FEATURE TERAHERTZ ➤
1,560nm fibre femtosecond laser, producing less than 100mW. Chips can work with a variety of different lasers.’ Menlo is working with a Fraunhofer institute developing a photoconductive antenna. These antennas allow the user to pump a chip with a laser to produce terahertz radiation. The low temperature antenna is typically made of gallium arsenide for 800nm signals or indium gallium/aluminium arsenide for 1,560nm. These lasers typically produce low power output, but cover a broad spectrum. The quantum cascade lasers used by the University of Leeds produce higher power with a smaller spectral range.
Linfield explained that the team at Leeds had achieved the high power output by improving the
semiconductor material quality and uniformity. This meant that all the parts of the structure produce as much light as possible. To improve the quality of the material, the team grew the semiconductor in a complete vacuum. This reduced the risk of contamination of the cell and allowed the team to take their time in building up the layers of gallium, aluminium, arsenic, and silicon, and allowed very accurate positioning of wells and barriers for the electrons to manoeuvre. These wells and barriers create artificial energy levels when the electrons pass through the material, releasing photons with each step. Linfield explained: ‘For every one electron going through the material, you can get as many photons out of it
The Menlo K15 terahertz system A pyroelectric detector that fits in a T05 can TERAHERTZ DETECTORS While Gentec-EO
predominantly produces pyroelectric detectors for terahertz applications, there are alternatives: Golay Cells and thermopiles are among those that can be used. A Golay Cell is a photo- acoustic device that works at ambient temperatures and has a broad spectral response. The sensors are however fragile and installation can be difficult. The central component of the cell is a small chamber containing gas, fronted by
a diamond window, and with a mirrored back wall. The chamber contains a metallic film, known as an optical microphone, which heats up as it absorbs terahertz radiation, heating the gas in turn. As the gas is heated, it expands and distorts the mirrored back wall. The distortion, which can be monitored by displacement sensors, is directly proportional to the amount of radiation absorbed. It has been used for many years but it is fragile, slow to respond,
and has a limited dynamic range. Thermopiles measure change in the voltage output as a series of thermocouples’ absolute temperature changes. They are also sensitive to the range between soft x-rays to terahertz radiation. Thermopiles can handle a higher average power than the pyroelectric detectors of up to 10kW but are inherently slow to produce readings and are not as sensitive for the lower power ranges.
Pyroelectric detectors contain a pyro crystal; the current across this crystal is directly proportional to the rate of temperature change. This means it can be used to measure power and is very sensitive between the nanowatt to 200mW range. They also have quite a broadband response, ranging from soft x-rays to terahertz. Pyroelectric detectors are relatively cheap when compared to other detectors, and take readings very quickly.
According to Don Dooley,
pyroelectric detectors are the workhorse of the terahertz detector field. He explained: ‘They work at room temperature; they are small and compact, to the point that we can mount a detector and amplifier in a TO5 transistor can. Pyroelectric detectors are very sensitive and low in price when compared to the other types of terahertz detectors such as microbolometers or Golay Cells.’
as there are wells.’ So, by placing the many layers very accurately on top of one another, a higher quality material, and higher power output, can be achieved. ‘That’s half the story,’ he said. ‘The actual device processing, the dimensions, and the packaging itself, are the other half.’ There are thermal issues with terahertz lasers. Linfield explained: ‘It’s not a very efficient process. This is partly because there is a large amount of heat that you can’t dissipate effectively. Driving the efficiency up would obviously be good for the commercialisation of the technology and the less heat you have to deal with, the better.’ However, it is not the quality of the semiconductor that Linfield believes is the issue: ‘You mount
the chip on some kind of holder and it’s actually down to how efficiently you can extract this heat from the active region. You have a substrate that has a few microns of semiconductor on it; now that is a resistance to heat getting out. We’re looking at making that better to improve efficiency.’ These issues with heat are also what make manufacturers and the researchers so keen to keep the lasers small. ‘As they get bigger, you get more power but you have to make compromises,’ said Linfield. Agate agreed that, from a commercial point of view, the size is very important: ‘It’s all very well having a high-powered system, but it’s difficult to make it relevant in the real world environment, if it’s not portable.’
16 ELECTRO OPTICS l APRIL 2014
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