Page 302 - Organic Electronics in Sensors and Biotechnology

Organic Semiconductor Lasers as Integrated Light Sources for Optical Sensors 279

devices where several laser materials on one substrate with a wide
range of different emission wavelengths are needed.
Different concepts can be used for tuning the laser emission
wavelength within the gain spectrum of one single material.
The most common method is to produce one substrate exhibiting
several resonators with varying grating periods. The active layer can
then be deposited onto all gratings simultaneously. Due to the different
feedback conditions the emission wavelength can be tuned. Figure 7.11
shows an example for Alq :DCM lasers where the grating constants
3
cover the range between 390 and 440 nm. The corresponding wave-
length range extends over more than 120 nm. 25
Additional emission wavelength tuning can be achieved by alter-
ing the film thickness. Increasing the film thickness results in a bigger
overlap of the guided mode into the active layer, increasing the effec-
tive refractive index n and thus shifting the emission wavelength
eff
spectrally into the red (see Sec. 7.2.1). With this method a tuning range
of 44 nm for Alq :DCM lasers could be demonstrated. 10
3
Such a laser can be fabricated also in a single pass by using a shut-
ter to cover the areas of lower thickness. Another way to alter the
effective refractive index is via a buffer layer between substrate and
active material with a different refractive index. For this approach it
is required to add an extra deposition step to the process chain before

h (nm)
FIGURE 7.11 Spectra of Alq :DCM lasers based on resonator gratings with
3
different periodicities. (Reprinted with permission from Ref. 25, “Organic
semiconductor lasers as integrated light sources for optical sensor systems,”
SPIE 2007.)