Laser THz emission nanoscopy and THz nanoscopy

Abstract

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 6th and the 10th harmonic 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.

Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures

“…Interlayer tunnelling should generate a time-dependent out-of-plane current of density j z (t) that produces coherent terahertz emission 20 , providing direct access to the ultrafast charge-transfer dynamics. This emission is scattered into the far field for electro-optic detection 38,43 (Fig. 3a).…”

“…Qualitatively, e-h pairs that are quantum confined into a single monolayer should screen external fields less efficiently than pairs delocalized over two monolayers. Such a difference in polarizability may leave a measurable fingerprint in the scattered terahertz field E scat , which we detect by electro-optic sampling 33,38,43 (see Methods).…”

“…Here, we use ultrafast scattering-type scanning near-field optical microscopy 29,32,33 (s-SNOM) at terahertz (THz) frequencies 27,[34][35][36][37][38][39][40][41][42][43] as a non-invasive, subcycle, and contact-free probe of charge-transfer dynamics in WSe 2 /WS 2 heterostructures on insulating substrates. Our approach builds on a critical change of the polarizability of e-h pairs during interlayer tunnelling, as explained by ab initio density functional theory and confirmed by terahertz emission directly linked to the ultrafast charge separation.…”

Review of THz-based semiconductor assurance

“…229 Recent advances in LTEM and s-SNOM were compared by imaging a lightly p-doped InAs sample, and we were able to record waveforms with detector signal components demodulated up to the 6th and the 10th harmonic of the tip oscillation frequency and measure a THz near-field confinement down to 11 nm. 230 A GaP solid immersion lens is attached to the back side of the LSI, to improve the spatial resolution. 223 The whole LTEM image size is 30 × 30 mm 2 , and a resolution estimated by the transition width from 20% to 80% of the amplitude is 360 nm as indicated in (d).…”

Quantitative terahertz emission nanoscopy with multiresonant near-field probes

“…Still, the approach lacks nanoscale spatial sensitivity. To overcome this challenge, TES has recently been merged with THz near-field microscopy [11][12][13][14][15][16][17][18] and termed terahertz emission nanoscopy (TEN) [11,14] paving the way for investigations of nanostructures. While the near-field probe can efficiently confine the emerging long-wavelength radiation to its sharp apex and subsequently scatter it to the far field, the metallic tip also alters the temporal structure of the detected waveforms.…”