The vibrational spectrum of calcite (CaCO3) is evaluated at an ab initio periodic quantum-mechanical level by using the CRYSTAL package. A localized basis set of Gaussian-type functions and the B3LYP hybrid Hamiltonian are adopted. The dynamical matrix is obtained by differentiating numerically the analytical first derivatives of the energy. The accuracy with respect to all computational parameters is documented. The calculated frequencies are compared with available IR and RAMAN data (16 and 5 peaks, respectively), the mean absolute error being less than 12 cm−1 (frequencies range from 100 to 1600 cm−1). Overall, the agreement with experiment is very satisfactory, and shows that simulation can produce at a relatively low cost the full spectra of crystalline compounds of mineralogical interest.

The vibrational spectrum of calcite (CaCO3): an ab initio quantum-mechanical calculation Note: Theoretically derived structure

Background Food quality and safety are very important from health as well as fiscal point of view. Surface-enhanced Raman spectroscopy (SERS) techniques have unique advantages in the field of food detection , with characteristics of fast analysis speed, high detection sensitivity, and without interference from the water phase. Scope and approach In this review, recent and potential advances in the application of SERS in food safety and quality from the perspective of SERS substrate and SERS composite systems are addressed. SERS systems such as molecular labeling, immunochromatographic assay, microfluidics, molecularly imprinted polymers, colorimetry and imaging are discussed, and their main advantages and limitations are highlighted. The applications of SERS in food safety are reviewed critically, with focus on the detection of microorganisms, pesticides, metal ions and antibiotics. Furthermore, applications of SERS in food quality regarding food freshness and ingredients are discussed. Key findings and conclusions SERS technology has been widely used in food testing, but it still has shortcomings. In order to establish SERS as a routine tool for the monitoring of food safety and quality, future research should focus on minimizing technical costs, standardizing experimental protocols, developing new SERS substrates, and integrating SERS with other methods to overcome its shortcomings.

Evolving trends in SERS-based techniques for food quality and safety: A review

A grating interferometer, called the “optical encoder,” is a commonly used tool for precise displacement measurements. In contrast to a laser interferometer, a grating interferometer is insensitive to the air refractive index and can be easily applied to multi-degree-of-freedom measurements, which has made it an extensively researched and widely used device. Classified based on the measuring principle and optical configuration, a grating interferometer experiences three distinct stages of development: homodyne, heterodyne, and spatially separated heterodyne. Compared with the former two, the spatially separated heterodyne grating interferometer could achieve a better resolution with a feature of eliminating periodic nonlinear errors. Meanwhile, numerous structures of grating interferometers with a high optical fold factor, a large measurement range, good usability, and multi-degree-of-freedom measurements have been investigated. The development of incremental displacement measuring grating interferometers achieved in recent years is summarized in detail, and studies on error analysis of a grating interferometer are briefly introduced.

Displacement measuring grating interferometer: a review

High concentration Eu3+-doped NaYb(MoO4)2 multifunctional material: Thermometer and plant growth lamp matching phytochrome PR

In dimensional metrology it is necessary to carry out multi-axis angle and displacement measurement for high-precision positioning. Although the state-of-the-art linear displacement sensors have sub-nanometric measurement resolution, it is not easy to suppress the increase of measurement uncertainty when being applied for multi-axis angle and displacement measurement due to the Abbe errors and the influences of sensor misalignment. In this review article, the state-of-the-art multi-axis optical sensors, such as the three-axis autocollimator, the three-axis planar encoder, and the six-degree-of-freedom planar encoder based on a planar scale grating are introduced. With the employment of grating reflectors, measurement of multi-axis translational and angular displacement can be carried out while employing a single laser beam. Fabrication methods of a large-area planar scale grating based on a single-point diamond cutting with the fast tool servo technique and the interference lithography are also presented, followed by the description of the evaluation method of the large-area planar scale grating based on the Fizeau interferometer.

Optical Sensors for Multi-Axis Angle and Displacement Measurement Using Grating Reflectors.

We propose a simple and compact reading head with the Littrow configuration that will increase measurement range and reduce the complexity of a two-dimensional grating-based interferometer. The reading head contains only a beam splitter, two polarizing beam splitter modules, and two mirrors. The theoretical resolutions in two directions are 0.27 nm and 0.18 nm, respectively. In comparison with a dual-frequency laser interferometer, the proposed interferometer can measure displacement from 3 nm to 10 mm with high accuracy. The 3σ values in two directions for the difference are 1.67 nm and 1.35 nm for a displacement of 9 nm. Repeatability for a displacement of 1 μm is better than 2 nm.

Simple and compact grating-based heterodyne interferometer with the Littrow configuration for high-accuracy and long-range measurement of two-dimensional displacement.

The designs of 300 mm ruling engine producing gratings and echelles under interferometric control (CIOMP-2) are presented. A new ruling-tool carriage system and its driving mechanism—which are easy to manufacture—are proposed. A new blank carriage system, controlled by a dual-frequency laser interferometer and a piezoelectric actuator, is designed. The CIOMP-2 ruling engine can now rule grating blanks with dimensions up to 300  mm×300  mm, and mainly rules gratings with grating constants between 10 and 2400  lines/mm. The wavelength range of CIOMP-2 gratings is from ultraviolet to mid-infrared wavelengths. Experiments show that the new ruling-tool carriage system works well and stability of motion is improved with the help of a flexure-hinge structure, and the threefold standard deviation values of the blank carriage positioning errors are less than ∼5  nm. For 600  line/mm gratings (diffraction order m=−1), the scatter intensities and ghosts reach 10−5 of the maximum intensity. The scatter intensities and ghost of gratings ruled with CIOMP-2 are low, and no Rowland ghosts are visible. The gratings ruled by CIOMP-2 have high diffraction efficiency and resolving power. CIOMP-2 can also produce varied-line-space, bend-line, and aberration-reducing gratings and echelles.

300 mm ruling engine producing gratings and echelles under interferometric control in China

Spatial heterodyne Raman spectroscopy is a spectroscopic detection technique that is particularly suitable for Raman measurements. The spectral range of traditional spatial heterodyne Raman spectrometer (SHRS) is limited by its spectral resolution and the number of detector elements. We propose an SHRS with an echelle-mirror structure that employs multiple diffraction orders to achieve a broad spectral coverage and high spectral resolution simultaneously. This SHRS is used to obtain the Raman spectra of organic liquids, inorganic solid targets, and mixed targets. Observations of aqueous solutions, and minerals are presented. In addition, anti-Stokes Raman shifts are also presented. The proposed SHRS technique shows good performance for broadband, high-resolution Raman measurements.

Development of a spatial heterodyne Raman spectrometer with echelle-mirror structure.

Groove error is one of the most important factors affecting grating quality and spectral performance. To reduce groove error, we propose a new ruling-tool carriage system based on aerostatic guideways. We design a new blank carriage system with double piezoelectric actuators. We also propose a completely closed-loop servo-control system with a new optical measurement system that can control the position of the diamond relative to the blank. To evaluate our proposed methods, we produced several gratings, including an echelle grating with 79  grooves/mm, a grating with 768  grooves/mm, and a high-density grating with 6000  grooves/mm. The results show that our methods effectively reduce groove error in ruled gratings.

Correcting groove error in gratings ruled on a 500-mm ruling engine using interferometric control.

Line curvature error greatly influences the quality of the diffraction wave fronts of machine-ruling gratings. To reduce the line curvature error, we propose a correction method that uses interferometric control. This method uses diffraction wave fronts of symmetrical orders to compute the mean line curvature error of the ruled grating, taking the mean line curvature error as the system line curvature error. To minimize the line curvature error of the grating, a dual-frequency laser interferometer is used as a real-time position feedback for the grating ruling stage, along with using a piezoelectric actuator to adjust the stage positioning to compensate the line curvature error. Our experiments show that the proposed method effectively reduced the peak-to-valley value of the line curvature error, improving the quality of the grating diffraction wave front.

Xiaotian Li

Papers

Simple and compact grating-based heterodyne interferometer with the Littrow configuration for high-accuracy and long-range measurement of two-dimensional displacement.

300 mm ruling engine producing gratings and echelles under interferometric control in China

Development of a spatial heterodyne Raman spectrometer with echelle-mirror structure.

Correcting groove error in gratings ruled on a 500-mm ruling engine using interferometric control.

Reducing the line curvature error of mechanically ruled gratings by interferometric control