Molecular spectroscopy and structure: LASERS

Question 1

What does LASER stand for?

Answer 1

light amplification by the stimulated emission of radiation

Question 2

When was stimulated emission predicted?

Answer 2

by Einstein in 1921

Question 3

When was stimulated emission observed experimentally?

Answer 3

1960

Question 4

What is ‘population inversion’?

Answer 4

many molecules in an excited state wait to be stimulated into emission in a cascade or domino-like process

Question 5

Name the basic components of a laser

Answer 5

• active medium
• pump
• optical resonator

Question 6

What is the active medium?

Answer 6

the luminescent material in which the electromagnetic wave is amplified

Question 7

What is the pump?

Answer 7

a source of energy which supplies the initial energy to active medium required to electronically excite the first molecules

Question 8

What different pumps are there?

Answer 8

• an optical pump is a high intensity light source such as a flash lamp or another laser
• an electrical pump supplies energy to the active medium by, for example, electrical discharge

Question 9

What is the optical resonator?

Answer 9

it contains the active medium and is essentially a box with a highly reflecting mirror at each end

Question 10

What is the active species in a ruby laser?

Answer 10

Cr3+ in a synthetic sapphire (Al2O3), doped with about 0.5% Cr2O3

Question 11

How does the laser work?

Answer 11

(1) the ruby crystal is illuminated with an intense flash of radiation of the requisite frequency to match the energy gap between the populated level [0] with energy E0 and an initially unpopulated level [1] with energy E1, exciting a number of the Cr3+ ions to level [1]
(2) state [1] in this system rapidly relaxes through a non-radiative process to a long-lived metastable state [2] with energy E. thus a large number of Cr3+ ions end up in state [2]
(3) when one of the ions in [2] naturally emits a photon, this is able to stimulate emission from another ion in level [2]. there are now two photons moving in phase and in the same direction, both of which can stimulate emission from other ions [2] and so the emission is amplified

Question 12

Why are molecules in [2] metastable?

Answer 12

although a molecule in [1] can be stimulated to emit by the pumping light source (ie. it has the correct frequency), the pumping light source is of too high a frequency to stimulate emission from [2]

(4) after a while, there is a large buildup of light (photons) in the cavity, regulated by the pumping of the system

Question 13

Which photons contribute to lasing?

Answer 13

only photons moving along the axis between the mirrors

Question 14

What would happen if the mirrors were perfectly reflecting the light?

Answer 14

the light built up in the cavity would remain trapped

Question 15

How should the length of the laser cavity be selected?

Answer 15

it must be carefully chosen to be an integer number of wavelengths of the laser radiation - otherwise the laser light would interact with itself destructively

Question 16

What is the problem with this process?

Answer 16

• each photon emitted from [2] can either be lost at non-reflective surfaces, lead to stimulated emission of another photon from [2] or be absorbed by a molecule in [0] taking it to state 2
• hence the process of stimulated emission is in direct competition with the process of absorption

Question 17

What is the requirement for population inversion?

Answer 17

in order for stimulated emission to outweight absorption, level [2] must have a higher population than [0]

Question 18

For the three-level system described here, what proportion of molecules must be excited to [2] for population inversion to be achieved?

Answer 18

if more than half the molecules in the active medium are excited to [2]

Question 19

What is the solution to these problems?

Answer 19

the 4-level laser:

• molecules are excited to [1] by pumping with a flash lamp or just a powerful continuum lamp (ie. continuous pumping)
• ions cross to a metastable state [2]
• stimulated emission occurs to an intermediate state [3] (not the ground state)
• [3] is then immediately depopulated non-radiatively to the ground state [0]

Question 20

What are the advantages of a 4-level laser?

Answer 20

• as the population inversion is between two excited states, it is much easier to achieve and control than a population inversion between an excited state and a heavily populated ground state
• the lower level of the lasing transition ( [3] in the example given) is very rapidly depopulated and so it is easy to maintain an excess population in the upper lasing level [2]. this means that much lower levels of pumping are needed in order for a population inversion to be sustained
• the laser can be operated continuously, giving out a continuous wave and the pumping transition can be provided by a continuous light source
• conversion efficiencies are very much higher

Question 21

Give two examples of organic species with electronic states ideally spaced to allow for their use in 4-level lasers

Answer 21

rhodamine 6G and rhodamine B

Question 22

What is the effect of incorporating a diffraction grating?

Answer 22

• the laser light can be tuned to produce a specific frequency within the range of energy levels, corresponding to just one of the vibrational levels of [3]
• thus we now have a continuum of frequencies available which can be chosen by careful tuning of the laser cavity

Question 23

Why do high power dye lasers need effective stirring and cooling?

Answer 23

high power dye lasers need effective stirring and cooling as level [3] is rapidly depopulated by collisions with the solvent molecules, which would otherwise lead to rapid heating of the laser

Question 24

What are the properties of laser light?

Answer 24

• highly monochromatic - all the energy is concentrated in essentially a single frequency
• radiation is coherent, meaning that the oscillations of the light waves are all in phase with one another
• radiation is highly parallel. this enables the laser light to be focussed to a very small spot, giving extremely high localised power density
• pulsed lasers, eg. ruby, can give radiation in extremely small pulses - nowadays shorter than 10^-15 s. this allows for techniques such as femtosecond spectroscopy, which directly probe the vibrations of bonds within molecules

Question 25

At what frequency does the ruby laser emit at?

Answer 25

694 nm