Molecular spectroscopy and structure: LASERS Flashcards
What does LASER stand for?
light amplification by the stimulated emission of radiation
When was stimulated emission predicted?
by Einstein in 1921
When was stimulated emission observed experimentally?
1960
What is ‘population inversion’?
many molecules in an excited state wait to be stimulated into emission in a cascade or domino-like process
Name the basic components of a laser
- active medium
- pump
- optical resonator
What is the active medium?
the luminescent material in which the electromagnetic wave is amplified
What is the pump?
a source of energy which supplies the initial energy to active medium required to electronically excite the first molecules
What different pumps are there?
- 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
What is the optical resonator?
it contains the active medium and is essentially a box with a highly reflecting mirror at each end
What is the active species in a ruby laser?
Cr3+ in a synthetic sapphire (Al2O3), doped with about 0.5% Cr2O3
How does the laser work?
(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
Why are molecules in [2] metastable?
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
Which photons contribute to lasing?
only photons moving along the axis between the mirrors
What would happen if the mirrors were perfectly reflecting the light?
the light built up in the cavity would remain trapped
How should the length of the laser cavity be selected?
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
What is the problem with this process?
- 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
What is the requirement for population inversion?
in order for stimulated emission to outweight absorption, level [2] must have a higher population than [0]
For the three-level system described here, what proportion of molecules must be excited to [2] for population inversion to be achieved?
if more than half the molecules in the active medium are excited to [2]
What is the solution to these problems?
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]
What are the advantages of a 4-level laser?
- 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
Give two examples of organic species with electronic states ideally spaced to allow for their use in 4-level lasers
rhodamine 6G and rhodamine B
What is the effect of incorporating a diffraction grating?
- 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
Why do high power dye lasers need effective stirring and cooling?
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
What are the properties of laser light?
- 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
At what frequency does the ruby laser emit at?
694 nm
