00:00:12
everyone has seen them and have probably
00:00:14
teased many cats with them maybe some of
00:00:17
you have had unwanted hair removed or
00:00:19
maybe you have built one and popped some
00:00:20
balloons with it bottom line lasers are
00:00:23
ubiquitous not only in scientific
00:00:25
research but also in Industry just how
00:00:28
do these little devices manage to put
00:00:30
out that nice powerful cated beam of
00:00:32
light all this and more coming up as
00:00:35
some may or may not know laser is
00:00:37
actually an acronym it stands for light
00:00:40
amplification by stimulated emission of
00:00:42
radiation however nowadays it is so
00:00:45
common that people don't bother to
00:00:46
capitalize it and simply write laser a
00:00:49
very brief history of the laser starts
00:00:51
in 1917 when Einstein introduced the
00:00:54
concept of stimulated emission which
00:00:56
will be explained shortly then in 195
00:00:59
before the first Mesa was demonstrated
00:01:02
by Charles towns the M standing for
00:01:05
microwave the ammonia Mesa was the first
00:01:08
device based on Einstein's predictions
00:01:10
and obtained the first amplification and
00:01:12
generation of electromagnetic waves with
00:01:14
a wavelength of about 1 cm which is in
00:01:17
the microwave range this is recognized
00:01:20
as the precursor to the laser it wasn't
00:01:23
until 1960 when Theodor mayam developed
00:01:26
the first working laser at Hughes
00:01:28
research lab mayon's early laser used a
00:01:31
powerful energy source to excite atoms
00:01:34
in a synthetic Ruby to higher energy
00:01:36
levels the development of the laser was
00:01:38
a collaborative effort by scientists and
00:01:40
Engineers who were leaders in Optics and
00:01:43
photonics okay so why are lasers useful
00:01:47
why are they ubiquitous the answer can
00:01:50
be broken down to three unique
00:01:51
properties the laser holds the first
00:01:54
being line width the purity of a laser
00:01:58
referred to as the line width can can be
00:02:00
quite narrow more so than any other
00:02:02
light source in layman's terms this is a
00:02:05
measure of what frequencies are
00:02:06
contained in the emitted light the
00:02:08
narrower the line withd the closer the
00:02:10
emitted light is to a single frequency
00:02:12
single color if you will thus a laser is
00:02:15
said to be monochromatic in reality it
00:02:18
does output a small range of
00:02:20
frequencies the smaller this range the
00:02:23
better the line width and quality of the
00:02:24
laser in contrast an incandescent bulb
00:02:27
has a very large line width and emits
00:02:29
the broad spectrum which is why the
00:02:31
emitted light is white white light is a
00:02:34
superposition of all the colors in the
00:02:36
visible
00:02:36
spectrum having a narrow line width is
00:02:39
useful because many scientific
00:02:40
experiments want to analyze stuff with
00:02:43
certain energies different wavelengths
00:02:45
of light corresponds to different
00:02:47
energies hence having a source with one
00:02:49
energy is helpful the second is
00:02:53
coherence the light emitted by a laser
00:02:55
is coherent light this means it is all
00:02:58
polarized in the same direction as well
00:03:00
as being in Phase the laser is said to
00:03:02
Output highly coherent monochromatic
00:03:05
light and led on the other hand is also
00:03:07
monochromatic one color but it emits
00:03:10
incoherent light an analogy with
00:03:12
synchronization and Harmony can be made
00:03:15
imagine an orchestra playing if the
00:03:17
orchestra is in sync and everyone is
00:03:19
playing the parts correctly it will be
00:03:21
pleasing to the ear the laser if some
00:03:24
players are playing out of sync but
00:03:26
still playing the parts correctly it
00:03:28
won't sound as good the D coherence is
00:03:31
important because all the photons add
00:03:33
their energies together and we can then
00:03:35
focus them on a small spot over some
00:03:38
distance lastly power lasers make it
00:03:41
possible to deliver High intense light
00:03:43
to a small area of course militaries are
00:03:46
particularly interested in this aspect
00:03:48
of the laser as well as medical
00:03:50
applications laser ey surgery for
00:03:53
example now let's take a look at how a
00:03:55
laser works the workings of a laser are
00:03:58
quite complex as it requires an
00:04:01
understanding of quantum mechanics there
00:04:03
are some commonalities behind every
00:04:05
laser the first part can be broken down
00:04:08
to three key pieces stimulated
00:04:10
absorption spontaneous emission and
00:04:12
stimulated emission which is what the SE
00:04:15
part of laser stands for let's take a
00:04:18
look at the first concept stimulated
00:04:20
absorption we will need a nucleus that
00:04:23
is made up of protons and neutrons that
00:04:25
has an overall positive charge and an
00:04:27
electron that has a negative charge
00:04:30
hey there little guy most textbooks show
00:04:34
electrons existing in discrete energy
00:04:36
states of a material but actually
00:04:39
electrons exist in probability density
00:04:41
clouds around the nucleus as they have
00:04:44
wave likee Behavior and the orbitals
00:04:46
represent the average distance one is
00:04:48
likely to find it let's use this average
00:04:51
distance to define the orbital and
00:04:53
ignore the probability distribution for
00:04:55
Simplicity mostly always electrons are
00:04:58
found in the lowest energy state or
00:05:01
ground state everything in nature wants
00:05:03
to be in a low energy State as it is
00:05:05
easier for it to exist at this level in
00:05:08
other words it minimizes energy think of
00:05:10
a ball on a hill and how easy it is for
00:05:13
it to roll down it wants to roll down
00:05:15
because the energy state is lower closer
00:05:17
to the Earth's core than further away in
00:05:20
this case potential
00:05:21
energy however it is possible to excite
00:05:24
electrons by some kind of external means
00:05:27
just like we can exert a force on the
00:05:29
ball that has rolled down and push it
00:05:31
back up light can be this push to excite
00:05:35
electrons if a photon of Light which is
00:05:37
one unit of light comes across an
00:05:39
electron in a low energy state it can
00:05:42
sacrifice itself and push the electron
00:05:44
to a higher energy State the photon is
00:05:47
annihilated but the energy of it is now
00:05:49
part of the excited electron it should
00:05:52
be noted that each material has
00:05:53
different levels of energy in other
00:05:56
words if the ground state is one unit
00:05:58
and the next energy level is 5 units
00:06:01
then the photon of light must have
00:06:02
exactly four units of energy to excise
00:06:05
the electron to that energy level
00:06:07
anything lower will not suffice and
00:06:08
anything higher would not as well as
00:06:11
there is nowhere for that extra energy
00:06:12
to go unless a higher energy State
00:06:15
exists if the incident photon is very
00:06:18
high in energy the electron would be
00:06:20
ionized to continue our analogy it would
00:06:23
be like trying to push the ball up the
00:06:25
hill with not enough Force the ball
00:06:27
would just roll back down too much force
00:06:29
and it would roll down the other side go
00:06:32
to another Plateau or be launched into
00:06:34
space an exact amount of energy is
00:06:37
required to elevate it to a particular
00:06:39
energy State again this process is
00:06:42
called stimulated absorption as we are
00:06:45
stimulating the electron and it absorbs
00:06:47
the photon's energy the next mechanism
00:06:50
we will look at is spontaneous emission
00:06:53
we now have an excited electron what
00:06:55
happens now well again this higher
00:06:58
energy level is quite unstable and after
00:07:00
a very very short time about 100 nond of
00:07:04
being there the electron will eventually
00:07:06
fall for some perspective light travels
00:07:10
about 29 m in 100 NS when it falls back
00:07:14
down it will release a photon with
00:07:16
energy equal to the difference in energy
00:07:18
levels the higher the fall the higher
00:07:21
the energy of the photon will be should
00:07:23
the energy value of the photon that is
00:07:25
released be in the visible range we
00:07:27
would perceive it as color you may be
00:07:30
thinking if the electron reaches the
00:07:31
higher energy level through the
00:07:33
previously mentioned stimulated
00:07:34
absorption mechanism why exactly does it
00:07:37
fall back down well referring back to
00:07:40
the ball example imagine the ball on a
00:07:42
hill but now with the top having zero
00:07:45
friction and a sharp point the ball can
00:07:48
remain there only if it is perfectly
00:07:49
balanced but any tiny little force in
00:07:52
either direction will cause it to start
00:07:54
rolling the electron in this higher
00:07:56
energy state is in a similar situation
00:07:59
the forces that push it are small
00:08:01
perturbations in vacuum energy this is a
00:08:04
quantum mechanical effect space or
00:08:06
vacuum is not as empty as we think
00:08:09
things are popping into and out of
00:08:10
existence constantly it is these vacuum
00:08:13
events that perturb the electron this is
00:08:16
also responsible for why things are
00:08:18
ferromagnetic that's a different story
00:08:20
though again this process is called
00:08:23
spontaneous emission as the process that
00:08:26
the electron falls back down to the
00:08:28
lower energy state is more or less
00:08:30
spontaneous the last Quantum process we
00:08:33
will talk about and the most important
00:08:35
for lasers is stimulated emission this
00:08:38
occurs when a photon interacts with an
00:08:40
electron that is already excited this
00:08:43
Photon can act as a type of pertubation
00:08:45
and force the electron to fall back down
00:08:48
to a lower energy State and emit a
00:08:49
photon we then will have two photons
00:08:52
photons actually like to be together so
00:08:55
if one comes near a situation where
00:08:57
another one could be present such as the
00:08:59
the electron falling back to a lower
00:09:00
energy State the situation usually will
00:09:03
play out the important part is that the
00:09:06
emitted Photon will be identical to the
00:09:08
one that stimulated it meaning same
00:09:10
frequency phase and polarization they
00:09:14
will be coherent with each other so if
00:09:17
we could somehow Avalanche this process
00:09:19
we would have a laser after all that is
00:09:21
basically what a laser is a zip tillan
00:09:24
identical coherent photons being emitted
00:09:27
in contrast if two electrons undergo
00:09:29
spontaneous emission the emitted photons
00:09:32
will unlikely be traveling in the same
00:09:34
direction nor be in Phase but in order
00:09:37
for electrons in the excited energy
00:09:39
level to be able to undergo stimulated
00:09:41
emission and not spontaneous emission
00:09:44
enough time has to be available the
00:09:46
lifetime of an electron in the excited
00:09:48
level is just too short however some
00:09:51
materials have so-called meta stable
00:09:54
States these are excited states with
00:09:56
slightly lower energy than the excited
00:09:58
States States these states allow the
00:10:01
electron to remain there for much longer
00:10:03
lifetimes milliseconds instead of Nan
00:10:06
seconds enough time that a passing
00:10:08
Photon can cause it to undergo
00:10:10
stimulated emission of course an initial
00:10:13
spontaneous emission from the metastable
00:10:15
state to the ground state must occur in
00:10:17
order to have the initial Photon that
00:10:19
can stimulate other excited electrons in
00:10:21
the metastable states to sum up if a
00:10:25
ground state electron is hit with a
00:10:27
photon it will absorb it and move from
00:10:30
the ground state to the excited state
00:10:32
the photon must have the energy equal to
00:10:35
the difference between these levels this
00:10:37
electron will then transition to the
00:10:39
metast stable state if one exists this
00:10:42
transition does not emit a photon and is
00:10:45
said to be a radiationless transition
00:10:48
the energy difference is dissipated in
00:10:50
other ways heat or phons now this
00:10:53
electron if a photon stimulates it will
00:10:56
emit a photon with equal energy phase
00:10:59
and Direction these are the ones that
00:11:01
make up the laser beam it should be
00:11:04
apparent that the photon which pumps the
00:11:06
electron from the ground state to the
00:11:08
excited state has a different energy
00:11:10
than the photons that are being lazed
00:11:13
this is because the energy difference
00:11:14
between the ground state and the excited
00:11:17
state is different than the difference
00:11:19
between the meta stable State and the
00:11:21
ground state the pumping photons are
00:11:23
always higher in energy than the photons
00:11:25
being
00:11:26
lazed we obviously want lots of
00:11:29
electrons in this meta stable State more
00:11:32
so than the ground state in order for
00:11:33
them to be in a situation where
00:11:35
stimulated emission can occur something
00:11:37
known as creating a population inversion
00:11:39
is required if we only had a two levels
00:11:43
we would reach a point of saturation
00:11:45
where 50% of the electrons are excited
00:11:47
and 50% are not the excited electrons
00:11:51
simply spontaneously emit to fast
00:11:53
essentially our medium becomes
00:11:55
transparent to photons by introducing
00:11:58
the metas stable State we force the
00:12:00
pumping photons to excite the ground
00:12:02
state electrons that then transition to
00:12:05
the metastable state so the photons that
00:12:07
are emitted by the transition from the
00:12:09
metastable state to the ground state are
00:12:12
primarily used to stimulate other
00:12:13
electrons in the metastable state enough
00:12:16
time exists for this to happen yes some
00:12:20
of these photons will excite ground
00:12:22
state electrons directly into the
00:12:23
metastable state but the pumping photons
00:12:26
should take care of the majority and
00:12:28
create a situation where there are more
00:12:30
excited electrons in the metast stable
00:12:32
State than ground state electrons a
00:12:34
population
00:12:35
inversion by the way the above is
00:12:38
describing a three-level laser four
00:12:40
level lasers exist and are more
00:12:45
efficient again we want to create an
00:12:47
avalanche effect where the spontaneously
00:12:50
emitted Photon that was created when an
00:12:52
electron transitioned from the
00:12:53
metastable state to the ground state get
00:12:56
Amplified through the means of
00:12:57
stimulated emission
00:12:59
we don't want just a single puny Photon
00:13:01
we want lots all working together it is
00:13:04
not practical to create a laser that is
00:13:06
extremely long so the solution is to put
00:13:09
the laser medium in a cavity let's take
00:13:11
a closer look at how a cavity will
00:13:13
influence the light waves and how
00:13:15
exactly this will create the
00:13:17
amplification we desire since light is a
00:13:19
wave it will be subject to constructive
00:13:21
and destructive
00:13:23
interference we want constructive
00:13:25
interference in our cavity to take place
00:13:27
in order to have a high intensity beam a
00:13:30
laser cavity has a mirror on one side
00:13:32
and a partial mirror on the other it is
00:13:35
partial because we want some of the beam
00:13:36
to escape that's the beam we see now
00:13:39
when light waves are created through
00:13:41
spontaneous emission they will initially
00:13:43
travel in random directions but the ones
00:13:46
traveling perpendicular to the mirrors
00:13:48
will reflect back and forth let's take a
00:13:50
look at one of these light waves it is
00:13:53
first emitted via spontaneous emission
00:13:55
and quickly becomes large in amplitude
00:13:57
through stimulated emission it travels
00:14:00
towards the mirror and is reflected back
00:14:03
because we continue to stimulate atoms
00:14:05
in the left and right directions we get
00:14:07
two waves in the cavity again one moving
00:14:11
to the left and one moving to the right
00:14:13
waves will add their amplitudes when
00:14:15
interfering with each other in this case
00:14:18
we will get a standing wave meaning
00:14:21
instead of a wave noticeably moving to
00:14:23
the left or right the combin wave will
00:14:25
appear to be going up and down rest sure
00:14:29
this is just an illusion this is the
00:14:31
effect of two waves hitting each other
00:14:33
head on and their left and right
00:14:35
components cancel out but their up and
00:14:37
down components add together so when the
00:14:40
wave looks flat this is a moment when
00:14:42
the two waves are destructively
00:14:44
interfering with each other and at the
00:14:46
maximum they are in a constructive
00:14:48
interference Point here are a few
00:14:51
examples of some standing waves in a
00:14:53
cavity that are resonating resonance is
00:14:55
just a fancy word for having these waves
00:14:57
being in a state where standing waves
00:14:59
are being produced a mode being just
00:15:02
what n you have Nal 1 is a mode Nal 2 is
00:15:05
another one n equal 3 Etc is there an
00:15:09
equation that will tell us what modes
00:15:11
can exist in the cavity sure there is
00:15:14
the left part is the frequency that
00:15:16
exists in the cavity n is the mode which
00:15:18
is always an integer V is the velocity
00:15:21
of the wave and L is the distance
00:15:23
between the two sides of the cavity the
00:15:26
Velocity in our equation is the speed of
00:15:28
light C which is 300,000
00:15:32
km/s the L is just the distance between
00:15:34
the mirrors light traveling from the
00:15:37
left of the cavity will now interfere
00:15:39
with light traveling from the right so
00:15:41
again we have these possible modes where
00:15:43
the light can produce standing waves and
00:15:46
be in resonance not all frequencies are
00:15:49
able to exist in a cavity but a lot are
00:15:52
also let's be clear that the standing
00:15:54
waves produces are a collection of
00:15:56
trillions and trillions of light waves
00:15:58
all working together they are produced
00:16:01
by stimulated emission and the cavity
00:16:03
allows them to keep amplifying each
00:16:05
other they are coherent with each other
00:16:08
recall this was one of the big reasons
00:16:10
why we care about lasers if we didn't
00:16:13
have this Synergy between light waves we
00:16:15
would just have an ugly LED I bet you
00:16:18
can't make your cat go crazy with a red
00:16:20
LED well maybe but you get my point
00:16:24
question what frequencies are allowed to
00:16:27
exist in a red Las a point of cavity
00:16:29
answer a cheap red laser pointer has a
00:16:31
cavity length of about 1 mm and the
00:16:34
speed of light is c 300 million
00:16:37
m/s plugging in these values to our
00:16:40
equation we would get a difference
00:16:42
between allowed frequencies of about
00:16:44
10050
00:16:46
GHz Now red light has a frequency of
00:16:49
about 400.0 5 terz which corresponds to
00:16:53
an N value of
00:16:56
2,667 recall n must be an integer so if
00:17:00
400.0 5 terz is an allowed frequency
00:17:03
then the next one would be when n equal
00:17:06
2,668 which is a frequency of 400.2
00:17:10
terz we can plot all allowed frequencies
00:17:14
as we know 150 GHz will separate them
00:17:18
the plot will look like this here we
00:17:20
have an equal to
00:17:23
2,667 and the corresponding frequency of
00:17:26
400.0 five terz here is
00:17:30
2,668
00:17:33
2,669 and so on these are the
00:17:35
frequencies that are allowed to resonate
00:17:37
in this laser cavity so if you wanted
00:17:40
your laser to have a frequency of 400.1
00:17:43
terz you would first have to change the
00:17:45
cavity length for this to be allowed as
00:17:48
it is not possible in this red Laser's
00:17:50
cavity about 2,600 frequencies in the
00:17:53
visible spectrum would be able to
00:17:55
resonate in this red lasers cavity
00:17:59
now there is slightly more to the story
00:18:01
about these allowed frequency lines we
00:18:04
have assumed the mirrors are perfect
00:18:06
which is practically impossible the
00:18:09
imperfectness of the mirrors and other
00:18:10
slight variations add a thickness to the
00:18:12
frequency lines the actual allowed
00:18:15
frequencies in a laser cavity looks like
00:18:17
this again this is due to
00:18:21
imperfections the last piece of the
00:18:23
puzzle is to mention the gain medium
00:18:25
itself gain medium is just the material
00:18:28
we are using for our laser different
00:18:30
materials will have different energy
00:18:32
levels hence photons of different energy
00:18:34
will be released during stimulated
00:18:36
emission for example different materials
00:18:39
will need to be used to create a blue
00:18:41
laser than that of a red laser since the
00:18:44
energy levels in a material are discrete
00:18:46
one would think that exactly one
00:18:48
frequency would be emitted out of a
00:18:50
laser but only if this is also a
00:18:52
frequency allowed in our laser cavity we
00:18:55
can superimpose these ideas on this
00:18:57
graph
00:18:58
we assume here that indeed the
00:19:00
stimulated emitted photon is a frequency
00:19:02
that is allowed in the cavity however
00:19:05
there is much more to the story The
00:19:07
frequencies being emitted out of the
00:19:09
laser actually takes a shape like this
00:19:12
this was briefly mentioned at the
00:19:13
beginning of this video when discussing
00:19:15
L width what is going on here are
00:19:18
complicated events such as the Doppler
00:19:20
effect Stark effect and other quantum
00:19:23
mechanical Behavior the takeaway is that
00:19:26
the gain medium does output a small
00:19:27
range of frequencies and has this gain
00:19:30
curve it is still extremely narrow and
00:19:33
said to be monochromatic it's not but
00:19:35
it's close enough to sum up certain
00:19:38
frequencies are allowed to exist in a
00:19:40
laser cavity there is some relaxation to
00:19:43
these frequencies as the mirrors and
00:19:45
such are not perfect the laser game
00:19:47
medium emits photons in a certain
00:19:49
frequency as well but again there is
00:19:51
some broadness to this as certain
00:19:53
effects influence this we can
00:19:56
superimpose these two frequency Plus
00:19:58
spots and get the following the
00:20:00
frequencies under the game curve that
00:20:01
have enough intensity to overcome other
00:20:03
cavity losses are the ones the laser
00:20:05
emits there are plenty of laser active
00:20:08
medium these days any frequency you wish
00:20:11
to lace is pretty much possible here is
00:20:13
a picture of different laser material
00:20:15
and the frequency they output some are
00:20:18
in the gas State Some solid and it is
00:20:20
even possible to use a liquid as a
00:20:22
Lessing
00:20:25
material this concludes this episode on
00:20:28
the laser if you enjoyed the content and
00:20:31
learned something please consider doing
00:20:33
all that stuff every other video asks
00:20:35
you to do you know what I am talking
00:20:43
about
