Page 289 - Organic Electronics in Sensors and Biotechnology

266 Chapter Seven

Organic lasers are a special type of organic light-emitting devices. In
7
contrast to OLEDs, these devices emit a highly directional, mono-
chromatic beam of light. By using different materials the whole visi-
ble wavelength range can be covered by the laser emission. 8–11 These
lasers can also be fabricated by low-cost methods, 12, 13 rendering them
interesting for an application in optical sensor systems. In the last
years the vision of a complete analysis system integrated “on a chip,”
a so-called lab-on-a-chip, has attracted an increasing number of users
and research groups. 14–18 Possible applications for such systems are in
the areas of medicine, drug discovery, and environmental monitoring.
Instead of shipping a sample for an analysis to a central laboratory, one
could use such small, portable systems for point-of-care diagnostics. 19, 20
Optical detection schemes such as laser-induced fluorescence, absor-
bance detection, or evanescent field sensing are commonly used
approaches in sensor systems. Today, these techniques are usually
based on external laser sources and complicated coupling optics being
bulky and expensive. The monolithic integration of laser light sources
onto the chip will result in small, portable, and far more flexible sys-
tems. 21–25 In this chapter we discuss first the fabrication and properties
of organic semiconductor lasers. Some focus is put on the aspects of
low-cost replication. Then we address the integration of organic lasers
into optical sensor chips and the choice of sensing principles.

7.2 Organic Semiconductor Lasers
The term organic semiconductor laser applies to a large variety of
devices that are currently under development. Common to most of
them is the use of the basic laser approach consisting of an optical
active material placed inside a resonator which provides the feed-
back required for efficient laser operation. In contrast to inorganic
semiconductors, their organic counterparts typically exhibit a large
spectral range with optical gain under optical pumping. This pro-
vides the possibility to tune the emission wavelength of the laser by
just modifying a wavelength selective resonator.
7.2.1 Distributed Feedback Resonators
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Among different approaches for the resonator design the distributed
feedback (DFB) resonator scheme is one of the most popular realiza-
tions due to its rather simple and efficient design. Typically, the organic
distributed feedback laser consists of a thin-film active material that is
deposited on top of a periodical corrugated substrate. The active mate-
rial is optically pumped, e.g., by an external pulsed UV laser. The layers
sequence forms a slab waveguide (thickness < 400 nm) confining the
optical wave in the substrate plane. The so-called distributed feedback
mechanism of the laser relies on Bragg scattering induced by a periodic
modulation of the refractive index induced by surface corrugations