Hsiang-Chu Wang

Ultrathin Planar Cavity Metasurfaces

An ultrathin planar cavity metasurface is proposed based on ultrathin film interference and its practicability for light manipulation in visible region is experimentally demonstrated. Phase of reflected light is modulated by finely adjusting the thickness of amorphous silicon (a-Si) by a few nanometers on an aluminum (Al) substrate via nontrivial phase shifts at the interfaces and interference of multireflections generated from the planar cavity. A phase shift of π, the basic requirement for two-level phase metasurface systems, can be accomplished with an 8 nm thick difference. For proof of concept, gradient metasurfaces for beam deflection, Fresnel zone plate metalens for light focusing, and metaholograms for image reconstruction are presented, demonstrating polarization-independent and broadband characteristics. This novel mechanism for phase modulation with ultrathin planar cavity provides diverse routes to construct advanced flat optical devices with versatile applications.

Ultra-thin film metasurface (Conference Presentation)

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.

Light manipulation with metasurface and meta-device (Conference Presentation)

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.

Fabrication of three-dimensional plasmonic structure and multilayer metamaterials by femtosecond laser-induced forward transfer

Plasmonic metamaterials, composed by arti?cial micro or nanostructures, exhibit extraordinary optical properties such as negative refraction, which cannot be observed in nature. The focused ion beam (FIB) and electron beam lithography (EBL) technique are frequently adopted to fabricate multilayer and three-dimensional plasmonic structures, but the alignment error, high cost and time consuming cannot be avoided. In contrast, the laser direct writing (LDW) technique is one of the best candidates to fabricate plasmonic structure owing to its low cost, high throughput and simple experimental setup. Here, we fabricate the plasmonic structures to manipulate the scattering of SPP waves by LDW technique. Both the backward and forward scattering of surface plasmonic waves can be generated by the arranged gold (Au) nanobumps on an Au thin film. Upon controlling the geometry of the plasmonic structures, the height, position, and pattern of scattered light are modified as desired. It provides a simple and efficient way to project a specific light pattern into free space, and demonstrate the capability of three-dimensional light manipulation. Furthermore, we demonstrated that, with tightly contacted donor-receiver pair, multilayer plasmonic structures such as multilayer split resonant rings and square-shaped multilayer resonant cavities, can be fabricated on donor by laser-induced forward transfer (LIFT) process as a kind of LDW. By using the contact-mode LIFT, the illuminated materials of sputtered multilayer thin films are ablated and transferred to the receiver, which would leave the uniform metamolecules with multilayer structures on the donor. Finally, we have analyzed their plasmonic modes by both measurement and simulated results. These properties may facilitate many applications in integrated optics, optical nonlinearities, and luminescence enhancement, etc..

Novel applications of plasmonic metamaterials

Metamaterials are the artificial materials composed of multiple individual elements in specially designed periodic subwavelength patterns. The properties of these metamaterials strongly depend on the featured shape or size of the unit of metamaterials rather than the original properties of the composition of materials. Their precise shape, geometry, size, orientation and arrangement can affect the light in an unconventional manner, creating material properties which are unachievable with conventional materials, such as negative refractive index, abnormal reflection, and so on. Due to these fascinating properties, we have designed and fabricated various plasmonic metamaterials with localized energy, electromagnetic field enhancement, and nonlinearity by the state of the art methods, such as e-beam lithography with multi-exposure technique, focus ion beam drilling, ultrafast laser direct writing, phase-change materials etching process, etc..

Coherent control of meta-device(Conference Presentation)

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.

Optical response of phase change material for metasurface(Conference Presentation)

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.

The extreme of optical recording(Conference Presentation)

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.