This paper presents the method of fabricating a frequency-selective surface (FSS) filter for terahertz frequency by short- and ultrashort laser ablation process. The FSS consists of a capacitive screen made up of the copper metallic structure supported by a Teflon dielectric substrate. The dielectric properties of the Teflon substrate at terahertz frequencies were evaluated using THz-time-domain spectroscopy technique. The numerical simulation of the designed structure was performed using Computer Simulation Technology microwave studio software with unit cell configurations and the resonance was observed at 0.135 THz. The desired metallic microstructure was created in copper using an 8-ns solid-state Nd: YAG and 150 fs Ti:sapphire laser operating at the second harmonic wavelength of 532 and 800 nm, respectively. The proposed structure provides polarization-independent operation. The experimental verification of the fabricated FSS using femtosecond ablation process was done using THz-TDS technique, and it is observed that the measured data well match with the simulated result.
Dipole induced quadrupole resonance leads to transmission peak within the broad dipole absorption as shown in plasmonic metamaterials. We show quadrupolar interactions as a new paradigm to control this classical analogue of electromagnetically induced transparency (i.e. plasmon induced transparency, PIT) in metamaterials. While the asymmetry factor of the resultant Fano line shape of a EIT spectrum limits the quality factor (Q) of the resonance and thus the sensing application, we show that quadrupolar interactions give a handle on the Q factor. A Q as high as 600 is seen in simulations at about 0.977 THz. Limited by the experimental resolution, a Q of about 100 is observed. Further plasmonic structures can be designed to make use of the quadrupolar interactions for high sensitive devices at THz frequencies.
We review here our efforts to make high power THz sources. We have developed different plasmonic structural designs for the confinement of incident excitation infra-red (800nm, 10fs) laser pulse on SI-GaAs surface. The SI-GaAs surface is modified so that incident radiation is less reflected and more absorbed in the substrate. Fabricating THz antenna structures on it increases the efficiency of the THz source. We have demonstrated this idea in its simplest form by increasing the overall surface area by fabricating trenches on the GaAs surface in the past. The new designs are expected to increase the THz source emission by at least a factor 2 to 4. We have also fabricated quasi-crystal pattern on SI-GaAs substrate to enhance the incident light confinement and checked THz emission from it. The simulated plasmonic structures will be fabricated on the SI-GaAs substrates as well as C-irradiated GaAs substrates. We have shown that our in-house fabricated THz Sources from C-irradiated SI-GaAs showed ~2 orders power increase. The detectors fabricated from these materials showed replica of the incident THz wave compared to the one detected using ZnTe. We will also present use of the C-irradiated substrates in the generation of Continuous Wave (CW) THz sources. All the aforementioned sources have been compared with the commercially available sources made of LT-GaAs.