250
7 Light in Biology and Medicine
A laser produces high intensity light waves with close ‘phase coherence’ 38
among the waves of nearly one frequency. In contrast, ordinary light sources,
including incandescent and fluorescent, produce light with little or no phase
coherence among the waves produced by the radiating atoms or molecules. Laser
devices rely on the existence of metastable states in a material which can emit light
when that material releases energy by changing to a lower energy state. This release
is stimulated by passing light through the material.
Generally, metastable states are energy states of a system which have a higher
energy than a lower energy state of that system, but the system does not ‘fall’ quickly
into the lower energy because the available forces which might carry the system to
the lower energy are weak, or there is a barrier that the system has to overcome in
order to reach the lower energy.
For a laser, the metastable states are usually electron excited states which have
‘weak coupling’ to the ground state of the system. Because of the nature of
electromagnetic interactions, if an excited electron state can be reached quickly by
an electron absorbing a photon from the ground state, that same excited state will be
‘strongly coupled’ to the ground state, and therefore is not suitable as the metastable
state of a laser. However, there may be an intermediate state the system may fall
into which is only weakly coupled to the ground state. Electron metastable states
may be excited by electrons ‘falling’ from higher-energy excited states or directly
by non-radiative electromagnetic interactions, such as collisions with other atoms.
Figure 7.19 can be used to picture the three levels used for laser operation. In this
case, electrons are pumped into state with energy shown as E 1 . The level with label
E 2 is the metastable state.
By populating metastable states in the atoms or molecules of a selected material,
a light wave near one frequency can be amplified as the wave reflects back and
forth through the material. On each pass, the light stimulates the metastable atoms
to emit, amplifying the wave. To maintain the amplification, more atoms must
be in the metastable state than in the lower ground state, producing a population
inversion. Otherwise, the wave passing by will have a greater chance of being
reduced in intensity, rather than amplified. Optical flashers, atomic collisions after
acceleration, and electron collisions after acceleration by an electric field have all
been successfully used as ‘laser pumps’ to keep more than 50% of the working
atoms in a metastable state.
Laser beams are said to be coherent, in that the electromagnetic waves have close
to a single phase. As such, the average intensity of the beam is in proportion to the
square of the sum of the component waves from the emitting atoms. If there are
N such waves coming from N atoms, each of amplitude A, the intensity will be
proportional to N 2 A 2 . By contrast, the sum of waves with no phase relationship
38 Phase coherence means that the phases of the waves produced at different locations have a
definite relationship to each other.
7 Light in Biology and Medicine
A laser produces high intensity light waves with close ‘phase coherence’ 38
among the waves of nearly one frequency. In contrast, ordinary light sources,
including incandescent and fluorescent, produce light with little or no phase
coherence among the waves produced by the radiating atoms or molecules. Laser
devices rely on the existence of metastable states in a material which can emit light
when that material releases energy by changing to a lower energy state. This release
is stimulated by passing light through the material.
Generally, metastable states are energy states of a system which have a higher
energy than a lower energy state of that system, but the system does not ‘fall’ quickly
into the lower energy because the available forces which might carry the system to
the lower energy are weak, or there is a barrier that the system has to overcome in
order to reach the lower energy.
For a laser, the metastable states are usually electron excited states which have
‘weak coupling’ to the ground state of the system. Because of the nature of
electromagnetic interactions, if an excited electron state can be reached quickly by
an electron absorbing a photon from the ground state, that same excited state will be
‘strongly coupled’ to the ground state, and therefore is not suitable as the metastable
state of a laser. However, there may be an intermediate state the system may fall
into which is only weakly coupled to the ground state. Electron metastable states
may be excited by electrons ‘falling’ from higher-energy excited states or directly
by non-radiative electromagnetic interactions, such as collisions with other atoms.
Figure 7.19 can be used to picture the three levels used for laser operation. In this
case, electrons are pumped into state with energy shown as E 1 . The level with label
E 2 is the metastable state.
By populating metastable states in the atoms or molecules of a selected material,
a light wave near one frequency can be amplified as the wave reflects back and
forth through the material. On each pass, the light stimulates the metastable atoms
to emit, amplifying the wave. To maintain the amplification, more atoms must
be in the metastable state than in the lower ground state, producing a population
inversion. Otherwise, the wave passing by will have a greater chance of being
reduced in intensity, rather than amplified. Optical flashers, atomic collisions after
acceleration, and electron collisions after acceleration by an electric field have all
been successfully used as ‘laser pumps’ to keep more than 50% of the working
atoms in a metastable state.
Laser beams are said to be coherent, in that the electromagnetic waves have close
to a single phase. As such, the average intensity of the beam is in proportion to the
square of the sum of the component waves from the emitting atoms. If there are
N such waves coming from N atoms, each of amplitude A, the intensity will be
proportional to N 2 A 2 . By contrast, the sum of waves with no phase relationship
38 Phase coherence means that the phases of the waves produced at different locations have a
definite relationship to each other.