Tera Hertz photoconductive antennas (THz PCAs) have significantly advanced the THz research by offering room-temperature operation, broad bandwidth and relatively low cost as both emitters and detectors. However, the primary limitation has been their low power output due to inefficient conversion. This article demonstrates a substantial improvement in efficiency (∼200%) by incorporating sub-micron photonic structures on the surface. These photonic structures enhance pump beam coupling, leading to increased photocarrier generation. They also facilitate efficient carrier recombination after THz emission, thereby suppressing carrier screening. Experimental and numerical studies confirm the enhanced photocarrier generation and controlled transport through defect-free paths, further reducing screening effects. The integration of photonic structures into large area emitters (LAEs) holds the potential to develop emitters and detectors suitable for real-world THz systems, overcoming the limitations of the current commercial LAEs that rely on plasmonic structures or antireflection coatings. This innovation has the potential to revolutionise THz technology, enabling the development of more powerful and efficient THz sources and detectors. This can lead to advancements in various fields, including wireless communication, imaging and sensing and spectroscopy.

We propose herein a metamaterial (MM) dual-band THz sensor for various biomedical sensing applications. An MM is a material engineered to have a particular property that is rarely observed in naturally occurring materials with an aperiodic subwavelength arrangement. MM properties across a wide range of frequencies, like high sensitivity and quality factors, remain challenging to obtain. MM-based sensors are useful for the in vitro, non-destructive testing (NDT) of samples. The challenge lies in designing a narrow band resonator such that higher sensitivities can be achieved, which in turn allow for the sensing of ultra-low quantities. We propose a compact structure, consisting of a basic single-square split ring resonator (SRR) with an integrated inverted Z-shaped unit cell. The projected structure provides dual-band frequencies resonating at 0.75 THz and 1.01 THz with unity absorption at resonant peaks. The proposed structure exhibits a narrow bandwidth of 0.022 THz and 0.036 THz at resonances. The resonant frequency exhibits a shift in response to variations in the refractive index of the surrounding medium. This enables the detection of various biomolecules, including cancer cells, glucose, HIV-1, and M13 viruses. The refractive index varies between 1.35 and 1.40. Furthermore, the sensor is characterized by its performance, with an average sensitivity of 2.075 THz and a quality factor of 24.35, making it suitable for various biomedical sensing applications.

This article presents a high-gain, noninvasive Vivaldi antenna array sensor for microwave breast tumor imaging, operating over a wide frequency range of 3.2–7.8 GHz with a compact footprint of 1.2 x 0.9λg. The antenna features a simplified microstrip-fed design that achieves a peak gain of 10 dBi while maintaining strong sensitivity in differentiating between healthy and malignant breast tissues. A key innovation of this work is the development of a realistic, biomimetic breast phantom modeled in CST Microwave Studio, whose anatomical shape, closely resembling an actual human breast, has not been previously reported in the literature. Comprehensive performance evaluation includes both simulated and measured S-parameters, along with specific absorption rate (SAR) analysis conducted at multiple tumor locations and sizes. The SAR results reveal significant contrasts between healthy and tumor-embedded regions, validating the sensor’s ability to detect tumors accurately. Additionally, power distribution analysis (PDA) is performed across each layer of the multilayer phantom over the entire operational bandwidth, offering detailed insight into the incident, reflected, and absorbed electromagnetic power. Experimental validation corroborates the simulation outcomes, demonstrating the proposed sensor’s potential for early-stage breast cancer detection through its high gain, broadband operation, and enhanced tissue sensitivity. Finally, a detailed implementation of the confocal microwave imaging (CMI) algorithm is presented, which reconstructs a high-contrast 2-D image that accurately localizes the tumor within the phantom.

In this report, the structural, morphological and electro-optical analysis of 2-D graphitic carbon nitride (g-C3N4) nano-sheets has been performed. The g-C3N4 nano-sheets were synthesized based on the thermal calcination process and characterized by transmission electron microscopy (TEM). X-ray diffraction studies (XRD) showed the inter-layer spacing to be 0.323 nm for the (002) plane which is 3.5% more dense than crystalline graphite and higher than literature reports for g-C3N4. For the evaluation of electro-optical properties, we have utilized time-domain spectroscopy for the frequency range 0.2 to 2 THz. The complex reflective indices (n, k) and permittivity (ϵ,ϵ′) for g-C3N4 have been determined. The complex conductivity has been observed to increase monotonically with an increase in frequency. The mobility of g-C3N4 has been theoretically estimated. The terahertz band properties such as plasma frequency, damping rate (0.095 THz), and collision time, were calculated for the synthesized material. The high permittivity value for g-C3N4 as reported in this work is promising for THz frequency selective components such as resonators, absorbers and collimators.

This paper introduces a compact, rhombus-shaped antenna based on a Half-Mode Substrate Integrated Waveguide (HMSIW) with a cavity-backed design, tailored to operate at 5.8 GHz within Industrial, Scientific, and Medical (ISM) band. The HMSIW cavity is designed to maintain the half-dominant TE110mode while reducing the cavity size by 50%. Initially, the antenna's bandwidth is insufficient for the entire ISM band. To address this, a rectangular slot is etched into the center of the cavity, which divides the dominant mode into TE110-odd and TE110-even modes resonating at 5.74 GHz and 5.8 GHz, respectively. This modification results in around 63% increase in bandwidth. For a Multiple Input Multiple Output (MIMO) application, the design extends from a single element to a four-element configuration, arranged orthogonally to achieve a high isolation level of approximately -23 dB in a compact footprint. The antenna has been prototyped and experimentally validated. The measured -10 dB impedance bandwidth spans 240 MHz (from 5.63-5.87 GHz), covering the full ISM band. The peak gain of the antenna is measured of 5.95 dBi, with an efficiency exceeding 90% across the operating frequency range. The 4-ports MIMO antenna diversity metrics parameters have been evaluated which are found in satisfactory limit.

Dipole antennae are commonly used radio frequency devices. They gained good prominence as a result of their efficiency, consistent performance and flexibility. Different optimization strategies such as particle swarm optimization, differential evolution and Machine Learning algorithms have been utilized in the past to design dipole antennae. This helps in creating a complete device profile and increases its efficacy. Due to the complexity of modern antennas in terms of topology and performance requirements, standard antenna design approaches are tedious and cannot be guaranteed to produce effective results. Out of the strategies that are widely being utilized, Machine Learning (ML) algorithms evolved rapidly due to their capabilities in extrapolating the dimensional and material profiles of the device. Antenna design optimization still faces several difficulties, even though machine learning-based design optimization complements traditional antenna design methodologies. The effectiveness and optimization capabilities of available ML approaches to address a wide range of antenna design problems, considering the increasingly strict specifications of current antennas, are the fundamental difficulties in antenna design optimization which need to be focused on. In our current work, the capability of ML algorithms in elucidating minor trends in device profiles is tested. A bootstrap aggregation model is proposed, concatenating Linear Regression, Support Vector Regression and Decision Tree Regression algorithms. The concatenated model was used to optimize the parameters of reflection coefficient, directivity, efficiency and operating frequency, depending on the feed length, dipole radius and dipole length of the antenna.

In this article, a novel, compact, dual-band 4-elements multi-input and multi-output (MIMO) antenna diplexer is investigated, prototyped, and tested. The proposed design is based on a planar half-mode Substrate Integrated Waveguide (HMSIW) technology, that diminishes size by 50% than of full-mode Substrate Integrated waveguide (FMSIW) cavity. Later, to enhance the bandwidth around 50%, a rectangular slot is introduced at the center of each cavity. The slot splits the dominant mode half-TE110 into an odd- and even-modes. The proposed antenna resonates at 3.3 GHz and 3.4 GHz when either Port-1 or Port-3 is fed, and Port-2 or Port-4 are terminated with matched loads. Similarly, the antenna operates at 4.2 GHz and 4.3 GHz, either Port-2 or Port-4 is fed, and the rest are matched terminated. The intrinsic isolation between any two ports is achieved below -23 dB with an antenna footprint of 1.0λ1 × 0.8 λ1. The average gain in the lower and upper frequency bands are around 5.15 dBi and 6.1dBi, respectively while radiation efficiency is more than 80% in both frequency bands. The envelope correlation coefficient (ECC) of the MIMO antenna has been achieved < 0.13, directive gain (DG) around 9.9 dBi, and mean effective gain (MEG) around -3.05 dB in both frequency bands.

In this article, a compact dual-band, 2-elements antenna-diplexer is investigated and extended to a 2×2 multi-input and multi-output (MIMO) antenna. The proposed design employs half-mode Substrate Integrated Waveguide (HMSIW) technology, which reduces the antenna footprint by 50%. To enhance the bandwidth, a rectangular slot is engraved at the center of each HMSIW cavity. The slot splits the dominant mode of the HM cavity into two odd-and even-half TE110 modes in a proximity, which leads to enhancement in the bandwidth by 50%. The antenna resonates around 3.4 GHz with a fractional bandwidth of 5% and around 4.3 GHz with a bandwidth of 4.7%, when corresponding ports are excited, respectively. Both the lower and upper frequency bands can be tuned individually, by simply altering the dimensions of each HMSIW cavity. This can be achieved in a common antenna, without employing filters, which satisfies the antenna-diplexer function. The isolation levels between any two radiating elements are obtained below-23 dB for the proposed MIMO antenna, and it occupies overall size of 1.0λg × 0.8λg. The peak gain of the antenna is obtained 5.35 dBi in the lower frequency band and 6.75 dBi in the upper frequency band while radiation efficiency is better than 80% in both frequency bands. The MIMO antenna properties have been evaluated and all found satisfactory. The envelope correlation coefficient (ECC) is obtained less than 0.13, DG around 9.9 dBi, and mean effective gain (MEG) around-3.05 dB. The 2×2 MIMO antenna is prototyped and experimentally verified. The measured results are closely following the simulation counterparts. The proposed 2×2 MIMO antenna is an appropriate alternative for LTE frequency bands in 5G wireless communication.

In this paper, a new periodic Leaky-Wave Antenna (LWA) based on Substrate Integrated waveguide (SIW) technology is proposed and investigated. To scan the beam in backward as well as forward plane, a periodic leaky LWA has been employed in this work. Periodic LWA also offers the advantage of larger bandwidth with higher gain. This LWA is realized using a sensibly developed SIW technology to achieve the proposed geometry in a planar form. In this antenna, multiple square-shaped slots are etched into the top plane for radiation. An investigative study on stop-band minimization and thereby increasing scan range, scan resolution has been performed by varying the periodic range between the slots. The antenna shows a scanning from -78° to 73° in the frequency range of 7.5 to 14.8 GHz with a peak gain of 11.8 dBi. It is observed from simulation studies that the period 20 mm is the best choice with respect to the stopband, scan resolution, and gain.

We report single and multiband linear polarisers for terahertz (THz) frequencies using cut-wire metamaterials (MM). The MMs were designed by finite-element method (FEM), fabricated by electron beam lithography, and characterised by THz time-domain spectroscopy. The MM unit cells consist of single or multiple length cut-wire pads of gold on semi-insulating gallium arsenide (GaAs) for single or multiple band polarisers. For example, a MM with a square unit cell of 50μm size on 1 mm GaAs substrate with a gold cut wire of 65μm length, 2μm width, and 150 nm height gives a resonance around 1.05 THz. The dependence of the resonance frequency of the single-band polariser on the length of the cut-wires was explained based on transmission line model.

A loop Yagi-Uda array can be used as an efficient, polarization independent and narrowband absorber in the THz regime. The elementary unit of the array consists of three-stacked gold micro-rings, which are separated from each other by thick SU-8 layer. Here, we present a topological study of a loop Yagi-Uda structure to examine the robustness of the narrow absorption peak against the misalignment among multiple layers and structural disorders. The study reveals that misalignment and variation of geometrical parameters with in the range of experimental errors can still protect the absorption peak with more than 70% absorption.

Terahertz (THz) frequencies, despite having the potential for several important applications, have been relatively underexplored in the past owing to the unavailability of proper sources and detectors. The scenario has been changing over the past few decades due to the advent of convenient THz sources and detectors. THz photoconductive antennas (PCA), due to their attractive features, such as cost effectiveness and room temperature operation, are playing a key role in current and future research prospect in the field of THz spectroscopy, both as sources and detectors. Complex PCA designs have been proposed and studied to boost the THz emission efficiencies. Elucidating the underlying physics in such devices requires a thorough investigation of a few physical parameters. This requires the integration of several experimental techniques under identical conditions. In this paper, we show such a study, including a parametric variation of pump polarization, conducted on a PCA with a nanopatterned active region, which boosts the emitted THz radiation. Through the set of measurements, we unravel the subtle interplay of the various physical processes responsible for the emission of THz radiation from the device.

Resonant transmission of electromagnetic radiation through an array of rectangular apertures occurs when electric field vectors of the incident wave are perpendicular to the long axis of the aperture. In this paper, we conduct a study on periodic array of asymmetric aperture and demonstrate that introducing a taper along long axis can yield transmission for both orthogonal polarizations. Specifically, we show that, spectral peak position of transmission spectra can be tuned by just flipping the polarization from one state to its orthogonal state. Moreover, highly directive transmitted beam results at far field as the inter-element spacing of the aperture is reduced to half of the wavelength at resonance.

In this article, we conduct a study on terahertz (THz) transmission through a periodic array of asymmetric apertures to demonstrate polarization controlled dual transmission windows. Specifically, we show that resonance peaks and Q-factors of these two transmitted bands are controllable via switching the incoming THz light from one polarization state to the orthogonal state. We investigate the origin and dispersion of two transmission peaks using a thorough study of numerical simulation. In our study, we show that depending on the polarization state, Q-factor can be changed from 3 to 100. Such a large variation of resonance quality can be utilized for a variety of applications like high resolution sensing and data communication.

In this article we have investigated the effect of surface nano-patterns in THz Photo-Conductive Antennae (PCA). The surface patterns help to localize the incident IR excitation thus reduces the reflection. It also reduces carrier recombination time by introducing defect states in current path. The combined effect enhances the THz emission from the device up to 220% when the electrical bias field is parallel to the optical pump polarization.

Due to the low optical-to-THz conversion efficiencies, applications of photoconductive antennas are limited in current scenario. In this paper, we report up to an order of enhancement in THz emission efficiency from conventional PCAs by coating a nano-layer of dielectric (TiO2) on the active area between the electrodes of a Semi-Insulating GaAs (SI-GaAs) based device. Effect of different thicknesses of the TiO2layer on THz power enhancement with different applied optical power and bias voltages were studied. Simulations in COMSOL Multiphysics® (RF and semi-conductor modules) were performed to validate the enhancement in the efficiency of the PCA due to TiO2 layer coating.

In this contribution, we demonstrate a new technique for fabricating a mechanically stretchable metasurface with tunable response on an elastic polydimethylsiloxane (PDMS) membrane operating at Terahertz (THz) frequencies. The tunability of the response of the metasurface is based on the change of the physical dimensions of individual micro-structures due to the strain caused by mechanical stretching. The technique is fast, easy and uses high resolution e-beam patterning in contrast to established screen-printing and other techniques reported in literature. Furthermore, the change in the response of the device shows linear dependence with strain over a broad range of strain with high repeatability. Thus, this technique has high potential for applications in communications technology, remote strain sensing and biological applications.

This article presents a novel, compact reflectarray antenna operating at 5 GHz. The array consists of two types of phasing element - square ring and complementary square ring. The complementary square ring is used for the phase values that are not covered by the single ring element. The grid size of the array is 0.28λ × 0.28λ at operating frequency 5 GHz that is much smaller than the conventional periodicity 0.5λ × 0.5λ. The aim of unit cell design is to have a slower slope of the reflection phase graph without sacrificing the phase range of 360° where in general there is a trade-off between these two goals. The maximum slope in reflection phase graph obtained here is 34°/mm. The proposed array is fabricated on a low loss PTFE substrate of thickness 3.175 mm (0.053λ at 5 GHz) and illuminated by a horn antenna. Radiation pattern results show a very precise far-field beam with 3-dB beamwidth of 7° and 7.3° for two principal planes respectively. The gain of the antenna is 26 dBi at 5 GHz.

Radiative dipolar resonance with Lorentzian line-shape induces the otherwise dark quadrupolar resonances resulting in electromagnetically induced transparency (EIT). The two interfering excitation pathways of the dipole are earlier shown to result in a Fano line shape with a high figure of merit suitable for sensing. In metamaterials made of metal nanorods or antennas, the plasmonic EIT (PIT) efficiency depends on the overlap of the dark and bright mode spectra as well as the asymmetry resulting from the separation between the monomer (dipole) and dimer (quadrupole) that governs the coupling strength. Increasing asymmetry in these structures leads to the reduction of the figure of merit due to a broadening of the Fano resonance. We demonstrate a PIT system in which the simultaneous excitation of two dipoles result in double PIT. The corresponding two quadrupoles interact and control the quality factor (Q) of the PIT resonance. We show an antiresonancelike symmetric line shape with nonzero asymmetry factors. The PIT resonance vanishes due to quadrupole-quadrupole coupling. A Q factor of more than 100 at 0.977 THz is observed, which is limited by the experimental resolution of 6 GHz. From polarization-dependent studies we show that the broadening of the Lorentzian resonance is due to scattering-induced excitation of orthogonally oriented dipoles in the monomer and dimer bars in the terahertz regime. The high Q factors in the terahertz frequency region demonstrated here are interesting for sensing application.

Photoconductive antennas (PCAs) are among the most conventional devices used for emission as well as detection of terahertz (THz) radiation. However, due to their low optical-to-THz conversion efficiencies, applications of these devices in out-of-laboratory conditions are limited. In this paper, we report several factors of enhancement in THz emission efficiency from conventional PCAs by coating a nano-layer of dielectric (TiO2) on the active area between the electrodes of a semi-insulating GaAs-based device. Extensive experiments were done to show the effect of thicknesses of the TiO2 layer on the THz power enhancement with different applied optical power and bias voltages. Multiphysics simulations were performed to elucidate the underlying physics behind the enhancement of efficiency of the PCA. Additionally, this layer increases the robustness of the electrode gaps of the PCAs with high electrical insulation as well as protect it from external dust particles.

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.

In this article, we explore a mechanically tunable metasurface on an elastic polydimethylsiloxane (PDMS) membrane operating at Terahertz (THz) frequencies synthesized using a "pattern and peel fabrication" technique. The tunability of the metasurface is based on the change of physical dimensions of the individual micro-structures due to the strain caused by mechanical stretching. The novelty of this technique is the ability to use high resolution e-beam patterning in contrast to established screen-printing techniques reported in the literature. The metasurface studied in this article is a periodic lattice of split-ring structures resonant at THz frequencies. The effect of mechanical stretching on the response of the metasurface is investigated thoroughly through experiments and numerical simulations. The response of the metamaterial to stretching manifests as a shift in the higher order mode by ~ 12% for an applied strain of ~ 25%. This tunability of the spectral response with macroscopic strain is not only substantial for the given structure, but also follows a linear behavior. This device can have potential applications in communications technology, remote strain sensing, chemical and biological sensing.

In this article we present a three-dimensional loop Yagi-Uda array for efficient, polarization independent and directional absorption of THz radiation over a narrow frequency range (f0 = 0.657 THz & Q factor = 7.5). Unit cell of the array consists of three vertically stacked gold micro rings separated from each other by 30 μm thick SU-8 layers. The proposed array also exhibits a filtering response in its transmittance spectrum. The characteristics are explained by plasmon hybridization method. The transmission, reflection and absorption spectra of the structure are measured and they show a good agreement with corresponding simulated results.

This work demonstrates the functioning of creating a micro-scale patch structure in copper and optically transparent Indium Tin Oxide thin film by a mode-locked 20ns and 650ps laser pulses with 1064nm and 532nm wavelength respectively. The proposed device uses Cu and ITO as conducting patches. A thin film of 0.4 micron Cu and 0.125micron ITO was deposited on Polyimide and PET substrate respectively. The crystalline of the deposited Cu and ITO films have been discussed. It is clear that the ablation threshold is dependent on applied power, pulse width, and scanning speed. The analysis of the results has been done using Scanning Electron Microscope, an Optical Profiler, and a white line interferometer. The ablation depth of the patch profiles was analyzed through an optical profiler. The temperature rise in the ITO by the applied laser pulse has been studied using FEM simulation and the laser ablation of ITO film from PET substrate by the delamination process was observed. The created structure finds application in Terahertz Antenna in making infrared detector and the performance metrics of the developed structures are studied using EM simulation software. The resonance nature of the fabricated structure has been validated experimentally by THz-TDS technique.

A compact reflectarray antenna is presented here, consisting of different element types. Periodicity of the proposed unit cell smaller than 0.5λ in both directions is studied here. Non-resonant behavior of unit cells leads to solve the problem of inherently narrow bandwidth of such antenna. Main aim of unit cell design is focused to get phasing elements fitted in subwavelength grid size as well as to get full range of 360 degree reflection phase angle. The proposed array is fabricated on a low loss PTFE substrate of thickness 0.053 λ at 5 GHz and illuminated by an offset feed horn antenna. Radiation pattern results show a very précised far-field beam with 3-db beamwidth of 7.5 and 7.3 degree for two principle planes respectively with a gain of 24 dB at 5 GHz.

Electromagnetically induced transmission (EIT) in THz domain is studied in Dolmen structures. The effect of dipole in exciting the otherwise dark quadrupole mode and the interaction of a 2nd dipole in controlling the EIT peak are studied. We show broad EIT peak in Dolmen as well as two EIT peaks and vanishing of EIT peak by varying the position of a 2nd dipole to form a ring like structure.

The design of a free-standing (substrate-less) metallic hole array is proposed for Terahertz frequencies. The bandpass filtering effect through free standing perforted Aluminium (Al) films is demonstrated using a square array of circular holes on Al films having thickness of ∼ 11 μm. The effect of variation of periodicity and hole diameter on the transmittance due to excitation of surface plasmons and coupling between resonant and non-resonant modes is studied.

Wide band MEMS resonators banking on the principle of amplitude dependent stiffness nonlinearity as reported in few recent publications can stretch the frequency response. A recent report by our group on four internal proof mass based membrane resonator has been observed to vibrate below 350 Hz with more than three closely placed peaks within 2kHz in the frequency response without introducing non-linearity. However, improvements in the design of such wide band MEMS resonators operating in the linear regime are required, so that the lower frequency range can be reduced further. In this paper we have optimized the lateral dimensions and displacements of the internal proof-masses by introducing a figure of merit to estimate the performance of these resonators. The figure of merit incorporates three parameters viz. power output per unit area, lower cut off frequency and no peaks in the operating frequency range within 2 KHz (most of the practical vibration sources lie within it). It has been observed that the resonator with four internal proof-masses each of dimensions 3×2mm 2 placed 0.5mm apart from each other shows highest figure of merit. © 2012 Springer-Verlag.