Thermal Radiation from Single Sub-λ Resonators and Devices

The thermal radiation scanning tunneling microscope (TRSTM) uses a scattering-tip modulated in a direction normal to the surface of a heated sample to measure its near-field thermal radiation [1]. This is achieved by lock-in demodulation of the infrared detected signal, which filters out the far-field radiation. In past experiments, it was shown that the method allows one to measure images and spectra of the near-field thermal radiation [1,2]. The latter shows strong deviations from black-body radiation for polaritonic materials [2].<br/><br/>Inspired by the TRSTM, we have recently developed a new spectroscopy method called infrared spatial modulation spectroscopy (IRSMS) to measure the far-field thermal radiation spectrum of heated sub-wavelength sized objects. As it is a far-field spectroscopy method, it does not require any scattering probe at the sample surface, unlike TRSTM. The principle of the IRSMS is to use a lateral modulation of the sub-λ sized object on the heated substrate in the field of view of an infrared microscope combined with a Fourier Transform Infrared Spectrometer. Due to the light fall-off effect on the detector, lock-in demodulation allows one to filter out the overwhelming background thermal radiation from the heated substrate and thus to extract the spectrum of the sub-λ sized object.<br/><br/>We have used IRSMS first to investigate the thermal radiation from single sub-λ sized metallic or dielectric resonators. Metal-insulator-metal antennas exhibit a narrowband thermal emission associated to the confinement of gap plasmons [3], with the appearance of hybrid modes when antennas are in near-field interactions [4]. Dielectric microspheres are also attractive to tailor the thermal radiation spectrum. For single SiO₂ microspheres, we evidence the excitation of both surface phonon-polariton (SPhP) modes and geometrical electric and magnetic Mie modes. The transition from a phonon-mode-dominated to a Mie-mode-dominated emission spectrum is observed when considering different sphere diameters [5].<br/><br/>Next, we have used modulation spectroscopy to investigate the radiation produced by high mobility graphene-hBN field effect transistors. The infrared radiation from these heterostructures under bias exhibits features which are typical for electroluminescence [6]. It starts at the threshold bias of Zener-Klein tunneling, an effect initially demonstrated in graphene with hyperbolic substrate in noise thermometry measurements [7]. It involves a specific cooling mechanism of the electron gas with radiative emission, due to the electron-hole recombination, in electromagnetic near-field modes of the hBN-graphene heterostructure.<br/><br/>[1] De Wilde et al., Nature <b>444</b>, 740 (2006).<br/>[2] Babuty et al., <i>Phys. Rev. Lett. </i><b>110</b>, 146103 (2013).<br/>[3] C. Li, et al., Phys. Rev. Lett. <b>121</b>, 243901 (2018).<br/>[4] L. Abou-Hamdan, et al., Opt. Lett. <b>46</b>, 981 (2021).<br/>[5] L. Abou-Hamdan, et al., ACS Photonics <b>9</b>, 2295 (2022).<br/>[6] W. Yang et al., Nature Nanotech. <b>13</b>, 47 (2018).<br/>[7] L. Abou-Hamdan, A. Schmitt et al., in preparation (2023).