Ultralow frequency Stokes and anti-Stokes Raman spectroscopy of single living cells and microparticles using a hot rubidium vapor filter

Development of a high-resolution, broadband spatial heterodyne Raman spectrometer based on field-widened grating-echelle structure

We propose a spatial heterodyne Raman spectrometer (SHRS) based on a field-widened grating-echelle (FWGE). A normal grating is combined with an echelle grating in a conventional spatial heterodyne spectrometer to eliminate ghost images without using masks, and prevents interference among the spatial frequencies of different diffraction orders. Mathematical expressions and derivation processes are given for the spectral parameters in the FWGE-SHRS and a verification breadboard system is fabricated. The FWGE-SHRS measures Raman spectra of single chemicals and mixed targets with different integration times, laser powers, concentrations, and transparent containers. The results of the experiments demonstrate that the FWGE-SHRS is suitable for high-resolution, broadband Raman measurements for a wide range of applications.

High-resolution, broad-spectral-range Raman measurement using a spatial heterodyne spectrometer with separate filters and multi-gratings

We propose a high-resolution, broad-spectral-range spatial heterodyne Raman spectrometer (SHRS) having separate filters and multi-gratings (SFMG). A prototype of the SFMG-SHRS is built using multi-gratings with four sub-gratings having groove densities of 320, 298, 276, and 254 gr/mm and separate filters with filter bands corresponding to the sub-gratings. We use the SFMG-SHRS to measure the Raman spectra of inorganic and organic compounds with various integration times, laser power, and transparent containers, compare measurements of microplastics with and without the separate filters, and measure mixtures of inorganic powders and organic solutions. The designed SFMG-SHRS makes high-resolution, broad-spectral-range Raman measurements with improved signal-to-noise ratios and visibility of weak Raman peaks even in the presence of fluorescence.

Broadband, high-resolution spatial heterodyne Raman spectroscopy measurement based on a multi-Littrow-angle multi-grating

This paper proposes a spatial heterodyne Raman spectrometer (SHRS) based on a multi-Littrow-angle multi-grating (MLAMG). Compared with a conventional multi-grating, the MLAMG not only provides higher spectral resolution and a broader spectral range, but is also easier to produce. A verification breadboard system is built using the MLAMG combined with four sub-gratings with a groove density of 300 gr/mm and Littrow angles of 4.6355°, 4.8536°, 5.0820°, and 5.3253°. This MLAMG-SHRS is used to obtain the Raman spectra of inorganic solids and organic solutions for different integration times, laser powers, suspension contents, and containers. The Raman spectra of mixed targets and minerals are also presented. The experiments demonstrate that the MLAMG-SHRS is suitable for broadband measurements at high spectral resolution in a wide range of potential applications.

A Simple Calibration Method of Anti-Stokes-Stokes Raman Intensity Ratios Using the Water Spectrum for Intracellular Temperature Measurements

Presented here is a facile and practical method for calibrating anti-Stokes-Stokes intensity ratios in low-frequency Raman spectra that is devised specifically for temperature measurements inside cells. The proposed method uses as an intensity standard the low-frequency Raman spectrum of liquid water, a major molecular component of cells, whose temperature is independently measured with a thermocouple. Rather than calibrating pixel intensities themselves, we obtain a correction factor at each Raman shift in the 20-200 cm^-1 region by dividing the anti-Stokes-Stokes intensity ratio calculated theoretically from the Boltzmann factor at the known temperature by that obtained experimentally. The validity of the correction curve so obtained is confirmed by measuring water at other temperatures. The anti-Stokes-Stokes intensity ratios that have been subjected to our calibration are well fitted with the Boltzmann factor within ∼1% errors and yield water temperatures in fairly good agreement with the thermocouple temperature (an average difference ∼1 ℃). The present method requires only 15 min of spectral acquisition time for calibration, which is 50 times shorter than that in a recently reported calibration method using the pure rotational Raman spectrum of N₂. We envision that it will be an effective asset in Raman thermometry and its applications to cellular thermogenesis and thermoregulation.

Vertical Heterostructure of SnS-MoS2 Synthesized by Sulfur-Preloaded Chemical Vapor Deposition

We synthesize a vertical heterostructure (HS) between tin sulfide (SnS) and molybdenum sulfide (MoS₂) by chemical vapor deposition based on the preferential adsorption of sulfur on MoS₂. Most of the SnS grains nucleate on MoS₂ nanosheets, formatting partially stacked HS with large overlapping regions. Photoluminescence quenching of MoS₂ is observed and illustrates effective charge separation in HS. The HS shows increased reverse saturable absorption relative to MoS₂ and SnS. The preferential adsorption of sulfur powders on MoS₂ and HS growth reported herein will provide a promising approach to the synthesis of other two-dimensional HS.

Atomic optical stimulated amplifier with optical filtering of ultra-narrow bandwidth

Taking advantages of ultra-narrow bandwidth and high noise rejection performance of the Faraday anomalous dispersion optical filter (FADOF), simultaneously with the coherent amplification of atomic stimulated emission, we propose a stimulated amplified Faraday anomalous dispersion optical filter (SAFADOF) at cesium 1470 nm. The SAFADOF is able to significantly amplify very weak laser signals and reject noise in order to obtain clean signals in strong background. We show that for a weak signal of 50 pW, the gain factor can be larger than 25000 (44 dB) within a bandwidth as narrow as 13 MHz. Having the ability to amplify weak signals with low background contribution, the SAFADOF finds outstanding potential applications in weak signal detections.

Broadband, high-resolution Raman observations from a double-echelle spatial heterodyne Raman spectrometer

A new broadband Raman spectrometer has been developed, to the best of our knowledge, using a double-echelle spatial heterodyne Raman spectrometer (DESHRS). The instrument is constructed by using two echelle gratings. Masks are used to remove the shadow ghosts caused by the different orders of the two echelle gratings. Raman spectra of inorganic solid targets and methanol are given, and Raman shifts of up to 3000  cm^-1 are obtained by the DESHRS. The instrument has shown that a broadband coverage and high resolution can be achieved simultaneously to meet the requirements of Raman measurements, covering 3590  cm^-1 with 1.21  cm^-1 spectral resolution.

Broadband transmission Raman measurements using a field-widened spatial heterodyne Raman spectrometer with mosaic grating structure

A field-widened spatial heterodyne Raman spectrometer with a mosaic grating structure is developed for the simultaneous sensitivity enhancement and broadband transmission Raman measurements. We optimize the etendue to maximize the signals collected from the samples by using field-widening prisms and employ two mosaic gratings to achieve broadband operation, covering 5638 cm^-1 with 2.865 cm^-1 spectral resolution. The signal-to-noise ratios are improved by a factor of more than 11 and show a good stability and fair repeatability. We investigate the effects of the sample thickness and outer layer depth and observe liquids, solids, mixed targets, and anti-Stokes shifts. The instrument exhibits good performance for wide-field, high-resolution broadband transmission Raman measurements.

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

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.

Low-frequency shift Raman spectroscopy using atomic filters

A Faraday anomalous dispersion optical filter (FADOF) and an atomic resonant absorption filter are used in tandem to demonstrate a low-frequency shift Raman measurement down to few cm^-1. The FADOF, with an ultralow bandwidth of 0.08  cm^-1 at 780 nm, serves as a bandpass filter, while the rubidium atomic cell acts as a notch filter which has a bandwidth of 0.3  cm^-1. A proof-of-concept study to measure a Raman signal generated from a silica optical fiber is performed, demonstrating a low-frequency measurement of both the Stokes and the anti-Stokes shift down to 3  cm^-1 at an equivalent signal level. These results indicate the prospect for gigahertz-terahertz low-energy Raman spectroscopy based on atomic filters.

Magnetically tuned, robust and efficient filtering system for spatially multimode quantum memory in warm atomic vapors

Warm atomic vapor quantum memories are simple and robust, yet suffer from a number of parasitic processes which produce excess noise. For operating in a single-photon regime precise filtering of the output light is essential. Here, we report a combination of magnetically tuned absorption and Faraday filters, both light-direction insensitive, which stop the driving lasers and attenuate spurious fluorescence and four-wave mixing while transmitting narrowband Stokes and anti-Stokes photons generated in write-in and readout processes. We characterize both filters with respect to adjustable working parameters. We demonstrate a significant increase in the signal-to-noise ratio upon applying the filters seen qualitatively in measurements of correlation between the Raman scattered photons.