Qiaofeng Tan

Three-dimensional optical holography using a plasmonic metasurface

Benefitting from the flexibility in engineering their optical response, metamaterials have been used to achieve control over the propagation of light to an unprecedented level, leading to highly unconventional and versatile optical functionalities compared with their natural counterparts. Recently, the emerging field of metasurfaces, which consist of a monolayer of photonic artificial atoms, has offered attractive functionalities for shaping wave fronts of light by introducing an abrupt interfacial phase discontinuity. Here we realize three-dimensional holography by using metasurfaces made of subwavelength metallic nanorods with spatially varying orientations. The phase discontinuity takes place when the helicity of incident circularly polarized light is reversed. As the phase can be continuously controlled in each subwavelength unit cell by the rod orientation, metasurfaces represent a new route towards high-resolution on-axis three-dimensional holograms with a wide field of view. In addition, the undesired effect of multiple diffraction orders usually accompanying holography is eliminated.

Dual-polarity plasmonic metalens for visible light

Surface topography and refractive index profile dictate the deterministic functionality of a lens. The polarity of most lenses reported so far, that is, either positive (convex) or negative (concave), depends on the curvatures of the interfaces. Here we experimentally demonstrate a counter-intuitive dual-polarity flat lens based on helicity-dependent phase discontinuities for circularly polarized light. Specifically, by controlling the helicity of the input light, the positive and negative polarity are interchangeable in one identical flat lens. Helicity-controllable real and virtual focal planes, as well as magnified and demagnified imaging, are observed on the same plasmonic lens at visible and near-infrared wavelengths. The plasmonic metalens with dual polarity may empower advanced research and applications in helicity-dependent focusing and imaging devices, angular-momentum-based quantum information processing and integrated nano-optoelectronics.

Dispersionless phase discontinuities for controlling light propagation.

Ultrathin metasurfaces consisting of a monolayer of subwavelength plasmonic resonators are capable of generating local abrupt phase changes and can be used for controlling the wavefront of electromagnetic waves. The phase change occurs for transmitted or reflected wave components whose polarization is orthogonal to that of a linearly polarized (LP) incident wave. As the phase shift relies on the resonant features of the plasmonic structures, it is in general wavelength-dependent. Here, we investigate the interaction of circularly polarized (CP) light at an interface composed of a dipole antenna array to create spatially varying abrupt phase discontinuities. The phase discontinuity is dispersionless, that is, it solely depends on the orientation of dipole antennas, but not their spectral response and the wavelength of incident light. By arranging the antennas in an array with a constant phase gradient along the interface, the phenomenon of broadband anomalous refraction is observed ranging from visible to near-infrared wavelengths. We further design and experimentally demonstrate an ultrathin phase gradient interface to generate a broadband optical vortex beam based on the above principle.

Helicity dependent directional surface plasmon polariton excitation using a metasurface with interfacial phase discontinuity

Researchers have now demonstrated a reconfigurable, unidirectional coupling scheme for excitation of collective oscillations of the electrons at a metal surface, the so-called surface plasmon-polaritons. Lingling Huang and co-workers achieved an efficient and controllable coupling of photons in free space to these surface states on a metal film — a task important for the future development of nanoscale optoelectronic circuitry — by employing a nanostructured thin metal film on a glass substrate. The nanostructured metal film features an array of rectangular nano-apertures arranged in a carefully designed orientation and pattern. Such a ‘plasmonic metasurface’ couples photons to surface plasmon-polaritons while depending crucially on the circular polarization state of the incident light. As a result, when circularly polarized light strikes the surface, the handedness of the light dictates the propagation direction of the resulting surface plasmon-polaritons.

Integrated plasmonic semi-circular launcher for dielectric-loaded surface plasmon-polariton waveguide

A semi-circular plasmonic launcher integrated with dielectric-loaded surface plasmon-polaritons waveguide (DLSPPW) is proposed and analyzed theoretically, which can focus and efficiently couple the excited surface plasmon polaritons (SPPs) into the DLSPPW via the highly matched spatial field distribution with the waveguide mode in the focal plane. By tuning the incident angle or polarization of the illuminating beam, it is shown that the launcher may be conveniently used as a switch or a multiplexer that have potential applications in plasmonic circuitry. Furthermore, from an applicational point of view, it is analyzed how the coupling performance of the launcher can be further improved by employing multiple semi-circular slits.

Security enhanced optical encryption system by random phase key and permutation key

Conventional double random phase encoding (DRPE) encrypts plaintext to white noise-like ciphertext which may attract attention of eavesdroppers, and recent research reported that DRPE is vulnerable to various attacks. Here we propose a security enhanced optical encryption system that can hide the existence of secret information by watermarking. The plaintext is encrypted using iterative fractional Fourier transform with random phase key, and ciphertext is randomly permuted with permutation key before watermarking. Cryptanalysis shows that linearity of the security system has been broken and the permutation key prevent the attacker from accessing the ciphertext in various attacks. A series of simulations have shown the effectiveness of this system and the security strength is enhanced for invisibility, nonlinearity and resistance against attacks.

Improved fast fractional-Fourier-transform algorithm

Through the optimization of the main interval of the fractional order, an improved fast algorithm for numerical calculation of the fractional Fourier transforms is proposed. With this improved algorithm, the fractional Fourier transforms of a rectangular function and a Gaussian function are calculated. Its calculation errors are compared with those calculated with the previously published algorithm, and the results show that the calculation accuracy of the improved algorithm is much higher.

Theories for the design of diffractive superresolution elements and limits of optical superresolution.

We suggest using the theory of linear programming to design diffractive superresolution elements if the upper bound of the intensity distribution on the input plane is restricted, and using variation theory of functional or wide-sense eigenvalue theory of matrix if the upper bound of the radiation flux through the input plane is restricted. Globally optimal solutions can be obtained by each of these theories. Several rules of the structure and the superresolution performance of diffractive superresolution elements are provided, which verify the validity of these theories and set some limits of optical superresolution.

Holographic display system of a three-dimensional image with distortion-free magnification and zero-order elimination

We propose a three-dimensional (3-D) holographic display system which consists of a phase-only spatial light modulator (SLM) and a modified 4-f system. The 3-D scene is generated from OpenGL, and the point source algorithm with anti-aliasing technique is used to generate the Fresnel hologram. A modified 4-f system is proposed to produce distortion-free magnification of the 3-D image and eliminate the zero-order interruption of the 3-D holographic imaging system. This method can make efficient utilization of the space-bandwidth product of the SLM, which promises the image quality and keeps the 3-D imaging zone unchanged. Numerical simulations and optical experiments are performed, and the results show that our proposed method can reconstruct enlarged 3-D optical image with correct magnification factor and low image noise.

Achromatic phase retarder applied to MWIR & LWIR dual-band.

The development of the dual-band IR imaging polarimetry creates the need for achromatic phase retarder used in dual-band. Dielectric grating with the period smaller than the illuminating wavelength presents a strong form-birefringence. With this feature, the combination of several subwavelength gratings can be used as achromatic phase retarders. We proposed a combination of 4 subwavelength structured gratings (SWGs) used as an achromatic quarter-wave plate (QWP) applied to MWIR & LWIR bandwidths. Design method using effective medium theory and optimization algorithms is described in detail. The simulation results led to the possibility of an dual-band achromatic QWP whose retardance deviates from 90 degrees by <+/-0.75 degrees with the fast axis unfixed and by <+/-1.35 degrees with the fast axis fixed over MWIR(3-5microm) & LWIR(8-12microm) bandwiths.