How lasers work

Summary

This 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

• 📚 Electrons orbiting an atom are described by quantum mechanics, with specific quantized orbits unlike the continuous orbits of planets around the sun.
• 🌌 The energy levels of electrons are fixed and can only take precise quantized values, with lower energy states closer to the nucleus on average.
• 🔄 The repulsive force between multiple electrons in an atom is strong, affecting the energy levels and complicating calculations compared to a single electron.
• 💡 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.
• 🕊 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.
• 🚀 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.
• 🌟 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.
• 🔍 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.
• 🔄 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'.
• 🔧 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.
• 🛠️ 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

00:00

🌌 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.

05:00

🚀 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.

10:02

🔋 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.

15:03

🔬 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.

20:04

🌈 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.

Keywords

💡Electron Orbits

Electron orbits refer to the paths followed by electrons around an atomic nucleus. In the video, it is compared to the orbits of planets around the sun to illustrate the concept of quantization in electron orbits, which is a fundamental principle in quantum mechanics. The script explains that unlike planets, electrons occupy specific, quantized energy levels or quantum states.

💡Quantum Mechanics

Quantum mechanics is the branch of physics that deals with the behavior of matter and energy at the atomic and subatomic level. The video emphasizes its role in governing the behavior of electrons in atoms, particularly how they occupy discrete energy levels or quantum states, which is a departure from classical physics where orbits can be arbitrary.

💡Quantum States

Quantum states are the specific energy levels that electrons can occupy around an atomic nucleus. The video script explains that these states are not defined with certainty in terms of position but are probabilistic, and the energy of an electron in a given state is fixed, unlike the continuous range of energy levels in classical orbits.

💡Energy Levels

Energy levels in the context of the video refer to the distinct energy states that an electron can have when orbiting an atomic nucleus. The video explains that these levels are quantized, meaning they are specific and not continuous, and that electrons can only exist in these precise energy states.

💡Ground State

The ground state is the lowest energy level that an electron can occupy in an atom. The video script mentions that this is the most stable state for an electron, and any increase in energy can excite the electron to higher energy levels or excited states.

💡Excited States

Excited states are energy levels higher than the ground state that an electron can be excited to. The video describes how an electron can be excited to these states through collisions with other atoms or free electrons, or by absorbing a photon with energy matching the energy gap between levels.

💡Spontaneous Emission

Spontaneous emission is the process by which an excited electron decays and emits a photon without the influence of an external photon. The video script explains this as a random process where the emitted photon's direction and timing are not controlled, unlike the stimulated emission in a laser.

💡Stimulated Emission

Stimulated emission is the process where an incoming photon of the correct energy causes an excited electron to emit a photon with the same properties, such as wavelength and direction. The video emphasizes that this process is not random and results in the synchronization of the emitted photon with the stimulating one, which is crucial for laser operation.

💡Gain Medium

The gain medium is the material within a laser that contains the atoms capable of producing stimulated emission. The video script describes how this medium can be in various states of matter and must contain atoms of the same type to ensure that the emitted photons have the same energy and therefore the same color.

💡Population Inversion

Population inversion is a condition where there are more atoms in the higher energy state (upper laser level) than in the lower energy state. The video script explains that achieving this condition is essential for a laser to operate because it allows for the amplification of light through stimulated emission, overcoming the natural tendency of atoms to minimize their energy.

💡Q-Switching

Q-switching is a technique used in lasers to create short, high-intensity pulses of light. The video script describes how this is achieved by momentarily removing the mirror from one end of the laser cavity to prevent the build-up of a continuous beam, allowing a large number of atoms to become excited, and then suddenly reintroducing the mirror to release a powerful burst of light.

💡Continuous Wave (CW) Laser

A continuous wave laser is a type of laser that emits a steady, uninterrupted beam of light. The video script explains that this type of laser operates as long as energy is provided for the pumping process, maintaining an equilibrium where atoms are continuously excited and photons are emitted and extracted at a constant rate.

💡Semiconductor Laser

A semiconductor laser, also known as a diode laser, is a type of laser that uses a semiconductor material as the gain medium. The video script describes how these lasers operate between energy bands rather than specific energy levels and how they can be made very inexpensively and efficiently, with applications in various technologies like barcode scanners and CD drives.

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.