The fundamental idea behind this mainstay of modern life was
published one hundred years ago by Albert Einstein. But blink and you’ll miss
it in his seminal paper, “The quantum theory of radiation”, published in German
in Physikalische Zeitschrift 18 121 (English translation here). Einstein is
trying to work out what Max Planck’s “quantum hypothesis” – that the energy of
an oscillator must take discrete values equal to some integer multiple of the
oscillation frequency times a constant h – implies for the way light interacts
with matter.
Einstein himself had shocked many physicists (including
Planck) by proposing in 1905 that we regard Planck’s formula as a physical
fact, not just a mathematical trick that avoids the complications of a
classical energy continuum. What’s more, Einstein said, this quantization of
energy applies not only to the “oscillators” (in essence, the vibrating atoms)
that cause “black-body” radiation from a warm object; it applies also to the
electromagnetic oscillations of light itself, chopping up a ray into discrete
energy packets called photons.
That was the start of the “quantum revolution” that
overturned classical physics at the scale of atoms and molecules. We now know
that classical laws are not an alternative to quantum laws, kicking in at
everyday scales, but are a consequence of them. But in 1917 the implications,
and indeed the meaning, of quantization were still unclear. There was no
formalism for doing quantum mechanics – Erwin Schrödinger did not write down
his iconic equation until seven years later – and so, to address the problem of
matter–light interaction, Einstein had to make do with ad hoc methods.
They were clear enough. Planck’s oscillators can emit a
photon of the allowed frequency spontaneously and at random with a certain
rate, in much the same way as a radioactive atom decays. And the oscillator can
also absorb a photon from the ambient radiation field with a different rate.
Einstein wrote the simple differential equations for these processes. But he
realized that photon emission can also be stimulated by that radiation field.
He went on to work out the equilibrium state between these processes of
spontaneous absorption and emission, and stimulated emission, and derived
Planck’s radiation law.
The implication was that, if one arranges for a large number
of atoms to be in identical excited states, a stray photon of the right energy
can stimulate one atom to emit another photon, which stimulates another… and
all the atoms release their excess energy in a sudden cascade. What’s more, the
photon released by stimulated emission will be in phase – coherent – with the
one that stimulated it, and so all the light produced in the cascade will be
coherent.
In 1955 American physicist Charles Townes of Columbia
University in New York, an expert in molecular spectroscopy, and his co-workers
showed how stimulated emission could be used to make a device for generating or
amplifying microwaves, which they called a maser (microwave amplified
stimulated emission of radiation). Three years later Townes and Arthur Schawlow
explained how to extend the idea to visible and infrared frequencies to make an
“optical maser” – in effect, the laser.
They proposed using ordinary (incoherent) light to pump
atoms into an excited state, setting up the “population inversion” in which the
atoms are primed to return to their ground state by emitting photons. And their
design used an optical cavity – basically two mirrors between which photons
would bounce – to trap the emitted photons while they stimulated more emission.
The device, they explained, would generate “extremely monochromatic
[single-wavelength] and coherent light”. Theodore Maiman of the Hughes Research
Laboratories in Malibu, California, described such a device, using a ruby
crystal (already used for masers) as the lasing medium, in 1960.
Myth has it that the laser was first dismissed as a
“solution looking for a problem”. But its potential was never in much doubt,
especially when the first compact semiconductor laser was produced at the
General Electric Laboratories in 1962, making the devices more easily
integrated with electronics. All the same, they got up to some tricks. In 1969
a laser was bounced off a reflector on the Moon put there by Apollo astronauts.
Schawlow even made an edible laser out of gelatin for a joke; more recently
they’ve been made from silk, offering the prospect of cheap, flexible
biodegradable optoelectronics, and potentially taking lasers to new frontiers.
There’s no evidence that Einstein had any inkling in 1917 of
the implications of his work for making a beam of coherent light, let alone the
extraordinary array of uses that might have. But that just goes to show once
again how practically and unexpectedly fruitful ideas in fundamental science
can be.
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