Mohammad Parvinnezhad Hokmabadi

Design and analysis of perfect terahertz metamaterial absorber by a novel dynamic circuit model

Metamaterial terahertz absorbers composed of a frequency selective layer followed by a spacer and a metallic backplane have recently attracted great attention as a device to detect terahertz radiation. In this work, we present a quasistatic dynamic circuit model that can decently describe operational principle of metamaterial terahertz absorbers based on interference theory of reflected waves. The model comprises two series LC resonance components, one for resonance in frequency selective surface (FSS) and another for resonance inside the spacer. Absorption frequency is dominantly determined by the LC of FSS while the spacer LC changes slightly the magnitude and frequency of absorption. This model fits perfectly for both simulated and experimental data. By using this model, we study our designed absorber and we analyze the effect of changing in spacer thickness and metal conductivity on absorption spectrum.

Equivalent-Circuit Interpretation of the Polarization Insensitive Performance of THz Metamaterial Absorbers

Polarization insensitive metamaterial perfect absorbers were investigated through finite element numerical method and a new equivalent-circuit electric model was proposed to interpret this polarization insensitivity. The devices were fabricated to validate the model and experimental measurements were shown to be in good agreement with the simulated results. This absorber device is suitable for future use in THz sensing and detection applications.

Plasmon-Induced Transparency by Hybridizing Concentric-Twisted Double Split Ring Resonators

As a classical analogue of electromagnetically induced transparency, plasmon induced transparency (PIT) has attracted great attention by mitigating otherwise cumbersome experimental implementation constraints. Here, through theoretical design, simulation and experimental validation, we present a novel approach to achieve and control PIT by hybridizing two double split ring resonators (DSRRs) on flexible polyimide substrates. In the design, the large rings in the DSRRs are stationary and mirror images of each other, while the small SRRs rotate about their center axes. Counter-directional rotation (twisting) of the small SRRs is shown to lead to resonance shifts, while co-directional rotation results in splitting of the lower frequency resonance and emergence of a PIT window. We develop an equivalent circuit model and introduce a mutual inductance parameter M whose sign is shown to characterize the existence or absence of PIT response from the structure. This model attempts to provide a quantitative measure of the physical mechanisms underlying the observed PIT phenomenon. As such, our findings can support the design of several applications such as optical buffers, delay lines, and ultra-sensitive sensors.

Impact of Substrate and Bright Resonances on Group Velocity in Metamaterial without Dark Resonator

Manipulating the speed of light has never been more exciting since electromagnetic induced transparency and its classical analogs led to slow light. Here, we report the manipulation of light group velocity in a terahertz metamaterial without needing a dark resonator, but utilizing instead two concentric split-ring bright resonators (meta-atoms) exhibiting a bright Fano resonance in close vicinity of a bright Lorentzian resonance to create a narrowband transmittance. Unlike earlier reports, the bright Fano resonance does not stem from an asymmetry of meta-atoms or an interaction between them. Additionally, we develop a method to determine the metamaterial “effective thickness”, which quantifies the influence of the substrate on the metamaterial response and has remained challenging to estimate so far. By doing so, very good agreement between simulated and measured group delays and velocities is accomplished. The proposed structure and method will be useful in designing optical buffers, delay lines, and ultra-sensitive sensors.

Terahertz metamaterials perfect absorbers for sensing and imaging

Devices operating at THz frequencies have been continuously expanded in many areas of application and major research field, which requires materials with suitable electromagnetic responses at THz frequency ranges. Unlike most naturally occurring materials, novel THz metamaterials have proven to be well suited for use in various devices due to narrow and tunable operating ranges. In this work, we present the results of two THz metamaterial absorber structures aiming two important device aspects; polarization sensitivity and broad band absorption. The absorbers were simulated by finite element method and fabricated through the combination of standard lift-off photolithography and electron beam metal deposition. The fabricated devices were characterized by reflection mode THz time domain spectroscopy. The narrow band absorber structures exhibit up to 95% absorption with a bandwidth of 0.1 THz to 0.15 THz.

Comprehensive study of terahertz metamaterial absorber by applying a hybrid approach on its circuit analogue

Here, we propose a hybrid approach to uniquely determine the elements of the circuit analogue of the terahertz metamaterial absorber that we previously reported. The proposed method is based on calculations, fitting, and physical mechanism of the absorption process interpreted by the model. In this work, the correlation between the model components and our designed absorber is comprehensively enlightened, and the dependence of the model elements to structural dimensions of the absorber is analyzed both qualitatively and quantitatively. By applying this approach on frequency selective surface (FSS) model, we are also able to interpret the polarization insensitivity of our designed absorber. The proposed model and approach is applicable for all metamaterial absorbers with any arbitrary FSS design.

Theoretical and experimental investigation of hybrid broadband terahertz metamaterial absorber

Among electromagnetic spectrum, terahertz region has been utilized less due to the lack of appropriate devices that works well in these frequencies But recently growing interest has been focused to design devices with functionality in terahertz region because of potential terahertz applications. We present a novel structure that broadens bandwidth of terahertz metamaterial absorber. Our structure takes a benefit of multiband absorber by making the bands close enough to each other but in a multilayer pattern. The absorber has composed of two concentric copper rings in two different layers followed by polyimide and a metal back layer. Simulation shows 100 GHz bandwidth which is double of that of a single layer single ring absorber.

Highly Efficient, Polarization Insensitive Terahertz Metamaterial Perfect Absorber and Imaging

We demonstrate performance and characteristics of a metamaterial absorber designed for operation in the THz regime. This absorber exhibits upto 90% absorbance and shows potential for use in sensor and detector devices.

Investigation of tunable terahertz metamaterial perfect absorber with anisotropic dielectric liquid crystal

In this letter, we report the unique design, simulation and experimental verification of an electrically tunable THz metamaterial perfect absorber consisting of complementary split ring resonator (CSRR) arrays integrated with liquid crystal as the subwavelength spacer in between. We observe a shift in resonance frequency of about 5.0 GHz at 0.567 THz with a 5 V bias voltage at 1KHz between the CSRR and the metal backplane, while the absorbance and full width at half maximum bandwidth are maintained at 90% and 0.025 THz, respectively. Simulated absorption spectrum by using a uniaxial model of LC matches perfectly the experiment data and demonstrates that the effective refractive index of LC changes between 1.5 and 1.7 by sweeping a 1 kHz bias voltage from 0 V to 5 V. By matching simulation and experiment for different bias voltages, we also estimate the angle of LC molecules versus the bias voltage. Additionally, we study the created THz fields inside the spacer to gain a better insight of the characterist...

Terahertz metamaterials: design, implementation, modeling and applications

Sub-wavelength metamaterial structures are of great fundamental and practical interest because of their ability to manipulate the propagation of electromagnetic waves. We review here our recent work on the design, simulation, implementation and equivalent circuit modeling of metamaterial devices operating at Terahertz frequencies. THz metamaterials exhibiting plasmon-induced transparency are realized through the hybridization of double split ring resonators on either silicon or flexible polymer substrates and exhibiting slow light properties. THz metamaterials perfect absorbers and stereometamaterials are realized with multifunctional specifications such as broadband absorbing, switching, and incident light polarization selectivity.