Distributed feedback lasing in a dye-doped chiral smectic C liquid crystal
Distributed feedback lasers are devices that do not use a mirror cavity to provide feedback to a system with gain in order to produce lasing.   Rather, they use feedback from the lasing medium itself.   The advantage of this kind of device over conventional lasers is their compact size and tunable wavelength output.


The Chiral Smectic C (SmC*) phase possess a helical superstructure forming a 1-D periodic structure, where the periodicity is temperature dependent.   As a result, circularly polarized light propagating parallel to the helical axis is selectively reflected if it has the same handedness as the liquid crystal helix and its wavelength is approximately equal to the pitch.   This is the feedback mechanism in this system.


In this experiment cells 50 µm thick were filled with a dye-doped SmC* material that was aligned homeotropically. The dye gives this system gain.


By overlapping the red edge of the photonic band gap with the fluorescence of the dye, spontaneous emissions are reduced while stimulated emission is enhanced.   This, plus the feedback and gain, make lasing possible.



[1] G. Strangi, V. Barna, R. Caputo, A. De Luca, C. Versace, N. Scaramuzza, C. Umeton, R. Bartolino, and G. N. Price,"Color-Tunable Organic Microcavity Laser Array Using Distributed Feedback," Physical Review Letters 94, 063903 (2005).

Text and images contributed by Gabriel Price.