I received my Ph.D. in 2017 from S. N. Bose National Center for Basic Sciences, Kolkata (Registration under the University of Calcutta) under the guidance of Prof. Rajib Kumar Mitra and Prof. Anjan Barman in "Manipulating Terahertz Radiation Using Nanostructures". I prepared various types of. nanomaterials (nanoparticles and nanowires) using electrochemical methods and studied their application in the THz frequency range.
THz polarizer
We report the polarizing behavior of aligned Ni nanoparticles (NPs) having an average diameter of 165±15 nm in ∼210 μm thick polyvinyl alcohol (PVA) matrix in the frequency range of 0.2–1.6 THz. The NPs have been prepared via a wet chemical route and then aligned in PVA film by using an external magnetic field. When the polarization of the THz electric field is parallel to the NP's alignment direction, a strong THz absorption is observed whereas a minimum THz absorption is noticed for the corresponding perpendicular configuration. The degree of polarization is calculated to be 0.9±0.08. Considering the good polarizing performance, ease of preparation, durability, and low maintenance, this aligned NP system is a perfect candidate to emerge as a potential THz polarizer.
“Polarizing effect of aligned nanoparticles in terahertz frequency region", D. Polley, A. Ganguly, A. Barman, and R. K. Mitra; Optics Letters 38, 2754 (2013).
EMI Shielding in THz Frequency Range
We investigate the shielding effectiveness and complex conductivity of single-walled carbon nanotubes (SWNT) distributed in a polyvinyl alcohol (PVA) matrix in the THz frequency range. SWNTs are dispersed in PVA matrices with varying SWNT content (keeping the thickness of SWNT/PVA film constant) using a slow-drying method, and terahertz time-domain spectroscopy (THz-TDS) is performed at room temperature in transmission geometry in the frequency range of 0.3–2.1 THz. The transmittance spectra show a possible application of SWNT/PVA composites as low-bandpass filters in the THz frequency region. The shielding effectiveness of all the samples is measured, and, at a particular probing frequency, they tend to follow a linear relationship with the SWNT weight fraction in the polymer matrices. THz conductivity of the composite system is described in the light of a.c. hopping conduction.
“EMI shielding and conductivity of carbon nanotube-polymer composites at terahertz frequency“, D. Polley, A. Barman, and R. K. Mitra; Optics Letters 39, 1541 (2014).
Controllable THz conductivity
Terahertz (THz) conductivity of single-walled carbon nanotube (SWNT)/poly-vinyl alcohol (PVA) composites has been studied in the frequency window of 0.3–2.0 THz. SWNT/PVA composite films with a constant thickness of 300 ± 20 μm are grown by dispersing required amount of SWNT in PVA solution via a slow drying process at room temperature under ambient conditions. THz time domain spectroscopic measurements have been performed in transmission geometry at room temperature under N2 atmosphere and THz conductivity spectra have been extracted from the time domain data. It is found that the conductivity of these samples can be efficiently tuned by changing the SWNTs' length and the SWNT weight fraction. For the highest weight fraction at a frequency of 1.5 THz, the more extended SWNT sample (average length ∼ 15 μm) showed 80% increased conductivity than its shorter counterpart (average length ∼ 2 μm) of the same diameter (1–2 nm). The shielding effectiveness of the samples has also been engineered by simply changing the effective length of SWNT inclusion in the polymer matrix. A modified Universal Dynamic Response model is applied to analyze the conductivity spectra of the samples.
“Controllable terahertz conductivity in single-walled carbon nanotube/polymer composites", D. Polley, A. Barman, and R. K. Mitra; Journal of Applied Physics 117, 023115 (2015).
Terahertz Conductivity Engineering in Surface Decorated Carbon Nanotube Films by Gold Nanoparticles
We report the controllable conductivity of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes with their surface walls decorated by gold nanoparticles (Au NPs) with varying concentration in terahertz (THz) frequency range. Colloidal Au NPs of nominal diameter ∼15 nm are synthesized by the reduction of gold chloride solution using tri-sodium citrate. A simple chemical route is followed to attach Au NPs on the surfaces of both types of carbon nanotubes (CNTs). The attachment of Au NPs on the sidewalls of CNTs is confirmed by UV-visible spectroscopy and scanning electron microscope images. THz spectroscopic measurements are carried out at room temperature in transmission geometry in the frequency range of 0.3–2.0 THz. It is found that the THz conductivity of the surface decorated SWNT composites can either be increased or decreased by ±15% than that of the as-prepared SWNT composites by carefully choosing the Au NP concentration. The conductivity variation is qualitatively explained in terms of carrier trapping potential for low Au NP density, and alternative carrier conduction pathways at higher Au NP density and analyzed with the help of a modified universal dielectric relaxation model.
“Terahertz Conductivity Engineering in Surface Decorated Carbon Nanotube Films by Gold Nanoparticles” D.Polley, A. Patra, A. Barman, and R. K. Mitra; Applied Optics 56, 1107 (2017).
Diameter Dependent Shielding Effectiveness and Terahertz Conductivity of Multi-walled Carbon Nanotubes
In the present contribution, we have experimentally demonstrated the diameter dependence of terahertz (THz) shielding and THz conductivity of multiwalled carbon nanotubes (MWNTs) and performed detailed theoretical analysis to extract the mechanism of shielding at different MWNT diameters. Self-standing films of three different types of MWNT having the same average length but different outer tube diameters (namely, MWNT_7, MWNT_25, and MWNT_40 nm) are prepared by the vacuum filtration technique. The shielding effectiveness (SE) of these films in the frequency range of 0.4 – 2.2 THz is measured at room temperature, and the results are analyzed using a theoretical model. Shielding due to absorption (SE𝐴) turns out to be the dominant shielding mechanism for the MWNT_7 and MWNT_25 nm films, while the contribution of shielding due to reflection (SE𝑅) dominates for the MWNT_40 nm films in the smaller frequency region (<0.8 THz). Considering the films as a composite of MWNTs and air gaps, we have modeled the dielectric properties of the films using a combination of Maxwell–Garnett effective medium theory and the Drude–Lorentz model. THz conductivity is found to be increasing with increasing MWNT diameter due to the increasing number of Drude-like free electrons. No systematic dependence of the THz conductivity peak (TCP) frequency has been observed on the diameter of the tubes, which negates the idea of a curvature-induced bandgap as the sole origin of the much-debated TCP in carbon nanotubes. Our results reveal intriguing aspects on THz response of MWNT films as a function of MWNT diameter.
“Diameter Dependent Shielding Effectiveness and Terahertz Conductivity of Multi-walled Carbon Nanotubes” D. Polley, K. Neeraj, A. Barman, and R. K. Mitra; JOSA B 33, 2430 (2016).
THz Emission From a [Co/Pd]8 Multilayer
We report the experimental observation of ultrafast demagnetization associated with a significant terahertz (THz) emission in [Co/Pd]8 multilayers (MLs) with strong perpendicular magnetic anisotropy (PMA) by using a time-resolved magneto-optical Kerr effect (TR-MOKE) magnetometer. The THz emission is associated with subsequent remagnetization which is coupled with a collective picosecond magnetization dynamics and the generation of acoustic waves in these multilayers. The demagnetization time increases slightly with decreasing Co layer thickness, while no such clear trend is observed in remagnetization timescales. The anisotropy field and corresponding precession frequency show a systematic increase with the decrease of Co layer thickness. However, the acoustic wave frequencies remain unchanged with Co layer thickness. The inverse of the full width at half maximum of the THz radiation varies systematically with the anisotropy field, whereas the THz peak frequency remains almost constant at around 2 THz. The origin of THz radiation is explained in terms of the excitation of THz transient phonons.
“Ultrafast Dynamics and THz Oscillation in [Co/Pd]8 Multilayers” S. Pal, D. Polley, R. K. Mitra, and A. Barman; Solid State Communication 221, 50-54 (2015).
Anti-Reflection Coating in THz Frequency Range
We report the use of micrometer-sized copper (Cu) anti-dot structures as a novel terahertz (THz) anti-reflection coating (ARC) material and their superior performance over conventionally used metallic (Cu) thin films. Cu anti-dot structures of two different thicknesses (7 and 10 nm) with varying anti-dot diameters (100, 200, and 300 μm, inter-anti-dot separation fixed at 100 μm) are deposited on silicon substrates by RF magnetron sputtering and e-beam evaporation. The anti-reflection performance of these samples is studied in the frequency range of 0.3–2.2 THz. While continuous metallic (Cu) thin film minimizes the Fabry–Perot (FP) peak, it also suppresses the primary transmission peak, reducing the advantage due to the former effect. On the contrary, the anti-dot arrays reduce both the absolute amplitude of the FP peak and the amplitude ratio (AR) of the FP peak to the primary peak, making them a superior material for ARC applications. The AR can be further manipulated by varying the anti-dot size. A universal conductivity phase-matching condition, which is a prerequisite for the disappearance of the FP peak, is observed in these samples. The enhanced anti-reflection performance promotes these anti-dot structures as an efficient terahertz ARC material.
“Efficient Terahertz Anti-Reflection Properties of Metallic Anti-Dot Structures” K. Neeraj, S. Choudhury, D. Polley, R. Acharya, J. Sinha, A. Barman, and R. K. Mitra; Optics Letters 42, 1764 (2017).
