Multiple Diffraction Rings Research Articles

Motion of submicrometer particles in micrometer-size channel measured by defocusing nano-particle image velocimetry

Understanding the motion of colloidal particles flowing in small spaces is a general issue in various fields such as thermal engineering and micro/nanofluidics. In the present study, we investigated the motion of fluorescent submicrometer particles in a 3-μm microchannel by defocusing nanoparticle image velocimetry. An optical measurement system with controlled spherical aberration and an algorithm for processing defocused particle images with multiple diffraction rings were developed. By detecting the centroid position and the diameter of the outermost diffraction ring, which is proportional to the distance between the focal plane and the particle, the position of particles was determined with the spatial resolutions of 154–204 nm in the streamwise direction and 76–311 nm in the depthwise direction, which are comparable to or smaller than the optical diffraction limit. A reusable microfluidic device containing a size-regulated microchannel made of glass was developed, which is suitable for optical measurements and precise flow control. By controlling the strength of low-temperature glass bonding, detachment of the bonded glass substrates, washing, and reuse were achieved. Based on this method and technology, the velocity of particles with diameters of 199, 457, and 1114 nm was successfully measured in pressure-driven laminar flow. Results suggested that for larger particles comparable to the channel size, the particle velocity is slowed from the flow velocity by particle–wall hydrodynamic interactions. Therefore, the motion of colloidal particles in 100-μm spaces is considered to be affected by particle–wall hydrodynamic interactions, as well as 102-μm spaces reported previously.

Determination of nonlinear refractive index of paprika oil and pepper oil under cw visible laser illumination

We report the observation of multiple diffraction rings patterns in paprika and pepper oils using continuous wave laser beams .The patterns occur at relatively low intensities . The number of rings together with outermost ring diameter of each pattern increases monotonically with increasing input power . The total changes in refractive index , ?n ,and nonlinear refractive index,n2 ,are determined using the number of rings observed in both samples and 473 nm and 532 nm laser beams . We obtained good values of ?n = 0.01 and n2 = 10-6 cm2/w . These large values are attributed to thermal effects .

Far-field diffraction patterns and optical limiting properties of bisdemethoxycurcumin solution under CW laser illumination

In this work, nonlinear optical properties of bisdemethoxycurcumin solution were studied using diffraction ring patterns method. Multiple diffraction rings observed as a continuous wave (CW) laser beam passes through a bisdemethoxycurcumin solution. The nonlinear refractive index, n2, and change in refractive index, Δn, for sample were determined from the number of observed rings, and are found to be 0.8327 × 10−6 cm2/W and 0.00946 respectively. The evolution of the diffraction ring patterns are numerically studied using the Fresnel–Kirchhoff diffraction integral theory. The experimental findings are in good agreement with theoretical analysis. Also the optical limiting properties of the sample were measured at 473 nm. The obtained results proved that the sample bear good optical limiting property.

Nonlinear absorption and spatial self-phase modulation in Ag2Se colloidal nanocrystals

The third-order optical nonlinearities of Ag2Se nanocrystals were investigated by using Z-scan techniques. Large optical susceptibility χ(3) with the imaginary and real parts of 5.0 × 10−11 esu and −(3.0 ± 0.2) × 10−9 esu has been obtained under 1 mW laser excitation, which is believed to originate from the intrinsic nonlinearity of Ag2Se nanocrystals. When 150 mW excitation power was applied, multiple diffraction rings were observed. The kinetic imaging results demonstrate that the rings originate from the self-phase modulations caused by the intrinsic nonlinearity and the thermal effect.

Effect of conjugation length on nonlinear optical parameters of anthraquinone dyes investigated using He–Ne laser operating in CW mode

Abstract We report the studies on third-order optical nonlinearity and optical limiting of anthraquinone dyes. Z-scan technique was employed to evaluate the nonlinear parameters such as nonlinear absorption coefficient βeff and nonlinear index of refraction n2. Continuous wave He–Ne laser was used as the source of excitation. The estimated values of βeff, n2 and χ(3) are of the order of 10−3 cm/W, 10−5 esu and 10−7 esu respectively. The presence of donor and acceptor groups in the structure results in increase in conjugation length. This resulted in the enhancement of nonlinear optical parameters values of the dye. Multiple diffraction rings were observed when the samples were exposed to laser beam due to thermal lensing. Dyes exhibited good optical limiting behavior under the experimental conditions. The results indicate that the dyes investigated here are materialise as candidates for photonics device applications such as optical power limiters.

XRD<sup>2</sup> Stress Measurement for Samples with Texture and Large Grains

Stress measurement on samples with texture and large grains is always a challenge. The diffraction peak intensity varies dramatically with different sample orientation. The macroscopic elasticity becomes anisotropic due to strong preferred orientation. The large grains may results in a big error in 2θ due to poor sampling statistics. The fitting results of the conventional sin2ψ method is extremely sensitive to texture and large grains. When stress is measured with a 2D detector, most of the above adverse effects can be minimized or eliminated. The data integration helps to smooth out rough diffraction profiles due to large grain size, texture, small sample area or weak diffraction. The large angular coverage and multiple diffraction rings can minimize the effect of the macroscopic anisotropy. The weighted least squares regression and intensity threshold can further reduce the effect of poor statistics associated with texture and large grains. Multiple {hkl} rings may be used to measure the stress to improve the statistics and minimize the elastic anisotropy effect.

Thermally induced nonlinear optical response and optical power limiting of acid blue 40 dye

We report the investigations of thermally induced third-order nonlinear optical and optical limiting characterizations for various concentrations of acid blue 40 dye in N,N-Dimethyl Formamide, studied by employing z-scan technique under cw He–Ne laser irradiation at 633 nm wavelength. The samples exhibited nonlinear absorption and nonlinear refraction under the experimental conditions. For lower concentration, the samples display both saturable absorption (SA) and reverse saturable absorption (RSA); whereas with increase in concentration, RSA behaviour prevails. The estimated values of the effective coefficients of nonlinear absorption βeff, nonlinear refraction n2 and third-order nonlinear susceptibility χ(3) were found to be of the order of 10−2 cm/W, 10−4 esu and 10−6 esu respectively. Multiple diffraction rings were observed when the samples were exposed to laser beam due to refractive index change and thermal lensing. The effect of concentration and the laser intensity on the self-diffraction ring patterns was studied experimentally. The acid blue 40 dye also exhibited strong optical limiting properties under cw excitation and reverse saturable absorption is found to be the dominant nonlinear optical process leading to the observed nonlinear behaviour.

Effect of annealing on the structural and nonlinear optical properties of ZnO thin films under cw regime

We report on the studies of the effects of annealing on the structural and third-order nonlinear optical properties of ZnO thin films deposited on quartz substrates by the RF magnetron sputtering technique. The films were annealed in the temperature range 400–1000 °C. The third-order nonlinear optical studies were carried out using the z-scan technique under continuous wave (cw) He–Ne laser irradiation at 633 nm wavelength. The effects of annealing on the structural properties were examined using x-ray diffraction and atomic force microscopy (AFM). The (0 0 2) preferred orientation increased with increase in annealing temperature up to 800 °C. The crystalline phases of SiO2 were observed at higher annealing temperatures. The appearance of an extraneous phase was confirmed by AFM images and optical transmittance spectra. The samples exhibited nonlinear absorption and nonlinear refraction under the experimental conditions. The negative sign of the nonlinear refractive index n2 indicated that the films exhibit self-defocusing property due to thermal nonlinearity. The nonlinear refractive index n2, the nonlinear absorption coefficient βeff and the third-order optical susceptibility χ(3) were found to be of the highest orders. The estimated value of third-order optical susceptibility χ(3) was of the order of 10−3 esu. Multiple diffraction rings were observed when the samples were exposed to the laser beam. The appearance of rings was due to the refractive index change and thermal lensing. With increase in laser intensity, the variations of the self-diffraction ring patterns were studied experimentally. The films also exhibited strong optical limiting properties under cw laser excitation, and reverse saturable absorption was the dominant process leading to the observed nonlinear behaviour.

χ(3) and optical power limiting measurements of Polyaniline and its derivative Poly (o-toluidine) under CW regime

We present the synthesis and results on the third-order optical nonlinearity and optical limiting properties of Polyaniline and Poly (o-toluidine) studied in N,N-Dimethyl Formamide solution using a single-beam z-scan technique. Experiments were performed using continuous wave He–Ne laser operating at 633nm wavelength as an excitation source. The samples exhibit absorption and refraction nonlinearities. The negative sign of the nonlinear refractive index n2 indicates that both materials exhibit self-defocusing optical nonlinearity. The nonlinear refractive index n2, the nonlinear absorption coefficient βeff and the third-order optical susceptibility χ(3) were determined and found to be of the order of 10−7cm2/W, 10−2cm/W and 10−7esu respectively. The FTIR and UV–visible characterization spectra confirm the expected molecular structure of the polymers. Multiple diffraction rings were observed when the samples were exposed to laser beam. The appearance of rings is due to the refractive index change and thermal lensing. The polymer samples with different concentrations exhibits strong optical power limiting under continuous wave laser at the experimental wavelength. Reverse saturable absorption is the dominating mechanism for the observed absorption nonlinearities.

Investigation of the optical limiting properties of acid blue29 in various solvents

The optical limiting properties of acid blue29 solutions in different solvents are investigated. Experiments are performed using a CW He–Ne Laser beam at 632.8 nm wavelength and 35 mW power. The strength of the optical limiting action is dramatically influenced by the solvent. Multiple diffraction rings were observed when the samples were exposed to laser radiation. The effect of concentration, solvent and laser intensity on the diffraction rings are studied experimentally. Our results show that the nonlinear refraction is the dominant mechanism to explain the optical limiting behavior in acid blue29.

Investigation of optical nonlinearities of an azo dye using a 532 nm diode-pumped Nd:YAG laser

Solid-state dye-doped polymers are attractive alternatives to the conventional liquid dye systems. In this paper, the third order nonlinear optical property of Sudan III, an azo dye in solvents and dye doped polymer films measured using the Z-scan technique is reported. Multiple diffraction rings were observed when the dye doped in both the liquid and solid media was exposed to a diode-pumped Nd:YAG laser at 532 nm.

Self-diffraction and optical limiting properties of organically modified sol–gel material containing palladium-octaisopentyloxy-phthalocyanine under cw laser illumination

Abstract Palladium-octaisopentyloxy-phthalocyanine-doped organically modified silicate (ormosil) has been prepared. Significant optical multiple diffraction rings of the sample under cw Ar + laser illumination at wavelengths of 457.9, 488.0 and 514.5 nm were observed. Correspondingly, the sample exhibited flexible optical limiting properties when a size-changeable aperture was used. 457.9 nm has the best limiting effect among the three wavelengths.

Laser-Induced Diffraction Rings from an Absorbing Solution

We observed multiple diffraction rings of a cw Ar+ laser beam from a nitrobenzene solution of saturable absorber BDN (bis-(4-dimethylaminodithiobenzil)-nickel) caused by the spatial self-phase modulation at low incident optical intensities. We obtained 37 rings for a 200-μm thick sample at an optical intensity of 38 W/mm2. The refractive-index change Δ;n and effective nonlinear refractive index n2 were determined from the number of observed rings and by the z-scan technique. We obtained large values of Δ;n∼0.1 and n2 = -2.9×;10-5 cm2/W. This large nonlinearity is attributed to a thermal effect resulting from linear absorption.

Single beam self-interaction in Langmuir-Blodgett films

Multiple diffraction rings are observed after a pulsed laser beam passes through an absorbing dye LB film. The theoretical analysis, based on the model of transverse self-phase modulation, is also given. The saturated nonlinear refractive index n 2 is estimated to be 2.0×10 −6 cm 2 W from the number of rings induced in the LB films. The main contribution to this larger nonlinearity is believed to be from laser-induced transformation between H-aggregates and monomers of molecules in the LB films.

Transient multiple diffraction rings induced by ultrafast laser from chinese tea

Transient multiple diffraction rings associated with self-defocusing from chinese tea solution in ethanol are observed during the time of laser “turn on”. A second harmonic of cw mode-locked Nd:YAG laser was used. The phenomenon is shown to be the result of laser induced refractive-index change. Diffraction ring patterns under different conditions are compared. The dependence of ultrafast laser induced fluorescence on the polarization of laser beams is also observed. A tentative explanation is given.

Laser-induced diffraction rings from a nematic-liquid-crystal film

Multiple diffraction rings appear as a cw laser beam passes through a homeotropic nematic film. The phenomenon is shown to be the result of spatial self-phase modulation that is due to the laser-induced Freedericksz transition.