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20 Apr 2007
Researchers in UK say that an imprinting technique is a fast and reliable way to make optics for terahertz wavelengths.
Using a silicon master to imprint a pattern into polypropylene is a cost-effective way to produce terahertz diffractive optics, say researchers in the UK. The team used the method to fabricate a 2 THz, 50 mm focal length Fresnel lens, measuring 25x25 mm with features as small as 51 microns. (Optics Letters 32 1141)
"The lens is made of a material with excellent transmission properties and will enable optical functions such as focusing," David Cumming of the University of Glasgow told optics.org. "Furthermore, multi-optic structures would enable collimation, beam expanding and so on."
Today, terahertz radiation has applications ranging from medical imaging and security scanning to communications. However, manufacturing optical components compatible with terahertz radiation has been problematic. For example, fabricating suitable optics in silicon leads to high Fresnel losses and is labor intensive.
Looking for alternative approaches, Cumming and colleagues investigated fabricating polymer lenses using an imprinting technique and a silicon master. This is shown to improve signal strength by over 70 times.
The 3 mm thick silicon masters used for imprinting are made using a binary fabrication process. The complexity and feature depth of the structure is increased by a factor of 2 with each stage. The final structure has eight phase levels with a theoretical diffraction efficiency of 95 %.
Imprinting involved applying a temperature of up to 180 degrees and a pressure of 5 bar to the silicon master and 3 mm thick polypropylene sheet. After 5 minutes, the pressure was reduced and the master and polypropylene optic were separated. The team used a scanning electron micrograph to reveal excellent replication of both the feature size and the surface finish.
To test the lens, the team placed it in a collimated 2 THz beam emitted by a quantum cascade laser measuring 3 mm long and 250 µm wide. Cumming says that the lens increased the signal strength around 70 times that of the unfocused beam.
"The fabrication process enables a much wider range of lens properties to be achieved," concluded Cumming. "We are now working on multi-element and holographics. If a cost-effective means of making high-quality, low-loss optics can be found, we can reduce the need for more conventional reflective optics (such as off-axis paraboloids) that are expensive and large."
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