Imprinting method makes its mark on terahertz optics
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.”
 Terahertz Optic |
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.”
Source: Optics.org News.