Terahertz sensing of reduced graphene oxide nanosheets using sub-wavelength dipole cavities.

Because of extraordinary optoelectronic properties, two-dimensional (2D) materials are the subject of intense study in recent times. Hence, we investigate sub-wavelength dipole cavities (hole array) as a sensing platform for the detection of 2D reduced graphene oxide (r-GO) using terahertz time-domain spectroscopy (THz-TDS). The r-GO is obtained by reducing graphene oxide (GO) via Hummer's method. Its structural characteristics are verified using X-ray diffraction (XRD) and Raman spectroscopy. We also assessed the morphology and chemistry of r-GO nanosheets by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAX), and Fourier Transformed Infrared (FTIR) spectroscopy. Further, we studied the surface plasmon resonance (SPR) characteristics of r-GO nanosheets hybridized dipole cavities using THz-TDS by varying the r-GO thickness on top of the dipole cavities, since these cavities are well known for sustaining strong SPRs. Based on these, we experimentally obtained a sensitivity of 12 GHz/µm for the porous r-GO film. Thus, a modification in SPR characteristics can be employed towards the identification and quantification of r-GO by suitably embedding it on an array of dipole cavities. Moreover, we have adopted a generic approach that can be expanded to sense other 2D materials like Boron Nitride (BN), phosphorene, MoS2, etc., leading to the development of novel THz nanophotonic sensing devices.

New insights into APCVD grown monolayer MoS2 using time-domain terahertz spectroscopy.

In modern era, wireless communications at ultrafast speed are need of the hour and search for its solution through cutting edge sciences is a new perspective. To address this issue, the data rates in order of terabits per second (TBPS) could be a key step for the realization of emerging sixth generation (6G) networks utilizing terahertz (THz) frequency regime. In this context, new class of transition metal dichalcogenides (TMDs) have been introduced as potential candidates for future generation wireless THz technology. Herein, a strategy has been adopted to synthesize high-quality monolayer of molybdenum di-sulfide (MoS2) using indigenously developed atmospheric pressure chemical vapor deposition (APCVD) set-up. Further, the time-domain transmission and sheet conductivity were studied as well as a plausible mechanism of terahertz response for monolayer MoS2 has been proposed and compared with bulk MoS2. Hence, the obtained results set a stepping stone to employ the monolayer MoS2 as potential quantum materials benefitting the next generation terahertz communication devices.

External bias dependent dynamic terahertz propagation through BiFeO3film.

Interactions of terahertz radiations with matter can lead to the realization of functional devices related to sensing, high-speed communications, non-destructive testing, spectroscopy, etc In spite of the versatile applications that THz can offer, progress in this field is still suffering due to the dearth of suitable responsive materials. In this context, we have experimentally investigated emerging multiferroic BiFeO3 film (∼200 nm) employing terahertz time-domain spectroscopy (THz-TDS) under vertically applied (THz propagation in the same direction) electric fields. Our experiments reveal dynamic modulation of THz amplitude (up to about 7% within 0.2-1 THz frequency range) because of the variation in electric field from 0 to 600 kV cm-1. Further, we have captured signatures of the hysteretic nature of polarization switching in BiFeO3film through non-contact THz-TDS technique, similar trends are observed in switching spectroscopy piezoresponse force microscope measurements. We postulate the modulation of THz transmissions to the alignment/switching of ferroelectric polarization domains (under applied electric fields) leading to the reduced THz scattering losses (hence, reduced refractive index) experienced in the BiFeO3film. This work indicates ample opportunities in integrating nanoscale multiferroic material systems with THz photonics in order to incorporate dynamic functionalities to realize futuristic THz devices.

Hybrid resonant cavities: A route towards phase engineered THz metasurfaces.

Coupled resonant cavities can enable strong photon energy confinement to facilitate the miniaturization of functional photonic devices for applications in designs of sensors, modulators, couplers, waveguides, color filters etc. Typically, the resonances in subwavelength plasmonic cavities rely on the excitation of surface plasmons at specific phase-matching conditions, usually determined by the lattice parameters and constituent material properties. Contrary to this notion, we experimentally demonstrate the control and manipulation of cavity resonances via suitably modifying the split ring resonator geometry in hybrid plasmonic-metasurface (dipole cavity-SRR) configuration without altering the lattice parameters. This results to the excitation of dual resonance peaks. Such dual channel characteristics demonstrate high quality (Q) factor, multi-band resonances, not permissible with typical (unhybridized) plasmonic dipole cavities. We envisage such hybrid meta-cavity designs can become important ingredients for futuristic terahertz devices that can hold the key for sixth generation (6G) communications, designer filters, dual channel sensors etc.