CAPTURE OF AN ENSEMBLE OF MICROPARTICLES IN THE FIELD OF LASER MULTIBEAM INTERFERENCE S. B. Bushuk, * Yu. A. Kal’vinkovskaya, and A. N. Rubinov UDC [535:621.373.8]:539 Capture of an ensemble of polymeric microspheres in a water suspension under the action of optical gradient forces in the field of interfering laser beams has been investigated for the case of two-, three-, and four-beam interference. The self-diffraction and diffraction of radiation on induced ordered spatial distribution of parti- cles have been obtained. Keywords: optical gradient fields, multibeam interference, control of microobjects. Introduction. Gradient light fields are widely used in laser tweezers to manipulate single microobjects [1]. Application of an interference field instead of a focused laser beam makes it possible to carry out spatial ordering and control of ensembles of microparticles [2, 3]. On the other hand, regular interference patterns may serve as a new in- strument for affecting biological systems [4, 5]. As shown in [6], the space structure of a laser beam is an extremely important factor of the influence of light on the genetic properties and metabolic processes in living cells and tissues. In addition to two-beam interference and interference in Bessel beams [2], more complex multibeam interfer- ence schemes can be used. This allows one to create structures of microobjects ordered in two and three measure- ments, which opens up new possibilities for micro- and nanotechnologies [7]. Creation of ordered structures from an ensemble of microparticles can be observed either directly by using a microscope or by diffraction of an external light beam on such structures. The second approach allows one to easily record the dynamics of the formation and disordering of such lattices. In this work, the results of experimental obser- vation of the spatial alignment of an ensemble of microparticles under the action of two-, three-, and four-beam inter- ference of laser beams are presented. Experimental. The experimental setup is shown in Fig. 1. As a source of radiation, we used a Spectra-Phys- ics 2017-04s argon laser (Spectra-Physics, USA) operating in a TEM 00 mode at a wavelength of 514.5 nm. To form two parallel beams of the same intensity, we used a system of two Fresnel biprisms. The distance between the beams can be smoothly changed by changing the distance between the biprisms. This optical system makes it possible to smoothly change the period of interference bands from 6 to 22 μm. To obtain more complex interference patterns, we used a system of four Fresnel biprisms, which made it possible to obtain three and four symmetric parallel beams of the same intensity. The beams are brought together in a sample by means of a collecting lens with a focus distance of 30 cm and a local spot diameter of about 100 μm. A cell with a sample of thickness 200 μm is placed in the focal plane of the lens. After the cell, two beams (hereafter called main ones) pass through a 50% divider, which makes it possible to directly control their intensities and to observe diffraction. The sample can be observed simultaneously through the microscope objective using a CCD camera. As the sample, a suspension of calibrated one-piece polymeric microspheres of diameter 10, 5, and 3 μm in doubly distilled water was used. In the majority of cases, the objects formed a monolayer on a glass substrate. The laser radiation intensity was 2000 W/cm 2 . The measurements were carried out at a temperature of 20 o C. * To whom correspondence should be addressed. B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68 F. Skorina Ave., Minsk, 220072, Belarus; e-mail: bushuk@ifanbel.bas-net.by. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 2, pp. 267–269, March–April, 2004. Original article submitted September 29, 2003. Journal of Applied Spectroscopy, Vol. 71, No. 2, 2004 0021-9037/04/7102-0291 ©2004 Plenum Publishing Corporation 291