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Laser: The Gift that Keeps on Giving

Writer's picture: Mishkat BhattacharyaMishkat Bhattacharya

This post is about the laser - the same one that's in your pointer. The laser was invented on May 16, 1960, so its anniversary is this coming week (UNESCO has declared the day as the International Day of Light), and I thought a discussion about it would be timely.


Why do I call the laser the gift that keeps giving? One place to look for the answer to that question is appendix 7 (page 283) to the biography of Ted Maiman, who made the first optical laser. The appendix lists thirty-eight Nobel prize winners, in physics and chemistry, whose awards cite the laser. We can add a few more, since after the book was published, Ashkin got part of the prize for inventing the optical tweezer, sharing it with Mourou and Strickland who won for making advances in pulsed laser physics in 2018; and Aspect, Zeilinger and Clauser were awarded for quantum information in 2022.


Here's a random sample of five other laser-related Nobel prizes:


i) 2017 - Detection of gravitational waves (R. Weiss, B. Barish, K. Thorne)

ii) 2009 - Fiber optics (C. Kao)

iii) 2005 - Coherence and precision measurement (R. Glauber, J. Hall, T. Hansch)

iv) 2001 - Bose-Einstein condensation (E. Cornell, W. Ketterle, C. Wieman)

v) 1999 - Femtosecond spectroscopy (A. Zewail)

What makes the laser so special, so versatile as a source of light? How is it different from other light sources, such as the sun, or an electric bulb, or an LED? What makes the laser capable of cutting steel, ablating cardiac tissue, constructing holograms, slowing atoms, reading barcodes, correcting vision, connecting the internet through fibers, - and pointing at slides? (The gift that keeps on giving!) Why is the worldwide market for lasers around $10 billion?


The answer lies in the fact that every source of light puts out waves. For the sun, an electric bulb or an LED, these waves wiggle randomly with respect to each other. The crest of one wave might align with the crest of another wave, or it might not. The waves come out in all directions. This may not be a bad thing in itself. When we turn on a light bulb, for example, we would like it to illuminate the whole room, not just a patch of it.


But for a laser, all the emitted waves are in lockstep - the technical term for this behavior is coherence - to cohere means to stay together. All the waves are identical. They want to do the same thing, move in the same direction, have the same frequency (i.e. color), carry the same momentum. This makes the laser, loosely speaking, a pure and concentrated source of light. This is what enables lasers to cut metals. This is what makes the light come out as a narrow beam from your pointer. Of course, what we have described is an idealization - if you let the pointer beam travel far enough (hundreds of meters), it becomes fatter, as the waves spread out.


How does the laser - unlike the sun or an electric bulb - produce identical waves of light? Identical waves of light are born from stimulated emission, a process discovered by Einstein. Stimulated emission is the process by which an atom emits light waves of the same kind as it is surrounded by (we can think of it as a kind of peer pressure). Fundamentally, this kind of emission can be traced to the fact that light consists of photons, which are bosons, and bosons like to be in the same state as other bosons. Now the more photons surround the atoms, the more stimulated emission there is, resulting in amplification of the light. All this explains the acronym LASER: Light Amplification by Stimulated Emission of Radiation.


But this is a simplified description, of course. For those more technically inclined, journals like Nature Photonics provide a 'laser questionnaire' which lists the several essential characteristics a device needs to satisfy if it is to qualify as a laser.


Lasers come in various species, broadly classified by the media they use as solid (e.g. titanium sapphire), liquid (e.g. Rhodamine) and gas (e.g. Helium-Neon). They can put out some beautiful colors, listed in the chart here, and displayed, for example at a Pink Floyd laser light show. Let's be thankful for this gift!



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