A laser system: armed to comb through a speckled array of substances


Sunlight or the light from your bedside lamp (white light), contains a mixture of all colours (thus a mixture of wavelengths).


Laser light, on the other hand, contains only a small range of colours, sometimes even only one particular wavelength. Infrared light allows to detect chemical substances.


In a molecule - the smallest unit of a substance - the movements of atoms resonate in the infrared spectrum. Thus, it is important to have light sources and detectors to read those movements.



The laser system produces many frequencies with equal spacing between them. (Image Courtesy: Johannes Hillbrand, TU Wien. 2)


Dr. Hermann Detz contributed within a team of researchers at TU Wien and recently, at CEITEC BUT as well, to develop a mid-infrared laser and detector technology that can be integrated on a single chip (1).


More recently, these lasers were brought to a distinct state, called frequency comb (2).



What are frequency combs?


Unlike conventional lasers, a complex high-precision tool called frequency comb, is a unique form of a light beam with multiple frequencies, all equally spaced from each other, similar to the teeth on a comb.


Once the multiple frequency light is sent through a molecule, the molecule absorbs some specific frequency rendering its precise chemical identity.


Frequency combs open doors to applications such as in biochemical analysis, medical diagnoses or environmental monitoring – detecting e.g. toxic pollutants, with the list of possible applications growing fast!



The team at TU Wien: Benedikt Schwarz, Aaron Maxwell Andrews, Gottfried Strasser, Johannes Hillbrand, Hermann Detz (left to right)



Quantum cascade lasers: sprouting the frequency combs


The researchers have generated the frequency combs with quantum cascade lasers (QCLs), a unique kind of lasers based on periodic layer sequences and different semiconductor materials that can be designed for a wide range of mid-infrared wavelengths.


Hence, the designed QCL based device could measure compounds absorbing frequencies across the so-called fingerprint region of the electromagnetic spectrum, replacing bulky Fourier-transform infrared spectrometers or setups with multiple lasers by a single device.



Prior problems are solved: towards an efficient frequency comb


Dr. Detz mentioned, “Generating frequency combs is a complex task itself. Once the laser light is emitted, it can be reflected back to the laser-source by other optical parts, and destroy the frequency comb state.”


“We have solved this issue, by injecting an electrical radio-frequency signal into the comb device, which makes it insensitive to back-reflected light from neighboring comb lasers, which enables dual-comb spectroscopy setups.”


“The resulting oscillation due to the interaction of the comb wavelengths can then be detected again as a radiofrequency signal using classical electronics”, he adds.



Everything on a single chip


Typical spectroscopy setups around a Fourier-transform infrared spectrometer filled a whole optical table. “We got rid of the Fourier spectrometer.” Dr. Detz states.


“Now we have the laser and the detector, both made from the same material, efficiently housed on the same chip, which is at present already working at room temperature.”


This chip can be easily integrated into portable electronic devices at the size of your mobile phone!


Dr. Hermann Detz (Photography by Emil Gallík)



The sky is the limit: myriads of applications are awaiting


“The blood oxygen saturation sensor is already known, it would be really cool to do the same to detect blood sugar levels. Our research could change many lives in the future."


"One has to stay realistic though as this proves to be a challenging task with many labs world-wide working on it without a major break-through at this point. Nevertheless, this sensor concept can be applied to a much larger variety of use-cases.” states Detz.


Apart from that, the designed device could measure the levels of greenhouse gases in the exhaust of a factory or trace substances in security check-points at airports.


“In principle, the chemical bonds of any organic molecule are under the radar of this device. But, we would need help from chemists/biologists to tailor the sensor, for a particular substance. In the future, we need more groups to be working on this, hopefully as our potential partners.”



The future is mapped


“We are starting to collaborate with labs in the life sciences sphere. Going step-by-step, essential future collaborators will be research groups in hospitals to perform real-world tests using our sensors.”


Dr. Detz affirms passionately “We showed that our device is working, and we have degrees of freedom to fine-tune it. These sensors are continuously optimized within the collaboration between TU Wien and CEITEC."


"Recently, the same principle was shown for interband cascade lasers, which open shorter mid-infrared wavelengths (3). Now it is about teaming up with partners from other fields like life-sciences, to focus on real-world applications.”



References

  1. B. Schwarz, P. Reininger, D. Ristanić, H. Detz, A.M. Andrews, W. Schrenk, G. Strasser, “Monolithically integrated mid-infrared lab-on-a-chip using plasmonics and quantum cascade structures”, Nat. Commun., vol. 5, p. 4085, Jun. 2014.
  2. J. Hillbrand, A. M. Andrews, H. Detz, G. Strasser, B. Schwarz, "Coherent injection locking of quantum cascade laser frequency combs", Nat. Photon., vol. 13, pp. 101-104, Feb. 2019.
  3. B. Schwarz, J. Hillbrand, M. Beiser, A.M. Andrews, G. Strasser, H. Detz, A. Schade, R. Weih, S. Höfling,    “A monolithic frequency comb platform based on interband cascade lasers and detectors”, Optica, vol. 6, p. 890-895, Jul. 2019.



Written by Somsuvro Basu


Publication date: 01.07.2019