Zhenwei Xie

Optical image encryption with spatially incoherent illumination

A simple optical image encryption system with spatially incoherent illumination is proposed. Only one Fourier lens and one random phase plate are contained in this system. The original image is perturbed by a random phase-only mask located on the aperture plane. The encrypted information is the only intensity distribution that can be directly recorded by a CCD or CMOS. The decryption procedure can be performed digitally and the random phase distribution works as the decryption key. Numerical simulations and experimental results demonstrate the validity of the proposed method.

Wavelength de-multiplexing metasurface hologram.

A wavelength de-multiplexing metasurface hologram composed of subwavelength metallic antennas is designed and demonstrated experimentally in the terahertz (THz) regime. Different character patterns are generated at the separated working frequencies 0.50 THz and 0.63 THz which determine a narrow frequency bandwidth of 130 GHz. The two working frequencies are around the central resonance frequency of the antennas where antennas behave strong wavefront modulation. Each antenna is fully utilized to control the wavefront of the metasurface at different frequencies by an optimization algorithm. The results demonstrate a candidate way to design multi-colors optical display elements.

Generation of terahertz vector beams with a concentric ring metal grating and photo-generated carriers

A scheme for vector terahertz (THz) beam generation is proposed. A subwavelength metal grating is utilized to adjust the polarization of the THz radiation. The amplitude and phase distributions of the THz beam are dynamically regulated by a THz computer-generated hologram (CGH) pattern of the photo-generated carriers. A radially polarized THz beam and a vector THz vortex beam with a topological charge of 1 are generated to demonstrate the validity and the effectiveness of the proposed scheme. Experimental results correspond to the theoretical simulations well. Moreover, the proposed method is applicative for a broadband THz radiation. These results could be applied in the THz sensing, THz imaging, and THz communication in the future.

Terahertz Tunable Metasurface Lens Based on Vanadium Dioxide Phase Transition

Based on the insulator-to-metal phase transition of vanadium dioxide (VO 2), a terahertz (THz) tunable metasurface lens (TML), which consists of a THz metasurface lens and a VO 2 film on Al 2O3 substrate, is proposed and experimentally verified. The focal intensity of the TML can be thermally controlled. The changes of the cross-polarized amplitude spectrum and the focal intensity during the heating and cooling processes are also investigated in detail. Any desired focal intensity can be obtained by adjusting the TML temperature. This TML and its approach will be of great significance for the development of the THz active devices.

Focusing and imaging of a virtual all-optical tunable terahertz Fresnel zone plate

A virtual all-optical tunable terahertz Fresnel zone plate is achieved utilizing the localized distribution of the transient electron plasma on a silicon wafer. Its focusing and imaging performance are experimentally demonstrated. Experimental results show that the effect of the virtual zone plate is the same as an actual one. Adjusting the spatial pattern of the electron plasma, the central wavelength and the focal length of the virtual zone plate can be all-optically dynamically steered. The research is a significant step to the development of tunable optical imaging elements.

Full vector measurements of converging terahertz beams with linear, circular, and cylindrical vortex polarization.

The complete vector information of converging terahertz (THz) beams with linear, circular, and cylindrical vortex polarization are precisely measured by using a THz digital holographic imaging system. The transverse (Ex, Ey) and longitudinal (Ez) polarization components of the THz fields around the focal point are separately obtained utilizing the detection crystals with different crystalline orientations. The measured results are in good agreement with the theoretical expectations. This imaging technique provides an effective way for revealing the vector diffraction properties of the THz electro-magnetic waves.

Terahertz polarization modulator based on metasurface

Based on the metasurface composed of C-shaped slit (CSS) antennas, a broadband transmitted terahertz (THz) polarization modulator (TPM) is proposed and experimentally verified. By rotating the designed TPM, the amplitudes and phase difference of the two orthogonal electric components can be tuned, and thus the polarization of the transmitted THz beam is modulated. The linear-to-linear and linear-to-elliptical polarization conversion is realized with the proposed TPM. Numerical simulations are also performed and agree well with the experimental measurements. The proposed concept can be applied to other wavebands simply by varying the structure parameters and the material of the substrate.

Abruptly autofocusing terahertz waves with meta-hologram

An abruptly autofocusing ring-Airy beam is demonstrated in the terahertz (THz) waveband with a meta-hologram. The designed meta-hologram is composed of gold C-shaped slot antennas, which can realize both phase and amplitude modulation of the incident THz wave. A THz holographic imaging system is utilized to measure the generated ring-Airy beam; an abrupt focus following a parabolic trajectory is subsequently observed. THz ring-Airy beams with different parameters are also generated and investigated. This method can be expanded to other wavebands, such as the visible band, for which the meta-hologram can replace traditional computer-generated holography to avoid undesirable multiple diffraction orders.

Secure optical verification using dual phase-only correlation

We introduce a security-enhanced optical verification system using dual phase-only correlation based on a novel correlation algorithm. By employing a nonlinear encoding, the inherent locks of the verification system are obtained in real-valued random distributions, and the identity keys assigned to authorized users are designed as pure phases. The verification process is implemented in two-step correlation, so only authorized identity keys can output the discriminate auto-correlation and cross-correlation signals that satisfy the reset threshold values. Compared with the traditional phase-only-correlation-based verification systems, a higher security level against counterfeiting and collisions are obtained, which is demonstrated by cryptanalysis using known attacks, such as the known-plaintext attack and the chosen-plaintext attack. Optical experiments as well as necessary numerical simulations are carried out to support the proposed verification method.

Optimization of the Rayleigh anomaly of metallic gratings for terahertz sensor applications

The frequency of the Rayleigh anomaly of a metallic grating varies with the refractive index of the surrounding medium. To employ this phenomenon in optical sensors it is essential that the feature, due to the Raleigh anomaly in the sensor’s optical response, is very distinct and stable. We study the reflectance of optical structures consisting of a metallic grating with a thin dielectric film on highly reflective silicon substrates in the terahertz (THz) range between 0.6 and 2 THz. A distinct reflectance peak due to the Rayleigh anomaly can only be observed if a dielectric spacer layer is inserted between the metallic grating and silicon substrate. The dielectric layer between the grating and substrate acts as a Fabry–Perot resonator with a low quality factor. Thus, the corresponding Fabry–Perot modes only couple weakly to the Rayleigh anomaly. At the frequency of the Rayleigh anomaly, the combined structure exhibits a distinct peak due to the Rayleigh anomaly, no matter whether there is a Fabry–Perot low or high reflection band at this frequency. In particular, the reflection background in the vicinity of the frequency of the Rayleigh anomaly is suppressed when the Rayleigh anomaly is in resonance with a Fabry–Perot low reflection band, whilst the Rayleigh reflectance peak itself is retained. The underlying physics is confirmed by a simple analytic model and by numerical calculations. This approach of optimizing the optical response of metallic gratings is important for the design of THz sensing devices based on Rayleigh anomalies.