Abstract-Longitudinal Component Properties of Circularly Polarized Terahertz Vortex Beams

 

Miao Wang, Xinke Wang, Peng Han, Wenfeng Sun, Shengfei Feng, Jiasheng Ye, Yan Zhang

https://www.frontiersin.org/articles/10.3389/fphy.2021.736831/full

A circularly polarized vortex beam possesses similar focusing properties as a radially polarized beam. This type of beam is highly valuable for developing optical manufacturing technology, microscopy, and particle manipulation. In this work, a left-hand circularly polarized terahertz (THz) vortex beam (CPTVB) is generated by utilizing a THz quarter wave plate and a spiral phase plate. Focusing properties of its longitudinal component Ez are detailedly discussed on the simulation and experiment. With reducing the F-number of the THz beam and comparing with a transverse component Ex of a general circularly polarized THz beam, the simulation results show that the focal spot size and intensity of its Ez component can reach 87 and 50% of Ex under a same focusing condition. In addition, the experimental results still demonstrate that the left-hand CPTVB can always maintain fine Ez focusing properties in a broad bandwidth, which manifest the feasibility of this class of THz beams.

Abstract-Time‐Resolved Terahertz Spectroscopy Studies on 2D Van der Waals Materials

Peng Han,  Xinke Wang,  Yan Zhang,

https://onlinelibrary.wiley.com/doi/10.1002/adom.201900533

Owing to the fascinating and technologically useful electronic and optical properties, 2D van der Waals (vdW) materials are viewed as the key component for the next‐generation optoelectronic, photovoltaic, and nanoelectronic devices. Fully understanding the ultrafast carrier dynamics in 2D vdW materials is essential to study the fundamental physics and realize potential applications. Time‐resolved terahertz (THz) spectroscopy is a powerful tool used to investigate the ultrafast carrier dynamics and transport properties in semiconducting materials. In recent years, the time‐domain THz spectroscopy measurements, with or without photoexcitation on samples, are widely used to investigate the carrier dynamics of 2D vdW materials. A brief summary on the recent progress of time‐domain THz spectroscopy studies on ultrafast dynamics processes is provided. These include exciton formation and relaxation processes in semiconducting 2D materials, the relaxation of hot electrons in graphene layers, the conductivity of 2D materials modulated by femtosecond laser pulses, and their related physical properties. Finally, the physical properties of 2D vdW materials observed using the time‐domain THz spectroscopy approach are highlighted and the future challenges for time‐resolved THz spectroscopy investigations on 2D vdW materials are presented.

Abstract-Contribution of the optical rectification in terahertz radiation driven by two-color laser induced plasma

Hang Li, Ying Zhang, Wenfeng Sun, Xinke Wang, Shengfei Feng, Peng han, Jiasheng Ye, and Yan Zhang

Schematic diagram of the experimental setup.

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-28-4-4810

In the terahertz (THz) generation driven by two-color laser pulses, the THz wave radiated from the BBO crystal as the effect of the optical rectification is always assumed to be less and negligible. In this paper, the contribution of the optical rectification in the THz radiation driven by two-color laser pulses has been determined quantitatively, by the crucial factors including BBO crystal rotation angle, the pump power of laser, and the numerical aperture of lens. The experimental and simulation results show that the above related factors have dramatically affected the intensity ratio of the THz waves from the plasma and BBO crystal. It is helpful for understanding the mechanism of THz generation from air plasma.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Active metasurface devices for terahertz wave front modulation

Yan Zhang, Jingying Guo, Teng Wang, and Xinke Wang

https://www.osapublishing.org/abstract.cfm?uri=CLEO_Europe-2019-cc_p_8

Terahertz (THz) radiation, which is sandwiched between the infrared and microwave in the electromagnetic spectrum, has become more and more attractive due to its special properties. Compared with the quick development of THz sources and detectors, THz functional devices are lagging behind, especially the devices for wave front modulation. Metasurface, a kind of artificial materials, is enable effective manipulation of the amplitude, phase and polarization state of electromagnetic waves with subwavelength spatial resolution. It has been used for ultrathin planar lenses, complex light field generation, spin orbital angular momentum coupling, and metasurface holograms. However, the function of this kind of metasurface based devices are fixed once they are fabricated, the spatial THz modulation which can dynamically modulate the wave front of THz radiation is demand in high resolution THz imaging and THz communication.

© 2019 IEEEhttps://www.osapublishing.org/abstract.cfm?uri=CLEO_Europe-2019-cc_p_8

Abstract-Thermally switchable terahertz wavefront metasurface modulators based on the insulator-to-metal transition of vanadium dioxide

Teng Wang, Jinwen He, Jinying Guo, Xinke Wang, Shengfei Feng, Florian Kuhl, Martin Becker, Angelika Polity, Peter J. Klar, and Yan Zhang

Fig. 1 Schematic image of the setup used for studying operational characteristics of the THz wavefront modulators as a function of temperature. (a) The device has no effect on the THz wave in its off-state (e.g., at T = 20 °C < TC). The THz device acts on the THz wave in its on-state (e.g., at T = 70 °C > TC) as (b) a multiple foci lens or (c) a Ariy beam generator. TTML: Temperature controlled THz multi-focus lens. TTAG: Temperature controlled THz Ariy beam gererator.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-15-20347

Active use of phase transition phenomena for reversibly tuning the properties of functional materials in devices currently is an attractive research area of materials science. We designed and fabricated two kinds of metasurface modulators for dynamically controlling the wavefront of terahertz (THz) radiation based on the temperature-induced insulator-to-metal phase transition of vanadium dioxide (VO2). The modulators designed are based on the C-shaped slot antenna array. The slot antennas are made of the VO2 films on c-sapphire substrates. The C-shaped slot antennas are active only when the VO2 is in its metallic phase, i.e. at temperatures T > TC ∼68 °C. At T > TC, the first kind acts as a THz multi-focus lens which converges an incident THz plane wave into four focal spots and the second kind as an Airy beam generator. We characterized the function of two THz wavefront modulators over a broad frequency range, i.e. from 0.3 to 1.2 THz. Such thermally switchable THz wavefront metasurface modulators with a capability of dynamically steering THz fields will be of great significance for the future development of THz active devices.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz image reconstruction based on compressed sensing and inverse Fresnel diffraction

Yingjie Shang, Xinke Wang, Wenfeng Sun, Peng Han, Jiasheng Ye, Shengfei Feng, and Yan Zhang

Fig. 1 (a) Schematic illustration of single-pixel THz imaging based on photoinduced dynamic masks. (b) Procedure of the proposed image reconstruction scheme. (c)-(h) give the simulation results, including (c) sample, (d) original THz temporal image, (e) THz diffraction image at temporal peak position, (f) THz temporal image recovered by CS, (g) THz spectral image at 0.3 THz, (h) THz image reconstructed by the IFD algorithm.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-10-14725

The introduction of compressed sensing (CS) effectively pushes the development of single-pixel THz imaging due to reducing the experimental time and avoiding raster scanning. In this work, a CS method based on photoinduced dynamic masks is employed to recover a THz diffraction field in the time domain, and an inverse Fresnel diffraction (IFD) integral is adopted to remove the influence of the diffraction and reconstruct the sharp THz spectral image in a single-pixel THz imaging system. The compatibility of the CS and IFD algorithms are validated on the simulation and experiment. Besides, the reconstruction effects are also systematically analyzed by reducing the measurement number and varying the diffraction distance, respectively. This work supplies a novel thinking for improving the practicability of single-pixel THz imaging.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Modulation of terahertz radiation from graphene surface plasmon polaritons via surface acoustic wave

Sichen Jin, Xinke Wang, Peng Han, Wenfeng Sun, Shengfei Feng, Jiasheng Ye, Chao Zhang,  Yan Zhang,

Fig. 1 (a) Three-dimensional and (b) side schematic views of a moving electron beam atop a graphene layer on a piezoelectric MoS2 flake under an applied surface acoustic wave (SAW) field. The vacuum layer, the MoS2 flake with the applied SAW field, and the substrate layer are labeled as regions I, II, and III, respectively. The distance between the electron beam and the graphene layer and the thickness of the MoS2flake are labeled 𝑏$b$ and d$\text{d}$, respectively. (c) Schematic illustration of the electron and hole distributions in the SAW-induced type-II band-edge modulation of the n-doped MoS2 flake.

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-27-8-11137

We present a theoretical study of terahertz (THz) radiation induced by surface plasmon polaritons (SPPs) on a graphene layer under modulation by a surface acoustic wave (SAW). In our gedanken experiment, SPPs are excited by an electron beam moving on a graphene layer situated on a piezoelectric MoS2 flake. Under modulation by the SAW field, charge carriers are periodically distributed over the MoS2 flake, and this causes periodically distributed permittivity. The periodic permittivity structure of the MoS2 flake folds the SPP dispersion curve back into the center of the first Brillouin zone, in a manner analogous to a crystal, leading to THz radiation emission with conservation of the wavevectors between the SPPs and the electromagnetic waves. Both the frequency and the intensity of the THz radiation are tuned by adjusting the chemical potential of the graphene layer, the MoS2 flake doping density, and the wavelength and period of the external SAW field. A maximum energy conversion efficiency as high as ninety percent was obtained from our model calculations. These results indicate an opportunity to develop highly tunable and integratable THz sources based on graphene device.https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-27-8-11137

Abstract-Reconfigurable Terahertz Metasurface Pure Phase Holograms

Jinying Guo, Teng Wang,  Huan Zhao, Xinke Wang,  Shengfei Feng, Peng Han, Wenfeng Sun,   Jiasheng Ye, Guohai Situ, Hou‐Tong Chen,Yan Zhang

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201801696?af=R

Terahertz (THz) radiation has many potential applications. However, comparing with the rapid development of THz sources and detectors, functional devices for THz modulation, especially the spatial modulation devices, are still insufficient. Here, a novel approach for generating arbitrary wavefronts of a THz beam is presented. By dynamically creating metasurface structures through illuminating a thin silicon wafer with femtosecond laser, which is spatially modulated, an array of reconfigurable subwavelength resonators is generated. The wavefront of the THz beam is then determined by forming spatial profiles of the Pancharatnam–Berry scattering phase by dynamically controlling the resonator orientation. Proof‐of‐concept experiments demonstrate that streaming holographic images and lenses of variable focal length can be realized in real time. The reconfigurable scheme demonstrated here is convenient and fast, and may lead to advances in a host of THz applications.

Abstract-Vectorial Properties of a Terahertz Bessel Beam

Xinke Wang,  Zhen Wu, Yan Zhang,

https://ieeexplore.ieee.org/document/8510365


As a kind of special beams, Bessel beams are always a research hot spot in optics due to its non-diffractive and self-healing properties. Here, a zero-order terahertz (THz) Bessel beam is generated by using a Teflon axicon. By applying a THz focal-plane imaging system, the evolutions of the transverse and longitudinal electric fields are coherently measured and analyzed during the propagation process of the THz Bessel beam. The work provides a comprehensive view for exactly understanding peculiar features of a THz Bessel beam.

Abstract-Efficient manipulations of circularly polarized terahertz waves with transmissive metasurfaces

Min Jia, Zhuo Wang, Heting Li, Xinke Wang, Weijie Luo, Shulin Sun, Yan Zhang, Qiong He, Lei Zhou,

Fig. 1: Working principle of the high-efficiency photonic spin Hall effect (PSHE) and background-free Bessel beam (BB) generation for circularly polarized (CP) waves in a transmission geometry.

https://www.nature.com/articles/s41377-019-0127-0

The unrestricted control of circularly polarized (CP) terahertz (THz) waves is important in science and applications, but conventional THz devices suffer from issues of bulky size and low efficiency. Although Pancharatnam–Berry (PB) metasurfaces have shown strong capabilities to control CP waves, transmission-mode PB devices realized in the THz regime are less efficient, limiting their applications in practice. Here, based on Jones matrix analysis, we design a tri-layer structure (thickness of ~λ/5) and experimentally demonstrate that the structure can serve as a highly efficient transmissive meta-atom (relative efficiency of ~90%) to build PB metadevices for manipulating CP THz waves. Two ultrathin THz metadevices are fabricated and experimentally characterized with a z-scan THz imaging system. The first device can realize a photonic spin Hall effect with an experimentally demonstrated relative efficiency of ~90%, whereas the second device can generate a high-quality background-free CP Bessel beam with measured longitudinal and transverse field patterns that exhibit the nondiffracting characteristics of a Bessel beam. All the experimental results are in excellent agreement with full-wave simulations. Our results pave the way to freely manipulate CP THz beams, laying a solid basis for future applications such as biomolecular control and THz signal transportation.

Abstract-Vector measurement and performance tuning of a terahertz bottle beam

Heting Li, Xinke Wang, Sen Wang, Wenfeng Sun, Jiasheng Ye, Peng Han, Shengfei Feng, Yue Yu,  Yan Zhang,

https://www.nature.com/articles/s41598-018-31250-7

A terahertz (THz) bottle beam is realized by adopting the combination of a Teflon axicon and a silicon lens. By using a THz imaging system with a focal-plane array, the vector characteristics of the THz bottle beam are coherently measured and detailedly analyzed, including the transverse (Ex) and longitudinal (Ez) components. The experimental phenomena vividly reveal the distribution characteristics and the formation origin of the THz optical barrier. A vectorial diffraction integral algorithm of a focusing optical system are utilized to exactly simulate the measured results. Besides, the features of the THz bottle beam are effectively tuned by varying the parameters of the Teflon axicon and the silicon lens. This work gives a full view to understand the evolution characteristics of the THz bottle beam and provide a solid experimental foundation for guiding the future applications of this type of THz beam.

Abstract-Vectorial diffraction properties of THz vortex Bessel beams

Zhen Wu, Xinke Wang, Wenfeng Sun, Shengfei Feng, Peng Han, Jiasheng Ye, Yue Yu, Yan Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-2-1506&origin=search

A vortex Bessel beam combines the merits of an optical vortex and a Bessel beam, including a spiral wave front and a non-diffractive feature, which has immense application potentials in optical trapping, optical fabrication, optical communications, and so on. Here, linearly and circularly polarized vortex Bessel beams in the terahertz (THz) frequency range are generated by utilizing a THz quarter wave plate, a spiral phase plate, and Teflon axicons with different opening angles. Taking advantage of a THz focal-plane imaging system, vectorial diffraction properties of the THz vortex Bessel beams are comprehensively characterized and discussed, including the transverse (Ex, Ey) and longitudinal (Ez) polarization components. The experimental phenomena are accurately simulated by adopting the vectorial Rayleigh diffraction integral. By varying the opening angle of the axicon, the characteristic parameters of these THz vortex Bessel beams are exhibited and compared, including the light spot size, the diffraction-free range, and the phase evolution process. This work provides the precise experimental and theoretical bases for the comprehension and application of a THz vortex Bessel beam.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-High-efficiency terahertz devices based on cross-polarization converter

• Huan Zhao, 
• Xinke Wang, 
• Jingwen He, 
• Jinying Guo, 
• Jiasheng Ye, 
• Qiang Kan,  
• Yan Zhan

• https://www.nature.com/articles/s41598-017-18013-6#Sec2

Metasurface-based devices have been investigated intensively because of their attractive properties but these devices generally suffer from low efficiency. Here we demonstrate several high-efficiency terahertz (THz) devices based on cross-polarization converters that is composed of bilayer metasurface-based structures. The converter can transfer the polarization states of transmitted THz waves from the x-direction into the y-direction with an experimental conversion efficiency of 85%. This high-efficiency transfer mechanism is investigated in detail. Furthermore, this kind of devices can be fabricated easily. A THz metalens is designed and fabricated and its focusing and imaging properties are investigated experimentally. A pure phase THz hologram that can generate different images on different propagation planes is also designed and the image reconstruction capabilities of the phase holograms are demonstrated experimentally. The performance levels of all designed devices show excellent agreement between the theoretical expectations and the corresponding experimental results. This technology may pave the way towards practical applications of such metasurface devices.

Abstract-Vector characterization of zero-order terahertz Bessel beams with linear and circular polarizations

https://www.nature.com/articles/s41598-017-12524-y

As a kind of special beams, Bessel beams are always a research hot spot in optics due to its non-diffractive and self-healing properties. Here, zero-order terahertz (THz) Bessel beams with linear and circular polarizations are generated by using a THz quarter wave plate and Teflon axicons with different opening angles. By applying a THz digital holographic imaging system, the evolutions of the transverse (E x , E y ) and longitudinal (E z ) electric fields are coherently measured and analyzed during the propagation processes of the THz Bessel beams. The vectorial Rayleigh diffraction integral is used to accurately reproduce the amplitude, phase, and non-diffractive feature of each polarization component for the THz Bessel beams. With varying opening angles of the axicons, the focal spots, diffraction-free ranges, and Gouy phase shifts of the THz Bessel beams are compared and discussed. The experiment and simulation results provide a comprehensive view for exactly understanding peculiar features of THz Bessel beams.

Abstract-Active modulation of the terahertz spectra radiated from two air plasmas

Ying Zhang, Wenfeng Sun, Xinke Wang, Jiasheng Ye, Shengfei Feng, Peng Han, and Yan Zhang

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-42-10-1907&origin=search

A simple and energy-saving method has been proposed to actively modulate the spectra of terahertz (THz) waves radiated from two serial plasmas, which uses the background light to generate one plasma to make full use of the energy of the femtosecond laser. With this method, the modulation of the central frequency, spectral bandwidth, and spectral profile of the output THz waves have been observed. The shifting of the amplitude dip has been manipulated by changing the distance of the two serial plasmas. The manipulation results agree with the ones simulated by the transition-Cherenkov model. This proposed method provides a useful tool for getting the modulated THz spectra that can be used in the THz remote sensing.

© 2017 Optical Society of America

Abstract-Observation of dehydration dynamics in biological tissues with terahertz digital holography [Invited]

Lihan Guo, Xinke Wang, Peng Han, Wenfeng Sun, Shengfei Feng, Jiasheng Ye, and Yan Zhang

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-56-13-F173&origin=search

A terahertz (THz) digital holographic imaging system is utilized to investigate natural dehydration processes in three types of biological tissues, including cattle, mutton, and pork. An image reconstruction algorithm is applied to remove the diffraction influence of THz waves and further improve clarity of THz images. From THz images of different biological specimens, distinctive water content as well as dehydration features of adipose and muscle tissues are precisely distinguished. By analyzing THz absorption spectra of these samples, temporal evolution characteristics of the absorbances for adipose and muscle tissues are described and compared in detail. Discrepancies between water retention ability of different animal tissues are also discussed. The imaging technique provides a valuable measurement platform for biological sensing.

© 2017 Optical Society of America

Abstract-Terahertz vortex beam generator based on a photopatterned large birefringence liquid crystal

Shijun Ge, Peng Chen, Zhixiong Shen, Wenfeng Sun, Xinke Wang, Wei Hu, Yan Zhang, and Yanqing Lu

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-11-12349

A terahertz (THz) q-plate is proposed and demonstrated to generate THz vortex beams. It is composed of a large birefringence liquid crystal (LC) with spatially-varying director distribution sandwiched by two pieces of fused silica glass. A polarization-sensitive alignment agent is photopatterned to carry out the specific LC director distribution. THz vortex beams with different topological charges are characterized with a THz digital holographic imaging system. The intensity and phase distributions consistent with theoretical analyses are obtained. Besides, an eight-lobed intensity distribution is observed corresponding to the vertical polarization component of a cylindrical vector beam. This work may inspire novel THz applications.

© 2017 Optical Society of America

Abstract-Ultra-wide band reflective metamaterial wave plates for terahertz waves

Shaojie Ma1, Xinke Wang2, Weijie Luo1, Shulin Sun3, Yan Zhang2, Qiong He1,4 and Lei Zhou1,4

Published 31 March 2017 • Copyright © EPLA, 2017 

http://iopscience.iop.org/article/10.1209/0295-5075/117/37007/meta

Terahertz (THz) electromagnetic waves have important applications in science and technology but available functional devices suffer from the issues of bulky size, low efficiency and narrow bandwidth. Here, based on Jones matrix and Poincaré sphere analyses, we present a set of general criterions to help design high-efficiency ultra-wide band THz wave plates using ultra-thin reflective metamaterials. Two half-wavelength and one quarter-wavelength THz wave plates are designed and fabricated based on the general criterions, and their excellent polarization manipulation capabilities are demonstrated experimentally. In particular, the realized devices, with thicknesses ~ λ/7, exhibit polarization-conversion efficiencies higher than 80% in ultra-wide working bandwidths (relative bandwidth >80% at about ~ 0.7 THz).