Taking the pulse of mystery materials

2019-02-28 09:17:06

By Jeff Hecht CHECK up on the chocolate in chocolate chip cookies, find out where the fat is in bacon, or read business cards through a sealed envelope. All this can be achieved with a new laser imaging technique which probes materials with broad-spectrum pulses of electromagnetic radiation lasting only a few millionths of a billionth of a second. “We’re still in the ‘play’ mode to see what we can see,” says developer Martin Nuss of AT&T Bell Laboratories in Holmdel, New Jersey. The pulses are generated by shining laser light on a tiny gallium arsenide antenna. Each laser pulse lasts around 10−15 seconds. The light releases current-carrying electrons and “holes”, which cause electricity to flow briefly in the antenna. The antenna in turn responds by radiating an electromagnetic pulse for the duration of the laser pulse. The shorter an electromagnetic pulse, the wider the range of frequencies it tends to contain. So the rapid pulses produced in the new technique contain a wide range of frequencies, from about 100 to 5000 gigahertz, centred around 1 terahertz (1000 gigahertz). As the pulses pass through the sample, their frequency profile alters, and these changes can be used to identify the sample. Previous attempts to use this approach took about two minutes to analyse a single point on the sample, which was far too slow to record images. To make imaging possible, Nuss and Binbin Hu increased the brightness of the pulse, improved the detectors, and took advantage of new fast-pulse solid-state lasers that generate pulses more rapidly than previous devices. They can now record pulse shapes at about 40 points per second, and obtain a 100 by 100 pixel image in just a few minutes. Existing detectors are too slow to profile all the transmitted pulses directly. Nuss and Hu instead collect readings at different stages of each pulse in a series of pulses. They then combine these readings to make a single composite pulse. This sampling process effectively shifts the very high frequencies in the pulses down to audio frequencies. Nuss and Hu use a speech-recognition technique to match measured wave shapes at these frequencies to those of known materials. The information can then be processed to generate clear falsecolour images. Hu highlighted the versatility of the technique at a conference in Baltimore at the end of last month. He described how bacon fat appears light, while the meat appears dark. Differences in fat levels also highlight chocolate chips in cookies. And a fresh leaf, which is still full of water, is darker than one picked two days previously. Terahertz pulses also show up the waxy inks often used on business cards,