Flexible Terahertz Research Articles

Flexible terahertz and FarIR polymer‐supported filter

AbstractWe present the design, fabrication, and measurement of a terahertz (THz) polymer‐supported filter. A periodic structure, frequency selective surfaces (FSS) is employed to provide bandpass characteristic for this filter design with polyimide (PI) film as a substrate. This PI film (13 μm thick) is very thin and thus provides a flexible support to the filter. The average extracted dielectric constant ε r is 3.35 and the average refractive index n s is 1.84. The final filter structure was fabricated using standard microfabrication process. The measured transmission response shows high percentage of transmission (~80%) with the peak at 0.58 THz and sharp roll‐off that covers the broad range of frequency from 0.1 to 1 THz (wavelength of 300 μm‐3000 μm).

A flexible terahertz waveguide for delivery and filtering of quantum-cascade laser radiation

We demonstrate that a specially designed microstructured polymer waveguide can improve the quality of quantum cascade laser radiation. It can filter out undesired side frequencies and transform the delivered radiation to a sub-millimeter size beam with a Gaussian-like cross-section. The spatial distribution of the bended waveguide mode is measured, and low bending losses of the fundamental mode are confirmed. Detailed optimization of a capillary structure for 3 THz frequency allows delivering single frequency radiation up to a 1 meter distance. The results of the experimental study are confirmed by calculations with an accurate mode solver that explains the achieved losses of ∼10 dB/m. A further improvement strategy is suggested.

Flexible terahertz modulators based on graphene FET with organic high-k dielectric layer

Graphene field-effect-transistor (GFET) based terahertz (THz) modulators usually possess an unfulfilling modulation depth (MD) of 15% ∼ 20%. In this work we developed a flexible GFET based THz modulator, where the graphene monolayer is coated with an organic high-K dielectric as the screening layer and an ion-gel layer as the gate. With this exquisite composite modulating structure, the new device possesses a significantly enhanced modulation depth (MD) up to 70% over a broad frequency band, an extremely low insert loss (IL) of 1.3 dB, and unexpected good structural and properties stability. The large intrinsic MD, low IL, as well as its flexibility, render this performance enhanced modulator versatile in fabrication of novel THz devices, such as multi-level modulator, for nonplanar or wearable applications.

Broadband terahertz metamaterial absorber based on simple multi-ring structures

Single-layer metallic rings are the effective structure cell which are widely used to design single-band and multiband perfect metamaterial absorbers owning to their electromagnetic resonance. However, the absorbers based on the single-layer metallic rings have a common shortcoming, that is the narrow absorption bandwidth. To overcome the limitations, here we proposed a single-layer, flexible and broadband terahertz metamaterial absorber, which consists of four sub-cells with multiple metal rings and a metal ground plane separated by a dielectric layer. By enhancing the coupling response between adjacent metallic rings and merging the adjacent resonant peaks of multi-resonators, we experimentally observed broadband characteristics at the terahertz band. The average absorption of 88% from 0.63 to 1.34 THz and the relative absorption bandwidth of 95% at the incident angle of 15o for TE polarization. Correspondingly, for TM polarization the absorption of more than 80% from 0.61 to 1.1 THz with the relative absorption bandwidth of 80% were also observed. The results went far beyond the previous single-layer absorbers based on metal rings and were much better than the fractal-cross structure reported recently [Kenney et al., ACS Photonics 4, 2604 (2017)]. We had reason to believe that the presented terahertz metamaterial absorber with broad absorption bandwidth and simple structure can find important applications in communication, stealth, energy harvesting systems and so on.

Fermi-Level-Controlled Semiconducting-Separated Carbon Nanotube Films for Flexible Terahertz Imagers

Carbon-nanotube-related (CNT-related) materials and structures are highly anticipated as potential building blocks for future flexible electronics and photonics. Despite the various promising applications of CNT-related materials, one obstacle is the lack of ability to globally control and tune the Fermi level of microscale-thick CNT films because these films require a certain thickness to maintain their free-standing shape and freely bendable flexibility. In this work, we report on Fermi-level-controlled flexible and bendable terahertz (THz) imagers with chemically adjustable Fermi-level-tuning methods for CNT films. By utilizing the electronic-double-layer technique with ionic liquids, we obtained an on/off resistance ratio (2758) for a semiconducting-separated CNT film with a thickness of 30 μm and tuned the Fermi level at an optimal gate voltage to maximize the THz detector performance. In addition, the development of a gate-free tunable doping technology based on a variable-concentration dopant solut...

Flexible and tunable terahertz all-dielectric metasurface composed of ceramic spheres embedded in ferroelectric/ elastomer composite.

Terahertz (THz) all-dielectric metasurfaces made of high-index and low-loss resonators have attracted more and more attention due to their versatile properties. However, the all-dielectric metasurfaces in THz suffer from limited bandwidth and low tunability. Meanwhile, they are usually fabricated on flat and rigid substrates, and consequently their applications are restricted. Here, a simple approach is proposed and experimentally demonstrated to obtain a flexible and tunable THz all-dielectric metasurface. In this metasurface, micro ceramic spheres (ZrO2) are embedded in a ferroelectric (strontium titanate) / elastomer (polydimethylsiloxane) composite. It is shown that the Mie resonances in micro ceramic spheres can be thermally and reversibly tuned resulting from the temperature dependent permittivity of the ferroelectric / PDMS composite. This metasurface characterized by flexibility and tunability is expected to have a more extensive application in active THz devices.

Electromagnetically induced transparency control in terahertz metasurfaces based on bright-bright mode coupling

We demonstrate a classical analogue of electromagnetically induced transparency (EIT) in a highly flexible planar terahertz metamaterial (MM) comprised of three-gap split ring resonators. The keys to achieve EIT in this system are the frequency detuning and hybridization processes between two bright modes coexisting in the same unit cell as opposed to bright-dark modes. We present experimental verification of two-bright mode coupling for a terahertz EIT-MM in the context of numerical results and theoretical analysis based on a coupled Lorentz oscillator model. In addition, a hybrid variation of the EIT-MM is proposed and implemented numerically in order to dynamically tune the EIT window by incorporating photosensitive silicon pads in the split gap region of the resonators. As a result, this hybrid MM enables the potential active optical control of a transition from the on-state (EIT mode) to the off-state (dipole mode).

Flexible and stackable terahertz metamaterials via silver-nanoparticle inkjet printing

There is presently much interest in tunable, flexible, or reconfigurable metamaterial structures that work in the terahertz frequency range. They can be useful for a range of applications, including spectroscopy, sensing, imaging, and communications. Various methods based on microelectromechanical systems have been used for fabricating terahertz metamaterials, but they typically require high-cost facilities and involve a number of time-consuming and intricate processes. Here, we demonstrate a simple, robust, and cost-effective method for fabricating flexible and stackable multiresonant terahertz metamaterials, using silver nanoparticle inkjet printing. Using this method, we designed and fabricated two arrays of split-ring resonators (SRRs) having different resonant frequencies on separate sheets of paper and then combined the two arrays by stacking. Through terahertz time-domain spectroscopy, we observed resonances at the frequencies expected for the individual SRR arrays as well as at a new frequency due to coupling between the two SRR arrays.

Thermal Device Design for a Carbon Nanotube Terahertz Camera.

Terahertz (THz) wave detectors are increasingly expected to serve as key components of powerful nondestructive and noncontact inspection tools in a large variety of fields. In contrast to conventional THz detectors based on rigid solid materials, we previously developed an uncooled and bendable THz camera based on the THz-induced photothermoelectric effect of carbon nanotube (CNT) array devices and demonstrated omnidirectional THz imaging of three-dimensional curved samples. Although this development opened a pathway to flexible THz electronics, the physical parameters that determine the performance of the CNT THz camera have not been fully investigated. As a result, the thermal device design has not been optimized in terms of the camera sensitivity and spatial resolution. In this work, we studied the underlying mechanism of the THz-induced photothermoelectric effect of the CNT camera and found physical factors related to the detector performance. Through simulation and experiments, we observed that the detection sensitivity and response time strongly depend on the CNT channel width and film thickness. We further identified that the irradiated wave penetration into the CNT film through the electrode materials deteriorates the detection area, which is directly linked to the camera spatial resolution. By utilizing the improved CNT device design fabricated based on these findings, we eliminated undesired signals generated via thermal diffusion and THz wave penetration and achieved higher-sensitivity THz detection and higher imaging resolution compared to our previously reported THz camera. The presented technologies are expected to contribute to future flexible THz imaging applications and will also be applicable to other types of photothermoelectric devices.

Dynamic multifunctional control of terahertz beam based on graphene coding metamaterial

Terahertz (THz) waves have aroused tremendous research interest due to its some unique features and widespread applications in broadband communication, military radar, non-destructive detection, biomedical, security check, etc. With the development of THz applications, dynamic control beam of THz wave with wide bandwidth and multifunction has become a key issue in the field THz technology. The metamaterial with a kind of artificial material provides an approach to controlling the terahertz beam. However, the characteristics of metamaterials based on the equivalent medium parameters are limited by the structural configuration, which usually causes disadvantageous problems including the real-time dynamic control, narrow bandwidth, modulating efficiency, complicated design, etc. The coding metamaterial based digital elements provide an approach to wideband and flexible control terahertz wave by different sequences. However, the THz waves are still hard to tune in dynamic ways due to the limitation of material properties and processing capability. Graphene with a new two-dimensional material has excellent photoelectric properties such as tunable band gap, flexibly dynamic performance, and low material loss. Therefore, the graphene with coding metamaterial can offer a new way of dynamically controlling beam. In this paper, we design a 1 bit coding metamaterial based on graphene ribbon, which can be expected to realize multi-modulation to the number of beams, frequency and amplitude of THz wavers. The mechanism of controlling electromagnetic wave by coding metamaterial can be explained by the reflective array antenna. And the characteristics of the proposed metamaterial based on the graphene ribbon and the far-field scattering of coding metamaterial are simulated using the CST Microwave Studio. A composite structure which consists of gold metal, polyimide, silicon, silicon dioxide, graphene ribbon is designed and characterized in the THz range. The simulation results show that by gating different graphene ribbons, the coding state (digital element) on each column can be independently controlled as well, thus the ‘0’ and ‘1’ digital elements with a phase difference of 180° in a certain frequency range can be realized, and then the coding sequence on metamaterials is dynamically modulated. Full-wave simulation results also show that different-sequence coding metamaterials can achieve the control of the number of scattering THz beams, from one, double, multi scattering in a wide frequency range (from 1.7 to 2.2 THz). For coding sequence ‘010101...’ realized by gating different voltages on coding elements ‘0’ and ‘1’, the frequency at which double scattering beams are produced, presents shift. For the coding metamaterial of periodic sequence of 000000 or 111111 with different voltage for different graphene ribbon, which can be expected to realize amplitude modulation from -12 dB to -23 dB of THz beam steering at f1=1 THz. Therefore, this graphene coding metamaterial can control the THz beam flexibly and may offer widespread applications in stealth, imaging, and broadband communication of THz frequencies.

Polarization-dependent electromagnetic responses of ultrathin and highly flexible asymmetric terahertz metasurfaces

We report the polarization-dependent electromagnetic response from a series of novel terahertz (THz) metasurfaces where asymmetry is introduced through the displacement of two adjacent metallic arms separated by a distance $\delta$. For all polarization states, the symmetric metasurface exhibits a low quality (Q) factor fundamental dipole mode. By breaking the symmetry, we experimentally observe a secondary dipole-like mode with a Q factor nearly $9\times$ higher than the fundamental resonance. As $\delta$ increases, the fundamental dipole mode $f_{1}$ redshifts and the secondary mode $f_{2}$ blueshifts creating a highly transmissive spectral window. Polarization-dependent measurements reveal a full suppression of $f_{2}$ for all asymmetries at $\theta \geq 60^\circ$. Furthermore, at $\delta \geq 60 \text{ }\mu\text{m}$, we observe a polarization selective electromagnetic induced transparency (EIT) for the fundamental mode. This work paves the way for applications in filtering, sensing and slow-light devices common to other high Q factor THz metasurfaces with EIT-like response.

Dual-surface flexible THz Fano metasensor

Sensing technologies based on terahertz waves have immense potential due to their non-destructive, transparent, and fingerprint spectral response of several materials that are opaque to other parts of the electromagnetic spectrum. Terahertz metasensors reported so far merely exploit the fringing electric field on the top of the subwavelength resonators. Here, we experimentally demonstrate an ultrathin flexible terahertz metamaterial sensor on a low refractive index substrate which enables sensing of analytes from the top and bottom surfaces of the metamaterial, opening up avenues for dual-surface sensing of analytes with fringing resonant fields on both front and rear sides of a metasurface. Since most of the real-world objects have 3D curvatures, the reported flexible metasensor with large mechanical strength and stability in free space would be an ideal platform for ultrasensitive sensing of dielectrics, chemicals, and biomolecules of extremely low concentrations with dual non-planar surfaces.

A flexible graphene terahertz detector

We present a flexible terahertz (THz) detector based on a graphene field-effect transistor fabricated on a plastic substrate. At room temperature, this detector reveals voltage responsivity above 2 V/W and estimated noise equivalent power (NEP) below 3 nW/Hz at 487 GHz. We have investigated the effects of bending strain on DC characteristics, voltage responsivity, and NEP of the detector, and the results reveal its robust performance. Our findings have shown that graphene is a promising material for the development of THz flexible technology.

Optically pumped terahertz sources

High-power terahertz (THz) generation in the frequency range of 0.1–10 THz has been a fast-developing research area ever since the beginning of the THz boom two decades ago, enabling new technological breakthroughs in spectroscopy, communication, imaging, etc. By using optical (laser) pumping methods with near- or mid-infrared (IR) lasers, flexible and practical THz sources covering the whole THz range can be realized to overcome the shortage of electronic THz sources and now they are playing important roles in THz science and technology. This paper overviews various optically pumped THz sources, including femtosecond laser based ultrafast broadband THz generation, monochromatic widely tunable THz generation, single-mode on-chip THz source from photomixing, and the traditional powerful THz gas lasers. Full descriptions from basic principles to the latest progress are presented and their advantages and disadvantages are discussed as well. It is expected that this review gives a comprehensive reference to researchers in this area and additionally helps newcomers to quickly gain understanding of optically pumped THz sources.

A ‘T’ shaped flexible multiband ultra-thin terahertz metamaterial with consistent curved transmission spectra

In this paper, we designed a flexible multiband ultra-thin terahertz metamaterial with four ‘T’ shaped beams deposited on parylene-C substrate to achieve sensing. The simulated and experimental investigations of transmission spectra showed two resonant peaks, respectively, which demonstrated a good numerical and experimental agreement. Due to this special ‘T’ shaped structure, the transmission spectra were nearly unchanged in the first resonant peak and slightly fluctuant in the second resonant peak after bending with a constant step, which successfully confirmed a pretty consistent curved transmission performance. A further study about the effect of polarization and incident angle direction of terahertz wave on transmission spectra indicated that the transmission of the sample did not vary with the change of polarization angles but would be influenced by incident angles. In order to explore the performance of the structure in terms of sensing phenomenon, a final contrast simulation on different surface analytes and without analyte was carried out, consequently suggesting a promising and remarkable function in sensing applications especially in detecting biomolecules.

Direct Writing of Flexible Barium Titanate/Polydimethylsiloxane 3D Photonic Crystals with Mechanically Tunable Terahertz Properties

Mechanically flexible 3D terahertz photonic crystals (3D‐TPCs) are created by the direct‐writing technology with a composite ink system composed of polydimethylsiloxane (PDMS) and barium titanate (BaTiO3) nanoparticles. The direct‐writing technology allows an easy creation of complex 3D structures with designed geometry, while the refractive indices of the composite ink can be modulated by varying the content of BaTiO3 nanoparticles. Thus, 3D‐TPCs with different terahertz properties are obtained by the direct‐writing technology. More interestingly, these 3D‐TPCs demonstrate a unique tunable terahertz property under external force field due to their mechanical flexibility from the PDMS matrix of the composite ink. Thus, their terahertz property is responsive to external force fields reversibly, which can find novel applications in terahertz technology and other related technological applications.

Solid analyte and aqueous solutions sensing based on a flexible terahertz dual-band metamaterial absorber

A high-sensitivity sensing technique was demonstrated based on a flexible terahertz dual-band metamaterial absorber. The absorber has two perfect absorption peaks, one with a fundamental resonance (f1) of the structure and another with a high-order resonance (f2) originating from the interactions of adjacent unit cells. The quality factor (Q) and figure of merit of f2 are 6 and 14 times larger than that of f1, respectively. For the solid analyte, the changes in resonance frequency are monitored upon variation of analyte thickness and index; a linear relation between the amplitude absorption with the analyte thickness is achieved for f2. The sensitivity (S) is 31.2% refractive index units (RIU−1) for f2 and 13.7% RIU−1 for f1. For the aqueous solutions, the amplitude of absorption decreases linearly with increasing the dielectric constant for the ethanol–water mixture of f1. These results show that the designed absorber cannot only identify a solid analyte but also characterize aqueous solutions through the frequency shift and amplitude absorption. Therefore, the proposed absorber is promising for future applications in high-sensitivity monitoring biomolecular, chemical, ecological water systems, and aqueous biosystems.

Terahertz endoscopic imaging for colorectal cancer detection: Current status and future perspectives.

Terahertz (THz) imaging is progressing as a robust platform for myriad applications in the field of security, health, and material science. The THz regime, which comprises wavelengths spanning from microns to millimeters, is non-ionizing and has very low photon energy: Making it inherently safe for biological imaging. Colorectal cancer is one of the most common causes of death in the world, while the conventional screening and standard of care yet relies exclusively on the physician’s experience. Researchers have been working on the development of a flexible THz endoscope, as a potential tool to aid in colorectal cancer screening. This involves building a single-channel THz endoscope, and profiling the THz response from colorectal tissue, and demonstrating endogenous contrast levels between normal and diseased tissue when imaging in reflection modality. The current level of contrast provided by the prototype THz endoscopic system represents a significant step towards clinical endoscopic application of THz technology for in-vivo colorectal cancer screening. The aim of this paper is to provide a short review of the recent advances in THz endoscopic technology and cancer imaging. In particular, the potential of single-channel THz endoscopic imaging for colonic cancer screening will be highlighted.

A flexible and wearable terahertz scanner

Imaging technologies based on terahertz (THz) waves have great potential for use in powerful non-invasive inspection methods. However, most real objects have various three-dimensional curvatures and existing THz technologies often encounter difficulties in imaging such configurations, which limits the useful range of THz imaging applications. Here, we report the development of a flexible and wearable THz scanner based on carbon nanotubes. We achieved room-temperature THz detection over a broad frequency band ranging from 0.14 to 39 THz and developed a portable THz scanner. Using this scanner, we performed THz imaging of samples concealed behind opaque objects, breakages and metal impurities of a bent film and multi-view scans of a syringe. We demonstrated a passive biometric THz scan of a human hand. Our results are expected to have considerable implications for non-destructive and non-contact inspections, such as medical examinations for the continuous monitoring of health conditions. A flexible and wearable terahertz scanner based on carbon nanotubes is demonstrated at room temperature over a frequency range 0.14 THz to 39 THz. The terahertz photothermoelectricity is enhanced by using different electrode materials.

Flexible single-mode hollow-core terahertz fiber with metamaterial cladding

A key requirement for achieving high-density integration of terahertz (THz) systems is a strongly confining single-mode and low-loss waveguide. Several waveguide solutions based on technologies from both electronics and photonics have been proposed; among these, hollow-core waveguides are one of the best options for guiding THz radiation due to their very low material absorption of air. However, to minimize reflection losses, hollow-core waveguides typically have a core diameter larger than the operating wavelength, and as a consequence are multimode. Here, we report on a single-mode, single-polarization hollow-core THz fiber with a metamaterial cladding, consisting of subwavelength-diameter metal wires embedded in a dielectric host. The idea of using metal–dielectric hybrid cladding relies on the extreme anisotropy of wire metamaterials, which reflects transverse magnetic (TM) waves and transmits transverse electric waves, leading to a waveguide structure that only confines TM modes—thus halving the number of modes from the outset. Numerical simulations and experimental measurements confirm a wide single-mode single-polarization window ranging from 0.31 to 0.44 THz, with a wavelength-sized core (0.88 mm diameter). Our work overcomes a stumbling block for achieving compact and flexible single-mode THz waveguides, which may be important for future THz systems with high-density integration.