Grating Groove Profile Research Articles

Experimental study and modeling of extreme ultraviolet 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers.

Multilayer coated diffraction gratings are crucial components for extreme ultraviolet (EUV) applications such as spectroscopy or spectro-imaging. However, for high groove density, the smoothening of the grating surface profile with multilayer deposition remains a limitation that requires further investigation. In this paper, we report on the design, characterization, and modeling of 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers for EUV radiation. Two types of gratings with different groove depths are compared. Multilayer coatings were designed using a genetic algorithm to maximize the first-order diffraction efficiency in the 17-21 and 19-23nm wavelength ranges at normal incidence. Periodic and aperiodic multilayers with different numbers of layers were deposited by magnetron sputtering on the two types of fused silica gratings, and the grating groove profile evolution was measured by atomic force microscopy and cross-section transmission electron microscopy. The first-order diffraction efficiency was measured in the EUV at 5° incidence using monochromatic synchrotron radiation and modeled using the rigorous coupled-wave analysis method. The simulation models refined by using the Debye-Waller factor to account for the multilayer interfacial roughness show good agreement with experimental data. The results reported in this study will allow for designing efficient EUV multilayer gratings for high-resolution spectro-imaging instruments.

High-quality and large-area echelle grating ruled on 500-mm ruling engine for the fiber array solar optical telescope (FASOT).

A 300 mm×500 mm large-area echelle grating with groove density of 79 grooves/mm is fabricated for the spectrometer of the fiber array solar optical telescope (FASOT). This paper focusses on measurement methods of the grating performance. We present a method to evaluate the grating's stray light intensity, which is measured to a level of 10-4. The directly measured grating efficiency is approximately 90% of the designed value, and an indirect measurement method based on the grating groove profile is proposed. Based on the Rayleigh criterion and the grating diffraction wavefront, a physical optics method and a geometric grating method are proposed and are used to calculate the actual grating resolving power; the calculated results exceed 95% of the grating's theoretical resolving power. These results show that the CIOMP-6 ruling engine has sufficient precision to fabricate high-quality, large-area echelle gratings.

ЗАЛЕЖНІСТЬ ЕФЕКТИВНОСТІ ЗБУДЖЕННЯ ПОВЕРХНЕВИХ ПЛАЗМОН-ПОЛЯРИТОНІВ ВІД ГЛИБИНИ РЕЛЬЄФУ АЛЮМІНІЄВОЇ ГРАТКИ

An experimental study of the excitation of surface plasmonpolaritons (SPP) on aluminum diffraction gratings with a fixed period of 519 ± 0,5 nm and a variable modulation depth h/a(where h is the grating depth, and a – its period) was carried out. Gratings with a sine-like profile were formed on vacuum chalcogenide photoresists films by interference lithography and covered with an opaque aluminum film. A Dimension 3000 Scanning Probe Microscope was used to determine the grating groove profile. The characteristics of the SPP were determinedfor28 gratings with h/a ranged from 0,018 to 0,20, by measuring the dependences of specular reflection of p-polarized radiation of He-Ne laser on the angle of incidence, which was defined as the angle between the normal to the substrate plane and the laser beam. It was found that there is an optimal grating relief depth for a given excitation wavelength, which provides the maximum transfer of the incident electromagnetic wave energy to the surface plasmon-polariton mode.The dependence of the SPP excitation efficiency on the grating modulation depth has a maximum at a relatively small value of h/a ≈ 0.086. At such modulation depth the absorption of electromagnetic radiation of the incident laser beam is more than two orders of magnitude higher than the absorption of aluminum film with flat surface at the same angle of incidence. The position of the angle of resonant excitation of SPP practically does not change from h/a= 0,018 up to h/a ≈ 0,06. With further increase of h/a it begins to shift to the region of smaller incidence angles, with the rate of the shift accelerating gradually. With an increase of h/a, a decrease in the depth of the plasmon resonance and a significant increase in its half-width are also observed, and the dependence of the half-width of the SPP band on the modulation depth is close to quadratic. Using this grating-coupled SPP technique, the estimated thickness of air-formed oxide layer on the aluminum gratings surface (about 3.9 nm) is close to the value obtained in the literature with a set of complicated techniques.

Improved control of electron computer-generated holographic grating groove profiles using ion beam gas-assisted etching.

In a transmission electron microscope, electrons are described by matter-waves with wavelengths five orders of magnitude smaller than optical electromagnetic waves. Analogous to optical holography, electron wavefronts can be shaped using nanoscale holographic gratings. Here we demonstrate a novel, scalable nanofabrication method for creating off-axis holographic gratings that demonstrate near ideal diffraction efficiencies for binary, sinusoidal, and blazed grating groove profiles. We show that this method can produce up to 50 µm diameter area gratings that diffract up to 68% of the transmitted electron wave into a desired diffraction order with less than 7% into any other order. Additionally, we find that the amount of inelastically scattered electrons from the material gratings remaining in the coherent diffraction orders from the gratings is negligible in the far field.

Integrated high-order surface diffraction gratings for diode lasers

High-order surface diffraction gratings acting as a distributed Bragg reflector (DBR) in mesa stripe semiconductor lasers have been studied theoretically and experimentally. Higher order interfering radiation modes (IRMs), which propagate off the plane of the waveguide, have been shown to have a crucial effect on the reflection and transmission spectra of the DBR. The decrease in the reflectivity of the DBR in response to the increase in the diffraction efficiency of these modes may reach 80 % and more. According to theoretical analysis results, the intensity of the higher order IRMs is determined by the geometry of the DBR groove profile. Experimental data demonstrate that the noncavity modes are responsible for parasitic light leakage losses in the laser cavity. It has been shown that, in the case of nonoptimal geometry of the grating groove profile, the overall external differential quantum efficiency of the parasitic laser emission may exceed 45 %, which is more than half of the laser output power. The optimal geometry of the DBR groove profile is trapezoidal, with the smallest possible lower base. Experimental evidence has been presented that this geometry considerably reduces the power of the higher order IRMs and minimises the parasitic light leakage loss.

Fabrication of holographic blazed grating with Fourier synthesis exposure

A simple, direct, and universal fabrication method for holographic blazed gratings is proposed and a 79 lines/mm holographic echelle grating with 4 μm depth quasi-triangular grooves experimentally demonstrated as an example of a blazed grating fabricated directly by Fourier synthesis exposure. With the method, we converted a periodic grating groove profile into the exposure energy spatial distribution on the surface of a photoresist with the response characteristics of the particular photoresist. Then the exposure energy could be decomposed into a series of sinusoidal functions using Fourier series, and realized by superposing a series of two-beam interference patterns. In contrast with mechanically ruled gratings, the fabrication process of holographic gratings is quite quick and low cost, and especially no Rowland ghosts appear in their dispersion spectrum. The reported work will offer a new approach for blazed grating fabrication with a holographic process.

Fabrication of extreme-ultraviolet blazed gratings by use of direct argon-oxygen ion-beam etching through a rectangular photoresist mask

We present a simple method to fabricate blazed gratings used in the extreme-ultraviolet wavelength region. The method uses an argon and oxygen ion beam to etch directly the fused silica substrate through a rectangular profile photoresist grating mask. The blaze angle can be significantly reduced by using oxygen, and the profile evolution under ion-beam bombardment can be simplified when a rectangular mask is used. A simple geometric model is built to analyze the etching process. The etched grating groove profile is approximately triangular with a sharp apex angle provided the aspect ratio of the mask ridge is properly chosen; the blaze angle is directly proportional to the ion-beam grazing incident angle for a fixed oxygen partial pressure. Gratings of a 2400 line/mm groove density and 0.5-3 degrees blaze angles have been fabricated, which confirms the convenience of this method and the effectiveness of the etching model.

High-efficiency multilayer-coated ion-beam-etched blazed grating in the extreme-ultraviolet wavelength region

We describe a simple method to fabricate blazed gratings used in the extreme ultraviolet wavelength region. The method uses an argon and oxygen mixed-gas ion beam to directly etch the grating substrate through a rectangular profile photoresist grating mask. With this method the etched grating groove profile can be well controlled. An Mo/Si multilayer-coated specimen with a blaze angle of 1.9 degrees was fabricated and measured. At an incident angle of 10 degrees and a wavelength of 13.62 nm, the diffraction efficiency of the negative second order reaches 36.2%.

Using symmetries of grating groove profiles to reduce computation cost of the C method

This work reduces the computation cost of the C method by taking into account the symmetries of grating grooves. All one-dimensionally periodic, single-interface, surface-relief gratings are classified into five categories according to the symmetries of the planar periodic curves describing the interface. The five categories are reflection symmetry, inversion symmetry, reflection-translation symmetry, complete symmetry (i.e., simultaneous existence of all three aforementioned symmetries), and no symmetry. Reductions of the eigenvalue problem in the C method are first carried out in real space and then in Fourier space by taking advantage of the four types of symmetries. The reflection-translation symmetry can be used without any restriction on the incident angle, but the other symmetries require a Littrow mounting; simultaneous use of the reflection-translation symmetry with any other symmetry further requires an even-order Littrow mounting. The types of eigenfunctions to be solved and boundary conditions to be matched, as well as the time reduction ratios in solving the eigenvalue problem, are given for all possible combinations of groove symmetries and incident configurations. The time reduction ratios range from 1/4 to 1/64.

Multilayer resonances sharpened by grating waveguide resonance

A multilayer planar structure comprising a highly reflective multilayer dielectric mirror and a corrugated waveguide is proposed for use as a narrow passband optical filter. The proposed filter has a much narrower linewidth than a usual Fabry–Perot cavity with two multilayer dielectric mirrors. It is shown that the narrowing of the linewidth is due to the strong spectral dependence of the phase of the wave reflected from the waveguide grating mirror. The shape of the pass band can be made symmetrical by a proper choice of the grating groove profile.

Investigation of the properties of an ion-etched plane laminar holographic grating

We have measured the efficiency over the range 125-225 A of a bare ion-etched plane laminar holographic grating made of fused silica and with 1000 grooves/mm. The measured efficiency of each order oscillates with wavelength because of constructive and destructive interference between radiation diffracted from the lands and the grooves. We measured the grating groove profile with an atomic force microscope, and the resulting groove depth of 434 ? 6 A agrees well with the values determined independently from the oscillatory behavior of the efficiency measurements. Grating efficiency in the +1 order peaked at values of 0.027%, 0.011%, and 0.005% at wavelengths of 191, 157, and 132 A, respectively; and the derived groove efficiencies are 27%, 25%, and 27%. The irregular shape at the land-groove edges dominates the large grating roughness of 23-45-A rms, but even regions far from the edges have a roughness of 10-18-A rms. The average groove profile was used to model the grating efficiency, and the resulting wavelengths predicted for different order maxima and minima agree well with measured wavelengths, although the calculated efficiencies are greater than the measured results by 10-20%.

Coherent and incoherent grating reconstruction

Recent progress in coherent and incoherent optical inverse-scattering problems is reviewed with emphasis on the construction of grating-groove profiles from known or desired diffraction efficiencies and the detection of phase gratings hidden by diffusers using optical correlation techniques. Retrieval and synthesis problems and the role of coherence are described. The coherent grating-profile reconstruction is revisited in terms of the full mathematical description of our previous procedure. The crucial role of prior phase information for uniqueness of the reconstruction is elucidated. The theoretical prediction and experimental verification of a provoking coherence effect are presented: The signature of a hidden phase grating can be seen in the far-zone coherence function but not in the far-zone intensity for a certain range of ratios of the diffuser correlation length to the grating period.

Radiative Bragg mirrors: Spectral characteristics versus grating groove profile

The spectral characteristics of radiative Bragg mirrors are theoretically investigated. The mirrors are assumed to be corrugated portions of an optical thin-film waveguide. It was found that the mirror bandwidth can be significantly varied by a slight change of the grating groove profile. The asymmetry of the reflection line is also found to be strongly dependent on the groove profile.

Use of the Talystep in investigating diffraction grating groove profiles

Groove profiles from a number of diffraction gratings have been examined with the Talystep using a 0.1 mu m stylus. Useful results have been obtained with groove densities up to 1200 mm-1. The method has some advantages over electron microscope techniques.