DyeLaser

An Introduction to Pulsed Dye Lasers
Introduction
Light amplification by stimulated emission of radiation, that is--lasers, have become an
important tool in chemistry. Lasers are ideal light sources for spectroscopy, chemical kinetics
studies, and light scattering studies of molecular motion. High-powered lasers are finding use as
light sources for photochemical synthesis, yielding products not available from other techniques.
Dye lasers are an important class of lasers because they can be tuned to a range of wavelengths.
The lasers you are probably familiar with, like He-Ne lasers, produce only fixed wavelengths and
are therefore not good sources for spectroscopic studies. In this lab you will measure the
absorption and fluorescence characteristics of several laser dyes and then set up a dye laser with
the dyes and compare the laser emission characteristics.
Theory
Coherence Laser light is unusual because it is coherent, intense, and formed in a narrowly
divergent beam. The light is also monochromatic, that is, the laser produces a narrow range of
wavelengths. Let us consider each of these characteristics. Figure 1 illustrates the difference
between coherent and incoherent light sources. Incoherent light sources, such as light bulbs,
produce many waves but the waves are out of step. Lasers produce light waves that are in step so
that the electric and magnetic fields of the waves oscillate in phase with each other. This
coherence increases the intensity of the light, in the following way. If the amplitude of a wave is
"a," then the intensity of that one wave is a2. The total intensity of an incoherent light source is
just the sum of the intensity of all of the waves. For n waves the sum is na2. In spatially
coherent radiation the fields oscillate in phase so that the amplitudes add, giving a total amplitude
of na. The total intensity is then the square of the total amplitude or n2a2. Thus, the increased
intensity of laser light is caused by its coherence. Coherence is also responsible for the ease of
forming beams of light, but the details are not necessary here. T he monochromatic nature of the
light is caused by the atomic or molecular transitions involved in the mechanism of laser action.
Spatially incoherent Spatially coherent
Intensity of one wave = a2
Total intensity = n a2
Total Amplitude = n a
Total intensity = (n a )2 = n2 a2
Figure 1. Coherent vs. incoherent radiation.
Mechanism In a laser the atoms or molecules of the medium are excited into excited states by
energy from an electric discharge, flash lamps (like a high powered photographic flash), or
another laser. The atoms or molecules then emit light while going back to their ground states.
Dye Lasers -2-
Three things are needed for laser emission: (1) a high probability of stimulated emission, (2) a
population inversion, and (3) a resonant cavity. Figure 2 compares spontaneous and stimulated
emission. After an atom or molecule absorbs light, it is left in an excited state. This state may
loose energy by emitting a photon or it may lose energy through collisions with other atoms or
molecules in the form of heat. In all that follows, we will assume that this latter pathway of loss
of energy as heat is not important. When an atom or molecule emits a photon it returns to its
lower energy state. In the absence of any other light source, this photon emission is a random
event with a certain probability, and is called spontaneous emission. Spontaneous emission is
familiar to chemists because it is the process responsible for fluorescence and phosphorescence.
Absorption Spontaneous
Emission
Stimulated
Emission
h? h? h?
h?
h?
Figure 2. Spontaneous vs. Stimulated emission.
However, if light at the frequency of the transition is present, an excited state may be
stimulated to emit a photon. Picture that if an oscillating electric field of just the right frequency
passes an excited atom, that the atom is jostled about and therefore finds it easier to emit its
energy. Therefore, if one photon is present at the start, after emission a new photon is present
along with the original. This process is called stimulated emission. One important characteristic
of stimulated emission is that the two photons are synchronized by the process; that is, they are
coherent. For laser action the probability of stimulated emission must be greater than that for
spontaneous emission.
The second requirement for laser emission is a population inversion. Under normal
circumstances, there are more atoms or molecules in low energy states than in high energy states.
In a population inversion the opposite is true. Different types of lasers create this population
inversion in several ways. The ruby laser operates by a "two level system" as shown in Figure 3.
The energy diagram includes the ground state for the Cr3+ ion and several excited states. The
population inversion is created when an intense flash of light excites Cr3+ ions from their ground
states into a variety of excited states. These excited states rapidly lose energy to produce ions in
the lowest energy excited state by a process called internal conversion. In internal conversion the
excess energy is lost as heat. The lowest energy excited state in ruby has an unusually long
lifetime, i.e. low probability for spontaneous emission. Therefore, as the higher excited states
depopulate many ions are caught in this lowest excited state, creating a population inversion with
the ground state. Stimulated emission can then start a cascade of emission back to the ground
state. This concerted cascade results in an intense burst