Page 292 - Organic Electronics in Sensors and Biotechnology

Organic Semiconductor Lasers as Integrated Light Sources for Optical Sensors 269

First-order resonators are of special interest for on-chip integration
(see Sec. 7.4) and enable lower laser thresholds.

7.2.2 Organic Semiconductor Energy Transfer Systems
Material systems with a large spectral gap between the absorption of
the excitation light and the laser emission are of particular interest for
organic semiconductor lasers. This reduces the self-absorption of the
emission, leading to a more efficient lasing operation. Such a system
can be created by doping the active material, thus forming a so-called
guest-host material.
The exciting radiation is absorbed in the host material. The absorbed
energy is then transferred by a radiationless process to the guest material,
lifting it into an electronically excited state (see Fig. 7.3). The predomi-
nantly utilized energy transfer mechanism for organic semiconductor
8
lasers is the Förster resonance energy transfer. This process is based on
a dipole-dipole interaction and therefore requires a spectral overlap of
the donor’s emission with the acceptor’s absorption. The effective
radius of this process can be up to 10 nm.
The second important energy transfer system is the Dexter transfer.
Here, an excited electron is transferred from the donor to the accep-
tor. In return, an electron in the ground state is moved from the accep-
tor to the donor. This particle exchange needs the atomic orbitals of
acceptor and donor to overlap. Therefore, this process is typically
only effective at distances of less than 1 nm.

7.2.3 Optical Pumping
There are several approaches for optically pumping an organic semi-
conductor laser. A high absorbance of the pump light in the laser
material is a basic requirement. Additionally, short pulses and a small
focus are advantageous. Depending on the material, the pulse dura-
tion has a critical influence on the threshold where lasing can be
observed. This condition depends on the ratio of fluorescence life-
time to the pump pulse duration. 27

Alq DCM
3

Photon Energy
transfer

FIGURE 7.3 Energy transfer in dye-doped organic semiconductors.