Wednesday, 17 September 2014
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Signal-boosting points to cheaper, better light sensors

New technology could allow light sensors to detect a greater range of signals, helping to make them cheaper and more effective.

Researchers from Britain and the US have found a way to increase the range of wavelengths that light sensors made from standard semiconductor materials can detect to include infrared and terahertz radiation as well as visible light.

The engineers from Leeds and Georgia State universities say this could lead to cheaper infrared sensors but also multi-band detectors that reduce false positives in devices that identify substances such as toxic gases.

‘If you have a broader range of wavelengths you can imagine getting detection over two or more frequencies simultaneously rather than having to have two different semiconductors,’ said Prof Edmund Linfield from the University of Leeds.

He added that in principle the technology could also allow solar panels to use a greater range of wavelengths of sunlight to generate electricity, although this would require substantial development work.

Existing photodetectors for detecting long-wavelength radiation tend to be made from more expensive materials such as indium-gallium-arsenide or are less efficient, limiting their applications.

The technology works by effectively boosting the energy of the signals by adding an extra beam of light, meaning lower-energy, longer-wavelength radiation can also be detected.

Standard photodetectors rely on light signals exciting electrons or positive charge carriers in a semiconductor to a specific energy level, which then produces an electrical signal indicating the presence of the light.

Using light to effectively pre-excite the charge carriers means less energy is needed raise them to the required level. This means common semiconductor materials such as gallium-arsenide could be used to detect signals that usually wouldn’t have enough energy to sufficiently excite the material’s charge carriers.

An alternative to this process has been to trap carriers in a special architecture known as ‘quantum wells’, which also reduces the amount of energy needed to excite the carriers but requires two types of semiconductor material.

The improved device can detect wavelengths up to at least the 55 micrometer range. Previously, the same detector could only see wavelengths of about four micrometers. The team has run simulations showing that a refined version of the device could detect wavelengths up to 100 micrometers long.

The experimental technology used a separate white light source to pre-excite the carriers but the researchers said the energy could also come from an LED integrated into the semiconductor or from electric current.

The researcher is published in the journal Nature Photonics.


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