Aligned Gratings Research Articles

APPLICATION OF OPTICAL DIFFRACTOMETRY FOR THE ERROR MONITORING OF MANUFACTURING OF PRECISION OPTICAL ELEMENTS

The work is devoted to an experimental study of the effectiveness of the method for measuring errors in manufacturing of computer-generated synthesized holograms using specialized marks. Each mark represents two aligned diffraction gratings with the same duty cycle and period, the first of which is formed before writing, and the second is formed in the process of forming the structure of the hologram. To determine the writing error, the method of optical diffractometry was used. The results of an experimental study of dependence of diffraction efficiency of the mark on the mutual displacement of parts of the microgratings are presented. The error manufacturing of a synthesized hologram arising during long-term writing of a diffractive optical element has been simulated experimentally.

Performance testing of an off-plane reflection grating and silicon pore optic spectrograph at PANTER

An x-ray spectrograph consisting of aligned, radially ruled off-plane reflection gratings and silicon pore optics (SPO) was tested at the Max Planck Institute for Extraterrestrial Physics PANTER x-ray test facility. SPO is a test module for the proposed Arcus mission, which will also feature aligned off-plane reflection gratings. This test is the first time two off-plane gratings were actively aligned to each other and with an SPO to produce an overlapped spectrum. We report the performance of the complete spectrograph utilizing the aligned gratings module and plans for future development.

Analytical alignment tolerances for off-plane reflection grating spectroscopy

Future NASA X-ray Observatories will shed light on a variety of high-energy astrophysical phenomena. Off-plane reflection gratings can be used to provide high throughput and spectral resolution in the 0.3–1.5 keV band, allowing for unprecedented diagnostics of energetic astrophysical processes. A grating spectrometer consists of multiple aligned gratings intersecting the converging beam of a Wolter-I telescope. Each grating will be aligned such that the diffracted spectra overlap at the focal plane. Misalignments will degrade both spectral resolution and effective area. In this paper we present an analytical formulation of alignment tolerances that define grating orientations in all six degrees of freedom. We verify our analytical results with raytrace simulations to fully explore the alignment parameter space. We also investigate the effect of misalignments on diffraction efficiency.

Radiative heat transfer between two dielectric nanogratings in the scattering approach

We present a theoretical study of radiative heat transfer between dielectric nanogratings in the scattering approach. As a comparision with these exact results, we also evaluate the domain of validity of Derjaguin's Proximity Approximation (PA). We consider a system of two corrugated silica plates with various grating geometries, separation distances, and lateral displacement of the plates with respect to one another. Numerical computations show that while the PA is a good approximation for aligned gratings, it cannot be used when the gratings are laterally displaced. We illustrate this by a thermal modulator device for nanosystems based on such a displacement.

Novel non-linear optical techniques for diagnostics: laser-induced gratings and two-colour four-wave mixing

Electrostriction, population transfer caused by absorption of laser energy and subsequent thermalization are the most important mechanisms responsible for the formation of laser-induced gratings. Applying time resolved light diffraction from electrostrictive gratings allows for the measurements of thermodynamic properties like temperature, gas composition, and flow velocity. Thermal gratings can be employed as a very sensitive spectroscopic tool. The Two-Colour Four-Wave Mixing (TCFWM) signal formed by a non-resonant pump laser and a resonant probe laser in an acetylene/air diffusion flame comprises of contributions due to soot and C 2, the latter is generated by vaporising the soot. Profile measurements across the flame have shown that the signal composition changes within the flame. Intermediate level labelling by Two-Colour Resonant Four-Wave Mixing Spectroscopy (TC-RFWM) is applied to reduce the spectral complexity of molecular spectra. In addition, we observe extra resonances forbidden by a conventional three-level scheme in OH- and NH-containing flames. A detailed investigation shows that the newly observed TC-RFWM resonances are induced by collisional energy transfer within the population, alignment and orientation gratings formed in both electronic states that are coupled to the laser fields.

Effects of different population, orientation, and alignment relaxation rates in resonant four-wave mixing

We present a combined theoretical and experimental study on the effects of different population, orientation, and alignment relaxation rates in resonant four-wave mixing (RFWM). Signal generation in RFWM can be viewed as the formation of and scattering from laser-induced population, orientation, and alignment gratings. We show that the relative contributions from the upper-state and lower-state population, orientation, and alignment gratings to the observed output signal can be changed by varying the polarizations of the three input fields. A theory is developed to account for these changes in collisional environments where the three multipole moments of the total angular momentum distribution, i.e., the population, the orientation, and the alignment, relax unequally. This theory is applied to the OH radical in an atmospheric-pressure H2/O2/He flame for which we have measured the line profiles using high-resolution degenerate and nearly degenerate four-wave mixing. We find that orientation and alignment gratings relax more rapidly than population gratings for low rotational levels of OH in the presence of He but at essentially the same rate for high rotational levels. A discussion is presented of the importance of this effect in the interpretation of RFWM experiments.

New Evaluation Method of Resist Coating Using Heterodyne Holographic Wafer Alignment

A new method to evaluate nonuniform resist coating using heterodyne holographic wafer alignment was proposed. In this method, the resist coating nonuniformity is measured as the displacement of the alignment position after resist coating on deep-UV-hardened resist alignment gratings. It has been confirmed that nonuniformity of resist coating was mainly caused by the nonuniform evaporation speed of the resist solvent, and the uniformity of resist coating was obtained by optimizing the solvent and the viscosity of the resist. The resultant overlay accuracy was improved to 59 nm/3σ for the optimized resist coating. The overlay error budget for processed wafers was analyzed and the overlay accuracy was confirmed to be better than 80 nm/3σ using this method and wafer scaling correction.

Grating coupled liquid crystal waveguide

Abstract A waveguide consisting of a thin (< 1 μm) layer of nematic liquid crystal sandwiched between two parallel aligned gratings is described. Grating coupling to guided modes is demonstrated by monitoring the reflection and transmission from the cell as a function of incident angle. A reduced birefringence is seen compared to the bulk case which is explained in terms of disordered regions of liquid crystal close to the grating surfaces. The movement of guided modes under electrical bias is shown and time resolved experiments demonstrate high contrast switching with rise times as fast as 18 μs.

Large-area, free-standing gratings for atom interferometry produced using holographic lithography

Interferometers based on matter waves promise orders-of-magnitude improvements in metrology over laser-based systems by virtue of the fact that the de Broglie wavelengths of atoms are about 104 times shorter. To date, the required matched set of four aligned gratings for such atom interferometers has been made using electron beam lithography and, as a result, such gratings suffer from a lack of spatial-phase coherence. We report on processes we have developed for fabricating free-standing gratings over large areas using conventional holographic lithography and achromatic holographic lithography to achieve spatial periods of 200 and 100 nm, respectively (i.e., nominal linewidths of 100 and 50 nm, respectively).

Interferometric Nanometer Alignment for a Wafer Stepper by Two Wave-Front Reconstruction onto Deformed Wafer Gratings

A discussion is presented on a reduction projection wafer stepper with an interferometric nanometer alignment detector by two wave-front reconstruction. The optics of the detector to form an interferometer comprises two coherent beams and a grating on a wafer. The alignment detector detects the relative position of the reticle grating to the wafer grating. It has been verified that the response sensitivity to the step movements of the wafer was 5 nanometers. The response signals received from various deformed alignment gratings indicated an ideal cosine wave form. The resultant overlay to the irregular surface gratings on an Al deposited wafer was improved to 0.1 µm/3 sigma.

Wood's anomaly angular alignment gratings

Wood's anomaly angular alignment gratings

Wood's anomaly angular alignment gratings

Wood's anomaly angular alignment gratings