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Wednesday, 20 February 2013

APL cover image article: lasing of nanoimprinted photonic crystals

ICN researchers and colleagues report fabrication of laser photonic crystals by nanoimprint lithography in Applied Physics Letters. Article honoured with cover image (18 Feb issue).

Lasing from nanoimprinted dye-doped polymers highlight the use of this fabrication technique for visible, more-compact, potentially low-cost lasers with improved power. This work may enable medical and sensing applications.

ICN researchers, led by Prof. Clivia M. Sotomayor Torres , ICREA Professor and Group Leader, together with colleagues from Ireland, Poland and Spain, have just published an article in Applied Physics Letters describing the fabrication  of photonic crystals of dye-doped polymers by nanoimprint lithography, and their subsequent lasing via optical-pumping. The article, entitled "Lasing in nanoimprinted two-dimensional photonic crystal band-edge lasers", has been honoured with the cover image of the 18 February issue of the journal.

All laser devices—from the now ubiquitous "red dot" handheld pointers used by lecturers to sophisticated experimental set-ups that occupy entire laboratories—function in basically the same way: electronic level transitions inside a material (the lasing medium) are excited via externally applied light, heat or electricity, and then relax to emit photons in the form of a highly coherent, unidirectional beam of light at one specific wavelength (the emission wavelength). Photonic crystals—crystals that enable strict control over the movement of photons, analogously to the way that semiconductors enable control over the movement of electrons—have become a hot topic in research on lasing media.

Dye-doped polymers have garnered attention as a lasing medium, owing to the ease with which dyes can be incorporated into polymers; the broad range of tuneable emission frequencies enabled by the ample variety of available dyes; and the fact that polymers can be patterned over large areas at relatively low-cost, using established micro- and nanofabrication methods.Dr Vincent Reboud*, Prof Dr Sotomayor Torres and colleagues have devised a method to combine the light control offered by photonic crystals with the practical advantages offered by dye-doped polymers, using known methods of nanofabrication.

The team mixed the polymer mr-NIL 6000 with the dye rhodamine 6G, spin-coated the mixture onto a glass substrate, and then used nanoimprint lithography to pattern photonic crystals in the dye-doped polymer. Nanoimprint lithography is like the traditional process of sealing and stamping envelopes with candle wax—only done at the nanoscale. The fabrication performed by the ICN researchers and their colleagues entailed two principal steps: spin-coating of the dye-doped polymer onto the substrate (like placing melted candle wax onto an envelope); and stamping of the spin-coated layer with a custom designed pattern (of different arrays of circular pillars) under elevated temperature and pressure (like stamping still-warm candle wax to create a pattern in it before it dries). The spin-coated layer was approximately 400 nm thick, and was stamped at a depth of approximately 350 nm.

The researchers then proceeded to study the optical properties of the imprinted polymers, by exciting them with pulses of laser light (a technique known as optical pumping) and subsequently measuring the intensity and wavelength of the emitted light. They found a lasing effect of the imprinted structures at wavelengths predicted by simulation depending on the geometry of the  arrays. They also found that their 2D photonic laser offered better performance than standard 1D lasers, as measured by its (2.5 times) lower lasing threshold and (50 times) smaller laser surface.

According to Prof Dr Clivia Sotomayor-Torres, who heads ICN's Nanophononics and Nanophotonics Group, "This work demonstrates the feasibility and precision of nanoimprint lithography for rapid, cost-efficient, one-step fabrication of polymer photonic crystals of diverse compositions, which should provide access to numerous practical laser applications in areas like medical analysis and sensing ."* Dr Reboud, a former member of ICN's Nanophononics and Nanophotonics Group, is currently a Research Engineer at CEA-LETI (Grenoble, France).

To access the article, "Lasing in nanoimprinted two-dimensional photonic crystal band-edge lasers", click here.