Energy Transfer

The main effects of energy transfers in laser gain media are:

• Excitation energy is transported within the gain medium via energy transfers between laser ions of the same species (energy migration) (Figure 1). This can occur e.g. in highly ytterbium-doped crystals. Such processes do not directly modify the stored energy, but nevertheless can affect the laser efficiency in positive and negative ways: the efficiency may be increased e.g. in single-frequency lasers with strong spatial hole burning (where the latter is reduced by energy migration), but it can also be severely reduced when the excitation energy is transported to crystal defects where non-radiative decay occurs. In other cases, energy migration facilitates energy transfer to other ions (e.g. Er³⁺, see below).

• Energy transfers between ions of different species are often exploited in lasers and amplifiers. For example, pump radiation may be efficiently absorbed by Yb³⁺ (ytterbium) ions, which then transfer their excitation energy to erbium (Er³⁺) ions, and these then non-radiatively decay into a lower-lying level (Figure 2). That level serves as the upper laser level for a transition to the ground state. The Yb³⁺ ions act as sensitizer ions, allowing for more efficient pumping of the Er³⁺ acceptor ions due to a higher allowed doping concentration and larger absorption cross sections of the Yb³⁺ ions. Erbium–ytterbium-doped fibers allow, e.g., the construction of very short (often single-frequency) 1.5-μm fiber lasers and amplifiers. Similarly, Tm³⁺/Ho³⁺-doped fibers can exploit energy transfer from thulium to holmium ions. In bulk lasers, Cr³⁺ ions (e.g. in Nd³⁺:Cr³⁺:GSGG) can absorb radiation from a flash lamp and transfer their energy to Nd³⁺ ions; chromium doping can thus make lamp-pumped lasers more efficient.

• Energy transfers can lead to cross relaxation of ions (Figure 3): an excited ion transfers part of its energy to another ion in the ground state, so that both end up in some intermediate level. Such processes are helpful e.g. in some thulium-doped 2-μm lasers, where a quantum efficiency well above unity can be achieved (more than one ion in the upper laser level per originally excited ion), but in other cases the laser efficiency can be degraded.
• In other cases, energy transfers help to depopulate the naturally long-lived lower laser level, which would otherwise lead to a self-terminating laser transition.

• Energy transfers can also be useful in upconversion lasers. Upconversion based on such energy transfers, where one ion enters a higher lying electronic state using energy from another ion (Figure 4), is often called Auger upconversion or cooperative upconversion. This kind of upconversion is an alternative to upconversion via subsequent absorption of pump photons. It has the advantage that only a single pump source is needed. On the other hand, additional unwanted energy transfer processes can easily occur. Cooperative upconversion processes are also detrimental in lasers where upconversion is not wanted, but in that case they can in principle be reduced by using a gain medium with a lower doping concentration.

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