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Metamaterials with active tunability of electromagnetic response have attracted much attention due to their flexibility and applicability in a wide range of optoelectronic applications. In this study, a mid-infrared (MIR) H-shaped metamaterial absorber (HMA) with high tunability is proposed. The structure of HMA is composed of one longitudinal and two transverse metal bars, forming an H-shaped resonator. By varying structural parameters, the HMA achieves perfect triple-band absorption. The resonant intensities of first and second resonances can be modulated linearly at the wavelength of 5.3 μm, while the resonant intensity of third resonance can be kept as stable at the wavelength of 7.3 μm. By moving the transverse metal bar, the maximum tuning range of resonance is 290 nm. Due to the non-rotationally symmetric configuration of HMA, it exhibits polarization-dependence characteristic. HMA exhibits the switchable function from triple-resonance to double-resonance. These results prove that the proposed HMA can be used for applications in gas sensing, polarization controlling, and IR filtering fields.

This study presents and demonstrates a design of a micro-electro-mechanical system (MEMS) reconfigurable terahertz (THz) metamaterial absorber (TMA) that shows tunable free spectra range with a maximum value as 166 GHz in transverse magnetic mode and switchable resonance from 1.24 THz to 1.53 THz. The unit structure of TMA consists of two movable rectangular resonators aside and an x-shaped resonator at the center comprising two c-shaped resonators connected upside down. The material configuration of TMA is typical of the metal–insulator-metal sandwich configuration. That is a bottom Au layer preventing the penetration of the THz wave, a dielectric SiO2 layer to accommodate and absorb the incident wave, and a patterned Au structure array to select the resonant frequency. The electromagnetic response of TMA could be flexibly modulated when the structure is reconfigured by changing the gaps between x-shaped resonators and rectangular resonators based on the MEMS platform. Meanwhile, owing to the asymmetry of resonant structure, the electromagnetic response of TMA could be tuned from single-, dual-, triple- quad-resonances by varying the polarization state of incidence. Furthermore, the sensitivity of TMA is explored and demonstrated with remarkable linearity under a variation of the surrounding environment refractive index from 1.0 to 2.0. The feasibility of TMA is validated as the experimental absorption spectra are well meet the simulation results. The above results indicate the proposed TMA possesses great potential applications in multiple resonance switches, polarization switches, and highly efficient environmental sensors.

An infrared (IR) tunable perfect meta-absorber (TPM) is presented, which is configured of metal-insulator-metal (MIM) structure. The unit cell of the TPM device is composed of two opposite T-shaped and two rod-shaped Au resonators on the reflective mirror layer. The optimized feature size of TPM can achieve perfect absorption and the resonant wavelength can be tuned from blue-shift first and then red-shift by increasing the gap size between two T-shaped resonators continuously. The tuning process of red shift is linear. By changing the distance between two rod-shaped resonators, the TPM device exhibits red shift linearly. Moreover, TPM device shows polarization-dependent characteristic that the main absorption resonance can be attenuated from 100% to 0% by changing polarization angle from 0° to 90° of incident IR light. The resonance of TPM device has an excellent linear relationship with the ambient refractive index, and the sensitivity and maximum figure of merit (FOM) are calculated as 1249 nm/RIU and 21 RIU−1, respectively. These results prove that the design of the proposed TPM device is very suitable for the use of IR sensing, polarization switching, and refractive index sensing applications.

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS0.91Se0.09 can obtain a high average thermoelectric figure of merit (ZTave≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS0.91Se0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS0.91Se0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced.

• A MEMS reconfigurable THz metamaterial absorber (TMA) is presented. • MEMS-based TMA shows tunable free spectra range (FSR) characteristic. • MEMS-based TMA also shows polarization-dependent characteristic. • Remarkable linear sensitivity to variations in surrounding environment is demonstrated. This study presents and demonstrates a design of a micro-electro-mechanical system (MEMS) reconfigurable terahertz (THz) metamaterial absorber (TMA) that shows tunable free spectra range with a maximum value as 166 GHz in transverse magnetic mode and switchable resonance from 1.24 THz to 1.53 THz. The unit structure of TMA consists of two movable rectangular resonators aside and an x-shaped resonator at the center comprising two c-shaped resonators connected upside down. The material configuration of TMA is typical of the metal–insulator-metal sandwich configuration. That is a bottom Au layer preventing the penetration of the THz wave, a dielectric SiO 2 layer to accommodate and absorb the incident wave, and a patterned Au structure array to select the resonant frequency. The electromagnetic response of TMA could be flexibly modulated when the structure is reconfigured by changing the gaps between x-shaped resonators and rectangular resonators based on the MEMS platform. Meanwhile, owing to the asymmetry of resonant structure, the electromagnetic response of TMA could be tuned from single-, dual-, triple- quad-resonances by varying the polarization state of incidence. Furthermore, the sensitivity of TMA is explored and demonstrated with remarkable linearity under a variation of the surrounding environment refractive index from 1.0 to 2.0. The feasibility of TMA is validated as the experimental absorption spectra are well meet the simulation results. The above results indicate the proposed TMA possesses great potential applications in multiple resonance switches, polarization switches, and highly efficient environmental sensors. [ABSTRACT FROM AUTHOR]

We present a design of MEMS-based tunable terahertz (THz) metamaterial (TTM) with polarization-dependent sensing characteristic. The MEMS-based TTM device is reconfiguration by using two electrostatic comb-drive actuators, which exhibits tunable dual-band resonances in both transverse electric (TE) mode and transverse magnetic (TM) modes. The tuning ranges of free spectra ranges are 42 GHz and 82 GHz for TE and TM modes, respectively by increasing the gap between the left and right semicircle-shaped ring resonators. The electromagnetic responses of the proposed MEMS-based TTM device can be switched between dual-resonance and triple-resonance by changing the distance between the left and right semicircle-shaped ring resonators along y-axis direction. These results indicate MEMS-based TTM device shows polarization-sensitive and switching characteristics. MEMS-based TTM device is particularly sensitive to the environmental refraction index, and there is a linear relationship between the shift of resonant frequency and the change of refraction index. The sensitivity is 0.186 THz/RIU while the maximum figure of merit and quality factor reach 208 and 248, respectively at the first TE resonance. These outstanding and unique results of MEMS-based TTM device provide an effective approach for the design of highly linear and sensitive gas and environmental applications.

A design of electric split-ring metamaterial (eSRM) with tunable terahertz (THz) free spectra range (FSR) characteristic is presented. The proposed eSRM is composed of double semicircle-shaped ring resonators with two connected metallic bars. By changing the radius of the semicircle-shaped ring resonators, the tuning ranges of FSR are 0.187 THz and 0.024 THz in transverse electric (TE) and transverse magnetic (TM) modes, respectively, which indicates eSRM could operate stably in TE and TM modes. Furthermore, eSRM exhibits switching and polarization-dependent characteristics by exploiting comb driver to control the distances between ring resonators along x- and y-axis directions. The electromagnetic responses could be tuned and switched between dual- and quad-resonance in both TE and TM modes. eSRM shows polarization angle-sensitive characteristic and the corresponding FSR could be tuned from 919 GHz to 336 GHz by changing the polarization angle of incident THz wave. In view of the above optical characteristics of eSRM, it is used for the sensing application exposed on different surrounding refraction index. The sensitivity, quality factor (Q-factor), and figure of merit (FOM) are calculated. The sensitivity is 0.457 THz/RIU and the average Q-factor and FOM are 40.65 and 35.47, respectively. These results indicate the proposed eSRM could be used in widespread applications, such as dual-/quad-resonance switches, polarization switches, and high-efficient environmental sensors. [2021-0006] [ABSTRACT FROM AUTHOR]

We propose reconfigurable metamaterials with multiband resonance in the terahertz (THz) frequency range. By modifying the pattern width and gap of metamaterial, these designs exhibit the characteristics of polarization-dependence, dual-band resonance, and triple-band resonance. Two cantilever metamaterial (CM) devices are demonstrated by using micro-electro-mechanical system (MEMS) based bimorph cantilevers array, which can be deflected toward the out-of-plane direction. The measured transmission spectra are greatly agreed well with the simulation results. By applying different direct current (DC) bias voltages, the bending degree and electromagnetic response of CM device could be controlled. The multiband resonance can be tuned in the range of 0.33 THz by applying a DC bias voltage of 30 volts. The relationship of the resonant shifts and the applied DC bias voltages is quite linear, which can assist the device to be more controllable and applied in more fields. Such designs are promising to be used in various applications, such as frequency-selective detectors, multispectra imagers, and multifunctional switches in the THz frequency range. [2020-0236] [ABSTRACT FROM AUTHOR]

We present four kinds of tunable terahertz (THz) metamaterials (TTMs) composed of spilt-ring resonators (SRRs) with different splits and inner movable structures. According to the numbers of splits, these four designs are based on the conventional SRR with single-, dual-, triple- and quad-split denoted as TTM-1, TTM-2, TTM-3, and TTM-4, respectively. These four TTMs exhibit polarization-dependent and polarization-independent characteristics depending on whether the SRR structures are symmetrical or asymmetrical configuration. By increasing the height of the inner movable structure, the tunability of the resonant frequency of TTMs is explored. These results show that these TTMs can achieve different tuning ranges in transverse electric (TE) and transverse magnetic (TM) modes. The resonances are stable for TTM-1 in TE and TM modes, and TTM-2 in TE mode. TTM-3 exhibits the tuning range of 0.202 THz in TE mode and 0.176 THz in TM mode, while TTM-4 exhibits the tuning range of 0.198 THz in both TE and TM modes. The maximum tuning range is 0.306 THz by implementing TTM-2 in TM mode. These designs possess anti-vibration and tunable characteristics, which provide a potential possibility for the use in the THz-wave optoelectronic devices with high tunability, flexibility, and applicability. [ABSTRACT FROM AUTHOR]

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS 0.91 Se 0.09 can obtain a high average thermoelectric figure of merit (Z T ave ≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS 0.91 Se 0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS 0.91 Se 0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced. • In this paper, we presented the generation and detection of coherent acoustic phonons in SnS 0.91 Se 0.09 bulk single crystal by ultrafast two-color pump-probe reflectivity method. • The experiment results show that the coherent phonons with an oscillation frequency of 39.2 GHz originate from the dominant mechanism of deformation potential coupling stress. • In addition, this work proposes the view of the unique carrier dynamics with optical anisotropic response in SnS 0.91 Se 0.09 coherent phonon oscillation. [ABSTRACT FROM AUTHOR]

• MEMS-based TTM shows polarization-dependent sensing characteristic. • The tuning ranges of FSR are 42 GHz and 82 GHz for TE and TM modes, respectively. • TTM device can be tuned and switched between dual-resonance and triple-resonance. • MEMS-based TTM device exhibits polarization-dependent, switching, and polarization angle-sensitive characteristics. • Sensitivity, Q-factor, and FOM of TTM device are 0.186 THz/RIU, 231 and 187, respectively. • TTM design provides an effective approach for the environmental sensors. We present a design of MEMS-based tunable terahertz (THz) metamaterial (TTM) with polarization-dependent sensing characteristic. The MEMS-based TTM device is reconfiguration by using two electrostatic comb-drive actuators, which exhibits tunable dual-band resonances in both transverse electric (TE) mode and transverse magnetic (TM) modes. The tuning ranges of free spectra ranges are 42 GHz and 82 GHz for TE and TM modes, respectively by increasing the gap between the left and right semicircle-shaped ring resonators. The electromagnetic responses of the proposed MEMS-based TTM device can be switched between dual-resonance and triple-resonance by changing the distance between the left and right semicircle-shaped ring resonators along y -axis direction. These results indicate MEMS-based TTM device shows polarization-sensitive and switching characteristics. MEMS-based TTM device is particularly sensitive to the environmental refraction index, and there is a linear relationship between the shift of resonant frequency and the change of refraction index. The sensitivity is 0.186 THz/RIU while the maximum figure of merit and quality factor reach 208 and 248, respectively at the first TE resonance. These outstanding and unique results of MEMS-based TTM device provide an effective approach for the design of highly linear and sensitive gas and environmental applications. [ABSTRACT FROM AUTHOR]

Abstract: In order to compare the carrier dynamics in λ-(BETS)2MCl4 (M=Ga, Fe) and κ-(ET)2X (X=Cu(NCS)2, Cu[N(CN)2]Br, Cu[N(CN)2]Cl), their polarized reflectance spectra were measured by a pump–probe method. Polarization-dependences were compared among the different materials at varying temperatures. The result indicated that each salt had anisotropy in the conduction planes corresponding to its crystallographic axes at all the temperatures of measurements. [Copyright &y& Elsevier]