Fabrication Of Diffractive Optical Elements Research Articles

Improved sol-gel thin film for fabrication of multilevel structures using a high-energy beam-sensitive gray-scale mask

A new photosensitive negative-tone inorganic-organic hybrid SiO 2 /TiO 2 glass with improved linear UV response property is investigated for fabrication of multilevel structures. The UV response of the sol-gel material is studied as a function of optical densities (ODs) of a high-energy beam-sensitive (HEBS) gray-scale mask. It is shown that the sol-gel material exhibits a good linear response to the OD values, which is ultimately used to define the surface profile height for a specific design. The linearity is especially useful for fabrication of diffractive optical elements with linear phase distributions. As an example, we apply the sol-gel process and the gray-scale mask to the fabrication of a blazed grating.

Fabrication of diffractive optical elements with arbitrary surface-relief profile by direct laser writing

A new method is developed to precisely control the key parameters for fabricating high quality diffractive optical elements with any surface-relief profiles. For the case of direct laser writing on photoresists, these parameters include the laser intensity, laser writing speed, etc. A computer control system has been programmed to compute the corresponding laser intensity incremental step and the spacing of adjacent scans, and control of the optimized process for fabricating diffractive optical elements. The compensation due to the laser variation over time has also been considered in the program. An example of a fabricated diffractive optical element and its optical performance is also demonstrated.

Fabrication of diffractive-optical elements by using halftone gray-scale masks

The fabrication of diffractive optical elements (DOEs), especially the DOEs with variable spatial frequency features, by using halftone gray-scale masks is investigated. Three aspects of the DOEs profile infidelity have been studied. The first two infidelities are, with the reduced periods, the decrease in maximum depth in photoresist and the increase in relative transition width between adjacent ramps. Imaging error is found to be responsible for the infidelities. The infidelities can be reduced by using a certain aperture or a larger photoreduction, while proximity-printing nearly eliminates the infidelites. The third infidelity is the uncertainty of the nonlinearity between the gray values in mask data and the final depth in photoresist. Both the nonlinearity and the uncertainty of the nonlinearity can be reduced to some extent by using a reduced gamut of gray values of 0.3–0.8 and an appropriate exposure dose from the primary mask to the secondary mask before the compensation function is finally measured and imposed on the gray-scale values.

Design and fabrication of diffractive optical elements for optical tweezer arrays by means of e-beam lithography

Optical tweezer arrays generated by diffractive optical elements can extend the capabilities to manipulate and organize microscopic particles into complex structures, to sort them intelligently and to study collective behavior in many-body systems materials. We describe new design methods, based on iterative algorithms, to calculate phase-only diffractive optical elements able to generate planar and three-dimensional arrays of tweezers. Experimental results showing the validity of the design approach are presented for an array of eight gaussian spots.

Single-step Photo fabrication of Kinoforms in use of Azobenzene-containing Polymer Films

ABSTRACTWe propose a new method of optically fabricating kinoforms using polyester containing cyanoazobenzene units in the side chain. This method utilizes the surface relief structures induced optically on the azopolymer films. Using a gray-tone amplitude mask with black and white lines as a spatial light modulator, we constructed the surface relief structures on the azopolymer films with an argon-ion laser beam (488nm). The relief depths constructed were deep enough to provide a wide range of visible light with a phase difference of 2π. In addition, irradiation on the azopolymer film with a spatially modulated light that had multilevel intensities inscribed the multilevel relief structure reflecting the intensity modulation. This property is applicable to the fabrication of diffractive optical elements, especially kinoforms, because the multilevel relief structure provides an incident light with phase modulation. We designed a phase pattern for a kinoform by computation and expressed it as a gray-tone amplitude mask. Irradiation on the film through the gray-tone mask with the pump beam inscribed a multilevel relief structure that diffracted an incident light to produce a desired image. The desired image appeared without the effect of the internal refractive index modulation of the film. We, therefore, confirmed a possibility of fabricating a kinoform with a multilevel relief structure on the azopolymer film. Since our fabrication method for kinoforms requires only exposure, it has a significant advantage over other techniques that require additional processes such as developing and etching.

Fabrication of Diffractive Optical Elements on a Si Chip by Imprint Lithography using Novel Mold

Fabrication of Diffractive Optical Elements on a Si Chip by Imprint Lithography using Novel Mold

Fabrication of diffractive optical elements using the CMOS process

In this paper we propose a novel diffractive optical element (DOE) which is fabricated by the conventional complementary metal-oxide semiconductor (CMOS) process. A simple post-CMOS process is employed to establish the relief pattern. The pattern can be used directly for its optical properties or can serve as a mould for subsequent replication. In this study we design a DOE with eight-phase-level gratings, which is compatible with the CMOS process. The eight-phase-level gratings have 81.4% diffraction efficiency calculated by computational and theoretical analyses. The experiment finished four-phase-level gratings.

Customization of a self-processing polymer for obtaining specific diffractive optical elements

Photopolymerizable materials are capable of recording volume holograms via a change in refractivity of the layer. On illumination by an interference pattern around 500 nm, polymerization and diffusion led to regions with various segment densities. A modulation of the refractive index took place, thus giving rise to permanent thick phase gratings created in situ in the emulsion. This modulation was estimated at 7×10 −3, corresponding to diffraction efficiencies of ca. 80% for transmission holography. Applications for the fabrication of diffractive optical elements can be expected in view of the experimental results and the simplicity of use. For instance, the duplication of computer-generated holograms on a photopolymerizable film led to thick phase holograms with higher diffraction efficiencies. Substrate-mode polarizing elements were also produced, taking advantage of the laminated property since this emulsion was embedded between two glass plates.

Photoresist characterization and linearization procedure for the gray-scale fabrication of diffractive optical elements

We present a procedure for the characterization and the linearization of the photoresist response to UV exposure for application to the gray-scale fabrication of diffractive optical elements. A simple and reliable model is presented as part of the characterization procedure. Application to the fabrication of surface-relief diffractive optical elements is presented, and theoretical predictions are shown to agree well with experiments.

Fabrication of diffractive optical elements for an integrated compact optical microelectromechanical system laser scanner

We describe the microfabrication of a multilevel diffractive optical element (DOE) onto a microelectromechanical system (MEMS) module as a key element in an integrated compact optical-MEMS laser scanner. The DOE is a four-level off-axis microlens fabricated onto a movable polysilicon shuttle. The microlens is patterned by electron beam lithography and etched by reactive-ion-beam etching. The DOE was fabricated on two generations of MEMS components, each of which is briefly described. The compact design of the laser scanner is based on mounting a MEMS module and a vertical cavity surface-emitting laser onto a fused silica substrate that contains the rest of the optical system. The calculated scan range of the system is ±4° with a spot size of 0.5 mm.

Single-step fabrication of diffraction gratings on hybrid sol–gel glass using holographic interference lithography

Single-step fabrication of diffractive surface relief gratings on an ultraviolet (UV) sensitive hybrid sol–gel glass is presented. The gratings are imprinted on the negative-tone sol–gel glass based on a holographic interference lithographic system. Characteristics of the sol–gel glass including refractive index and film thickness have been discussed and experimental results are presented as a function of baking temperature and UV exposure time respectively. The grating relief profiles are measured by an atomic force microscope and the measured groove depths are found in good agreement with the theoretical results determined by the Gerchberg–Saxton phase retrieval algorithm. It is promising to utilise the new material for various diffractive optical elements fabrication.

Excimer laser fabrication of diffractive optical elements

Excimer lasers are well known for their ability to machine features on the micron scale. Recently, they have been used to fabricate diffractive optical elements (DOEs). These elements are patterns of pixels machined to the depth required, in order that a reconstructed pattern is formed in the image plane when the DOE is illuminated by a coherent source. The patterns are designed using computer phase retrieval code. These designs are used to create XY co-ordinate data files for conversion to machining routines where the pixels are machined individually. The laser used is an ArF excimer operating at 193 nm. We describe research undertaken on the fabrication and testing of some typical DOEs. The measurements include efficiency of reconstruction as well as the measurements of profile and surface roughness. The work compares the reconstruction results with the topographical measurements made on the DOE for 2, 4 and 8 phase-level designs. Newly developed simulation code was used to generate reconstructions without machining imperfections.

On the linearity of holographic recording in amorphous As2S3 films

Optical recording in amorphous arsenic trisulfide was investigated for amplitude to phase conversion to be exploited in applications such as holographic recording and the fabrication of diffractive optical elements. The main result of this study is the demonstration that it is possible to obtain a linear relation between the spatial variation of the recording intensity and the recorded phase modulation. The extent of the linear recording region depends strongly on the recording wavelength. At short wavelength (458 nm in these experiments), where the rate of the photoinduced structural changes is strong, the recording is practically linear from zero exposure up to about 25% of the maximum value of the refractive index change. With higher exposure, saturation effects start to play an important role and degrade the linearity of the response. Recording with longer wavelength (488 and 514.5 nm) is less effective (absorption becomes weaker) and then relaxation processes in the material become significant. If the competition between photoinduced processes and relaxation processes is strong, the linear relationship between exposure and refractive index change can no longer be linear. Nevertheless, even for recording with the longer wavelengths, a region of exposures can be found within which variations of intensity can be converted into variations of phase with a linear relationship.

Excimer laser micro machining: fabrication and applications of dielectric masks

Excimer laser ablation is a versatile tool for the micro machining of various materials. Mask projection configurations employing dielectric masks allow fast and flexible high power processing. Due to their high reflectivity at the laser processing wavelength, these dielectric masks show superior performance concerning damage threshold and durability compared to chrome on quartz or metal stencil masks. Dielectric masks are fabricated by laser ablation patterning. The highly reflective (HR) layer stack (e.g., HfO 2/SiO 2 for HR 248 nm) is ablated with high spatial resolution from the quartz substrate applying 193 nm laser irradiation. A resolution of about 2 μm is achieved allowing for sub-μm patterning when using this mask in a demagnifying image projection set up for ablation at 248 nm. The application of these masks for laser micro machining is shown. For example single pulse fabrication of diffractive optical elements (DOE) in transparent polymer material is demonstrated.

Excimer laser patterned dielectric masks for the fabrication of diffractive optical elements by laser ablation

Masks for laser processing are generated by laser ablation patterning of dielectric layer systems. The application of these masks for the rapid fabrication of diffractive optical elements (DOEs) is presented.

Polar coordinate laser pattern generator for fabrication of diffractive optical elements with arbitrary structure

A precision laser pattern generator for writing arbitrary diffractive elements was developed as an alternative to Cartesian coordinate laser/electron-beam writers. This system allows for the fabrication of concentric continuous-relief and arbitrary binary patterns with minimum feature sizes of less than 0.6 microm and position accuracy of 0.1 microm over 300-mm substrates. Two resistless technologies of writing on chromium and on amorphous silicon films were developed and implemented. We investigated limit characteristics by writing special test structures. A 58-mm f/1.1 zone plate written directly is demonstrated at a lambda/50 rms wave-front error corresponding to a 0.06-microm pattern accuracy. Several examples of fabricated diffractive elements are presented.

Data preparation and fabrication of DOE using electron-beam lithography

The application of electron-beam lithography to the generation of diffractive optical elements topology is examined. The formula for the estimation of exposure data volume for variable-shaped electron-beam lithography is presented as a function of diffractive optical element parameters and approximation accuracy. A special software dedicated to preparing exposure data for fabrication of diffractive optical elements is developed. Diffractive optical elements with an artificial refractive index are manufactured with a feature size much less than the wavelength. Design and experimental results on photomasks fabrication for an optical element focusing radiation into a ring with pregiven parameters are presented. The photomask set is manufactured for a reflecting optical element focusing the high power CO 2 laser beam into two points with required parameters for laser welding.

Design and fabrication of diffractive optical elements

An introduction to the design and fabrication of diffractive optical elements is presented. Design techniques for diffractive optic in the two theoretical design areas, the scalar and resonance domains, and the dominant methods of fabrications are described. Theoretical and experimental examples are given in each section.

Excimer laser micromachining for rapid fabrication of diffractive optical elements

We demonstrate the fabrication of binary, multilevel, and blazed diffractive structures by a fast and flexible direct-write process by using an excimer-laser-based tabletop micromachining workstation with an integrated optical surface profiler.

Rapid fabrication of diffractive optical elements by use of image-based excimer laser ablation

We describe a method of fabricating multilevel diffractive optics by excimer laser ablation. A portion of a chrome mask containing many patterns is illuminated by 193-nm laser light and imaged by an objective lens onto a poly(imide) substrate. Ablation of an entire single pattern is achieved in a single laser pulse. Multiple pulses are used to vary the ablation depth, and multiple patterns are used to create a variety of multilevel optics. We have successfully fabricated arrays of eight-level diffractive microlenses with varying focal lengths and decenters. The optics performed with diffraction-limited focusing and near-theoretical diffraction efficiency (92%).