Intracavity optical trapping with fiber laser

Abstract

After Ashkin's seminal works, optical trapping has been a powerful technique for capturing and manipulating sub micro particles not only in physics research fields but also in biology and photonics. Standard optical tweezers consists of a single beam with Gaussian or profile which focused by a high numerical aperture (NA) water or oil immersion microscope objective. Typically, objective with NA>1.2 is used to provide strong enough gradient forces being able to overcome Brownian

uctuations and gravity and trap the particle stably. On the other hand, compare with high NA, trapping with low NA, has its own advantage and among all the advantages, low local heating of the sample has a particular interest in molecular biology and manipulating living cells. The main concern is that the interaction of trapping laser beam and biological object induces a damage on the specimen which is mainly due to light absorption of the sample. It is, therefore, recommended to use NIR (near infrared ) wavelength due to its minimal absorption by water and biological objects. Other important factors that must be considered, to secure the viability of the cell, are spot size of the focused beam and laser power at the sample plane. Thus, it deserves an effort to look for new configurations with low NA with the capability of creating 3D confinement. Standard optical tweezers rely on optical forces that arise when a focused laser beam interacts with a microscopic particle: scattering forces, which push the particle along the beam direction, and gradient forces, which attract it towards the high-intensity focal spot. Importantly, the incoming laser beam is not affected by the particle position because the particle is outside the laser cavity. Here, we demonstrate that intracavity nonlinear feedback forces emerge when the particle is placed inside the optical cavity, resulting in orders-of-magnitude higher confinement per unit laser intensity on the sample. We first present a toy model that intuitively explains how the microparticle position and the laser power become nonlinearly coupled: The loss of the laser cavity depends on the particle position due to scattering, so the laser intensity grows whenever the particle tries to escape. We describe a simple toy model to clarify how the nonlinear feedback forces emerge as a result of the interplay between the particle's motion and the laser's dynamics. It also quantifies how and to what extent this scheme reduces the average laser power to which a trapped particle is exposed. In this model, the power and hence trapping force are considered to be zero for small particle displacements. However, in reality they have small values that do operate the trap even when the particle is near the equilibrium position. Thus, we need an accurate description of the coupling between the laser and the trapped particle thermal dynamics at equilibrium to compare with experiments. In particular, accurate simulations can help to associate an effective harmonic potential to the optical trap for small displacements from the equilibrium position, and hence to define a meaningful stiffness using the standard calibration methods based on the thermal uctuations of a trapped particle. We therefore present a series of numerical simulations based on an extended theoretical model, including highly realistic descriptions of the laser dynamics, optical losses incurred by the particle, and the particle's Brownian motion in order to gain a quantitative understanding of the dynamics of intracavity optical trapping and to guide the experiments. Finally, guided by the simulation results, we have built an experimental setup to prove the operational principle of intracavity optical trapping and experimentally realize this concept by optically trapping microscopic polystyrene and silica particles inside the ring cavity of a fiber laser. One of the major advantages of the intracavity optical trapping scheme is that it can operate with very low-NA lenses, with a consequent large field-of-view, and at very low average power, resulting in about two orders of magnitude reduction in exposure to laser intensity compared to standard optical tweezers. When compared to other low-NA optical trapping schemes, positive and negative aspects can be considered, such as in terms of trap stiffness and average irradiance of the sample. These features can yield advantages when dealing with biological samples. Ultra-low intensity at our wavelength can grant a safe, temperature controlled environment, away from surfaces for micro uidics manipulation of biosamples. Accurate studies on Saccharomices cerevisiae yeast cells in near-infrared counterpropagating traps and standard optical tweezers have found no evidence for a lower power threshold for phototoxicity. We observed that we can 3D trap single yeast cells with about 0:47 mW, corresponding to an intensity of 0:036 mW m􀀀2, that is more than a tenfold less intensity than standard techniques.