Optical Dispersion Measurements Research Articles

Shedding light on exciton's nature in monolayer quantum material by optical dispersion measurements.

Strong light-matter interactions based on two-dimensional excitons formed in quantum materials such as monolayer transition-metal dichalcogenides have become a major subject of research in recent years. Particularly attractive is the extraordinarily large oscillator strength as well as binding energy of the excitonic quasiparticles in these atomically-thin crystal lattices. Numerous theoretical studies and experiments have been devoted to the exploration of the excitonic systems that could be exploited in future nano-scaled optoelectronic devices. To obtain unique insight into the exciton's characteristics in an archetype monolayer quantum material, we directly measure the quasiparticle energy-momentum dispersion for the first time optically. Our results for h-BN encapsulated single-layer WSe2 clearly indicate an emission regime with a dispersion in the meV range in within the light cone at cryogenic temperatures. The amount of dispersion agrees well with calculations for an exciton-polariton based on the material's monolayer exciton, or energetic modifications caused by exciton exchange interactions predicted for this material family. The measurable dispersion slightly weakens for elevated excitation densities, whereas at elevated temperatures, it even becomes immeasurable. The obtained reduction in dispersion is attributed to an enhanced role of uncorrelated charge carriers as well as the formation of phonon sidebands above 100 K.

Method for Determining Average Iron Content of Ferritin by Measuring its Optical Dispersion.

We report a method where the refractive index increments of an iron storage protein, ferritin, and apoferritin (ferritin minus iron) were measured over the wavelength range of 450–678 nm to determine the average iron content of the protein. The protein used in this study had ∼3375 iron atoms per molecule. The measurement of optical dispersion over the broad wavelength range was enabled by the use of mesoporous leaky waveguides (LWs) made of chitosan. We present a facile approach for fabricating mesoporous chitosan waveguides for improving the measurement sensitivity of macromolecules such as ferritin. Mesoporous materials allow macromolecules to diffuse into the waveguide, maximizing their interaction with the optical mode and thus increasing sensitivity by a factor of ∼9 in comparison to nonporous waveguides. The sensitivity was further improved and selectivity toward ferritin was achieved by the incorporation of antibodies in the waveguide. The method presented in this work is a significant advance over the state of the art method, the enzyme linked immunosorbent assay (ELISA) used in clinics, because it allows determining the average content of ferritin in a single step. The average iron content of ferritin is an important marker for conditions such as injury, inflammation, and infection. Thus, the approach presented here of measuring optical dispersion to determine the average iron content of ferritin has a significant potential to improve the point of care analysis of the protein for disease diagnosis and screening.

Phase sensitive chirped laser dispersion spectroscopy under high absorbance conditions

A whole-fiber methane sensor under high absorbance based on phase sensitive chirped laser dispersion spectroscopy is presented in this paper. The laser source of the sensor is a tunable distributed feedback diode laser with a frequency of 1653.7 nm. A telecom-based electro-optical intensity Mach-Zehnder modulator working in carrier suppression mode is adapted to modulate the single frequency laser beam for generating a dual-sideband spectrum beside the carrier wave. Unlike previous proposed phase sensitive chirped laser dispersion spectroscopy scheme, the beatnote signal generated by the two sidebands is detected experimentally. The refractive index fluctuation around the 23 transition of methane is measured by detecting the phase variation of the dual-sideband beatnote signal through using the heterodyne interferometric method. A lock-in amplifier is employed in the phase demodulation process. By connecting the refractive index (the real part of the complex refraction index) and the absorption coefficient (the imaginary part of the complex refraction index) via Kramers-Kroning relation, the gas concentration information is retrieved from the optical dispersion measurement. Absorption-based wavelength modulation spectroscopy measures the gas concentration encoded in the optical intensity based on Beer-Lambert's law. However, the signal sensitivity of wavelength modulation spectroscopy decreases, and the signal even decreases while the gas concentration is raised in high absorbance condition, which leads to an uncertainty in concentration measurement. Experimental results demonstrate that wavelength modulation spectroscopy has better performance in low absorbance condition. The detection limit is about 38.1 ppmm. However, because the sensitivity decreases in high absorbance conditions, the upper detection limit of wavelength modulation spectroscopy is only 1500 ppmm. The dynamic range is defined through dividing the upper detection limit by the detection limit. Therefore, the wavelength modulation spectroscopy obtains a linear measurement dynamic range of 16 dB. Nevertheless, under the same experimental condition, the phase sensitive chirped laser dispersion spectroscopy has a much larger linear measurement range from 47.3 ppmm to 174825 ppmm with a dynamic range higher than 35 dB. Absorption-based gas measurement technique such as wavelength modulation spectroscopy can achieve a low detection limit by using long optical path at the expense of lower upper limit concentration. Phase sensitive chirped laser dispersion spectroscopy appears to be effective in high absorbance condition, which may be caused by high concentration or long optical path. Furthermore, by combining phase sensitive chirped laser dispersion spectroscopy and long optical path technique such as multi pass cell in sensor design, large linear measurement dynamic range and low detection limit can be obtained at the same time.

Three-wavelength parallel optical fiber dispersion measurement using dual-heterodyne mixing

We propose a novel optical dispersion measurement system using dual-heterodyne mixing to measure the relative phase. The system can realize parallel measurement of the relative phases between adjacent frequencies by introducing optical modulators to generate optical sidebands from a laser light source and an arrayed waveguide grating to separate the sidebands. To realize a wide dispersion range, different frequency intervals for the adjacent frequencies were combined in the system. One is the three-frequency optical dispersion measurement system (three-frequency measurement), which has been developed to measure the relative phase between adjacent peaks of an optical frequency spectrum with intervals of 25 GHz generated without any frequency scanning. The other is the four-frequency optical dispersion measurement system (four-frequency measurement) with intervals of 2 GHz generated from the three-frequency sets to expand the measurement range. The experimental results using single-mode optical fibers of different lengths from 0 to 90 km indicated the dispersion slope to be 16.8 ps/nm/km with a measurement range of 2500 ps/nm and an uncertainty of less than 1 ps/nm. The proposed system provides advantages to enable parallel measurement on a frequency axis without a high-speed (GHz) photodetector, even though GHz spacing on the optical scale is used, thus reflecting the dual-heterodyne mixing.

Single-shot quantitative dispersion phase microscopy.

We present an imaging modality capable of providing high-speed optical dispersion measurements of live cells. The technique permits wide-field measurement of the optical phase shifts introduced by a sample for multiple discrete wavelengths in a single image capture. Utilizing spatial modulation and the wavelength dependence of the interference-fringe spacing, average refractive index as a function of wavelength is obtained, yielding optical dispersion measurements of the sample under observation. Because of its simple and low-cost design, the technique can be readily integrated into a standard microscope to collect additional diagnostic information about biological cells.

Chirped laser dispersion spectroscopy with harmonic detection of molecular spectra

Chirped laser dispersion spectroscopy (CLaDS) has been introduced recently as a technique that performs molecular detection based on measurement of optical dispersion. In this paper, a new detection scheme based on chirp modulation (CM) and subsequent phase-sensitive detection is described. CM-CLaDS inherits the full advantages of conventional CLaDS and additionally overcomes some of its limitations. A prototype CM-CLaDS instrument has been developed and characterized in laboratory conditions. The system is based on a distributed feedback quantum cascade laser which operates around 4.52 µm and can probe the most intense nitrous oxide (N2O) ro-vibrational transitions. Preliminary performance tests are presented and provide a path/bandwidth normalized minimum N2O detection limit below 100 ppbv m/Hz1/2.

Nematic Liquid Crystal Optical Dispersion in the Visible-Near Infrared Range

A commercial variable angle spectroscopic ellipsometer from J. A. Woollam Company is modified in order to make possible measurements in guiding structures. The half leaky guided mode technique is realised for liquid crystal optical dispersion measurements. Features and problems related to experimental data interpretation and error sources in the refractive index measurements are carefully analyzed, achieving a final accuracy of ±0.0002. We present the measured optical dispersion curves in the wavelength range from 0.5 to 1.7 μm for two widely studied liquid crystals, namely E7 and 5CB.

Application of optical dispersion measurements using small-angle laser scattering for standardization and quality control of medicinal preparations

Application of optical dispersion measurements using small-angle laser scattering for standardization and quality control of medicinal preparations

Van der Waals attraction between St�ber silica particles in a binary solvent system

Hydrophilic Stober silica particles are stable in ethanol, but flocculation may be induced by the addition of sufficient cyclohexane. Low-shear rheological measurements indicated non-Newtonian behaviour beyond the critical cyclohexane concentration. The thickness and composition of the solvation layer around the particles were calculated from the adsorption excess isotherm on the basis of a multilayer adsorption model. The composition dependences of the Hamaker constants of the dispersion medium and the adsorption layer were obtained from optical dispersion measurements. A single-sheet, hard-sphere model predicted a weak van der Waals attraction in the ethanolic regime, but a strong attraction in the cyclohexane-rich region, in good accordance with the rheological properties of the dispersions and visual observations.

The properties of the adsorption layer and the stability of aerosil dispersions in binary liquids

From a knowledge of the adsorption, immersion and wetting properties, we have examined the influence of particle—particle and particle—liquid interactions on the stability and structure formation of suspensions of hydrophobic and hydrophilic aerosil particles in the mixtures benzene— n-heptane, methanol—benzene and 1-propanol—water. For the binary mixtures, the Hamaker constants have been determined by optical dispersion measurements over the entire composition range. The Hamaker constant of the adsorption layer, of composition different from that in the bulk, has been calculated for several mixture compositions on the basis of the above results. Having the excess isotherms available enabled us to determine the adsorption layer thickness as a function of the mixture composition. For interparticle attractive potentials, model calculations have been effected on the basis of the Vincent model. In the case of hydrophobic particles dispersed in benzene— n-heptane and methanol—benzene mixtures, it was established that the change in the attractive potential was in accordance with the interactions obtained from rheological measurements. However, for silica particles dispersed in 1-propanol—water mixtures, the structure-forming and rheological properties of the dispersed system are determined by the alcohol—water structure, i.e. by propanol—water clusters.

Thermodynamic properties of the S/L interfacial layer: Stabilization of the colloidal system in binary liquids

Abstract

Precise measurements of optical dispersion using a new interferometric technique

Measurements of the dispersion of the refractive index using a nonlinear interferometer are described. A sample of the optical material to be measured is interposed between two optically nonlinear crystals, and a moderately intense laser is passed through the combination. By observing the interference between the second harmonics produced in the two nonlinear crystals, the difference between the refractive indices of the sample at the laser frequency and its second harmonic frequency can be determined very precisely. We have used the interferometer to measure the dispersion in several gases between 1064 and 532 nm. We have also used it to determine the dispersion of two transparent solids. The method can measure the index difference to better than 0.1%, which for gases such as helium represents an absolute accuracy of ∼10−9.