Self-trapping of light through red blood cell suspensions
Rekha GautamWe observe self-trapping and deep penetration of a laser beam in human red blood cell (RBC) suspensions under isotonic, hypotonic, and hypertonic conditions, in spite of the intrinsic absorption and scattering loss due to RBCs. OCIS codes: (170.4520) Optical confinement and manipulation; (190.5940) Self-action effects Optical manipulation in strongly-scattering suspensions such as blood remains a challenge, largely because light diffuses and loses directionality in biological suspensions. Formulation of various approaches to achieve low-loss deep penetration of light in soft-matter environments is requisite for many applications. Recently, in a few types of soft-matter systems, including dielectric and plasmonic nanosuspensions, we have achieved low-loss deep penetration of light [1]. Despite significant efforts in the study of optical properties of biological media, it is commonly thought that light cannot penetrate deeply into biological environments due to strong scattering loss and weak optical nonlinearity. Here, we demonstrate robust propagation and enhanced transmission of self-trapped light over long distances through suspensions of human red blood cells (RBCs). By deliberately altering the host environments, we show dramatic change of nonlinear response and propagation of a light beam, while the cells remain intact and viable. Our results may open up new opportunities in developing bio-soft-matter systems with tunable optical nonlinearities for various applications such as in optofluidics, bio-fabrication, deep tissue imaging, and bio-fuels. It is well known that RBCs exhibit distinctive deformability from the application of an external force. This intrinsic property of RBCs enables them to modify their shape as they flow through microcapillaries [2]. Deformation can also be elicited by the liquid buffer osmolarity. In a recent study which hints at the potential of RBCs, researchers adjusted the liquid buffer's chemistry and utilized RBCs as tunable optofluidic lenses [3]. In the present study, human blood samples (obtained from anonymous donors through the Blood Centers of the Pacific) were collected in EDTA tubes, centrifuged, and washed before diluted in phosphate-buffered saline (PBS) to a concentration of about 10 7-10 8 cells mL-1 in a 2 cm glass cuvette. To study the effect of shapes on the optical forces, RBCs were dispersed intentionally in buffers with different osmolarity. In the isotonic PBS, RBCs exhibit biconcave disc shapes; in the hypotonic buffer, RBCs swell up to spherical shapes; and in the hypertonic buffer, RBCs shrink [4]. A collimated laser beam (í µí¼ = 532 nm) was focused close to the interior front surface of the sample, and the output beam profiles were recorded by a CCD camera. We studied nonlinear beam propagation in human RBC suspensions in isotonic, hypotonic and hypertonic PBS buffer solutions, and found that the RBCs exhibit a strong self-focusing nonlinearity that can be fully controlled by the liquid buffer's chemistry. Typical experimental results are presented in Fig. 1. The focused laser beam diffracts normally in the PBS media alone (i.e., without RBCs) at all laser powers tested ranging from 50 to 800 mW (Fig. 1a,i). However, once RBCs are added into PBS suspensions, nonlinear self-trapping of light is observed. In all three RBC suspension conditions, the beam first diffracts normally at a low power of about 50 mW (Fig. 1f-h), and then undergoes nonlinear self-focusing as the power increases (Fig.4j-l). The nonlinear self-action of the beam suggests that the RBCs have a strong nonlinear response, possibly due to their positive polarizability (the refractive index of the RBCs varies between 1.38-1.42, higher than that of the water 1.33). As the power is further increased above the self-trapping power, the beam cannot remain in a self-trapped channel and it begins to deteriorate into multiple filaments. Interestingly, the power required to self-trap the beam to the minimal beam size depends on the host media for the RBCs (Fig. 4m). In hypotonic condition (Fig. 1k), nonlinear self-trapping occurs at slightly lower power (100mW less) than in an isotonic condition. The reduced power requirement may be ascribed to the shape change in RBCs: In a hypotonic solution, RBCs take approximately a spherical shape, as opposed to a concave disk shape in an isotonic condition [3, 4]. Thus, the optical forces acting on hypotonic RBCs could be stronger. On the other hand, in the hypertonic RBC suspension (Fig. 1l), self-trapping of light requires slightly higher power (100mW more) than that in the isotonic condition due to the asymmetric shape of RBCs under the hypertonic condition that produce an enhanced diffusion of light. The transmission percentage through three different buffer conditions (Fig. 4n) clearly shows a higher transmission in the case of the hypotonic RBCs suspension as compared to the other two conditions.