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Narrowband detector helps spot airborne toxins

15 Jun 2007

A UV photodetector with a bandwidth of only 6 nm could allow real-time detection of biological and chemical agents.

An international team from Germany, Spain and India has demonstrated a photodetector operating at 360 nm that has a bandwidth of 6 nm, five times narrower than that of comparable devices. The detector is also sensitive to polarized light, reducing the influence of scattered background radiation (Applied Physics Letters 90 091110).

The detector is based on a 0.4 µm photo-sensitive film of non-polar gallium nitride (GaN) on a substrate of lithium aluminate (LiAlO2), and its unique characteristics arise from controlled strain in the GaN layer. "A mismatch between the lattice constants and thermal expansion coefficients of the film and the substrate causes in-plane anisotropic strain," Sandip Ghosh of the Tata Institute, India, explained to optics.org. "The result is that the optical bandgap of the film is different for light polarized parallel and perpendicular to the film's c-axis. It's this polarization sensitivity that's crucial for achieving the narrow detection bandwidth."

Many airborne agents emit at one or a few characteristic wavelengths when excited by laser, but making interference filters that work deep into the UV region can be difficult. A narrowband detector sensitive to the correct UV wavelength should be more efficient, especially if the polarization sensitivity allows background interference to be minimized.

The team's demonstration opens up the prospect of employing banks of detectors, each one using a different film composition to detect a particular chemical. "To make the configuration work at a particular wavelength just requires an M-plane nitride film with an appropriate bandgap," said Ghosh. "Using a binary alloy of AlN and GaN should allow detection of wavelengths shorter than 360 nm, while an alloy of GaN and InN will detect longer wavelengths."

A separate detector would be needed for each target agent, but this could still mean faster real-time detection and lower costs compared to a broad spectral scan with a spectrometer.

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