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We present an experimental and theoretical comparison of two different scattering-type scanning near-field optical microscopy (s-SNOM) based techniques in the terahertz regime; nanoscale reflection-type terahertz time-domain spectroscopy (THz nanoscopy) and nanoscale laser terahertz emission microscopy, or laser terahertz emission nanoscopy (LTEN). We show that complementary information regarding a material’s charge carriers can be gained from these techniques when employed back-to-back. For the specific case of THz nanoscopy and LTEN imaging performed on a lightly p-doped InAs sample, we were able to record waveforms with detector signal components demodulated up to the 6thand the 10thharmonic of the tip oscillation frequency, and measure a THz near-field confinement down to 11 nm. A computational approach for determining the spatial confinement of the enhanced electric field in the near-field region of the conductive probe is presented, which manifests an effective “tip sharpening” in the case of nanoscale LTEN due to the alternative geometry and optical nonlinearity of the THz generation mechanism. Finally, we demonstrate the utility of the finite dipole model (FDM) in predicting the broadband scattered THz electric field, and present the first use of this model for predicting a near-field response from LTEN.
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Anomalous contrast in broadband THz near-field imaging of gold microstructures
THz scattering-type scanning near-field microscopy (s-SNOM) has become a powerful technique for measuring carrier dynamics in nanoscale materials and structures. Changes in a material’s local THz reflection or transmission can be correlated to changes in electrical conductivity. Here, we perform tip-based THz nano-imaging of subwavelength gold nanostructures and demonstrate image contrast unrelated to any spatially varying material properties. We show that the specific physical configuration of the gold structures can have a strong influence on local excitations which can obscure the sample’s true dielectric response, even in cases where the relevant structures are far outside of the spatial region probed by the AFM tip.
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- Award ID(s):
- 1904280
- PAR ID:
- 10225770
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optics Express
- Volume:
- 29
- Issue:
- 10
- ISSN:
- 1094-4087; OPEXFF
- Format(s):
- Medium: X Size: Article No. 15190
- Size(s):
- Article No. 15190
- Sponsoring Org:
- National Science Foundation
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