Manjakavahoaka Razanoelina

Probing low-density carriers in a single atomic layer using terahertz parallel-plate waveguides

As novel classes of two-dimensional (2D) materials and heterostructures continue to emerge at an increasing pace, methods are being sought for elucidating their electronic properties rapidly, non-destructively, and sensitively. Terahertz (THz) time-domain spectroscopy is a well-established method for characterizing charge carriers in a contactless fashion, but its sensitivity is limited, making it a challenge to study atomically thin materials, which often have low conductivities. Here, we employ THz parallel-plate waveguides to study monolayer graphene with low carrier densities. We demonstrate that a carrier density of ~2 × 10(11) cm(-2), which induces less than 1% absorption in conventional THz transmission spectroscopy, exhibits ~30% absorption in our waveguide geometry. The amount of absorption exponentially increases with both the sheet conductivity and the waveguide length. Therefore, the minimum detectable conductivity of this method sensitively increases by simply increasing the length of the waveguide along which the THz wave propagates. In turn, enabling the detection of low-conductivity carriers in a straightforward, macroscopic configuration that is compatible with any standard time-domain THz spectroscopy setup. These results are promising for further studies of charge carriers in a diverse range of emerging 2D materials.

Magnetic resonance of terahertz metamaterials in parallel plate waveguides

As new designs of metamaterials rapidly emerge, methods of characterizing their fundamental electromagnetic properties become increasingly important. Here, we utilize the parallel plate waveguide associated with terahertz time-domain spectroscopy experiments to analyze the coupling of terahertz radiation to ultrathin electric split-ring resonators located halfway between the waveguide plates. Our observations determine that the magnetic response dominates across the frequency range of the system. The experimental results are confirmed by simulations, emphasizing the usefulness of the proposed approach for further investigations of magnetic coupling in metamaterials in the terahertz regime.

Measurable lower limit of thin film conductivity with parallel plate waveguide terahertz time domain spectroscopy

Parallel plate waveguide (PPWG) terahertz (THz) time domain spectroscopy (TDS) is a powerful tool to investigate the properties of thin and low conductive materials. In this Letter, we determine the lower limit of detection of the PPWG-THz-TDS approach. We provide a closed-form expression of the minimal measurable conductivity by the system. The experimental results of amorphous YBa2Cu3O7-δ films indicate that the factor limiting the spectroscopic modality is the waveguide device misalignment. On the other hand, the expression of the minimal detectable conductivity provides a clear scheme of optimization by increasing the waveguide length and therefore enhancing the sensitivity of the system.

Parallel plate waveguide time domain spectroscopy to study terahertz conductivity of utltrathin materials

The newly discovered atomically thin and layered materials which host electronic system that respond to longwavelength light in extraordinary manner can lead to a major breakthrough in the field of terahertz (THz) optics and photonics. However, their low conductivities due to either low densities or low mobility make it challenging to characterize their basic THz properties with the standard spectroscopic method. Here, we develop a THz spectroscopic technique based on parallel plate waveguide (PPWG) to overcome the limitations of the conventional THz time domain spectroscopy (TDS) technique. The present method is particularly suitable to ultrathin conductive materials with low carrier density. We report in details the derivation of the dispersion equations of the terahertz wave propagation in a PPWG loaded by a thin conductive materials with zero-thickness. These dispersion equations for transverse magnetic (TM) and transverse electric (TE) waveguide modes are the core of the optical parameters extraction algorithm in the THz-PPWG-TDS analysis. We demonstrate the effectiveness of the waveguide approach by characterizing low conductive CVD graphene. The high sensitivity of THz-PPWG-TDS technique enables us to study the carrier dynamics in graphene with Drude and Drude-Smith model.