Abstract-Terahertz time-domain spectroscopy of zone-folded acoustic phonons in 4H and 6H silicon carbide

Abebe T. Tarekegne, Binbin Zhou, Korbinian Kaltenecker, Krzysztof Iwaszczuk, Stewart Clark, and Peter Uhd Jepsen

Fig. 1 Experimental setup. BS – R/T 80/20 beamsplitter; SHG – second harmonic generation crystal; HWP – dual wavelength half-wave plate; Si – silicon plate; BD – beam dump; BPF – 400-nm band pass filter; APD – avalanche photodiode; L – focusing lenses. The setup is driven by 35-fs, 800 nm, 1.5 mJ laser pulses at 1 kHz repetition rate.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-3-3618

We investigate the dielectric properties of the 4H and 6H polytypes of silicon carbide in the 0.1-19 THz range, below the fundamental transverse-optical phonons. Folding of the Brillouin zone due to the specific superlattice structure of the two polytypes leads to activation of acoustic phonon modes. We use a combination of ultrabroadband terahertz time-domain spectroscopy and simulations based on density-functional perturbation theory to observe and characterize these modes, including band splitting due to the dissimilar carbon and silicon sublattices of the structures, and an indirect measurement of the anisotropic sound velocities in the two polytypes.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz near-field imaging of dielectric resonators

Wendy S. L. Lee, Korbinian Kaltenecker, Shruti Nirantar, Withawat Withayachumnankul, Markus Walther, Madhu Bhaskaran, Bernd M. Fischer, Sharath Sriram, and Christophe Fumeaux

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-4-3756

As an alternative to metallic resonators, dielectric resonators can increase radiation efficiencies of metasurfaces at terahertz frequencies. Such subwavelength resonators made from low-loss dielectric materials operate on the basis of oscillating displacement currents. For full control of electromagnetic waves, it is essential that dielectric resonators operate around their resonant modes. Thus, understanding the nature of these resonances is crucial towards design implementation. To this end, an array of silicon resonators on a quartz substrate is designed to operate in transmission at terahertz frequencies. The resonator dimensions are tailored to observe their low-order modes of resonance at 0.58 THz and 0.61 THz respectively. We employ a terahertz near-field imaging technique to measure the complex near-fields of this dielectric resonator array. This unique method allows direct experimental observation of the first two fundamental resonances.

© 2017 Optical Society of America

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Abstract-Terahertz Localized Surface Plasmon Resonances in Coaxial Microcavities

http://onlinelibrary.wiley.com/doi/10.1002/adom.201300021/abstract

1. Withawat Withayachumnankul^1,2,*, 
2. Charan Manish Shah², 
3. Christophe Fumeaux¹,
4. Korbinian Kaltenecker^3,4, 
5. Markus Walther³, 
6. Bernd M. Fischer^1,4, 
7. Derek Abbott¹,
8. Madhu Bhaskaran², 
9. Sharath Sriram^2,*

Coaxial microcavities etched into the surface of a doped silicon substrate are shown to support localized surface plasmon resonances at terahertz frequencies. The underlying mechanism involves coupling freely propagating terahertz waves with surface plasmon polaritons (SPPs), which propagate in a coaxial mode along the cavity walls in the axial direction. A Fabry–Pérot resonance is built up when the SPP wavenumber appropriately relates to the cavity depth. Owing to the Ohmic loss of the silicon at terahertz frequencies, the energy of the resonating SPPs is largely dissipated, leading to a modified reflection spectrum. Strong field enhancement is observed inside the cavities at resonance. The theoretical analysis is supported by numerical and experimental results. This study is a promising pathway for development of terahertz devices with applications in the areas of photonic integrated circuits, molecular sensing, and subwavelength imaging.