The functionalities of traditional optical component are mainly based on the phase accumulation through the propagation length, leading to a bulky optical component such as converging lens and waveplate. Metasurfaces composed of planar structures with artificial design have attracted a huge number of interests due to their ability on controlling the electromagnetic phase as well as amplitude at a subwavelength scale. The feasible applications based on metasurfaces include nonlinear dynamics, light beam shaping, quantum interference etc. Beside those promising characteristics, people now intend to discover the field of meta-devices, where we can attain optical properties and functionalities through changing the feature characteristics of metasurfaces in demand. They therefore pave a potential way for the development of flat optical devices and integrated optoelectronic systems and toward the far-reaching applications which are impossible previously. In this talk, four research topics for photonic applications with metasurfaces and meta-devices will be performed and discussed: high efficiency anomalous beam deflector, highly dimensional holographic imaging, versatile polarization control and metadevices with active property.
Metasurfaces, the two-dimensional (2D) sub-wavelength artificial structures, where light is not required to have a deep penetration, have shown the ability to tailor the amplitude, phase and polarization of light. The functionalities of various optical components can be realized by metasurface-based design, such as beam splitters, filters, waveplates, deflector, lens and holograms. Here, we propose a new type of metasurface based on the concept of ultra-thin film interference and experimentally demonstrate its feasibilities in beam deflector, light focusing and broadband meta-hologram in visible spectrum. Considering an ultra-thin thin film interference system, a sandwich structure, composed of air, a lossy material layer and a metallic mirror, the reflection of this system can be regarded as the linear superposition of the partial reflections from first interface and from the cavity after several roundtrips. First, we calculate the phases and reflections of various thicknesses of amorphous silicon (a-Si) on top of aluminum layer under normal illumination of an unpolarized light in the wavelength region from 400 to 850nm. 2 π phase coverage can be achieved by changing the film thickness of a-Si within 50 nanometers. We select two thicknesses (2-level phase modulation) for the demonstration of meta-devices. The ultra-flat grating metasurface for beam steering are designed. The reflection angles of grating metasurface can be modulated by changing its period, while the specular reflection is inhibited. We further demonstrate computer-generated holograms (CGH) based on ultra-thin interference metasurface. The holographic images are reconstructed by the combinations of phase- and amplitude- modulation. These devices show the great potential and CMOS-compatibility in the application of optics, display, security printing, and metasurface-based optical storage system.
Increasing the nonlinear optical response at nanometer length scale is a very important issue due to the wide applications in various disciplines such as information science, bio-medicine and quantum computation technology. Second harmonic generation (SHG) arising from the metal nanostructures has provide a very powerful tool in studying the surface and interface properties of these materials. The SHG from various kinds of asymmetric geometric configurations such as V and L shape structures, imperfect nano-spheres, metal/insulator/metal multilayer structures, and planar split ring resonators have been proposed. However, all the previous studies in plasmonic nonlinear optical behavior rely on the enhancement of the electric field and seldom considered the magnetic field effect.
In this work, we present a vertical split ring resonator (SRR) based metamaterial to generate SHG. By adopting such a novel structure, both the electric and magnetic field will be significantly enhanced due to the localized surface plasmon resonance, hence the generation of the second-harmonic and its re-emission into the far field are dramatically increased several orders comparing with that of the planar SRR. We simulated and fabricated the reflective type vertical SRR, and optimized the aspect ratio to maximize the SHG signal. We further systematically studied the nonlinear optical response in the vertical SRR dimers and trimers and found that the gap distance between two SRRs plays a very important role in the SHG intensity. This work paves a new way in increasing the nonlinear transition quantum efficiency and provides a new insight in designing new nonlinear sources.
Metamaterials, with the ability of tailoring optical properties of materials, have been applied to holograms recently, which has shown the priorities of switchable polarization and multicolor image comparing with the conventional holograms. However, the current metasurface based multicolor holograms have suffered the problems of narrow band and low efficiency in phase modulation for gold and silver when their feature dimensions are in few tens of nanometers. Interestingly, aluminum with higher plasma frequency could yield surface plasmon resonance across a broader range of the spectrum ranging from visible to UV. Metasurfaces incorporating with the aluminum offer the unique opportunity to extend the working wavelength to cover the entire visible spectrum for the generation of full color meta-holograms.
Here we demonstrated a phase modulated multicolor meta-hologram that is polarization dependent and capable of producing images in red, green and blue colors. The metahologram is made of aluminum nanorods that are arranged in a two-dimensional array of pixels with surface plasmon resonance in the visible to UV range. The aluminum nanorod array is patterned on a 30 nm thick SIO2 spacer layer sputtered on top of a 130nm thick aluminum mirror. With proper design of the structure, we obtain resonances of narrow bandwidths to allow for implementation of multicolor scheme. Taking into account of the wavelength dependence of the diffraction angle, we can project images to specific locations with predetermined size and order. With tuning of aluminum nanorod size, we demonstrate that the image color can be continuously varied across the visible spectrum.
Upconversion fluorescence from Lanthanide-doped nanocrystals has attracted widespread interests because of its greatly potential applications in various fields, such as photonic crystal lasers, material science, biological therapy, and so on. However, the relatively low quantum yield (typically < 5%) is the major limitation for upconversion nanocrystals. Meanwhile, in addition to the chemical methods, plasmonic structures have been adopted as another strategy to improve the radiation efficiency and control the relaxation process of the upcovnersion nanocrystals. We designed the anti-symmetric split ring resonators with various periods and the fishnet structures. The surface plasmon resonance peaks of the structure shift as the periods varies. For example, in a multi-layered plamsonic metasurface with the period of 250nm, both the electric and magnetic modes could be generated simultaneously when excited by the incident light with proper polarization. This plasmonic structure provides two different channels for the enhancement of upconversion fluorescence. The resonance peak of 650nm is magnetic resonance mode, while the peak of 980nm is electric resonance mode. The resonance peak of 980nm coincides with the absorption band of the Lanthanide-dopoed nanocrystal, and the peak of 650nm matches with its emission band. We found that the upconversion fluorescence intensity could be enhanced more than 10 times when the electric resonance frequency of the metasurface matches with the absorption band of the upconversion nanocrystals, while the magnetic mode overlaps with its emission band. This is due to the local density of optical states was significantly enhanced by the plasmonic metasurface. The detailed results and mechanism will be discussed.
Selective excitation of specific multipolar resonances in matter can be of great utility in understanding the internal make-up of the underlying material and, as a result, in developing novel nanophotonic devices. Many efforts have been addressed on this topic. For example, the emission spectra related to the different multipolar transitions of trivalent europium can be modulated by changing the thickness of the dielectric spacer between the gold mirror and the fluorescent layer. In this talk, we reported the results about active control of the multipolar resonance in metadevices using the coherent control technique. In the coherent control spectroscopy system, the optical standing wave constructed from two counterpart propagation coherent beams is utilized as the excitation. By controlling the time delay between two ultrafast pulses to decide the location of metadivce as the electromagnetic field node or antinode node of standing wave, the absorption related to the specific multipolar resonance can be controlled. Using this technique, with the 30-nm-thick metadevice, the broadband controlling light with light without nonlinearity can be realized. The switching contrast ratios can be as high as 3:1 with a modulation bandwidth in excess of 2 THz. The active control of the high order and complex optical resonance related to the magnetic dipole, electric quadrupole, and toroidal dipole in the metamaterial is reported as well. This research can be applied in the all ultrafast all-optical data processing and the active control of the resonances of metadevice with high order multipolar resonance.
Conventional optical data storage such as digital versatile disc (DVD) and Blu-ray disc (BD), provide us inexpensive and compact media for satisfying information storage requirement for decades. As the knowledge and information increase rapidly, the requirement cannot be already satisfied by current data storage systems. As far as we know, the size of recording mark, the critical storage density indicator, depends on recording energy, writing strategies, opto-thermal threshold plane and thermal conductivity. Readout is limited by optical resolution limit, the wavelength of readout laser and numerical aperture (N.A.) of objective lens. In this talk, I will introduce several means to increase the optical storage density. A powerful tool, conductive-tip atomic force microscopy (C-AFM), with the advantages of high spatial resolution, high contrast of conductivity and non-destructive method to help us better understand the formation of recording marks is also presented. Finally, I will show our recent efforts on realizing the extreme of recording mark.
Phase change materials are used as the recording layer in optical data storage, electronic storage and nanolithography due to the enormous physical difference between crystalline and amorphous states. In recent years, they are demonstrated to exploit in various tunable plasmonic devices, such as perfect absorber, planar lenses, plasmonic antenna, Fano resonance and so on. However, in these researches, the phase change material merely plays a role as a refractive index switchable substrate. In this paper, we study the intrinsic optical properties of phase change material Ge2Sb2Te5 (GST) in the near-infrared regime. A clear insight into the dipole resonance system of GST is provided. The reflection phase retardation and intensity of each unit cells depending on the phase state and geometry are estimated. Further, we introduce the concept of reconfigurable gradient metasurface, which has different anomalous reflection angles by switching the combination of nanorods with different geometries and phase states. The research has great potential in the area of tunable metamaterial device (metadevice) in the future.