Their paper “Flexible and ultra-lightweight polymer membrane lasers” published in Nature Communications details a seemingly simple fabrication process that yields membrane-based, substrate-less ultrathin organic distributed feedback lasers with an excellent mechanical flexibility. Because the bare-bone laser stack is built on a 50nm-thin water soluble PEDOT:PSS layer on top of a glass substrate, a simple immersion suffices to dissolve the sacrificial layer and float the hydrophobic laser membrane. After lift-off, the laser stack merely consists of a UV curable imprint resist defining distributed feedback (DFB) gratings, covered with a 180nm thin layer of an organic semiconducting polymer as the gain material.
Unlike most organic lasers reported so far, the membrane laser was proven operational even without a substrate or being flexed.
In various experiments, the authors transferred their membrane laser to unlikely substrates, such as a banknote, a bovine eye, and a finger nail. When excited with pulsed blue light (at 450nm), the distributed feedback gratings emitted a visible beam at 540nm (green).
Because the emission spectrum of these membrane lasers can be accurately tuned with an emission linewidth well under 0.5nm (using different grating periodicities and gain materials), the researchers anticipate that several lasers could be designed on a same membrane, emitting at distinct wavelengths only 1nm apart. The idea is to encode a well-defined and discrete lasing spectrum, very much like a binary barcode unique to each membrane laser and easily read out optically. Spread over the wide gain spectrum of the organic polymer used in this experiment, at least 50 independent spectral channels could be encoded, translating to about 250 combinations or roughly 1015 unique labels.
The highly transparent membrane lasers could be mass-produced and laminated to uniquely secure banknotes and ID documents or customized to augment biometrics such as finger scans and iris recognition. In an experiment, a bovine eye covered with the laser membrane (embedded in a contact lens) beamed a green light at a white screen. The researchers say the pump thresholds and emission intensity of their membrane lasers are well within the permissible exposures for ocular safety, so they could be worn as contact lenses as a novel form of wearable security tags.
University of St Andrews – www.st-andrews.ac.uk