Oct 2, 2003
Nanoimprint lithography stamps organic lasers
Researchers in Italy have used a technique known as nanoimprint lithography to fabricate an organic distributed feedback (DFB) laser at room temperature. The resulting laser has a peak single-mode emission at 637 nm and a linewidth of less than 0.7 nm (Applied Physics Letters 83 2545).
The laser is based on a patterned thin-film of an oligomer called T5oCx. Dario Pisignano and colleagues have tried various methods to pattern the oligomer. However its low molar mass and poor thermoplastic behaviour mean that conventional hot-embossing techniques cannot be used.
Unlike hot-embossing, nanoimprinting works in air at room temperature. The team believes that this straightforward method could be used to develop a range of optoelectronic devices based on non-thermoplastic organic semiconductors.
The first step in making the laser is to etch a pattern onto a silicon master, which acts as a template. T5oCx films, between 400 and 600 nm thick, are then deposited on a glass substrate using spin-coating.
Pisignano says the crucial step is pressing the master into the film. “The applied pressure is the key factor to achieve a representative pattern without loss of adhesion between the organics and the substrate,” he told Optics.org.
The team says the optimum applied force is around 5 kN. This leaves a film containing a grating with a period of 600 nm. The pattern extends some 120 nm into the film.
The patterned film is then placed in a vacuum and optically excited by the third harmonic (355 nm) of a Q-switched Nd:YAG laser.
“We have applied this technique to a large number of low-molar-mass organic semiconductors, including electron and hole-transporting layers and gain compounds,” said Pisignano. “Electrical pumping of organic lasers is still a challenge of the current research, and it is the crucial step in order to made the organic lasers competitive compared with their inorganic counterparts.”
About the author
Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.