PHOTONIC FRONTIERS: ORGANIC SEMICONDUCTOR LASERS
The pump is
the challenge
JEFF HECHT, CONTRIBUTING EDITOR
O
rganic semiconductors have opened up
new possibilities for optoelectronics as
well as electronics. Organic light-emit-
ting diodes (OLEDs) have gained rapid
acceptance in portable displays be-
cause of their bright emission and low power consumption.
ey’re common on mobile phones and portable audio play-
ers and are scaling upward. Organic semiconductor lasers
would seem a logical next step, but where are they?
Optically pumped versions are in the laboratory, where developers have demonstrated a variety of devices and lately have made striking progress in reducing pump powers. At the University of St. Andrews (Fife, Scotland), Ifor Samuel recently pumped a polymer laser with a gallium nitride LED. But electrical pumping of organic semiconductor diode lasers has proved to be extremely di cult.
Organic semiconductors
Light-emitting organic semiconductors fall into two broad categories: small molecules and conjugated polymers. Both types are used in OLED fabrication, but laser developers have focused on conjugated polymers, which can be deposited from solution or printed by ink-jet-like devices.
Organic semiconductors have important attractions
that extend beyond the ability to fabricate them in inex-
Pump beam Output beam
Output mirror
Thin organic semiconductor
Total reflector
FIGURE 1. Optical pumping of a polymer thin lm sandwiched in a Fabry-Perot cavity shows that gain can be high, but power is low because the cavity is very thin.
pensive arrays for OLED displays. In polymers, the semiconducting properties arise from overlap in electron orbits along carbon chains where single and double bonds alternate. Both emission and absorption bands are inherently wide, and wavelength ranges can be chosen by selecting the compounds that make up the polymer. Like laser dyes, both emission bands and absorption bands are broad, allowing both wavelength tuning and short-pulse generation. e absorption bands are strong and widely separated from uorescence bands, as required for high gain.
In laser dyes, interactions with neighboring molecules can quench excited states and prevent emission, limiting the usable concentrations of laser dyes in solution. is problem can be overcome in conjugated polymers by adding side groups that separate the excited groups along the molecule from each other.
However, organic semiconductors have other inherent limitations that particularly impact electrical excitation. Electron mobility in them is less than in inorganic semiconductors. Polymers don’t hold up well at the high cur-
References:
Archives