Lasers

Simplest energy level

2 level system

Types of level systems

2 level system 3 level system 4 level system

Why are lasers used in laser spectroscopy

Able to perform high resolution spectroscopy As the lasers are able to have very narrow line-widths (10 MHz)

What is a 4 level state system, four level laser

A group of electrons that occupy four energy levels E1, E2, E3, E4

What is a three level state system, three level laser

A group of electrons with three energy levels E1 E2 E3

Examples of lasers in everyday goods

DVD CD players Blu-ray Bar code readers Telecommunications

Spectroscope

An instrument that separates light into a spectrum.

Characteristics of light sources

Broadband Incoherent High divergence Low power flux

Examples of four level state systems

CO2 laser Nd: YAG laser

Properties of laser

Coherent (photons are dependent on each other), high intensity, goes in one direction, one wavelength Monochromatic output Coherent output Highly directional output Output is very powerful (powerful pulses or continuous output) Output can be either pulsed or continuous

How does continuous lasers occur?

Continuous waves (cw)

Examples of lasers used in engineering

Cutting Welding Drilling Micromachining

What are the units for continuous lasers

Cw

What lasers are used for laser spectroscopy

Diode lasers

What is the disadvantages of three level state system

Do work but not very efficient Difficult to create and maintain a population inversion between E1 and E2 as this requires fast radioactive decay between E3 and E2 (energy released radioactively) A lot of energy is needed to create and maintain a population inversion

Can we get a population inversion in a two-level system?

Does not have a population inversion Impossible to get more than 50% of the population on the upper level (as population inversion is more electrons in the excited state)

How can a four level laser system be more efficient?

Due to the population inversion More efficient if there is quick radioactive decay between level 3 and level 0 (ground state)

What does E1/E2/E3 and N1/N2/N3 mean?

E meaning energy level N1 = Number of electrons in E1 (energy level 1) N2 = Number of electrons in E2 (energy level 2) N3 = Number of electrons in E3 (energy level 3)

How does population inversion occur in a three level system? (Population inversion required for a laser to function remember)

E1 is the most populated The higher the energy level the less populated Only small amount of electrons in E2 and E3 Supply light energy which is equal to the energy difference between E1 and E3 The E1 electrons gain energy and jump to E3 The process of supplying energy is pumping The flow of electrons from E1 to E3 is called pump transition The lifetime of electrons in E3 is very small compared to lifetime of electrons in E2 Electrons in E3 fall to E2 realising radiation Small amount of electrons in E3 Electrons stay in E2 a large number of electrons will start to accumulate Overall E2 > E1 > E3 Overall N2 > N1 > N3 N2 has the highest population of electrons So a population inversion is achieved through energy levels E2 and E1

Diagram for three level system

E3 top level, excited state E2 Middle level, meta stable state E1 bottom level, ground state

Uses of lasers

Engineering Everyday goods Medical uses Weapons LIDAR Laser Spectroscopy Laser photolysis LIF laser induced fluorescence

How often do pulsed lasers occur?

Femtoseconds Milliseconds This means that the pulses are so frequent that it cannot be measured in seconds

How does the LASER exits in laser spectroscopy

In a gas phase As the absorption line widths are broad in liquids and solids

Where is laser spectroscopy used?

In absorption spectroscopy / in MIR for fundamental vibration UV - Visibke for electronic transitions

What are examples of conventional light sources

Incandescent lamps

Advantages of a three level system over a two level system

Increased quantum effects Population inversion

Explain the four level laser system and how population inversion occurs?

LEVEL 1 is the ground state LEVEL 3 is the highest excited state Supply light energy which is equal to the energy difference of level 0 and level 3 The electrons in level 0 (lowest energy state) gain enough energy and jumps to level 3 The lifetime of electrons in level 3 is very small so fall to level 2 a lower energy level Releases non radiation energy Lifetime of level 2 is larger than level 3 and level 1 (level 3 and 1 have very short lifetimes) A large number of electrons will start to accumulate in level 2 After their completion of their lifetime electrons drop from level 2 to level 1 Releases energy as photons Lifetime of level 1 is very small Electrons will quickly fall to level 0 Releases non radiation energy A population inversion is established between level 2 and level 1

Directionality

Laser beam emits light that is highly directional that means low divergence. Laser light is emitted as a relatively narrow beam in a specific direction. Ordinary light, such as coming from the sun, a light bulb, or a candle, is emitted in many directions away from the source. This is a direct consequence of the fact that laser beam comes from the resonant cavity, and only waves propagating along the optical axis can be sustained in the cavity. The directionality is described by the light beam divergence angle. Please try the figure below to see the relationship between divergence and optical systems.

Overall population inversion means

Laser emission

Overall population inversion results in

Laser emission

Examples of lasers in medical uses

Laser eye surgery (correction of short sightedness, removal of cataracts) Removal of birthmarks and tattoos Photodynamic therapy (treatment of skin cancers) Laser dentistry

High directionality means

Laser light has a very small divergence

Diagram for four level state system

Level 3 highest energy level Level 2 Level 1 Level 0 ground energy levels

What is LIDAR?

Lidar is a surveying method that measures distance to a target by illuminating that target with a laser light.

What does LiDAR stand for?

Light Detection and Ranging

What are pulsed lasers measured in?

Measured as energy Units being joules (J) in one pulse (How many joules are in one pulse)

What are continuous wave lasers measured in?

Measured as power Units being watts (W) 1 joule per second

Examples of lasers in weapons

Might sound like science fiction but U.S. military are developing a high powered laser mounted on a jumbo jet to shoot down ICBMs Chemical laser based on creating a population inversion in I2 from the reaction of I and O atoms Will generate 1 mega joule per pulse when operational

Why is population inversion needed?

More atoms are needed in the ground state than in the excited state A photon emitted by a spontaneous emission can lead to a collection of stimulated emission If the excited state has too short of a lifetime, not enough electrons will be around for stimulated emsission (will all be in the ground state) Excited state needs a longer lifetime and the absorption of a passer by photon is minimised To do this the smaller the ratio of atoms in the ground state to excited state The smaller the ratio, the more likely that the passer by photon will cause a stimulated emission rather than getting absorbed, (ground to excite but not excited to ground) So a population inversion is needed (more photons in the excited state)

Why is a 4 level state system better than a three levels state system

More efficient Relatively easy to maintain a population inversion between E1 and E2

Lasers characteristics

Must emit monochromatic light High directionality Small divergence The light that the laser produces must be coherent high brightness Concentrated light beam Efficient (higher ratio of output energy to input energy)

2 level system

One energy level which is the ground state Second energy level which is the excited state

Divergence

Simply put, it tells you how the beam grows from the source to the target. Divergence is defined as the angular measure of how the beam diameter increases with the distance from the laser aperture More precisely, divergence is defined as the angle at which the beam expands in the far field

What can be quoted for pulsed lasers?

Peak power This is pulsed energy / pulse width

What requirements are needed for a laser to function

Population inversion

What does a laser need to function?

Population inversion between 2 levels to function (not a two level system however)

What two ways can optical power be classified in

Pulsed lasers (optical power appears in pulses of some duration at some repetitive rate, concentrate their output in high power bursts) Continuous wave lasers (optical power appears constant over a interval of time) Output is either pulsed (optical power appear in pulses of some duration at some repetition rate) Or continuous waves (optical power appears constant over a interval of time)

Have do atoms and molecules have defined energy levels

Quantum theory

Why doesn't population inversion exist in two level systems?

Require a strong pumping action which is inpractical and inefficient, costly The pumping transition will need to work against a lasing transition More atoms will end up in the ground state

Example of a three level laser system

Ruby laser (First optical laser) By Theodore Maiman

Where is LIDAR used

Self driving cars Meteorology

stimulated emission

Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. (stimulated emission produces identical photons that are of equal energy and phase and travel in the same direction) (for stimulated emission to take place a passer by photon whose energy must approach the excited atom before it de-excites via spontaneous emission)

Example of peak power

The 20 nanosecond, 300 mili joule pulse from a 1064 manometer Nd:YAG laser Has a peak power of 1.5x10(7) W

One of the conditions for lasing is coherence, how is this condition satisfied (possible)

The electron transition occurs through stimulated emission (stimulated emission produces identical photons that are of equal energy and phase and travel in the same direction) (for stimulated emission to take place a passer by photon whose energy must approach the excited atom before it de-excites via spontaneous emission)

Coherence

The light from a laser is said to be coherent, which means the wavelengths of the laser light are in phase in space and time. electron transfer occur in a specific time and artificially (instead of naturally and random in time) Because of this coherence, a large amount of power can be concentrated in a narrow space. emit photons that are all in phase with one another

Stimulated emission

The process by which an incoming photon of a specific frequency can interact with an excited atomic electron causing it to drop to a lower energy level.

For a two level system how does population transfer occur?

Thermally Optically

Explain the usefulness of diode lasers

Tuneable However only have a narrow range, only available at selected wavelengths (Selected MIR, NIR, visible wavelengths and uv wavelengths)

What is LIDAR used for

Used to measure the distance to a target using a laser Similar to a radar

How does LiDAR work?

Uses time of flight to measure distance using short pulsed lasers Then scanning over the laser can produce a 3D map of the scene

Coherence laser

We know that visible light is emitted when excited electrons (electrons in higher energy level) jumped into the lower energy level (ground state). The process of electrons moving from higher energy level to lower energy level or lower energy level to higher energy level is called electron transition. In ordinary light sources (lamp, sodium lamp and torch light), the electron transition occurs naturally. In other words, electron transition in ordinary light sources is random in time. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colors. Hence, the light waves of ordinary light sources have many wavelengths. Therefore, photons emitted by an ordinary light source are out of phase. In ordinary light sources (lamp, sodium lamp and torch light), the electron transition occurs naturally. In laser, the electron transition occurs artificially. In other words, in laser, electron transition occurs in specific time. All the photons emitted in laser have the same energy, frequency, or wavelength. Hence, the light waves of laser light have single wavelength or color. Therefore, the wavelengths of the laser light are in phase in space and time. In laser, a technique called stimulated emission is used to produce light. In laser, the electron transition occurs artificially. In other words, electron transition in laser occurs in specific time. Thus, light generated by laser is highly coherent. Because of this coherence, a large amount of power can be concentrated in a narrow space.

Absorption

When atoms absorb energy of a photon to promote electrons to a higher energy level

Can we get a population inversion in a three level system

Yes, it is the simplest way for a population inversion can be created between two levels

absorption spectrum

a graph plotting a pigment's light absorption versus wavelength

Define population inversion

a necessary condition for laser operation, in which more members of a collection of atoms are in an excited state than are in lower energy states More electrons in the excited state than the ground state

incandescent

brilliant; giving off heat or light

monochromatic

having only one color A laser beam will be in a single wavelength Width of the laser is extremely narrow Photons with the same frequency or wavelength

Stimulated emission is important for

lasers

Laser

light amplification by stimulated emission of radiation

Where does population inversion occur and doesn't occur in three level systems

occurs in energy level E2 and E1 as E2 is the most populated Does not occur in energy level E1 and E3

optical pumping

one way to excited electrons in a laser is to shine light on the lasing medium (the crystal, gas, etc. inside the laser), causing radiative transitions upward

Diode laser

portable compact lasers used in clinical applications

A laser is a device that

produces a narrow beam of coherent light

What did Einstein propose in 1915?

that all electromagnetic waves are quantized Electromagnetic waves interact with atoms or molecules

Quantum

the amount of energy needed to move an electron from one energy level to another

Example of monochromaticity

the line width of laser beams are extremely narrow. The wavelengths spread of conventional light sources is usually 1 in 106, whereas in case of laser light it will be 1 in 1015. i.e., if the frequency of radiation is 1015 Hz, then the width of line will be 1 Hz. So, laser radiation is said to be highly monochromatic. The degree of non-monochrotmaticity has been expressed as where dλ or dν is the variation in wavelength or variation in frequency of radiation. ...

stimulated emission

the process in which an incoming photon causes an excited atom to drop to a lower energy level, thereby emitting a photon in the same direction as the incident photon

spontaneous emission

the random emission of a photon due to an atomic electron moving to a lower energy state quantum system goes from an excited energy state to a lower energy state and emits a quantized amount of energy in the form of a photon

Spectroscopy

the study of the interaction of electromagnetic radiation with matter the branch of science concerned with the investigation and measurement of spectra produced when matter interacts with or emits electromagnetic radiation.

quantum theory

the theory that describes matter and energy at very small (atomic) sizes