Our research activities mainly concern the investigation of infrared radiation at subwavelength scales. The radiation originates either from thermal fluctuations with the sample, or from electrical pumping of a semiconductor device, or from an external source. We also investigate thermal transport phenomena at small scales.
Our research activities are often interdisciplinary and go from fundamental to applied research. Our results mostly arise from important collaborative efforts with partners who may originate from the academic, the aerospace, or the industrial sector.
Applications to internships, PhD and positions are more than welcome (see “Jobs” section). For more information, please contact the group leader, Dr.Yannick De Wilde.
email: 
Research topics
Thermal radiation at subwavelength scales
Part of our activities deal with the investigation of thermal radiation at distances from the sample which are much smaller than the thermal wavelength given by Wien’s law (10 µm at room temperature). In this regime, novel and outstanding physical phenomena occur. Being linked with evanescent fields, they are unobservable at large distance from the sample. We have developed a scanning probe microscope which has no equivalent, called thermal radiation scanning tunneling microscope, or TRSTM. It allows to measure the thermal radiation at nanometer distances from the sample which constitute a thermal source around room temperature. With the instrument, we have measured for the first time superresolved images of the infrared near-field thermal radiation, which represent the spatial variations of the electromagnetic local density of states (EM-LDOS), and we have demonstrated the existence of confined modes of thermally excited surface plasmon polaritons [1]. Recently, we have combined the TRSTM with a Fourier transform infrared spectrometer (FTIR) in order to be able to measure the frequency dependence of the near-field thermal radiation, and thus of the EM-LDOS as well. We have measured quasi-monochromatic near-field thermal radiation spectra on silicon carbide (SiC). This strongly non-Planckian behavior results from the contribution of surface phonon polaritons which produce a peak in the EM-LDOS in the near-field zone of a SiC/air flat interface [2, 3]. This effect is responsible of an anomalous increase of heat transfers at short distances.
The TRSTM combined with the FTIR operates beyond the diffraction limit. Being capable to detect the evanescent fields associated to high spatial frequencies with the TRSTM, we have demonstrated an improvement of the spatial resolution by a factor 100 with respect to classical far-field instruments such as infrared microscopes and spectrometers. Importantly, the TRSTM/FTIR also does not require any external source, since the thermal fluctuations of the sample are responsible of the signal which is detected. We have first demonstrated the high resolution capabilities of our hybrid instrument by measuring spectral images at a Silicon/gold (SiC/Au) interface [3,4]. Note that the scattering scanning probe has also allowed one to achieve a resolution of 100 nm in the infrared or the terahertz, by coupling it with the infrared beamline of the synchrotron SOLEIL. This demonstrates the high potential of the scattering-tip method to improve the spatial resolution in this type of large instrument [4].
Near-field thermal radiation from doped/undoped semiconductor multilayers
We have recently also studied with our hybrid TRSTM/FTIR instrument the cleaved facet of a multilayered stack in which doped and undoped semiconductor layers alternate, each layer having a thickness of approximately 300 nm. In some regions of the infrared spectrum, this stack behaves as a hyperbolic metamaterial. We have recorded TRSTM images in the infrared, while scanning the probe at different heights H above the stack from 200 nm to contact. While 200 nm is much smaller than the thermal wavelengths, we have observed that at that height the stack behaves as a homogeneous material as far as its thermal emission is concerned. It is only below 100 nm that each layer contributes individually to the near-field thermal radiation. The recording of spectral images with the TRSTM/FTIR has demonstrated a maximum of near-field thermal radiation around 1000 cm-1 due to the contribution to the EM-LDOS of surface plasmon polaritons which propagate at the interface between the doped and undoped semiconductor layers [5].
Near-field infrared imaging one active plasmonic devices
Another part of our research concerns active plasmonics. Using infrared near-field microscopy, we had formerly evidenced the generation of surface plasmon polaritons on the metallic electrode of a mid-infrared quantum cascade laser at a wavelength of 7.5 µm [6]. We have continued these investigations at shorter wavelengths by developing a new near-field microscopy setup. With this instrument, we have demonstrated the generation and control of surface plasmon polaritons at the surface of the nanostructured metal electrode of an integrated device based on a telecom laser diode at 1.3 µm. We have also been able to measure superresolved images on the facet of semiconductor laser devices on which a distributed cavity is produced by means of a metallic grating, or on the facet of lasers with a continuous metal coating on the top of the cavity. This has allowed to directly observe the coupling between the laser cavity modes and plasmonic modes [8].
Research connected to aerospace and industrial laboratories
The competencies which we have acquired at the Langevin Institute in the field of thermics at small scales and thermal radiation have triggered the interest of Industrial laboratories. Since recently we collaborate with the industrial group Saint-Gobain on thermal insulation materials, and the aerospace agency ONERA on plasmonic antennas means to improve the performances of infrared imaging systems. The image on the left shows a 3D tomographic image of an insulation material made of fibers which we investigate with methods inspired from the field of nanoscience and nanotechnology. The image has been recorded by optical coherent tomography (OCT) microscopy.
REFERENCES (full list here )
[1] Thermal Radiation Scanning Tunnelling Microscopy
Y. De Wilde, F. Formanek, R. Carminati, B. Gralak, P.-A. Lemoine, J.-P. Mulet, K. Joulain, Y. Chen, J.-J. Greffet,
NATURE 444, 740 (2006).
[2] Blackbody spectrum revisited in the near-field
A. Babuty, K. Joulain, P.-O. Chapuis, J.-J. Greffet, Y. De Wilde
PHYSICAL REVIEW LETTERS, 110, 146103 (2013).
[3] Electromagnetic Density of States in Complex Plasmonic Systems
R. Carminati, A. Cazé, D. Cao, F. Péragut, V. Krachmalnicoff, R. Pierrat, Y. De Wilde
SURFACE SCIENCE REPORTS, 70, 1 - 41 (2015).
[4] Infrared near-field imaging and spectroscopy based on thermal or synchrotron radiation
F. Peragut, J.-B. Brubach, P. Roy, Y. De Wilde
APPLIED PHYSICS LETTERS, 104, 251118 (2014).
[5] Hyperbolic metamaterials and surface plasmon polaritons
F. Peragut, L. Cerruti, A. Baranov,J.P. Hugonin, T. Taliercio, Y. De Wilde, J.J. Greffet
OPTICA, 4, 1409-1415 (2017).
[6] Semiconductor Surface Plasmon Sources
A. Babuty, A. Bousseksou, J.−P. Tetienne, I. Moldovan Doyen, C. Sirtori, G. Beaudoin, I. Sagnes, Y. De Wilde, R. Colombelli,
PHYSICAL REVIEW LETTERS, 104, 226806 (2010).
[7] In Situ Generation of Surface Plasmon Polaritons Using a Near-Infrared Laser Diode,
D. Costantini, L. Greusard, A. Bousseksou, R. Rungsawang, T. P. Zhang, S. Callard, J. Decobert, F. Lelarge, G.-H. Duan, Y. De Wilde, R. Colombelli,
NANO LETTERS, ISSN: 1530-6984, v. 12, 4693–4697 (2012).
[8] Near-field analysis of metallic DFB lasers at telecom wavelengths,
L. Greusard, D. Costantini, A. Bousseksou, J. Decobert, F. Lelarge, G.-H. Duan, Y. De Wilde, R. Colombelli
OPTICS EXPRESS, ISSN: 1094-4087, v. 21, 10422-10429 (2013).
