So, by this point, we've seen several types of lasers, some inexpensive, some characteristically efficient and some relatively powerful. The unique characteristic of a Dye laser is that it is tunable - the user can select a desirable output frequency within a certain range characteristic of the dye. The above picture is that of the Dye laser that Jim and Derek used in their multi-photon absorption and raman scattering lab.
As we saw with the CO2 laser, the lasing/amplifying medium of the Dye laser is a molecule. With each electronic energy level of the molecule are associated vibrational and rotational energy levels. These dye molecules are hydrocarbon molecules with carbon-carbon double bonds and sequences of alternating single and double bonds. All of these molecules have the hexagonal carbon ring structures. These rings have associated loosely bound electrons that can move from different nuclei within the plane of the ring. It is these electrons that provide the energy level structures of the dye.
At any time, there is always an even number of electrons in the ground state. These are grouped in pairs: one with its spin momentum vector pointed up and the other with its vector pointed down. Therefore, the total angular momentum is zero (remember they are in the ground state).
A single electron from one pair can be excited into a series of states. In the following diagram, S1 and S2 are singlet states, T1 and T2 are triplet states and S0 is the ground state:
or, more simply:
The difference between these two 'genres' of states is that in a singlet state, the excited electron has moved to a higher energy level, but retains its original spin. Therefore, its total angular momentum remains 0. In a triplet state, when excited electrons transition to a higher energy state, the spin flips. Thus there is a net angular momentum for these states.
Each singlet and triplet state has, as previously mentioned, associated rotational and vibrational energy states. Vibrational and rotational levels are close together relative to the electronic-level spacing. The vibrational levels are on the order of 1200-1700 cm-1, and the rotational levels are even smaller by two orders of magnitude. At room temperature, most dye molecules are in the vibrational state of So. If light of appropriate energy is incident on these molecules, excitation to a higher singlet state will probably occur.
The S0 to S1 transition covers a broad range of frequencies due to the rotational-vibrational energy levels. Once excited to the S1 state, these molecules can de-excite to the lowest vibrational level of S1 in a transition which takes ~ a pico-second. This non-radiative decay is referred to as internal conversion. Transitions from this lowest vibrational level of S1 to the vibrational levels of S0 can occur spontaneously. This is called fluorescence. A further possibility is that molecules nonradiatively move from S1 to T1, or S2 to T2, etc. This process, intersystem crossing occurs relatively slowly relative to internal conversion, but up to 10 ns. Phosphorescence, an unlikely process as it involves electrons de-exciting and flipping their spin, is the spontaneous emission from T1 to S0.
Jim and Derek used the Nd-YAG laser to excite the dye. Their setup looked like:
The pumping process works like this:
1. Rapid excitation of a vibrational-rotational level of S1
2. Internal conversion to the lowest vibrational level of S1
3. Laser emission to the vibrational-rotational levels of S0 sufficiently far above the ground state that their populations are low (Davis 251).
The system needs the S1 state to be heavily populated because of the broad fluorescence spectrum. Furthermore, intersystem crossing populates T1 and absorption can occur from the same wavelengths as emitted in the fluorescence. This loss must be minimized and is done so by adding substances such as oxygen or detergents to the dye. This can further be done by using a dye solution in which intersystem crossing is unlikely to occur. Using the Nd-YAG laser, however, the transfer to the triplet state can be ignored because of the short laser pulses. When the pump intensity is sufficient to achieve an upper level population density, the lower level is scarcely populated.
Dye molecules have a broad emission curve which is responsible for their tunability. This characteristic allows a range of emission wavelengths from 10-60Å. To tune the laser, manipulate the cavity so that the gain of most frequencies outside of the desired frequency is less than the loss. Often this is accomplished by replacing on of the cavity's mirrors with a diffraction grating. Tuning the laser is done by rotating the grating as shown:
Wavelengths satisfying the relation , where m = 1,2,... and d is the spacing between the lines on the grating are amplified within the laser.
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Davis, Christopher C. Lasers and Electro-Optics. New York: Cambridge University Press, 1996.
Milonni, Peter W. and Joseph H. Eberly. Lasers. New York: John Wiley & Sons, 1988.