Interactions of Terahertz radiations with matter can lead to the realization of functional devices related to sensing, high-speed communications, non-destructive testing (NDT), 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 THz to 1 THz frequency range) because of the variation in electric field from 0 kV/cm to 600 kV/cm. Further, we have captured signatures of the hysteretic nature of polarization switching in BiFeO3 film through non-contact THz-TDS technique, similar trends are observed in SS-PFM (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 BiFeO3 film. 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.

Tunable slow light systems have gained much interests recently due to their efficient control of strong light-matter interactions as well as their huge potential for realizing tunable device applications. Here, a dynamically tunable polarization independent slow light system is experimentally demonstrated via electromagnetically induced transparency (EIT) in a terahertz (THz) metasurface constituted by plus and dimer-shaped resonators. Optical pump-power dependent THz transmissions through the metasurface samples are studied using the optical pump terahertz probe technique. Under various photoexcitations, the EIT spectra undergo significant modulations in terms of its resonance line shapes (amplitude and intensity contrast) leading to dynamic tailoring of the slow light characteristics. Group delay and delay bandwidth product (DBP) values are modulated from 0.915 ps to 0.42 ps and 0.059 to 0.025 as the pump fluence increases from 0 to 62.5 〖nJ cm〗^(-2). This results in tunable slow THz light with group velocities ranging from 2.18×〖10〗^5 〖m s〗^(-1) to 4.76×〖10〗^5 〖m s〗^(-1), almost 54% change in group velocity. The observed tuning is attributed to the photo-induced modifications of the optoelectronic properties of the substrate layer. The demonstrated slow light scheme can provide opportunities for realizing dynamically tunable slow light devices, delay lines, and other ultrafast devices for THz domain.

Resonance excitation of surface plasmons in sub-wavelength periodic apertures (popularly known as hole arrays) is typically decided by its lattice configurations and the constituent material characteristics. Therefore, the excitation frequency of surface plasmon resonances (SPRs) in hole arrays is not easy to alter without modifying these basic structural parameters. However, we experimentally demonstrate modulation of SPR frequency by carefully incorporating an additional hole of similar geometry. By suitably modifying the relative positions between the holes inside the unit cell (fixed lattice parameters), we have tailored the SPR excitation frequency. Predominantly, we attribute such frequency detuning to near-field Coulomb interactions in between the holes that can modify the effective permittivity of the hole arrays, hence SPR characteristics. In totality, our experiments demonstrate a 7.6% shift in the SPR frequency. Further, all the experimental findings are explained through elaborate electromagnetic simulations that helped to acquire deeper physical insights related to the SPR excitation. We believe such near-field effect-based resonance tuning can find potential applications in realizing SPR-based sensors, tunable filters, and tunable non-linear devices operating in the terahertz (THz) domain.

The integration of active materials in terahertz (THz) metasurfaces is pivotal for the realization of functional device applications in diverse fields like sensing, imaging, communication, etc. In this context, ferroelectric materials endowed with tunable electro-optic properties have recently emerged as a novel candidate for achieving actively tuned THz metasurfaces. Here, we experimentally investigate temperature tuning of electromagnetically induced transparency (EIT) effects in a THz metasurface based on ferroelectric barium titanate (BaTiO3) thin film. We characterize tunable dielectric properties of the BTO thin film under variable temperatures (25 0C to 100 0C) at THz frequencies by utilizing THz time domain spectroscopy (THz-TDS) technique. Based on this aspect, we intelligently design a THz metasurface capable of displaying the EIT effect. THz transmissions through the metasurface sample are then probed for different applied temperatures. The EIT features undergo frequency shifts along with amplitude modulations owing to the temperature induced variations of the dielectric properties of the BTO thin film. A total red shift ~ 27 GHz in EIT resonance dip is observed experimentally as the temperature increases from 25oC to 100oC. Therefore, we demonstrate utilities of ferroelectric platform towards the development of temperature tunable EIT metasurfaces.

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