

Track membranes with open pores used as diffractive filters for space-based x-ray and EUV solar observations

We describe the fabrication and performance of diffractive filters designed for space-based x-ray and EUV solar observations. Unlike traditional thin film filters, diffractive filters can be made to have a high resistance against the destructive mechanical and acoustic loads of a satellite launch. The filters studied are made of plastic track-etched membranes that are metal-coated on one side only. They have all-through open cylindrical pores with diameters as small as 500 nm, limiting their transmittance to very short wavelengths. The spectral transmittance of various diffractive filters with different pore parameters was measured from the soft x-ray to the near IR range (namely, from 1-1100 nm).

X-ray diffraction filters based on track membranes

The principle of operation of X-ray filters made of polymer track membranes with through cylindrical pores is described. The relationship between the transmission of track membranes and the parameters of pores and optical constants of the polymer matrix is established. The way in which the interface roughness improves the spectral selectivity of diffraction filters is recognized.

Convex objective function-based design method developed for minimizing side lobe

The existence of multiple local solutions makes it very difficult to search for filter parameters to achieve a desired side lobe level during the design of superresolution pupil filters. To deal with the difficult issue of side lobe control in the designing process, a convex objective function-based design method is developed through phase rotation and variable replacement to transform the complicated solving process with multiextreme subintervals into a simple optimization process with a convex interval. A group of constant annular complex superresolving filters are designed using the developed method. The comparison of the superresolving filters designed in this way with the well-known continuous phase filter and 3-zone multiphase diffractive superresolution filters proves the validity of the developed method.

Magnetostatic interactions between carbon nanotubes filled with magnetic nanoparticles

The magnetostatic interactions between carbon nanotubes filled with magnetic particles have been experimentally and theoretically studied. By making nanotubes uniformly magnetized, one eliminates the attraction caused by periodicity of nanoparticles in magnetic chains. The discreteness of individual nanoparticles in the nanoneedles is not observed and these nanoneedles interact by their magnetic poles. Since the attraction/repulsion events are predictable, the suspensions of magnetic nanotubes are attractive candidates for active elements in changeable diffraction gratings, filters, and polarizers.

Influence of the divergence of a light beam on the characteristics of collinear diffraction

Two types of anisotropic collinear diffractions of light on a diffraction grating are theoretically investigated, including that where the light is scattered forward in the direction of the incident radiation and that where the light is scattered backward in the direction toward the incident radiation. For both types, two-dimensional transfer functions are calculated, and the character of their transformation upon variation of the light wavelength and the period of the diffraction grating is analyzed. The dependence of the integrated diffraction efficiency and transmission band of diffraction filters on the divergence angle of the light beam is studied.

Correctability limitations imposed by plane-wave scintillation in multiconjugate adaptive optics

Plane-wave scintillation is shown to impose multiconjugate adaptive optics (MCAO) correctability limitations that are independent of wavefront sensing and reconstruction. Residual phase and log-amplitude variances induced by scintillation in weak turbulence are derived using linear (diffraction-based) diffractive MCAO spatial filters or (diffraction-ignorant) geometric MCAO proportional gains as open-loop control parameters. In the case of Kolmogorov turbulence, expressions involving the Rytov variance and/or weighted C(2)(n) integrals apply. Differences in performance between diffractive MCAO and geometric MCAO resemble chromatic errors. Optimal corrections based on least squares imply irreducible performance limits that are validated by wave-optic simulations.