Terahertz Phase Research Articles

Electrically tunable dual-layer twisted nematic liquid crystal THz phase shifters with intermediate composite polymer thin film

We demonstrate electrically tunable twisted nematic (TN) aligned liquid crystal (LC) terahertz phase shifters with a novel structure, in which a single LC composite polymer thin film is inserted in the middle of the LC cell. For designing such types of phase shifters, we have applied the Mauguin formalism used in LC-based switches for applications in the visible. Experimental results are in good agreement with those predicted by the theoretical model. Using this design, the phase shift has been increased by 14.3° at 1.2 THz in comparison to the conventional design (98.1°). The threshold voltage has been lowered from 0.81 VRMS to 0.48 VRMS while the driving voltage for quarter-wave (π/2) phase shift operation has been reduced from 8.4 VRMS to 6.3 VRMS at 1.2 THz. The dynamic response of devices also enhances significantly. Such design has also successfully been adopted to demonstrate a 2π phase shifter with similar improvement. Full-wave operation is achieved at a bias of 25 VRMS at 1.2 THz using a highly birefringent LC material. Order parameter calculations show that the polymer film improves significantly LC molecular alignment in the thick LC cell for THz applications. We have also compared the figure of merits (FOMs) of the present devices with previously reported THz π/2 and 2π phase shifters.

Electrically active control of terahertz waves in a hybrid metasurface-grating structure with a liquid crystal.

A reflective terahertz phase shifter for wide-range dynamic and continuous phase modulation is proposed. By injecting a tunable liquid crystal between the slotted metasurface and the grating microstructure, the phases of reflected waves can be modulated with different electrically driven methods. Numerical simulations show that the proposed terahertz phase shifter has a phase difference of more than 360° between unbiased and biased states. Furthermore, an array of 35×35 patch elements was designed and fabricated. The performance of the phase shifter provides more than 360° between 379.6 and 391.8 GHz, where the maximum phase shift reaches 422.4° at 385.9 GHz. Moreover, the fully electrically controlled phase modulation of more than 180° is achieved between 382.0 and 394.1 GHz, with a maximum phase modulation of 248.4° at 383.3 GHz. This work may provide a reflective terahertz phase modulator for beam steering.

Ultrafast Terahertz Self-Induced Absorption and Phase Modulation on a Graphene-Based Thin Film Absorber

Interaction of intense terahertz (THz) waves with graphene based modulation devices holds great potential for the optoelectronic applications of the future. Here, we present a thin film graphene-based THz perfect absorbing device whose absorption and phase characteristics can be modulated through THz self-actions in the subps time scale. The device consists of a single-layer graphene placed on an ionic liquid substrate, back-plated by a metallic back-reflector, with the graphene doping level mediated through electrostatic gating. We experimentally record an absorption modulation of more than 3 orders of magnitude from the initial perfect absorption state, when the device is illuminated with THz field strengths in the range of 102 to 654 kV/cm. Furthermore, an absolute phase modulation of 130° is recorded. Detailed theoretical analysis indicates that the origin of the THz nonlinear response is the THz-induced heating of the graphene’s carriers that leads to a reduction of its conductivity and, consequently, to reduced absorption of the THz radiation. Our analysis also maps the temporal dynamics of the THz-induced temperature elevation of the graphene’s carriers, illustrating the ultrafast, subps nature of the overall process. These results can find applications in future dynamically controlled flat optics and spatiotemporal shaping of intense THz electric fields.

Active control of terahertz amplitude and phase based on graphene metasurface

Metasurfaces with actively tunable features are highly demanded for advanced applications in terahertz (THz) systems. However, realizing dual-tunability is still challenging. In this paper, we demonstrate a dual function modulator based on graphene metasurface. It can actively control the amplitude and phase of THz wave. The unit cell of the device is made up of two T-shaped resonators and a graphene patch between two resonators. By electrically tuning the Fermi level of the graphene patch, the maximum amplitude modulation depth at the central frequency of 0.55 THz can be up to 92%. Meanwhile, the dynamic phase modulation at 0.3 THz can change from −20° to 70°, and the maximum phase difference reaches to 90°. When the incident angle is up to 60°, the amplitude modulation depth is still high than 80%, and the phase difference is more than 70°. Therefore, the device has good performance for incident angles. Our work provides deeper insight into multifunctional integrated THz devices, which will enable a variety of practical applications, such as THz imaging, tunable lens and phase array antenna . • A dual function modulator based graphene metasurface was proposed. • The amplitude and phase of terahertz wave can be actively controlled by changing the Fermi level of graphene. • The modulation depth is high to 92%, and the phase modulation is 90°. • The maximum transmission is 0.7 when the Fermi level is 1 eV.

Rapid terahertz wave manipulation in a liquid-crystal-integrated metasurface structure.

A terahertz phase shifter based on liquid-crystal-integrated metasurface is proposed, which contains a three-slotted array structure and comb grating. The orientation of the liquid crystal molecules can be completely controlled by the direction of the electric field. From the acquired experimental results, it was demonstrated that the phase shift exceeds 300° in the range of 378.6 - 390.8 GHz, whereas the maximum phase shift reaches 374.1° at 383.1 GHz. The molecular reorientation transient process induced by the external electric field in the liquid crystal was measured and analyzed. Based on the molecular reorientation mechanism, which can be divided into three processes, a rapid modulation mechanism was demonstrated. From the performance of the proposed device, an actively controllable phase delay and reflectance with a cycle switching time of approximately 0.3 s was achieved, which is remarkably faster than the usual cycle time that exceeds 8 s. Our work provides useful ideas for improving the response speed of LC-based terahertz devices, which is considered of great significance for several applications, in terms of terahertz reconfigurable devices.

Tunable terahertz phase shifter based on GaAs semiconductor technology

We devised an electronically controllable plasmonic modulator capable of changing the phase of the transmitted electromagnetic wave. It is based on a well-established GaAs semiconductor technology. We demonstrate the phase tunability of the device over the range of up to 41° at the insertion loss of −2.2 dB. The phase shifter operates at frequencies of up to 0.27 THz and temperatures of up to 80 K. The design is readily scalable to a planar phased array—a key component in beamforming technologies used in THz communication.

Sub‐THz Phase Shifters Enabled by Photoconductive Single‐Walled Carbon Nanotube Layers

Materials with tunable dielectric properties are highly relevant for terahertz (THz) applications. Herein, the tuning of the dielectric response of single‐walled carbon nanotube layers by light illumination is studied for applications to THz phase shifters. The dependence of the length of individual nanotubes on the THz photoconductivity of the network is experimentally investigated in the frequency range of 0.2–1 THz by time‐domain spectroscopy (TDS). The effective conductivity of the networks is described by a theoretical model that fits the measured dielectric function. Terahertz phase shifters are realized with the carbon nanotube layers as the optically tunable element deposited on the wall of rectangular dielectric waveguides. The phase of the electromagnetic wave propagating in the waveguide is shown to be tunable by illuminating the nanotubes. A linear phase shift with the frequency is measured between 75 and 500 GHz with a low change in amplitude due to the illumination.

STO/PDMS composite film achieving continuously tunable terahertz phase modulation via mechanical and thermal regulation

Phase is the basic property of electromagnetic waves, and controlling the phase is one of the eternal research topics in the field of electromagnetic waves. Here, a broadband terahertz phase shifter made of polydimethylsiloxane (PDMS) and strontium titanate (STO) has been theoretically proposed and experimentally demonstrated. The mixed material design gives the phase shifter the dual properties of PDMS and STO. Therefore, the phase shifter can not only change the thickness through stretching but also change the effective refractive index through temperature changes. The thickness and refractive index changes directly cause the transmission phase of the phase shifter to move synchronously. Because the material does not have frequency selection characteristics, the phase shifter realizes customized phase control in a wide range of the terahertz band. In the experiment, any phase shift can be achieved by adjusting the doping ratio, stretching ratio, and temperature. The proposed broadband phase shifter is simple to fabricate and has various control methods, which provides a variety of modes of terahertz wave phase modulation.

Chocolate inspection by means of phase-contrast imaging using multiple-plane terahertz phase retrieval.

Terahertz (THz) radiation has shown enormous potential for non-destructive inspection in many contexts. Here, we present a method for imaging defects in chocolate bars that can be extended to many other materials. Our method requires only a continuous wave (CW) monochromatic source and detector at relatively low frequencies (280 GHz) corresponding to a relatively long wavelength of 1.1 mm. These components are used to construct a common-path configuration enabling the capturing of several images of THz radiation diffracted by the test object at different axial depths. The captured diffraction-rich images are used to constrain the associated phase retrieval problem enabling full access to the wave field, i.e., real amplitude and phase distributions. This allows full-field diffraction-limited phase-contrast imaging. Thus, we experimentally demonstrate the possibility of identifying contaminant particles with dimensions comparable to the wavelength.

Dynamic and Active THz Graphene Metamaterial Devices.

In recent years, terahertz waves have attracted significant attention for their promising applications. Due to a broadband optical response, an ultra-fast relaxation time, a high nonlinear coefficient of graphene, and the flexible and controllable physical characteristics of its meta-structure, graphene metamaterial has been widely explored in interdisciplinary frontier research, especially in the technologically important terahertz (THz) frequency range. Here, graphene’s linear and nonlinear properties and typical applications of graphene metamaterial are reviewed. Specifically, the discussion focuses on applications in optically and electrically actuated terahertz amplitude, phase, and harmonic generation. The review concludes with a brief examination of potential prospects and trends in graphene metamaterial.

Enhanced Terahertz Phase Retrieval Imaging by Unequal Spaced Measurement.

Terahertz lensless phase retrieval imaging is a promising technique for non-destructive inspection applications. In the conventional multiple-plane phase retrieval method, the convergence speed due to wave propagations and measures with equal interval distance is slow and leads to stagnation. To address this drawback, we propose a nonlinear unequal spaced measurement scheme in which the interval space between adjacent measurement planes is gradually increasing, it can significantly increase the diversity of the intensity with a smaller number of required images. Both the simulation and experimental results demonstrate that our method enables quantitative phase and amplitude imaging with a faster speed and better image quality, while also being computationally efficient and robust to noise.

Single-scan multiplane phase retrieval with a radiation of terahertz quantum cascade laser

Terahertz phase retrieval from a set of axially separated diffractive intensity distributions is a promising single-beam computational imaging technique that ensures the obtention of high spatial resolutions and phase wavefronts, but remains restricted by time-consuming data acquisition processes. In this work, we have adopted an approach, relying on the radiation of a quantum cascade laser and the implementation of an express single-scan measurement of intensity distributions through the continuous on-the-go displacement of a high-sensitivity antenna-coupled microbolometer sensor array. In addition to the simplicity of this practical implementation and the minimization of measurement times, such an approach overcomes the problem of preliminary optimal selections of transverse intensity distributions used in the iterative phase retrieval algorithm and guarantees the required data diversity for high-quality wavefront reconstruction.

Terahertz dynamic π-phase modulation with high transmittance using graphene-metal metamaterials

Dynamic phase modulation of electromagnetic waves is desirable in many applications such as beaming, tunable focusing, and holography. However, it remains challenging to achieve both large dynamic phase shift and high transmittance based on terahertz metamaterials. Here we propose a double-layered graphene-metal metamaterial for achieving transmissive terahertz phase modulator with large dynamic phase modulation range and meanwhile high transmittance. Numerical results show that a large dynamic transmission phase shift reaching up to 180∘ and relatively high transmittance above 22% can be achieved simultaneously by varying the graphene Fermi level from 0 to 1 eV. We attribute this striking performance to the transition of the resonance type when the graphene Fermi level is tuned across a threshold. We expect that the proposed dynamic phase modulator will find applications in the tunable lens, holographic imaging and phased array antenna in terahertz regime.

Large Terahertz Phase Shift Induced by Photothermal Effect of Gold Nanoparticles Incorporated with Toluene

AbstractPhase and amplitude modulation of propagating terahertz (THz) waves in free space is essential for many rapidly developing THz technologies. Herein, an all‐optical, high‐performance, and ultra‐broadband THz phase shifter is demonstrated. Tunable phase shift is achieved by the photothermal effect of plasmonic gold nanoparticles in toluene, which can induce variations in the THz refractive index of toluene as the temperature increases. A phase shift of 270° at 1 THz can be achieved under a laser irradiation of 2.5 W (3.9 W cm−2) on the mixture solution. Constructive and destructive interference of THz waves can be achieved by tuning the relative phase of two split THz waves, which can be controlled by the power of the irradiation laser. This is the first broadband THz phase modulator in free space driven by optical stimuli, and it provides a new solution for tuning the phase shift of THz waves.

Terahertz dynamic π-phase modulation with high transmittance using graphene-metal metamaterials

Abstract Dynamic phase modulation of electromagnetic waves is desirable in many applications such as beaming, tunable focusing, and holography. However, it remains challenging to achieve both large dynamic phase shift and high transmittance based on terahertz metamaterials. Here we propose a double-layered graphene-metal metamaterial for achieving transmissive terahertz phase modulator with large dynamic phase modulation range and meanwhile high transmittance. Numerical results show that a large dynamic transmission phase shift reaching up to 180” and relatively high transmittance above 22% can be achieved simultaneously by varying the graphene Fermi level from 0 eV to 1 eV. We attribute this striking performance to the transition of the resonance type when the graphene Fermi level is tuned across a threshold. We expect that the proposed dynamic phase modulator will find applications in the tunable lens, holographic imaging and phased array antenna in terahertz regime.

Lensless Fourier-transform terahertz digital holography for real-time full-field phase imaging

With the development of continuous-wave terahertz (THz) sources and array detectors, the pursuit of high-fidelity real-time imaging is receiving significant attention within the THz community. Here, we report a real-time full-field THz phase imaging approach based on lensless Fourier-transform THz digital holography. A triangular interferometric layout is proposed based on an oblique illumination of 2.52 THz radiation, which is different from other lensless holographic configurations at other frequencies. A spherical reference beam is generated by a reflective parabolic mirror with minor propagation loss. The complex-valued images are reconstructed using a single inverse Fourier transform of the hologram without complex calculation of the diffraction propagation. The experimental result for a Siemens star validates the lateral resolution of ∼ 346 μm in the diagonal direction. Sub-pixel image registration and image stitching algorithms are applied to enlarge the area of the reconstructed images. The dehydration process of an aquatic plant leaf (Hottonia inflata) is monitored for the first time, to the best of our knowledge, at the THz band. Rapid variations in water content and morphology are measured with a time interval of 0.6 s and a total time of 5 min from a series of reconstructed amplitude and phase images, respectively. The proposed method has the potential to become a powerful tool to investigate spontaneous phenomena at the THz band.

Single- Versus Multicarrier Terahertz-Band Communications: A Comparative Study

The prospects of utilizing single-carrier (SC) and multi-carrier (MC) waveforms in future terahertz (THz)-band communication systems remain unresolved. On the one hand, the limited multi-path components at high frequencies result in frequency-flat channels that favor low-complexity wideband SC systems. On the other hand, frequency-dependent molecular absorption and transceiver characteristics and the existence of multi-path components in indoor sub-THz systems can still result in frequency-selective channels, favoring off-the-shelf MC schemes such as orthogonal frequency-division multiplexing (OFDM). Variations of SC/MC designs result in different THz spectrum utilization, but spectral efficiency is not the primary concern with substantial available bandwidths; baseband complexity, power efficiency, and hardware impairment constraints are predominant. This paper presents a comprehensive study of SC/MC waveforms for THz communications, utilizing an accurate wideband THz channel model and highlighting the various performance and complexity trade-offs of the candidate schemes. Simulations demonstrate that discrete-Fourier-transform spread orthogonal time-frequency space (DFT-s-OTFS) achieves a lower peak-to-average power ratio (PAPR) than OFDM and OTFS and enhances immunity to THz impairments and Doppler spreads, but at an increased complexity cost. Moreover, DFT-s-OFDM is a promising candidate that increases robustness to THz impairments and phase noise (PHN) at a low PAPR and overall complexity.

Terahertz liquid crystal phase shifter based on metamaterial composite structure

In order to solve the problems of large, uncontrollable insertion loss and small phase shift of the existing terahertz phase shifters, in this work a kind of terahertz phase shifter realized by a simple metamaterial composite structure is designed. The device is composed of four layers in structure, they from top to bottom, being an L-shaped metal resonance layer, a liquid crystal layer, a bow-shaped metal layer, and a quartz substrate layer. By applying a bias voltage to the upper metal layer and the lower metal layer, the deflection angle <i>α</i> of the director of liquid crystal molecules in the liquid crystal cell is changed, so that the effective refractive index of the liquid crystal changes, and the phase of the device also changes accordingly, thereby achieving the purpose of dynamic phase control. The performances of the upper metal layer, the lower metal layer and liquid crystal layer are optimized and compared with each other. The performance characteristics of the phase shifter under different values of deflection angle <i>α</i> and different values of incident angle <i>θ</i> are analyzed by frequency domain finite integration method. Through the simulation optimization and comparison of the size of the upper and lower metal layers and the thickness of the liquid crystal layer, the optimum is obtained. The simulation results show that the transmittance of the terahertz liquid crystal phase shifter can reach 0.968 in a frequency between 1.68–1.78 THz, and the insertion loss can be as low as 0.3 dB. When the frequency is 1.7396 THz, the maximum phase shift of the terahertz phase shifter is 352.625°. The phase shift exceeds 352° in a frequency range of 1.7315–1.7396 THz (Bandwidth is 8.1 GHz). This simple metamaterial multilayer structure provides a new method of controlling terahertz waves, and has broad application prospects in terahertz imaging, sensing and other fields.

A review of terahertz phase modulation from free space to guided wave integrated devices

Abstract In the past ten years, terahertz technology has developed rapidly in wireless communications, spectroscopy, and imaging. Various functional devices have been developed, such as filters, absorbers, polarizers, mixers, and modulators. Among these, the terahertz phase modulation is a current research hotspot. It is the core technology to realize flexible control of the terahertz wavefront, beam scanning, focusing deflection. It is indispensable in terahertz wireless communication, high-resolution imaging, and radar systems. This review summarizes the research progress of terahertz phase modulators from the two major types: free space and guided wave integration. Among these, the free space terahertz phase modulator is realized by combining the tunable materials and artificial metasurfaces. Based on different types of tunable materials, the terahertz free space phase modulator combining the semiconductor, liquid crystal, phase change materials, graphene, and other two-dimensional materials are introduced, and the influence of different materials on the phase modulation performance is discussed and analyzed. The monolithic integration and waveguide embedding methods are introduced separately, and the characteristics of different forms of terahertz-guided wave phase modulation are also discussed. Finally, the development trends of terahertz phase modulators, possible new methods, and future application requirements are discussed.

A 420-GHz Sub-5-μm Range Resolution TX–RX Phase Imaging System in 40-nm CMOS Technology

This article presents a 420-GHz phase imaging system designed in a 40-nm CMOS technology. It consists of a transmitter (TX), based on a multiplier chain, and a receiver (RX), based on a two-step IQ down-conversion. Those chips share an external, 17.5 GHz, reference to secure frequency synchronization between them. To increase the overall system signal-to-noise ratio (SNR), the TX is modulated with a low-frequency sine-wave signal, while a two-way LO power combining is implemented for the RX first mixer. Those techniques result in a measured TX effective isotropic radiated power (EIRP) of 10 dBm and RX noise figure (NF) of 27 dB, leading to the overall SNR of 52 dB (at a distance of 25 cm and a resolution bandwidth (RBW) of 100 kHz). Furthermore, the measured phase-detection root-mean-square (rms) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\sigma $ </tex-math></inline-formula> precision is equal to 1.7° (on a 400 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\circ $ </tex-math></inline-formula> range, at a distance of 25 cm and processing time of 500 ns), which leads to 3.4- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> range resolution. In addition, the system operation is illustrated in two imaging demonstrations, recognition of the printed text on paper and 3-D imaging. Those demonstrations illustrate the potential of the presented system and terahertz (THz) phase imaging in general.