How lasers work
Summary
TLDRThis video script explores the principles of lasers, comparing electron orbits to planetary motion and delving into quantum mechanics. It explains how electrons occupy quantized energy levels and how lasers use stimulated emission to produce coherent light. The script covers various types of lasers, including gas, solid-state, semiconductor, and free-electron lasers, and discusses techniques like population inversion and Q-switching. The goal is to provide an engaging and informative overview of laser technology and its applications.
Takeaways
- π Electrons orbiting an atom are described by quantum mechanics, with specific quantized orbits unlike the continuous orbits of planets around the sun.
- π The energy levels of electrons are fixed and can only take precise quantized values, with lower energy states closer to the nucleus on average.
- π The repulsive force between multiple electrons in an atom is strong, affecting the energy levels and complicating calculations compared to a single electron.
- π‘ An electron can be excited to a higher energy level by collision or by absorbing a photon with energy matching the energy gap between levels.
- π Spontaneous Emission is the process where an excited electron emits a photon after a random period, while Stimulated Emission is the synchronized emission of a new photon caused by an existing photon.
- π A laser operates on the principle of Stimulated Emission, where a chain reaction of photon emissions is initiated by a single photon, resulting in a coherent beam.
- π The Gain Medium in a laser is the material with atoms capable of emitting light through the process of Stimulated Emission, and it can be in various states of matter.
- π To produce a laser beam, the Gain Medium is often shaped into a cylinder to favor photons traveling the long axis, which can then be amplified by mirrors at either end.
- π Population Inversion is a crucial condition for lasing, where more atoms are in the excited state than the lower energy state, achieved through the process of 'pumping'.
- π§ Lasers can be Continuous Wave, maintaining a steady output as long as energy is supplied for pumping, or they can use techniques like Q-switching to produce powerful, short pulses.
- π οΈ Various types of lasers exist, including solid-state, gas, semiconductor, and free-electron lasers, each with unique methods of pumping and applications.
Q & A
How does the electron orbital model in an atom differ from the planetary model of the solar system?
-Electrons in an atom are governed by quantum mechanics and can only occupy specific quantized orbits or quantum states, unlike planets which can orbit at any distance from the sun.
What is the significance of the term 'quantum states' in the context of electron orbits?
-Quantum states refer to the specific, discrete energy levels that an electron can occupy around an atomic nucleus, as opposed to the continuous range of orbits available to planets around the sun.
Why is it necessary for a photon to have nearly the exact energy as the gap between electron levels to excite an electron?
-The energy of the photon must match the energy difference between the electron levels to provide the necessary energy for the electron to transition to a higher state, which is a requirement for excitation.
What is the concept of 'population inversion' in lasers?
-Population inversion is a condition where there are more atoms in the higher energy state (upper laser level) than in the lower energy state, which is necessary for the amplification of light in a laser.
How does the mutual repulsion between electrons in an atom affect the energy levels?
-The repulsive force between multiple electrons in an atom is quite strong, which changes the energy levels due to their mutual repulsion and makes it more difficult to calculate them compared to a single electron.
What is the role of the 'gain medium' in a laser?
-The gain medium is the material that contains the atoms capable of producing the laser light through the process of stimulated emission of radiation. It is where the energy transfer and photon amplification occur.
How does a laser differ from a simple light source like a lamp?
-A laser produces a coherent beam of light with a single wavelength and direction due to the process of stimulated emission, whereas a lamp emits incoherent light with photons emitted in random directions and wavelengths.
What is the purpose of the mirrors at the ends of a laser's gain medium?
-The mirrors are used to reflect the light back and forth through the gain medium, allowing the light to interact with the excited atoms multiple times and amplifying the light through stimulated emission.
What is the process called 'pumping' in the context of lasers?
-Pumping is the process of energizing atoms into an excited state. It is necessary to continuously supply energy to the gain medium to replace the atoms that have emitted photons and returned to a lower energy state.
What is the difference between a continuous wave (CW) laser and a Q-switched laser?
-A continuous wave laser operates steadily as long as energy is provided for the pumping, emitting a constant beam of light. A Q-switched laser, on the other hand, temporarily blocks the laser beam to build up a large population of excited atoms and then releases a powerful, short pulse of light when the block is removed.
How do semiconductor or diode lasers produce light?
-Semiconductor lasers produce light through the recombination of electrons and holes at a junction between P-type and N-type materials. When a voltage is applied, the recombination process can be stimulated by existing photons, leading to the emission of new photons and the creation of a laser beam.
Outlines
π Electron Orbits and Quantum Mechanics
The script begins by comparing the traditional model of electron orbits around an atom to the solar system, highlighting the differences introduced by quantum mechanics. It explains that electrons exist in quantized energy levels or quantum states, with closer orbits to the nucleus corresponding to lower energies. The probabilistic nature of electron position and the fixed energy levels are emphasized, contrasting with the continuous orbits and variable energies of planets. The script also touches on the strong repulsive forces between electrons and the concept of ground and excited states, leading to the discussion of how atoms can emit or absorb photons, either through collisions or by absorbing the correct energy photon, a process integral to the functioning of lasers.
π The Workings of a Laser: Gain and Emission
This paragraph delves into the specifics of how a laser operates, starting with the concept of 'Gain', which requires a sufficient number of atoms with excited electrons in a 'Gain Medium'. The medium can be in various states of matter and may be mixed with other materials. The script explains how the shape of the Gain Medium, typically a long cylinder, helps to direct the emission of photons into a coherent beam, and how mirrors at either end can enhance this effect. It also introduces the concept of 'pumping' to continuously energize atoms into an excited state, leading to an equilibrium in a 'Continuous Wave' laser. Furthermore, the paragraph discusses the technique of Q-switching to create powerful but short laser pulses by momentarily removing the mirror and allowing a buildup of excited atoms, which are then rapidly depleted to release a burst of laser light.
π Pumping and Population Inversion in Lasers
The script continues by discussing the process of 'pumping' and the necessity of achieving 'population inversion', where more atoms are in the higher energy state than the lower one, to ensure a net gain of photons. It explains the challenges of using simple two-level atoms and introduces the concept of multi-level atoms to overcome these issues. The paragraph provides examples of three-level and four-level laser systems, illustrating how more efficient lasers can be designed by using additional energy levels to minimize the number of atoms that need to be excited at any given time. It also touches on the practical aspects of exciting atoms using electric currents or light sources and the importance of managing the energy levels to maintain lasing action.
π¬ Common Laser Designs and Their Applications
This section of the script provides an overview of various common and less common laser designs, starting with the Ruby laser, which was the first to operate and uses a crystal structure as its Gain Medium. It then discusses the more powerful neodymium lasers, which use a four-level system for higher efficiency. The script also mentions the Helium-Neon laser, which serves as an educational tool, and the CO2 laser, which uses a gaseous medium. The most prevalent type, the semiconductor or diode laser, is highlighted for its simplicity and cost-effectiveness, with a brief explanation of its operation based on energy bands and the recombination of electrons and holes. The paragraph concludes with a mention of more unconventional laser types that operate in the far ultraviolet or x-ray spectrum, requiring a plasma medium and special pumping methods.
π Advanced Laser Techniques and Their Engineering
The final paragraph explores advanced laser techniques, including the use of capillary discharges and Free Electron Lasers (FELs). It describes how a capillary discharge laser can quickly ionize a noble gas and amplify photons traveling down its length, producing a short, intense pulse of light. The script then explains the FEL, which uses a beam of fast-moving electrons that emit radiation when accelerated by a series of magnets. This radiation can stimulate further photon emission, resulting in an exponentially growing beam of light. The FEL is noted for its tunability, allowing for the precise selection of photon wavelengths. The paragraph also briefly mentions the technique of Lasing Without Inversion, suitable for lasers with large energy gaps, and concludes by summarizing the fundamental principles behind laser operation, including the release of energy as light and the physical engineering required to produce a tight beam.
Mindmap
Keywords
π‘Electron Orbits
π‘Quantum Mechanics
π‘Quantum States
π‘Energy Levels
π‘Ground State
π‘Excited States
π‘Spontaneous Emission
π‘Stimulated Emission
π‘Gain Medium
π‘Population Inversion
π‘Q-Switching
π‘Continuous Wave (CW) Laser
π‘Semiconductor Laser
Highlights
Electrons orbiting an atom are governed by quantum mechanics, occupying quantized orbits with fixed energy levels.
Electrons in atoms experience a strong repulsive force, unlike planets in the solar system.
An electron can be excited to a higher energy level by collision or photon absorption.
The process of an electron decaying and emitting light is called Spontaneous Emission.
Stimulated Emission occurs when an existing photon causes the emission of a synchronized photon.
A laser's Gain Medium consists of atoms of the same type, which can be in various states.
Laser light is produced by a chain reaction of photons emitted through Stimulated Emission.
A laser requires a population inversion for efficient photon emission, achieved by pumping atoms into an excited state.
Different types of lasers use different methods for pumping, such as electric current or light.
Three-level and four-level laser systems differ in efficiency and the percentage of atoms required to be excited.
Ruby lasers were the first to operate, using a Ruby rod as the Gain Medium and pumped by a Xenon flash lamp.
Neodymium lasers are among the most powerful, using neodymium ions embedded in a crystal or glass.
Helium-Neon lasers use a mixture of gases where excited Helium atoms transfer energy to Neon atoms.
Semiconductor or diode lasers operate between energy bands and can be built inexpensively.
Unconventional lasers include those in the far ultraviolet or x-ray spectrum, requiring a plasma Gain Medium.
Capillary discharge lasers use a noble gas in a capillary tube with a brief electrical discharge for ionization.
Free Electron Lasers utilize a beam of fast-moving electrons accelerated by magnets to emit photons.
Lasing Without Inversion is a technique for lasers with large energy gaps, requiring minimal pump energy.
Lasers produce light by carefully selecting the shape of the beam and using mirrors, and can operate in continuous wave or pulsed modes.
Transcripts
This video, more than most that I've made,
starts off simple and then ramps up
quickly. So, as always, I encourage you to
skip ahead or rewind as necessary.
The way that electrons orbit
an atom is frequently compared to the
way that planets orbit the sun. It's
worthwhile briefly looking at the
similarities and differences between
these two structures. With the solar
system, a planet or artificial satellite
can orbit arbitrarily close to or far
away from from the Sun. The closer an
object orbits the Sun, the more tightly
it is held by gravity, the less energy it
has. To go from a small orbit out to a
further one, energy must be expended just
as you would have to perform work
against gravity to lift an object from
the surface of the Earth. So in short,
objects can orbit the Sun at any
distance, but the further out they are
the more total energy they have.
Electrons orbiting an atomic
nucleus are different different in that
they are governed by quantum mechanics.
An electron can take one of a number of
specific or quantized orbits, more
accurately called quantum states. For an
electron in a given state, the position
is no longer defined with certainty, but
rather is probabilistic in nature. The
energy, however, is fixed. Just as with the
planets, the states where the electrons
orbit closer to the nucleus on average
have lower energies. So in short, electron
energies can take only precise quantized
values and nothing in between. This
picture is further complicated by the
fact that, whereas the planets exert only
a weak force on one another so for
example the gravitational pull of
Jupiter on Earth is quite small and that
of Mercury is downright negligible, the
repulsive force between multiple
electrons in an atom is quite strong.
This means that adding an additional
electron to an atom changes the energy
levels because of their mutual repulsion
and also makes it much harder to
calculate them compared to just a single
electron orbiting with no
repulsion. The long and short of all this
is that an electron in an atom can only
take one of a set of fixed energy levels.
The lowest possible energy is called the
ground state and above that are excited
states. An electron can be excited up in
energy if another atom or free electron
collides with it. The colliding particle
loses energy, which the electron gains.
The electron can also be excited by
absorbing a photon, but this photon must
be carrying almost exactly the same
energy as the gap between the levels in
question. All of these processes are
reversible, so a free electron may come
in, de-excite a bound electron and carry
off the excess energy. If an excited
bound electron is left alone for long
enough, it will decay after a certain
time which is random.
By carefully managing these
atomic processes, it is possible to
generate useful light. In the United
Kingdom, for example, we have a lot of
street lamps where an electric current -
in other words the movement of energetic
electrons - is passed through Sodium vapor.
The moving free electrons will collide
and excite the bound electrons in the
Sodium atoms. The excited electrons then
decay, emitting photons of yellow light
at 589 nanometers in wavelength and an
energy corresponding exactly to the
difference between the ground and
excited states. But this is not a laser.
Each of the atoms emits a photon at
random times and moving in random
directions, so this does not produce the
kind of tight beam that is
characteristic of a laser. Normally, a
Sodium light will have a reflector
behind it so the light goes broadly in
one direction, but it still spreads out
into a wide area.
The phenomenon we have seen so far
is more precisely called Spontaneous
Emission, where an excited electron left
alone for some amount of time will emit
a photon in isolation. But, if there's
already a photon with the right energy
passing by the atom, it might cause
Stimulated Emission of a new one. The
oscillating electric field of the
original photon interacts with the
electron, leading to the emission. Because
of this, stimulated emission is no longer
random. The two photons are synchronized
in time and moving in the same direction.
Also, like all quantum processes,
stimulated emission is not guaranteed,
but merely has a probability to happen.
The energy of the photon has to match the
gap between energy levels very closely,
which typically means you have to have
atoms of the same element for this to
work. The energy levels of Sodium are
different to Neon and so on. Anyway, at
the end of all that, you just need to
understand that an excited electron will
emit a photon by Spontaneous Emission if
it's left alone or by Stimulated
Emission if an existing photon comes
in. These processes are harnessed within a
laser to create a chain reaction. One
photon emitted spontaneously stimulates
the emission of a second. Two soon turn
to four, then eight and so on. The photons
are all moving at the same time in the
same direction. The growth in strength of
the laser light is exponential, where the
speed of the growth is called the Gain.
The crucial ingredient to have Gain is
to have plenty of atoms with excited
electrons and I'll come on to how
they're excited in a moment, but in a
laser these atoms are assembled into a
material referred to to as the Gain
Medium. The Gain Medium has to have atoms
of the same type, but they can be in a
solid, liquid, gaseous or even plasma
state. The atoms may, however, be mixed in
with other material. So, for example, in a
moment we will look at a type of laser
where individual Neodymium atoms - usually
metallic - are embedded in glass.
So, a gain medium will send out
bunches of photons. After all, laser is an
acronym for Light Amplification by
Stimulated Emission of Radiation. This is
good, but we are back to the problem of
the Sodium light: the initial spontaneous
emission is oriented randomly, so these
groups of photons are going to be
leaving the gain medium traveling in all
the different directions. However, there
is a way around this. The Gain Medium is
made into a long, but narrow cylinder. If
a photon is spontaneously emitted
traveling in the short dimension of the
cylinder, it will double itself only a
few times and then quickly leave the
Gain Medium. If it's traveling the long
way through the cylinder, it will double
itself for very many times and form a
strong beam when it finally leaves. The
difference between these two is literally
exponential. Another way to make this
effect even stronger is to put mirrors
at either end of the cylinder, so the
beam will go back and forth through the
Gain Medium many times and grow even
larger. In order to actually be useful,
one of the mirrors is partially
reflective and partially transparent.
Some of the light will then leave the
laser for its intended purpose. All the
photons are traveling in largely the
same direction in a tight beam and each
one carries almost the same energy,
meaning they all have the same
color. As we have seen, the laser only
creates more photons at the expense of
excited electrons in atoms. Eventually
the Gain Medium will run out of these
atoms and that initial exponential
growth will taper off. More atoms must be
continuously energized into an excited
state in a process called "pumping".
Eventually, as the strength of the laser
beam grows, it will reach an equilibrium.
Atoms will be pumped into the excited
state at a rate of R atoms per second,
new photons will be emitted from the
Gain Medium at a rate of R per second
and photons will leave through the
output mirror at a rate of R per second.
This is usually referred to as a
Continuous Wave laser and it will work
steadily as long as energy is provided
for the pumping.
There is a way to create a much more
powerful, but short laser pulse with a
technique called Q-switching. Imagine
removing the mirror at one end of the
laser. Now photons can no longer make
continuous round trips through the Gain
Medium to build up their number. Because
there is no longer a powerful beam, there
is nothing to deplete those excited
atoms and so more and more of them
become pumped up to become excited. If
the mirror is suddenly put back, a beam
will quickly grow to much larger
intensity than before. Because there are
now so many more excited atoms, you're
quite literally "chargin' your laser"
above the level it would normally be at
and then releasing that energy in a big
burst. It will, of course, quickly deplete
the number of excited atoms, so the pulse
will not last very long. The only caveat
is that the mirror has to be put back
quickly - perhaps on the order of
microseconds - much faster than it can be
physically moved. Much faster, also, than a
shutter can open.
Instead, there are electronic methods,
such as applying a voltage to make
crystals transparent, and so on. The
technique is called Q-switching because
the Quality factor of the mirror - how
well it reflects light - has to be quickly
switched from low to high.
So, back to pumping. How do you make
the electrons in atoms excited to the
right level? And, as a shorthand, I will
just say that the whole atom is excited
rather than the electron. Firstly, let me
clarify that when I said that an excited
atom can emit a photon, it doesn't
necessarily have to drop down all the
way to the ground or lowest energy level.
In fact, for reasons I'll explain in a
moment, most don't. All that is needed is
that an atom starts in a high energy
state called the upper laser level and
then drops down an energy to a lower
laser level, but it doesn't necessarily
need to be the lowest possible. As we've
seen, it's possible to excite atoms with
an electric current or with incoming
light. For the latter, it's possible to
use an ordinary light source like a lamp,
which also takes electrical input
directly. The problem with a laser is
that it's not enough to just have some
atoms in the upper level. Remember what I
said about quantum processes: they're
reversible. When photons encounter an
atom in the upper level, stimulated
emission will create a new photon. When
they encounter an atom in the lower
level, a photon will be lost by
stimulated absorption, the inverse
process. Things normally try to minimize
their energy, so ordinarily there are
more atoms in the lower energy levels
than in the upper ones. A laser beam will
then lose more photons by absorption
than it gets back by emission. There is
no longer Gain.
The pumping needs to flip this
situation, attaining what is called
population inversion: more atoms in the
upper level than the
lower. OK, let's do that, let's excite up
more than half the atoms. The inverse
processes are going to hit us hard once
again. Let's say we put a bunch of
photons from from a lamp to try and
excite our gain medium. Once the fraction
of atoms in the upper level gets above
50%, the incoming light is going to start
causing more losses by stimulated
emission than they put back. It's the
opposite problem of needing population
inversion. The way to solve this problem
is to make use of more than just two
atomic energy levels. Let's look at an
example system where the ground state is
the lower laser level, above that in
energy is the upper laser level and
there is a third level a little higher
in energy still. As usual, I will pull
some numbers out of my behind purely for
illustrative purposes. 40% of atoms are
in the lower level and 10% in the
highest level. This is perfectly fine to
pump. Once they're pumped to the third
level, atoms decay down to the upper
lazing level, giving up the excess energy
as waste heat. This is also fine, because
this loss of energy happens so quickly
atoms don't stay on the third level, but
a large number ends up in the upper
laser level; 50% of the total. Now for the
lasing: with a 50/40 split, there are more
atoms in the upper level, therefore there
is a population inversion and Gain. The
laser works. The downside of such a
three level laser is that a total of 60%
of the atoms must be kept excited above
the ground state. That requires a lot of
power. More efficient is a four level
laser. The two laser levels are both
above the ground state and there is a
four fourth level above them again. Get
ready for some more butt numbers. 95% in
the ground state and 1% in level four.
Pumping is fine. Quick decay down to
level three, the upper laser level,
leaving 3% of all atoms in that state.
Any atoms in the lower laser level decay
down to the ground state quickly, so only
1% are in this level. As a result, there
is still a population inversion and
therefore Gain, but only 5% of the atoms
are are in an excited state at any one
time. So, the energy required to run this
four level laser is lower than the
three level laser.
With all that in mind, let's look
at some common and some not so common
laser designs. The Ruby laser was the
first type to ever operate. The Gain
Medium is a classic crystal structure
associated with lasers: a cylindrical
Ruby rod with the same chemical
composition as the gemstone, much longer
than it is wide. Ruby is most made of
Aluminum Oxide with some Chromium mixed
in. These atoms give it the red color and
are the ones which produce the laser
light. Ruby lasers might typically be
pumped by a Xenon flash lamp. A very high
current is passed through Xenon gas -
actually it becomes a plasma - and it
emits light which excites the Chromium.
Usually, there are mirrors all around the
flash lamp to direct the light into the
Ruby. It is a quintessential three level
laser. A similar design with neodymium
has been used to make the most powerful
lasers in existence, such as those at the
National Ignition Facility. Neodymium
atoms, or actually ions as they are each
missing three electrons, are embedded in
an Yttrium Aluminum Garnet crystal, or simply
in glass. Again, there are different ways
to pump it, but the key difference is
that this is a four-level laser and
therefore more efficient
Something which is not so
technologically useful, but which you
will probably encounter as a teaching
tool if you ever take a proper course on
lasers, is the Helium-Neon laser. Both
these noble gases are mixed together. An
electric current excites the Helium to a
particular level. What's interesting is
that this level matches a particular
energy level in Neon which happens to be
the fourth level in a four level laser
system. When excited Helium collides with
Neon, it gives up its energy and
effectively pumps the Neon. There are
other lasers with a gaseous gain medium,
like the CO2 laser as well.
By far the most common type of
laser in the world, present in barcode
scanners and in CD drives when they used
to be a thing, is the semiconductor or
diode laser. Semiconductors are a lot
more complicated than even the quantum
picture of a single atom I talked about
earlier. Rather than specific levels,
there are energy bands, so the laser
operates between those instead. Usually a
semiconductor like Silicon has a small
amount of impurity atoms added or doped,
which have either too many or too few
electrons. An N-type material is one with
too many electrons which are then free
to move. A P-type material is one with too
few electrons, leading to so-called holes
left behind. The holes act and move like
positive particles. If a neighboring
electron moves left into the hole, a new
one is left behind as if the old one had
simply moved right.
An ordinary diode is made by
sandwiching P-type and N-type materials
next to each other. These two regions
meet at a junction where the electrons
and holes recombine to cancel out. This
all works a bit like transitions between
levels in atoms: they can recombine
spontaneously or be stimulated by a
photon. Normally, a photon would have to
give up its energy by stimulated
absorption to cause this recombination.
However, applying a voltage across the
junction makes it energetically
favorable for an electron and hole to
recombine, so then existing photons can
stimulate the emissions of new ones.
Feeding electrical energy in creates
Gain and therefore lasing. Obviously, I've
simplified a lot here and the diode
laser requires a lot more engineering
than I've discussed, but this type of
laser can now be built for pennies. The
design is simple in that partial
reflection happens from the output faces
of the semiconductor crystal itself. Huge
arrays of semiconductor lasers can then
be used to pump other more powerful
types of lasers.
Now for some more unconventional
laser types. Light in the electromagnetic
spectrum beyond ultraviolet is strongly
ionizing, which means it would be
absorbed by any material including air
until it has turned that material into a
plasma. That's not to say that there
aren't any reasons to create a laser in
the far ultraviolet or x-ray part of the
spectrum, but it would have to exist in
space or a vacuum chamber. The Gain
Medium must also be a plasma, or it would
son become one. The Gain Medium for this
type of laser can be pumped by other
lasers in the visible part of the
spectrum, by intense electric currents or
even nuclear explosions.
The idea of Ronald Reagan's
Star Wars program was to detonate a
nuclear warhead in space, which would
obviously release a lot of energy. This
energy would somehow be channeled to
pump a laser system creating an x-ray
beam to shoot down enemy
missiles. A more controlled way to create
a laser with short wavelengths is
called a capillary discharge. A long
cylindrical capillary has a noble gas
being pumped through it. A strong
electrical current is briefly discharged
through the gas, quickly ionizing it and
exciting those remaining electrons that
are still in the ions. At this point,
there are no mirrors at either end of
the Gain Medium. Any photons traveling
the long way down the capillary get
amplified extremely quickly and in a
matter of nanoseconds the pulse is
over. Another proven technique to make
laser light at any wavelength is the
Free Electron Laser. When very fast
moving electrons accelerate, they emit
radiation. This is actually a problem for
accelerators like the LHC because all
charged particles lose energy in this
way. Why not harness this radiation?
A beam of electrons is generated by a
linear accelerator. This beam is then
passed through a region of alternating
magnets, so that the magnetic force
causes a side to side acceleration and
hence a a tight forward beam of photons.
The initial photons effectively
stimulate further emission if the
wavelength matches the spacing of the
magnets and the electrons bunch
themselves up with the same spacing. The
output of such a free electron laser
also grows exponentially up to a point.
The neat thing is that the wavelength of
the photons depends on the energy of the
electrons coming out of the linear
accelerator. Not only can the latter be
designed with a huge range of energies
in mind, but those energies can be varied
on the fly. A free electron laser can be
built to work with anything from
microwaves to x-rays, and then dial in
the precise wavelength of photons
desired, within a certain range. This is
useful for experiments and even weapons.
The color of the beam can be rapidly
varied to the best atmospheric
conditions. There is also a specialist
technique called Lasing Without
Inversion. If a laser has a very large
energy gap between its lower and upper
levels, this would mean that it would
take a lot of pump energy to create a
population inversion, especially if it's
not possible to have a four-level laser.
The gain medium needs to only be pumped
by a small amount - not enough to create a
population inversion. Then a relatively
small amount of energy can be injected
to the atoms in the lower laser level,
not to take them all the way up, but to
transfer some of them to a low-lying
energy level not involved in the lasing.
This is the best way I can explain it in
brief, but if you've done the equivalent
of a university level quantum mechanics
course, here is a paper with all the
details which should be
understandable. So to sum up, lasers make
use of the fact that energy is released
as light - as photons - when an electron
orbiting a nucleus goes from one orbit
high up in energy to one lower down.
Photons stimulate the emission of more
photons of exactly the same wavelength,
or color, moving in the same direction.
A laser is physically engineered to put
out a tight beam of light by carefully
selecting its shape and adding mirrors
as required. It can be continuous or
pulsed. A given laser uses one type of
atom to actually create the light, but
these atoms may be on their own, in a gas
or a plasma, embedded in a crystal or
some other arrangement, and in some cases
like the free electron laser, atoms are
not even required. Thank you for watching.
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