Laser, laser burning bright,
In the lab, through the night
What immortal ideas
Could frame thy emission process?
–modified from William Blake’s ‘The Tyger’
Light Amplification by Stimulated Emission of Radiation or LASER is a light source which is ubiquitous in the world around us. They are extensively used in scientific laboratories to study phenomenon spanning various sizes: from astronomy to sub-atomic particles. A laser has three vital characteristics: they are a monochromatic, coherent and highly directional in nature, which make them unique emitters of light. Unlike conventional light sources, such as tube-lights, lasers carry a relatively large amount of energy in a confined volume, and hence can propagate over a longer distance and strongly interact with matter. These properties of lasers have not only made them a fascinating topic of fundamental research, but also play a critical role in various applications.
Vital Quest: Almost all the lasers that have been produced are made of abiotic matter, that is, matter that does not have life. An interesting question to ask is: can we create a laser out of a living system, such as biological cell?
The answer is YES. In 2011, two optical physicists, Malte C. Gather and Seok Hyun Yun, then at Harvard Medical School, came up with an interesting experiment. They transfected green florescent proteins inside a human embryonic kidney cell, and placed the cell between two high-quality mirrors, and optically pumped the cells with blue pulsed light (see figure 1(a)). Interestingly, this experiment resulted in lasing action at around 513nm wavelength from various parts of the biological cell (see figure 1(b)), and thus a biological laser was realized. The green laser light originated from the transfected green florescent protein inside the cell, which essentially acted as a gain media inside an optical cavity. The emitted light had rich spatial structure as evidenced in figure 1(b), and this structure depended on the local distribution of the green fluorescent proteins. This is the first report, to my knowledge, where a living system, such as a biological cell, has emitted laser light, and has literally created tremendous ‘excitement’. So one may ask, what the uses of such a system are.
Bright Future: The research on biological lasers is in its infancy. There are many interesting prospects of such lasers, and I outline a few of them:
- They can act as a localized source of light and heat, which may further drive certain mechanical, thermal and chemical reactions in organelles and compartments of a cell.
- If placed in an appropriate location inside living systems, biological laser can be harnessed as light sources in biomedical applications, where the reach of certain surgical instruments is constrained.
- The emanating light is due to stimulated emission, which has narrow spectral width. In contrast to spontaneous emission, which is the process behind conventional light emitters, lasers have very narrow spectral widths, and hence can be utilized for spectral multiplexing and discrimination of species inside a cell.
Voyage ahead: There are still many unexplored aspects of a biological laser. Below I mention three of them from different viewpoints:
- From optical physics viewpoint, a biological laser can be treated as an optical cavity with a randomly-scattering gain media. It would be interesting to explore the localization, propagation and directional emission of light in such a randomized medium inside a living system.
- From chemistry viewpoint, it is interesting to ask if one can design and synthesize bio-compatible macromolecules or nano-materials which can be placed inside a cell such that it leads to efficient self-lasing without external stimuli.
- From biology viewpoint, it is vital to know what will be the fate of a cell if it keeps emitting laser light. Specifically, it would be interesting to know how cells can adapt to an in-built laser source. Furthermore, if a cell with biological laser splits into to two, will it carryover its ability of lasing to the next generation?
As with all interesting inventions and discoveries, biological lasers have opened many interesting questions to be explored. It has room for contribution from various branches of science and technology, and may open new avenues by bringing together biology and laser photonics. Let me conclude by quoting Feynman:
“…..A biological system can be exceedingly small. Many of the cells are very tiny, but they are very active; they manufacture various substances; they walk around; they wiggle; and they do all kinds of marvelous things—all on a very small scale. Also, they store information…..”
Well Mr. Feynman, now they can even emit laser light!