Phase Of Terahertz Wave Research Articles

Terahertz metallic waveguide with meta-holes for bidirectional conversion between two-dimensional guided waves and free-space waves

The conversion between guided and free-space waves is crucial for achieving integrated terahertz (THz) communication and signal processing. Herein, a bidirectional conversion mechanism is proposed for bridging two-dimensional (2D) guided waves and free-space waves, which is demonstrated by the wave manipulation of a metallic waveguide with meta-holes (MWMH). Compared with the conventional conversion between one-dimensional guided waves and free-space waves, in the proposed bidirectional conversion process, meta-holes can arbitrarily manipulate the phase of THz waves in higher dimensions, which enables stronger beam-manipulation capability and a higher gain. When used as a transmitting antenna, the MWMH exhibits excellent performance, i.e., a high gain (33.3 dBi), a high radiation efficiency (∼90%), and flexible beam manipulation. When the MWMH is reversely employed as a receiving antenna to obtain the focus of 2D guided waves, it achieves a gain of 27 dB and a focusing efficiency of 50.4%. The measured results for both the transmitting and receiving antennas agree well with the simulation results. The proposed bidirectional conversion mechanism facilitates the development of THz integrated photonic devices and is promising for application in the sixth-generation mobile communication, radar detection, and nondestructive testing.

High-Aspect-Ratio Silicon Metasurfaces: Design, Fabrication, and Characterization

Unrestricted manipulations on terahertz (THz) waves are highly desired in integration-optics, but THz devices based on conventional materials are usually bulky in size. Although all-silicon metasurfaces have exhibited great capabilities in manipulating THz waves, most of them are less efficient and have limited functionalities. In this work, we first design a silicon meta-atom structure consisting of a high-aspect-ratio (AR) micro-pillar that exhibits nearly perfect transmission and large transmission phase of THz wave, and systemically study how the fabrication quality (e.g., steepness of the sidewall and the vertical thickness distribution) may influence the final performance of a functional metasurface constructed with such meta-atoms. After experimentally examining how two working phases in the deep-reactive-ion-etch technology (i.e., the etch and passivation phases) influence the quality of the fabricated meta-devices, we develop an optimized Bosch process to realize high-AR (~20:1) all-silicon metasurfaces by balancing two working phases. We finally design/fabricate a high-AR silicon metasurface and experimentally demonstrate that it behaves as a high-efficiency half-wave-plate for THz waves in transmission geometry. Our results pave the avenue to realize ultra-compact THz meta-devices with high performance in transmission geometry, which is highly desirable for THz applications.

Amplitude and phase independent modulation based on transmission-type metasurfaces

We propose an ultra-thin I-type metasurface that independently controls the amplitude and phase of transmitted waves simultaneously by changing the geometry and direction of rotation of I-shaped metal particles. It is found that the direction of rotation and the size of the geometrical opening of the I-shaped copper particle independently control the amplitude and phase of the incident wave. In order to better demonstrate the independent regulation of the amplitude and phase of the metasurface, we arranged the element structure and constructed a grating structure. We can adjust the diffraction order of grating freely by arranging different element structures. Using classical lithography techniques, we have experimentally prepared the array structure samples and compared the theoretical results with the experimental measurements. Theory and experiment agree well. We further propose a two-layer structure to simultaneously regulate the amplitude and phase of terahertz waves. The amplitude and phase of transmitted terahertz waves can be independently regulated by the rotation angle and opening angle of the double-layer cell structure. We further prepared the double-layer structure sample, and the theoretical calculation results are in good agreement with the experimental measurement results.

Function switchable broadband wave plate based on the Au-VO2 hybrid metasurface.

In recent years, the integration of active materials into a metasurface to achieve tunable devices has attracted much attention. Here, we design an Au-VO2 hybrid metasurface, which can switch between quarter-wave plate and half-wave plate due to the phase transition of VO2. At 298 K, the proposed structure acts as a quarter-wave plate in the 0.87-1.2 THz band, achieving the mutual conversion between linear polarization and circular polarization. Raising the temperature to 358 K, it works as a broadband half-wave plate in the range of 0.65-1.45 THz, with the reflective chirality preservation of circular polarization and the cross-polarization conversion of linear polarization. In the above cases, the response efficiencies are both above 90%. The switchable multifunction results from the tunable geometric phase of the metasurface, where the elaborately designed Au and VO2 blocks separately bring the phase of π/2. Furthermore, the electric field and current density distributions are employed to explain the physical mechanisms leading to the different functions. Such an active broadband metasurface is expected to find applications in tunable and multifunction devices manipulating the polarization and phase of terahertz waves.

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.

Active and Smart Terahertz Electro-Optic Modulator Based on VO2 Structure.

Modulating terahertz (THz) waves actively and smartly through an external field is highly desired in the development of THz spectroscopic devices. Here, we demonstrate an active and smart electro-optic THz modulator based on a strongly correlated electron oxide vanadium dioxide (VO2). With milliampere current excitation on the VO2 thin film, the transmission, reflection, absorption, and phase of THz waves can be modulated efficiently. In particular, the antireflection condition can be actively achieved and the modulation depth reaches 99.9%, accompanied by a 180° phase switching. Repeated and current scanning experiments confirm the high stability and multibit modulation of this electro-optic modulation. Most strikingly, by utilizing a feedback loop of "THz-electro-THz" geometry, a smart electro-optic THz control is realized. For instance, the antireflection condition can be stabilized precisely no matter what the initial condition is and how the external environment changes. The proposed electro-optic THz modulation method, taking advantage of strongly correlated electron material, opens up avenues for the realization of THz smart devices.

Phase control of terahertz waves moves on chip

The phase of terahertz waves can now be precisely modulated electronically using a chip-based digitally coded phase shifter. The achievement is a step towards chip-scale integrated terahertz technology.

Multi-parameter tunable phase transition based terahertz graphene plasmons and its application

The active modulation of the amplitude and phase of terahertz wave has been widely adopted in terahertz functional devices. The current metal-insulator-metal metasurface structure combined with two-dimensional materials such as graphene can realize dynamic control of terahertz amplitude/phase, but it has some disadvantages such as less freedom of control (voltage or light intensity), complex processing technology and high price of metasurface structure. In this article, we propose a prism-coupled matel-insulator-graphene (MIG) phase regulation structure. This structure can not only control the phase by adjusting the Fermi level in the usual way, but also change the intrinsic loss and radiation loss of the structure by adjusting the thickness of the air gap and the number of layers of pre-spread graphene, so that the phase of the structure can be controlled, which is determined by the difference between intrinsic loss and radiation loss of the fabric, which is closely related to this structure staying in the under-coupling/over-coupling state. The adjustment of the structural phase can also lead the magnitude of the terahertz Goos–Hänchen(GH) displacement and its positive sign and negative sign to be selected. Furthermore, it is shown that the under-coupling state and the over-coupling state of the structure have an important effect on the coincidence of the Goos–Hanchen (GH) displacement. The results show that by dynamically adjusting the thickness of the air gap and the Fermi level of graphene, and changing the eigenloss and radiation loss of the system, the phase regulation can be achieved. Finally, the transition from overdamped to underdamped state is realized. In this physical process, the GH displacement of the system will also change obviously. This paper puts forward the structure of the process with simple processing technology (no need to microstructure), tunable high degrees of freedom (available graphene Fermi level and air gap dynamic regulation, also could be regulated and controlled by controlling the graphene layers) in comparison with the phase modulator of metal-insulator-metal super surface structure. The results of this paper open up a new way of developing the multi-parameter tunable terahertz sensor components.

Photoinduced Broad-band Tunable Terahertz Absorber Based on a VO2 Thin Film.

The demand for terahertz (THz) communication and detection fuels continuous research for high performance of THz absorption materials. In addition to varying the materials and their structure passively, an alternative approach is to modulate a THz wave actively by tuning an external stimulus. Correlated oxides are ideal materials for this because the effects of a small external control parameter can be amplified by inner electronic correlations. Here, by utilizing an unpatterned strongly correlated electron oxide VO2 thin film, a photoinduced broad-band tunable THz absorber is realized first. The absorption, transmission, reflection, and phase of THz waves can all be actively controlled by an external pump laser above room temperature. By varying the laser fluence, the average broad-band absorption can be tuned from 18.9 to 74.7% and the average transmission can be tuned from 9.2 to 69.2%. Meanwhile, a broad-band antireflection is obtained at 5.6 mJ/cm2, and a π-phase shift of a reflected THz wave is achieved when the fluence increases greater than 5.7 mJ/cm2. Apart from other modulators, the photoexcitation-assisted dual-phase competition is identified as the origin of this active THz multifunctional modulation. Our work suggests that advantages of controllable phase separation in strongly correlated electron systems could provide viable routes in the creation of active optical components for THz waves.

Coherent Control of Terahertz Wave from Coherent Longitudinal Optical Phonon in a GaAs/AlAs Multiple-Quantum-Well Structure

We report on the coherent control of terahertz (THz) waves emitted from coherent longitudinal optical (LO) phonons in a GaAs/AlAs multiple-quantum-well structure at room temperature using two-pulse excitation. When the time difference between the first pump pulse and second weak control pulse is increased, the intensity of THz waves from the coherent phonons is cancelled and enhanced by the control pulse, and the phase of THz waves shows periodic changes accompanied by a phase jump. The time-difference dependences of the intensity and phase of THz waves are explained in terms of the superposition of two coherent oscillations.