Case Study

World-First Alignment-Free
Ti:S Laser Supports TechnicalAdvances in Holographic OpticalTweezing at Glasgow University

Holographic Optical Tweezers
Optical Tweezers are a scientific instrument now routinely used in the physical manipulation of
micrometer-scale objects such as individual cells, unicellular organisms such as bacteria and algae,
and molecular-scale cellular components,inorganic particles and nanotools. Originally termed thesingle-beam gradient force trap, optical tweezers take advantage of the phenomenon whereby tiny
particles can be trapped by the strong electric field gradient found at the focus of a laser beam (the“beam waist”) created by sending the beam through a microscope objective. The firstobservation of optical scattering and gradient forces was made at Bell Labs in the early 1970’s by
Arthur Ashkin. Since then, the use of force microscopy has become a widely used tool in bioscience.
During the 1980’s the technique was used to trap and study the E-Coli virus and in the 1990’s
molecular scale biological motors, the engines of intracellular logistics. The same principle has been
applied to the trapping of atoms (sub-nanometer scale) using resonant laser light and a magnetic
gradient trap.

Two forces work on the microscopic dielectric
particles trapped in the beam waist: their attraction
down the gradient to the location of the strongest
electric field and the buffeting they experiencefrom the light itself in the direction of its path. The
latter - scattering - force tends to push the trapped
particle slightly downstream of the heart of thebeam waist. Optical Tweezers therefore act like aspring which follows Hooke’s Law, with the trapped
particle held in a stable position close to the trapcentre. (During a recent visit to the Optics Group atGlasgow University, we also saw how the trapped
particles vibrated slightly as they were jostled byBrownian motion on the sample slide).

Glasgow University Optics Group
Our customer, the Optics Group in the School of Physics and Astronomy at Glasgow University, isa pioneer of optical tweezing technique and technology, including the use of haptic feedback incontrollers and the recent development of an iPad-based controller - iTweezers - downloadable
now from the App Store. Glasgow University Optics Group collaborates with non-specialist groups
in institutions around the world who want to use the latest techniques in optical tweezing fortheir own research purposes but who do not themselves have expertise in the associated opticaltechnology.

The first video below illustrates the iTweezers iPad application for controlling holographic opticaltweezers developed by the GU Optics Group. In the second video, we see a 1-micron diameter glass
bead being manipulated into a fissure in a microscopic fragment of chrome. When a haptic controller is used, the user experiences resistance at the point of insertion, as well as sensing the water inthe sample “feeling like treacle.”

The latest apparatus used by the GU Optics Group includes a SolsTiS CW Ti-S laser and an ICEcontroller, both of which are built into a set-up which also includes the microscope and server. This
was a key requirement of the group, that the laser would not need to be extracted from the system
for manual realignments, maintenance or adjustment. The main components in the holographic
optical tweezers are the Group’s proprietary controller, the microscope, the SLM which generates
the hologram and makes it possible to generate multiple focal points in a single beam (and therebymultiple traps) the SolsTiS laser and the ICE controller. In the photograph below you can see theSolsTiS enclosed within the apparatus which also includes the microscope.

SolsTiS cw Ti:S Laser
The SolsTiS is the world’s only alignment-free ultra-compact CW Ti:S laser. Tuning range and power
settings are all adjusted remotely via Ethernet. <read more...> “This has been important to us,” said
Dr. Graham Gibson, a post-doc researcher in the group who demonstrated the system for us. “Wehaven’t needed to touch the laser since it was installed into the system.”On the basis of its extreme reliability, usability and Ethernet Control, the SolsTiS is the tool of choice
for researchers pushing at the frontiers of holographic optical tweezing.