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-film of dielectrics with much longer terahertz waves (1 THz = 0.3 mm) is challenging. Here, we demonstrate a quasi-bound state in the continuum (BIC) resonance for sensing of nanometer scale thin analyte deposited on a flexible metasurface. The large sensitivity originates from 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 length scale of λ/43000, where λ is the resonance wavelength. The low-loss, flexible and large mechanical strength of the quasi-BIC micro structured metamaterial sensor could be an ideal platform for developing ultrasensitive wearable terahertz sensors.

A bound state in the continuum (BIC) is a nonradiating state of light embedded in the continuum of propagating modes providing drastic enhancement of the electromagnetic field and its localization at micro–nanoscale. However, access to such modes in the far‐field requires symmetry breaking. Here, it is demonstrated that a nanometric dielectric or semiconductor layer, 1000 times thinner than the resonant wavelength (λ/1000), induces a dynamically controllable quasi‐bound state in the continuum (QBIC) with ultrahigh quality factor in a symmetric metallic metasurface at terahertz frequencies. Photoexcitation of nanostrips of germanium activates ultrafast switching of a QBIC resonance with 200% transmission intensity modulation and complete recovery within 7 ps on a low‐loss flexible substrate. The nanostrips also form microchannels that provide an opportunity for BIC‐based refractive index sensing. An optimization model is presented for (switchable) QBIC resonances of metamaterial arrays of planar symmetric resonators modified with any (active) dielectric for inverse metamaterial design that can serve as an enabling platform for active micro–nanophotonic devices.

on distinctive spectral signatures resulting from vibrational modes of covalent and hydrogen bonds. [8,[13][14][15][16] The focus on this spectrally rich region is attributed to the highly coherent and nonionizing nature of terahertz radiation, wide unallocated frequency bands, distinctive wavelengths, and their penetration through a significant depth of dielectric materials. Terahertz technology is geared toward realizing devices that can efficiently manipulate the phase, amplitude, and polarization of terahertz radiations for the above-mentioned myriad of applications.However, progress in this domain is held back by low terahertz power available from compact sources, high free-space path loss, and limited choice of materials that exhibit less absorption of terahertz waves. [17] Owing to the fact that natural materials demonstrate weak wave-matter interaction at terahertz frequencies, terahertz devices with engineered subwavelength resonant metallic inclusions on dielectric spacers have been realized, to interact strongly with an incident electromagnetic wave.In the past, metamaterials have been a promising route to building terahertz devices. These devices have in essence been engineered as 3D resonating elements that effectively manipulate both permittivity and permeability of an effective medium to couple to free space. The underlining disadvantage of these structures is the difficulty involved in the fabrication of 3D geometrical structures using standard semiconductor fabrication techniques. Standard fabrication techniques are generally amenable to 2D design with extrusion of these structures into thickness (the third, vertical dimension). Moreover, the choice and availability of dielectric materials and metals that provide sufficient interaction while retaining device efficiency has proved challenging. [13,[18][19][20] Recently, effective manipulation of electromagnetic wave has been widely demonstrated with 2D metasurfaces, which were originally employed as building blocks for 3D metamaterials. In these lower-dimension designs, effective manipulation of terahertz waves for various applications is achieved by carefully designing sub-wavelength resonant structures as is evident in published review articles. [21,22] Metasurfaces present high degree of compactness that enhances radiation efficiency. Additionally, the planar form factor of metasurfaces enables Manipulation of terahertz radiation opens new opportunities that underpin application areas in communication, security, material sensing, and characterization. Metasurfaces employed for terahertz manipulation of phase, amplitude, or polarization of terahertz waves have limitations in radiation efficiency which is attributed to losses in the materials constituting the devices. Metallic resonators-based terahertz devices suffer from high ohmic losses, while dielectric substrates and spacers with high relative permittivity and loss tangent also reduce bandwidth and efficiency. To overcome these issues, a proper choice of low loss and low relative permittivi...

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Polarization conversion of terahertz waves is important for applications in imaging and communications. Conventional wave plates used for polarization conversion are inherently bulky and operate at discrete wavelengths. As a substitute, we employ reflective metasurfaces composed of subwavelength resonators to obtain similar functionality but with enhanced performance. More specifically, we demonstrate low-order dielectric resonators in place of commonly used planar metallic resonators to achieve high radiation efficiencies. As a demonstration of the concept, we present firstly, a quarter-wave mirror that converts 45° incident linearly polarized waves into circularly polarized waves. Next, we present a half-wave mirror that preserves the handedness of circularly polarized waves upon reflection, and in addition, rotates linearly polarized waves by 90° upon reflection. Both metasurfaces operate with high efficiency over a measurable relative bandwidth of 49% for the quarter-wave mirror and 53% for the half-wave mirror. This broadband and high efficiency capabilities of our metasurfaces will allow to leverage maximum benefits from a vast terahertz bandwidth.

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A high-pressure gravimetric apparatus using a quartz spring for measuring solubility and diffusivity of CO2 in ionic liquids (ILs) was established for the first time. The time-dependent amounts of CO2 were recorded with a telescopic cathetometer and analyzed by using a one-dimensional diffusion model to obtain diffusion coefficients of CO2 in two ILs, namely, 1-n-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF6]) and 1-butyl-3-methyl imidazolium tetrafluoroborate ([bmim] [BF4]) at pressures up to 10 MPa. Solubility data of CO2 in the two ILs up to 20 MPa were also obtained from its equilibrium masses and compared with those reported in the literature. The Peng–Robinson equation of state with the van der Waals one-fluid mixing rules was employed to correlate the experimental solubility data, revealing satisfactory calculation results. The measured diffusion coefficients of CO2 in [bmim][PF6] and [bmim][BF4] separately increase from 3.550 × 10–10 to 6.064 × 10–10 m2/s and from 7.184 × 10–10 to 9.880 × 10–10 m2/s following the pressure increase from 2.0 to 10.0 MPa at 323.2 K, while those at 5.0 MPa and different temperatures follow the Arrhenius equation, providing the diffusion activation energies of 25.53 and 20.30 kJ/mol for the [bmim][PF6]–CO2 and [bmim][BF4]–CO2 systems, respectively.

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