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Ajinkya Punjal

Bio: Ajinkya Punjal is an academic researcher. The author has contributed to research in topics: Terahertz radiation & Optics. The author has an hindex of 2, co-authored 20 publications receiving 11 citations.

Papers
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TL;DR: In this paper , a toroidal excitation-based metamaterial that is capable of converting terahertz from its linearly polarized state to an orthogonally polarized state over a broad spectrum was investigated.
Abstract: The development of metamaterial-based photonic components has acquired a significant interest in technological developments at terahertz frequencies. The manipulation of the state of polarization is an important parameter in optical devices. In this study, we have investigated, both numerically and experimentally, a toroidal excitation-based metamaterial that is capable of converting terahertz from its linearly polarized state to an orthogonally polarized state over a broad spectrum. The meta-molecule unit of the proposed geometry is comprised of a pair of resonators connected to each other having a split gap in each arm. We have studied both the horizontal and vertical components of transmission for numerous in-plane rotations of the proposed geometry. A multipolar analysis confirms a significant contribution of the toroidal component. Polarization conversion of nearly 40% is observed over a broad spectrum of 1.19–2.5 THz. Such a broadband cross-polarization converter could have remarkable implications for the development of terahertz toroidal metamaterial devices.

4 citations

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TL;DR: In this paper , a dynamically tunable polarization independent slow light system is experimentally demonstrated via electromagnetically induced transparency (EIT) in a terahertz (THz) metasurface constituted by plus and dimer-shaped resonators.
Abstract: Tunable slow light systems have gained much interests recently due to their efficient control of strong light–matter interactions as well as their huge potential for realizing tunable device applications. Here, a dynamically tunable polarization independent slow light system is experimentally demonstrated via electromagnetically induced transparency (EIT) in a terahertz (THz) metasurface constituted by plus and dimer-shaped resonators. Optical pump-power dependent THz transmissions through the metasurface samples are studied using the optical pump THz probe technique. Under various photoexcitations, the EIT spectra undergo significant modulations in terms of its resonance line shapes (amplitude and intensity contrast) leading to dynamic tailoring of the slow light characteristics. Group delay and delay bandwidth product values are modulated from 0.915 ps to 0.42 ps and 0.059 to 0.025 as the pump fluence increases from 0 to 62.5 nJ cm−2. This results in tunable slow THz light with group velocities ranging from 2.18 × 105 m s−1 to 4.76 × 105 m s−1, almost 54% change in group velocity. The observed tuning is attributed to the photo-induced modifications of the optoelectronic properties of the substrate layer. The demonstrated slow light scheme can provide opportunities for realizing dynamically tunable slow light devices, delay lines, and other ultrafast devices for THz domain.

3 citations

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TL;DR: In this paper , the terahertz optical properties of vanadium doped (100) β-Ga 2 O 3 were investigated using TDS in the 0.2-2.4 GHz range.
Abstract: We report the terahertz optical properties of vanadium doped (100) β-Ga 2 O 3 using terahertz time-domain spectroscopy (THz-TDS). The V-doped β-Ga 2 O 3 crystal shows strong birefringence in the 0.2-2.4 THz range. Further, phase retardation by the V-doped β-Ga 2 O 3 has been measured over the whole THz range by terahertz time-domain polarimetry (THz-TDP). It is observed that the V-doped β-Ga 2 O 3 crystal behaves both as a quarter waveplate (QWP) at 0.38, 1.08, 1.71, 2.28 THz, and a half waveplate (HWP) at 0.74 and 1.94 THz, respectively.

2 citations

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TL;DR: In this article , the Ni3N nanosheets catalyze CO2 hydrogenation with a high CO production rate (1212 mmol g-1 h-1) and selectivity (99%) using visible light.
Abstract: The majority of visible light-active plasmonic catalysts are often limited to Au, Ag, Cu, Al, etc., which have considerations in terms of costs, accessibility, and instability. Here, we show hydroxy-terminated nickel nitride (Ni3N) nanosheets as an alternative to these metals. The Ni3N nanosheets catalyze CO2 hydrogenation with a high CO production rate (1212 mmol g-1 h-1) and selectivity (99%) using visible light. Reaction rate shows super-linear power law dependence on the light intensity, while quantum efficiencies increase with an increase in light intensity and reaction temperature. The transient absorption experiments reveal that the hydroxyl groups increase the number of hot electrons available for photocatalysis. The in situ diffuse reflectance infrared Fourier transform spectroscopy shows that the CO2 hydrogenation proceeds via the direct dissociation pathway. The excellent photocatalytic performance of these Ni3N nanosheets (without co-catalysts or sacrificial agents) is suggestive of the use of metal nitrides instead of conventional plasmonic metal nanoparticles.

1 citations

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TL;DR: In this article , Fourier transformed terahertz spectroscopy (FTTS) was used to detect evanescent orders from a 1-dimensional dielectric grating for both TE and TM configurations.
Abstract: Although evanescent orders offer interesting physics, the experimental detection of these modes are very challenging task because of their inherent characteristics. Evanescent orders are non-propagating in nature making it impossible to detect in the far-field. At the same time, near-field detection is also complicated because of the concurrence of propagating and non-propagating diffraction orders. In order to overcome these hindrances, we introduce Fourier transformed terahertz spectroscopy (FTTS) through near-field scanning terahertz microscopy (NSTM) to experimentally detect the non-propagating evanescent orders originating from a 1-dimensional dielectric grating for both the TE and TM configurations. Our studies demonstrate exponential decay of mode strengths away from the grating-air interface confirming them as evanescent waves. Identification of evanescent orders demonstrate the sound competences that FTTS can offer in exploring near-field systems. We believe, FTTS based approaches can pave new avenues on studying other near-field phenomena in terahertz photonic systems.

1 citations


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TL;DR: In this paper , a temperature tuning of electromagnetically induced transparency (EIT) effects in a terahertz (THz) metasurface based on ferroelectric barium titanate (BaTiO3) thin film was investigated.
Abstract: The integration of active materials in terahertz (THz) metasurfaces is pivotal for the realization of functional device applications in diverse fields like sensing, imaging, communication, etc. In this context, ferroelectric materials endowed with tunable electro-optic properties have recently emerged as a novel candidate for achieving actively tuned THz metasurfaces. Here, we experimentally investigate temperature tuning of electromagnetically induced transparency (EIT) effects in a THz metasurface based on ferroelectric barium titanate (BaTiO3 (BTO)) thin film. We characterize tunable dielectric properties of the BTO thin film under variable temperatures (25 °C–100 °C) at THz frequencies by utilizing THz-time domain spectroscopy technique. Based on this aspect, we design a THz metasurface capable of displaying the EIT effect. THz transmissions through the metasurface sample are then probed for different applied temperatures. The EIT features undergo frequency shifts along with amplitude modulations owing to the temperature induced variations of the dielectric properties of the BTO thin film. A total red shift ∼27 GHz in EIT resonance dip is observed experimentally as the temperature increases from 25 °C to 100 °C. Therefore, we demonstrate utilities of ferroelectric platform toward the development of temperature tunable EIT metasurfaces.

2 citations

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TL;DR: In this article , a unique toroidal metasurface that modulates a broad resonance into a sharp mode, independent of the polarization of the incident terahertz (THz) radiation, by coupling the inherent toroidal dipole excitation to the lattice mode was demonstrated.
Abstract: The toroidal dipole excitation is important for metamaterial research because of its low-loss attribute. In this study, we demonstrate numerically and experimentally, a unique toroidal metasurface that modulates a broad resonance into a sharp mode, independent of the polarization of the incident terahertz (THz) radiation, by coupling the inherent toroidal dipole excitation to the lattice mode of the metasurface. The advantage of polarization independence enables the excitation of ‘lattice-coupled toroidal mode’ for both the orthogonally polarized states of the incident THz radiation in the metasurface. The interaction of the two resonances results in the enhancement of the quality factor of the metasurface at the point of resonance matching. The surface current profile as well as multipole analysis of scattered powers by electric, magnetic, and toroidal dipoles confirm the domineering effect of toroidal dipole excitation for both the polarization states of incident THz radiation. Such a lattice-matched toroidal excitation-based device has the potential to impact the development of polarization-independent THz components for ultrasensitive sensors, lattice-enhanced equipment, and slow light devices for light–matter interaction.
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TL;DR: In this paper , photosensitive silicon (Si) is chosen to control the state of the device, realizing the mutual conversion between EIT-like and EIA-like in a single device.
Abstract: Electromagnetically induced transparency-like (EIT-like) and electromagnetically induced absorption-like (EIA-like) can be easily generated separately in metastructure (MS), but it is rare to achieve both states simultaneously in a single device, let alone broadband EIT-like and EIA-like. Notably, the responses of the left-handed circularly polarized waves and the right-handed circularly polarized waves to the MS are the same, demonstrating the generation of polarization-insensitive EIT-like and EIA-like. The photosensitive silicon (Si) is chosen to control the state of the device, realizing the mutual conversion between EIT-like and EIA-like. With the incidence of circularly polarized (CP) waves, the broadband EIT-like can be observed between 0.505 THz and 1.403 THz, where the maximum group delay and group index can arrive at 1.57 ps and 7.26, respectively. Besides, the transition from broadband EIT-like to broadband EIA-like can be successfully achieved in the same frequency band when Si is excited by the pump light, where Si-based ladder structure (SBLS) is adapted to enhance absorption efficiency to realize broadband EIA-like. The relative bandwidths of EIT-like and EIA-like have reached 26.5% and 15.2%, respectively. The theoretical RLC (resistance, inductance, and capacitance) equivalent circuit model demonstrates the feasibility of broadband EIT-like and EIA-like implementation.
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TL;DR: In this paper , a multi-element metastructure unit cell consisting of split ring and dipole resonators is proposed to explore the intricate effects of the polarization dependency of these hybridized modes.
Abstract: Plasmonic metasurfaces have been quite a fascinating framework to invoke transformation of incident electromagnetic waves for a while now. Oftentimes, the building block of these metasurfaces or the unit cells consists of two or more meta-resonators. As a consequence, near-field coupling amongst these constituents may occur depending upon the spatial and spectral separation of the individual elements (meta-resonators). In such coupled structures resonance mode-hybridization can help in explaining the formation and energy re-distribution among the resonance modes. However, the coupling of these plasmonic modes is extremely sensitive to the polarization of the incident probe beam and offers ample amount of scope to harness newer physics. A qualitative understanding of the same can be attained through mode-hybridization phenomena. In this context, here, we have proposed a multi-element metastructure unit cell consisting of split ring and dipole resonators aiming to explore the intricate effects of the polarization dependency of these hybridized modes. Multi-resonator systems with varied inter-resonator spacing (sp = 3.0, 5.0, and 7.0 μm) are fabricated and characterized in the terahertz domain, showing a decrement in the frequency detuning (δ) by 30% (approx.) for a particular polarization orientation of THz probe beam. However, no such detuning is observed for the other orthogonal polarization configuration. Therefore, modulation of the resonance-hybridization is strongly dependent on the terahertz beam polarization. Further, as an outcome of the strong near-field coupling, the emergence of dual toroidal modes is observed. Excitation of toroidal modes demands thoughtful mode engineering to amplify the response of these otherwise feeble modes. Such modes are capable of strongly confining electromagnetic fields due to higher Quality (Q-) factor. Our experimental studies have shown significant signature of the presence of these modes in the Terahertz (THz) domain, backed up by rigorous numerical investigations along with multipole analysis. The calculated multipole decomposition demonstrates stronger scattering amplitude enhancements (∼7 times) at both the toroidal modes compared to off-resonant values. Such dual toroidal resonances are capable of superior field confinements as compared to single toroidal mode, and therefore, can potentially serve as an ideal testbed in developing next-generation multi-mode bio-sensors as well as realization of high Q-factor lasing cavities, electromagnetically induced transparency, non-radiating anapole modes, novel ultrafast switching, and several other applications.
Journal ArticleDOI
TL;DR: In this article , a metamaterial (MM) design capable of showing linear broadband polarization conversion over the terahertz (THz) frequency range is presented. But the performance of the proposed MM is limited by the fact that the distance between the strip and the split ring resonators is gradually increasing.
Abstract: We demonstrate a metamaterial (MM) design capable of showing linear broadband polarization conversion over the terahertz (THz) frequency range. The building block of the proposed MM structure is composed of a strip and four split ring resonators (SRRs), which are coupled through their near fields. To examine co- and cross-polarization transmission amplitudes, we gradually increase the distance between the strip and SRRs. When the SRRs are near (S = 2 [Formula: see text]m) the strip, maximum cross-polarization conversion is attained with a resonance mode hybridization effect in the co-polarization transmission due to strong near-field coupling between the strip and SRRs. When the SRRs moved away from the strip (S = 22 [Formula: see text]m), minimum cross-polarization conversion is attained due to weak coupling between the strip and SRRs. This MM system exhibits a transition from a strongly coupled state to a weakly coupled state with the rise in displacement between the strip and SRRs. The ability to tune the linear polarization conversion can be useful in the improvement of efficient THz polarization rotation devices. The proposed MM structure can be used in other frequency domains, like the microwave and visible range, by scaling up/down the structure.