With signatures of high photon energy and short wavelength, ultraviolet (UV) light enables numerous applications such as high-resolution imaging, photolithography and sensing. In order to manipulate UV light, bulky optics are usually required and thereby do not meet the fast-growing requirements of integration in compact systems. Recently, metasurfaces, with subwavelength or wavelength thicknesses, have shown unprecedented control of light, enabling substantial miniaturization of photonic devices from Terahertz to visible regions. However, material limitations and fabrication challenges have hampered the realization of such functionalities at shorter wavelengths. Herein, we theoretically and experimentally demonstrate that metasurfaces, made of highly scattering silicon (Si) antennas, can be designed and fabricated to manipulate broadband UV light. The metasurface thickness is only one-tenth of the working wavelength, resulting in very small height-to-width aspect ratio (~ 1). Peak conversion efficiency reaches 15% and diffraction efficiency is up to 30%, which are comparable to plasmonic metasurface performances in infrared (IR). A double bar structure is proposed to further improve the metasurface’s diffraction efficiency to close to 100% in transmission mode over a broad UV band. Moreover, for the first time, we show photolithography enabled by metasurface-generated UV holograms. We attribute such performance enhancement to the high scattering cross-sections of Si antennas in the UV range, which is adequately modeled via a circuit. Our new platform will deepen our understanding of light-matter interactions and introduce even more material options to broadband metaphotonic applications, including those in integrated photonics and holographic lithography technologies.
Metamaterials have revolutionized the ways we control light and design optical materials. Researchers have been highly successful in designing sub-wavelength “meta-atoms” to achieve new optical functionalities. On the other hand, “meta-atoms” have to be made of natural materials, therefore one can further tune the responses of metamaterials by varying the constituent natural materials and achieve even broader range of optical properties.
In my research lab, we conduct extensive exploration of optical materials beyond the widely used noble metals to achieve novel capabilities of metaphotonic systems. Here I would like to share some of our recent progresses on implementing new material platforms in metaphotonics:
First I will present an all-solid, rewritable metacanvas using phase change materials, on which arbitrary photonic devices can be rapidly and repeatedly written and erased for real-time manipulation of light waves. Different patterns can be written and erased on the same metacanvas with a low-power laser. Dynamic manipulation of optical waves is demonstrated with the metacanvas, specifically light propagation, polarization, and reconstruction. This dynamic optical system without moving parts opens possibilities where photonic elements can be field-programmed to deliver complex, system-level functionalities.
We have also demonstrated a simple bottom-up approach to create self-assembled, nanostructured metamaterials with controllable structural geometry and temperature-tunable optical response from spinodally-decomposed VO2-TiO2 epitaxial thin films. As-grown solid solution films are driven to phase separate upon post-annealing and we demonstrate the ability to deterministically create horizontally- or vertically-aligned lamellae consisting of Ti- and V-rich phases. The optical iso-frequency surface of the self-assembled nanostructures can be made to exhibit a temperature-tunable transition from elliptic to hyperbolic dispersion in the near-infrared range and thus the formation of hyperbolic metamaterial response.
Lastly, I will talk about our recent efforts in achieving functional metasurfaces in the UV range by introducing new material platforms and new light-manipulation mechanisms.
Two-dimensional (2D) materials with natural layer structures have been proven to provide extraordinary physical
and chemical properties. Bismuth chalcogenides are examples of such two-dimensional materials. They are strongly
bonded within layers and weak van der Waals interaction ties those layers together. Such naturally layered structure
allows chemical intercalation of foreign atoms into the van der Waals gaps. Here, we show that by adding large
number of copper atoms into van der Waals gaps of bismuth chalcogenides, we observed counter-intuitive
enhancement of optical transparency together with improved electrical conductivity, which is on the contrary to
most bulk materials in which doping reduces the light transmission. This surprising behavior is caused by substantial
tuning of material optical property and nanophotonic anti-reflection effect unique to ultra-thin nanoplates. With the
intercalation of copper atoms, large number of electrons have been added into the semiconducting material system
and effectively lift the Fermi level of the resulting material to its conduction band, as proved by our densityfunctional-
theory computations. Occupied lower states in the conduction band do not allow the optical excitation of
electrons in the valence band to the bottom of the conduction band, leading to an effective widening of optical band
gap. Optical transmission is further enhanced by constructive interference of reflected beams as bismuth
chalcogenides have large permittivity than the environment. The synergy of these two effects in two-dimensional
nanostructures can be exploited for various optoelectronic applications including transparent electrode. The
reversible intercalation process allows potential dynamic tuning capability.
Surface plasmon polaritons, sometimes referred to as Surface Plasmons (SPs) have brought us great opportunities to
work in nanoscale at optical frequencies. The SPs at the two surfaces of a thin metal film interact with each other, hence
generate new modes which are either symmetric or anti-symmetric. For anti-symmetric modes, the dispersion curve turns
to be of negative slope at large wave vectors, so two different anti-symmetric modes can be excited at the same
frequency. These two modes can form beats with novel features. The envelope (profile) of the beating SP waves could be
stationary, which means its shape will not change in time. Our simulation results clearly showed such phenomena, which
is a strong evidence of the SPs dispersion relations at the thin metal film. It is a proof of the existence of negative group
velocity of SPs. Beats can help us determine the difference in k and the amplitudes ratio of the two beating waves. We
also studied beating between anti-symmetric mode and symmetric mode SPs with the same frequency. The study of the
energy density distribution showed that the output from such system can be well controlled through beats formation.
Example by using NSOM (Near-field Scanning Optical Microscopy) has been simulated. The beating phenomena have a
potential application in the integrated optical circuits.