Laser-Induced Diffraction Rings from an Absorbing Solution

Abstract

We observed multiple diffraction rings of a cw Ar+ Iaser beam from a nitrobenzene solution 0L saturable absorber BDN (bis-(4-dimethylaminodithiobenzil)-nickel) caused by the spatial selL-phase modulation at low incident optical intensities. We obtained 37 rings for a 200-/Im thick sample at an optical intensity of 38 W/mm2. The refractive-index change An 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 An-0.1 and n2=-2 9xlO cm IW This large nonlinearity is attributed to a thermal effect resulting from linear absorption.The propagation of a laser beam in third-order nonlinear media with the intensity-dependent refractive index and absorption coefficient is accompanied by a variety of interesting phenomena. Among these are the selL-focusing and -defocusing, self-phase modulation, spatial ring formation, and bearn break-up. The spatial ring formation is understood to be induced by the spatial self-phase modulation arising from the laser-induced refractive index change. The multiple diffraction rings have been demonstrated in a film made of (or a cell containing) a wide variety of materials, i.e., dense atomic vapors such as potassium and sodium,1~5) nematic liquid crystals,6~ll) solids,12) and liquids.13) In reported experimental demonstrations using nematic liquid crystals,6,8,9) several tens of diffraction rings have been observed at an optical intensity of several hundred watts per square centimeter for a sample thickness of several hundred micrometers; but for all the materials except for liquid crystals, the number of observed rings is fewer than two or three. The number of rings depends generally on the on-axis nonlinear phase shift suffered during the passage through the sarnple. The phase shift depends on an optical intensity, magnitude and saturation value of the nonlinear refractive index, sample thickness, etc. The attainable refractive index change of nematic liquid crystals is about 0.1. The large nonlinearity is due to molecular reorientations of liquid crystals induced by an optical field and resulting birefringence.In this letter, we demonstrate the multiple diffraction rings from a solution of saturable absorber at low input powers and discuss the ring formation and the origin of the nonlinearity used. In fact, we accidentally found that diffraction rings were easily obtained in a solution of saturable absorber bis-(4-dimethylarnino-dithiobenzil)-nickel (BDN) during a z-scan measurement of its nonlinearity. We are using this dye as a nonlinear material to develop a high-speed fiber-type bistable device, the prototype of which the authors proposed earlier.14) BDN dye is a saturable absorber developed for Q-switching and modelocking of an Nd: glass laser operating at infrared.15) This dye has also been used for demonstrating a bistable FabryPerot resonator with a switching time of the order of nanoseconds.16) We used nitrobenzene as a solvent in our experiment. This solvent was selected for its durabi...

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“…Therefore, we can infer that the increase of H 0 results in a larger OPD and creates a larger number of diffraction rings. This conclusion coincides with conclusions drawn from nematic crystal research, 12,15 where the number of rings N ring was estimated as…”

“…To provide quantitative validations between theory and experiment, correlations between three different approaches were investigated: estimation from nematic crystal research, 12,15 estimation from the proposed model, and the experimental result ͑Fig. 9͒.…”

“…This implies that the magnitude of the slope of the wavefront is the key factor to determining the maximum diffraction angle, a theory that was also suggested by other researchers using nematic crystals. [12][13][14][15][16] Previous research from nematic LCs reported the equation for estimating the maximum half cone angle / 2 max as,…”

“…[11][12][13][14][15][16][17][18][19][20] LC molecules were rotated in proportion to the intensity of the incident laser, which results in refractive index variations with a Gaussian profile. When this laser passes through an LC film of several hundred micrometer thickness, the optical wavefront is modulated to create a Gaussian wavefront that is propagated to a screen to create several diffraction rings.…”

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“…The correlation of phase modulation and the observed diffraction rings was also commented in the Nematic liquid crystal research (Bloisi et al, 1988;Chen et al, 1998;Khoo et al, 1987;Ogusu et al, 1996). The authors pointed out the relationship of the phase modulation and the number of diffraction rings as:…”