Chemical sensor on a chip

Infrared laser beams are ideal for analysing liquids and gases, and scientists in Vienna have miniaturised the technique to develop a new type of sensor. The method sees a quantum cascade laser and detector housed on one single chip in such a way that the wavelength of the laser perfectly matches the wavelength to which the detector is sensitive. The gap between the laser and detector is only 50 micrometers and bridged by a plasmonic waveguide made of gold and silicon nitride. This approach allows for simple and cheap production of tiny sensors for many different applications. Quantum cascade lasers are made of a perfectly optimised layer system of different materials, allowing properties such as the wavelength of the laser to be tuned. When a voltage is applies to the layer structure, the laser emits light, but it can also work the other way around – when irradiated by light, an electric signal is created. The bifunctional material was created atomic layer by atomic layer at the Centre for Micro- and Nanostructure at the Vienna University of Technology. “As both parts are created in one step, laser and detector do not have to be adjusted,” said Benedikt Schwarz. In a conventional system, the laser light has to be transmitted to the detector using carefully placed lenses, but the new system, the optical connection works completely differently – a plasmonic waveguide made of gold and silver. “The light interacts with the electrons in the metal in a very special way, so that the light is guided outside the gold surface,” said Schwarz. “That is why the light can be absorbed by the molecule on its way between laser and detector.” By immersing the sensor chip in liquid and measuring the decrease in detected light intensity due to the presence of the light absorbing molecules, the composition of the liquid can be determined. The sensor was tested with both water and alcohol, with the water concentration measured with an accuracy of 0.06%. The sensor can be applied to a wide variety of molecules such as carbohydrates or proteins by changing the design of the layered structure, which influenced the wavelength of light. The work has been published in Nature Communications. Monolithically integrated mid-infrared lab-on-a-chip using plasmonics and quantum cascade structures