arXiv subjects

Vivek Dwij

Publications and source records attributed to Vivek Dwij.

7 recordsLinked to original sources

arXiv2026

Terahertz emission and detection using Ge-on-Si photoconductive antennas

Germanium-on-Silicon (Ge-on-Si) is a promising, CMOS-compatible platform for integrated terahertz (THz) photonics, offering a low-cost alternative to III-V semiconductors. A primary challenge for Ge-based photoconductive antennas (PCAs), however, has been the long carrier lifetime of bulk Ge, preventing its use as a detector. Here, we demonstrate that amorphous Ge (a-Ge) films overcome this limitation, possessing inherent ultrashort carrier lifetimes ~ 1.11-1.38 ps. We leverage this property to demonstrate, for the first time to our knowledge, coherent THz pulse detection using undoped a-Ge-on-Si PCAs. We present a comparative study of devices fabricated on a-Ge films grown by plasma-enhanced chemical vapor deposition (PECVD) and DC magnetron sputtering. The PECVD-Ge device, with better homogeneity and a smoother morphology in the films, demonstrates superior performance for both THz emission and detection. As an emitter, the PECVD-Ge PCA achieves a 40 dB signal-to-noise ratio (SNR) with a bandwidth of ~ 3 THz. As a detector, it achieves a 32 dB SNR and a ~ 2 THz bandwidth, representing a ~2.5-fold increase in detected signal amplitude over the sputtered-Ge device. These results establish amorphous Ge-on-Si as a viable and scalable platform for both THz generation and detection, paving the way for fully integrated Si-based THz time-domain systems.

physics.optics↗

arXiv2025

Phonon-polariton mediated dual electromagnetically induced transparency-like response in a THz metamaterial

Light and matter can intertwine to create entirely new quantum states in the so-called strong-coupling regime, allowing unprecedented control over electromagnetic waves. In this work, strong-coupling mediated polaritonic states are exploited to demonstrate tunable dual electromagnetically induced transparency (EIT) like response in the terahertz (THz) frequency range using a micron-sized metamaterial system coupled with the phonon mode of a lead halide perovskite film. This architecture allows us to reversibly switch between the single and the dual EIT-like behavior without modifying the metamaterial structures. The dual EIT-like nature is further confirmed through the in-plane electric field distributions and the slow-light effects. The specific structural symmetry further allowed us to effectively switch between the dual EIT-like response and the conventional strong-coupling responses. Such tunability bears potential implications for developing metamaterial-phonon-based tunable THz devices such as switches, filters, and slow-light devices.

physics.optics↗

arXiv2023

Ultrafast Carrier Relaxation and Second Harmonic Generation in a Higher-Fold Weyl Fermionic System PtAl

In topological materials, shielding of bulk and surface states by crystalline symmetries has provided hitherto unknown access to electronic states in condensed matter physics. Interestingly, photo-excited carriers relax on an ultrafast timescale, demonstrating large transient mobility that could be harnessed for the development of ultrafast optoelectronic devices. In addition, these devices are much more effective than topologically trivial systems because topological states are resilient to the corresponding symmetry-invariant perturbations. By using optical pump probe measurements, we systematically describe the relaxation dynamics of a topologically nontrivial chiral single crystal, PtAl. Based on the experimental data on transient reflectivity and electronic structures, it has been found that the carrier relaxation process involves both acoustic and optical phonons with oscillation frequencies of 0.06 and 2.94 THz, respectively, in picosecond time scale. PtAl with a space group of $P$$2_{1}$3 allows only one non-zero susceptibility element i.e. $d_{14}$, in second harmonic generation (SHG) with a large value of 468(1) pm/V, which is significantly higher than that observed in standard GaAs(111) and ZnTe(110) crystals. The intensity dependence of the SHG signal in PtAl reveals a non-perturbative origin. The present study on PtAl provides deeper insight into topological states which will be useful for ultrafast optoelectronic devices.

cond-mat.mes-hall↗

arXiv2022

Optical control of in-plane domain configuration and domain wall motion in ferroelectric and ferroelastic

The sensitivity of ferroelectric domain walls to external stimuli makes them functional entities in nanoelectronic devices. Specifically, optically driven domain reconfiguration with in-plane polarization is advantageous and thus highly sought. Here, we show the existence of in-plane polarized sub-domains imitating a single domain state and reversible optical control of its domain wall movement in a single-crystal of ferroelectric BaTiO3. Similar optical control in the domain configuration of non-polar ferroelastic material indicates long-range ferroelectric polarization is not essential for the optical control of domain wall movement. Instead, flexoelectricity is found to be an essential ingredient for the optical control of the domain configuration and hence, ferroelastic materials would be another possible candidate for nanoelectronic device applications.

cond-mat.mtrl-sci↗

arXiv2020

Revisiting 70 years of lattice dynamics of BaTiO3: Combined first principle and experimental investigation

BaTiO3 is a classical ferroelectric studied for last one century for its ferroelectric properties. Lattice dynamics of BaTiO3 is crucial as the utility of devices is governed by phonons. In this work, we show that traditional characterization of the polar phonon modes is ambiguous and often misinterpreted. By combining Raman, Neutron and X-ray diffraction, dielectric spectroscopic observations with first principle calculations, we have re-examined the character of the normal modes of phonons of BaTiO3. We obtained Eigen displacements of vibrational modes through DFT calculations and reclassified the polar modes being Slater (Ti-O), Last (Ba-TiO3) and Axe (BO6) vibrations by correlating experimental and theoretical calculations. The study thus provides correct nomenclature of the polar modes along with the evidence of presence of short range polar distortions along (111) directions in all the phases shown by BaTiO3. The Burns temperature and absence of second order contributions have been witnessed in the temperature dependent Raman study.

cond-mat.mtrl-sci↗

arXiv2019

Unfolding femtoscale ionic movement in CuO through polarized Raman spectroscopy

Recently, CuO has been proposed as a potential multiferroic material with high transition temperature. Competing models based on spin current and ionic displacements are invoked to explain ferroelectricity in CuO. The theoretical model predicting ionic displacement suggested that the shift in ions is essentially along b-axis with very small amplitude (~10-5 Å). Experimentally detecting displacements of such a small amplitude in a particular direction is extremely challenging. Through our detailed polarized Raman spectroscopy study on epitaxial film of CuO, we have validated the theoretical study and provided direct evidence of displacement along the b-axis. Our study provides important contribution in the high temperature multiferroic compounds and showed for the first time, the use of the polarized Raman scattering in detecting ionic displacements at the femto-scale.

cond-mat.mtrl-sci↗

arXiv2019

Raman spectroscopic investigation of relaxor behavior in Pr doped SrTiO3 and Origin of Fano resonance

Detailed Raman spectroscopy studies on polycrystalline Sr1-xPrxTiO3 (x=0.01, 0.025, 0.05, 0.075, 0.09, 0.13, 0.15, 0.17) samples are reported elucidating the microscopic mechanism of relaxor ferroelectrics. The polar mode was observed upto very high temperature ~1000 K suggesting that the dipoles exists at temperatures well above the characteristic relaxor temperatures and they develop a short range correlation at 505 K leading to formation of PNRs. The TO2 polar mode showed anomalous softening in cooling below T~505 K supporting the growth of PNRs. Fano resonance is reported in the Pr doped compounds which decreases with increasing doping. Our work on Pr doped SrTiO3 reveals that the local TiO6 octahedral tilt scales with the intensity of the polar mode and hence can be used as an order parameter for relaxor transition. The study showed that the paraelectric to relaxor ferroelectric phase transition in this compound is random polarizability instability driven phenomenon which is correlated with the local octahedral tilt angle. The study supports competition and cancelation between lattice and polar instabilities at global length scale while cooperation between the two at local length scale. The modulation in local structure of the material in temperature interval related to dielectric anomaly has been observed which is not been investigated previously by any structural tool.

cond-mat.mtrl-sci↗