Photon-Force Controlled Molecular Assembling in Solution

Abstract

Laser light can be focused to a small spot of the order of the wavelength of the light, which gives lasers the potential to be used for a variety of chemical and physical applications in the micrometer to submicrometer domains. Spectroscopic characterization and studies of molecular photochemical and photophysical processes at these dimensions under microscopes have become routine in recent years [1–3]. Another characteristic of a focused laser beam is the “photon force”, which was originally proposed by Newton, treated theoretically by Maxwell, and confirmed experimentally by Lebedev [4], and has been traditionally called “radiation pressure”. The photon force is generated by changes in photon momentum when light is refracted at the interface between a medium and a μm-sized particle with different refractive indices [5–9]. This force enables noncontact and nondestructive manipulation of microparticles in solution; a phenomenon developed recently in the “laser trapping” technique. The photon force has received much attention in the field of optics and was first applied for trapping μm-sized particles by Ashkin [5]. In his series of studies the phenomena have been clarified and its application as a laser trapping method has been widely explored in optical measurements of microscopic systems. The potential of laser trapping was demonstrated particularly in investigations of biological systems and also the superiority of using near-infrared (IR) laser light (1064 nm) instead of a visible beam (514.5nm) was confirmed [9]. The three-dimensional micromanipulation of microparticles with the use of near-IR laser trapping was demonstrated more recently by us [10] and extended to particles with high reflection coefficient or with refractive index lower than that of the surrounding medium [11]. Thus this technique is now recognized as having a potential in physical chemistry. The spatial pattern formation, size selection, and assembling of microparticles in solution were achieved in noncontact mode [12–15]. Also studies of chemical processes like laser ablation [16] as well as the spectroscopic investigations of manipulated individual microparticles [17–21] were made possible in small domains. More sophisticated output is work on the lasing of a single microparticle and its application to intracavity spectroscopy [22].