Position fluctuations of 1.1 um trapped bead.

Optical tweezers

Optical tweezers can be simply described as a golf course made up of light, where there may exist single or multiple potential wells (akin to holes in the golf course) of spatial dimension of the order of the wavelength of the light used to construct the tweezers. Dielectric or even metal particles fall into this potential ‘hole’ (they essentially feel a restoring force towards the center of the hole if they try to come out) and are trapped stably for hours. One needs a liquid environment (most commonly water) for the particles – since otherwise, the particles diffuse about too quickly (and thus have large kinetic energy) to fall into the well – just as a golf-ball putted too hard would skip over a hole! In fact, the viscosity of the liquid where the particles diffuse around is so high that the inertia of the particles plays no role in their dynamics, since the kinetic energy is damped out much faster than the experimental time scales typically employed to measure the dynamics of the particles. Thus, a single particle confined in tweezers and executing Brownian motion while confined, obeys an over-damped linear harmonic oscillator equation with the inertial term dropped and a stochastic noise term (Brownian motion) on the right hand side of the equation. This is essentially a Langevin equation of first order, whose solution for particle displacement is an exponential that damps out extremely fast so that what one measures experimentally is just the fluctuations. You will find details on the math here.

Experiments

Optical tweezers (OT) was first demonstrated by Arthur Ashkin at Stanford in 1986 where he showed the confinement of latex micro-beads in water using a focused laser. Over the next three decades, the field has really expanded a lot, but mostly as a singularly useful means to manipulate single micro or nano-particles ranging from metal nano-particles to biological cells or even DNA. However, in most research involving OT, light is used as a work-horse to manipulate particles. What is often neglected is that the configuration of OT allows interesting studies of the diverse properties of light itself. This is because the trapping light is focused to diffraction limited spot sizes, where one of the most important assumptions of electromagnetic theory – namely the paraxial approximation breaks down. This leads to a number of diverse effects, chiefly involving the spin orbit interaction (SOI) (give link to expts on SOI) of light, where the polarization, intensity, and trajectory of light all become coupled to each other. Thus, while OT can understandably be used for diverse applications involving micro-manipulation of small particles, they also lead to very interesting fundamental studies of light. Our laboratory, which launched off in 2009, with the first trapping even occurring in July 2010, focuses on both aspects of OT, fundamental studies in optics and soft matter, and applications. We do not simply perform experiments, but develop the associated theory as well. Read about our experiments here.

Set-up

We have presently three optical tweezers systems. The instrumentation of OT at the foremost involves a microscope – we typically use inverted microscopes for most of our OT systems, though we do have an upright microscope as well for some of our experiments. The set-up is rather standard, and described in detail in our paper in RSI. We have two inverted microscopes – one from Zeiss (Axiovert.A1), and the other from Olympus (IX71), and one upright indigenous microscope from Debro Engineers. The lasers we use are all low-cost solid state lasers from Chinese companies – and all of them have worked pretty well! We make our sample chambers in a very simple manner, we take a cover-slip (glass or polymer, depending on the application), put a drop of sample (around 20 ul), and sandwich it with a glass slide (standard microscope slide). At times we put a sticky-tape to increase the chamber thickness. Thus, our normal, non-sticky-tape chambers are about 25-30 um in depth/height, while the sticky-taped ones are around 100 um.

Our Research and Experiments

While the lab predominantly focuses on OT, we are also involved in other spectroscopy and precision interferometry experiments – hence the name ‘Light Matter lab”! To read about our specific work, please click on the links below.

  1. Trap calibration and dynamics of trapped particles

  2. Fundamentals of Light

  3. Soft matter physics using Optical Tweezers

  4. Physics of Micro-bubbles

  5. Bio-experiments

  6. Optical trapping of absorbing mesoscopic particles in air

  7. Adaptive optics based imaging and spectroscopy