Shrinking conventional optical systems to chip-scale dimensions will benefit custom applications in imaging, displaying, sensing, spectroscopy, and metrology. Towards this goal, metasurfaces — planar arrays of subwavelength electromagnetic structures that collectively mimic the functionality of thicker conventional optical elements — have been exploited at frequencies ranging from the microwave up to the visible. Here, we demonstrate high-performance metasurface optical components operating at ultraviolet frequencies, including down to the record-short deep ultraviolet range, and performing representative wavefront shaping functions, namely high numerical-aperture lensing, accelerating beam generation, and hologram projection. The constituent nanostructured elements of the metasurfaces are formed of hafnium oxide — a lossless, high-refractive-index dielectric material deposited using low-temperature atomic layer deposition and patterned using high-aspect-ratio Damascene lithography. This study opens the way towards low-form-factor, multifunctional ultraviolet nanophotonic platforms based on flat optical components, enabling diverse applications including lithography, imaging, spectroscopy, and quantum information processing
The ultraviolet (UV) range (wavelength of light: 200 nm to 400 nm) is a technologically important spectral regime hosting diverse applications such as photolithography, high-resolution imaging, spectroscopy, quantum optics, atomic trapping, etc. Current technology for manipulating UV light waves is largely based on "conventional" optical elements, whose working principles are based on light refraction or diffraction. These elements suffer from issues including large footprint, complicated (dedicated) manufacturing process, limited functionalities or operational bands (in contrast to their counterparts for the visible and infrared regimes), reduced operational efficiencies towards short wavelengths (e.g., for the deep-UV range), etc. In this invention, we employ a novel approach to construct high-performance optical elements operating in the UV, moreover, into the deep-UV regime. Our technology is based on metasurfaces, where we design nanoantennas with sizes of a fraction of the scale the wavelength of UV light, and arrange them over a planar surface. These nanoantennas collectively imprint arbitrary amplitude, phase or polarization manipulations an incident UV light.
Metasurface technology has been demonstrated for the infrared and visible regime, using dielectric materials such as silicon (Si), titanium oxide (TiO2), gallium nitride (GaN), etc. However, utilization of such technology for the UV regime has been impeded by the scarcity of transparent dielectric materials in this spectral range and the required high-aspect-ratio nanopatterning techniques at this wavelength scale. To overcome such material and fabrication limitations, we identify a suitable metasurface constituent dielectric — hafnium oxide (HfO2), justified by (i) an exceptionally low extinction coefficient, k ≈ 0, down to a free-space wavelength of 217 nm; (ii) a relatively large refractive index n > 2.1 for free-space wavelength < 400 nm; and (iii) material compatibility with CMOS production. Simultaneously, we develop a unique high-aspect-ratio nanopatterning technique based on a Damascene lithography process incorporating low-temperature atomic layer deposition of high-optical-quality HfO2 and ion milling. In contrast to the above-mentioned bulky conventional optical components with spatially varying thickness profiles, our devices feature a much smaller footprint with a planar thickness profile. Also, we can freely design an optical element working at an arbitrary UV wavelength with an arbitrary function, moreover with high operation efficiencies. We experimentally demonstrated representative wavefront shaping UV optical elements including lenses, accelerating beam generators and holograms, which operate at wavelengths spanning the near- to deep-UV, and exhibit efficiencies up to 72% (see the attached manuscript). Furthermore, our technology enables optical elements with complicated functionalities such as multiplexing, which conventional UV optical technology cannot easily achieve. We experimentally demonstrate spin-multiplexed hologram projection at 364 nm (near-UV range) and 266 nm (deep-UV range), as well as spin-controlled accelerating beam generation at 364 nm (please see the attached manuscript). This invention opens opportunities for creation of low-loss, multifunctional, and “flat” optical elements for operation in the near-, mid- and deep-ultraviolet regimes, that are ideally suited for integration into compact nanophotonic systems.