The Quantum Snooper

The laser on the right sends its light through a gas container, then the beam is reflected by a mirror. (Source: TU Wien)

The laser on the right sends its light through a gas container, then the beam is reflected by a mirror. (Source: TU Wien)

As humans, we sniff out different scents and aromas using chemical receptors in our noses. In techno­logical gas detection, however, there are a whole host of other methods available. One such method is to use infrared lasers, passing a laser beam through the gas to an adjacent separate detector, which measures the degree of light attenua­tion it causes. TU Wien’s tiny new sensor now brings together both sides within a single component, making it possible to use the same micro­scopic structure for both the emission and detection of infrared radia­tion.

“The lasers that we produce are a far cry from ordinary laser pointers,” explains Rolf Szedlak from the Institute of Solid State Elec­tronics at TU Wien. “We make what are known as quantum cascade lasers. They are made up of a sophis­ticated layered system of different materials and emit light in the infrared range.” When an elec­trical voltage is applied to this layered system, electrons pass through the laser. With the right selection of materials and layer thicknesses, the electrons always lose some of their energy when passing from one layer into the next. This energy is released in the form of light, creating an infrared laser beam.

“Our quantum cascade lasers are circular, with a diameter of less than half a millimetre,” reports Gottfried Strasser, head of the Center for Micro- and Nano­structures at TU Wien. “Their geometric pro­perties help to ensure that the laser only emits light at a very specific wavelength.” “This is perfect for chemical analysis of gases, as many gases absorb only very specific amounts of infrared light,” explains Bernhard Lendl from the Institute of Chemical Techno­logies and Analytics at TU Wien. Gases can thus be reliably detected using their own indi­vidual infrared ‘finger­print’. Doing so requires a laser with the correct wavelength and a detector that measures the amount of infrared radia­tion swallowed up by the gas.

The new sensor type: Two concentric quantum cascade rings are fitted for a sensor, which can both emit and detect light. (Source: TU Wien)

The new sensor type: Two concentric quantum cascade rings are fitted for a sensor, which can both emit and detect light. (Source: TU Wien)

“Our micro­scopic structure has the major advantage of being a laser and detector in one,” Rolf Szedlak says. Two concentric quantum cascade rings are fitted for this purpose, which can both emit and detect light, even doing so at two slightly different wave­lengths. One ring emits the laser light which passes through the gas before being reflected back by a mirror. The second ring then receives the reflected light and measures its strength. The two rings then immediately switch their roles, allowing the next measure­ment to be carried out.

In testing this new form of sensor, the TU Wien research team faced a truly daunting challenge: they had to differen­tiate isobutene and isobutane. The micro­scopic sensors passed this test with flying colours, reliably iden­tifying both of the gases. “Combining laser and detector brings many advantages,” says Gottfried Strasser. “It allows for the produc­tion of extremely compact sensors, and concei­vably, even an entire array – i.e. a cluster of microsensors – housed on a single chip and able to operate on several different wave­lengths simul­taneously.” The application possi­bilities are virtually endless, ranging from environ­mental techno­logy to medicine. (Source: TU Vienna)

Reference: Rolf Szedlak et al.: Remote Sensing with Commutable Monolithic Laser and Detector, ACS Phot. 3, 1794 (2016); DOI: 10.1021/acsphotonics.6b00603

Link: Center for Micro- and Nanostructures, TU Wien, Vienna, Austria

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