Credit: © 2007 AIP

Nanoimprint lithography is a versatile technique for producing multiple finely featured structures using a single mould. This printing method works by bringing a hard pre-patterned 'mould' into contact with a soft polymer that is heated above its glass transition temperature. The master pattern is transferred into the polymer, which hardens as it cools down.

Led by Axel Scherer, a group at the California Institute of Technology now shows that this technique can tackle a fairly sophisticated optical structure — a two-dimensional circular laser cavity1. The process begins by depositing a polymer called poly(methylmethacrylate) — commonly known as PMMA — onto a substrate that has a lower index of refraction than the PMMA to ensure that light stays confined in the grating once made. To make the cavity, which is actually a diffraction grating, circular rings, 400 nm apart and 400 nm deep, are formed in the polymer layer with a nanoimprint mould. The dye-doped PMMA emits light that spreads out in the grating when it is stimulated with another laser, but is only enhanced at a wavelength that depends on the spacing between the concentric rings.

The circular grating laser has a peak wavelength at 618.52 nm (orange light) and a narrow width of 0.18 nm. Although it is found that the laser emission in the grating starts to decrease over time, it is claimed that the process and materials are so inexpensive that a new one can be made when required.