Thermal and blackbody radiations are tightly entangled with the material’s quantum properties for any finite temperature above 0 K. Such radiation is incoherent, both spatially and temporally. However, this characteristic change abruptly in the optical near-field region where it is related to the local density of photonic states (LDOS) [1]. Near-field thermal emission experiences huge amplification close to the epsilon-near-zero (ENZ) spectral region, i.e. at those specific LDOS phonon resonances where the material’s complex dielectric permittivity ε approaches zero. It may lead to narrow-band emission, directionality and coherence properties. Here, we investigate the near-field of anisotropic two-dimensional ENZ materials (Hexagonal boron nitride and α-Molybdenum Trioxide) with Synchrotron Infrared Nano-Spectroscopy and s-SNOM imaging using quantum cascade lasers. Theoretical and numerical investigations confirm the observed enhanced oscillating behaviour of the LDOS around the ENZ frequency. Tuning of the emission properties of 2D ENZ material via external control of the temperature is also demonstrated using a setup for nano-imaging at low temperature.
KEYWORDS: Near field, Microscopy, Optical imaging, Mid-IR, Switching, Near field scanning optical microscopy, Crystals, Terahertz radiation, Switches, Super resolution microscopy
This talk advertises scattering-type scanning near-field infrared micro-spectroscopy (s-SNIM) in the spectral range of 75 to 1.3 THz [1], as provided by the free-electron laser FELBE, the narrow-band laser-light source at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany. We demonstrate the -independent s-SNIM resolution of a few 10 nm by exploring structured Au samples, Graphene-transistors, meta-materials [2], and local ferroelectric phase-transitions down to LHe [3]. s-SNIM secondly was integrated into a THz pump-probe experiment for the inspection of excited states in structured SiGe samples. We developed a novel demodulation technique with high temporal resolution [4] hence achieving an excellent Signal-to-Noise Ratio. Thirdly using the super-radiant TELBE light source [5], HZDR recently extended the wavelength range down to 100 GHz radiation. We adapted our s-SNIM to this TELBE photon-source as well, achieving an equally high spatial resolution as with FELBE. Moreover, the superb 30-fs temporal resolution of TELBE will allow us to study a multitude of physical phenomena with sub-cycle resolution [5,6], such as spin-structures, magnons and phonon polaritons.
[1] F. Kuschewski et al., Appl. Phys. Lett. 108 (2016) 113102.
[2] S.C. Kehr et al., ACS Photonics 3 (2016) 20.
[3] J. Döring et al., Appl. Phys. Lett. 105 (2014) 053109.
[4] F. Kuschewski et al., Sci. Rep. 5 (2015) 12582.
[5] B. Green et al., Sci. Rep. 6 (2016) 22256.
[6] S. Kovalev et al., Struct. Dyn. 4 (2017) 024301.
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