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Terahertz QCLs image samples in 3D

28 Mar 2006

Researchers combine high-power terahertz quantum cascade lasers with a three dimensional image reconstruction technique.

A team from the UK has unveiled what it says is the first imaging system to combine high-power terahertz (THz) quantum cascade lasers (QCLs) with a three dimensional image reconstruction technique. Having tested their system on polystyrene samples, the researchers believe future applications include characterisation of pharmaceutical products, security and quality control. (Optics Express 14 2123)

Studies into terahertz imaging systems have tended to concentrate on two-dimensional scanning of thin samples. In this work, Lynn Gladden from the University of Cambridge and her colleagues from the universities of Leeds and Manchester, adopted the same approach to that used in X-ray computed tomography. This involves acquiring a series of cross-sections through a sample and processing them to obtain a 3D reconstruction.

"Important considerations when designing the system were the available power, the optical design to provide a well-collimated, small diameter beam through the sample and the analysis of the data," Gladden told optics.org. "Using a QCL addressed the issue of available power directly."

The researchers grew a THz QCL based on a GaAs-AlGaAs heterostructure by molecular beam epitaxy. Emitting at 2.9 THz, the laser produced 250 ns pulses at a repetition rate of 80 kHz and offered a peak pulse power of 70 mW.

The QCL was mounted on a cryostat and its emission was focused on to the sample using parabolic mirrors. Motorised stages translated the sample and transmitted radiation was collected by a Golay cell.

"A Golay cell is a relatively high-sensitivity, cheap, room temperature opto-acoustic detector," explained Gladden. "It was chosen because it does not require cryogenic cooling plus the portability it offers may be useful."

Gladden's team tested its set-up on a range of samples including a low density polystyrene formed into the shape of a clown's head and phantoms made from polytetrafluoroethylene (PTFE).

According to Gladden, the image resolution is typically 800 to 1100 microns and is governed by the beam focus over the thickness of the sample. Although it takes around 15 minutes to acquire one slice of an image, the complete reconstruction takes only a few seconds.

"The limitation of the current system is very much its ability to account for reflection and refraction of THz radiation," explained Gladden. "The Radon transform algorithm used in X-ray tomography limits us to studying materials with a refractive index of less than 1.5 if artefact-free images are to be obtained."

The team is now developing the reconstruction algorithm and improving its experimental design. It hopes to find ways to account for scattered radiation in the image reconstruction and increase the refractive index of materials that can be studied quantitatively.

"Employing more advanced optics giving a non-diffracting pencil beam closer to the diffraction limit will be a future development," said Gladden. "We are also investigating micro-bolometer arrays for parallel detection. This would remove the need to scan the beam across the sample. Increasing THz QCL powers will also decrease data acquisition times."

This project was performed under the RCUK Basic Technology Programme.

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