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Carbon nanotubes, quintessentially one-dimensional quantum objects, possess a variety of electrical, optical, and mechanical properties that are suited for developing devices that operate on quantum mechanical principles. The states of one-dimensional electrons, excitons, and phonons in carbon nanotubes with exceptionally large quantization energies are promising for high-operating-temperature quantum devices. Here, we discuss recent progress in the development of carbon-nanotube-based devices for quantum technology, i.e., quantum mechanical strategies for revolutionizing computation, sensing, and communication. We cover fundamental properties of carbon nanotubes, their growth and purification methods, and methodologies for assembling them into architectures of ordered nanotubes that manifest macroscopic quantum properties. Most importantly, recent developments and proposals for quantum information processing devices based on individual and assembled nanotubes are reviewed.

Thin film optical coatings have a wide range of industrial applications from displays and lighting to photovoltaic cells. The realization of electrically tunable thin film optical coatings in the visible wavelength range is particularly important to develop energy efficient and dynamic color filters. Here, we experimentally demonstrate dynamic color generation using electrically tunable thin film optical coatings that consist of two different phase change materials (PCMs). The proposed active thin film nanocavity excites the Fano resonance that results from the coupling of a broadband and a narrowband absorber made up of phase change materials. The Fano resonance is then electrically tuned by structural phase switching of PCM layers to demonstrate active color filters covering the entire visible spectrum. In contrast to existing thin film optical coatings, the developed electrically tunable PCM based Fano resonant thin optical coatings have several advantages in tunable displays and active nanophotonic applications. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) Accepted version The authors acknowledge the funding support from Advanced Manufacturing and Engineering (AME) Programmatic grant (A18A5b0056) by Agency for Science, Technology and Research (A*STAR) and the National Research Foundation Singapore (Award No.: NRF-CRP23-2019-0005).

We experimentally demonstrated an optical controlled terahertz modulator based on the 3D Dirac semimetal Cd3As2 thin film (~100 nm, λ/3000). Utilizing the time-resolved terahertz spectroscopy, the photoactive characters of Cd3As2 thin film are investigated. The measurements reveal that Cd3As2 modulator exhibits a low optical threshold (< 63.5 μJ/cm2) and ultrafast (< 20 ps) broadband (0.2-1.5 THz) modulation of terahertz waves. Moreover, integrated with the metamaterials structures, Cd3As2 could potentially be used as photo-sensitive and ultrafast reconfigurable terahertz resonant plasmonic/meta-device.

Extreme terahertz (THz) science and technologies, the next disruptive frontier in nonlinear optics, provide multifaceted capabilities for exploring strong light-matter interactions in a variety of physical systems. However, current techniques involve the need for an extremely high-field free space THz source that is difficult to generate and has limited investigations to a rather weak and linear regime of light-matter interactions. Therefore, new approaches are being sought for the tight confinement of THz waves that can induce nonlinear effects. Here, a nonlinear “tera-nano” metasurface is demonstrated exhibiting extremely large THz nonlinearity and sensitive self-modulation of resonances at moderate incident THz field strengths. A record deep-subwavelength (≈λ/33 000) confinement of strongly enhanced (≈3200) THz field in a nano-gap (15 nm) exhibits remarkable THz field-tailored nonlinearity. Further, ultrafast injection of photocarriers reveals a competition between nonlinear THz field-induced intervalley scattering and optically driven interband excitations. The results on “tera-nano” metasurfaces enable a novel platform to realize enhanced nonlinear nano/micro composites for field-sensitive extreme THz nonlinear applications without the need for intense THz light sources. Ministry of Education (MOE) This work was supported by the National Natural Science Foundation of China (61905007, 11827807), the National Key R&D Program of China (2019YFB2203102), and the Open Project Program of Wuhan National Laboratory for Optoelectronics No. 2018WNLOKF001, and the Open Fund of Guangdong Provincial Key Laboratory of Information Photonics Technology (Guangdong University of Technology, No. GKPT20). M.M. and R.S. acknowledge research funding support from the Ministry of Education, Singapore (AcRF Tier 1, Grant RG191/17, MOE2017-T2-1-110, and MOE2016-T3-1-006(S)).

Bismuth double perovskite Cs2AgBiBr6 strong electron–acoustic phonon interactions derail its photovoltaic aspirations., Bismuth-based double perovskite Cs2AgBiBr6 is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10−11 cm3 s−1) and low charge carrier mobility (around 0.05 cm2 s−1 V−1). Besides intermediate Fröhlich couplings present in both Pb-based perovskites and Cs2AgBiBr6, we uncover evidence of strong deformation potential by acoustic phonons in the latter through transient reflection, time-resolved terahertz measurements, and density functional theory calculations. The Fröhlich and deformation potentials synergistically lead to ultrafast self-trapping of free carriers forming polarons highly localized on a few units of the lattice within a few picoseconds, which also breaks down the electronic band picture, leading to efficient radiative recombination. The strong self-trapping in Cs2AgBiBr6 could impose intrinsic limitations for its application in photovoltaics.

This thesis discusses the study and implications of near- field interactions mediated by radiant and sub-radiant resonant modes in passive metamaterials (MMs), MEMS actuated active MMs and semiconductor-metamaterial heterostructures at terahertz frequencies. The thesis is divided into two parts, where the fi rst part of the thesis includes fi rst three chapters (Chaps. 2, 3 and 4) that discusses on the phenomena of metamaterial induced transparency and lattice induced transparency in the metamaterial structures at terahertz frequencies. Theoretical analysis using classical coupled oscillators model is proposed to unravel the interaction mechanisms that reveals the radiant and subradiant type of Fano interference effects in the system. These systems show strong slow light effects with large enhancement in the group delay of the pulse through the medium. Further, the implications of the competing electric and magnetic near- field interactions on the transmission characteristics of the medium have shown to exhibit resonant invisibility effects showing an active control of effective permittivity and permeability of the medium. Second part of the thesis (Chap. 5 and Chap. 6) focuses on the demonstration of excitation and active modulation of sharp Fano resonances in a MEMS metamaterial and semiconductor-metamaterial hybrid heterostructure systems. Excitation of Fano resonance in MEMS metamaterial by introducing out-of plane structural asymmetry exhibits anisotropic coupling that results in the multiple-input-output (metahysteresis) characteristics in its near- and far-fi eld optical properties. This exhibits exciting features such as NAND and XOR logical operations, where XOR function can show direct implications in the one-time pad (OTP) secured cryptographic channel for sub-terahertz wireless communications. Further, an active control of Fano resonance in a heterostructure consisting of solution processed CH3NH3PbI3/PbI2 semiconductors spin coated on a metamaterial structure is discussed that exhibits ultrasensitive and ultrafast modulation of Fano resonance in the metamaterial structure, respectively (Chap. 6). Additionally, a new signature of localized plasmon-phonon quasiparticles sensing and interference effects are observed and elucidated using the coupled oscillator model. This phenomenon reveals a strong resonant interactions between the elementary excitations that can be actively controlled by optically pumping the sample using a femtosecond pulse. The thesis concludes by discussing the importance and the future prospectives of the conducted studies. Doctor of Philosophy

The quantum well in 2D perovskite provides an additional channel for photoexcited electrons to relax back at ultrafast time scale., 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.

Metamaterials are well-known for their tunable and unnatural properties that are not usually accessible in natural materials. Recently, there is a large growing interest in the actively tunable metamaterials, where their structural/optical can be actively tuned using an external means, such as optical pulse, thermal and electrical controls. Among them the microelectromechanical systems (MEMS) based metamaterials have given useful features of multiple controls in engineering their structural geometry in all the three-spatial directions of the sample at the THz frequencies. This allows to probe and engineer unique and intriguing near-field coupling phenomena in metamaterials, thereby obtaining the electro-optical properties on demand.

In terahertz (THz) photonics, there is an ongoing effort to develop thin, compact devices such as dielectric photonic crystal (PhC) slabs with desirable light–matter interactions. However, previous works in THz PhC slabs have been limited to rigid substrates with thicknesses ∼ 100 s of micrometers. Dielectric PhC slabs have been shown to possess in-plane modes that are excited by external radiation to produce sharp guided-mode resonances with minimal absorption for applications in sensors, optics, and lasers. Here we confirm the existence of guided resonances in a membrane-type THz PhC slab with subwavelength ( λ 0 / 6 − λ 0 / 12 ) thicknesses of flexible dielectric polyimide films. The transmittance of the guided resonances was measured for different structural parameters of the unit cell. Furthermore, we exploited the flexibility of the samples to modulate the guided modes for a bend angle of θ ≥ 5 ∘ , confirmed experimentally by the suppression of these modes. The mechanical flexibility of the device allows for an additional degree of freedom in system design for high-speed communications, soft wearable photonics, and implantable medical devices.

We experimentally show the excitation of sharp Fano resonances in a MEMS reconfigurable metasurface exhibiting multiple-input-output states in its electro-optical properties. Further, a set of composite logic gates such as exclusive-OR and XNOR are readout using THz pulse.

Solution‐processed lead iodide (PbI2) governs the charge transport characteristics in the hybrid metal halide perovskites. Besides being a precursor in enhancing the performance of perovskite solar cells, PbI2 alone offers remarkable optical and ultrasensitive photoresponsive properties that remain largely unexplored. Here, the photophysics and the ultrafast carrier dynamics of the solution processed PbI2 thin film is probed experimentally. A PbI2 integrated metamaterial photonic device with switchable picosecond time response at extremely low photoexcitation fluences is demonstrated. Further, findings show strongly confined terahertz field induced tailoring of sensitivity and switching time of the metamaterial resonances for different thicknesses of PbI2 thin film. The approach has two far reaching consequences: the first lead‐iodide‐based ultrafast photonic device and resonantly confined electromagnetic field tailored transient nonequilibrium dynamics of PbI2 which could also be applied to a broad range of semiconductors for designing on‐chip, ultrafast, all‐optical switchable photonic devices. MOE (Min. of Education, S’pore) Accepted version

We show exciton mediated ultrafast feature in 2D perovskite arises due to quantum confinement of free carriers. Integrating it with metamaterials exhibit 93 % modulation of terahertz field at ultrafast time scales (~ 20 ps).

1 wileyonlinelibrary.com C o m m u n iC a io n and their possible applications. The ultimate form of active manipulation of EIT phenomenon will be when all three primary parameters are controlled independently. The independent control of individual resonators demands for the controllability at unit cell level, and conventional approaches such as optical pumping of photoconductive elements or thermally controlled superconductor are restricted to provide only global control. Recently, microelectromechanical systems (MEMS) based tunable metamaterials have been reported to achieve controllability at unit cell level, along with the added advantage of being electrically controlled, miniaturized size and enhanced electrooptic performance. The versatility of MEMS design has enabled active manipulation of numerous THz properties such as magnetic resonance,[19–22] electrical resonance,[23–25] anisotropy,[26] broadband response,[27] isotropic resonance switching[28] multiresonance switching,[29–31] and coupling strength between resonators.[32] The enhanced controllability and direct integration of MEMS actuators into metamaterial unit cell geometry is an ideal fit for the realization of selective control of coupled mode resonators. In this Communication, reconfigurable metamaterial with independently controlled bright and dark mode resonators is proposed for advanced manipulation of the classical analog of EIT and slow light effects in THz spectral region. The active control of bright mode resonator enables modulation of EIT intensity, while the tuning of dark mode resonance causes the EIT peak to tune in frequency. Furthermore, simultaneous switching of bright and dark mode resonators results in dynamic switching of the system between coupled and uncoupled states. The proposed approach of selective reconfiguration can be scaled for multiresonator systems, which can be coupled either through inductive, capacitive, or conductive means. The metamaterial consists of 80 × 80 periodic array of cut wire resonator (CWR) with closely placed split ring resonators (SRRs), as shown in Figure 1 and Figure 2. The periodicity of unit cell is 100 μm along both axial directions. The CWR has length, lC = 60 μm and width, wC = 5 μm, respectively. The SRRs have a base length, bS = 30 μm, side length, lS = 20 μm, and split gap, gS = 4 μm. The SRRs are placed at a distance of S = 2 μm from the CWR. When the polarization of the excitation field is along the CWR arm, the dipole mode resonance of the CWR will be the bright mode and the inductive-capacitive (LC) mode of SRR resonance acts as the dark mode. Thus for the incident THz polarization, the direct excitation of the bright mode induces image charges on the nearby SRRs through nearfield inductive coupling, thereby exciting the LC resonance of the SRRs. These bright-dark resonances have contrasting line widths with identical resonance frequencies and under a strong coupling regime they experience an EIT-type of interference that gives rise to a sharp transmission peak. Thus, through Active Control of Electromagnetically Induced Transparency Analog in Terahertz MEMS Metamaterial

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.

Incorporating semiconductors as active media into metamaterials offers opportunities for a wide range of dynamically switchable/tunable, technologically relevant optical functionalities enabled by strong, resonant light–matter interactions within the semiconductor. Here, a germanium‐thin‐film‐based flexible metaphotonic device for ultrafast optical switching of terahertz radiation is experimentally demonstrated. A resonant transmission modulation depth of 90% is achieved, with an ultrafast full recovery time of 17 ps. An observed sub‐picosecond decay constant of 670 fs is attributed to the presence of trap‐assisted recombination sites in the thermally evaporated germanium film. MOE (Min. of Education, S’pore) Accepted version

The mechanism of Cooper pair formation and its underlying physics has long occupied the investigation into high temperature (high-Tc ) cuprate superconductors. One of the ways to unravel this is to observe the ultrafast response present in the charge carrier dynamics of a photoexcited specimen. This results in an interesting approach to exploit the dissipation-less dynamic features of superconductors to be utilized for designing high-performance active subwavelength photonic devices with extremely low-loss operation. Here, dual-channel, ultrafast, all-optical switching and modulation between the resistive and the superconducting quantum mechanical phase is experimentally demonstrated. The ultrafast phase switching is demonstrated via modulation of sharp Fano resonance of a high-Tc yttrium barium copper oxide (YBCO) superconducting metamaterial device. Upon photoexcitation by femtosecond light pulses, the ultrasensitive cuprate superconductor undergoes dual dissociation-relaxation dynamics, with restoration of superconductivity within a cycle, and thereby establishes the existence of dual switching windows within a timescale of 80 ps. Pathways are explored to engineer the secondary dissociation channel which provides unprecedented control over the switching speed. Most importantly, the results envision new ways to accomplish low-loss, ultrafast, and ultrasensitive dual-channel switching applications that are inaccessible through conventional metallic and dielectric based metamaterials. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Strongly coupled metamaterial resonances typically undergo mode-splitting by which there is an exchange of energy between matter excitations and photons. Here, we report a strong coupling of the lattice mode with the structural eigen-resonances of an asymmetric split-ring metamaterial associated with mode-splitting and resonance line-narrowing that gives rise to high quality factor (Q-factor) resonances. We demonstrate selective control of the resonance strength, line-width, and Q-factor of individual split-ring modes by tailoring the coupling of the fundamental lattice mode to each of the hybridized resonances. A three-coupled-oscillator model shows lattice-mediated strong coupling in the form of an anti-crossing behavior between the hybridized metamaterial resonances. Such schemes of strong coupling between the lattice and the hybrid modes of the metamaterial unit cell offer an avenue to invoke lattice induced transparency, high-Q resonances and strong field confinement, which could find applications in designing slow light devices, ultrasensitive sensors, and multiband narrow filters. National Research Foundation (NRF) Published version The authors acknowledge research funding support from the Singapore National Research Foundation (NRF), the French National Research Agency (ANR, Grant No. NRF2016-NRF-ANR004), and the UK's Engineering and Physical Sciences Research Council (EPSRC, Grant No. EP/M009122/1).

Advances in plasmonic metamaterials have been rapidly evolving with innovations aimed at developing metadevices for real-world applications. In reality, energy losses in plasmonic systems are prevalent and it is of paramount importance to come up with solutions that could overcome the limitations that impede further advancements toward the miniaturization of optoelectronic metadevices. High-Q Fano resonance as a scattering phenomenon can be easily triggered by introducing asymmetry into plasmonic systems, and thus it offers a simple approach for reducing radiative losses through lineshape engineering. High-Q Fano resonance possesses narrow linewidth and intensely confined electromagnetic fields, which makes it viable for widespread applications. The purpose of this review is to consolidate the current advances and contributions that high-Q Fano resonance has made in the metamaterial community. Two general modes of energy loss including radiative and nonradiative losses are introduced and possible ways to overcome these challenges are examined. Furthermore, applications based on high-Q Fano resonance including sensors, lasing spasers, and optical switches are discussed, embracing the future of Fano resonance based high performance photonic technologies. MOE (Min. of Education, S’pore) Accepted version

Localized electromagnetic excitation in the form of toroidal dipoles has recently been observed in metamaterial systems. The origin of the toroidal dipole lies in the currents flowing on the surface of a torus. Thus, the exotic toroidal excitations play an important role in determining the optical properties of a system. Toroidal dipoles also contribute towards enabling high quality factor subwavelength resonances in metamaterial systems which could be an excellent platform for probing the light matter interaction. Here, we demonstrate sensing with toroidal resonance in a two-dimensional terahertz metamaterial in which a pair of mirrored asymmetric Fano resonators possesses anti-aligned magnetic moments at an electromagnetic resonance that gives rise to a toroidal dipole. Our proof of concept demonstration opens up an avenue to explore the interaction of matter with toroidal multipoles that could have strong applications in the sensing of dielectrics and biomolecules. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

Electromagnetically induced transparency (EIT) arises from coupling between the bright and dark mode resonances that typically involve subwavelength structures with broken symmetry, which results in an extremely sharp transparency band. Here, we demonstrate a tunable broadband EIT effect in a symmetry preserved metamaterial structure at the terahertz frequencies. Alongside, we also envisage a photo-active EIT effect in a hybrid metal-semiconductor metamaterial, where the transparency window can be dynamically switched by shining near-infrared light beam. A robust coupled oscillator model explains the coupling mechanism in the proposed design, which shows a good agreement with the observed results on tunable broadband transparency effect. Such active, switchable, and broadband metadevices could have applications in delay bandwidth management, terahertz filtering, and slow light effects. MOE (Min. of Education, S’pore) Published version

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. MOE (Min. of Education, S’pore) Accepted version

We experimentally report the bidirectional reconfiguration of an out-of-plane deformable microcantilever based metamaterial for advanced and dynamic manipulation of terahertz waves. The microcantilever is made of a bimaterial stack with a large difference in the coefficient of thermal expansion of the constituent materials. This allows for the continuous deformation of microcantilevers in upward or downward direction in response to positive or negative temperature gradient, respectively. The fundamental resonance frequency of the fabricated microcantilever metamaterial is measured at 0.4 THz at room temperature of 293 K. With decreasing temperature, the resonance frequency continuously blue shifts by 30 GHz at 77 K. On the other hand, with increasing temperature, the resonance frequency gradually red shifts by 80 GHz and saturates at 0.32 THz for 400 K. Furthermore, as the temperature is increased above room temperature, which results in the downward actuation of the microcantilever, a significant resonance line-narrowing with an enhanced quality factor is observed due to tight field confinement in the metamaterial structure. The thermal control of the microcantilever possesses numerous inherent advantages such as enhanced tunable range (∼37.5% in this work compared to previously reported microcantilever metamaterials), continuous tunability, and repeatable operations. The microcantilever metamaterial also shows high robustness to operate at cryogenic conditions and hence opens up the possibility of using meta-devices in harsh environments such as space, polar, and deep sea applications. Published version

Classical analogue of electromagnetically induced transparency mediated by near field coupling in meta-atom resonators form the basis for design of slow light metamaterial devices. Here, we experimentally demonstrate the active control of individual resonators in the near-field coupled system. This allows for active modulation and spectral tuning of the transparency peak and hence the slow light behavior in terahertz spectral range.

Metamaterials operating in terahertz (THz) region is an attractive platform for the detection of micro-sized particles. However, only certain location in the metamaterial pattern offers maximum sensitivity. In this work, microfluidic structure was integrated with THz metamaterial in order to precisely handle the position of particles. This enables sensitive and effective detection of microorganism detection in the THz spectral range.

We report the passive as well as active modulation of the sensitive Fano resonances in terahertz asymmetric metamaterial structures. The strength of the Fano resonances can be completely switched off passively by changing the asymmetry of the structures and also actively by shininh the sample with very low pump powers. These devices can be used as ultrasensitive sensors, low threshold modulators and ultrafast switches in real world device based applications.

2D micro‐nanostructured metal films with hole arrays show promising features such as the extraordinary transmission of light. Such systems are interesting in the field of subwavelength photonics and nonlinear optics due to their high field confinement in addition to their inherent spectral scalability and frequency selective response. Several active schemes to control the extraordinary transmission are recently demonstrated. However, these dynamic devices do not reveal any obvious color‐dependent modulation of the resonant transmission behavior. Here, color‐sensitive ultrafast modulation of extraordinary resonant transmission of terahertz (THz) waves through 2D metallic hole arrays is demonstrated. Pumping the silicon beneath the metallic array with light of different colors and identical fluences exhibit significantly different ultrafast switching dynamics and modulation. The color‐dependent sensitivity and control of THz waves at an ultrafast timescale provide an extra degree of freedom that opens up new opportunities for future applications in active subwavelength optics, optoelectronics, and all‐optical switching of THz photonic devices. MOE (Min. of Education, S’pore) Accepted version

We report experimental results of the active switching of electromagnetically induced transparency (EIT) analogue by controlling the dark mode excitation pathways in a microelectromechanical system based tri-atomic metamolecule, operating in the terahertz spectral region. The tri-atomic metamolecule consists of two bright cut wire resonators (CWRs) on either side of the dark split ring resonators (SRRs). Each of the CWRs can independently excite the dark inductive-capacitive resonance mode of the SRRs through inductive coupling, and this allows for the dual pathways of dark mode excitation. The CWRs are made movable along the out-of-plane direction and electrically isolated to achieve selective reconfiguration. Hence, by controlling the physical position of these CWRs, the excitation pathways can be actively reconfigured. This enables the strong excitation of EIT analogue at 0.65 THz, only when one of the pathways is made accessible. Moreover, the transparency peak is completely modulated when both pathways are made either inaccessible or equally accessible. The proposed approach of realizing independent control of constituent resonators in a multi-resonator coupled system, enables the realization of efficient slow light devices and tunable high-Q resonators in terahertz spectral region. NRF (Natl Research Foundation, S’pore) Published version

Lattice modes are intrinsic to periodic structures and they can be easily tuned and controlled by changing the lattice constant of the structural array. Previous studies have revealed the excitation of sharp absorption resonances due to lattice mode coupling with the plasmonic resonances. Here, we report an experimental observation of a lattice-induced transparency (LIT) by coupling the first-order lattice mode (FOLM) to the structural resonance of a terahertz asymmetric split ring resonator. The observed sharp transparency is a result of the destructive interference between the bright mode and the FOLM assisted dark mode. As the FOLM is swept across the metamaterial resonance, the transparency band undergoes a large change in its bandwidth and resonance position. We propose a three-oscillator model to explain the underlying coupling mechanism in LIT system that shows good agreement with the observed results. Besides controlling the transparency behavior, LIT also shows a huge enhancement in its Q factor and exhibits a high group delay of 28 ps with an enhanced group index of 4.5 x 104, which could be pivotal in ultrasensitive sensing and slow-light device applications. MOE (Min. of Education, S’pore) Published version

We study the lensing effect experienced by a weak probe field under conditions of electromagnetically induced transparency (EIT) involving a Rydberg state. A Gaussian coupling beam tightly focused on a laser-cooled atomic cloud produces an inhomogeneity in the coupling Rabi frequency along the transverse direction and makes the EIT area acting like a gradient-index medium. We image the probe beam at the position where it exits the atomic cloud, and observe that a red-detuned probe light is strongly focused with a greatly enhanced intensity whereas a blue-detuned one is de-focused with a reduced intensity. Our experimental results agree very well with the numerical solutions of Maxwell-Bloch equations., 7 pages, 4 figures

We experimentally study the effect of near field coupling on the transmission of light in terahertz metasurfaces. Our results show that tailoring the coupling between the resonators modulates the amplitude of resulting electromagnetically induced transmission, probed under different types of asymmetries in the coupled system. Observed change in the transmission amplitude is attributed to the change in the amount of destructive interference between the resonators in the vicinity of strong near field coupling. We employ a two-particle model to theoretically study the influence of the coupling between bright and quasi-dark modes on the transmission properties of the system and we find an excellent agreement with our observed results. Adding to the enhanced transmission characteristics, our results provide a deeper insight into the metamaterial analogues of atomic electromagnetically induced transparency and offer an approach to engineer slow light devices, broadband filters, and attenuators at terahertz frequencies. Published version

In recent years, the stunning performance of transition metal dichalcogenides (TMDCs) has been utilized in the area of field effect transistors, integrated circuits, photodetectors, light generation and harvesting, valleytronics, and van der Waals (vdW) heterostructures. However, the optoelectronic application of TMDCs in realizing efficient, ultrafast metaphotonic devices in the terahertz part of the electromagnetic spectrum has remained unexplored. The most studied member of the TMDC family, i.e., MoS2, shows an ultrafast carrier relaxation after photoexcitation with near-infrared femtosecond pulse of energy above the bandgap. Here, this study investigates the photoactive properties of MoS2 to demonstrate an ultrasensitive active switching and modulation of the sharp Fano resonances in MoS2-coated metamaterials consisting of asymmetric split ring resonator arrays. The results show that all-optical switching and modulation of micrometer scale subwavelength Fano resonators can be achieved on a timescale of hundred picoseconds at moderate excitation pump fluences. The precise and active control of the MoS2-based hybrid metaphotonic devices open up opportunities for the real-world technologies and realization of ultrafast switchable sensors, modulators, filters, and nonlinear devices.

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-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. MOE (Min. of Education, S’pore) Accepted version

We study phase-sensitive amplification of electromagnetically induced transparency in a warm $^{85}\mathrm{Rb}$ vapor wherein a microwave driving field couples the two lower-energy states of a $\ensuremath{\Lambda}$ energy-level system thereby transforming into a $\ensuremath{\Delta}$ system. Our theoretical description includes effects of ground-state coherence decay and temperature effects. In particular, we demonstrate that driving-field-enhanced electromagnetically induced transparency is robust against significant loss of coherence between ground states. We also show that for specific field intensities, a threshold rate of ground-state coherence decay exists at every temperature. This threshold separates the probe-transmittance behavior into two regimes: probe amplification vs probe attenuation. Thus, electromagnetically induced transparency plus amplification is possible at any temperature in a $\ensuremath{\Delta}$ system.

We experimentally demonstrate the integration of a microfluidic trap array on top of metamaterial resonators for size selective trapping and remote sensing of microparticles. A split-ring resonator (SRR) design supports strongly confined electric field in the capacitive split gap at the fundamental inductive-capacitive resonance mode. The tightly confined electric field in the SRR gap forms a hot-spot that has become an enabling platform for sensing applications. Here, we extend the concept of metamaterial sensing to “trapping and sensing” by fabricating trapezoidal shaped structures near the split gap that enables trapping of microparticles in the split-gap region of each SRR. The proposed microfluidic metamaterial sensor enables sensing of different refractive index microparticles in terms of change in the transmitted amplitude and resonance frequency of the fundamental resonance mode operating in the terahertz spectral region. The proposed approach exploits the advantages offered by microfluidics, metamaterials, and terahertz technologies to form an ideal platform for ultra-sensitive, label-free, remote, and non-destructive detection of micro-substances. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

Fano resonances offer exciting features in enhancing the non-linearity and sensing capabilities in metamaterial systems. An active photoswitching of Fano resonances in a terahertz metadevice at low optical pump powers is demonstrated, which signifies the extreme sensitivity of the high-quality-factor resonant electric field to the external light illumination. MOE (Min. of Education, S’pore) Accepted version

We experimentally report a structurally reconfigurable metamaterial for active switching of near-field coupling in conductively coupled, orthogonally twisted split ring resonators (SRRs) operating in the terahertz spectral region. Out-of-plane reconfigurable microcantilevers integrated into the dark SRR geometry are used to provide active frequency tuning of dark SRR resonance. The geometrical parameters of individual SRRs are designed to have identical inductive-capacitive resonant frequency. This allows for the excitation of classical analogue of electromagnetically induced transparency (EIT) due to the strong conductive coupling between the SRRs. When the microcantilevers are curved up, the resonant frequency of dark SRR blue-shifts and the EIT peak is completely modulated while the SRRs are still conductively connected. EIT modulation contrast of ∼50% is experimentally achieved with actively switchable group delay of ∼2.5 ps. Electrical control, miniaturized size, and readily integrable fabrication process of the proposed structurally reconfigurable metamaterial make it an ideal candidate for the realization of various terahertz communication devices such as electrically controllable terahertz delay lines, buffers, and tunable data-rate channels. MOE (Min. of Education, S’pore) Published version

We experimentally observe a non-linear absorption resonance in one of the two probe beams of a double Λ system which is interacting with three optical beams. This probe beam counterpropagates to the other two beams, both of which are co-propagating. The absorption resonance in this probe beam arises due to interaction between the dark states of the double Λ created by the co-propagating beams. The double Λ configuration of levels is in the D2 hyperfine manifold of 87Rb atoms, and the interaction is studied at room temperature. The non-linear absorption resonance is shown to be a three-photon process arising whenever the two co-propagating beams satisfy the two-photon Raman condition, necessary for electromagnetically induced transparency. Through a dressed state analysis, we show that the absorption feature is a result of interference between the absorption pathways to the excited states, from the ground states of the double Λ. The non-linear nature of the probe beam absorption enables it to perform like a directional optical switch. This absorption switch operates conditioned on non-absorption of a single photon of the same frequency due to a coherent process, in a beam traveling in the opposite direction.