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Metasurfaces with a strongly enhanced local field are envisioned as a powerful platform for ultrasensitive optical sensors to significantly amplify imperceptible differences between compatible bioanalytes. Through the use of phototunable silicon-based terahertz (THz) metasurfaces, we experimentally demonstrate ultrafast switchable sensing functions. It is found that the THz responses of the coupled-resonances in the metasurfaces shift from Lorentz-lattice mode to electromagnetism-induced transparency (EIT) mode under optical pumping within an ultrashort time of 32 ps, enabling an ultrafast sensitive sensor. For the Lorentz-lattice mode, the THz time-domain signal directly shows a highly sensitive response to detect tiny analytes without extra Fourier transformation as the mismatch between the two modes increases. Once the metasurfaces are switched to the EIT mode, the silicon-metal hybrid structure supports frequency-domain sensing ability due to strong field confinement with a sensitivity of 118.4 GHz/RIU. Both of the sensing configurations contribute to more subtle information and guarantee the accuracy of the sensor performance. Combined with the aforementioned advantages, the proposed metasurfaces have successfully identified colorectal cells between normal, adenoma, and cancer states in experiments. This work furnishes a new paradigm of constructing reliable and flexible metasurface sensors and can be extended to other optics applications.

Topological photonic crystals with robust pseudo-spin and valley edge states have shown promising and wide applications in topological waveguides, lasers, and antennas. However, the limited bandwidth and intrinsic coupling properties of a single pseudo-spin or valley edge state have imposed restrictions on their multifunctional applications in integrated photonic circuits. Here, we propose a topological photonic crystal that can support pseudo-spin and valley edge states simultaneously in a single waveguiding channel, which effectively broadens the bandwidth and enables a multipath routing solution for terahertz information processing and broadcasting. We show that distorted Kekulé lattices can open two types of bandgaps with different topological properties simultaneously by molding the inter- and intra-unit cell coupling of the tight-binding model. The distinct topological origins of the edge states provide versatile signal routing paths toward free space radiation or on-chip self-localized edge modes by virtue of their intrinsic coupling properties. Such a powerful platform could function as an integrated photonic chip with capabilities of broadband on-chip signal processing and distributions that will especially benefit terahertz wireless communications.

Losses are ubiquitous and unavoidable in nature inhibiting the performance of most optical processes. Manipulating losses to adjust the dissipation of photons is analogous to braking a running car that is as important as populating photons via a gain medium. Here, we introduce the transient loss boundary into a photon populated cavity that functions as a ‘photon brake’ and probe photon dynamics by engineering the ‘brake timing’ and ‘brake strength’. Coupled cavity photons can be distinguished by stripping one photonic mode through controlling the loss boundary, which enables the transition from a coupled to an uncoupled state. We interpret the transient boundary as a perturbation by considering both real and imaginary parts of permittivity, and the dynamic process is modeled with a temporal two-dipole oscillator: one with the natural resonant polarization and the other with a frequency-shift polarization. The model unravels the underlying mechanism of concomitant coherent spectral oscillations and generation of tone-tuning cavity photons in the braking process. By synthesizing the temporal loss boundary into a photon populated cavity, a plethora of interesting phenomena and applications are envisioned such as the observation of quantum squeezed states, low-loss nonreciprocal waveguides and ultrafast beam scanning devices., Manipulating photon dynamics is becoming increasingly important especially for quantum optics applications. In this work, the authors provide a mechanism that functions as a ‘photon brake’ which relies on the concept of temporal loss boundary to manipulate the dynamics of cavity photons.

Bound state in the continuum (BIC) refers to the trapped state in the radiation continuum of a system. In the terahertz band, BIC provides a unique and feasible method to design devices with ultra-high quality factor (Q factor) and to achieve intense terahertz-matter interaction, which is of great value to terahertz science and technology. Here, multiple BICs protected by the resonance symmetry in the terahertz metasurface consisting of metallic split ring resonators (SRR) is demonstrated. The evolution from the BIC to the quasi-BIC (QBIC) is induced by changing the gap width of the SRRs. The proposed BICs are experimentally demonstrated and analyzed by the coupled mode theory along with the numerical simulation. It is found that the leakage behavior of these QBICs is strongly affected by the intrinsic Ohmic loss in the SRRs while it is quite robust to the tilted incidence.

Miniaturized ultrafast switchable optical components with an extremely compact size and a high-speed response will be the core of next-generation all-optical devices instead of traditional integrated circuits, which are approaching the bottleneck of Moore’s Law. Metasurfaces have emerged as fascinating subwavelength flat optical components and devices for light focusing and holography applications. However, these devices exhibit a severe limitation due to their natural passive response. Here we introduce an active hybrid metasurface integrated with patterned semiconductor inclusions for all-optical active control of terahertz waves. Ultrafast modulation of polarization states and the beam splitting ratio are experimentally demonstrated on a time scale of 667 ps. This scheme of hybrid metasurfaces could also be extended to the design of various free-space all-optical active devices, such as varifocal planar lenses, switchable vector beam generators, and components for holography in ultrafast imaging, display, and high-fidelity terahertz wireless communication systems., Active metasurfaces: Terahertz control All-optical control and manipulation of terahertz waves is now possible thanks to the development of custom-designed, dynamic reconfigurable metasurfaces. Realized by Longqing Cong and coworkers from Nanyang Technological University in Singapore and Tianjin University in China, the metasurfaces enable ultrafast (sub-nanosecond) polarization switching and beam splitting. They consist of an array of miniature aluminum-coated silicon split-ring resonators on a sapphire substrate. Illumination with short pulses of an infrared “pump beam” causes charge carriers in the silicon patch of the microring to transition from the valence to the conduction band, temporarily changing the photoconductivity and thus switching the transmission of the metasurface between “off” and “on” states for a polarized terahertz wave. Such high-speed switching of polarization is expected to be useful for applications such as data-encoding and multiplexing in a terahertz communications system as well as holography.

Metasurface-empowered bound state in the continuum (BIC) provides a unique route for fascinating functional devices with infinitely high quality factors. This method is particularly attractive to the terahertz community because it may essentially solve the deficiencies in terahertz filters, sensors, lasers, and nonlinear sources. However, most BIC metasurfaces are limited to specified incident angles that seriously dim their application prospects. Here, we propose that a dual-period dielectric metagrating can support multiple families of BICs that originate from guided mode resonances in the dielectric grating and exhibit infinite quality factors at arbitrarily tilted incidence. This robustness was analyzed based on the Bloch theory and verified at tilted incident angles. We also demonstrate that inducing geometric asymmetry is an efficient way to manipulate the leakage and coupling of these BICs, which can mimic the electromagnetically induced transparency (EIT) effect in our dual-period metagrating. In this demonstration, a slow-light effect with a measured group delay of 117 ps was achieved. The incidence-insensitive BICs proposed here may greatly extend the application scenarios of the BIC effect. The high Q factor and outstanding slow-light effect in the metagrating show exciting prospects in realizing high-performance filters, sensors, and modulators for prompting terahertz applications.

Solution-processed organic–inorganic hybrid halide perovskite has emerged as an excellent material for harnessing solar energy. The ease of processing offers a fairly straightforward approach to integrate methylammonium lead iodide perovskite with diverse technological platforms. Metamaterials, with an array of artificially designed meta-atoms and extraordinary electromagnetic properties, have enabled several functionalities in the passive mode, especially for flexible photonic devices, which have found application in the areas of flexible displays and wearable sensors. However, it is extremely challenging to induce a dynamic response in flexible photonic devices. Here, we experimentally demonstrate that the solution-processed perovskite with an intrinsic ultrafast response could be an ideal platform for developing active flexible photonic devices. A thin perovskite layer on a flexible metadevice enables an ultrafast all-optical switching of Fano resonance with a time constant of 500 ps. The hybrid flexib...

The remarkable emergence of all‐dielectric meta‐photonics governed by the physics of high‐index dielectric materials offers a low‐loss platform for efficient manipulation and subwavelength control of electromagnetic waves from microwaves to visible frequencies. Dielectric metasurfaces can focus electromagnetic waves, generate structured beams and vortices, enhance local fields for advanced sensing, and provide novel functionalities for classical and quantum technologies. Recent advances in meta‐photonics are associated with the exploration of exotic electromagnetic modes called the bound states in the continuum (BICs), which offer a simple interference mechanism to achieve large quality factors (Q) through excitation of supercavity modes in dielectric nanostructures and resonant metasurfaces. Here, a BIC‐driven terahertz metasurface with dynamic control of high‐Q silicon supercavities that are reconfigurable at a nanosecond timescale is experimentally demonstrated. It is revealed that such supercavities enable low‐power, optically induced terahertz switching and modulation of sharp resonances for potential applications in lasing, mode multiplexing, and biosensing. MOE (Min. of Education, S’pore) Accepted version

Recent advances in graphene photonics reveal promising applications in the technologically important terahertz spectrum, where graphene-based active terahertz metamaterial modulators have been experimentally demonstrated. However, the sensitivity of the atomically thin graphene monolayer towards sharp Fano resonant terahertz metasurfaces remains unexplored. Here, we demonstrate thin-film sensing of the graphene monolayer with a high quality factor terahertz Fano resonance in metasurfaces consisting of a two-dimensional array of asymmetric resonators. A drastic change in the transmission amplitude of the Fano resonance was observed due to strong interactions between the monolayer graphene and the tightly confined electric fields in the capacitive gaps of the Fano resonator. The deep-subwavelength sensing of the atomically thin monolayer graphene further highlights the extreme sensitivity of the resonant electric field excited at the dark Fano resonance, allowing the detection of an analyte that is λ/1 000 000 thinner than the free space wavelength.

A microelectromechanically reconfigurable digital metamaterial for dynamic switching of terahertz anisotropy is experimentally demonstrated. Out-of-plane reconfigurable microcantilevers placed in orthogonal direction with isolated control allows active making and breaking of unit cell symmetry. The electrical control, miniaturized size, and ease of process integration make the reported microelectromechanical system metamaterial ideal for the realization of THz polarimetric devices. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

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.

Next-generation devices for low-latency and seamless communication are envisioned to revolutionize information processing, which would directly impact human lives, technologies, and societies. The ever-increasing demand for wireless data traffic can be fulfilled by the terahertz band, which has received tremendous attention as the final frontier of the radio spectrum. However, attenuation due to atmospheric humidity and free-space path loss significantly limits terahertz signal propagation. High-gain antennas with directional radiation and reconfigurable beam steering are indispensable for loss compensation and terahertz signal processing, which are associated with spatial and temporal dimensions, respectively. Here, experimental demonstration of a spatiotemporal dielectric metasurface for unidirectional propagation and ultrafast spatial beam steering of terahertz waves is shown. The spatial dimension of the metasurface provides a solution to eliminate backscattering of collimated unidirectional propagation of the terahertz wave with steerable directionality. Temporal modulation of the spatial optical properties enables ultrafast reconfigurable beam steering. Silicon-based spatiotemporal devices amalgamate the rich physics of metasurfaces and technologies that are promising for overcoming the bottlenecks of future terahertz communication, such as high-speed and secure wireless data transmission, beamforming and ultrafast data processing. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) The authors 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)) and the funding support from Advanced Manufacturing and Engineering (AME) Programmatic grant (A18A5b0056) by Agency for Science, Technology and Research (A*STAR).

The interaction between microscopic particles has always been a fascinating and intriguing area of science. Direct interrogation of such interactions is often difficult. Structured electromagnetic systems offer a rich toolkit for mimicking and reproducing the key dynamics that governs the microscopic interactions, and thus provide an avenue to explore and interpret the microscopic phenomena. In particular, metamaterials offer the freedom to artificially tailor light-matter coupling and to control the interaction between unit cells in the metamaterial array. Here we demonstrate a terahertz metamaterial that mimics spin-related interactions of microscopic particles in a 2D lattice via complex electromagnetic multipoles scattered within the metamaterial array. Fano resonances featured by distinct mode properties due to strong nearest-neighbor interactions are discussed that draw parallels with the 2D Ising model. Interestingly, a phase transition from single Fano resonance to hyperfine splitting of Fano spectrum is observed by manipulating the 2D interactions without applying external magnetic or electric fields, which provides a potential multispectral platform for applications in super-resolution imaging, biosensing, and selective thermal emission. The dynamic approach to reproduce static interaction between microscopic particles would enable more profound significance in exploring the unknown physical world by the macroscopic analogues.

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

We report the observation of extremely high-quality factors in all-dielectric subwavelength terahertz metasurfaces. We further demonstrate an active ultrafast switching of the high-Q resonances via direct optical excitation of the supercavities.

Controlling the phase of local radiation by using exotic metasurfaces has enabled promising applications in a diversified set of electromagnetic wave manipulation such as anomalous wavefront deflection, flat lenses, and holograms. Here, we theoretically and experimentally demonstrate an active phase transition in a micro-electromechanical system-based metadevice where both the phase response and the dispersion of the metamaterial cavity are dynamically tailored. The phase transition is determined by the radiative and the absorptive losses in a metal-insulator-metal cavity that obeys the coupled-mode theory. The complete understanding of the phase diagram in a reconfigurable configuration would open up avenues for designing multifunctional metadevices that can be actively switched between different phases leading to a plethora of applications in polarization control, beam deflectors, and holographic metamaterials.

We investigate the dominant role of toroidal dipole in modulating the Fano resonance in planar terahertz metasurfaces by reducing the radiative loss from electric dipole.

Metamaterials offer exciting opportunities that enable precise control of light propagation, its intensity and phase by designing an artificial medium of choice. Inducing birefringence via engineered metamolecules presents a fascinating mechanism to manipulate the phase of electromagnetic waves and facilitates the design of polarimetric devices. In this paper, a high-efficiency, broadband, tunable and flexible quarter-wave plate based on a multilayer metamaterial is presented. Excellent achromatic π/2 phase retardance with high transmission is observed upon terahertz propagation through the quarter-wave plate. The calculated Stokes parameter represents the output polarization state numerically, indicating an excellent broadband conversion of linearly polarized light into circularly polarized light. The metamaterial-based quarter-wave plate demonstrated in this work could be an important step forward in the development of functional terahertz polarization conversion devices for practical applications.

The stress-optic law is fundamental to photoelasticity, and has been a prevalent method for measuring stress. However, its effectiveness in the terahertz (THz) regime has not been completely characterized. In this paper, the effects of uniaxial tensile stress on THz transmission through a polytetrafluoroethylene (PTFE) sample are investigated. Specifically, THz time domain spectroscopy (THz-TDS) has been used to measure the phase delay of transmitted THz radiation for different PTFE stress states. Differences in the refractive index of the PTFE are calculated from the measured phase delay. The calculated refractive index grows linearly with the applied stress, which can be considered a verification of the uniaxial stress-optic law in the terahertz frequency regime.

Bound state in the continuum (BIC) is a mathematical concept with an infinite radiative quality factor (Q) that exists only in an ideal infinite array of resonators. In photonics, it is essential to achieve high Q resonances for enhanced light-mater interactions that could enable low-threshold lasers, ultrasensitive sensors, and optical tweezers. Hence, it is important to explore BICs in different photonic systems including subwavelength metamaterials where symmetry-protected dual BICs exist. The spectral features of dual BICs are experimentally verified in the terahertz domain by breaking the C2 symmetry that invokes a leakage channel in the form of weakly radiating Fano resonance and electromagnetically induced transparency. The radiative Q factors tend to infinity at discrete symmetry-restoring points and obey an inverse square dependence on the structural asymmetry. BICs in metamaterials allow extreme field confinement with small mode volumes, thereby improving the rate of spontaneous emission in the cavity with much larger Purcell factor. In addition, the topological nature enables a robust existence of BICs with a vector beam profile that is ideal for lasing. The symmetry-protected BICs in metamaterials also possess a unique advantage of scalability at different wavelengths for potential applications in sensing, lasing, switching, and spectral filtering.

A novel scheme of dispersion-free metamaterial is proposed to control the polarization states of terahertz waves in a broadband regime. The artificially engineered phase delay is tunable with the space-variant P-B phase.

Toroidal multipoles, which along with the familiar electric and magnetic multipoles is necessary for the complete multipole representation of an arbitrary radiating source. We have designed a planar metasurface with resonant response in the terahertz range, which is easy to fabricate and exhibits the signature of a toroidal excitation.

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.

A toroidal dipole in metasurfaces provides an alternate approach for the excitation of high-Q resonances. In contrast to conventional multipoles, the toroidal dipole interaction strength depends on the time derivative of the surrounding electric field. A characteristic feature of toroidal dipoles is tightly confined loops of oscillating magnetic field that curl around the fictitious arrow of the toroidal dipole vector.

Metamaterials have recently enabled coupling induced transparency due to interference effects in coupled subwavelength resonators. In this work, we present a three dimensional (3-D) metamaterial design with six-fold rotational symmetry that shows electromagnetically induced transparency with a strong polarization dependence to the incident electromagnetic wave due to the ultra-sharp resonance line width as a result of interaction between the constituent meta-atoms. However, when the six-fold rotationally symmetric unit cell design was re-arranged into a fourfold rotational symmetry, we observed the excitation of a polarization insensitive dual-band transparency. Thus, the 3-D split-ring resonators allow new schemes to observe single and multi-band classical analogues of electromagnetically induced transparencies that has huge potential applications in slowing down light, sensing modalities, and filtering functionalities either in the passive mode or the active mode where such effects could be tuned by integrating materials with dynamic properties. MOE (Min. of Education, S’pore) Published version

Inductive coupling of the identical bianisotropic meta-atoms is demonstrated to be dependent on the spatial distance. Here, a nonintuitive orientation-dependent switchable coupling phenomenon is experimentally studied and the applications in polarization control of the proposed metasurface are shown. Accepted version

The near field meta-molecular interactions in a lattice play an important role in determining the collective behavior of the metamaterials. Here, we exploit the nearest neighbor inter unit cell interactions and the intra near-field coupling in metamolecules to manipulate the co- and the cross-polarized light. We observed large enhancement in the bandwidth and the amplitude of the transmitted light in the strongly coupled meta-molecular lattice. We further show that the proposed metasurface could function as a broadband achromatic quarter-wave plate. The chosen meta-molecular design also enhances the cross-polarized light when integrated with a ground plane to operate in the reflection mode. MOE (Min. of Education, S’pore) Published version

Polarization manipulation is essential in developing cutting-edge photonic devices ranging from optical communication displays to solar energy harvesting. Most previous works for efficient polarization control cannot avoid utilizing metallic components that inevitably suffer from large ohmic loss and thus low operational efficiency. Replacing metallic components with Mie resonance-based dielectric resonators will largely suppress the ohmic loss toward high-efficiency metamaterial devices. Here, we propose an efficient approach for broadband, high-quality polarization rotation operating in transmission mode with all-dielectric metamaterials in the terahertz regime. By separating the orthogonal polarization components in space, we obtain rotated output waves with a conversion efficiency of 67.5%. The proposed polarization manipulation strategy shows impressive robustness and flexibility in designing metadevices of both linear- and circular-polarization incidences.

It is extremely challenging to control the phase of light at will in free space. Here, Pancharatnam-Berry-phase-enabled, tunable phase control of free-space light is experimentally demonstrated in an ultrathin flexible dispersion-free metadevice. This metadevice enables the broadband conversion of linearly polarized light into any desired output polarization.

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

Planar metasurfaces and plasmonic resonators have shown great promise for sensing applications across the electromagnetic domain ranging from the microwaves to the optical frequencies. However, these sensors suffer from lower figure of merit and sensitivity due to the radiative and the non-radiative loss channels in the plasmonic metamaterial systems. We demonstrate a metamaterial absorber based ultrasensitive sensing scheme at the terahertz frequencies with significantly enhanced sensitivity and an order of magnitude higher figure of merit compared to planar metasurfaces. Magnetic and electric resonant field enhancement in the impedance matched absorber cavity enables stronger interaction with the dielectric analyte. This finding opens up opportunities for perfect metamaterial absorbers to be applied as efficient sensors in the finger print region of the electromagnetic spectrum with several organic, explosive, and bio-molecules that have unique spectral signature at the terahertz frequencies. Published version