Patanjali V. Parimi

Negative Refraction and Left-Handed Electromagnetism in Microwave Photonic Crystals

We demonstrate the negative refraction of microwaves in a metallic photonic crystal prism. The spectral response of the photonic crystal prism, which manifests both positive and negative refraction, is in complete agreement with band-structure calculations and numerical simulations. The validity of Snells law with a negative refractive index is confirmed experimentally and theoretically. The negative refraction observed corresponds to left-handed electromagnetism that arises due to the dispersion characteristics of waves in a periodic medium. This mechanism for negative refraction is different from that in metamaterials.

Focusing by planoconcave lens using negative refraction

We demonstrate focusing of a plane microwave by a planoconcave lens fabricated from a photonic crystal having a negative refractive index and left-handed electromagnetic properties. An inverse experiment, in which a plane wave is produced from a source placed at the focal point of the lens, is also reported. A frequency-dependent negative refractive index, n(ω)<0 is obtained for the lens from the experimental data which match well with that determined from band structure calculations.

Microwave photonic crystal with tailor-made negative refractive index

Negative refraction and left-handed electromagnetism in a metallic photonic crystal (PC) wedge are demonstrated in free space for both transverse magnetic and electric mode propagation. The experimental results are in excellent agreement with numerical calculations based on the band structure with no fit parameters used in modeling. The results demonstrate precision control on the design and fabrication of the PC to achieve tailor-made refractive indices between −0.6 and +1.

Q-band tunable negative refractive index metamaterial using Sc-doped BaM hexaferrite

A simple structured tunable negative refractive index (n) metamaterial (TNIM) has been designed, fabricated and tested in a Q-band rectangular waveguide. The structure consists of one slab of single crystalline scandium-doped barium hexaferrite (Sc-BaM), aligned parallel to two rows of periodic copper wires. The magnetic field tunable passband is measured indicating the occurrence of negative n. The centre frequency of the 5 GHz wide passband, having a transmission peak of −13 dB, is shifted linearly from 40.9 to 43.9 GHz by varying the bias field (H) from 4.0 to 7.0 kOe. The impact of ferrite volume factor (FVF) of the Sc-BaM slab upon the performance of the TNIM composite has been studied qualitatively. A tradeoff effect is illustrated in which the desirable negative permeability (μ) of the ferrite is offset by the detrimental impact of its dielectric property in suppressing the negative permittivity (e) of the nearby plasmonic wires.

A microstrip tunable negative refractive index metamaterial and phase shifter

A tunable negative refractive index metamaterial and miniature phase shifter have been designed and fabricated in a microstrip configuration for applications in radio frequency integrated circuits. The metamaterial consists of plasmonic copper wires and yttrium iron garnet slabs having a low insertion loss of 5dB at the center of the transmission band. The yttrium iron garnet material enables the magnetic field tuning of the negative refractive index in a dynamic frequency band from 7.0to11.0GHz. The insertion phase can be tuned by 45° continuously by varying the bias field from 3.8to4.6kOe at 9.0GHz.

Microwave properties of superconducting MgB2

Measurements of 10 GHz microwave surface resistance, Rs, of dense MgB2 wire and pellet are reported. Significant improvements are observed in the wire with reduction of porosity. The data lie substantially above the theoretical estimates for a pure Bardeen–Cooper–Schrieffer s-wave superconductor. However, the Rs (20 K) of the wire is an order of magnitude lower than that of polycrystal YBa2Cu3O6.95 and matches with single-crystal YBa2Cu3O6.95. The results show promise for the use of MgB2 in microwave applications.

Machine learning for stress detection from ECG signals in automobile drivers

Physiological sensor analytics is becoming an important tool to monitor health as the availability of sensor-enabled portable, wearable, and implantable devices becomes ubiquitous in the growing Internet of Things (IoT). Physiological multi-sensor studies have been conducted previously to detect stress. In this study, we focus on ECG monitoring that can now be performed with minimally invasive wearable patches and sensors, to develop an efficient and robust mechanism for accurate stress identification. A unique aspect of our research is personalized individual stress analysis including three stress levels: low, medium and high. Using machine learning algorithms from the ECG signals alone, we could achieve 88.24% accuracy in detecting the three classes of stress. We also find that high stress can be successfully detected for a person in comparison to his or her rest period with 100% accuracy.

Nonlinear microwave impedance of short and long Josephson junctions

The nonlinear dependence on applied ac field (bv) or current ( i v) of the microwave ~ac! impedance Rv 1iXv of both short and long Josephson junctions is calculated under a variety of excitation conditions. The dependence on the junction width is studied, for both field symmetric ~current antisymmetric! and field antisymmetric ~current symmetric! excitation configurations. The resistance shows steplike features every time a fluxon ~soliton! enters the junction, with a corresponding phase slip seen in the reactance. For finite widths the interference of fluxons leads to some interesting effects which are described. Many of these calculated results are observed in microwave impedance measurements on intrinsic and fabricated Josephson junctions in the high temperature superconductors. When a dc field (bdc) or current ( i dc) is applied, interesting phase locking effects are observed in the ac impedance Zv . In particular an almost periodic dependence on the dc bias is seen similar to that observed in microwave experiments at very low dc field bias. These results are generic to all systems with a cos(f) potential in the overdamped limit and subjected to an ac drive. @S0163-1829~99!00610-4#

Onset of dielectric modes at 110 K and 60 K due to local lattice distortions in nonsuperconductingYBa2Cu3O6.0crystals

We report the observation of two dielectric transitions at 110K and 60K in the microwave response of non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals. The transitions are characterized by a change in polarizability and presence of loss peaks, associated with overdamped dielectric modes. An explanation is presented in terms of changes in polarizability of the apical O atoms in the Ba-O layer, affected by lattice softening at 110K, due to change in buckling of the Cu-O layer. The onset of another mode at 60K strongly suggests an additional local lattice change at this temperature. Thus microwave dielectric measurements are sensitive indicators of lattice softening which may be relevant to superconductivity.

Wearable RF plethysmography sensor using a slot antenna

This paper presents a wrist wearable wireless plethysmography sensor based on the variation of reflection coefficient (S11) of a planar H-slot antenna and a compact, efficient signal processing circuit. The sensor detects changes in the volume of human blood in the radial artery and reflects it as a change in the reflection coefficient, and hence a change in the body pulse waveform. The sensor is designed to operate at 2.4 GHz ISM band. A salient feature of the present work is improved sensitivity through slot antenna and an efficient T/R architecture with better SNR design.