Abstract-Excitons in 2D perovskites for ultrafast terahertz photonic devices

Abhishek Kumar,  Ankur Solanki,  Manukumara Manjappa,  Sankaran Ramesh,  Yogesh Kumar Srivastava, Piyush Agarwal,  Tze Chien Sum. Ranjan Singh

https://advances.sciencemag.org/content/6/8/eaax8821

In recent years, two-dimensional (2D) Ruddlesden-Popper perovskites have emerged as promising candidates for environmentally stable solar cells, highly efficient light-emitting diodes, and resistive memory devices. The remarkable existence of self-assembled quantum well (QW) structures in solution-processed 2D perovskites offers a diverse range of optoelectronic properties, which remain largely unexplored. Here, we experimentally observe ultrafast relaxation of free carriers in 20 ps due to the quantum confinement of free carriers in a self-assembled QW structures that form excitons. Furthermore, hybridizing the 2D perovskites with metamaterials on a rigid and a flexible substrate enables modulation of terahertz fields at 50-GHz modulating speed, which is the fastest for a solution-processed semiconductor-based photonic device. Hence, an exciton-based ultrafast response of 2D perovskites opens up large avenues for a wide range of scalable dynamic photonic devices with potential applications in flexible photonics, ultrafast wavefront control, and short-range wireless terahertz communications.

Abstract-Terahertz sensing of 7 nm dielectric film with bound states in the continuum metasurfaces

Applied Physics Letters

Yogesh Kumar Srivastava,  Rajour Tanyi Ako,  Manoj Gupta, Madhu Bhaskaran, Sharath Sriram,  Ranjan Singh,

Nanometer-scale thin film sensing with quasi-BIC Fano resonance. (a) Optical image of the fabricated TASR metamaterial. The inset depicts the geometrical parameters of the unit cell, gap g = 3 μm, width w = 6 μm, length l = 60 μm, periodicity p = 75 μm, and asymmetry d = 10 μm. (b) Image of the fabricated metamaterial showing robustness and flexibility. (c) The Q factors of quasi-BICs of an ideal (dots, PEC) and a realistic (star, metallic) metamaterial array with varying asymmetry d. Inset: simulated transmission spectra of the metallic TASR metamaterial with an asymmetry of d = 10 μm. (d) and (e) Change in the simulated transmission amplitude (ΔT) and phase (Δϕ, degree) on coating Ge of thicknesses ranging from 7 to 20 nm on the TASR metamaterial.

https://aip.scitation.org/doi/abs/10.1063/1.5110383

The fingerprint spectral response of several materials with terahertz electromagnetic radiation indicates that terahertz technology is an effective tool for sensing applications. However, sensing few nanometer thin-films of dielectrics with much longer terahertz waves (1 THz = 0.3 mm) is challenging. Here, we demonstrate a quasibound state in the continuum (BIC) resonance for sensing of a nanometer scale thin analyte deposited on a flexible metasurface. The large sensitivity originates from the strong local field confinement of the quasi-BIC Fano resonance state and extremely low absorption loss of a low-index cyclic olefin copolymer substrate. A minimum thickness of 7 nm thin-film of germanium is sensed on the metasurface, which corresponds to a deep subwavelength scale of λ/43 000, where λ is the resonance wavelength. The low-loss, flexible, and large mechanical strength of the quasi-BIC microstructured metamaterial sensor could be an ideal platform for developing ultrasensitive wearable terahertz sensors.

The authors acknowledge valuable and timely assistance from Zhang Qiannan in performing the thickness measurements of the analyte layer using Atomic Force Microscopy. Y.K.S., M.G., and R.S. acknowledge the research funding support from the Ministry of Education AcRF Tier 1 Grant No. RG191/17 and Tier 2 Grant No. MOE2017-T2-1-110. S.S. and R.S. acknowledge support from an RMIT Foundation Research Exchange Fellowship. This work was performed in part at the Micro Nano Research Facility at RMIT University in the Victorian Node of the Australian National Fabrication Facility (ANFF).

Abstract-Guided‐Mode Resonances in All‐Dielectric Terahertz Metasurfaces

Song Han,  Mikhail V. Rybin, Prakash Pitchappa,  Yogesh Kumar Srivastava,  Yuri S. Kivshar,  Ranjan Singh,

Low‐loss silicon cuboid based all‐dielectric terahertz metasurfaces act simultaneously as diffraction grating and in‐plane slab waveguide, thereby enabling excitation of guided mode resonances (GMRs). At normal incidence, destructive interference between counter‐propagating GMRs give rise to symmetry‐protected bound state in the continuum. Terahertz GMRs are multifunctional devices that can enable narrow‐band filters, ultrafast modulators, and free‐space couplers.

https://onlinelibrary.wiley.com/doi/10.1002/adom.201900959

Coupling of diffracted waves in gratings with the waveguide modes gives rise to the guided mode resonances (GMRs). The GMRs provide designer linewidth and resonance intensity amidst a broad background, and thus have been widely used for numerous applications in visible and infrared spectral regions. Here, terahertz GMRs are demonstrated in low‐loss, all‐dielectric metasurfaces, which are periodic square lattices of silicon cuboids on quartz substrates. The silicon cuboid lattice simultaneously acts as a diffraction grating and an in‐plane slab waveguide, thereby resulting in the formation of terahertz GMRs. At oblique incidence, two distinct frequency detuned GMRs are observed. The frequency difference between these two GMRs increases at larger angle of incidence. However, extremely small angle of incidence causes destructive interference between these counter‐propagating GMRs that leads to a nonradiative symmetry‐protected bound state in the continuum. GMRs in all‐dielectric silicon metasurfaces can have potential applications in the realization of efficient terahertz devices such as high‐Q transmission filters with angular spectral selectivity, ultrafast modulators, and free‐space couplers.

Abstract-Asymmetric gap resonator based near-field coupling in terahertz metamaterials (Conference Presentation)

S. Jagan Mohan Rao, Yogesh Kumar Srivastava, Gagan Kumar,  Dibakar Roy Chowdhury

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10983/109830Z/Asymmetric-gap-resonator-based-near-field-coupling-in-terahertz-metamaterials/10.1117/12.2519820.short?SSO=1

The metamaterial is an arrangement of artificial structural elements designed to achieve advantageous and unusual electromagnetic properties. In the unit cell level, metamaterials are composed of an array of small structured elements called split ring resonators (SRRs). Recently, a lot of emphases has been given to the realization of terahertz metamaterials owing to its significance in the construction of terahertz photonic components. In this context, near-field coupling in terahertz metamaterials is extremely crucial. The short-range coupling in metamaterials occurs via the electric and magnetic fields due to the close proximity of the neighboring resonators. The electric field mainly couples through the gaps of SRRs, while the magnetic field couples through the circumference. In this work, we experimentally investigate near-field gap to gap capacitive coupling between a pair of single split gap ring resonators (SRRs) in a terahertz metamaterial. This has been achieved by manipulating the near field electric interactions via changing one resonator split gap with respect to the other resonator split gap for several inter resonator separations. Introducing asymmetry by changing the split gap in one resonator with respect to the other resonator, results in the split in the fundamental resonance mode when operated in the strong near-field coupled regime. The split occurs because of the strong near field capacitive/electric interactions between the resonators. We have further calculated Q factor for the lower and higher resonance modes for different inter resonator separations. The modulation of resonances in capacitive coupled planar terahertz metamaterial systems studied through this work has great potential in manipulating and controlling electromagnetic waves which can ultimately result in novel applications for terahertz frequency domain.

Abstract-Modulating Fundamental Resonance in Capacitive Coupled Asymmetric Terahertz Metamaterials

S. Jagan Mohan Rao, Yogesh Kumar Srivastava, Gagan Kumar & Dibakar Roy Chowdhury

https://www.nature.com/articles/s41598-018-34942-2

In this work, we experimentally investigate near-field capacitive coupling between a pair of single-gap split ring resonators (SRRs) in a terahertz metamaterial. The unit cell of our design comprises of two coupled SRRs with the split gaps facing each other. The coupling between two SRRs is examined by changing the gap of one resonator with respect to the other for several inter resonator separations. When split gap size of one resonator is increased for a fixed inter-resonator distance, we observe a split in the fundamental resonance mode. This split ultimately results in the excitation of narrow band low frequency resonance mode along with a higher frequency mode which gets blue shifted when the split gap increases. We attribute resonance split to the excitation of symmetric and asymmetric modes due to strong capacitive or electric interaction between the near-field coupled resonators, however blue shift of the higher frequency mode occurs mainly due to the reduced capacitance. The ability of near-field capacitive coupled terahertz metamaterials to excite split resonances could be significant in the construction of modulator and sensing devices beside other potential applications for terahertz domain.

Abstract-Magnetic annihilation of the dark mode in a strongly coupled bright–dark terahertz metamaterial

Manukumara Manjappa, Shuvan Prashant Turaga, Yogesh Kumar Srivastava, Andrew Anthony Bettiol, and Ranjan Singh

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-42-11-2106&origin=search

Dark mode in metamaterials has become a vital component in determining the merit of the Fano type of interference in the system. Its strength dictates the enhancement and suppression in the amplitude and 𝑄$Q$-factors of resulting resonance features. In this work, we experimentally probe the effect of strong near-field coupling on the strength of the dark mode in a concentrically aligned bright resonator and a dark split ring resonator (SRR) system exhibiting the classical analog of the electromagnetically induced transparency effect. An enhanced strong magnetic field between the bright–dark resonators destructively interferes with the inherent magnetic field of the dark mode to completely annihilate its effect in the coupled system. Moreover, the observed annihilation effect in the dark mode has a direct consequence on the disappearance of the SRR effect in the proposed system, wherein under the strong magnetic interactions, the LC resonance feature of the split ring resonator becomes invisible to the incident terahertz wave.

© 2017 Optical Society of America

Abstract-Hybrid Lead Halide Perovskites for Ultrasensitive Photoactive Switching in Terahertz Metamaterial Devices

Manukumara Manjappa, Yogesh Kumar Srivastava, Ankur Solanki, Abhishek Kumar, Tze Chien Sum, Ranjan Singh

http://onlinelibrary.wiley.com/doi/10.1002/adma.201605881/abstract

The recent meteoric rise in the field of photovoltaics with the discovery of highly efficient solar-cell devices is inspired by solution-processed organic–inorganic lead halide perovskites that exhibit unprecedented light-to-electricity conversion efficiencies. The stunning performance of perovskites is attributed to their strong photoresponsive properties that are thoroughly utilized in designing excellent perovskite solar cells, light-emitting diodes, infrared lasers, and ultrafast photodetectors. However, optoelectronic application of halide perovskites in realizing highly efficient subwavelength photonic devices has remained a challenge. Here, the remarkable photoconductivity of organic–inorganic lead halide perovskites is exploited to demonstrate a hybrid perovskite–metamaterial device that shows extremely low power photoswitching of the metamaterial resonances in the terahertz part of the electromagnetic spectrum. Furthermore, a signature of a coupled phonon–metamaterial resonance is observed at higher pump powers, where the Fano resonance amplitude is extremely weak. In addition, a low threshold, dynamic control of the highly confined electric field intensity is also observed in the system, which could tremendously benefit the new generation of subwavelength photonic devices as active sensors, low threshold optically controlled lasers, and active nonlinear devices with enhanced functionalities in the infrared, optical, and the terahertz parts of the electromagnetic spectrum.

Abstract-Accessing the High-Q Dark Plasmonic Fano Resonances in Superconductor Metasurfaces

http://onlinelibrary.wiley.com/doi/10.1002/adom.201600354/abstract

Superconducting metamaterials at terahertz frequencies has provided a platform for designing switchable plasmonic metamaterial devices. However, since metals at terahertz frequencies are excellent conductors, the superior role of superconductors in designing low-loss, high quality factor metamaterials remains unclear. In this work, a low asymmetry Fano resonant split-ring-resonator is considered in which a regime of extremely low radiative loss is identified where the high temperature yttrium barium copper oxide superconductor meta-atom supports a sharp high quality factor Fano resonance while an identical metallic resonator does not show the resonance behavior. The radiative and the nonradiative losses are comparable in low asymmetry Fano resonant meta-atoms. The observation of high quality factor Fano resonance behavior clearly establishes the utility of superconductors over metals in designing lower loss plasmonic metamaterials at terahertz frequencies that may have multifunctional applications in areas that require strong light–matter interactions.

Abstract-Ultrahigh-Q Fano Resonances in Terahertz Metasurfaces: Strong Influence of Metallic Conductivity at Extremely Low Asymmetry

1. Yogesh Kumar Srivastava^1,2, 
2. Manukumara Manjappa^1,2, 
3. Longqing Cong^1,2, 
4. Wei Cao³, 
5. Ibraheem Al-Naib⁴, 
6. Weili Zhang³and
7. Ranjan Singh^1,2,*

Article first published online: 14 DEC 2015

DOI: 10.1002/adom.201500504

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

http://onlinelibrary.wiley.com/doi/10.1002/adom.201500504/abstract


Fano resonances in metasurfaces are important due to their low loss subradiant behavior that allows excitation of high-quality (Q) factor resonances extending from the microwave to the optical regime. High-Q Fano resonances have recently enabled applications in the areas of sensing, modulation, filtering, and efficient cavities for lasing spasers. Highly conducting metals are the most commonly used materials for fabricating the metasurfaces, especially at the low-frequency terahertz region where the DC, Drude, and perfect electric conductivity show similar resonant behavior of the subwavelength meta-atoms. Here, it is experimentally and theoretically demontrated that the Q factor of a low asymmetry Fano resonance is extremely sensitive to the conducting properties of the metal at terahertz frequencies. Large differences in the Q factor and figure of merit of the Fano resonance is observed for perfect electric conductors, Drude metal, and a DC-conducting metal, which is in sharp contrast to the behavior of the inductive–capacitive resonance of meta-atoms at terahertz frequency. Identification of such a low asymmetry regime in Fano resonances is the key to engineer the radiative and nonradiative losses in plasmonic and metamaterial-based devices that have potential applications in the microwave, terahertz, infrared, and the optical regimes.