Page 14 - Science Focus (Issue 017)

Through this mechanism, scientists are able to move
cells and their constituent molecules around, with
micro- or nano-meter precision. Technically, spherical
particles whose wavelengths are “out of the range” for
optical tweezers can still be captured and manipulated.
To do so, scientists need to split the force of optical
tweezers into two: the gradient force, which is linearly
proportional to the gradient of the light intensity and
the dispersion force, which is linearly proportional to
the light intensity. Although the latter hinders capture,
it is the fundamental force in optical manipulation
and LASER cooling. Additional challenges came from
objects that are not spherical in shape. Ultimately, the
success of optical trapping is dependent on the precise
shape and composition of the experimental subject.
Despite of these obstacles, not only did the scientists
overcome them, but they also applied the technique to
nanotechnology, biology, random thermodynamics in
physics, spectroscopy, Casimir force, and active matter.

Among all applications that employ optical
tweezers, the most crucial one to medical development
is its application in spectroscopy. When combined
with Raman spectroscopy, which is a technique for the
detection of the vibrational and rotational modes of
molecules or crystal lattices, it is possible to diagnose
whether a cell is healthy or sick. The said combinational
technology is called Raman tweezers or laser tweezers
Raman spectroscopy (LTRS).
Using Raman tweezers, physicists Aseefhali Bankapur
et al. have conducted research on single living red
blood cells and white blood cells. They used high-
focused near-infrared (1064 nm) LASER to capture a
single cell, and used 785-nanometer light beam with
a power of several milliwatts as the incident light to
By Chih-yu Lee 李致宇 achieve Raman excitation. By using such highly sensitive
double-wavelength spectrophotometer, the researchers
succeeded in obtaining a signature Raman spectrum for
red blood cells. Unsurprisingly, some signals in the

When it comes to LASER (aka light spectrum were derived from hemoglobin. The same
amplification by stimulated emission of radiation), technology was used in the analysis of white blood
what will you think of? Is it the LASER gun in Star cells, including granulocyte and lymphocyte. Again,
Wars, or its everyday applications in scanner, optical characteristic vibrational spectra were obtained based
communication and CD player? Although we may on their constituting proteins and nucleic acids. What is
take it for granted, three excellent physicists, Arthur the significance of this study? The shape and content
Ashkin from Nokia Bell Labs, Gérard Mourou from École of blood cells can change when they are damaged
Polytechnique and Donna Strickland from University of or under stress. The ability to detect these changes
Waterloo, were awarded the Nobel Prize in Physics 2018 by Raman tweezers may therefore contribute to the
because of this technology. This article will introduce the accurate diagnosis of blood cell disorders.
optical tweezers invented by Arthur Ashkin.
After reading this article, I hope you appreciate
By the use of a highly focused light source, optical that a technology that warrants a Nobel Prize in
tweezers are a tool that allows optical trapping, and physics can also have a huge impact on other
the manipulation of many micro- and nano-scale fields. In this case, optical tweezers gave biologists a
materials. It harnesses the radiation pressure of light, precious tool to analyze cells in an unprecedented
which generates force to move tiny transparent objects. way. Can you think of any other examples?