The bonds in molecules will rotate or vibrate at frequencies which correspond to mid-infrared wavelengths (roughly 2 µm to 20 µm). Therefore these molecules absorb light and vibrate at these frequencies which provides a physical mechanism to identify the molecules by identifying which bonds between different atoms are in the molecule. The wavelength region between 6.7 µm and 20 µm is called the fingerprint region as most of the absorption lines are unique for specific molecules allowing unique identification.

Mid-infrared sensors have a wide range of applications which includes diagnosis of certain cancers from breath analysis, environmental monitoring and the detection and identification of explosives, chemical weapons and biological agents.

We have been working in mid- and far-infrared wavelengths for over 25 years and have two Bruker Fourier Transform Infrared (FTIR) spectrometers to allow the characterisation of molecules, materials, devices and sensors. FTIR is the gold standard for mid-infrared spectroscopic identification of many molecules and the only technique allowed in mid-infrared forensic evidence in UK courts.

We have demonstrated a wide range of mid-infrared photonic components including photodetectors, microbolometer detectors, low loss waveguides (< 1 dB/cm at 10 µm wavelength), plasmonic amplifiers, polarisaton rotation and non-linear wavelength conversion.

Our main vision is to integrate these photonic components together to build integrated sensors for healthcare, environmental monitoring and security applications.

1. "Ge-on-Si waveguides for sensing in the molecular fingerprint regime" Optics Express 28(4), pp. 5749 - 5757 (2020)- DOI: https://doi.org/10.1364/OE.382356
   U. Griškevičiūtė, R.W. Millar, K. Gallacher, J. Valente and D.J. Paul


2. "Ultra-Broadband Mid-infrared Ge-on-Si Waveguide Polarization Rotator" APL Photonics 5(2), 026102 (2020) - DOI: 10.1063/1.5134973
   K. Gallacher, R.W. Millar, U. Griškevičiūtė, M. Sinclair, M.Sorel, L. Baldassarre, M. Ortolani, R. Soref, and D.J. Paul


3. "Characterization of integrated waveguides by atomic-force-microscopy-assisted mid-infrared imaging and spectroscopy" Optics Express 28(15), pp. 22186 - 22199 (2020) - DOI: 10.1364/OE.393748
   K Gallacher, R.W. Millar, D.J. Paul, J. Frigerio, A. Ballabio, G. Isella, F. Rusconi, P. Biagioni, V. Giliberti, A. Sorgi, L. Baldassarre and M. Ortolani


4. "Plasmonic mid-infrared third harmonic generation in germanium nanoantennas" Light Science and Applications 7, 106 (2018) - DOI: 10.1038/s41377-018-0108-8
   M.P. Fischer, A. Riede, K. Gallacher, J. Frigerio, G. Pellegrini, M. Ortolani, D.J. Paul, G. Isella, A. Leitenstorfer, P. Biagioni and D. Brida


5. "Low loss Ge-on-Si waveguides operating in the 8 to 14 μm atmospheric transmission window" Optics Express 26(20) pp. 25667 - 25675 (2018) - DOI: 10.1364/OE.26.025667
   K. Gallacher, R.W. Millar, U. Griškevičiūtė, L. Baldassarre, M. Sorel, M. Ortolani and D.J. Paul


6. "Benchmarking the Use of Heavily Doped Ge for Plasmonics and Sensing in the Mid-Infrared" ACS Photonics 5(9) pp. 3601 - 3607 (2018) - DOI: 10.1021/acsphotonics.8b00438
   G. Pellegrini, L. Baldassare, V. Giliberti, J. Frigerio, K. Gallacher, D.J. Paul, G. Isella, M. Ortolani and P. Biagioni


7. "Mid-infrared light emission > 3 μm wavelength from tensile strained GeSn microdisks" Optics Express 25(21), pp. 25374 - 25385 (2017) - DOI: 10.1364/OE.25.025374
   R.W. Millar, D.C.S. Dumas, K.F. Gallacher, P. Jahandar, C. MacGregor, M. Myronov and D.J. Paul


8. "Optical Activation of Germanium Plasmonic Nanoantennas in the Mid Infrared" Physical Review Letters 117, 047401 (2016) - DOI: 10.1103/PhysRevLett.117.047401
   M.P. Fischer, C. Schmidt, E. Sakat, J. Stock, A. Samarelli, J. Frigerio, M. Ortolani, D.J. Paul, G. Isella, A. Leitenstorfer, P. Biagioni, and D. Brida


9. "Tunability of the dielectric function of heavily doped germanium thin films for mid-infrared plasmonics" Physical Review B 94, 085202 (2016) - DOI: 10.1103/PhysRevB.94.085202
   J. Frigerio, A. Ballabio, G. Isella, E. Sakat, P. Biagioni, M. Bollani, E. Napolitani, C. Manganelli, M. Virgilio, A. Grupp, M.P. Fischer, D. Brida, K. Gallacher, D.J. Paul, L. Baldassarre, P. Calvani, V. Giliberti, A. Nucara and M. Ortolani


10. "Mid-infrared intersubband absorption from p-Ge quantum wells grown on Si substrates" Applied Physics Letters 108, 091114 (2016) - DOI: 10.1063/1.4943145
    K. Gallacher, A. Ballabio, R. W. Millar, J. Frigerio, A. Bashir, I. Maclaren, G. Isella, M. Ortolani, and D.J. Paul


11. "8-band k.p modelling of mid-infrared intersubband absorption in Ge quantum wells" Journal of Applied Physics 120, 043103 (2016) - DOI: 10.1063/1.4959259
    D.J. Paul


12. "Mid-infrared plasmon-enhanced spectroscopy with germanium antennas on silicon substrates" Nano Letters 15(11), pp. 7225 - 7231 (2015) - DOI: 10.1021/acs.nanolett.5b03247
    L. Baldassarre, E. Sakat, J. Frigerio, A. Samarelli, K. Gallacher, E. Calandrini, G. Isella, D.J. Paul, M. Ortolani, P. Biagioni