Don't you have, uh, ray guns?
-The Terminator (1984)
The Nobel Prize winners for Physics this year have all contributed significantly to the physics of Lasers - 'Light Amplification by Stimulated Emission of Radiation'. Laser is not merely related to a 'B coefficient' from Einstein, it has moved beyond the theoretical field to contribute in many areas. The wonderful aspect is that the winners' contributions to laser have demonstrated applications in molecular biology, chemistry and medical science alike.
Professor Arthur Ashkin is awarded the Nobel Prize for the invention of optical tweezers. That is a laser-based approach for trapping and manipulating particles. Optical tweezers rely on the radiation pressure of light (which generates force) to do the job. While it may appear weird or may sound like something from an episode of Star Trek, radiation pressure exists in all electromagnetic radiation. While this type of pressure appears weak for a human being, it certainly is not so for a particle. Ashkin has discovered that a particle interacting with a laser beam will be pushed towards a center with the highest intensity, due to the action of radiation pressure. By using a lens to properly focus the laser light, the particle is further drawn towards the point with the greatest laser intensity. In a sense, it looks as if the laser beam is 'holding' the particle at that point. Ashkin's light trap will eventually be called the 'optical tweezers' as we know nowadays.
It is through the use of laser that optical tweezers can be devised and they have been applied in fields like biophysics and chemical biology, where single molecule or particle-sized materials have to be 'held up' and investigated.
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| Figure 1. From Ref. [1]. |
A famous use of optical tweezers in biochemistry is the study of molecular motors of the cytoskeleton system (Figure 1). Kinesin, a motor protein which is involved in moving cargo along the microtubules, has served as a testament to the use of optical tweezers in molecular biology.
The kinesin protein is first attached to a bead which is held up by the optical tweezers. When the kinesin starts moving along the microtubules, the bead is pulled along and it can be visualized and quantitatively established how far the molecular motor has gone. In a sense, the displacement of the bead is like its escape from the clutch of the optical tweezers. At the point when the kinesin has moved too far away, the bead will then be 'bounced back' into the focus point of the optical tweezer - like a spring restoring to its equilibrium.
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| Figure 2. From Ref. [2]. |
The other half of the Nobel Prize was awarded to the approach known as 'chirped pulse amplification', which has led to the generation of high intensity and ultra-short optical pulses. In a nutshell, it is a stress-and-compress process. First, an ultra short laser pulse is stretched in time, reducing its peak power. The pulse is linearly chirped through a dispersion, and it is amplified in a laser material, afterwards it is compressed in time to its original duration. The result is an significant increase in peak power.As a result, the laser pulses have enhanced intensity.
The high intensity laser is important because whenever we want to deal with situations requiring good precision, the energy source must be very focused to minimize undesirable thermal energy around the target, leading to collateral damage. That is indeed why high intensity laser has been used in surgery, because the ultra short pulse can give very precise cuts on the area.
Ultra short laser pulses can also be applied in spectroscopy. For example, the generation of charges in semiconductor devices can be monitored by spectroscopic approaches, yet there are challenges associated with that. Because the time scale of the photophysical processes are very short, therefore we need fast and accurate spectroscopic techniques – known as transient absorption spectroscopy, to deal with that. A well-established approach is known as the pump-probe spectroscopy, which often uses laser as an energy source. From the name, we can realize there are 2 beams that leads to the process. A pump beam is used to excite the (semiconductors) material to the excited energy state; and a probe beam is used to monitor the excited state. The advantage of pump-probe is that because the pump and probe beam can be fine-tuned, through the pulse length of the laser and the photon density of the probe beam, therefore it can monitor processes with a reasonable wide time-scale, leading to its versatility as a tool for photophysics.
All in all, it is great to see that the contributions from physics can also be nicely applied in chemistry and biology too!
by Ed Law
Reference:
1. http://www.iiserpune.ac.in/~cathale/lects/2011monsoon/bio322-student/014.pdf
2. Zeitoun et. al., X-ray Chirped Pulse Amplification: towards GW Soft X-ray Lasers
Appl. Sci. 2013, 3(3), 581-592.
