Amplifying the energy within a laser, over and over, won't get you an infinite amount of energy. There's a fundamental limit due to physics.
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JILA’s three-dimensional (3-D) quantum gas atomic clock
consists of a grid of light formed by three pairs of laser beams. A stack of
two tables is used to configure optical components around a vacuum chamber.
Shown here is the upper table, where lenses and other optics are mounted. A
blue laser beam excites a cube-shaped cloud of strontium atoms located behind
the round window in the middle of the table. Strontium atoms fluorescence
strongly when excited with blue light, creating the sight viewed here. |
Back in the middle of the 20th century, there was really no
good way to create purely monochromatic light: where all of the photons
possessed precisely the same wavelength. Sure, you could break up white light
into its component colors, such as by passing it through a prism or color
filter, and selecting for only a narrow range of wavelengths, but that wouldn’t
be truly monochromatic. However, the fact that atoms, molecules, lattices, and
other structures only admit a specific set of electron transitions brought
forth a fascinating possibility: if you could stimulate the same transition
over and over, you could make truly monochromatic light.
Since 1958, we’ve managed to do precisely that with the
invention of the laser. Over time, lasers have become more powerful, more
widespread, and come in an enormous variety of wavelengths. By having photons
of a specific wavelength build up in the lasing cavity, that same-frequency
emission gets stimulated over and over again. But you cannot simply build up
photons forever to get an arbitrarily large energy density in your laser; once
you cross a certain threshold, the laws of physics themselves will stop you.
Here’s why there’s an ultimate limit to laser energy, and we’ll never be able
to exceed it.
A variety of energy levels and selection rules for electron
transitions in an iron atom. There are only a specific set of wavelengths that
can be emitted or absorbed for any atom, molecule, or crystalline lattice. If
the same transition can be stimulated over and over, a laser can be created. |
Let’s first get to the basics of atoms, transitions, and
energy levels. In very simple terms, an atom is a positively charged nucleus
with a number of electrons orbiting it. These electrons typically exist in a
number of finite configurations, only one of which is optimally the most
stable: the ground state. There are only a finite set of wavelengths of light
that an electron within an atom can absorb, and if you strike that electron
with a photon of such a wavelength, it will jump: entering a higher energy
configuration, or an excited state.
If all other conditions could be ignored, that excited state
would spontaneously decay to a lower energy state — either all-at-once to the
ground state or in a chain — after a finite amount of time, emitting a photon
of a very particular energy (or set of energies) when it does so.
But if you can stimulate a ground-state atom (or a molecular
or lattice analogue, with, say, a valence electron) to excite into a particular
excited state, you can often coax it to de-excite (and emit radiation) at one
particular frequency, very consistently. The big idea of a laser is that you
pump energy in, and pretty much every emitted photon that comes out from
de-excitations all happen at the same wavelength.
The very idea of a laser itself is still relatively novel,
despite how widespread they are. The laser itself was only first invented in
1958. Originally an acronym standing for Light Amplification by Stimulated
Emission of Radiation, lasers are a bit of a misnomer. In truth, nothing is
really being amplified. They work by taking advantage of the structure of
normal matter, which has atomic nuclei and various energy levels for its
electrons to occupy. In molecules, crystals, and other bound structures, the
particular separations between an electron’s energy levels dictate which
transitions are allowed.
The way a laser works is by oscillating the electrons
between two allowable states, causing them to emit a photon of a very
particular energy when they drop from the higher-energy state to the lower one.
The addition of energy, which “pumps” the electrons into those desired excited
states, then leads to a spontaneous de-excitation, creating more and more
photons of that desired monochromatic frequency. These oscillations are what
cause the emission of light. We call them lasers, perhaps, because no one
involved thought it was a good idea to use the acronym Light Oscillation by
Stimulated Emission of Radiation.
A set of Q-line laser pointers showcase the diverse colors
and compact size that now are commonplace for lasers. By ‘pumping’ electrons
into an excited state and stimulating them with a photon of the desired
wavelength, you can cause the emission of another photon of exactly the same
energy and wavelength. This action is how the light for a laser is first
created: by the stimulated emission of radiation. |
The “spontaneous emission” part, however, is of paramount
importance, and what makes a laser, for lack of a better word, lase. If you can
produce either multiple atoms-or-molecules in the same excited state and
stimulate their spontaneous jump to the ground state, they’ll emit the same
energy photon.
These transitions are extremely fast (but aren’t infinitely
fast), and so there is a theoretical limit to how quickly you can make a single
atom (or molecule) hop up to the excited state and spontaneously emit a photon;
the system takes time to reset.
Normally, some type of gas, molecular compound or crystal is
used inside a resonant-or-reflective cavity to create a laser, but recent years
have uncovered other methods for stimulating this exact type of radiation. Free
electrons can also be used to make lasers, as can semiconductors, optical
fibers, and possibly even positronium: bound states of electrons and positrons.
The wavelength that lasers can emit light in range from extremely long radio
waves to incredibly short X-rays, with gamma rays theoretically possible as
well. The laser process even occurs naturally in space, at both microwave and
visible light frequencies.
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This composite Hubble (blue/white/dark) and ALMA (red) image
shows not only the colliding galaxy system Arp 220, but also the double nucleus
which contains the bright emission from both water and hydroxyl megamasers. |
As new methods and techniques are developed, the amount of
energy lasers produce has continued to rise over time, with intensities limited
only by the practicalities of modern technology. In 2018, the Nobel Prize in
Physics was awarded for advances in laser technology, with half of the prize
going specifically towards controlling the power and pulse frequency of your
laser. We think of laser light as being continuously emitted, but that isn’t
always necessarily the case. Instead, another option is to save up that laser
light you’re producing and to emit all of that energy in a single, short burst.
You can either do this all in one go, or you can do it repeatedly, potentially
with relatively high frequencies.
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